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Project Title:  ACME: EFIELD – Electric Field Effects On Laminar Diffusion Flames Reduce
Images: icon  Fiscal Year: FY 2021 
Division: Physical Sciences 
Research Discipline/Element:
Physical Sciences: COMBUSTION SCIENCE--Combustion science 
Start Date: 11/18/2016  
End Date: 11/17/2020  
Task Last Updated: 01/10/2021 
Download report in PDF pdf
Principal Investigator/Affiliation:   Dunn-Rankin, Derek  Ph.D. / University of California - Irvine 
Address:  Department of Mechanical & Aerospace Engineering 
4200 Engineering Gateway Bldg, EG3224 
Irvine , CA 92697-3975 
Email: ddunnran@uci.edu 
Phone: 949-824-8745  
Congressional District: 48 
Web:  
Organization Type: UNIVERSITY 
Organization Name: University of California - Irvine 
Joint Agency:  
Comments:  
Co-Investigator(s)
Affiliation: 
Karnani, Sunny  Ph.D. Army Research Labs 
Project Information: Grant/Contract No. NNX17AC51A 
Responsible Center: NASA GRC 
Grant Monitor: Stocker, Dennis P 
Center Contact: 216-433-2166 
dennis.p.stocker@nasa.gov 
Unique ID: 11207 
Solicitation / Funding Source: NOT AVAILABLE 
Grant/Contract No.: NNX17AC51A 
Project Type: FLIGHT 
Flight Program:  
No. of Post Docs:
No. of PhD Candidates:
No. of Master's Candidates:
No. of Bachelor's Candidates:
No. of PhD Degrees:  
No. of Master's Degrees:  
No. of Bachelor's Degrees:  
Program--Element: COMBUSTION SCIENCE--Combustion science 
Flight Assignment/Project Notes: NOTE: End date changed to 11/17/2020 per NSSC information (Ed., 11/7/19)

Task Description: NOTE this is a successor agreement to “Electric Field Control of Flames (NNX11AP42A)” with the same Principal Investigator Dr. Derek Dunn-Rankin, in Microgravity Combustion Science per D. Stocker, NASA Glenn Research Center.

This final project year at the University of California, Irvine completed the provision of microgravity experimental data to be used in the exploration of using electric fields in combustion for improving the performance of energy conversion systems. The year was a no-cost extension with the hope of obtaining a third phase of experiments on the International Space Station (ISS); this third phase did not occur so we completed our project. As an actuator, electric fields can improve flame stability and soot formation, while, as a sensing device, the electrical response at saturation is an inherent flame characteristic. In addition to completing the proposed Advanced Combustion via Microgravity Experiments (ACME) E-FIELD experiments aboard the ISS, we have continued to enhance our understanding of the experimental methods and the dynamic interaction between the flame and chemi-ions. We have finalized the methodology for extracting time-stamped experimental results of ion current and images, and we have made progress in our implementation of computational models to predict chemi-ion concentrations in flames, including a comparison between enhanced body forces from electric fields and those from enhanced gravity. This final annual report (following a no-cost extension) summarizes our findings in 2 main areas:

1.) Experimental – There are two major components of our experimental progress: (a) the extraction and analysis of the first phase of E-FIELD Flames microgravity combustion experiments aboard the ISS. These experiments were with the co-flow burner, with positive and negative polarities, methane and ethylene as fuels, different fuel dilutions, different flow velocities, and cases with and without a small coflow of air. These were the first electric field experiments completed in the combustion integrated rack (CIR). And (b) the second phase of E-FIELD Flames microgravity combustion experiments aboard the ISS were completed and all data verified and submitted for inclusion in the Physical Sciences Informatics (PSI) database. These experiments were with a simple jet burner, with positive and negative polarities, methane and ethylene as fuels, different fuel dilutions, and different flow velocities. These were the first jet diffusion flame experiments under the influence of an electric field in zero gravity. Although the experiments had been completed in the prior year, the additional no-cost extension permitted more intensive evaluation of the subtleties contained in the results.

2.) Developing a computational model to predict ion concentrations in flames and the effects of ion driven winds in coflow and jet flames. A focus has been on a full Computational Fluid Dynamics (CFD) simulation with appropriate reduced chemistry using the Open Foam Optics And Mechanics (FOAM) platform of computation. This final year completed a study confirming a potentially appropriate reduced chemical mechanism that provides accurate chemi-ion concentrations with a sufficiently small number of reactions to allow a comprehensive CFD model that includes electrical forces. The complete CFD was not accomplished, but the current simplified modeling approach where ion production is tuned via experimental results was employed and provided some insights regarding the relative value of electric versus gravitational body forces. The configuration for these computations was the coflow geometry but without coflowing gas. The comparisons were accomplished against 1-g flame data available in the literature. Future work will enhance the modeling to a full CFD and then compare the results to the newly acquired ISS E-FIELD Flames data.

Because this year was a no-cost extension used for refining the results already reported last year, we refer to the prior annual report for most of the details. Only salient summary information of outcomes beyond those reported last year are included. In addition, details of all the work will appear in the publications and conference proceedings identified in the Bibliography section (Ed. note: use the Cumulative Bibliography link). The COVID-19 pandemic interrupted substantially the dissemination of our findings, but in the past year we have produced: 1 PhD dissertation; 2 invited technical presentations, and a public presentation.

Research Impact/Earth Benefits: The control of combustion has the potential to improve efficiency and reduce emissions from burning fuels. Since high power density often requires combustion, these improvements will be important no matter what the fuel source. Electric fields acting on flames have the potential to aid in combustion control both for sensing and actuation. For example, electrical properties of flames can identify poor performing boiler flames that release poisonous carbon monoxide. Our studies show that a flame's electric signature can capture incipient quenching before dangerous emissions result. Electrically driven ions can produce local convection that changes combustion behavior. Understanding the links between electrical character and flame behavior may allow improved sensing of poor performing combustion systems.

Task Progress & Bibliography Information FY2021 
Task Progress: The project includes both experimental and computational parts. The experiments employ a coflow burner and a jet burner that are aboard the ISS. The computational work employs the OpenFOAM framework.

1. ISS Experiments. The major accomplishment of the prior year was the extraction and evaluation of the data from the first and second phase set of tests from the ACME E-FIELD Flames experiment. The raw results were reported in the last annual technical report (Ed. note: available to NASA management). The first phase employed the same co-flow burner aboard the ISS as was used for the CLD Flames experiments, but an electrode mesh was installed downstream of the burner. The second phase set of tests used a jet flame burner with the electrode mesh downstream of the jet tip.

No report can be comprehensive since the ISS experiments covered a wide range of conditions and outcomes, with 131 different tests run, and more than 120 of them successful in phase I with the coflow burner and 104 different tests run in phase II with the single-jet burner for a total of 235 testpoints. There are, therefore, too many conditions to share fully, though the data is available on the Physical Sciences Informatics website. Color camera images, intensified camera images, total flame luminosity, and ion current, along with all test parameters and flow monitoring, were recorded in all cases.

The most significant results from the past year are the understanding and evaluation of the subtleties of the data collected, including limitations driven by the experimental hardware, and there are then important data analysis and manipulation developments and details needed to bring the experimental information to the conditions appropriate for comparison with theory or numerical simulation. E-FIELD Flames data were recorded with the Greenwich Mean Time Zone (GMT) up to the millisecond, and the data acquisition capability of the voltage output (i.e., electric field strength), ion current, and the voltage signal of the Photomultipliers Tubes (PMT) collecting the flame illumination in different wavelength regions, are stored at 100 Hz (10 ms/data point). The image data requires variation based on the exposure time driven by the luminosity of the flame, and so the image data is not at regular intervals, and is at most recorded at video rates (30 Hz). This means that the image data and the quantitative data are not synchronized, which requires special attention for uniformity.

This experimental work reports the initial flame appearance and voltage-current (V-I) plots for the unique condition of a nonpremixed flame under the influence of an electric field in microgravity conditions using a coflow burner (without turning on coflow) and a jet burner. The flame images in microgravity are distinct in the change of size and reaction zone location while there are relatively small differences observed in normal-gravity laboratory experiments. The ion current results at low gravity do not exhibit the typical sub-saturation, saturation, and supersaturation trend as recorded in terrestrial tests, suggesting that buoyancy driven flows play an important role in this characteristic ion current behavior. This work will continue and further compare with similar carbon content at different fuel flow rates in 0g as a marker to research how the soot, ion current and flame luminosity vary with each other.

2. OpenFOAM Simulations. Electric fields can affect flame shape, burning velocity, temperature profile, speed of propagation, lift-off distance, species diffusion, stabilization, and extinction. The primary reason is that combustion of hydrocarbon fuels involves chemi-ionization, which generates ions and electrons that can be manipulated by the field producing a body force also referred to as an ion wind. To fully explore the links between these various aspects, a comprehensive computational model is needed. We have made some progress towards this goal, but there remain some significant challenges (that we hope to address in proposed future research).

There are four major parts in the computational study. The first one relates to the chemical kinetic model for the flame chemistry with charged and excited species; the second looks into the effects that two different jet burner geometries have on a flame that is exposed to different gravity environments; the third explores the implementation of applied electric fields in the simulations and how this affects a non-premixed flame at 1g; and the last aspect carries out a comparison between the effects on flame behavior when body forces of different nature, buoyancy and electric field force, are present. The first 3 parts were already described in some detail in the prior annual report, and so this report includes only on the last part. This last component also provides insight into the possible similarities and regimes when both forces could be equivalent. The final goal is to fully understand how flame behavior is altered by different electric fields and to compare those effects to the behaviors generated by gravitational body forces.

Part 4 – Comparing Buoyancy and Electric Body Forces. An analogy between body forces (buoyancy and electric) and the possible similarities among them is examined. This comparison corroborates previous literature mentioning that, even though electric and buoyancy forces are from a different nature, they can be considered equivalent when applying an electric field in a 1g flame to achieve an equivalent supergravity flame. It is concluded that the 2g flame resembles the 1g flame with 0.5kV/cm applied. However, a large regime of supergravity conditions where both forces are equivalent is yet to be explored.

The results of the computations show that, in fact, electric field magnitude predominates in comparison to buoyancy forces in the center axis of symmetry. Buoyancy forces are larger than the electric field moving further from the center line. These results reinforce the fact that the influence of small electric fields applied to a 1g flame – when the electric field is applied in the same direction as the buoyant plume – is often masked by the buoyant convection since both body forces have very similar values.

