Dynamics of a transcritical coaxial flame under a high-frequency transverse acoustic forcing: Influence of the modulation frequency on the flame response

Abstract

The present investigation is motivated by the problem of high-frequency instabilities in liquid rocket engines. The objective is to gain detailed information on the dynamical response of a transcritical coaxial jet flame submitted to transverse acoustic modulations. Large-eddy simulations (LES) are carried out to examine the effects of the modulation frequency on the flame dynamics when the flame lies in the vicinity of a transverse velocity anti-node, corresponding to a pressure node. It is found that the flame configuration and dynamics notably change with the modulation frequency and that the Strouhal number based on the dense core characteristics essentially determines the response of the system. In the case of a frequency close to the natural frequency of the oxygen jet (St=0.8), the flame motion may be assimilated to that of a flag flapping in the wind. When the flame is modulated at a higher frequency (St=3.2), it features a corrugated surface extending in the axial direction and executing a bulk motion in the transverse direction with no large scale deformation. For both frequencies, the flame is shortened, flattened in the spanwise direction and periodically displaced in the transverse direction following the acoustic field. These simulations are used in a second step to determine the unsteady rate of heat release and the Rayleigh term arising in the acoustic energy balance, giving access to the corresponding source of acoustic energy and to a possible driving mechanism of acoustic instabilities.