Impinging jet heat transfer in the transitional wall jet region

Convective heat transfer to an impinging air jet is known to yield high local and area averaged heat transfer coefficients. Such jets are of interest in the cooling of electronic components and of turbine blades and in manufacturing processes such as grinding. The current research is concerned with the measurement of heat transfer to an impinging air jet over a wide range of test parameters. These include Reynolds numbers, Re, from 10000 to 30000 and nozzle to impingement surface distance, HID, from 0.5 to 8. The current research reports both mean and fluctuating heat transfer distributions up to 6 diameters from the geometric centre of the jet. The heat transfer results are compared to local velocity data. At low nozzle to impingement surface spacings the heat transfer distributions exhibit peaks at a radial location that varies with both Re and HID. These peaks are shown to be due to an abrupt increase in turbulence in the wall jet boundary layer. At certain test configurations vortices that initiate in the shear layer impinge on the surface and move along the wall jet before being broken down into smaller scale turbulence. The effects of the vortical flow on the heat transfer mechanisms in an impinging jet flow are discussed.