Study of turbulent burning velocity using laser diagnostics in turbulent premixed flames

Turbulent burning velocity plays an important role in turbulent premixed combustion. After many years of study it still remains an unresolved problem. One goal of the present study was to determine if the burning velocity curve has nonlinear "bending" as turbulence intensity level increases and to study reasons why the non-linear behavior exists. The nonlinear dependence of the turbulent burning velocity on the mean flow velocity and the turbulence intensity level was studied using Mie scattering technique. A stoichiometric methane-air slot burner with 2-D mean flow and surrounding outer flames was used. Mean velocity was varied from 3 to 12 m/s and turbulent intensity levels were varied from 5 to 25%. Images of the wrinkled flame surface were recorded. It was found that nonlinear bending behavior does occur. Mean velocity was found to be a governing parameter in addition to turbulent intensity level. To understand stretch effects and possible extinction processes that can contribute to the bending phenomenon simultaneous CH-PLIF/PIV diagnostics were employed. The technique allowed studying the flame interaction with the reactant flow. The CH layer images show that flame surface loss by local extinction does not occur, and thus is not a cause of the bending observed. The average CH layer thickness was about the same for all cases studied. Flame-vortex interactions were observed: pairs of counter-rotating vortices exerted strain on the flame in ways similar to a counterflow flame; single vortices made the flame roll around them straining the flame. The stretch effects were investigated along the flame for different flow conditions.