Simulating the blowoff transient of a swirling, bluff body-stabilized kerosene spray flame using detailed chemistry

The lean blowoff transient of a Jet-A spray flame in a lab-scale swirl burner is simulated using Large Eddy Simulation (LES) and the Conditional Moment Closure (CMC) combustion model. The objectives are: (i) verify the capability of LES-CMC to predict the blowoff of heavy hydrocarbon spray flames, (ii) to investigate local flame behavior such as local extinction and flame lift-off, and (iii) to analyze species behavior during the blowoff transient. The kerosene in this study is a conventional Jet-A reference fuel standardized under the USA National Jet Fuels Combustion Program. A detailed hybrid chemistry (HyChem) mechanism based on the lumped pyrolysis assumption for high temperatures was used. The flame shape changes as it experiences the transient, shrinking down and then retreating from the edges of the bluff body until mixture fraction is present only in regions along the spray cone. During the simulation of the blowoff transient, local extinctions are identified both visually along the flame stoichiometric isosurface and quantitatively in mixture fraction space by regions of low OH and temperature and high fuel and formaldehyde (CH2 O) mass fractions. Pyrolysis species benzene (C6 H6 ) and ethylene (C2 H4 ) are analyzed and shown to reduce during blowoff, due to reduced temperatures preventing vaporized kerosene from undergoing pyrolysis. Without sufficient pyrolysis products available, the flame experiences fuel starvation. The combination of fuel starvation and increased number of local extinctions causes the global lean blowoff event. CH2 O notably builds up around the edges of the combustion chamber toward the end of blowoff, as less OH mass fraction is present to consume the formaldehyde. LES-CMC simulates blowoff to occur at a bulk air velocity within 5% of the experimental value, affirming that the LES-CMC approach coupled with detailed chemistry is able to accurately predict the blow off of heavy hydrocarbon flames