Flow and heat transfer characteristics of turbulent swirling impinging jets

Abstract

• PIV to resolve the time-averaged flow field of turbulent swirling jets. • Flow field of free and impinging swirl jets are compared. • Flow features correlated to heat transfer characteristics. • Vortex breakdown examined at Re = 11,600–35,000 at low and high swirl. This paper addresses both the upstream and downstream flow domain in turbulent impinging jets, which have widespread applications in cooling, heating, and drying. The role of impingement on swirl induced vortex breakdown as well as the interplay between heat transfer (at the impingement plane) and the downstream flow domain are analysed. Particle Image Velocimetry (PIV) is used to resolve the flow field in a range of jets, covering three Reynolds numbers (Re = 11,600, 24,600, and 35,000), two swirl numbers (S = 0.3 and 0.74), and two impingement distances (H/D = 2 and 4) as well as free jets. With regard to the upstream flow field at low swirl (S = 0.3), both free and impinging jets do not exhibit vortex breakdowns. However, for high swirl (S = 0.74), vortex breakdown in the upstream is observed at Re = 11,600 and Re = 24,600, but not Re = 35,000. Impingement distance affects the size, position, and strength of the recirculation bubble (vortex breakdown). The recirculation bubble is more spherical and stronger at Re = 11,600 in contrast to the Re = 24,600. Near-field impingement (H/D = 2) significantly affects the size and shape of the bubble. In free jets, the width of the vortex breakdown bubble at Re = 11,600 spans −0.4 < r/D < 0.4, but shrinks to −0.2 < r/D < 0.2 at Re = 24,600. Correlating the velocity field near the impingement plane with the heat transfer characteristics shows that at low swirl conditions (S = 0.3), that do not lead to upstream vortex breakdown, the spatial extent of a recirculation zone stabilised onto the impingement plane reduces in size at a higher Reynolds number for both H/D = 2 and 4. However, higher swirl conditions (S = 0.7) which typically lead to upstream vortex breakdown, do not coincide with the appearance of any downstream recirculation zones that stabilise onto the impingement plane.