Detailed Measurement of Oxidizer-Rich Staged Combustion Injector Dynamics in Model Rocket Combustors

Abstract

This paper presents experimental results from a model rocket combustor that uses a gas-centered swirl-coaxial injector element injecting warm oxygen and ambient temperature RP-2 fuel with 900 psia chamber pressure, representative of an oxidizer-rich staged combustion cycle. Results from the two injector geometries are analyzed and compared. One was a flat-faced injector with the element ending at a sudden expansion into the chamber. The other used a truncated cone at the injector exit: a design feature of specific interest that appears in drawings for the RD-170 main-chamber injector. To improve spatial resolution of high-frequency pressure measurements, the model injector was scaled up from RD-170 dimensions to facilitate measurements in the injector and head end of the combustor. Measured pressure spectra of the flat-face combustor showed a well-organized first axial mode and harmonics. The truncated-cone geometry showed a more diffuse spectra with higher-frequency modes, with a reduction in amplitude further upstream in the cone. Intermittency in measured dynamics is characterized via multiscale analysis. Adopting concepts from fractal theory, the stability margin of the injector configurations was quantified using the Hurst exponent of pressure oscillations. Comparatively, a larger range of fractal dimensions with smaller fluctuations in the initial reacting zone were observed for the flat-face injector.