Jet array impingement heat transfer: Separate effects of Mach number, Reynolds number, temperature ratio, and hole spacings

Limited available data suggest a substantial impact of Mach number on the heat transfer from an array of jets impinging on a surface at fixed Reynolds number. Many jet array heat transfer correlations currently in use are based upon tests in which the jet Reynolds number was varied by varying the jet Mach number. Hence, these data may be inaccurate for high Mach numbers. Results from the present study are new and innovative because they separate the effects of jet Reynolds number and jet Mach number for the purposes of validating and improving correlations which are currently in use. The present study provides new data on the separate effects of Reynolds number and Mach number for an array of impinging jets in the form of discharge coefficients, local and spatially-averaged Nusselt numbers, and local and spatially-averaged recovery factors. The data are unique because data are given for impingement jet Mach numbers as high as 0.74 and impingement jet Reynolds numbers as high as 60,000, and because the effects of Reynolds number and Mach number are separated.; Data are also provided as the ratio of wall temperature to impingement jet temperature is varied, as both the Mach number and Reynolds number are held constant. The results show that spatially-averaged Nusselt numbers decrease as the Tw/Tj temperature ratio increases. Provided the impingement jet static temperature is held constant, this means that spatially-averaged Nusselt members then decrease as the target plate temperature becomes larger.; Also varied is the spanwise and streamwise spacings of the impingement array jets. Experimental results show that Nusselt numbers generally decrease as x/D increases when X/D=8, whereas Nusselt numbers are generally about constant as x/D increases, when X/D=12. This is partially due to larger hole spacing which makes each jet discharges like a single jet. Experimentally measured Nusselt numbers also show that spatially-averaged Nusselt numbers for 8D hole spacing are generally higher than values for 12D hole spacing.