An acoustical model of the pulse combustion process

Abstract

Pulse combustors derive many of their advantages as thermal devices from the large acoustic oscillations that are characteristic of the pulse combustion process. There have been numerous attempts to model this process and predict the frequency and amplitude of the oscillations. Many of these modeling efforts have relied heavily upon computation while providing little in the way of explanation of the underlying physical mechanism. The present model treats the pulse combustor as an acoustical system with feedback. The dependence of the operating frequency on various system parameters is studied through stability analysis. The dynamics of the combustion event itself are characterized as a time delay. The presence of this delay in the feedback loop causes the operating frequency to differ significantly from the frequency one would calculate by treating the combustor as a Helmholtz resonator at resonance. Preliminary experimental verification of the theory is presented, as follows. Both the operating frequency and combustion time delay are measured for a pulse combustion boiler, as are temperatures in the unit to establish sound speeds. When the measured temperatures and delay are inserted into the model, it predicts the operating frequency to within 4%. [Work supported by The Gas Research Institute.]