## 4 Results and Discussion  
### Simulation Results  
Figure 6 shows the results of the 9 simulation groups listed in Table 4. Among them, the oil flow rates for groups A, B, and C were 5 L/min, and the water flow rates were 5, 10, and 15 L/min, respectively. In groups D, E, and F, the oil flow rates were 10 L/min, and the water flow rates were 5, 10, and 15 L/min, respectively. Groups G, H, and I had an oil flow rate of 15 L/min, and water flow rates of 5, 10, and 15 L/min, respectively. The range of 5-15L/min covers the working range of the flow rate for the plate-fin heat exchanger used in this study. For heat exchangers, the higher the heat transfer quantity, the better the performance of the heat exchanger. The simulation results of heat transfer quantity for the heat exchanger are shown in Figure 6a. As the fin angle decreased, the amount of heat transfer quantity decreased. When the fin angle was reduced from 90\({}^{\circ}\) to 60\({}^{\circ}\), the heat transfer quantity is almost unchanged for the same flow rate. When the fin angle was reduced to 45\({}^{\circ}\), the heat transfer quantity decreased compared to the large angle fins at the same flow rate. When the fin angle was further reduced to 30\({}^{\circ}\), the heat transfer quantity decreased significantly. Overall, the heat transfer quantity showed a rising trend with the increase of the flow rate on the oil side and water side. When the oil flow rate was less than or equal to 10 L/min, it is obvious that the oil flow rate had more influence on the heat transfer quantity than the water flow rate. The heat transfer rate showed a significant increase with the increase of the oil flow rate (group C-D). However, when the oil flow rate increased to 15 L/min, the increase of the water flow rate also caused a dramatic change of heat transfer quantity (Group G-I). That is, when the oil flow rate was less than or equal to 10 L/min, the change of heat transfer mainly depended on the oil flow rate. Additionally, when the oil flow rate was more than 10 L/min, the change of heat transfer mainly depended on the flow rate on the water side. Additionally, the larger the fin angle was, the more obvious this law was. Figure 6: Simulation result, inlet flow on oil and water sides corresponding to simulation group numbers A to I is listed in Table 3: (**a**) heat transfer quantity of the heat exchangers, (**b**) oil outlet temperature, (**c**) oil pressure drop, (**d**) water pressure drop. The closer the oil outlet temperature was to the water inlet temperature (65 \({}^{\circ}\)C), the better the performance of the heat exchanger. The simulation results of the oil outlet temperature for the heat exchanger are shown in Figure 6b. As the fin angle gradually decreased, the oil outlet temperature gradually increased at the same flow rate. When the fin angle was less than 45\({}^{\circ}\), the trend of increasing oil outlet temperature would be very obvious. It is important to note that a higher heat transfer quantity did not mean a lower oil outlet temperature (groups C-D, F-G). In fact, there was no inevitable relationship between the oil outlet temperature and the heat transfer quantity, and almost the same oil-side outlet temperature may correspond to a very different heat transfer quantity (groups B, E, I). The lower the pressure drop in the heat exchanger, the lower the cost of the oil pump and motor. The simulation results of the oil pressure drop and the water pressure drop are shown in Figure 6c, respectively. It was obvious that the flow rate is directly proportional to the pressure drop. The oil flow rate had a significant effect on the oil pressure drop, while the water flow rate had almost no effect on the oil pressure drop. The same was true for the water pressure drop, which was almost independent of the change in oil flow rate. This is because the pressure drop is not transferred between the oil and water sides like heat. At the same time, the oil pressure drop was much higher than that of the water side at the same flow rate.