Promising work in electric field forces computational studies has been performed. The electric field simulations still need to be developed to acquire more reliable results that match before and after the saturation point (i.e., the saturation plateau) in order to assure and assume that the fluid dynamics, chemistry, and body forces phenomena that are happening in the flame and are changing its behavior are well captured by the simulation. In addition, while the global ion current results align with the literature, the detailed distribution of the ions and the local forces they produce requires additional work. Once 1g flame simulations show comprehensible results matching with the literature for all the burner geometries, these simulations can be run for comparison with the experimental results obtained on the ISS.

Bibliography: Description: (Last Updated: 02/12/2024) 

Show Cumulative Bibliography
 
Abstracts for Journals and Proceedings Chien Y-C, Stocker D, Hegde U, Dunn-Rankin D. "Microgravity Experiments Examining Electric Field Effects on Laminar Methane Gas-Jet Diffusion Flames." 35th Annual Meeting of the American Society for Gravitational and Space Research, Denver, CO, November 20-23, 2019.

Abstracts. 35th Annual Meeting of the American Society for Gravitational and Space Research, Denver, CO, November 20-23, 2019. , Nov-2019

Dissertations and Theses Lopez-Camara C. "Numerical Study of Non-Premixed Methane/Air Flames Behavior under Different Body Forces: Buoyancy and Electric Field." Dissertation, University of California, Irvine, December 2020. , Dec-2020
Project Title:  ACME: EFIELD – Electric Field Effects On Laminar Diffusion Flames Reduce
Images: icon  Fiscal Year: FY 2020 
Division: Physical Sciences 
Research Discipline/Element:
Physical Sciences: COMBUSTION SCIENCE--Combustion science 
Start Date: 11/18/2016  
End Date: 11/17/2020  
Task Last Updated: 10/31/2019 
Download report in PDF pdf
Principal Investigator/Affiliation:   Dunn-Rankin, Derek  Ph.D. / University of California - Irvine 
Address:  Department of Mechanical & Aerospace Engineering 
4200 Engineering Gateway Bldg, EG3224 
Irvine , CA 92697-3975 
Email: ddunnran@uci.edu 
Phone: 949-824-8745  
Congressional District: 48 
Web:  
Organization Type: UNIVERSITY 
Organization Name: University of California - Irvine 
Joint Agency:  
Comments:  
Co-Investigator(s)
Affiliation: 
Karnani, Sunny  Ph.D. Army Research Labs 
Project Information: Grant/Contract No. NNX17AC51A 
Responsible Center: NASA GRC 
Grant Monitor: Stocker, Dennis P 
Center Contact: 216-433-2166 
dennis.p.stocker@nasa.gov 
Unique ID: 11207 
Solicitation / Funding Source: NOT AVAILABLE 
Grant/Contract No.: NNX17AC51A 
Project Type: FLIGHT 
Flight Program:  
No. of Post Docs:
No. of PhD Candidates:
No. of Master's Candidates:
No. of Bachelor's Candidates:
No. of PhD Degrees:  
No. of Master's Degrees:  
No. of Bachelor's Degrees:  
Program--Element: COMBUSTION SCIENCE--Combustion science 
Flight Assignment/Project Notes: NOTE: End date changed to 11/17/2020 per NSSC information (Ed., 11/7/19)

Task Description: NOTE this is a successor agreement to “Electric Field Control of Flames (NNX11AP42A),” in Microgravity Combustion Science per D. Stocker, NASA Glenn Research Center.

This project at the University of California, Irvine continues to explore using electric fields in combustion for improving the performance of energy conversion systems. As an actuator, electric fields can improve flame stability and soot formation, while, as a sensing device, the electrical response at saturation is an inherent flame characteristic. In addition to preparing for the proposed Advanced Combustion via Microgravity Experiments (ACME) E-FIELD experiments aboard the International Space Station (ISS), we have continued to enhance our understanding of the experimental methods and the dynamic interaction between the flame and chemi-ions. We have measured the distribution of ions at the downstream electrode in order to characterize the likely body forces resulting, and we have begun to develop computational models to predict chemi-ion concentrations in flames, including a comparison between enhanced body forces from electric fields and those from enhanced gravity. This report summarizes recent findings in 2 main areas:

1.) Experimental – There are two elements in the experimental progress: (a) the unreported first phase of E-FIELD Flames microgravity combustion experiments aboard the ISS. These experiments are with the co-flow burner, with positive and negative polarities, methane and ethylene as fuels, different fuel dilutions, different flow velocities, and cases with and without a small coflow of air. These are the first electric field experiments completed in the combustion integrated rack (CIR). And (b) the second phase of E-FIELD Flames microgravity combustion experiments aboard the ISS. These experiments are with a simple jet burner, with positive and negative polarities, methane and ethylene as fuels, different fuel dilutions, and different flow velocities. These are the first jet diffusion flame experiments under the influence of an electric field in zero gravity.

2.) Developing a computational model to predict ion concentrations in flames and the effects of ion driven winds in coflow and jet flames. A focus has been on a full Computational Fluid Dynamics (CFD) simulation with appropriate reduced chemistry using the Open Foam Optics And Mechanics (FOAM) platform of computation. This past year has concentrated on the appropriate reduced chemical mechanism that provides accurate chemi-ion concentrations with a sufficiently small number of reactions to allow a comprehensive CFD model that includes electrical forces. The configuration is for the coflow geometry but without coflowing gas.

Summaries of these findings appear in this report with the details of all the work appearing in the publications and conference proceedings identified at the end of this document. In the past year we have produced: 2 peer-reviewed journal articles; 2 invited technical presentations and a public presentation at a high school in Spain, and 7 conference papers.

Research Impact/Earth Benefits: The control of combustion has the potential to improve efficiency and reduce emissions from burning fuels. Since high power density often requires combustion, these improvements will be important no matter what the fuel source. Electric fields acting on flames have the potential to aid in combustion control both for sensing and actuation. For example, electrical properties of flames can identify poor performing boiler flames that release poisonous carbon monoxide. Our studies show that a flame's electric signature can capture incipient quenching before dangerous emissions result. Electrically driven ions can produce local convection that changes combustion behavior. Understanding the links between electrical character and flame behavior may allow improved sensing of poor performing combustion systems.

Task Progress & Bibliography Information FY2020 
Task Progress: ISS Experiments: The major accomplishment of the prior year was the completion of the first and second phase set of tests from the ACME E-FIELD Flames experiment. The first phase employed the same co-flow burner aboard the ISS as was used for the CLD Flames experiments, but an electrode mesh was installed downstream of the burner. The second phase set of tests used a simple jet flame configuration with the electrode mesh downstream of the jet tip. The experimental details for the coflow burner have been described in prior reports. The ISS experiments covered a wide range of conditions and outcomes, with 131 different tests run, and more than 120 of them successful in phase I with the coflow burner and 104 different tests run in phase II with the single-jet burner for a total of 235 testpoints. Color camera images, intensified camera images, total flame luminosity, and ion current, along with all test parameters and flow monitoring, were recorded in all cases.

Results and Discussion

The key measurement for the E-FIELD Flames experiments is the ion current as a function of applied voltage (the V-I curve). This curve provides information on the flames ability to produce chemi-ions based on its saturation ion current, and it provides a measure of combustion performance in that ion production is a function of carbon influx into the flame and the vigorousness of the combustion process. As was reported last year, more compact, higher temperature flames produce more chemi-ions than do larger more diffuse reaction zones of lower overall temperature for the same fuel flow. Another interesting feature of these microgravity V-I curves is the peak value. Traditional electrical aspects of flames studies look for a saturation ion current plateau, where the ion current remains fixed despite an increase in applied electric field. For these zero gravity flames, there is a pronounced peak in the ion current at saturation and then a gradual decrease to a global saturation plateau. This is a strikingly different behavior than is seen in 1-g, and the behavior is even more pronounced for the case where there is zero coflow.

The first data from the ISS experiments used 100% methane as fuel, while later experiments varied the methane concentration with the balance nitrogen. The flame images of methane diluted with nitrogen at positive field strength and negative field strength are taken with different shutter speed based on the flame luminosity and subtracted with several background images in average. There is no coflow in the outer concentric region to help enhance the air/fuel mixing, and hence the flame is relatively less stable and less bright as compared to the equivalent coflow flames, in general. For both positive and negative field strength, the ion current peaks when the flame is the most compact. For the positive field, the voltage-current (V-I) curve in microgravity shows a very similar trend as is observed from the very first sets of results with the 100% methane fuel condition but with some additional information over a larger range of voltage. The ion current increases parabolically with the nominal field strength and then reaches a peak. With the enhancement of voltage, the current undergoes another parabolic like decrease and slowly increases back up again. The second increase of the ion current is not complete and clear due to the very low fuel concentration with the limitation of the capability of the power supply and the constrained fixed distance of the mesh electrode. The flame images also relate to the ion current to field strength curve, showing that the flame reduces in size as the ion current increases and, as the current drops, the flame size increases. The flame size reduces again as the ion current climbs back up close to the end of voltage-current curve.

With the negative field, the overall ion current remains low (below 0.3 microamps) for both the 70% and 40% methane conditions in comparison to the positive field condition. In contrast to tests with the positive electric field, the ion current also reaches its peak sooner, at around 0.4 kV/cm, and then decays immediately with the increasing of electric field strength until the flame extinguishes. The ion current decay also corresponds to the flame appearance. For both of the fuel dilutions, the flame extends wider and the flame base is gradually lifted after the saturation peak. The flammability limit (i.e., just before flame blowout with the field) in the perspective of nominal electric field strength is 1.46 kV/cm and 1.30 kV/cm for 70% and 40% methane, respectively.

Single Jet Burner: In a positive field, the jet flames are relatively more stable initially and less stable under high voltage than the previously reported flames using a coflow burner with no coflow. As the positive field is applied to the jet, the flame becomes compact along the width and height. The immediate change observed is the bright soot aggregating around the flame tip. With an increase of electric field strength, the soot distributes less at the tip as the flame shrinks. The ion current corresponds to the flame appearance in that the current increases and reaches a peak at the most compact flame. With further electric field increase, the flame starts fluctuating and a distinct dancing phenomenon can be seen from the V-I curve where the ion current oscillates widely at the same field strength. These very high peak values may represent an incipient corona or discharge event.

