A Comparison of Bluff-Body and Swirl-Stabilized Flames

Abstract

Bluff-body and swirl-stabilized flames are similar in that they represent, in simplest terms, the fundamental interaction between a fuel jet and a surrounding toroidal vortex. The vortex in this case is the recirculation vortex which affects the properties of the flames. It is found, not surprisingly, that the two most important fundamental parameters that govern both types of flames are (1) the vortex circulation (Γ), and (2) the fuel jet momentum. Comparisons are made of the properties of the two types of flames using the proper nondimensional parameters, including the fuel-to-air momentum flux ratio and the properly nondimensionalized vortex strength. Such comparisons can help to illustrate the tradeoffs between the degree of swirl and the choice of bluff-body size in devices such as industrial burners, gas turbines, and ramjets. The data also show how one can control flame properties by controlling the vortex strength Γ and fuel momentum and thus gain a degree of control that is not provided by simple jet flames.

For flames that extend beyond the recirculation zone, the flame lengths of both types of flames are found to scale with the square root of fuel-to-air momentum flux ratio and with the inverse of vortex circulation. Thus, the length of these flames can be easily controlled and are found to scale in a different manner than the length of a simple jet flame. The location of the forward stagnation point of the recirculation zone, which is important in determining flame stability and the heat transfer to the fuel nozzle, also scales with the square root of fuel-to-air momentum flux ratio for both flames. For the same vortex strength and jet momentum, both flames display remarkably similar flow structure, including the presence of counter-rotating vortices. One vortex is driven by the air flow while the other is driven by the fuel flow. For both flames increasing the vortex strength significantly increases the fuel-air mixing rate (i.e., shortens flame length), which is desired in certain applications.