The choice of hole diameter in impingement cooling requires the number of holes to be specified and design information is provided for this purpose. The correlations for impingement cooling usually take geometry effects into account by using the pitch-to-diameter ratio (X/D) and this is independent of the number of holes and specified purely by the desired pressure loss at a given flow rate. Impingement heat transfer from a square array of holes was studied for a range of coolant flows G from 0.1 to 1.8 kg/sm2 at a fixed X/D of approximately 10. The number of holes per unit surface area N was varied by a factor of 70 at a constant gap-to-hole diameter ratio Z/D of 4.5 and constant gaps of 3 mm and 10 mm. It was shown that there was a range of N over which there was only a small influence on heat transfer at constant G. However, heat transfer fell at large N due to crossflow effects and at low N due to inadequate surface coverage of the impingement flow.

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