Experimental investigation on the effects of swirl in the initial region of isothermal free swirling jets by stereoscopic PIV

Swirling jets are characterized by having a velocity component in the azimuthal direction and used in many technical applications e.g. gas turbines, vortex burners, furnaces, and jet engines. In this paper, the near exit region of unconfined isothermal swirling jets has been investigated by stereoscopic particle image velocimetry with a focus on the effect of swirl intensity on the formation of recirculation zone, vortex breakdown, and the occurrence of the precessing vortex core. Stereoscopic particle image velocimetry measurements were carried out on an axial plus tangential entry swirl burner with Reynolds number of 21,800 and five different geometric swirl numbers (S-g). The appearance of intermittent vortex breakdown is revealed by the instantaneous flow field at S-g = 1.15 before it appears in the time-averaged flow field at S-g = 2.6. Results show that the swirl intensity increases the backflow rate in the recirculation zone and jet spreads almost linearly with a higher spread rate as compared to non-swirling flow. The frequency characteristics have been measured with a capacitive microphone for various Reynolds numbers and geometric swirl numbers which indicates the periodic oscillation presence related to the existence of precessing vortex core. The Strouhal number associated with the frequency of the precessing vortex core vary monotonically with swirl intensity after the occurrence of vortex breakdown. The study also revealed the presence of coherent vortical structures in the high turbulent kinetic energy region (z approximately equal to 0.02D-1.5D).