Spectral analysis of turbulent ammonia/hydrogen/nitrogen-air premixed flames at ambient and elevated pressure

Multi-scale spectral analysis of reactive scalar (co)-variance and turbulent kinetic energy is presented as a function of pressure for recent direct numerical simulation (DNS) of ammonia/hydrogen/nitrogen-air premixed flames in preheated strong shear layer turbulence. The objective is to understand the length scale-dependent behavior of different reactions and transport mechanisms for diffusive-thermally unstable flames. In particular, we compare two cases at 1 atm and 10 atm. Overall, the contribution of small-scale modes (SSMs) with wavelength smaller than 10δ_L is stronger in all state variables for the 10 atm case. The spectral densities of the mass fraction variations, E_Y², of products and reactants show peaks at large-scale modes (LSMs) greater than 10δ_L corresponding to the peaks of the turbulent kinetic energy spectra. The spectral behavior of E_Y² of intermediate species is found to vary with the molar weights. E_Y² of heavy molecules including H₂O₂, NO, NO₂, for example, shows peaks at LSM at 1 atm but at SSM at 10 atm. Also, E_Y²s for some species, such as HO₂, exhibit multiple peaks at different length scales and locations. The potential mechanisms causing species-specific spectral behaviors, e.g. mixing, mass-diffusion, and reaction rates will be discussed.