Geometric Scaling of Cylindrical Rotating Detonation Rocket Engine Combustors

Abstract

A Rotating Detonation Rocket Engine (RDRE) is a propulsion device employing detonation waves moving circumferentially around an annular channel to consume axially-fed propellants. Theoretically, this device provides benefits with respect to combustion pressure loss and thermodynamic efficiency when compared to deflagration-based combustors, although dominant design characteristics are not well identified. The relationships between geometric parameters, performance, and wave dynamics in particular are not thoroughly explored. This dissertation aims to provide empirical relations between combustor geometry, feed conditions, and engine operation, as well as correlation to thermodynamic parameters calculated with chemical equilibrium codes, including detonation cell size.The experimental effort included over 400 tests across six engine configurations utilizing gaseous methane-oxygen propellant and flat-faced impinging injectors, and focused on two studies with separate variations in combustor geometry. The first concentrated on radius of curvature effects through changes in the cylindrical combustor’s outer diameter, where larger engine scales were linked to higher wave counts during operation as mass flux and equivalence ratio were varied. The second study encompassed the consequences of altering the annular gap width at the smallest diameter scale of the original study and included a coreless configuration as the upper limit. Gap width primarily affected wave stability, denoted by the presence or absence of a counter wave, and detonative operating range. Combustor axial pressures in both studies were found to be dominated by feed conditions rather than geometry. For further comparisons across both studies, values for detonation cell size and energy flux based on combustion heat release were calculated with chemical equilibrium codes. Numerous relations between these calculated variables, combustor geometry, and experimental inputs and outputs were then explored across all six engine configurations. Mass flux combined with the ratio between the inner and outer radius, the latter of which was related to other geometric ratios, provided boundary estimates for operating modes, and the ratio between the inner radius and detonation cell size presented insight for wave stability. Combustor pressure showed a close relation to energy flux, although some geometric variance was still present. Finally, lessons from both the listed and additional comparisons were applied to experiments outside the original mass flux and equivalence ratio sweeps in an attempt to predict RDRE operation based solely on combustor geometry and fill conditions.