Influence of ratio of nozzle length to diameter on local heat transfer study of an unconfined circular air jet impingement

The influence of ratio of nozzle length to diameter on local heat transfer characteristics of an unconfined circular air jet impingement on flat plate is studied experimentally. The ratio of nozzle length to diameter (L/D = 0.5 to 83), nozzle to plate surface (z/D = 0.2 to 2) and Reynolds number (Re = 12,000 to 28,000) is investigated. The local heat transfer characteristics are reported at stagnation and wall jet region by using IR thermal imager and thin foil technique. Peak Nu occurs at wall jet area for z/D = 0.2 rather than at stagnation point for all L/D ratios and is substantially dependent on z/D and Re. It is caused by flow acceleration due to venacontraction effect between the nozzle and target surfaces which results in higher Nu in wall jet area than in stagnation area. For all Reynolds number, long pipe nozzle gives higher stagnation Nu value at z/D < 0.75 due to fully developed flow at nozzle outlet and L/D = 0.5 gives higher stagnation Nu value at z/D > 0.75 due to higher turbulence motion at nozzle outlet. At z/D < 0.4, L/D = 0.5 has a lower Nu at all L/D ratios in wall jet area (R/D > 2) and it decreases as Re increases. The influence of L/D ratio in wall jet zone (R/D > 1.8) is nearly comparable at all Re for z/D > 0.75. The L/D ratios are least influenced at lower Re and higher z/D value. The correlation of stagnation Nu was obtained with maximum deviation of +/- 16% to the maximum heat transfer rate for optimum conditions.