Effects of the Equivalence Ratio and Reynolds Number on Turbulence and Flame Front Interactions by Direct Numerical Simulation

Direct numerical simulation (DNS) of a two-dimensional premixed turbulent syngas flame (50% H₂ and 50% CO) was performed to investigate turbulence–flame interactions. An in-house DNS code, based on the low-Mach-number version of Navier–Stokes equations, was used to predict the influences of the equivalence ratio and Reynolds number on the turbulence–flame interactions using the constant volume enclosure method. The reaction between syngas and air was modeled with the Davis mechanism, which includes 14 species and 38 elemental reactions, and the species had distinct Lewis numbers. Syngas–air flames at combinations of four equivalence ratios and three Reynolds numbers were investigated. The flame structures at turbulence time scales were analyzed to demonstrate the influence of the turbulence on the flame structure, such as localized deformation of the flame front and isolated pockets of flame. The phenomenon was based on real effects detected by planar laser-induced fluorescence imaging of premixed syngas combustion under different turbulence conditions. The mechanism behind flame front disturbance was associated with various flame physical descriptors, e.g., flame thickness, tangential strain rate, curvature, displacement speed, flame length, and OH mass fraction. The flame thickness at the location of a bulge was usually thin, and the flame thickness on the gully was usually thick. A negative tangential strain rate was more likely to appear at lean stoichiometric ratios. Flashback occurred when the flame displacement speed was larger than the laminar flame speed. The flame length, another characteristic variable, had a positive correlation with the Reynolds number. The OH mass fraction, which is an important reaction intermediate, showed a strong negative correlation to flame front curvature. Correlation analysis indicated that the OH mass fraction was a better parameter to characterize the interaction of the turbulence and flame than the flame length.