On the effect of heat release in turbulence spectra of non-premixed reacting shear layers

Velocity, mixture fraction and temperature spectra obtained from five direct numerical simulations of non-reacting and reacting shear layers, using the infinitely fast chemistry are analysed. Two different global chemical reactions corresponding to methane and hydrogen combustion with air, respectively, are considered. The effect of heat release, i.e. density variation, on the inertial and dissipation turbulence subrange of the spectra is investigated. Analysis of the database supports the experimentally available measurements of spectra in turbulent reacting flows showing that heat release effects can be scaled out by utilizing Favre-averaged (density-weighted) large-scale turbulence quantities. This is supported by the simulation results for velocity and mixture fraction in our moderate-Reynolds-number flows but it appears to be less supported in the dissipation subrange of the temperature spectra. The departure from universal scaling using Favre-averaged quantities in the temperature spectrum, which is evident in the dissipation subrange, appears to be caused by the strong nonlinearity of the state relationship relating the mixture fraction to the temperature, as has been suggested previously. These effects are less pronounced at intermediate wavenumbers. Analysis suggests that the nonlinear state relationship and the spectra of mixture fraction moments can be used to reconstruct the temperature spectrum across the flow. Moreover, the governing equation for the temperature variance is analysed to identify a possible surrogate for the overall rate of dissipation of temperature fluctuations and their corresponding dissipation length scale. This scaling analysis is then used to separate planes across the shear layer where the temperature dissipation length scale is alike that of the mixture fraction from regions where smaller length scales are present, and are evidenced in the dissipation subrange using Kolmogorov scaling. In our simulations, these regions correspond to the centre of the shear layer and the mean flame location. The new estimate for the temperature dissipation length scale is able to collapse the compensated spectra profiles at all planes across the shear layer for all simulations.