Orthogonally impinging axisymmetric jet investigation - Heat transfer and near wall thermal flow field characteristics

The thermal flow field of a turbulent axisymmetric semi-confined impinging jet has been investigated using cold-wire resistance thermometry. The jet nozzle was integral to an upper confinement plate and the cold air jet impinged orthogonally onto a flat heated surface. Nozzle to plate spacings of 2 and 6,5 diameters, at a Reynolds number of 20000, have been considered. Data has been compared with complementary Nusselt number profiles, determined using a transient wail heating technique with liquid crystal thermography. At low nozzle to plate spacings, local maxima in heat transfer at the edge of the impinging jet an attributed to a thinning of the thermal boundary layer and the impingement of vortex structures, initiated at the jet exit, on the developing wall jet. Results also show that, further downstream, transition to turbulence occurs at the smaller. spacing and that high turbulence levels are coincident with secondary maxima in Nusselt number. At the larger spacing a similar transition was not observed.