Cyclic modulation characteristics of pulse combustors

Abstract

Pulse combustors have been recognized for some time as highly efficient combustors inducing high rates of heat transfer while producing relatively low levels of pollutants. However, the highly transient nature of the interaction between the fluid dynamic and combustion processes within the combustor means that adequately modeling such a system is a difficult proposition. In this paper, experimental results are presented for a 250-kW aerovalved pulse combustor that identify further complications to the problem. The phase difference between pressure and heat release oscillations is observed to modulate from cycle to cycle, and associated frequency variations are also identified. The characteristics of the inlet flow of reactants are also shown to exhibit periodic behavior, and it is postulated that this is responsible for the phase-shift modulations observed within the combustor. Several variations of a phenomenological spatially averaged model are introduced in an attempt to induce the cyclical modulations and hence substantiate the hypothesis. First, characteristics of a basic model with steady inlet flow are established and compared against mean values obtained from the experiments. A range of inlet flows for which continuous oscillations are induced is found, outside of which either flame extinction or steady combustion regimes prevail. Within the oscillatory regime, characteristics of pressure and temperature compare favorably with mean values measured. However, no phase modulation is predicted. Introduction of a sinusoidally varying flow of reactants induces modulations of the type observed in the experiments. Finally, coupling the frequency of the periodic flow of reactants with pressure oscillations within the combustor results in more chaotic oscillations, as observed in the experiments. There is no obvious similarity between pressure and heat release signatures for this model, but consistent with Rayleigh's Criterion, excitation of the pressure oscillations occurs when the phase difference between pressure and heat release is at a minimum.