FLOW INDUCED CAVITY RESONANCE FOR TURBULENT COMPRESSIBLE MIXING ENHANCEMENT IN SCRAMJETS

Abstract

In a Scramjet combustor, flow residence time is very short and fuel-air mixing can be adversely affected by compressibility effect. Thus, it is important to study mixing enhancement techniques for reducing the characteristic mixing time. It is also important to examine the feasibility of using them in practical settings. One of the promising mixing enhancement techniques is based on flow-induced cavity resonance, which generates large-scale coherent structures in the shear layer for faster mixing. Of particular interest is whether this technique, which is passive in nature, can be used over a wide range of flow conditions, expected in Scramjet operation. In this thesis, physical mechanisms governing the use of flow-induced cavity resonance were examined experimentally using Schlieren visualization of the flowfield and spectral analysis of resulting pressure oscillations. Various cavities with the length between 0.125 and 1.25 inch and the depth between 0.125 and 0.25 inch were placed inside a Mach 2 flow tunnel, which simulated the Scramjet internal flowfield. The properties of supersonic flow were further modified in the inlet, upstream of the cavity section, by changing the upstream stagnation pressure between 35 psig and 120 psig, which resulted in inlet shock trains of different strength. The objective was to characterize and compare the enhancement mechanism under various off-design conditions. In all, nine different cavity cases were tested under six different stagnation pressure settings. For each case, spark Schlieren images were taken and pressure oscillations inside the cavity were measured. The Schlieren images provided qualitative understanding of the physics while the pressure measurements were used to quantify the amplitude and frequency of dominant oscillations. Also from the images, inlet Mach number was deduced by measuring the Mach wave angles. The data were summarized to shed more light on reliability of the mixing enhancement mechanism under off-design inlet conditions. The results indicated that flow-induced cavity resonance mechanism was robust over a wide range of flow conditions. Also, mode-switching behavior of the cavities was observed, which could modify the mixing enhancement rate. Further, helium injection studies were conducted to gain qualitative assessment of the effect of cavity resonance on mixing.