Fractal characteristics of turbulent premixed hydrogen-enriched flames at different pressures

The combustion of ammonia/hydrogen/nitrogen blends and pure hydrogen in air presents an attractive carbon-free alternative to conventional natural gas firing in gas turbines. However, lean combustion of such fuels poses challenges due to the thermo-diffusively unstable nature of hydrogen-enriched fuel burning. We present results from Direct Numerical Simulations (DNS) of premixed ammonia/hydrogen/nitrogen-air flames in temporally-evolving shear layer configurations at atmospheric and elevated pressure under lean conditions and in a planar jet configuration at different equivalence ratios and atmospheric pressure. In addition, we present results of various lean hydrogen-air flames subjected to isotropic turbulence. These DNS show a fundamentally different turbulent combustion behavior at different pressures, with cases at elevated pressure exhibiting stronger effects of preferential diffusion and enhanced thermo-diffusively unstable behavior. The analysis will focus on the influence of pressure, preferential/differential diffusion and intrinsic instabilities on the fractal dimension of those flames, which is an important parameter in turbulent premixed flame modeling. The analysis clearly shows increased fractal dimensions as pressure increases and instabilities are amplified.