Internal flame structures of thermo-diffusive lean premixed H2/air flames with increasing turbulence

Abstract

Turbulent flow fields, instantaneous flame structures, and internal thermo-chemical states of lean premixed hydrogen/air jet flames at an initial equivalence ratio of 0.4 are experimentally investigated by simultaneous laser-induced fluorescence of hydroxyl radicals (OH-LIF) and particle image velocimetry (PIV), and quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering measurements over a range of Karlovitz numbers (Ka) from 50 to 730. At low Ka, intense burning characterized by elevated local equivalence ratio, high water mole fraction (XH2O), and super-adiabatic flame temperature is mainly observed in post-flame regions surrounded by positively curved flame surfaces, where the fast diffusive hydrogen is locally focused. The flame features stronger differential diffusion and curvature effects than that in the planar laminar flame, which indicates that both molecular and turbulent mixing play significant roles, and thermo-diffusive instabilities have synergistic interactions with turbulence at low turbulence level. With increasing Ka, the burning intensity in corresponding regions is weakened, even though the flame surface is more disturbed by the turbulence. At the highest Ka, no intense burning region is observed in the jet flame as the turbulent transport dominates over the molecular diffusion. In the temperature domain, the conditional means of hydrogen mole fraction (XH2) and local equivalence ratio feature effects of diffusive instabilities with broad distributions at low-Ka conditions. Elevated XH2O and local equivalence ratios with super-adiabatic flame temperatures are observed, which is attributed to differential diffusion in hydrogen-containing mixtures and the fuel focusing effect near positively curved flame surfaces. At high Ka, the XH2 shows a more linear decreasing trend and the local equivalence ratio profile becomes flatter over temperature with a narrower distribution, indicating the dominance of turbulent mixing.