Turbulent flame speed of NH3/CH4/H2/H2O/air-mixtures: Effects of elevated pressure and Lewis number

Abstract

This study investigated the turbulent flame speed (ST) of NH3/CH4/H2/air mixtures subjected to differential-diffusion effect characterized by sub-unity Lewis number (Le) at elevated pressures (1 and 5 atm), temperature (373 K), lean equivalence ratios (ϕ = 0.6–0.91) with and without 10% volumetric H2O dilution. The experiments were conducted in a fan-stirred constant volume combustion vessel, with a homogeneous turbulence intensity of u′ = 0 - 2.34 m/s. Schlieren imaging was used to derive the instantaneous turbulent flame speed. Morphology analysis shows that at atmospheric-pressure conditions, the laminar diffusional-thermal unstable flame surface is subjected to cellular wrinkling, this instability is increased at elevated-pressure conditions. Whereas, external turbulence will aggravate the wrinkling but the increase in turbulence intensity suppressed differential-diffusion effects on the flame structure. Flame speed development at sub-unity Le cases show differential diffusion effect and propagates faster than unity Le cases even though they have similar laminar flame speed. Then, the turbulent spherical flame speeds are scaled with a power-law fitting of flame Reynolds number, ReT,flame. The normalized turbulent propagation speed (ST/SL) is much higher for the sub-unity Le flames but it decreases as hydrogen mole fraction (XH2) increases in the fuel mixture. This was explained by the synergistic effect of differential diffusion and turbulence. Water dilution and elevated pressure via increase of Karlovitz number (Ka) and turbulent ReT,flow affect the ST/SL increasing. At sufficiently low Damköhler numbers, Da, a ratio of normalized turbulent flame speed ST/SL is mainly controlled by ReT and scales as a half power law, in line with the classical hypothesis by Damköhler. Additional turbulent stretch modification with Ka or Da can correlate with different hydrogen content conditions, this is because the independent pair of scaling parameters from a set of (u′/SL, lT/lf, Da, Ka, ReT) is considered. The scaling parameter ((u′/SL)α(lT/lf)β(Le)γ) is insensitive to Lewis number, because of the dominance of relative turbulent intensity and turbulent length scale.