Propellant Injection Strategy for Suppressing Acoustic Combustion Instability

Abstract

Shear-coaxial injector elements are often used in liquid-propellant-rocket thrust chambers, where combustion instabilities remain a significant problem. A conventional solution to the combustion instability problem relies on passive control techniques that use empirically-tested hardware such as acoustic baffles and tuned cavities. In addition to adding weight and decreasing engine performance, these devices are designd using trial-and-error empirical science, which does not provide the capability to predict the overall system stability characteristics in advance. In this thesis, two novel control strategies that are based on propellant fluid dynamics were investigated for mitigating acoustic instability involving shear-coaxial injector elements.

The new control strategies would use a set of controlled injectors allowing local adjustment of propellant flow patterns for each operating condition, of which the instability could become a problem. One strategy relies on reducing the oxidizer-fuel density gradient by blending heavier methane to the main fuel hydrogen. Another strategy utilizes modifying the equivalence ratio to affect the acoustic impedance through the mixing and reaction rate changes. To provide the scientific basis, unit-physics experiments were conducted to explore the potential effectiveness of these strategies. Two different model combustors, simulating a single-element injector test and a double-element injector test, were designed and tested for flame-acoustic interaction. For these experiments, the Reynolds number of the central oxygen jet was kept between 4700 and 5500 making the injector flames sufficiently turbulent. A compression driver, mounted on one side of the combustor wall, provided controlled acoustic excitation to the injector flames, simulating the initial phase of flame-acoustic interaction. Acoustic excitation was applied either as a band-limited white noise forcing between 100 Hz and 5000 Hz or as a single-frequency, fixed-amplitude forcing at 1150 Hz which represented a frequency least amplified by any resonance. Effects of each control strategy on flame-acoustic interaction were assessed in terms of modifying the acoustic resonance characteristics subject to white-noise excitation and changes in flame brush thickness under single-frequency excitation.

In the methane blending experiments, the methane mole fraction was varied between 0% and 63%. Under white noise excitation, up to 16% shift in a resonant frequency was observed but the acoustic pressure spectrum remained qualitatively similar. For the fixed frequency forcing, the spatial extent of flame-acoustic interaction was substantially reduced. In the other experiments, the equivalence ratio of the control injector was varied between 0 and infinity, causing up to 40% shift in a resonant frequency as well as changes in the acoustic pressure spectrum. These results open up the possibility of employing flow-based control to prevent combustion instabilities in liquid-fueled rockets.