Liquid water jet impingement cooling was investigated experimentally for both free-surface jet arrays and confined submerged jet arrays. The jet arrays consisted of straight holes of 1.0 mm diameter arranged in rectangular arrays with spacings of 3, 5 and 7 jet diameters between adjacent jets. For the impingement surface area of 780 mm2, these jet array configurations can be considered well populated, with a total of 21, 45 and 121 jets impinging on the surface. Average heat transfer and pressure drop measurements are presented for volumetric flow rates in the range of and dimensionless jet-to-target spacings between 2 ⩽ H/dn ⩽ 30. For the submerged jet arrays a strong dependence on both jet-to-target and jet-to-jet spacing is observed and correlations are presented that adequately predict the experimental measurements. The free-surface jets show a non-monotonic change with jet-to-target spacing with a local minimum in the heat transfer coefficient at approximately H/dn = 10. Here a transition from a submerged to a free jet flow configuration occurs. Once again, a correlating equation is presented that adequately predicts the free-surface jet array heat transfer data. The pumping power required to form the submerged and free jet flows show a different relationship to the heat transfer coefficient. Generally, submerged jets have a higher heat transfer coefficient for a given pumping power requirement.
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