Unlike in the positive field, the methane jet encounters the downward ion wind around the flame and does not sustain after 1.76 kV/cm for 24 cm/s, being blown out by electric field. The 19 cm/s flame lasts until 1.93 kV/cm. When the flame experiences a negative field, its height decreases and the full flame is moved onto the tube, similar to a downward pointing jet flame in 1g. The change of the flame width is not as clear and obvious as with a positive field but still changes. The ion current also increases with the field strength but differently than with the positive field without a clear curved slope. Instead, it decreases parabolically after reaching the current peak. The current peak corresponds to the most compact flame at 0.32 kV/cm for 19 cm/s and 0.41 kV/cm for 24 cm/s. Soot also appears at the flame tip but with a wider distribution along the reaction sheet. The soot reduces with increasing negative field strength, and it disappears at 0.80 kV/cm along the parabolic decay line after passing the ion current peak. The flame width and height also expand after the most compact flame. There are several sparkler-like soot particles at 1.18 and 1.93 kV/cm. Further evaluation of this soot sparking is underway.

Experimental Conclusions: This experimental work reports the initial flame appearance and voltage-current (V-I) plots for the unique condition of a nonpremixed flame under the influence of an electric field in microgravity conditions using coflow burner (without turning on coflow). The flame images in microgravity with no coflow are distinct in the change of size and reaction zone location while there are relatively small differences observed in normal-gravity laboratory experiments. The ion current results at low gravity do not exhibit the typical sub-saturation, saturation, and supersaturation trend as recorded in terrestrial tests, suggesting that buoyancy driven flows play an important role in this characteristic ion current behavior. This work will continue and further compare with similar carbon content at different fuel flow rates in 0g as a marker to research how the soot, ion current, and flame luminosity vary with each other.

The positive field allows the soot to concentrate and compresses the flame into a high current density thermal source and leading to an unpredicted oscillation before extinguishing with the increasing field strength. The negative field allows the flame to maintain its spherical characteristic in microgravity. The yellow soot appears for both of the electric field directions within the flame around the tip and disappears with increasing field strength after the reaching the current peak. The role of soot in the V-I behavior of these flames requires further investigation.

OpenFOAM Simulations: Electric fields can affect flame shape, burning velocity, temperature profile, speed of propagation, lift-off distance, species diffusion, stabilization, and extinction. The primary reason is that combustion of hydrocarbon fuels involves chemi-ionization, which generates ions and electrons that can be manipulated by the field producing a body force also referred to as an ion wind.

Chemiluminescence of the flame is considered in these simulations. Chemiluminescence is an important feature in flame diagnostics and is an electronically excited species present in flames. The reactions involved in chemiluminescence were not included in the original GRI-Mech 3.0 because excited species are in low concentration and are not significant contributors to the major chemical pathways. Recent work has suggested, however, that there is a strong link between excited species and chemi-ion production. Excited species are also important as an indicator for many flame properties including reaction zone position. It is valuable, therefore, to add reactions involved with important excited species, such as excited hydroxyl, OH*, and methylidyne, CH*. At the moment, there is no published model for non-premixed co-flow methane/air flames that contains both chemi-ions and excited species. Hence, a combination of chemical kinetic models that contains both, excited species and chemi-ions, is part of this study.

The final goal is to fully understand how flame behavior is altered by different electric fields. For that purpose, the effect of the electric field force to the 1g flame has been studied for a range of different applied electric fields.

The axisymmetric geometry models for the different jet methane flames studied are a burner with the inlet tube (from which fuel is flowing) extruded from the burner and a jet burner.

The chemical kinetic model used in this study contains neutral and charged species. H3O+ has been reported as the most long-lived ion in the flame and HCO+ is generated by the primary reaction creating ions and electrons. Similar boundary conditions have been applied to the jet case (which differs on the height of the burner to be 10 cm instead of 3 cm).

Results and discussion: For the flush and jet burner geometries, I-V curves are not matching with the ones in the literature. Further investigation must be performed, starting by revising the boundary conditions and the electric force and flow field mapping without flame. Calculations of the z-current densities for both burner geometries studied show physically plausible values, but deeper analysis has to be performed from the numerical simulation results in order to understand the physical meaning of these results.

Conclusions and Future Work: Promising work in electric field forces studies has been performed. The electric field simulations still need to be developed to acquire more reliable results that match after the saturation point (i.e., the saturation plateau) in order to assure and assume that the fluid dynamics, chemistry, and body forces phenomena that are happening in the flame and are changing its behavior are well captured by the simulation. However, the results obtained up to now align with the literature. Once 1g flame simulations show comprehensible results matching with the literature for all the burner geometries, these simulations will be re-run at a 0g environment in order to compare the obtained results with ISS experimental results.

Reduction of Chemical Kinetic Mechanisms: Currently, experiments alone cannot explain the phenomena contributing to the flame behavior when an electric field is applied since ion chemistry and the transport effects produced by the application of an electric field cannot be easily decoupled. As discussed before, simulations that accurately reproduce and interpret the experimental observations can provide insight into the key features of ion effects in combustion, especially when an electric field is applied. However, the appropriate ion chemistry to include in these simulations remains uncertain. Moreover, simulations of flames with applied electric field require a comprehensive small chemical kinetic model in order to be able to be performed in a cluster within a reasonable amount of computing resources and time.

To tackle these issues, a chemical kinetic model that captures the interactions occurring during methane-air combustion processes is developed. Achieving a reduced-order model that is faithful to the ion production but also captures the appropriate flame chemistry for this fuel will help to decouple the effects of ion chemistry and applied electric fields for flames, providing a fundamental understanding for further development of devices that might actively control combustion.

Once the full chemical kinetic model for non-premixed methane-air flames is achieved, the next step is to reduce it until it is of a size appropriate for use in a 2D Computational Fluid Dynamics Simulation (CFD simulation) of a real burner. This reduction is needed because large chemical kinetic models (i.e., with more than 35 species) cannot be practically applied in a 2D CFD simulation due to the high computational cost related to data storage and simulation time.

The last step will be to validate the reduced chemical kinetic model. To do so, a 2D OpenFOAM CFD simulation is performed with reduced chemical kinetic model developed. The validation proceeds by comparing the results obtained with the experimental data available from Earth experiments gravity (1g) and International Space Station experiments (0g). This comparison will include flame shape as marked by the chemiluminescent species locations, the ion current as measured at the downstream electrode of the applied electric field, and the soot location and luminosity as recorded photographically. As for the validation at this step, the acceptable tolerances will be of a 1% difference in flame structure of major species and temperature and less than 20% difference in minor species of importance (ions and excited species) and soot precursors. The reduction has been made using the PSR module in Chemkin-Pro® and only for The San Diego Mechanism with the excited species submechanism addition. This constitutes a detailed model of 330 reactions and 70 species. The charged species submechanism has been added later as it is without further reductions (the number of reactions and species has increased from with the addition of the submechanism for charged species included). The target species in the reduction have been: C2H, H2O, O, CH, O2, and OH. The species to keep during the reduction have been: CH4, C2H, CH, CH*, CO, CO2, H2O, N2, O, O2, OH, OH*.

A reduced model containing excited species CH* and OH* and neutral species has been achieved. The detailed model (330 reactions/70 species) has been reduced to have 152 reactions and 31 species. Validation between the reduced and the detail models has been done by comparing the results gotten from using each of the models (detailed and reduced) using PSR and counterflow burner geometries in Chemkin-Pro® and at temperatures of 1500K, 2000K, and 2500K. After these comparisons, the submechanism for charged species was added to both detailed and reduced model. Further comparison between the reduced model and the detailed model has been made for validation purposes using the counterflow geometry in Chemkin-Pro®. Once the results using 0D and 1D burner geometries were showing differences within an acceptable tolerance for all cases, a 2D OpenFOAM axisymmetric simulation for the extruded geometry was run for 1g and no electric field applied in order to compare these results against experiments.

Numerical Conclusions: A new reduced chemical kinetic model for methane/air flames has been developed and promising results have been obtained when comparing this reduced model to the detailed mechanism. Further validations have to be made in order to assure that this reduced model for methane/air is capable to accurately predict the flame behavior, including the changes when an electric field is applied to the flame and the quantity of soot produced. Comparison with the ion current as measured at the downstream electrode of the applied electric field and the soot location and luminosity are planned to be carried out in the near future.

Bibliography: Description: (Last Updated: 02/12/2024) 

Show Cumulative Bibliography
 
Abstracts for Journals and Proceedings Chien YC, Tinajero J, Stocker D, Hegde U, Dunn-Rankin D. "Microgravity Experiments of Electric Field Effects on Laminar Ethylene/Air Diffusion Flames." 34th Annual Meeting of the American Society for Gravitational and Space Research, Bethesda, MD, October 31-November 3, 2018.

Abstracts. 34th Annual Meeting of the American Society for Gravitational and Space Research, Bethesda, MD, October 31-November 3, 2018. , Nov-2018

Abstracts for Journals and Proceedings Lopez-Camara CF, Belhi M, Im HG, Dunn-Rankin D. "Numerical Simulations of Laminar Nonpremixed CH4-Air Flames Varying Buoyancy and Applied E-Field." 34th Annual Meeting of the American Society for Gravitational and Space Research, Bethesda, MD, October 31-November 3, 2018.

Abstracts. 34th Annual Meeting of the American Society for Gravitational and Space Research, Bethesda, MD, October 31-November 3, 2018. , Nov-2018

Articles in Peer-reviewed Journals Tinajero J, Dunn-Rankin D. "Non-premixed axisymmetric flames driven by ion currents." Combustion and Flame. 2019 Jan;199:365-76. https://doi.org/10.1016/j.combustflame.2018.10.036 , Jan-2019
Papers from Meeting Proceedings Lopez-Camara CF, Belhi M, Im HG, Dunn-Rankin D. "Numerical Simulations of Laminar Nonpremixed CH4-Air Jet Flames Influenced by Varying Electric Fields." 11th U.S. National Combustion Meeting, Pasadena, CA, March 24-27, 2019.

11th U.S. National Combustion Meeting, Pasadena, CA, March 24-27, 2019. Paper 2F16 , Mar-2019

Papers from Meeting Proceedings Chien YC, Tinajero J, Stocker D, Hegde U, Dunn-Rankin D. "Current and Flame Changes with Electric Fields in Microgravity." 11th U.S. National Combustion Meeting, Pasadena, CA, March 24-27, 2019.

