Investigation of the Effects of Oxidizer Temperature on the Stability of a Gas-Centered Swirl Coaxial Injector
Rocket engines achieve extraordinary high energy densities within the chamber in the form of high pressure turbulent combustion. Successful design of these engines requires sustained, stable operation of a combustor exposed to extreme thermal loads. Slight deviations in operating conditions can then incur consequences ranging from reduced performance up to catastrophic failure in the face of excess heat loading. Sustained periodic oscillations, termed combustion instabilities, are often encountered during development, as fluctuations produced by combustion noise couple with heat release modes by way of modulation of the feed system, injector hydrodynamics, chemical kinetics, and mixing and atomization process. Successful development of reliable, high performance rocket engines can be achieved either through a thorough understanding of both injector and combustor dynamics to mitigate these instabilities or through the laborious design/test iteration process. This document describes a two-fold work by the author. The first objective con- siders the acquisition of high-fidelity data sets of a single gas-centered swirl coaxial injector for use in the validation of computational models. Secondly, the stability of this injector was studied at two oxidizer inlet temperatures. Combustion stability was assessed through variation of the combustor geometry. Previous research shows that varying this geometry can either drive or dampen pressure oscillations. Testing was conducted on an experimental test bed equipped with modular sections to ac- commodate changing oxidizer post and chamber lengths. A single gas-centered swirl coaxial injector was used, with operating parameters based on the RD-180 injector element, such as equivalence and momentum flux ratios. Two oxidizer inlet temperatures were chosen. The first was oxygen combusted with gaseous hydrogen at lean conditions in a preburner to produce hot oxidizer near 700 K. The second was pure oxygen delivered at room temperature. Results from the test campaign revealed the system to be classically stable across all configurations and inlet conditions tested, with pressure perturbations less than 10% of the mean chamber pressure. Discriminating behavior was observed between the two oxidizer inlet temperatures. At elevated temperatures, peak-to-peak pressure oscillations observed throughout the system were small at less than 4% of the mean chamber pressure. There was no observed dependency of the amplitude on geome- try. At ambient temperatures, the pressure oscillations ranged from 4% up to 7%. The increase in amplitudes were similar to that of the acoustic reflection coefficient between the oxidizer and chamber gas, based on their acoustic specific impedance. An increase in the acoustic transmission coefficient was also observed, going from hot to ambient oxidizer. The increase in these two values would not necessarily lead to enhanced coupling between the chamber and resonance behavior in the post, but is expected to amplify pressure oscillations. At the ambient condition, clear variation in amplitudes were generated through manipulation of the geometry. The general trend matched previous experiments but was not followed by all tested configurations. It was determined that a methodology solely based on the effective resonator wavelength was not sufficient to predict the amplitude of pressure oscillations. Instead, a better predictor of amplitude was found based on the alignment of the system with postulated vortex generation from the injector face and impingement on the chamber walls. The time between local pressure oscillations and final impingement of the resulting vortices fell between one and two cycles of the fundamental longitudinal chamber mode, increasing linearly in strength as phase lag increased.
Anderson, Purdue University.
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