Stirred Reactor Analysis of Cavity Flame Holders for Scramjets

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 limit residence time. This model also showed that under certain conditions, the solution for combustion systems was not unique. It was found that ignition delay times and lower limit 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. The methodology developed in this research provides a design guide for the size of cavity required to provide flame holding for a scramjet combustor.