11th U.S. National Combustion Meeting, Pasadena, CA, March 24-27, 2019. Paper 2F18. , Mar-2019

Papers from Meeting Proceedings Chien YC, Stocker D, Hegde U, Dunn-Rankin D. "Microgravity Experiments with Methane Jet Flames under Electric Field Influence On-board the International Space Station (ISS)." 12th Asia-Pacific Conference on Combustion, Fukuoka, Japan, July 1-5, 2019.

ASPACC paper. 12th Asia-Pacific Conference on Combustion, Fukuoka, Japan, July 1-5, 2019. , Jul-2019

Project Title:  ACME: EFIELD – Electric Field Effects On Laminar Diffusion Flames Reduce
Images: icon  Fiscal Year: FY 2019 
Division: Physical Sciences 
Research Discipline/Element:
Physical Sciences: COMBUSTION SCIENCE--Combustion science 
Start Date: 11/18/2016  
End Date: 11/17/2019  
Task Last Updated: 10/23/2018 
Download report in PDF pdf
Principal Investigator/Affiliation:   Dunn-Rankin, Derek  Ph.D. / University of California - Irvine 
Address:  Department of Mechanical & Aerospace Engineering 
4200 Engineering Gateway Bldg, EG3224 
Irvine , CA 92697-3975 
Email: ddunnran@uci.edu 
Phone: 949-824-8745  
Congressional District: 48 
Web:  
Organization Type: UNIVERSITY 
Organization Name: University of California - Irvine 
Joint Agency:  
Comments:  
Co-Investigator(s)
Affiliation: 
Karnani, Sunny  Ph.D. Army Research Labs 
Project Information: Grant/Contract No. NNX17AC51A 
Responsible Center: NASA GRC 
Grant Monitor: Stocker, Dennis P 
Center Contact: 216-433-2166 
dennis.p.stocker@nasa.gov 
Unique ID: 11207 
Solicitation / Funding Source: NOT AVAILABLE 
Grant/Contract No.: NNX17AC51A 
Project Type: FLIGHT 
Flight Program:  
No. of Post Docs:
No. of PhD Candidates:
No. of Master's Candidates:
No. of Bachelor's Candidates:
No. of PhD Degrees:  
No. of Master's Degrees:  
No. of Bachelor's Degrees:  
Program--Element: COMBUSTION SCIENCE--Combustion science 
Task Description: NOTE this is a successor agreement to “Electric Field Control of Flames (NNX11AP42A),” in Microgravity Combustion Science per D. Stocker, NASA Glenn Research Center.

This project at the University of California, Irvine continues to explore using electric fields in combustion for improving the performance of energy conversion systems. As an actuator, electric fields can improve flame stability and soot formation, while, as a sensing device, the electrical response at saturation is an inherent flame characteristic. In addition to preparing for the proposed Advanced Combustion via Microgravity Experiments (ACME) ACME E-FIELD experiments aboard the International Space Station (ISS), we have continued to enhance our understanding of the experimental methods and the dynamic interaction between the flame and chemi-ions. We have measured the distribution of ions at the downstream electrode in order to characterize the likely body forces resulting, and we have begun to develop computational models to predict chemi-ion concentrations in flames, including a comparison between enhanced body forces from electric fields and those from enhanced gravity. This report summarizes recent findings in 2 main areas:

1.) The first phase of E-FIELD microgravity combustion experiments aboard the ISS. These experiments are with the co-flow burner, with positive and negative polarities, methane and ethylene as fuels, different fuel dilutions, different flow velocities, and cases with and without a small coflow of air. These are the first electric field experiments completed in the combustion integrated rack (CIR).

2.) Developing a computational model to predict ion concentrations in flames and the effects of ion driven winds in coflow and jet flames. A focus has been on a full CFD simulation with appropriate chemistry using the Open FOAM platform of computation. This past year has concentrated on the coflow geometry.

Summaries of these findings appear in this report with the details of all the work appearing in the publications and conference proceedings identified at the end of this document. In the past year we have produced: 1 peer-reviewed journal article, 5 invited technical presentations, and 7 conference papers.

The project includes both experimental and computational parts. The experiments employ a coflow burner that is aboard the ISS.

Research Impact/Earth Benefits: The control of combustion has the potential to improve efficiency and reduce emissions from burning fuels. Since high power density often requires combustion, these improvements will be important no matter what the fuel source. Electric fields acting on flames have the potential to aid in combustion control both for sensing and actuation. For example, electrical properties of flames can identify poor performing boiler flames that release poisonous carbon monoxide. Our studies show that a flame's electric signature can capture incipient quenching before dangerous emissions result. Electrically driven ions can produce local convection that changes combustion behavior. Understanding the links between electrical character and flame behavior may allow improved sensing of poor performing combustion systems.

Task Progress & Bibliography Information FY2019 
Task Progress: ISS Experiments – First Phase

The major accomplishment of the prior year was the completion of the first phase set of tests from the ACME E-FIELD Flames experiment. The first phase employed the same co-flow burner aboard the ISS as was used for the Coflow Laminar Diffusion Flame (CLD Flames) experiments, but an electrode mesh was installed downstream of the burner. The experimental details have been described in prior reports. The burner consists of two stainless steel plenums threaded together. Fuel, introduced in the bottom plenum, passes through a stainless-steel tube (2.13 mm ID) and exits the top surface as a fully developed parabolic flow. Air entering the top chamber flows through straightening beads and a honeycomb mesh. Ground-based characterization studies show that the air exits the top surface with an approximately top hat flow profile at the exit. The center fuel jet was slighting extruded above the top surface of the coflow burner to more closely approximate a jet burner and to offset the flame ignition region from the coflow surface. A variable high-voltage power supply connected between the burner and the downstream mesh electrode produces an applied electric field. Although the electric field distribution is deformed locally by the flame’s space charge, defining an applied field strength, E = V/H (where V is the voltage applied and H is the distance from the burner to the electrode) is useful when describing experimental results. Ion current is calculated using Ohm’s law and the potential drop across a shunt resistor between the electrically isolated burner and ground.

The hardware on the ISS comprises the ACME insert and the combustion integrated rack (CIR), and the complexity of this system is far greater than the burner component identified above. The detailed description of operations and hardware provides the context under which the ACME experiments are conducted, and their complexity makes the flame conditions trivial in comparison. In short, however, we have a simple coflow flame, with just a small jet of fuel surrounded by a weak (or sometimes non-existent) coflow of ‘air’ which also fills the chamber. A test includes starting the gas flows to prime the system, energizing the igniter to initiate a flame, retracting the igniter, and allowing the flame to steady slightly before energizing the electric field to its initial value. We found that in some cases the electric field assisted in the stabilization and so this first stabilization time was often kept short. The flame is again allowed to steady before beginning a voltage sweep over a specified range with steps or smooth ramps of specified voltage changes and durations. The sweep can be in one direction or can include cycles of sweeps. After each test, a duplicate field sweep is made to ensure that there are no current leaks, i.e., other than the ion current through the flame.

The first phase tests in E-FIELD Flames employ the coflow burner geometry, with the fuel tube extending slightly above the burner surface. The results in this reporting period are for tests conducted with 100% methane in a chamber atmosphere and coflow (when used) of synthetic ‘air’ (i.e., 21/79 oxygen/nitrogen by volume) at a nominal pressure of 100 kPa. The mesh is held at a negative potential relative to the burner in these tests. The sweeps shown generally span the full range to -10 kV, where the voltage is increased in small steps (e.g., of 100 V), with each step lasting 1-2 seconds.

The ISS experiments covered a wide range of conditions and outcomes, with 131 different tests run, and more than 120 of them successful. Color camera images, intensified camera images, total flame luminosity, and ion current, along with all test parameters and flow monitoring, were recorded in all cases.

The key measurement for the E-FIELD Flames experiments is the ion current as a function of applied voltage (the V-I curve). This curve provides information on the flames ability to produce chemi-ions based on its saturation ion current, and it provides a measure of combustion performance in that ion production is a function of carbon influx into the flame and the vigorousness of the combustion process. More compact, higher temperature flames produce more chemi-ions than do larger more diffuse reaction zones of lower overall temperature for the same fuel flow. The first effect (i.e., the relationship between carbon influx and peak ion current) can be seen as the flow of 15 cm/s methane produces slightly above 2 microamps of ion current at the peak (1.5 kV/cm) and the flow of 19 cm/s methane has a peak of 2.75 microamps. The peak ion current is nearly proportional to the flow velocity. Traditional electrical aspects of flames studies look for a saturation ion current plateau, where the ion current remains fixed despite an increase in applied electric field. For the zero gravity flames, there is a pronounced peak in the ion current at saturation and then a gradual decrease to a global saturation plateau. This is a strikingly different behavior than is seen in 1-g, and the behavior is even more pronounced for the case where there is zero coflow.

As is apparent from the flame images, the flame is most compact at the voltage consistent with the peak ion current. With increasing voltage, the flame becomes less compact and the ion current decreases. We presume that the flame remains at saturation throughout this period (that is, the flame is producing as many ions as it can, and those ions are being removed from the reaction zone as soon as they are created) so the decrease in ion current results from a change in combustion intensity. We can explore this hypothesis further using the PMT data collected in the experiments.

In all of the curves, the jagged features are spikes following a voltage step change that are mostly associated with the transient capacitance of the system when rapid steps are requested. These overshoots notwithstanding, the V-I curves follow smooth and reproducible trajectories. The results are too recent to allow a detailed analysis and discussion of the differences, but some significant observations from these results and the comparison with 1-g experiments follow.

1. The ion current reaches a peak at approximately the same field strength for both velocities in 1-g while in microgravity the ion current peak for a lower fuel velocity occurs at a lower field strength. The field strength at the peak current seems to scale linearly with the fuel velocity.

2. In microgravity, the ion current does not smoothly increase to saturation nor does it show a gentle peak with a mild decrease as is seen in the normal gravity results. Instead, in microgravity there is a distinct peak in the ion current with a severe decrease throughout the saturation condition, with a final saturation plateau observed at voltages substantially above the voltage at peak ion current. That is, the microgravity final saturation is not the maximum ion current observed. This difference is more dramatic when there is no coflow, which suggests that entrainment plays an important role whether caused by buoyancy or a forced coflow.

3. The peak ion current is higher for microgravity than comparison 1-g flames. The reason is not obvious at this point but may result from a more focused upward flow from the electric body force and ion-driven wind as compared to the combined effects of the ion wind and buoyancy.

