Flow field investigation of Confined Coaxial Swirling Jets by Stereo-PIV

This work analyses the isothermal flow field of a research burner exploiting both swirl and radially staged combustion air. The burner is composed of two coaxial co-rotating swirling jets, and the swirl level of the two jets can be controlled independently of each other. The influence of the swirl levels on the flow field is experimentally investigated under isothermal conditions using the Stereo-PIV technique, and the results evidence significant differences in the mean flow field at the different swirl levels. The vortex breakdown occurs in all the investigated cases, except for the combination of the lowest outer swirl level and the intermediate inner swirl level. Besides, the central recirculating flow region shape and size are affected by the momentum ratio and swirl level of the central jet. When the swirl intensities of the two coaxial jets are at their maximum, LDV measurements evidence on the burner axis tangential velocity fluctuations at a frequency of about 86 Hz; a POD-based phase average of the PIV maps allows recognizing the presence of a Precessing Vortex Core (PVC) which induces a periodic merging of the two jets. Those results can be of significance in the design of double swirl burners and for the control of the combustion process. Nevertheless, further tests are required to extend the current analysis to combustion conditions.