The effect of inflow conditions on the development of non-swirling versus swirling impinging turbulent jets

Abstract

In this paper, axisymmetric turbulent jets are investigated numerically with the aim of improving our understanding of the effect of inflow conditions, swirl and near-field impingement on jet development. After presenting an extensive overview of the works published on swirling and non-swirling impinging jets, the Reynolds-Averaged Navier–Stokes (RANS) approach (with different turbulence models) is used to study these flows and validated against experimental data sets. This study then explores the effects of the inlet swirl velocity profiles, namely a Uniform Profile (UP), Solid Body rotation (SB) and Parabolic Profile (PP) on impinging jet development in weakly swirling jets (S⩽0.3) where no vortex breakdown occurs at Re=23,000 in the near-field (x/D<2). These conditions are selected so as to avoid the perturbation induced to the flow by a stagnant recirculating (vortex breakdown) bubble and the additional effects, on jet spread, which would likely occur at larger swirl number or delayed jet impingement (x/D⩾2).The results show that in comparison to free jets, the streamwise development of non-swirling impinging jets generally follows their corresponding free jet counterparts up to about x/D=1 downstream from the nozzle exit plane. In comparison with a non-swirling impinging jet of equal Reynolds number, the introduction of low levels of swirl into an impinging jet results in similar centreline velocity decay but a significant reduction in wall shear stress and turbulent kinetic energy at the wall jet region. The reduction in wall shear stress becomes more pronounced as the swirl number increases. The increase in turbulence intensity at the inlet significantly affects turbulence characteristics of swirl flows in the central impingement region. Within the range of swirl numbers investigated, different inlet swirl profiles do not drastically change the static pressure behaviour on the impingement surface compared to non-swirling jets.