Area increase and stretch factor in lean hydrogen-air turbulent flames

Journal article, 2024

Analyzed in this paper are three-dimensional Direct Numerical Simulation (DNS) data obtained earlier by the present authors from (i) 16 statistically planar and one-dimensional, lean complex-chemistry hydrogen-air turbulent flames propagating in forced turbulence in a box and (ii) nine counterpart equidiffusive flames. The simulation conditions cover a wide range of non-dimensional turbulent combustion characteristics. Specifically, root-mean-square turbulent velocity is varied from 0.3 to 55 laminar flame speeds, integral length scale of turbulence is varied from 0.5 to 2.6 laminar flame thicknesses, Damköhler and Karlovitz number are varied from 0.01 to 5.2 and from 0.7 to 1300, respectively. Three equivalence ratios, 0.7, 0.5, and 0.35, are addressed. Results of complementary two- and three-dimensional simulations of thermodiffusively unstable laminar flames are also reported. They show that neutral wavelength of laminar flame instabilities (both hydrodynamic and thermodiffusive ones are enabled in the simulations) is smaller (larger) than computational domain wide in ten (three, respectively) cases characterized by a low Lewis number. Accordingly, in three of these cases, the instabilities are suppressed. The focus of the study is placed (i) on contributions of flame surface area increase and stretch factor to turbulent burning velocity and (ii) on the influence of differential diffusion effects on these contributions. The computed results show that the flame surface area (i) is substantially increased by both turbulent rms velocity and length scale, (ii) is turbulent velocity, but (iv) a notable influence of turbulence length scale on the stretch factor is not observed.