Continued power increases in devices, such as processors and IGBTs are requiring high capacity cooling methods to remove excess heat. One such method is the jet impingement of a liquid or gas onto a surface on a continuous basis. This mode of heat transfer has been tested extensively for many years and is still an ongoing pursuit. Some issues, among them noise reduction, are still being improved.
Impingement jets can either be air-powered or use some form of liquid, typically water. High speed jet impingement on a component surface creates a thin boundary layer, and thus a high heat transfer equation. There are three common jet configurations: the free-surface jet, which uses dense liquid in a medium that is less dense, such as air; the submerged jet, which allows the fluid to impinge in the same medium fluid; and the confined submerged jet, which is shown in Figure 1.
It has been shown that the submerged jet has higher a heat transfer coefficient than the free-surface jet for Reynolds numbers greater than 4000.  Other research shows that the confining wall can reduce the heat transfer coefficient due to the circulation regions between the top plate and the bottom surface. 
Determining heat transfer through jet impingement is very complicated, and depends on many factors. Among the most critical are the Reynolds number, the Prandtl number, jet diameter, and wall-to-nozzle spacing.
It has been shown that for the same Reynolds number, decreasing the jet diameter will increase the heat transfer coefficient due to the higher speed. For a constant diameter jet, the heat transfer coefficient is a function of . For certain values of jet distance to jet diameter, reducing the distance does not make an appreciable difference in the heat transfer. This is because the potential core is very close to the surface.
In part 2 we’ll cover jet shape and it’s effect on cooling. To reach part 2 click to What is Jet Impingement Cooling and How is it applied for Thermal Management of Electronics (Part 2 of 2)
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- Womac, D., Ramadhyani, S., Incropera, F., Correlating equations for impingment cooling of small heat sources with single circular jets, Transactions of the ASME, Vol. 115, PP 106-115, 1993.
- Fitzgerald, J., Garimella, S., Flow field effects on heat transfer in confined jet impingement, Transactions of the ASME, Vol. 119, pp. 630-632, 1997.