Direct Numerical Simulation and modeling of the flame propagation characteristics in lean premixed turbulent H2/CO/air combustion

The combustion of hydrogen and carbon-monoxide mixtures, known as syngas, is critical in the context of safety for non-fossil energy generation, particularly in ammonia synthesis and nuclear power plants. Preventing flame acceleration during severe accidents involving ignited syngas mixtures is crucial. Besides experimental investigations, characterizing and simulating the (partially) premixed combustion process of syngas/air mixtures at industrial scales is essential, necessitating the development of subgrid modeling approaches. This work analyzes the effects of partially substituting hydrogen by carbon-monoxide, and the addition of carbon-monoxide to hydrogen in the fuel blend, on the unstable combustion characteristics of statistically planar, premixed turbulent hydrogen/air flames using Direct Numerical Simulations (DNS). For case variations with a constant equivalence ratio, it is shown that the partial substitution of hydrogen by carbon-monoxide in the fuel composition induces stronger thermo-diffusive effects compared to the respective pure hydrogen/air flame. However, adding carbon-monoxide while keeping the molar hydrogen fraction constant reduces thermo-diffusive effects. This aligns well with the flame visualization by means of the instantaneous atomic hydrogen distribution, which indicates that the substitution by carbon-monoxide results in more pronounced atomic hydrogen accumulations, suggesting stronger preferential diffusion effects. The variation in normalized turbulence intensity reveals a strong interaction between the turbulent flow field and the flame, further influencing hydrodynamic and thermo-diffusive effects, with higher Karlovitz numbers leading to elevated unstable flame behavior. A modeling approach is proposed that determines the increase in normalized combustion rate per unit flame area using a power law framework, which is initially inspired by fractal theory and incorporates dispersion relations for theoretical predictions of flame propagation characteristics, showing promising results. The suggested model reasonably captures the enhanced normalized burning rate per unit flame area across various fuel compositions, equivalence ratios, and turbulence intensities.