VORTICITY GENERATION AND ATTENUATION AS VORTICES CONVECT THROUGH A PREMIXED FLAME

A sequence of PIV images shows the time history of both the vorticity field and the velocity field as vortices of different strength convect through a premixed flame. The vortices represent individual eddies in turbulent flow; the goal is to understand how each eddy wrinkles the flame and how the flame also may alter the eddy. It is found that weak vortices are completely attenuated primarily due to volume expansion. Strong vortices do survive flame passage, but only if they can weaken the flame due to stretch effects. Intense flame-generated vorticity i s measured which has a magnitude that exceeds that of the incident vor tex in some cases. The flame-generated vorticity in the products induc es a velocity field that tends to reduce the amplitude of flame wrinkl ing; thus it acts as an additional flame-stabilizing mechanism. This m echanism affects the wrinkling process and should be included in model s. A new nondimensional vorticity enhancement parameter (E) is suggest ed as a way to estimate the effect of vortex size, strength, Reynolds number, and Froude number on vorticity attenuation and production. Mea surements are made for E approximately equal to 0, -1, and -2, corresp onding to no change in vorticity, total attenuation of the vortex, and flame-generated vorticity, respectively. Buoyancy forces are importan t in one case that is considered, but not in other cases. The results can be used to quantify the size of the small eddies that can be negle cted in large eddy simulations; the role of small eddies is estimated in one example. (C) 1998 by The Combustion Institute.