Local heat transfer distribution on a flat plate impinged by a compressible round air jet

Abstract

An experimental investigation is carried out to study the local heat transfer distribution on a flat plate orthogonally impinged by a single round compressible impinging jet using thin metal foil technique. The surface temperature of the impingement plate is measured using thermal infrared imaging technique. A circular pipe of diameter 10 mm is used to obtain Mach numbers of 0.2, 0.4, 0.6, 0.8 and 1 for the corresponding Reynolds numbers of 44 000, 88 000, 133 000, 176 000 and 221 000 respectively. The circular pipes of diameters 6 mm, 7.3 mm, 10 mm and 15 mm are used to obtain Mach numbers of 1, 0.83, 0.6 and 0.4 respectively for a constant Reynolds number of 133 000. The jet-to-plate distance ( z/ d) is varied from 1 to 12 nozzle diameters. The adiabatic wall temperature is used as a reference temperature for the calculation of the local Nusselt number. It is found that the heat transfer rate increases with increase in Mach number (Reynolds number) for all nozzle-to-plate distances at all radial locations. The stagnation point Nusselt number is maximum at z/ d = 6 for incompressible flow and at z/ d = 8 for compressible flow. The substantial increment in average Nusselt number is observed as Mach number is increased from 0.2 to 0.4 as compared to the increment in average Nusselt number for Mach number increase from 0.8 to 1.0. Recovery factor variation with radial location is almost independent of the Reynolds number and the Mach number but varies with jet-to-plate distance. Recovery factor more than unity at stagnation point is reported for larger jet-to-plate distances ( z/ d > 8).