Effect of Confinement on Recirculation Structures in Isothermal Coaxial Swirling Jet

Abstract

In this work, an experimental and numerical investigation of effect of two confinement ratios (CR=1.81, and 2.7) on two different recirculation zones (RZs) viz., prevortex breakdown bubble (PVB) and central toroidal recirculation zone (CTRZ) observed in coaxial swirling jet in isothermal/non-reacting conditions is presented. As a part of experimental study 2-D laser Doppler velocimetry (LDV) has been used to acquire mean axial velocity values at different points within the non-reacting/isothermal flow field. 3D unsteady RANS simulations are carried out to evaluate the performance of different turbulent models in predicting the two types of RZs in confined condition. Radial profiles of mean axial velocity and extent (dimensions) of recirculation zones (RZs) obtained experimentally are used as parameters to compare the numerical results consequently to evaluate suitable turbulent models in simulating two types RZs. Finally, the numerical results obtained by employing best suitable turbulent model are used to gain more physical insights into the flow field of confined coaxial swirling jets. Results showed that flow confinement has a significant effect on the two types RZs studied. Furthermore, the effect of confinement ratio CR=2.7 is found more similar as that of unconfined case whereas, the wall/confinement effect is strongly witnessed in CR = 1.81 case for both PVB and CTRZ flow states.