Combustion dynamics in a high aspect ratio engine

A large eddy simulation (LES) based design tool for modeling rectangular cross-section, high aspect ratio (AR) combustors has been developed and tested against experimental results. The combustor is part of a miniature power generator in which a free piston fitted with permanent magnets is oscillated by the out-of-phase heat release from two opposed high AR combustors. This device is placed inside a permanent magnetic coil, where the piston motion generates electric power. The combustor geometry is governed by the desire to mass-produce the device using microelectromechanical systems (MEMS) techniques. Since ignition, flame propagation, heat losses, and quenching have been shown to be seriously affected by this combustor's geometry, standard design tools developed for low AR engines can not be used for these devices. Instead, LES methodology has been developed to address this. A wall heat transfer model is used for effectively handling the intense heat transfer in the system. The piston motion is treated by a Cartesian grid scheme. A miniaturized, high AR power generator in which the piston is moved by a single combustor and a spring was designed and built. The LES model provided significant guidance for the location of the inlet and exhaust ports during the design process. Comparison of the experimental results and predictions shows that flame behavior is well captured by the model, as are wall and exhaust temperatures. However, because mass losses between piston and cylinder were neglected and because the use of global kinetics did not permit the proper treatment of wall quenching, the model tends to overpredict both pressure pulse frequency and amplitude, as well as work generated by the device. A more sophisticated treatment of combustion chemistry is expected to improve agreement between experimental results and the model.