# Experimental Study on Cooling Performance of a Hybrid Microchannel and Jet Impingement Heat Sink
## 4 Results and Discussion
### Cooling Performance of the Hybrid Microchannel Heat Sink  
In the closed-loop test system, a water-cooled condenser is used to continuously remove heat from the system, which keeps the inlet temperature of the heating module in a certain range of 3.6-4.8 \({}^{\circ}\)C. The initial operating pressure is 3.0 bar. There are three volumetric flow rates (0.86, 1.30 and 1.90 L/min). The range of heat flux is 50-400 W/cm\({}^{2}\)  
Figure 5: Temperature distribution of six measuring points. Figure 6: Comparison of the thermal resistance correlation with experiment data. Figures 7 and 8 show the temperatures of the heating wire and the outside wall and inside wall. It is shown that there is a temperature drop of about 20 \({}^{\circ}\)C for larger mass fluxes. \(T_{ow}\) is 82.7 \({}^{\circ}\)C at 300 W/cm\({}^{2}\) and 1.90 L/min whereas the one is 99.9 \({}^{\circ}\)C at 300 W/cm\({}^{2}\) and 0.86 L/min. The heat transfer characteristics of the test module are investigated by analyzing the temperature distribution of the heating wire and the outer and inner walls. The temperature rise between the heating wire and the outside wall accounts for most of the overall temperature increase, up to 80%. It is indicated that thermal resistances of silicone grease and AlN substrates affect the cooling capacity of the test module significantly. The cooling performance is greatly improved when the volumetric flow rate is increased to 1.90 L/min. \(T_{iw}\) is 60.2 \({}^{\circ}\)C at 300 W/cm\({}^{2}\) whereas the one of 0.86 L/min is 77.5 \({}^{\circ}\)C at 300 W/cm\({}^{2}\). The temperature of outlet R134a is shown in Figure 9. When heat flux is increased from 50 W/cm\({}^{2}\) to 300 W/cm\({}^{2}\), the temperature rise is about 8 \({}^{\circ}\)C. The temperature of R134a increases from 4.2 \({}^{\circ}\)C to 16.3 \({}^{\circ}\)C when R134  
Figure 10 shows the outlet vapor quality at different heat fluxes. At 0.86, 1.30 and 1.90 L/min, the two-phase flow appears with heat fluxes of 100, 150 and 250 W/cm\({}^{2}\), respectively. The outlet vapor quality of 0.86 L/min is 0.12 at 350 W/cm\({}^{2}\) heat flux whereas the one of 1.90 L/min is 0.015. It is known that the cooling capacity is enhanced with the appearance of a two-phase flow. The thermal resistance for evaluating the cooling capacity of the heat sink is shown in Figure 11. There is a valley of thermal resistance at different heat fluxes, which represents the appearance of a two-phase flow. The cooling capacity is impaired when the heat flux increases continuously in the two-phase state. Therefore, there is an optimal heat flux at a certain flow rate at which the thermal resistance of the heat sink reaches the minimum. The thermal resistances are 0.15, 0.13 and 0.11 K/W for the cases of 0.86, 1.30 and 1.90 L/min, respectively. In addition to the thermal resistance of the heat sink, the thermal resistance of the test module also includes thermal interfacial resistance (AlN substrate and silicon grease). The thermal resistance distributions of the heat sink, AlN substrate and silicone grease are shown in Figure 12. The thermal resistances of the AlN substrate and silicone grease are almost always 0.14 K/W under different conditions, which accounts for 30-50% of the overall thermal resistance. It can be seen that the AlN  
Figure 8: The temperatures of the heat sink at different heat fluxes: (**a**) outside wall and (**b**) inside wall. Figure 9: The temperatures of outlet R134a. substrate and silicone grease have a great influence on the cooling performance of the test module. Improper handling of the thermal interfacial resistance between the heat sink and the heating device can greatly reduce the performance of the heat sink. High thermal conductivity materials and great packaging processes are the key points to improve the performance of the heat sink. The pressure drop is another important parameter of microchannel heat sinks.