In this project, it is hypothesized that burning behaviors and flammability boundaries of solid materials in partial gravity can be predicted using data collected in normal and microgravity. The overall goal of this project is to leverage the rich dataset collected in previous microgravity experiments to advance understanding of fire behaviors in partial gravity. The microgravity data will be complemented by existing and new ground data to lead to new science. Ultimately, we want to answer the following questions. “What can we learn from the microgravity data obtained in different experiments and different ambient conditions regarding material flammability in partial gravities?” “How do we predict burning behaviors in partial gravity using data collected in normal and microgravity?”

The project consists of three major components. The first component is analysis of previous microgravity data. Tools will be developed to analyze data available on the NASA’s Physical Sciences Informatics (PSI) system for microgravity combustion experiments (Burning and Suppression of Solids/BASS, BASS-II; Spacecraft Fire Safety Experiments: SAFFIRE-I, SAFFIRE-II, and SAFFIRE-III). Flame characteristics including spread rate, length, appearance, color, and flammability boundaries will be extracted, documented, and compared. Other microgravity datasets (e.g., International Space Station/ISS Confined Combustion project) will also be considered. This will result in a comprehensive inter-project meta-analysis of the current knowledge on microgravity combustion. The second component of the project is ground experiments. Burning experiments will be performed in a combustion chamber in the Principal Investigator (PI) lab with different environmental conditions. The experimental setup and sample selections will resemble the PSI microgravity experiments, enabling direct comparisons of ground and space data. This will allow us to correlate the gravity-induced buoyancy flows (in normal gravity) to the imposed flows in microgravity, in terms of their effects on the burning process. The third component will be numerical simulations. Both microgravity and ground experiments will be simulated. After the model is validated against the experimental data, the detailed profiles obtained in the numerical simulations will be used to understand underlying physics of the experimental observations. The model will then be able to simulate solid burning in different partial gravities, leading to a more complete understanding of the role of buoyancy flow on fire behavior.

Through these efforts, we will identify invariant dimensionless parameters for flame spread and develop correlations between burning characteristics and the identified dimensionless parameters. These dimensionless parameters will incorporate various variables, including sample dimensions (e.g., width, thickness); gravity; and ambient conditions (pressure, flow speed, oxygen percentage). All generated outcomes will be made available for future investigators to test theories or train models.

All generated outcomes will be made available for future investigators to test theories or train models. This directly contributes to the advancement of fire science. The developed understanding of burning behaviors in partial gravity will help improve the test methods and design of future space vehicles. The ultimate goal of this project is to reduce tragic loss of life and property both on earth and in space.

1) Analysis of the PSI data

The major component of this project is the analysis of data available on the PSI system for microgravity combustion experiments. Undergraduate student (Nathan Kralik) was recruited to support this project. He joined the PI’s lab in June 2024 as a summer student researcher and continued working on the project as a BS/MS student researcher in Fall 2024.

In this reporting period, we focused on conducting literature review, understanding the structure of the PSI, and developing initial MatLab code to process the PSI data. The initial focus for the PSI analysis was the Burning and Suppression of Solids II (BASS II) experiment. In this experiment, various materials (e.g., polymethyl methacrylate/PMMA, cotton-based fabric) were burned under controlled, concurrent air flow; and images and video were taken during the burning process. The main goal was to develop a MatLab code to track the flame progression from the images and videos on the database. This was accomplished by creating an edge tracing program to trace the flame in each image, and then a location tracking program to track the movement of the trace over time. Note that the detected flame edge depends on a threshold value set in the code. We are currently working to determine the optimal value for accurately detecting the flame edge while minimizing interference from background noise. Additionally, a subroutine was written to process the texts overlaid on each flame image and to obtain information on the real-time air flow speed in the experiments.

Going forward, the goal is to continue to refine the existing MatLab code for the BASS and BASS II experiments, as well as develop new code to analyze other aspects of flame spread (e.g., flame length, flame color) from other experiments on the PSI database.

2) Ground Experiments

Burning experiments were performed in a combustion chamber in the PI’s lab with different environmental conditions. A Ph.D. student (Robin Neupane) updated and restored the functionality of an old combustion chamber on loan from NASA Glenn Research Center. We conducted burning experiments in reduced pressures and in different oxygen percentages. The goal was to artificially lower the buoyant flows (which occurs in a partial gravity environment) via reducing the ambient gas density and to explore the burning characteristics of solid materials near the extinction limits.

Experiments were performed at pressures (ranging from 100 mbar to 1,000 mbar) and different oxygen concentration environment (9% - 21%). As pressure increased, different burning behaviors were observed: no ignition, partial flame spread, steady flame spread, and accelerating flame spread. Similar trend was observed as the ambient oxygen concentration increased. In partial pressure conditions (e.g., 250-500 mbar), flames exhibited characteristics that are typically observed in micro- and partial gravity environments: blue and dim. Flame spread rate and sample burnt length were deduced and compared between different pressure and oxygen levels. Overall, the burning intensity and the flame spread rate decreased with the decrease in ambient pressure and oxygen. The decrease in flame spread rate at reduced pressure was attributed to increase in flame standoff distance and decrease in convective heat transfer to the solid; whereas the decrease in flame spread rate in reduced oxygen concentration environment was attributed to decrease in flame temperature.

