Heat transfer and flow structures in axisymmetric impinging jet controlled by vortex pairing

Abstract

An experimental study is conducted to investigate the flow and heat transfer characteristics of an impinging jet controlled by vortex pairing. Two kinds of vortex control methods of secondary shear flow and acoustic excitation are applied. Local Nusselt numbers are measured on the impingement surface. Flow visualization, measurements of velocity and turbulence intensity and FFT analysis are used to understand the flow structures. The velocity ratio is changed from 0.45 to 1.75 for the shear flow control and the tested Strouhal number (excitation frequency, St D) is 1.2, 2.4, 3.0 and 4.0 for the acoustic excitation. Enhancement or reduction in heat transfer is obtained by the control of vortex pairing due to the change of flow structures. When the vortex pairing is promoted by the secondary counter-flowing (suction flow) and St D=1.2, low heat transfer rates are obtained at large nozzle-to-plate distances. Conversely, the jet flow has an extended potential core length with the secondary co-flowing (blowing flow) and St D=2.4 and 3.0. Thus high heat transfer rates are obtained at large gap distances.