Structures of tetrabrachial flames in two-stage autoigniting dimethyl ether (DME)/air mixture under diesel engine relevant conditions are investigated by direct numerical simulation. Three cases with different initial turbulent integral length scales are studied. Results show that the first stage of autoignition initiates in lean mixtures, and subsequently develops into a diffusion-supported cool flame propagating into rich mixtures; the second-stage autoignition features spatially distributed kernels in fuel-rich mixtures, followed by hybrid auto-ignition/tetrabrachial flames. The detailed chemical structures of the tetrabrachial flames are analyzed in terms of reactant concentrations and the reaction rate profiles. The cool flame branch is dominated by low temperature reactions, while the other branches are mainly involved in high temperature oxidation of the remaining fuel and intermediate species. The excess DME is consumed in the premixed flame branches and decomposed into more stable fuels including H-2, CH4 and CO in the trailing diffusion flame, where H-2 and CO are mainly oxidized by intermediate species OH and O. The structures and reaction rates in the tetrabrachial flame exhibit significant asymmetry, which is more distinct in the mixture fraction-temperature phase space. Effects of turbulence on the timing and location of two-stage ignition are then studied. In this study, turbulence tends to advance ignition compared with laminar cases, while the first high-temperature ignition time is similar for the three cases with different initial turbulence integral length scales.

This paper presents three-dimensional direct numerical simulations of lean premixed turbulent H-2/air flames in the thin and distributed reaction zones, with the Karlovitz numbers at 60, 110, 150 and 1000, and pressures at 1 and 5 atm, respectively. Flame front structures and chemical pathways are examined in detail to investigate the effects of pressure and turbulence on flames. There is an increasing number of finer structures on the flame front with increased Karlovitz number. Eddy structures are observed downstream of the reaction zone under high turbulence intensity and thus Karlovitz number, indicating that the turbulent eddies are small and energetic enough to break through the distributed reaction zone. Statistical analysis indicates that the probability of high curvatures increases with increasing Karlovitz number at a constant pressure. When the Karlovitz number is kept constant, the probability of high curvatures is significantly higher at the atmospheric pressure than at elevated pressure. The approximation of Schmidt number (Sc = 1) in theoretical analysis introduces errors in the estimation of the smallest flow scale and the Karlovitz number. Accordingly, in the turbulent flame regime diagram, the boundary between the thin reaction zone and the distributed reaction zone should be modified at the elevated pressure. Moreover, the decorrelation of heat release and H-2 consumption is directly related to turbulence intensity, and the decorrelation is reduced at the elevated pressure. Due to the enhanced radical transport at high Karlovitz number, chemical pathways can be locally changed, which is more significant at elevated pressure.

This paper presents three-dimensional direct numerical simulations of lean premixed turbulent H-2/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+O-2(+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.