Low-frequency oscillations of combustion products in the chamber of a low-thrust liquid rocket engine manufactured using additive technologies

Abstract

Oscillations in engine thrust are due to nonstationary dynamic processes in the combustion chamber. The presence of vortexes in the combustion product flow in the combustion chamber can lead to liquid rocket engine (LRE) dynamic instability with a relatively high level of pressure oscillation. In a number of studies, this type of instability is associated with vortex formation (Surface Vortex Shedding (SVS) - process) during the spatial movement of LRE propellant gaseous products in the combustion chamber, and its development is often considered as a consequence: surface vortex formation with different chamber walls roughness, vortex formation after an obstacle in the gaseous product movement, and angular vortex formation in the combustion chamber structure elements. The study of the above-described SVS dynamic processes during the flow of gaseous products in the combustion chamber of a low-thrust LRE was carried out, and the influence of surface vorticity formation on the hydrodynamic processes in the chamber was evaluated for different chamber walls roughness. Mathematical modeling of the unsteady gas flow in the engine chamber was performed, which was considered a two-dimensional axisymmetric in the solved problem. Numerical analysis was performed using the ANSYS Fluent finite element analysis software (LES - Large Eddy Simulation Model). Based on the results of the numerical study of the LRE combustion chamber dynamics, it was determined that the movement of the combustion gaseous products along the walls of the cylindrical chamber occurs with the formation of vortices, which are destroyed in the process of moving the gas flow to the chamber throat section. Vortex formation largely depends on the chamber cylindrical walls roughness: with the roughness of the chamber cylindrical walls, the circulation zone of high-amplitude gas flow is formed at the chamber inlet, which is stretched along the chamber axial length and oscillates with a frequency of 1 Hz - 2 Hz, As follows from the analysis of the calculation results, the amplitudes of the pressure self-oscillating process (that develops in the LRE chamber during oscillations in the chamber) can reach 3 bar (that is value of 40 % of the nominal chamber pressure value), which indicates SVS instability of the combustion chamber. The results of the numerical analysis were confirmed by the fire test data of the engine combustion chamber for cases in which the chamber inner surface was polished and without chamber polishing.