A DNS study on turbulence-chemistry interaction in lean premixed syngas flames

Abstract Syngas is a mixture of hydrogen and CO and it is considered as a cleaner alternative fuel for engines. Hydrogen, a component of syngas, is a sub-unity Lewis number (Le) fuel and the influence of Le on pure hydrogen flames has been documented in the past. However, the effect of Lewis number and turbulence on heat release rate (HRR) in a turbulent premixed syngas flames has not received enough attention. DNS of a turbulent premixed syngas flame in a radially expanding flame kernel is performed to investigate the effect of Lewis number, turbulence, and initial curvature on HRR. An equimolar mixture of hydrogen and CO is considered for most of the simulations except for one simulation which had higher mole fraction of CO. Enhancement in HRR at low temperature due to the chain terminating reaction H + O2 + M↔HO2 + M is observed. Higher diffusivity of hydrogen-containing species, viz. H2, H, and OH, is found to be responsible for the enhancement in HRR. Among these species, diffusion of atomic hydrogen is shown to have maximum influence on HRR at low temperature. The influence of differential diffusion of hydrogen is observed to diminish when the mole fraction of CO is raised to 70%. Syngas flame at high temperature and high pressure is also studied to mimic the conditions seen in SI engine. The shift of HRR towards low temperature zone observed in the atmospheric flames is absent under high temperature and pressure conditions. Non-unity Lewis number of hydrogen-containing species increases the conditional fluctuation of mass fraction and HRR, in the progress variable space. A care has to be taken while using first-order conditional moment closure for such flames, where conditional fluctuations cannot be neglected.