Structure of Turbulent, Swirling Round Jets

Abstract

The present study deals with the numerical analysis of the effect of the swirl in the self-preservation region of the turbulent round jet. However, a large number of literature exists for the analysis of near-exit regions—very few deals with the self-preservation region of the jets far downstream. The present study attempts to provide insights into the effect of swirl on the turbulent mixing and jet spread rate by examining the self-similar solution in the far-field region of the jet. The study is divided into two main portions: a comparison of the turbulent swirling and non- swirling jets and the comparison between the turbulent jets having low to moderate values of swirls.

A standard computation for a non-swirling jet is used to validate the flow solver. Simulations are carried out at a Reynolds number of 2,400 for the top-hat velocity profile at the inlet. All flow characteristics are computed in detail and compare the results with existing DNS data. Velocity profiles at different streamwise locations collapse on a single curve and closely match the available data. The jet decay and spread rates also align with the standard computed data.

Large eddy simulation has been performed for non-swirl (S = 0), weak swirls (S = 0.3, 0.5) and moderate swirl (S = 0.7) at a Reynolds number of 11,000. In both the non-swirling and swirling cases, special care is taken to ensure that the computational domain is large enough to study the jet’s behaviour in a self-similar region. The research presents the effects of the swirl on a turbulent flow and compares the simulation results with available experimental data. Comparing the swirling and non-swirling cases indicates a changed turbulence structure to the effect that the swirling jet spreads and mixes faster than the non-swirling. With increasing degrees of swirl, the angle of spread of the jets is increased, and correspondingly, the decay of the maximum values of velocity components along the lengths of the jets is faster. Flow entrainment shows that the entrainment increases with swirls. The numerical simulations showed that the flow quickly achieved a self-similarity for the mean axial velocity. In contrast, the radial and azimuthal mean velocities reached a self-similar state after a longer period of jet development. Results of the decay of velocity and jet spread rate in the self-similar region of the swirling jet without vortex breakdown were found to vary linearly with the streamwise direction of the jet irrespective of the magnitude of swirl number, which is in line with the findings from experiments of Rose (1962), Chigier & Chervinsky (1967) & Pratte & Keffer (1972). In contrast, Craya & Darrigol (1967) has theoretically shown that axial velocity decay varies as three halves along the length of the jet. Additionally, mass flux shows higher mixing in swirling jets compared with non-swirling. The integrated axial fluxes of linear and angular momentums were conserved along the jet’s axis in the self-preserving region.