Flames in nature are not flat. Studies in opposed-jet flat cool flames lack the presence of curvature found in practical internal combustion engines. The curvature of tubular flames is expected to change where the cool flames transition to warm and hot flames, as well as stretch rate extinction values. Curvature and stretch rate are also known to produce cellular structure in flames. In this study, cool premixed and diffusion flames in tubular flames will be investigated both computationally and experimentally to determine the effect of curvature on cool flame regimes, transition to warm and/or hot flames, flame structure, multi-stage flame formation, and extinction stretch rates.

The project will use data from PSI system listed as Investigation #12 in Table A of the Program Element entitled, “Quantitative Studies of Cool Flame Transitions at Radiation/Stretch Extinction using Counterflow Flames” ( https://nspires.nasaprs.com/external/viewrepositorydocument/cmdocumentid=807808/solicitationId=%7B3B58C0CA-6FF7-C12A-B5F1-75920A1809B1%7D/viewSolicitationDocument=1/E.8%20Physical%20Sciences%20Informatics_Amend36.pdf ). The computational and experimental study will expand upon this original investigation to determine the additional effect of curvature on the cool flames in terms of their transitions and structure, including cellular formation. In this study, cool flames in the tubular flame geometry will be investigated both computationally and experimentally. Gaseous fuels (dimethyl ether) and liquid fuels (dibutyl ether) will be investigated in the tubular burner to parallel earlier studies in flat, opposed-jet cool diffusion and premixed flames. The regime diagrams, flame transitions, flame structure, multi-stage formation, and extinction conditions will be determined computationally using the Vanderbilt Tubular Flame Code as a function of pressure. The detailed numerical simulation (DNS) code includes detailed molecular transport, complex chemical kinetics, and radiation heat loss. The cool tubular flame structure will be measured with advanced laser diagnostics. Raman scattering will be used to measure the flame temperature and major species concentrations. Planar laser-induced fluorescence (PLIF) of CH2O and chemiluminescence will be used as a marker of the cool diffusion flame. The tubular flame experimental and computational results will be compared to the previous cool opposed-jet results to determine the effect of curvature on the cool flame transitions, structure, multi-stage flame formation, and extinction.

Cool premixed and diffusion flames are found in diesel engines and other modern internal combustion engine concepts such as homogeneous charge compression ignition (HCCI), reactivity controlled compression ignition (RCCI), and partially premixed compression ignition (PCCI). Understanding the effects of curvature and stretch rate on cool flame transitions and structure will lead to better insight into cool flame propagation in practical internal combustion engines.

The in-house tubular flame code was modified to accept the larger chemical kinetic mechanisms associated with more complex fuels such as dimethyl ether (DME) and dibutyl ether (DBE). This code can simulate both negative curvature (concave to the fuel) and positive curvature (convex to the fuel) effects to further determine the important role curvature may play in non-unity Lewis number fuels. These simulations are underway for N2-DME vs. N2-O2 diffusion flames with multiple chemical kinetic mechanisms, and show promising results for cool, warm, and hot flame structure when compared to previous opposed-jet results from other research groups under similar conditions. The simulations cover a range of curvature values and pressures up to eight atmospheres. Continued computational work involves building a more complete dataset for cool flame regimes and transitions into warm or hot flames along with temperature and species profiles at the various cool flame conditions.

Initial equipment to experimentally produce tubular cool flames at atmospheric conditions has been acquired. This equipment includes in-line gas heaters and nozzles to deliver a uniform and consistent flow at the desired temperature to produce cool flames. The computational results will provide essential information in experimentally reproducing these cool flames.

Flames in nature are not flat. Studies in opposed-jet flat cool flames lack the presence of curvature found in practical internal combustion engines. The curvature of tubular flames is expected to change where the cool flames transition to warm and hot flames, as well as stretch rate extinction values. Curvature and stretch rate are also known to produce cellular structure in flames. In this study, cool premixed and diffusion flames in tubular flames will be investigated both computationally and experimentally to determine the effect of curvature on cool flame regimes, transition to warm and/or hot flames, flame structure, multi-stage flame formation, and extinction stretch rates.

The project will use data from PSI system listed as Investigation #12 in Table A of the Program Element entitled, “Quantitative Studies of Cool Flame Transitions at Radiation/Stretch Extinction using Counterflow Flames” ( https://nspires.nasaprs.com/external/viewrepositorydocument/cmdocumentid=807808/solicitationId=%7B3B58C0CA-6FF7-C12A-B5F1-75920A1809B1%7D/viewSolicitationDocument=1/E.8%20Physical%20Sciences%20Informatics_Amend36.pdf ). The computational and experimental study will expand upon this original investigation to determine the additional effect of curvature on the cool flames in terms of their transitions and structure, including cellular formation. In this study, cool flames in the tubular flame geometry will be investigated both computationally and experimentally. Gaseous fuels (dimethyl ether) and liquid fuels (dibutyl ether) will be investigated in the tubular burner to parallel earlier studies in flat, opposed-jet cool diffusion and premixed flames. The regime diagrams, flame transitions, flame structure, multi-stage formation, and extinction conditions will be determined computationally using the Vanderbilt Tubular Flame Code as a function of pressure. The detailed numerical simulation (DNS) code includes detailed molecular transport, complex chemical kinetics, and radiation heat loss. The cool tubular flame structure will be measured with advanced laser diagnostics. Raman scattering will be used to measure the flame temperature and major species concentrations. Planar laser-induced fluorescence (PLIF) of CH2O and chemiluminescence will be used as a marker of the cool diffusion flame. The tubular flame experimental and computational results will be compared to the previous cool opposed-jet results to determine the effect of curvature on the cool flame transitions, structure, multi-stage flame formation, and extinction.

Cool premixed and diffusion flames are found in diesel engines and other modern internal combustion engine concepts such as homogeneous charge compression ignition (HCCI), reactivity controlled compression ignition (RCCI), and partially premixed compression ignition (PCCI). Understanding the effects of curvature and stretch rate on cool flame transitions and structure will lead to better insight into cool flame propagation in practical internal combustion engines.