Analysis of local entrainment rate in the initial region of isothermal free swirling jets by Stereo PIV

Entrainment is of crucial importance in many engineering applications, while the relevance of swirling flows is related to their ability to improve mixing in the initial region of a jet, as a consequence of a larger entrainment of ambient air. This is particularly effective at swirl levels above the critical value associated with the onset of vortex breakdown and the generation of the instability referred to as the precessing vortex core (PVC). Despite the importance of the entrainment process in swirling flows, there have been very few quantitative data in the literature. In view of the above observation, the initial region of a swirling gas jet was investigated experimentally using Stereo Particle Image Velocimetry (SPIV) in order to evaluate the local entrainment rate as a function of swirl number and axial distance from the nozzle. The experiments were carried out on a model burner used to generate isothermal swirling air jets at Reynolds number, Re = 21,800. This configuration was chosen due to its relevance to industrial burners and the ability to reproduce a round, turbulent gas jet issuing into ambient air, since this is the most investigated case and allows comparison with previously published results with and without swirl. Measurements were made for the region extending from the nozzle exit to 2.5 nozzle diameters downstream. Several swirl conditions were investigated, characterized by swirl numbers ranging from S = 0 to S = 2.2. Time-averaged SPIV allowed simultaneous evaluation of the swirl number and characterization of the structure of swirling flows in the near-field region before and after the development of the inner recirculation zone. The entrainment rate was evaluated by using the integral method applied to the measured SPIV velocity maps. The results provided quantitative evidence that the entrainment rate in the near field Is enhanced considerably by the swirl, especially after the onset of vortex breakdown and PVC, reaching values significantly larger than those measured in non-swirling jets. The analysis of the average local entrainment rate revealed a large variability with both the swirl number and the axial distance from the nozzle, leading to the conclusion that the swirl affects the entrainment process in a complex way. The significance of the present results consists in a better understanding of the mixing in the initial region of a typical burner and the possible contribution to validation of numerical modelling of the combustion process.