Quantitative Measurements in the Exhaust Flow of A Rotating Detonation Combustor Using Rainbow Schlieren Deflectometry

Abstract

This study employs rainbow schlieren deflectometry (RSD) to characterize the unsteady, supersonic/subsonic exhaust plume of a rotating detonation combustor (RDC). First, RSD images are analyzed to quantify the frequency and strength of flow oscillations and their relationship to the detonation wave. Secondly, a three-dimensional (3D) tomographic algorithm is used to obtain the local 3D density field across the whole region of interest (ROI). The tomographic analysis relies upon wave rotation to infer projection data of the 3D exhaust plume at multiple view angles using a single RSD camera system and was previously validated using phantom data from computational fluid dynamics analysis of an RDC. The annular RDC operated on methane and 2/3 O2–1/3 N2 oxidizer mixture is equipped with a converging nozzle to pressurize the combustion chamber. The product flow exiting the nozzle throat expands across an unoptimized conical aerospike attached to the center body of the RDC. RSD images provide a temporal resolution of 369 ns and spatial resolution of 100 μm in a 6.4 mm high and 25.6 mm wide ROI of the exhaust plume. A rainbow filter is calibrated to convert hue in color schlieren images into deflection angle data. These data are used to characterize the unsteady flow oscillations that show excellent agreement with PCB pressure probe measurements acquired inside the combustion chamber. Tomographic analysis yields a 3D local density field that shows distinct features, consistent with published numerical simulations of the RDC exhaust plume. For the first time, this work demonstrates the ability of high-speed nonintrusive RSD diagnostics to acquire whole-field density measurements in an operational RDC. Such data would be valuable to validate high-fidelity numerical simulations and gain a further understanding of the exhaust flow to help with RDC-turbine integration. Further improvements to the RSD hardware and analysis procedures would enhance present capabilities to ultimately infer other thermodynamic properties such as temperature and pressure from density measurements.