4. The peak ion current is approximately proportional to fuel flow velocity, which, as described in prior studies, reflects the relationship between ion production and the carbon flux into the system. The peak ion current also increases with the coflow which suggests that healthier oxidizer transport helps increase the ion production.

OpenFOAM Simulations

Electric fields can affect flame shape, burning velocity, temperature profile, speed of propagation, lift-off distance, species diffusion, stabilization, and extinction. The primary reason is that combustion of hydrocarbon fuels involves a chemi-ionization process, which generates ions and electrons that can be manipulated by the field producing some alteration of the chemical kinetics and generation of a body force. The former arises because the chemistry of the system is affected by the redistribution of charges under the applied electric field; the latter generates an ion wind. The predominance of the literature indicates that changes in chemistry are minor, though the effects on soot formation and transport remain somewhat uncertain.

The simulations in this project also consider chemiluminescence of the flame. Chemiluminescence is an important feature in flame diagnostics and is an electronically excited species present in flames. The reactions involved in chemiluminescence were not included in the original GRI-Mech 3.0 because excited species, like charged species, are in low concentration and are not significant contributors to the major chemical pathways. Recent work has suggested, however, that there is a link between excited species and chemi-ion production. Excited species are also important as an indicator for many flame properties including reaction zone position, and chemiluminescence is one of the measurements being obtained on the ISS in ACME. It is valuable, therefore, to add reactions involved with important excited species, such as excited hydroxyl, OH*, and methylidyne, CH*. At the moment, there is no published model for non-premixed co-flow methane/air flames that contains both chemi-ions and excited species. Hence, combining chemical kinetic models that contains both excited species and chemi-ions is part of this study.

The final goal of the numerical simulation component of the research is to understand how flame behavior is altered by different buoyancy force environments and external electric fields. For that purpose, two separate studies have been performed using three different burner geometries. The first study focuses on the effect of the buoyancy force on the flame, observing the flame behavior in different gravity environments. The second focuses on the effect of the electric field force, studying the 1g flame behavior for a range of different applied electric fields.

The axisymmetric geometry models for the different jet flames studied are the burner with the inlet tube (from which fuel is flowing) extruded from the burner, the burner with a flush inlet tube, and the jet burner (i.e., no surrounding coflow). The geometries are based on the geometry of burners on the International Space Station (ISS), as part of the Advanced Combustion via Microgravity Experiments (ACME) project equipment which will eventually provide experimental data against which the described simulations can be compared.

Notice that the extruded and the flush tube burner geometries would accept an inlet of air from the burner outer ring, creating then a co-flow flame. However, the jet flame without the co-flow air (i.e., natural convective/diffusive flow only) was considered for this study. Further work will include co-flow air coming out from the outer ring.

For the buoyancy study, CH* is taken as a marker for the luminosity of the flame; H3O+ has been reported as the most long-lived ion in the flame and HCO+ is generated by the primary reaction creating ions and electrons (CHO -> CHO+ + e-), well-known as the principal chemi-ionization reaction. Hence, these three species are the minor species that have been taken into consideration for the buoyancy study and will be discussed later in this report. For the electric field study, the methodology followed to achieve comparable results with the literature has been developed along with our informal collaborators at KAUST, which models the ion production without the chemi-ionization reaction included formally in the chemical kinetics mechanism.

OpenFOAM was chosen as the numerical solver. The solver used was a modified version of the reactingFoam solver in OpenFOAM. The mass transport equation was modified by imposing a Schmidt number equal to 0.7 condition. Previous studies showed that this approximation was suitable for combustion processes. Maxwell’s equations have been implemented in the solver by Professor H.G. Im’s research group at the Clean Combustion Research Center (KAUST), as part of this project’s informal collaboration.

In OpenFOAM it is easy to deal with axially symmetric geometries; for cylinders it is sufficient to solve the problem in a 5 degree wedge. To do that, a cylindrical control volume has been considered; in this way, properties vary only along the radial coordinate x and the axial coordinate z. A large external domain must also be included to make sure that the boundaries do not affect the core combustion processes and behaviors.

The boundary conditions were the same for all geometries studied. The horizontal top wall (35 mm higher than the base of the mesh) has a zero-gradient condition. A non-uniform parabolic velocity profile of the inlet methane was imposed following previous literature procedures. The gravity force considered has been applied in the Z-direction, in contra-direction with the fuel jet exit flow.

For the temperature, a condition of zero gradient --i.e., normal gradient of temperature is zero-- has been set on all boundary walls. Since only steady state is examined for all cases, the simulations were run from ignited simplified cases in which air and fuel were entering the system at 800K. Once the simplified computation reached steady state, the temperatures were changed to 300K and the simulation cases were continued until they reached steady state again. The air was considered as molar fractions of 0.76, 0.2395, and 0.0005 for nitrogen (N2), oxygen (O2), and carbon dioxide (CO2), respectively. The initial pressure is atmospheric for all cases.

The burner acts as positive electrode for the all cases studied and the metallic upper mesh acts as negative electrode, since only negative voltages have been considered so far for this study. Positive voltages will be considered in future simulations, having to modify the Poisson and charged species boundary conditions accordingly.

The buoyancy environment conditions that have been considered so far are microgravity (0g), partial gravity (0.5g), gravity (1g), and supergravity (range from 1.125g to 3g).

The geometry of the burner plays an important role when buoyancy forces are applied. This might have been expected since other body forces (such as those produced by the electric field) have been shown experimentally to also be strongly conditioned by the burner geometry used.

Also, simulations for a jet burner geometry of different tube heights have been performed in order to check if there is a height where the flame at the jet burner geometry acts similarly to the extruded tube burner geometry. No clear correlation has been found between the concentrations observed (CH*, OH*, H3O+, and HCO+) and the height of the jet burner tube extrusion.

The electric field conditions that have been considered in this study vary depending on the burner geometry used. However, in all cases, the maximum electric field applied in the simulation was the one found experimentally at saturation.

For the flush and jet burner geometries, I-V curves are not matching with the ones in the literature. Further investigation must be performed, starting by revising the boundary conditions and the electric force and flow field mapping without flame.

Promising work with both buoyancy and electric field forces studies has been performed. The study of the buoyancy effects shows that different burner geometries will affect the flame when gravity changes. Deeper understanding on the fundamental reasons that cause these differences is still to be achieved. Electric field simulations still need to be completed for the geometries matching the experimental conditions to acquire more reliable results that match with the literature in order to assure and assume that the fluid dynamics, chemistry, and body forces phenomena that are happening in the flame and are changing its behavior are well captured by the simulation. Once 1g flame simulations show reliable results matching with the literature for all the burner geometries, these simulations will be run for the 0g environment in order to compare the obtained results with ISS experimental results.

Bibliography: Description: (Last Updated: 02/12/2024) 

Show Cumulative Bibliography
 
Articles in Peer-reviewed Journals Chien Y-C, Dunn-Rankin D. "Electric field changes a diffusion flame near an impinging surface: Heat and thermal perspective." Energies. 2018 May;11(5):1135. https://doi.org/10.3390/en11051235 (in Special Issue Electric Fields in Energy & Process Engineering) , May-2018
Papers from Meeting Proceedings Lopez-Camara CF, Dunn-Rankin D. "Numerical Simulations of a Co-Flow Methane/Air Flame Including Ions and Excited Species under Different Gravity Conditions." Western States Section of the Combustion Institute Fall Technical Meeting 2017, Laramie, WY, October 2-3, 2017.

Western States Section of the Combustion Institute Fall Technical Meeting 2017, Laramie, WY, October 2-3, 2017. , Oct-2017

Papers from Meeting Proceedings Chien Y-C, Tinajero J, Stocker D, Hegde U, Dunn-Rankin D. "Electric Field Effects on Flames in Microgravity on the International Space Station." 2018 Spring Technical Meeting of the Central States Section of The Combustion Institute, Minneapolis, Minnesota, May 20–22, 2018.

2018 Spring Technical Meeting of the Central States Section of The Combustion Institute, Minneapolis, Minnesota, May 20–22, 2018. , May-2018

Project Title:  ACME: EFIELD – Electric Field Effects On Laminar Diffusion Flames Reduce
Images: icon  Fiscal Year: FY 2018 
Division: Physical Sciences 
Research Discipline/Element:
Physical Sciences: COMBUSTION SCIENCE--Combustion science 
Start Date: 11/18/2016  
End Date: 11/17/2019  
Task Last Updated: 09/24/2017 
Download report in PDF pdf
Principal Investigator/Affiliation:   Dunn-Rankin, Derek  Ph.D. / University of California - Irvine 
Address:  Department of Mechanical & Aerospace Engineering 
4200 Engineering Gateway Bldg, EG3224 
Irvine , CA 92697-3975 
Email: ddunnran@uci.edu 
Phone: 949-824-8745  
Congressional District: 48 
Web:  
Organization Type: UNIVERSITY 
Organization Name: University of California - Irvine 
Joint Agency:  
Comments:  
Co-Investigator(s)
Affiliation: 
Karnani, Sunny  Ph.D. Army Research Labs 
Key Personnel Changes / Previous PI: September 2017 report: Dr. Alice Chien is joining the project.
Project Information: Grant/Contract No. NNX17AC51A 
Responsible Center: NASA GRC 
Grant Monitor: Stocker, Dennis P 
Center Contact: 216-433-2166 
dennis.p.stocker@nasa.gov 
Unique ID: 11207 
Solicitation / Funding Source: NOT AVAILABLE 
Grant/Contract No.: NNX17AC51A 
Project Type: FLIGHT 
Flight Program:  
No. of Post Docs:
No. of PhD Candidates:
No. of Master's Candidates:
No. of Bachelor's Candidates:
No. of PhD Degrees:
No. of Master's Degrees:  
No. of Bachelor's Degrees:  
Program--Element: COMBUSTION SCIENCE--Combustion science 
Task Description: NOTE this is a successor agreement to “Electric Field Control of Flames (NNX11AP42A),” in Microgravity Combustion Science per D. Stocker, NASA Glenn Research Center.

This project at the University of California, Irvine continues to explore the use of large electric fields in combustion for improving the performance of energy conversion systems. As an actuator, electric fields can improve flame stability and soot formation, while, as a sensing device, the electrical response at saturation is an inherent flame characteristic. In addition to preparing for the proposed ACME E-FIELD experiments aboard the International Space Station (ISS), we have continued to enhance our understanding of the experimental methods and the dynamic interaction between the flame and chemi-ions. We have measured the distribution of ions at the downstream electrode in order to characterize the likely body forces resulting, and we have begun to develop computational models to predict chemi-ion concentrations in flames, including a comparison between enhanced body forces from electric fields and those from enhanced gravity. This report summarizes recent findings in 4 areas:

1.) A study on the effect of burner geometry on the flame response to electric fields, such as flame shape changes and ion current differences.

