Numerical studies of open swirl-stabilized turbulent premixed flames

Abstract

In the current study, numerical methods are used to investigate an open swirl-stabilized turbulent premixed flame. It is assumed that swirl distributes uniformly along the circumference, and that the turbulent reacting flow is two-dimensional, symmetric, steady and incompressible. Flame stabilization by swirl flow is a common feature of many combustors. However, most of the swirl combustors are enclosed, there have been relatively few studies of open swirl-stabilized flames. Presently, freely propagating open swirl-stabilized turbulent premixed flames are studied systematically. The purpose of this work is to obtain a better understanding of the flame properties by simulating the flow field, combustion and heat transfer. Spalding's Stretch-Cut-Slide model is modified to determine the mixing controlled fuel burning rate, Sfu, T, which is defined by Sfu,T=-pmin[Mfu,Mot/s](0.5 |u/y+v/x| + SL/lo) The results reveal that intense combustion occurs in a narrow region. A stationary planar flame is maintained above the burner exit, where the turbulent flame speed is equal to local flow velocity. Although the flame is stabilized by swirl, the flame zone is in fact free of swirl. Compared with previous experiments, predictions reveal that a central re-circulation zone is located downstream of the flame. However, flame stabilization does not rely on re-circulation, but on flow divergence. Combustion helps to drive the re-circulation: the re-circulation zone becomes wider and longer under combustion than in cold swirling jets. The maximum reversal velocity also increases when combustion occurs. Strong impingement occurs between swirling jet and reverse flow, which makes the flame planar. Effects of swirl intensity, fuel to air equivalence ratio and burner configuration on flame properties are also presented in this thesis. It is found that chemical reaction itself has stronger influence on re-circulation zone length than swirl intensity and equivalence ratio. A more planar flame can be obtained by modifying the burner configuration.