Numerical analysis of two and three dimensional recessed flame holders for scramjet applications

Scramjet combustor technology is currently under development by the Air Force and a key component of scramjet combustors is the flame holder. This study investigated the flame holding properties of recessed cavities in supersonic flow using numerical analysis techniques. The numerical models developed for this analysis included several perfectly stirred reactor models. A simplified analytical model indicated that an important property for flame holding was the lower residence time. This model also showed that under certain conditions, the solution for combustion systems was not unique. It was found, that the ignition delay time and lower residence times varied by orders of magnitude with reaction mechanism. The perfectly stirred reactor model also indicated that trace species diffusion should increase flame spreading rate, and that heat loss reduces flame holding limits. Reduced mechanisms for hydrocarbons were also shown to have orders of magnitude variation in lower residence times. After nonreacting calibration, two-dimensional simulations confirmed the perfectly stirred reactor results for blowout limits. Also, the effect of trace species diffusion on flame spreading was shown to be negligible, and the reduced flammability with heat loss was confirmed. Lowering the temperature of the inflow boundary layer was shown to reduce the flammability limits. Cavity sweep and cavity sweep with variable aspect ratio were shown to generate axial vorticity. The variable aspect ratio cavity was demonstrated to slightly enhance flame spreading. The methodology developed in this research provides a design guide for the size of cavity required to provide flame holding for a scramjet combustor. Also, reduction of heat losses was shown to be a method to improve flame holding performance without increasing the cavity size.