2.) Research and developing computational models to predict ion concentrations in flames, specifically on the effects of ion driven winds. In particular, different ion chemistry models are surveyed and reviewed in detail in order to provide in-depth understanding of how ions are produced in hydrocarbon flames. In this case, the simulation study is carried out at multiple gravity levels to observe the variation of physical properties such as species concentration in the active regions under different body forces in flames.

3.) Examining the linkage between experimental flame geometries under the influence of various electric fields on Earth (at 1-g condition) and computational results under various enhanced gravity conditions. This study is focused on investigating the distribution and location of CH chemiluminescence species.

4.) Exploring alternative numerical approaches that will potentially lead to high efficiency computation of flames under the influence of electric fields.

Summaries of these findings appear in this report with the details of all the work appearing in the publications and conference proceedings identified at the end of this document. In the past year we have produced: 1 peer-reviewed journal article; 3 invited technical presentations, 3 conference papers, and 1 Ph.D. dissertation.

Understanding electric field interactions with flames is central to combustion control, particularly in situations near limit operation of the flame where small effects are amplified dramatically. Electric fields have been shown to modify burning rates as well as to enhance and reduce flame propagation even to the point of flame extinguishment. Our experiments provide a unique ability to make simultaneous measurements of both physical and electrical properties of flames.

Research Impact/Earth Benefits: The control of combustion has the potential to improve efficiency and reduce emissions from burning fuels. Since high power density often requires combustion, these improvements will be important no matter what the fuel source. Electric fields acting on flames have the potential to aid in combustion control. In addition, electrical properties of flames can identify poor performing boiler flames that release poisonous carbon monoxide. Our studies show that a flame's electric signature can capture incipient quenching before dangerous emissions result. Understanding the links between electrical character and flame behavior may allow improved sensing of poor performing combustion systems.

Task Progress & Bibliography Information FY2018 
Task Progress: Both experiments and computations are part of the project. The experiments employ a coflow burner that is dimensionally equivalent to the ISS coflow burner but with a fuel jet matching the dimension of the ISS jet burner. The burner consists of two stainless steel plenums threaded together. Fuel, introduced in the bottom plenum, is passed through a stainless-steel tube (2.13 mm ID) and exits the top surface fully developed. Air entering the top chamber passes through flow straightening beads and a honeycomb mesh. The air exits the top surface with a top hat flow profile at the exit. In many experiments, the center fuel jet was slighting extruded above the top surface of the coflow burner to more closely approximate a jet burner. A variable high-tension power supply connected between the burner and downstream mesh electrode produces an applied electric field. Although the electric field distribution is deformed locally by the flame’s space charge, defining an applied field strength, E = V/H, is useful when describing experimental results. Ion current is calculated using Ohm’s law and the potential drop across a shunt resistor between the electrically isolated burner and the building ground. An acrylic chamber shields the flame from room air currents.

(1) Electric Field Effects On Flame Geometry

External electric fields have long been known to modify the geometry of the flame. It has been demonstrated that flames may deflect in the direction of an electrode with a negative electrical bias. This study shows how an electric field affects the behavior of a non-premixed, conical shaped flame that is stabilized on a coflow burner when the central tube is extruded 3mm above the coflow surface. The burner acts as the ground electrode in the electrical circuit.

Chemiluminescence images for this study were captured with a Nikon D90 digital camera and a 430 nm (10 nm full width at half max) filter. Details on the procedure of imaging and quantifying of flame chemiluminescence can be found in the literature, e.g., Walsh et al. [1998], Giassi et al. [2016], and Tinajero [2017].

After the external field was initiated, the flames re-stabilized before the images for this study were taken. These images were deconvolved with an onion-peeling Abel inversion to reveal the two-dimensional flame contour. Details of the Abel inversion algorithm can be found, e.g., in Dasch [1992]. What can be extracted from this study is that the geometry of the flame subtly responds to the external electric field with a slight decrease in flame height and a slight decrease in flame width. The more significant change observed is the lifting of the flame's base and increased CH* light emission at high field strengths. These two flame characteristics were quantified as a function of field strength.

Previous simulations have predicted that the enhancement to saturated ion currents may be caused by enhanced mixing of fresh oxidizer with fuel in this flame base region, which results in a partially premixed flame as described by Yamashita et al. [2009]. The mixing is explained by entrainment of ambient air into the gas ow forced by the ion wind. The lifting of the flame seen during this study may be the result of this mixing.

A summary of the analysis for all experimental configurations shows that for the burner with the protruding fuel tube, a clear trend can be seen between the height of the flame's base, the intensity of light emitted from the flame by CH*, and the chemi-ion production (identified by the ion current above 1 kV/cm). For a flush fuel tube, on the other hand, the height of the flame's base is uncoupled from the CH* chemiluminescence and chemi-ion production. Regardless of the unchanged height of the flame, the CH* chemiluminescence and the chemi-ion production both demonstrate enhanced levels when the electric field strength rises above 3 kV/cm.

(2) OpenFOAM Simulations - I

As discussed earlier, electric fields can affect flame shape, burning velocity, temperature profile, speed of propagation, lift-off distance, species diffusion, stabilization, and extinction. The primary reason is that combustion of hydrocarbon fuels involves a chemi-ionization process, which generates ions and electrons that can be manipulated by the field producing some alteration of the chemical kinetics and generation of a body force. The former arises because the chemistry of the system is affected by the redistribution of charges under the applied electric field; the latter generates an ion wind.

Chemiluminescence of the flame is considered in these simulations. Chemiluminescence is an important feature in flame diagnostics and is an electronically excited species present in flames. Excited species are important as an indicator for many flame properties including reaction zone position. At the moment, there is no published model for non-premixed co-flow methane/air flames that contains both chemi-ions and excited species. Hence, a new combination of models that contains both, excited species and chemi-ions, is part of this study.

Moreover, a study of the behavior of the flame under different gravity conditions is performed to better understand how buoyancy affects flame behavior. This study could provide interesting insights should we choose to use combustion processes in current and future missions to Mars and other environments where gravity conditions differ from those on Earth, as well as helping elucidate the contribution of the buoyancy forces to the flame.

Chemical Kinetics and Reaction Mechanism

The chemical kinetic mechanism used is a new combination chemical kinetic model (i.e., a selected collection of reactions from published models) for the prediction of major species and minor species such as ions and excited species. It contains 299 reactions, which reactions 1 to 280 come from the model proposed by Prager et al. [2007] except that the parameters have been reviewed and checked with the references proposed in the original document. A submodel proposed by K.T. Walsh [1998] that predicts excited species (OH* and CH*) has been added to the combined chemical kinetic model proposed. Species transport coefficients were predicted by using Cantera® software. The excited species transport coefficients were presumed to be the same as their respective ground state specie. The ionic species transport was presumed to be the same as for air, as a first approximation. Refinements to the model will include using transport coefficients for ions that match their neutral counterpart but for the current work this was not necessary because the concentration of these species is so small.

Numerical Model

OpenFOAM was chosen as the numerical solver to analyze the effects of chemi-ionization and electric fields on coflow flames. The solver used was a modified version of the reactingFoam solver in OpenFOAM. The mass transport equation was modified by imposing a Schmidt number equal to 0.7 condition.

Implementing Maxwell’s equations into the model is still work in progress. The coflow burner has been modeled as two concentric cylinders, whose upper bases correspond to the exit area of the nozzle; the origin of the reference frame has been set here. Pure (fuel) and pure air (oxidizer) are ejected respectively from the internal nozzle and the external annulus. In OpenFOAM it is easy to deal with axially symmetric geometries; for cylinders it is sufficient to solve the problem in a wedge of small aperture. To do that, a cylindrical plate has been considered; in this way, properties vary only along the radial coordinate and the axial coordinate. A large external domain must also be included to make sure that the boundaries do not affect the core combustion processes and behaviors.

Simulations

The simulations of this study predict flames behavior under microgravity (0G), partial gravity (0.5G), gravity (1G), and supergravity (1.5G, 2G, and 3G). Hence, the gravity conditions used in the simulations are in all the range of the gravity values for the Earth-Solar System.

Results and discussion.

The profiles computed are the temperature in the flame as well as the CH* (which acts as the visible flame marker) profile and the profile of the two main ions produced in the flame, H3O+ and HCO+ (which highlights the source of electric body forces if a field were to be applied).

1. Temperature profile. Due to the co-flow air that is surrounding the inside fuel jet, the completely spherical shape expected from a 0G flame is slightly modified in the wings since air is coming from the sides. As gravity increases, buoyancy forces pull the flame upward and make it narrow. The effects observed in the temperature profiles were not unexpected in response to changing gravitational acceleration. New experimental data is expected to be available from the ACME project to validate the 0G profile.

2. CH* profile. Chemiluminescence associated with the relaxation of CH* to CH is directly correlated with the luminosity of the flame and how the flame looks to the eye. In the 1G flame, the flame height correlates with previous experimental results and the location of the CH* is well predicted.

3. HCO+ and H3O+ profiles. The main ions naturally produced by the flame, H3O+ and HCO+, are shown to be in similar or higher concentration than the CH*. This fact strengthens the hypothesis of modification of flame behaviors by using electric fields and interacting with these species, since it has been observed that electric fields change the luminosity and flame shape of laminar diffusion flames (Lawton and Weinberg [1970], Karnani [2011], and Tinajero [2017]).

On the other hand, as previously mentioned for CH* profiles, the height of the flame does not seem to be affected by the change in buoyancy forces but the flame width does shrink to create a thinner flame. This might be the explanation of the higher production of H3O+ species at super-gravity, since the flame shrinking more tightly packs the species.

Conclusions and Future Work

Simulations including excited species and ionic species reproduced successfully many of the main characteristics of the flame, such as flame height, flame temperature, as well as major and minor species naturally produced by the flame. Flame height was shown to not be affected by a change in gravity field while flame width was affected by producing a thinner plume, as was expected. However, detachment from the burner tip was observed numerically though it was not seen experimentally, and this detachment seems to result from the numerical challenge related to the prediction of these boundary conditions.

