Impact of shear-coaxial injector hydrodynamics on high-frequency combustion instabilities in a representative cryogenic rocket engine

Abstract

The excitation mechanism of a thermoacoustic instability in a 42-element research rocket thrust chamber with representative operating conditions with respect to European cryogenic rocket engines is investigated in detail. From previous research it was known that the chamber 1T mode can be excited by persistent heat release rate oscillations which are modulated by the resonant modes of the liquid oxygen injectors. The excitation source of the longitudinal injector eigenmodes is investigated in this study. Fibre-optical probes measuring the OH* dynamics from the recess volume of two injectors showed additional frequency content which could neither be explained by the chamber acoustics, nor the acoustics of the injection system. Instead, the temporal evolution of these frequencies correlate with the oxidizer flow velocity. In this work we show that the additional flame modulation originates from a hydrodynamic effect in the injection system. Even though the exact process cannot be precisely identified, an effect designated orifice whistling at the injector inlet orifice seems to be a likely candidate. Combining the new results with previous publications about this combustor, it is now possible to explain past and present observations in terms of the hydrodynamic and thermoacoustic conditions which are necessary for the combustion instability to appear. The conditions, which lead to an injection-driven excitation of the 1T mode are matching frequencies of the 2L mode of the injectors and the chamber 1T mode as well as a Strouhal number between 0.2 and 0.4 based on the length and flow velocity of the injector inlet orifice.

Orifice Flow Dynamics in a Rocket Injector as an Excitation Source of Injector-Driven Combustion Instabilities

“…In previous studies, the flow dynamics inside injector elements with orifices have been reported as the excitation source of the combustion instability under certain conditions. High-frequency (HF) instabilities of liquid oxygen/hydrogen (LOX/H 2 ) combustion were observed when the resonance mode of the LOX-post matched the first transverse acoustic mode (1T) in the combustion chamber of the research thrust chamber model 'D' (or "BKD") [2][3][4] as shown in Figure 1. Armbruster et al [3] concluded that the orifice whistling phenomenon at a Strouhal number of approximately 0.35 caused the excitation of the acoustic eigenmodes in the LOX injectors, which modulated the heat release rate, leading to an excitation of the 1T mode of the combustion chamber.…”

Selection Rules for Resonant Longitudinal Injector-Coupling in Experimental Rocket Combustors

“…Experimental studies in the past indicated a strong effect of the hydrogen temperature on the stability margins [4] of a rocket combustor. A recent study [5] assessed the role of hydrodynamic effects, e.g., vortex shedding, behind shear coaxial injectors that may also lead to a modulation of the heat release rate in the combustion chamber. Another possible instability mechanism that has been observed in rocket combustion chambers [6], as well as air-breathing combustion systems (cf.…”