Current and previous studies from literature performed at different ambient environments were correlated using the concept of flame standoff distance (df), which was estimated using the theoretical viscous boundary layer thickness (dv). It was found that approximating df ~ dv for forced flow and df ~ 1/3 dv for natural flow can predict the flame spread rate reasonably well for data obtained in micro-, partial, and normal gravities, for a wide range of environmental conditions away from extinction limits.

In the next reporting period, the PI will hire a new Postdoctoral researcher to continue this effort. For new experiments, sample selections and sample setup used in the PSI microgravity experiments will be considered, enabling direct comparisons of ground and space data.

3) Numerical Simulations

The third component of this project is numerical simulation. For this effort, we use an in-house Computational Fluid Dynamics (CFD) combustion model that has been validated against various previous microgravity and ground experiments. A Ph.D. student (Jiaxuan Han) conducted two-dimensional transient numerical simulations to understand concurrent-flow flame spread over a thin solid sample in NASA’s Spacecraft Fire Safety Spacecraft Fire Safety Experiments (Saffire) project (some data available in the PSI). The sample material, model geometry, and ambient conditions were chosen based on the Saffire VI experiment. A 94 cm-long cotton-fiberglass fabric blend (SIBAL fabric), situated at the center of a flow duct was burned in a concurrent forced flow of 20 cm/s. The ambient conditions were at 30 % oxygen and 0.54 atm pressure. The model consisted of both transient gas and solid phases. In the gas phase, full Navier-Stokes Equations were solved and a one-step, second-order Arrhenius reaction was included. In the solid phase, a two-step solid decomposition was considered. Combustion product dissociation effect at elevated oxygen conditions was also considered. The simulation results demonstrated that the flame spread rate, the location of the pyrolysis region, and the solid surface temperature distribution agreed well with those observed in the Saffire VI experiment. The simulation results were also compared with simulation results of Saffire IV for the same sample material burning at a different condition (23% oxygen and 1 atm pressure) to investigate the effect of ambient conditions on the fire behavior in microgravity. The numerical results successfully captured the increased flame spread rate observed in Saffire VI despite a lower partial pressure of oxygen, compared to Saffire IV. We are in the process of further analyzing the results and understanding the effects of partial pressure of oxygen on flame spread.

In addition to simulations for Saffire, the former Postdoctoral researcher, Dr. Ankit Sharma, used the three-dimensional version of the model to simulate upward flame spread over a thin solid sample in normal gravity. We first focused on validating the model against literature data. In these experiments, machine papers were burned in upward configuration in different oxygen (20-40 %) and ambient pressure at 0.2 atm in normal gravity. Without the consideration of combustion product dissociation, the model over-predicted the flame temperatures and the flame spread rates at elevated oxygen environments. After the combustion product dissociation effects were considered and accommodated, the model was able to predict the flame spread rate reasonably well. Dr. Sharma left the PI’s lab in February 2024. A new postdoctoral researcher (to be hired) will continue this effort, validate the model against a wider range of environmental conditions (e.g., pressure, gravity, oxygen), perform numerical simulations for both ground and microgravity experiments, and finally use the detailed profiles obtained in simulations to understand underlying physics of the experimental observations and the role of buoyancy flow on fire behavior.

In this proposal, it is hypothesized that burning behaviors and flammability boundaries of solid materials in partial gravity can be predicted using data collected in normal and microgravity. The overall goal of this project is to leverage the rich dataset collected in previous microgravity experiments to advance understanding of fire behaviors in partial gravity. The microgravity data will be complemented by existing and new ground data to lead to new science. Ultimately, we want to answer the following questions. “What can we learn from the microgravity data obtained in different experiments and different ambient conditions regarding material flammability in partial gravities?” “How do we predict burning behaviors in partial gravity using data collected in normal and microgravity?”

The project consists of three major components. The first component is analysis of previous microgravity data. Tools will be developed to analyze data available on the NASA’s Physical Sciences Informatics (PSI) system for microgravity combustion experiments (Burning and Suppression of Solids/BASS, BASS-II; Spacecraft Fire Safety Experiments: Saffire-I, Saffire-II, and Saffire-III). Flame characteristics including spread rate, length, appearance, color, and flammability boundaries will be extracted, documented, and compared. Other microgravity datasets (e.g., International Space Station/ISS Confined Combustion project) will also be considered. This will result in a comprehensive inter-project meta-analysis of the current knowledge on microgravity combustion. The second component of the project is ground experiments. Burning experiments will be performed in a combustion chamber in the Principal Investigator (PI) lab with different environmental conditions. The experimental setup and sample selections will resemble the PSI microgravity experiments, enabling direct comparisons of ground and space data. This will allow us to correlate the gravity-induced buoyancy flows (in normal gravity) to the imposed flows in microgravity, in terms of their effects on the burning process. The third component will be numerical simulations. Both microgravity and ground experiments will be simulated. After the model is validated against the experimental data, the detailed profiles obtained in the numerical simulations will be used to understand underlying physics of the experimental observations. The model will then be able to simulate solid burning in different partial gravities, leading to a more complete understanding of the role of buoyancy flow on fire behavior.

Through these efforts, we will identify invariant dimensionless parameters for flame spread and develop correlations between burning characteristics and the identified dimensionless parameters. These dimensionless parameters will incorporate various variables, including sample dimensions (e.g., width, thickness); gravity; and ambient conditions (pressure, flow speed, oxygen percentage). All generated outcomes will be made available for future investigators to test theories or train models.