More work must be done to accurately predict the bottom of the flame and the interaction of the flame with electric fields. Nevertheless, the combination chemical kinetic model proposed containing excited species and ions has shown an ability to predict important characteristics of the non-premixed methane laminar jet flame. To better understand the reasons why the observed concentration profiles using the simulations differ from the experimental results, the combination of chemical kinetic models used was previously tested for 1D simulations by using CHEMKIN software.

(3) OpenFOAM Simulations - II

This study case tested the effect of boundary conditions and compared multiple important flame parameters, e.g., flame geometry via CH* chemiluminescence and predicted Schlieren profiles, alongside experimental results without external electric fields. The experiments are available for the electric field case but not yet the simulation. This work examines the use of simulated experiments, rather than simulated flames, as the key comparator with experimental results.

GRI-Mech 3.0 from Smith et al. [1999] was used as the base mechanism for all neutral molecules and their reactions. The files containing the reaction parameters, transport properties and thermodynamic properties can be found from http://combustion.berkeley.edu/gri-mech/ . Detailed studies were performed on the validation of various chemiluminescence mechanisms by Panoutsos et al. [2009] and Kathrotia [2011]. The reaction parameters for excited species CH* were borrowed from Walsh [2000] and Hall et al. [2005]. Separate simulations were performed with both chemiluminescence mechanisms for comparison; however, Walsh [2000] was used as the baseline mechanism.

Numerical Validation

1. Temperature. Temperature profiles were predicted by the simulations with two different inlet velocity profiles. The differences between the non-uniform and the uniform velocity profiles are very subtle. Perhaps the only real difference is that a non-uniform velocity profile produces a slightly higher profile near the burner tube wall.

2. CH*. The peak CH* mole fractions calculated from the simulations were on the order of 5e-12. This is about 1 order of magnitude lower than the peak mole fractions experimentally obtained and computed by Walsh et al. [1998] and Walsh [2000] but the current flame is much smaller. Therefore, the CH* mole fractions shown in this study are reasonable.

A qualitative comparison between the simulated prediction of CH* and the experimentally obtained results was made. In both cases, the numerically predicted CH* profiles match well with the experimentally obtained profile. However, the non-uniform velocity profile shows a better spatial match, qualitatively. Specifically, the flame heights match better between the non-uniform velocity simulation and the experimentally obtained flame compared to the uniform velocity simulation. Also, the shape of the flame base-edge produced by the non-uniform velocity profile simulation matches qualitatively better with the experimental flame base-edge. In both simulated cases, the calculated flames sheets are thinner than the experimentally obtained flame sheets. Walsh et al. [2000] studied the effects of optics geometry (particularly the effect of f-number) on the Abel inversion results. Giassi [2017] expanded on this study and showed the effect of other excited species that could potentially emit light within the range of the light filters. Both cases, optical geometry and unknown chemiluminescence, result in broader flame thicknesses. A reasonable f-number of 5.6 and a narrow band filter (10 nm full width at half max) centered at 431 nm were chosen to minimize these effects. However, their contribution in the uncertainty of the experimental flame thicknesses should be noted.

3. Stoichiometric mixture fraction. Here, Cst (stoichiometric proportion) is tested with the numerical model to examine how well it matches with the visual marking considered to be the flame. CH* is the most logical marker for the reaction zone since it is what can most easily be seen (in most flame applications) by the human eye. Therefore, it makes sense to compare Cst with the CH* mole fraction profile. The CH* profile is found to lie just outside of the Cst line (on the oxidizer side). The Cst line stands about 0.1 to 0.2 mm from the peak mole fraction along the CH* curve. This is true everywhere except at the upstream leading edge of the flame. The edge of the flame diverges from the Cst line because the Cst is a property of non-premixed flames. Even though conical co-flow flames are considered non-premixed, the flame's edge contains many premixed flame characteristics because of the higher level of mixing occurring in this region.

The stoichiometric mixture fraction may not be a perfect marker for the CH* profiles but the spatial off-set distance (1/10th mm) is small enough for the theoretical simplification to be justified with typical experimental spatial uncertainty. Another test for the Cst line is to also compare it to the region of substantial local heat-release of the flame. Like the CH* comparison, the local heat-release is on the oxidizer side of the Cst line. The offset distance between the line and the peak locations along the heat-release curve is again on the order of 1/10th of a millimeter.

Schlieren Prediction

Schlieren imagery is an age-old technique with applications in many scientific disciplines. However, the difficulty in using schlieren is determining how to make quantified measurements. For instance, a schlieren image can be used to determine the location of a shock-wave formed by an extremely fast moving object through a fluid but acquiring measurements of temperature or density from the schlieren image is much more difficult to achieve because it requires derivatives of potentially noisy signatures that then amplifies the noise. Here, simple schlieren and light theory was applied to the numerical prediction of an axially symmetric co-flow flame in order to provide another form of validation to the OpenFOAM simulation and to see what the experimental schlieren images tell us about chemi-ion driven flows.

The simulated schlieren profiles showed good qualitative agreement with the experimental profiles. This positive comparison between measurement and calculation is important because it confirms the reliable prediction of the thermal field around the flame (i.e., the broader convective transport) in addition to the appropriate location of the reaction zone.

Effect of body forces

Simulations were performed on testing the effect of enhanced gravity on various non-premixed coflow flame parameters, e.g., flame geometry, flame tip height, flame base-edge height, CH* chemiluminescence, and schlieren profiles. While it cannot be expected that enhanced gravity will produce exactly what would be produced if chemi-ion chemistry and Maxwell's equations were employed, parallels can be drawn where applicable to assess the performance of enhanced body forces as the source of the observed flame/flow dynamics seen previously. Qualitatively the effect of enhanced gravity produces what was found to happen when external electric fields are used in experiments. That is, the flame height decreases slightly, the radius of the flame decreases slightly, the base of the flame increases significantly, and the peak mole fraction increases with increasing gravity. The gravity level at which point the flame could not sustain itself (extinction limit) was not exactly determined in this study, but as a general guide the flame was not sustainable (blow out) around and above 3G. In all cases, the spatial comparison of simulated flame location with experimental results shows very good agreement.

The comparison of the flame's base height shows that the enhanced gravity levels between 1-2 times that of normal gravity could increase the base height to the same levels as seen with external electric fields. This increase in flame base lifting would increase the local mixing in this region which would be the cause of the observed increase in CH* light emission from the flame. This is seen to happen in the simulations, however, the percent increase in peak CH* mole fraction is much less than the percent increase in pixel light intensity from the CH* chemiluminescence images taken from the experiments. The incompatibility between the two results could be due to multiple causes.

1. the similarities between ion wind body forces and buoyancy forces are enough to explain transport but are unable to capture CH* chemistry, 2. the simulation is unable to capture the proper CH* mole fractions at enhanced gravity, 3. the simulations are predicting CH* correctly and the CH* experiments need to be quantitatively calibrated to ensure linearity.

The results of the predicted schlieren radius under different gravity levels also shows good correspondence. The peak axial velocity predicted by the OpenFOAM simulations under increasing gravity fields shows, as would be expected, the axial velocity increases, but only by about 40%. The velocity measurements underpredict what was suggested by the experiments using external fields. This finding seems reasonable in that thermally driven buoyancy is likely to be a more diffuse body force as compared to the more narrowly focused ion wind body force effect. While the overall influence is similar, the local acceleration will be more sensitive to streamline compression from the tightly directed electric field driven forces.

Future Work

While it is possible that experimental effects are partly responsible, the very significant uncertainties in chemical kinetics and the extremely low concentrations of excited species suggest that a quantitative comparison with computations is a major challenge.

Conclusions

The results showed that enhancing gravity was enough to explain nearly all the experimental observations, especially spatial variations in flame shape. The CH* peak mole fraction and peak velocity were found to have disparities when compared to the experimental results by significantly underpredicting the experimental results. It is not yet clear what is the reason for the difference. Thus, further studies should be made to clarify if the issue is in the hypothesis, the experiment, or the simulation.

(4) High Performance Computing Approach – Collaboration with LBNL

Previous experiences suggest that it will be difficult to couple the small concentration ions with the overall flow field in a numerical simulation. To have a better chance at accomplishing this coupling, we are exploring a highly adaptive mesh refinement approach pioneered at Lawrence Berkeley National Laboratory (LBNL).

Overview

The code under development at the LBNL uses AMReX, a software library containing the functionalities (written either in C++ and Fortran90) required to perform adaptive mesh refinement (AMR); the basic idea of this technique is that the governing equations can be solved, at each timestep, on a multilevel grid. In a multilevel grid, the mesh is refined at each level but only where it is needed (typically where high gradients occur), and the solution is recomputed only in these zones. This allows to refine the solution without having to recompute it on the entire domain, which saves time and computational costs. AMReX can solve problems with simple geometry in 2D and 3D, and in either Cartesian and cylindrical coordinates. The applications range would go from combustion to astrophysics and cosmology, from porous media to fluctuating hydrodynamics.

In this case study, we are interested in laboratory-scale experiments involving time-dependent open systems running at atmospheric pressure, in which the characteristic velocity of the fluid is much smaller than the speed of sound. The low Mach number conservation equations used were the ones proposed in M.S. Day and J.B. Bell [2000], with the addition of the Poisson’s equation and the electric field contribution in the mass, momentum, species, and enthalpy equations. In this regime, conservation equations take the form of a system of differential algebraic equations representing coupled advection-diffusion-reaction-electrostatic processes. These equations evolve subject to a constraint which is represented by the Equation of State (EOS) written in the form of a divergence constraint on the velocity; this constraint equation can be derived by differentiating the EOS in the Lagrangian reference frame of the moving fluid, and enforcing that here the thermodynamic pressure pEOS remains constant. The model used allows a ~ 1/Mach # increase (typically, 10-100) in the admissible timestep with respect to a fully compressible method.

Numerical Model

In such a stiff regime, chemistry and diffusion occur at faster timescales than advection; that is what imposes the restriction on the timestep. Subsequently, reaction-diffusion can be modeled implicitly, while advection requires explicit treatment. Furthermore, an explicit treatment of the Poisson’s equation introduces a severe limitation on the timestep, which is represented by the dielectric relaxation time.

The presence of an electric field has been modeled through a modified version of the Poisson’s equation, whose implicit coupling with the species diffusion fluxes ensures that the predicted electric potential is numerically consistent with the iteratively-lagged diffusion-induced charge separations. This procedure is necessary to eliminate the physics induced by the dielectric relaxation of unphysical numerically-induced charge separations.

