Flow field and thermal behaviour in swirling and non-swirling turbulent impinging jets

Abstract

The fundamental interaction between the mean velocity field, turbulence and the impingement surface characteristics are presented. The nozzle used achieves a seamless transition from non-swirling (S = 0) to highly swirling jets (S = 1.05). Convective heat transfer measurements on the impingement surface are performed using infrared thermography. Numerical simulations are carried out using ANSYS FLUENT 14.5 via SST k-ω turbulence model. The effect of swirl number and impingement distance (H = 2D and 6D) on the heat transfer characteristics are investigated at a Reynolds number (Re) of 35,000.Results show the effects of swirl on impingement heat transfer depend on impingement distance. In the near-field (H = 2D), high jet turbulence (u'u' and w'w') close to the surface (0.8 mm upstream) correlate very well with Nusselt number peaks resolved on the heated surface. The occurrence of any pockets of low turbulent kinetic energy (k) near the surface may cause localised Nu trough, which can also be correlated with the presence of swirl induced recirculation zones if they stabilise on the surface. Alternatively, in the case of far-field impingement (H = 6D), swirl causes wider jet spread and hence turbulence levels are reduced. At this distance, non-swirling jets thus yield higher Nu zones at the surface.