Influence of Unsteadiness on the Analysis of Pressure Gain Combustion Devices

Abstract

Pressure gain combustion has been the subject of scientific study for over a century due to its promise of improved thermodynamic efficiency. In many recent application concepts, pressure gain combustion is used as a component in an otherwise continuous, normally steady flow system, such as a gas turbine or ramjet engine. However, pressure gain combustion is inherently unsteady. Failure to account for the effects of this periodic unsteadiness can lead to misunderstanding and errors in some performance calculations. This paper clarifies the accounting by presenting a consistent method of thermodynamic cycle analysis for a device using pressure gain combustion technology. The incorporation of the unsteady pressure gain combustion process into the conservation equations for a continuous flow device is presented. Most important, the appropriate method for computing the conservation of momentum is presented. It will be shown that proper, consistent analysis of cyclic conservation principles produces more realistic performance predictions. Specifically, for the ramjet example used, appropriate predictions of both the thermal efficiency (from energy conservation) and specific thrust (from momentum conservation) are developed.