Destabilization of the shear layer in the post chamber of a hybrid rocket

Abstract

Recent studies have determined that the combustion pressure in a hybrid rocket engine has another peak frequency of approximately 500 Hz aside from the dominant peak frequency of around 20 Hz; these two peak frequencies exhibit in-phase coupling upon low-frequency instability occurrence. In this study, we suspect that the unburned fuel inflow to the post chamber causes the flow characteristics to activate the peak frequency of about 500 Hz in the post chamber. Numerical calculation was performed with large eddy simulation method to investigate the effect of the influx of unburned fuel flow on the evolution of flow dynamics. Two different boundary conditions, namely, without blowing and with blowing, were employed for the inlet wall surface, which simulates evaporative fuel flow. Results show that the effect of wall blowing from the inlet wall surface does not significantly affect the time-averaged flow field but further increases turbulent kinetic energy to the downstream part. However, wall blowing produces unique flow dynamics, showing a peak frequency that approximates the Strouhal number, that is, St = 0.25-0.3. Proper orthogonal decomposition analysis reveals that the appearance of the peak frequency of St = O(0.3) is closely associated with the destabilization of small-scale motions in the shear layer caused by the influx of unburned fuel flow. The instantaneous axial profiles of velocity and passive scalar are qualitatively similar. Therefore, passive scalar distribution is closely related to flow characteristics, such that mixing is influenced by flow characteristics change.