Compressibility and heat release effects in high-speed reactive mixing layers I.: Growth rates and turbulence characteristics

A new set of three-dimensional direct numerical simulations (DNS) of spatially-developing high-speed H2/air mixing layers is presented and analysed. It corresponds to three distinct values of the convective Mach number Mc for both inert and reactive conditions. The results obtained from this new set of DNS data confirms that the mixing layer growth rate decreases according to pressure-dilatation Π11. However, an interesting point is that, in contrast to previous analyses, the decay of pressure fluctuations may not be the sole reason for this reduced growth rate, since strain-rate fluctuations decrease in a similar amount. For reactive mixing layers, depending on the value of Mc, the thermal runaway occurs in either the mixing layer development zone (small value of Mc) or the fully developed turbulence region (larger value of Mc). The examination of the mixture fraction probability density functions shows that the influence of engulfment processes, which play an important role at small Mc, tends to disappear in the presence of heat release as well as for increasing values of Mc. The present database of spatially-developing mixing layers featuring both detailed chemistry and detailed transport (i.e., Soret, Dufour and bulk viscosity effects) thus confirms some of the conclusions that were previously drawn from results issued from temporal mixing layer DNS studies conducted with single-step chemistry.