Impingement heat transfer in an under-expanded axisymmetric air jet

This paper is concerned with the heat transfer that occurs when an underexpanded jet impinges onto a heated surface. The heat transfer in the impingement zone is extremely high and when the surface interferes with the expansion of the jet, the radial distribution of the heat transfer coefficient becomes more complex. If the jet impinges upon the surface before the core of the jet has decayed, there is no longer a maximum stagnation heat transfer coefficient on the geometric axis of the jet, instead a stagnation ‘ring’ is formed with a radius of about one nozzle diameter. Experimental results are presented for nozzle pressure ratios up to 5.08, and nozzle-to-surface spacings of 3, 6 and 10 nozzle diameters. In addition to the measured data, a clear outcome of the work is that the usual method of representing Nusselt number as a function of Reynolds number is inadequate in compressible flows where the dimensional analysis shows that the nozzle Mach number, or pressure ratio, should also be included.