Jet Shape and its effect on cooling
Jets are deployed in different shapes that impact the eventual heat transfer from the impinged jet surfaces. These shapes include round and square. There can be a single jet or multiple smaller jets where the data has shown to have better performance than a single jet. Figure 2 shows the temperature gradient in a jet impingement application.  The jet was seeded with liquid crystal to show the thermal transport, its interaction, and the temperature gradient.
The heat transfer coefficient is maximized at the center, and the distribution is shown in Figure 3. 
Figure 4 shows the local heat transfer coefficient for air jet impingement as a function of dimensionless distance for a jet diameter of 1 mm, Re = 10,000 at different H/d. It is seen that at H/d = 1, H is higher than the other two values. In this experiment, an H = 2,500 W/m2K was achieved.
Figure 5 shows the heat transfer coefficient for a water jet, as reported by Garimella  that has attained a value of 60,000 for confined submerged jets. The x axis is the ratio of jet to target spacing to nozzle diameter. The top curve was for 4.6 m/sec (10 gram/sec), the middle curve was for 6.9 m/sec, and the bottom was for 9.2 m/sec.
Recently, Motakef, et al, described the achievement of a heat transfer coefficient of 500,000 with a water microjet, and 20,000 with an air microjet.  In their design, they manufactured a 3-D structure with hundreds of microjets that were 300 microns size. The jets are kept at a distance of a few hundred microns from the surface. The special manifold design allows the returned flow to exhaust without interfering with the main jets. This design significantly increases the heat transfer coefficient. Without the manifolding the heat transfer coefficient degrades to that of a macro-jet. Figure 6 shows a sample of this honeycomb structure.
Even though a very high heat transfer coefficient can be achieved using jet impingement, the packaging of such a system is very challenging. The following must be carefully considered and studied:
- What type of fluid should be used? Is it air or liquid? If it is air, what is the noise implication? If it is liquid, how will the liquid be drawn out of the system without damaging the electronics parts if there is any leakage?
- What type of compressor is needed to generate the high speed jet? Is its size practical for commercial use? What is its life span?
- What type of filter should be used to prevent the nozzle from clogging? What is the effect of the filter on the fluid line pressure drop? What would be the impact of semi-clogged nozzles on the pressure drop?
- What is the cost of such a system, and does it justify its application for that specific application?
To reach part 1 of this series click to: What is Jet Impingement Cooling and How is it applied for Thermal Management of Electronics (Part 1 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.
- Ashforth-Frost, S., Ridel, U., Thermal and hydrodynamic visualization of a water jet impinging on a flat surface using microencapsulated liquid crystals, International Journal of Fluid Dynamics, Vol 7, Article, 1-7, 2002.
- Glynn, C., ODonovan, T. and Murray, D., Jet impingement cooling, Department of Mechanical and Manufacturing Engineering, Trinity College, Dublin.
- Motakef, S., Overholt, M., Micro-fabricated solutions to management of high heat flux systems, CapeSym, Inc., Natick, Massachusetts.