Heat Transfer to an Obliquely Impinging Air Jet Tadhg S. O’Donovan, Darina B. Murray, Andrew A. Torrance Department of Mechanical & Manufacturing Engineering, Trinity College Dublin, Ireland odonovts@tcd.ie Abstract The current research is concerned with the measurement of convective heat transfer to an im- pinging air jet for a range of test parameters which include Reynolds numbers, (Re) of 10000 and 20000; nozzle to impingement surface distance, (H/D) from 0.5 to 2, and angle of impinge- ment, (α) from 45 ◦ to 90 ◦ (normal impingement). Both time-averaged and fluctuating heat transfer is investigated. In this range of low nozzle to impingement surface distances, the wall jet undegoes transition from laminar to turbulent. The transitional boundary layer is identified from the time-averaged heat transfer profiles. A flow structure initiates in the shear layer of the free jet and then impacts on the plate and moves along the wall jet. The corresponding fluctuating heat transfer is reported. It is shown that the flow structure grows initially as it moves radially from the stagnation point and eventually fades with further increasing radial position as the boundary layer becomes fully turbulent. Introduction Convective heat transfer to an impinging air jet is known to yield high local and area averaged heat transfer. Such a jet is of interest for the cooling of electronic components and gas turbine blades and for manufacturing processes such as grinding. A grinding process produces very high local temperatures, which, if not cooled, would have an adverse affect on the metallurgical composition of the work-piece. For this reason the enhancement of local or stagnation region heat transfer is investigated in the current research. The turbulence induced by mixing from entrainment does not penetrate to the centre of the free jet at low nozzle to plate spacings, normalised by the nozzle diameter (H/D). An in- vestigation by Gardon and Akfirat [1] has shown that at low nozzle to plate spacings, H/D < 2, the wall jet region transitions from laminar to turbulent. It is for this reason, according to Goldstein and Timmers [2], that the stagnation point Nusselt number is a local minimum at 1