## 4 Results and Investigations
### Cooling Analysis Characteristics  
The hot plate cooling used a direct cooling method in which cooling holes are created and coolant is employed, as well as an indirect cooling method in which a heat pipe is used. First, the results of the direct cooling method are described as follows. Figure 10 shows the maximum temperature and minimum temperature of the hot plate during analysis of uni-directional cooling using nitrogen as a refrigerant, and Figure 11 shows the difference between the minimum and maximum temperatures. The analysis used the same 9 measuring points that were initially used. The analysis' fluid inlet speeds were 10, 20, and 30 m/s, and the UDF code was used to enter a fully developed flow profile of the inlets and outlets. As the refrigerant temperature increased, the maximum and minimum temperatures showed a decreasing trend, and the amount of heat transport increased, causing the cooled places to become cooler faster. As shown in Figure 10 there was a great temperature difference between the maximum and minimum, and then as time passed, the temperature difference showed a decreasing trend. Even when flow occurred for 300 s, the initial target cooling speed and temperature deviation were not achieved. In this study, it was found that stability is secured as the speed increases as a result of rapid stabilization in the temperature drop according to the inflow speed. Figure 9: Temperature distribution on the surface of hot plate with cooling holes at t = 200s after heating with PID control. Figure 12 shows the hot plate's maximum and minimum temperatures during analysis using uni-directional cooling and water as a refrigerant, and Figure 13 shows the differences between maximum and minimum temperatures. Using the same method as before, measurements were taken at 9 points, and fluid inlet speeds of 10, 20, and 30 m/s were used. When water is used as a refrigerant, its heat transport capacity is greater because the density of water is greater than that of nitrogen. Therefore, the temperature fell relatively rapidly within 5 s, and the temperature deviation occurred within 10 s. In the initial 5 s, the target values were achieved except when the temperature deviation was different than the target value. Figure 11: Variation of maximum temperature difference in Figure 10.  
Figure 10: Numerical results on the temporal variation of maximum and minimum temperature at the surface of hot plate after turning on uni-directional N\({}_{2}\) flow in the cooling holes. Figure 14 shows the hot plate's maximum and minimum temperatures as found by the computational analysis during bi-directional cooling using nitrogen as a refrigerant, and Figure 15 shows the differences between maximum and minimum temperatures. Using the same method as before, measurements were taken at 9 points, and fluid inlet speeds of 10, 20, and 30 m/s were used. The overall trends were similar to when uni-directional cooling was performed with nitrogen. However, it is difficult to make the judgment that relatively even cooling and fast cooling were achieved by performing bi-directional cooling. During cooling, a maximum temperature deviation of 4 degrees occurred, and a deviation of less than one degree was maintained after 300 s. Compared to uni-directional cooling, the temperature deviation was reduced. Figure 12: Numerical results on the temporal variation of maximum and minimum temperature at the surface of hot plate after turning on uni-directional water flow in the cooling holes. Figure 13: Variation of maximum temperature difference in Figure 12.  
Figure 16 shows the hot plate's maximum and minimum temperatures as found by the computational analysis during bi-directional cooling using water as a refrigerant, and Figure 17 shows the differences between the maximum and minimum temperatures. Using the same method as before, measurements were taken at 9 points, and fluid inlet speeds of 10, 20, and 30 m/s were used.