| The flow structure of compressible reacting mixing layers is dominated by instability modes. In addition to the more common Kelvin-Helmholtz ‘central’ mode that exists unaccompanied in incompressible nonreacting conditions, compressible reacting mixing layers can develop two supplemental instabilities. These modes are termed ‘outer’ instabilities because of their association with the fast and slow free streams. Unfavorable changes in the mixing characteristics of the large-scale flow structure occur when these outer mode instabilities become dominant over the central mode. Thus, the presence of these modes has important consequences for applications in supersonic combustion.; The objective of this research is to study the flow structure in different regimes of the compressible reacting mixing layer. Focus is placed on developing a better understanding for the combined effects of compressibility, heat release and the flowstream ratios of density, equivalence, and velocity on the stability characteristics of each mode and on the predicted structure of a turbulent reacting mixing layer. First, the density-weighted vorticity profile is used to explain the role of heat release and compressibility to promote the outer modes, and to anticipate the effect of each parameter on the flow instabilities. Linear stability theory is then used to provide more quantitative insight. As part of a complete parametric study, the dominating effects of compressibility, heat release, and the density ratio are identified and regime charts are constructed to demonstrate their combined effect on flow structure. Colayer conditions, where two modes have equal growth rate and the mixing layer is formed by two sets of vortices, are also identified.; After establishing the trends above, the parabolized stability equations (PSE) are used to investigate issues of nonlinear flow development and mixing. Analysis of scalar probability density functions in flows with dominant outer modes demonstrates the ineffective, one-sided nature of mixing that accompanies these flow structures. Colayer conditions offer some opportunity for mixing enhancement, but their extent in the parameter space is found to be limited. The PSE technique is also shown to accurately predict vortex pairing, and this capability is used to explain the delay in pairing that is an observed effect of compressibility. |
