Direct numerical simulation of high pressure turbulent lean premixed CH4/H2 – Air slot flames

Abstract

A numerical study on lean turbulent premixed methane/hydrogen-air slot flames at high pressures is conducted through two-dimensional Direct Numerical Simulation (DNS). A single equivalence ratio flame at Φ=0.7 and 50% of hydrogen content is explored for three different pressures (0.1,1,4 MPa respectively). Due to the decreased kinematic viscosity with increasing pressure, the turbulent Reynolds numbers increase with an associated decrease of the smallest turbulence scales that wrinkle the flame front.Finite difference schemes were adopted to solve the compressible Navier-Stokes equations in space (compact sixth-order in staggered formulation) and time (third-order Runge-Kutta). Accurate molecular transport properties were also taken into account and, a detailed skeletal chemical mechanism for methane/hydrogen-air combustion, consisting of 23 transported species and 124 elementary reactions, was used.A general description of the three flames is provided, evidencing their macroscopic differences in terms of turbulent displacement speed, flame surface areas and mean flame brush thickness. Furthermore, topological features of the flames are explored by analyzing the probability density functions of several quantities: curvature, curvature shape factor, alignment between vorticity and principal strain rate vectors with flame surface normal, displacement speed and its components. Finally the differential diffusivity effect on the local equivalence ratio in the three flames is investigated showing a strong effect of the turbulent flame thickness of the high pressure flames on light species differential diffusivity.