The role of the recirculation vortex in improving fuel-air mixing within swirling flames

Abstract

An important example that shows how a flame interacts with a vortex is the case of a swirlstabilized flame. Such a flame essentially is a toroidal vortex into which a jet of fuel is injected along the torus centerline. Because of the central toroidal vortex, the overall fuel-air mixing rate within a swirl-stabilized flame is found to be a factor of five times greater than that of a simple jet flame, as evidenced by a fivefold shortening of flame length. Swirl flames are unique in that the fuel-air mixing rate can be varied by controlling the amount of swirl. Flow visualization shows how internal recirculation helps to enhance the mixing. The englufment mechanisms is similar to that of a simple jet flame but is enhanced because of two factors that increase the fuel-air contact area. First, the recirculation zone acts like a large eddy with a characteristic velocity and length scale that are much larger than those associated with eddies in a simple jet. The active role of the recirculation zone contradicts a previouslyheld concept that the recirculation zone is a passive obstacle and that its internal velocity is not important. Air is entrained into the toroidal vortex primarily in the downstream region of the vortex. A second reason why mixing is more intense within swirling flames than within jet flames is that there is impingement of opposed jets at the forward stagnation point. Thus pressure gradients, which are not present in simple jets, enhance the mixing rates. Flame length, which is a measure of overall fuel-air mixing, was found to have a different scaling in swirl flames than for simple jet flames. The physical reasons are explained by a scaling which was developed and which explains four trends in the flame length data. It is proposed to use a general, nondimensional circulation parameter that allows for general comparison of mixing efficiencies of swirl, bluff-body, and dump-combustor flames.