To the best of our knowledge, an entirely implicit treatment of the charged species has not been implemented yet. Here, a fully implicit treatment of the drift fluxes of both ions and electrons in the low Mach number regime is introduced. The drift fluxes are then coupled with the diffusion fluxes, which have also been modeled implicitly.

The low Mach number combustion code (PeleLM; previously known as LMC) developed at LBNL is a finite volume code that implements the variable density projection in the low Mach number regime.

In order to test the new algorithm, a simple well-known configuration is considered, for which a lot of experimental and numerical results are available in literature. This configuration and algorithm is determined for a 1D premixed methane/air flame, using a reaction mechanism from J. Hu et al. [2000] that includes neutral and charged species H3O+, HCO+, e-. The thermal and transport properties are based on GRIMech-3.0 and on the CHEMKIN-III database

Numerical results show that a flame is generated around ~ 0.3cm from the base, with a peak temperature of about 2200K. However, the applied electric field does not affect the flame temperature too much, so changes were not reported for the temperature profile. The focus of this study was, then, on the effects of the electric field on the most important charged species.

The time evolution of the electric field and electric potential for 1000V shows that the code captures the generation of plasma sheaths in the flame zone and at the outflow electrode: charges move to the electrode of opposite charge so they mask the effects of the electric field inside the domain. In fact, there is a strong electric field in the flame zone, but as time passes and charges move to the opposite electrode (ions to the left, electrons to the right) the electric field becomes almost zero inside the domain. Hence, the flame is charge neutral in that region.

Summary and Future Work

In summary, these preliminary results show that the code:

• correctly couples the different processed involved (advection-diffusion-reaction-electrostatic) ; • reproduces the physics associated with the formation of plasma sheaths.

Additional work is required in order to: • better manage the timescales of charged species transport; • further increase the timestep (up to 1e-6); • make the code more robust and suitable for AMR; • extend the code to 2D and 3D, in both Cartesian and cylindrical coordinates; • optimize the code to run on CORI/EDISON (tiling); • implement more complex geometry.

References

Walsh, K.T., Long, M.B, Tanoff, M.A, and Smooke, M.D. Experimental and computational study of CH, CH*, and OH* in an axisymmetric laminar diffusion flame. Symposium (International) on Combustion, 27(1):615–623, January 1998.

Giassi, D., Cao, S., Bennett, B.A.V, Stocker, D.P, Takahashi, F., Smooke, M.D., and Long, M.B. Analysis of CH* concentration and flame heat release rate in laminar coflow diffusion flames under microgravity and normal gravity. Combustion and Flame, 167:198–206, 2016.

Giassi, D. Optical Diagnostics Applied to Quantitative Characterization of Coflow Laminar Diffusion Flames in Microgravity and Normal Gravity. PhD thesis, Yale Univeristy, 2017.

Dasch, C.J. One-dimensional tomography: a comparison of Abel, onion-peeling, and filtered backprojection methods. Applied optics, 31:1146–1152, 1992.

Yamashita, K., Karnani, S., and Dunn-Rankin, D. Numerical prediction of ion current from a small methane jet flame. Combustion and Flame, 156(6):1227–1233, 2009.

Prager, J., Riedel, U., and Warnatz, J. Modeling ion chemistry and charged species diffusion in lean methane-oxygen flames. Proc. Combust. Inst.1129:31, 2007.

Lawton, J., and Weinberg, F.J. ‘Electrical aspects of combustion’. Oxford University Press (1970).

Karnani, S.V. PhD Dissertation. Electric Field-Driven Flame Dynamics. University of California at Irvine (2011).

Tinajero, J.A. PhD Dissertation. Flame Dynamics and Chemi-Ion Flows Driven by Applied Electric Fields. University of California at Irvine (2017).

Smith, G.P., Golden, D.M., Frenklach, M., Moriarty, N.W., Eiteneer, B., Goldenberg, M., Bowman, C.T., Hanson, R.K., Song, S., Gardiner, Jr., W.C., Lissianski, V.V., and Qin, Z. Gri-mech 3.0. http://www.me.berkeley.edu/gri_mech/ , 1999.

C. S. Panoutsos, Y. Hardalupas, and M. K. P. Taylor. Numerical evaluation of equivalence ratio measurement using OH* and CH* chemiluminescence in premixed and non-premixed methane-air flames. Combustion and Flame, 156(2):273–291, 2009.

Kathrotia, T. Reaction Kinetics Modeling of OH*, CH*, and C2* Chemiluminescence. PhD thesis, 2011.

Walsh. K.T. Quantitative Characterizations of Coflow Laminar Diffusion Flames in a Normal Gravity and Microgravity Environment. PhD thesis, Yale University, 2000.

Walsh, K.T., Fielding, J., and Long, M.B. Effect of light-collection geometry on reconstruction errors in Abel inversions. Optics letters, 25(7):457–459, 2000.

Hall, J.M., De Vries, J., Amadio, A.R., and Petersen, E.L. Towards a Kinetics Model of CH Chemiluminescence. (January), 2005.

Day, M.S., Bell J.B. ‘Numerical simulation of laminar reacting flows with complex chemistry’, Combustion Theory Modeling, 4, 535-556, 2000.

Hu, J., Rivin, B., Sher, E. ‘The effect of an electric field on the shape of coflowing and candle-type methane-air flames’, Experimental Thermal and Fluid Science, 21, 124-133, 2000.

Bibliography: Description: (Last Updated: 02/12/2024) 

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Articles in Peer-reviewed Journals Chien YC, Escofet-Martin D, Dunn-Rankin D. "CO emission from an impinging non-premixed flame." Combustion and Flame. 2016 Dec;174:16-24. https://doi.org/10.1016/j.combustflame.2016.09.004 , Dec-2016
Articles in Peer-reviewed Journals Tinajero J, Bernard G, Autef L, Dunn-Rankin D. "Characterizing I-V curves for non-premixed methane flames stabilized on different burner configurations." Combustion Science and Technology. 2017;189(10):1739-50. http://dx.doi.org/10.1080/00102202.2017.1331218 , Jun-2017
Dissertations and Theses Tinajero J. "Flame Dynamics and Chemi-Ion Flows Driven by Applied Electric Fields." Dissertation, University of California, Irvine, September 2017. , Sep-2017
Project Title:  ACME: EFIELD – Electric Field Effects On Laminar Diffusion Flames Reduce
Images: icon  Fiscal Year: FY 2017 
Division: Physical Sciences 
Research Discipline/Element:
Physical Sciences: COMBUSTION SCIENCE--Combustion science 
Start Date: 11/18/2016  
End Date: 11/17/2019  
Task Last Updated: 03/08/2017 
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Principal Investigator/Affiliation:   Dunn-Rankin, Derek  Ph.D. / University of California - Irvine 
Address:  Department of Mechanical & Aerospace Engineering 
4200 Engineering Gateway Bldg, EG3224 
Irvine , CA 92697-3975 
Email: ddunnran@uci.edu 
Phone: 949-824-8745  
Congressional District: 48 
Web:  
Organization Type: UNIVERSITY 
Organization Name: University of California - Irvine 
Joint Agency:  
Comments:  
Co-Investigator(s)
Affiliation: 
Karnani, Sunny  Ph.D. Clearsign Combustion Incorporated 
Project Information: Grant/Contract No. NNX17AC51A 
Responsible Center: NASA GRC 
Grant Monitor: Stocker, Dennis P 
Center Contact: 216-433-2166 
dennis.p.stocker@nasa.gov 
Unique ID: 11207 
Solicitation / Funding Source: NOT AVAILABLE 
Grant/Contract No.: NNX17AC51A 
Project Type: FLIGHT 
Flight Program:  
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Program--Element: COMBUSTION SCIENCE--Combustion science 
Task Description: NOTE: This is a successor agreement to "Electric Field Control of Flames," grant NNX11AP42A, per D. Stocker, NASA Glenn Research Center.

This project at the University of California, Irvine explores the use of large electric fields in combustion for the purpose of improving the performance of energy conversion systems. As an actuator, electric fields can affect flame stability and soot formation, while, as a sensing device, the electrical response at saturation is an inherent flame characteristic. In addition to preparing for the proposed ACME E-FIELD experiments aboard the International Space Station (ISS), we have performed an evaluation of an experimental method of measuring ion number density in a flame using a Langmuir probe, studied the dynamic interaction between the flame and chemi-ions, analyzed the source of increased saturation ion currents found in I-V curves, used electric fields to control flames impinging on a surface, and begun to develop computational models to predict chemi-ion concentrations in flames. This report summarizes recent findings. Specific activities include:

1.) A study on the dynamic interaction between the flame and the electrical body force created by a flux of chem-ions. This was done in several ways, by analyzing the dynamic response of the gaseous flow field in response to a sudden application of an electric field, by comparing transient flame behavior using a high speed camera and the transient ion current collected when an electric field is turned on.

2.) A study of the interaction between the flame and the DC electric fields. This was done by studying the flame geometry to varies steady electric field strengths and analyzing I-V curves for different burner geometries.

3.) A study to construct a framework and approach for building computational models to predict ion concentrations in the flame, including the effects of ion driven winds. Different burner geometries were tested. Different ion chemistry models were used to test the current level of understanding of how ions are produced in the flames.

Understanding electric field interactions with flames is central to combustion control, particularly in situations near limit operation of the flame where small effects are amplified dramatically. Electric fields have been shown to modify burning rates as well as to enhance and reduce flame propagation even to the point of flame extinguishment. Our experiments provide a unique ability to make simultaneous measurements of both physical and electrical properties of flames.

Research Impact/Earth Benefits: The control of combustion has the potential to improve efficiency and reduce emissions from burning fuels. Since high power density often requires combustion, these improvements will be important no matter what the fuel source. Electric fields acting on flames have the potential to aid in combustion control. In addition, electrical properties of flames can identify poor performing boiler flames that release poisonous carbon monoxide. Our studies show that a flame's electric signature can capture incipient quenching before dangerous emissions result. Understanding the links between electrical character and flame behavior may allow improved sensing of poor performing combustion systems.

Task Progress & Bibliography Information FY2017 
Task Progress: New project for FY2017.

Continuation of "Electric Field Control of Flames," grant NNX11AP42A. See that project for previous reporting.

Bibliography: Description: (Last Updated: 02/12/2024) 

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 None in FY 2017