Pressure-gain combustion for gas turbines based on shock-flame interaction

Abstract

One method to significantly improve the performance of gas turbine engines is to use the thermodynamically more efficient unsteady, pressure-rise combustion. In this work it is proposed to exploit the interaction of shock waves with a pre-mixed flame to achieve a time-averaged, combustion-induced pressure rise. The physical phenomena occurring in the course of shock-flame interaction are very complex and yet not understood in detail. In order to shed additional light onto the underlying mechanism and to gain understanding of the changes in gas state achievable due to a single interaction event, passage of shock waves through a pre-mixed flame was studied both experimentally and analytically. Pre-mixed combustion of a nearly-stoichiometric methane-oxygen-argon mixture was used in the experiments performed on a shock tube test rig. It was shown that both the heat release rate of the flame and the pressure are temporally amplified due to passage of a shock wave through the flame. Both the increase in pressure and the heat release of the flame were demonstrated to grow parabolically with the Mach number of the incident shock. Considerably higher increases in pressure and heat release were observed when the shock approached the flame from the burned gas side (called fast-slow mode of interaction) for the same incident shock strength. Further, the existence of regions with positive coupling between unsteady pressure and heat release oscillations was demonstrated after each transition of a shock wave through the flame front. Subsequently, an analytical quasi-one-dimensional model of the interaction between a shock wave and a sinusoidal flame was developed. Given known initial flow field and flame geometries as well as the incident shock Mach number, the model allows the calculation of a fully defined one-dimensional flow field that is formed at the end of a single shock-flame interaction event. The analytical model was successfully verified using experimental data. It was found that a single shock-flame interaction event generates a dramatic increase in pressure compared to isobaric combustion with the same unburned gas conditions. In contrast, the according increase in temperature remains at a relatively moderate level. Further, the combustion entropy is significantly reduced through a single shock-flame interaction event compared to the reference isobaric combustion process. The resulting changes in pressure, temperature and entropy rise with increasing incident shock strength and growing curvature of the flame front. They are significantly stronger in the fast-slow mode of interaction. This is a consequence of higher rates of gas compression and flame surface growth in this interaction type. Finally, a theoretical configuration of a shock-combustor enhanced high-pressure engine core was proposed and applied to two types of baseline engines: a twin-spool industrial gas turbine and a twin-spool high-bypass turbofan engine. The performance of the topped engines was evaluated using two variables: the combustor pressure…