POD-based effect of flow variations on the combustion modes of hypergolic pre-combustor

Combustion instability in liquid rocket engines is one of the major challenges faced in the development process, characterized by strong, destructiveness, randomness, and multiple coupling factors, which can reduce engine performance, damage the engine structure and lead to engine failure. The propellant combustion in the engine will generate pressure oscillations, and the pressure oscillations can cause propellant flow oscillations in the upstream supply system, further exacerbating combustion instability. In order to investigate the effect of hypergolic propellant oscillation frequency on combustion instability, using the UDMH/NTO pre-combustor as a research object, a 3-D combustion simulation model of a liquid-liquid swirl coaxial pre-combustor was established, and the pressure distribution under different propellant flow frequencies were obtained, which is processed and analyzed by the proper orthogonal decomposition (POD) method. The POD results show that: when the propellant flow rate oscillates at a frequency of 500Hz, the peak frequency of the first-order mode of the pressure field of the pre-combustor is 8950Hz, and the second-order longitudinally unstable combustion mainly occurs. When the propellant oscillation frequency is 1000Hz and 5000Hz, the peak frequencies of the first order mode of the pressure field in the pre-combustor are 16030Hz and 435Hz, respectively. The peak frequency of the 5000Hz oscillating flow field is much lower than 1000Hz, which may due to the transition stage from longitudinal mode to radial mode. When the propellant flow rate oscillation frequency is 10000Hz, the peak frequency of the first order mode of the pressure field in the pre-combustor is 6670Hz, and the main radial combustion instability occurs. The results of this paper provide a basis for the design of the engine, which is conducive to reducing the occurrence of the combustion instability.