The Structure of Premixed Flames in a Spatially Evolving Turbulent Flow

Abstract

Large eddy simulations of spatially propagating premixed flames in a three-dimensional dump combustor have been conducted at relatively high Reynolds numbers. A thin flame model is used to simulate the propagating flame front and the propagation speed is estimated using the subgrid turbulence intensity obtained from a one-equation subgrid turbulence model. Analysis of the simulations show that various statistics, such as flame front shape factor, stretch effects, vorticity/strain rate, and strain rate/surface normal alignments, agree reasonably well with data from constant density, direct numerical simulations of passive scalars and premixed flame propagation in simpler, temporally evolving low Reynolds number flows. This suggests that the general characteristics of propagating scalars in turbulent flows are relatively independent of Reynolds number and further that these features can be captured using relatively coarse grid LES at high Reynolds numbers. However, detailed analysis showed that in spatially developing flows, there is a significant dependence on the spatial location for some statistical properties. As the flame propagates downstream the probability density of the shape factor showed a decreasing probability for cylindrical shaped flames and the strain rate/flame normal alignment showed a transition from the alignment seen in shear driven flow to alignment seen in isotropic turbulence. Strain rate (in the plane of the flame) was maximum near the flame holder (the rearward facing step) and decreases in the downstream direction with the pdf becoming more symmetric similar to isotropic flows. The implications of the observed spatial dependence for investigating flame stretch effects are discussed.