AN EXPERIMENTAL-STUDY OF COMPRESSIBLE TURBULENT MIXING ENHANCEMENT INSWIRLING JETS

Compressible jets with various amounts of swirl and compressibility ar e investigated experimentally. The mixing-layer growth rate is obtaine d from time-averaged images of the mixing layer using the planar laser scattering (PLS) technique, and the swirl is quantified with laser Do ppler velocimetry and intrusive probes. The results conclusively demon strate that the addition of swirl to the jet increases entrainment by up to 60% compared to a corresponding non-swirling case. Instantaneous PLS images reveal modified turbulent structure in the mixing layer of the swirling-jet cases. In particular, analysis of these images indic ates that both the spatial extent and amplitude of the largest turbule nt fluctuations are increased when swirl is added. Based upon these re sults, a parameter beta that correlates the observed growth-rate enhan cement is proposed. This parameter is derived assuming that the stream wise vorticity, generated in the mixing layer by the addition of small amounts of swirl, causes additional turbulent mixing that increases t he growth rate. When the available growth-rate data for swirling jets are plotted against this parameter, they collapse to a single curve wi th increased enhancement for higher values of beta. This result implie s that the degree of enhancement actually increases with compressibili ty, although the dimensional growth rates for the present compressible swirling-jet cases are still less than those of their incompressible counterparts.