Experimental Study of Instabilities in Hydrogen-Air Fueled Rotating Detonation Combustion

Abstract Conventional gas turbine engines rely on an idealized constant pressure combustion process that in reality produces a pressure decrease as a result of viscous and other non-reversible losses. An alternative approach is rotating detonation combustion (RDC) which is a form of pressure gain combustion in which one or more detonation waves propagate an annular channel resulting in an increase in pressure across, subsequently providing greater work availability compared to deflagration ultimately leading to opportunities for greater thermodynamic efficiency when used in gas turbine engines that conventionally relies on constant. Modern gas turbine engines often rely on pre-mixed reactants to limit NOx emissions, although this may result in greater susceptibility to instabilities such as flashback and thermoacoustic oscillation, particularly for applications that utilize hydrogen as the fuel. Research in RDC has focused on non-premixed reactants thus limiting the occurrence of flashback, and high frequency detonation wave propagation (kHz) may interfere with the occurrence of thermoacoustic oscillations. Thermal NOx emissions are controlled through rapid combustion and sudden expansion of the working fluid. Although RDC may not be susceptible to instabilities encountered in conventional state of the art gas turbine engine combustion, there may be other mechanisms occurring that support instabilities that could be detrimental to performance. The design of systems utilizing RDC often incorporates a choked inlet and exit boundary conditions, but since the RDC is an inherently unsteady process it is unlikely these boundaries remain choked over the course of a complete cycle (complete rotation of a detonation wave). The presence of multiple waves could further exacerbate changes in choked conditions at inlet and exit boundaries. As a result of restrictive and changing conditions at the boundaries, reflected shock waves may occur traveling upstream from the exit or downstream from the inlet that interfere with the reactant inlet flow, mixing, and subsequent formation of a detonation wave (for example, variable wave height). The objective of this study is to utilize experimental data from multiple dynamic pressure transducers located along the length of an RDC combustor, supplemented with ion probe (chemical ionization) and OH* chemiluminescence acquired within the detonation region, to identify occurrences of instabilities during 20–30 second tests. The length of the test provides an adequate time for the combustor to reach a thermally stable condition. The axial distribution of dynamic pressure transducers provides a means of tracking reflected shock waves within the combustion channel. Experimental data are compared to a limited number of computational results that include both stable and unstable conditions with the detonation channel.