Simulation of Spherically Expanding Turbulent Premixed Flames

Statistically spherical expanding turbulent premixed flames are computed using an unsteady Reynolds-averaged Navier-Stokes (URANS) approach. Mean reaction rate is closed using strained and unstrained flamelet models and an algebraic model. The flamelets are parametrized using the scalar dissipation rate in the strained flamelet model. It is shown that this model is able to capture the measured growth rate of methane-air turbulent flame ball, which is free of thermo-diffusive instability. The spherical flames are observed to accelerate continuously. The flame brush thickness grows in time and the role of turbulent diffusion on this growth seems secondary compared to the convection due to the fluid velocity induced by the chemical reaction. The spherical flames have larger turbulent flame speed, the leading-edge displacement speed s(t), compared to the planar flames for a given turbulence and thermochemical condition. The computational results suggest s(t)/s(L)(0) approximate to Re-t(n) with 0.57 <= n <= 0.58, where Re-t is the turbulence Reynolds number and s(L)(0) is the unstrained planar laminar flame speed, for both spherical and planar flames.