MEASUREMENT OF ENTRAINMENT RATE IN THE INITIAL REGION OF SWIRLING JETS

The entrainment rate in the initial region of axisymmetric turbulent swirling air jets has been measured by Particle Image Velocimetry (PIV) for a number of swirl strengths from S = 0 up to S = 1.5 and two Reynolds numbers. From the complete PIV maps of the jet and its surroundings, entrainment has been evaluated by using two methods: a) integration of the radial profiles of time-mean axial velocity of the jet; b) direct measurements of the radial velocity component of ambient air entrained into the jet. The entrainment rate increases with increasing axial distance from the nozzle in a non linear way and shows a rapid enhancement with the swirl number, particularly after inception of vortex breakdown and when the processing vortex core (PVC) is observed. The results allow to better understand the entrainment process in swirling jets and related effects on mixture formation and flame structure in industrial burners.