FLOW INDUCED CAVITY RESONANCE FOR TURBULENT COMPRESSIBLE MIXING ENHANCEMENT IN SCRAMJETS
FLOW INDUCED CAVITY RESONANCE FOR TURBULENT COMPRESSIBLE MIXING ENHANCEMENT IN SCRAMJETS
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Date
2002
Authors
Nenmeni, Vijay Anand Raghavendran
Advisor
Yu, Kenneth H.
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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.