Combustion enhancement by axial vortices

Abstract

A tapered slot jet was studied experimentally in nonreacting and reacting tests using hot-wire anemometry, watertunnel flow visualization, and planar laser-induced fluorescence imaging. The tapered slot jet is a modified elliptic jet with a conical contraction leading to its elliptic outlet. The added contraction changed the entire flowfield relative to a regular elliptic jet. The jet spread in the major axis plane was larger than in the minor axis plane, which is the opposite behavior of an elliptic jet. The elliptic jet spreads much faster at the minor axis plane relative to the major axis plane, leading to axis switching downstream of the nozzle. In the tapered slot jet, no axis switching was observed. The turbulence amplification in the jet core was higher than in circular and elliptic jets. The different behavior was attributed to the change in flow direction, inside the nozzle, from the conical section to the slot outlet. During this transition, the flow acquired angular momentum, thereby generating axial vorticity. The contraction angle and the outlet aspect ratio were found to affect the spreading rate of the jet at both axis planes and to vary the turbulence augmentation. The effect of the increased turbulence on reactive flow was tested in a premixed flame. The combustion rate was augmented in both the core and edges of the flame relative to a circular burner.