Three-dimensional (3D) unsteady Reynolds-averaged Navier–Stokes (URANS) simulations are conducted in a coaxial swirling jet. Two distinct types of recirculation zones (RZ) relevant to coaxial swirling jets are considered based on the modified Rossby number Rom, which is known to represent the ratio of axial velocity deficit between two coaxial streams to the characteristic tangential velocity at the nozzle exit. The two flow states studied are Rom>1 and Rom≤1. The former is characterized by regions of high strain (especially in the shear layer between central and coaxial jet). It is found in this study that renormalization group (RNG) k−ɛ model is the suitable model for 3D URANS simulation of Rom>1 flow state. This is attributed to the model's ability to simulate flow regions that are heavily strained. The simulated results are compared with two-dimensional laser Doppler velocimetry measurements conducted as part of this study. For the flow states Rom≤1, which are characterized by the dominance of radial pressure gradient arising due to rotational (swirling) effect over the pressure gradient due to axial velocity deficit, the Reynolds stress model (RSM) is found best to simulate the flow topologies and mean and turbulent quantities. The time-averaged results obtained from optimized turbulent models are employed to gain insights into the fluid mixing phenomenon in these RZs. The unsteady axial velocity fluctuations obtained from both experiments and URANS simulations are analyzed in the frequency domain to gain insights into dominant axial oscillations prevalent in RZs.

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.