Pressure effects on flame structures and chemical pathways for lean premixed turbulent H2/air flames: Three-dimensional DNS studies

This paper presents three-dimensional direct numerical simulations of lean premixed turbulent H2/air flames over a range of pressures using a detailed chemical mechanism. Effects of pressure on flame front structures and heat release from pressure-dependent pathways are analysed. Under the same initial turbulence at different pressures, the Kolmogorov length scale and local flame thickness decrease with increasing pressure. Thinner and sharper structures are found on the flame front at elevated pressures. As the pressure is increased from 1 atm to 5 atm, heat release is greatly enhanced at convex regions but weakened at concave regions of the flame fronts, which indicates that the effect of Darrieus-Landau instability is becoming stronger. The correlation of heat release and fuel consumption is also strengthened as pressure is elevated. A main pressure-dependent heat release reaction, H + O2(+M) = HO2(+M), is found to contribute less to the total heat release with increasing pressure for turbulent flames, which is contrary to the trend in laminar flames. In the low temperature zones, this is due to the decreased H radical pool at elevated pressure. In the high temperature regions, the reaction is less competitive compared with H + OH+M = H2O+M, thereby reducing its contribution to the heat release.