Acoustic characteristics of pulse combustor mixing chambers

Abstract

This paper compares the contrasting behaviors of the apparently similar Helmholtz and the Schmidt type pulse combustors. A “modified impedance tube technique” which takes into account the non-uniform temperature in the impedance tube was used for determining the acoustic characteristics of the components which make up these combustors. These impedance tube studies have shown that there exists an optimum range of air flapper valve settings over which the acoustic driving by the combustion process is the strongest. When a Helmholtz combustor was operated under conditions of strongest driving, the sound pressure level in the combustor was maximized. However, a Schmidt tube produced the smallest pressure amplitudes under similar conditions. This phenomenon can be explained in terms of the acoustic properties of the components which make up these combustors. Cold flow studies indicated that damping by the air flapper valve and, therefore, the mixing chamber assembly decreased with increasing pressure amplitude. However, once the combustion chamber was added its acoustic properties dominated and the damping increased with amplitude. In the limit cycle, the pressure amplitudes in these combustors adjust until the damping in the entire system exactly balances the driving by the combustion process. Hence, strong driving requires strong damping which occurs at high amplitudes in the Helmholtz combustor and at low amplitudes in the Schmidt combustor.