Liquid jet impingement is a very effective way of cooling of simple and complicated geometry objects. The attainable cooling rate is radically enhanced when using liquids as coolant due to the possibility of having boiling to occur during the impingement process. Bubble activity on the surface and the resulted mixing with the fluid bulk produces an additional factor of enhancement which at some levels of surface temperature dominates other convective mechanism due to the coolant flow perpendicular or parallel to the surface. The efficient nucleate boiling heat transfer regime can be divided into: partial nucleate boiling and fully developed nucleate boiling. The heat transfer capacity of each and the range of surface temperature over which each of these two boiling regimes up to the critical heat flux (CHF) are experimentally investigated in this research for different coolant temperature and velocity. For this purpose, single planar jet is used to provide the cooling medium of a flat surface that is being heated steadily. The boiling surface temperature was thus controlled by a feed back computer program to allow for steady state operation. So, at each level of boiling surface temperature observation of boiling mode and heat transfer mechanisms was elongated and verified. The experiments were conducted using degassed water jet velocity range between 0.75 and 1.7 m/s and degree of sub-cooling range from 10 to 28 °C at atmospheric pressure. The variation of the heat flux with those factors at different surface superheat up to the CHF point is presented. A physical interpretation is introduced to explain the effects of the input parameters on the heat transfer changes in these regimes.

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