In the swirl-stabilized flame studied, the heat release and the swirl were systematically varied in order to quantify their effects on the internal recirculation zone, the mixing, the velocity PDFs and the flame blowoff limits. The amount of heat release was varied by changing the overall equivalence ratio, and the swirl number was varied up to 4.0 to assess the advantages or disadvantages of very high swirl operation. The velocity fields in nine non-premixed flames and three cold flows were mapped out using laser velocimetry. It was found that increasing the heat release resulted in a number of benefits, including an increase in the recirculation, the turbulence levels, and the flame stability. Heat release helped to drive the recirculation, since in some cases the cold flow did not have recirculation but the corresponding flames did. Turbulent kinetic energy levels increased by a factor of three as heat release increased.

The effect of increasing the swirl was to improve the mixing and flame stability for swirl numbers up to approximately one; further increases in swirl actually reduced the turbulence levels and flame stability. Excessive swirl also had the disadvantage of forcing the flame to move upstream to a position closer to the burner walls, resulting in excessive wall heating.

The velocity PDF was bimodal near the zero axial velocity line and was distinctly trimodal where there is mixing between the primary air flow, the reversed flow, and the entrained air. These PDFs indicate that large scale mixing and intermittency are important. The results suggest that computer models should include bimodal velocity PDFs in addition to bimodal scalar PDFs. The Bray-Moss-Libby model includes a bimodal velocity PDF to model intermittency in premixed flames; such a model is needed for non-premixed flames.