Large eddy simulations of multiple transcritical coaxial flames submitted to a high-frequency transverse acoustic modulation

Abstract

This article describes a numerical investigation that explores the dynamics of multiple cryogenic jet flames interacting with high frequency transverse acoustic modes. This research is motivated by the numerous issues associated with high-frequency instabilities in liquid rocket engines. Large eddy simulations are carried out in a complex flow configuration which is turbulent, reactive, transcritical and in the absence or presence of a large-amplitude acoustic motion. The geometry is that of a model scale experimental configuration. Results obtained by exploiting high end computational resources demonstrate the feasibility of such calculations, provide insight in the coupling process, exhibit features which are found in experiments and complement the experimental data. Depending on the acoustic environment (dominated by velocity or pressure oscillations), selective responses of the cryogenic jets and flames can be observed and analyzed. It is found that the flames are more compact when they are made to interact with the transverse acoustic motion and that the dense core length is notably reduced. In the span-wise direction, the flames and dense cores are flattened, a feature which is also found in experiments. The unsteady motion observed experimentally is well retrieved numerically. The simulations highlight the mechanisms that can feed energy in the transverse mode and suggest possible descriptions of the instability driving process. The light methane jet is shaken by the acoustic motion and impacts the dense oxygen stream alternatively on its top and bottom sides. The unsteady heat release rate and the corresponding acoustic Rayleigh term produced by the flames prove to be different according to the flame position regarding the acoustic environment.