## 4 The Simulation Comparison of the Oval Discharge Port
The temperature attenuation distance of the thermal jet in the Z-Axis direction is called the floating height of the thermal jet, and the floating height of the thermal jet is far less than the temperature attenuation distance in the X-Axis direction. The initial buoyancy of the thermal jet is not obvious. When the temperature of the thermal jet drops to 300 K, the height of the thermal jet is 0.02 m. For every 1 K of temperature drop in the middle section of the thermal jet, the floating height increases by 0.0187 m. In addition, the floating height in the rear section of the thermal  
When the underwater vehicle thermal jet diffuses and floats to a certain height, it no longer rises, but the process of temperature transfer and attenuation still exists, which means that the thermal jet influences the temperature field of the free surface of the environmental water to a certain extent, thus leading to the temperature rise phenomenon of the free surface of the environmental water. Figure 8 shows the characteristics of the temperature changes caused by the thermal jet from the circle discharge port on the free surface of the environmental water. Figure 8: Surface temperature of the thermal jet from the circle discharge port. Figure 7: Temperature attenuation deviation characteristics of the thermal jet from the circle discharge port: (**a**) X-Axis; (**b**) Z-Axis. In Figure 8, the water surface forms part of the rising temperature area affected by the temperature of the thermal jet. The rising temperature area spreads around a certain point as the center. Due to the phenomenon of diverging thermal jets, there are two temperature centers in the rising temperature area of the water surface. The temperature diffusion area of the water surface is dominated by diffusion along the X-Axis. Table 2 lists the values of the relevant characteristics of the temperature diffusion area formed on the water surface by different radius ratios of oval discharge ports. The characteristic value of the circle (Str 4) discharge port is taken as the standard value. When the radius ratio of the discharge port is less than one, the maximum temperature of the water surface decreases, and the maximum decrease is 52.03%. Compared with the standard value, when the discharge port radius ratio changes, the maximum diffusion distance of the surface temperature diffusion region on the X-Axis changes little, and the maximum diffusion distance decreases by 25.82-34.38% on the Y-Axis. By changing the radius ratio of the discharge port, the maximum distance of the temperature diffusion on the Y-Axis in the rising temperature region formed by the thermal jet on the water surface is reduced, and the temperature diffusion trend of the thermal jet on the water surface is inhibited on the Y-Axis. Figure 9 shows the maximum temperature difference in the rising temperature region formed by the thermal jet on the water surface for different radius ratios of oval discharge ports. As can be seen, except for the Str 6 and 7 discharge ports, the maximum temperature difference is less than the standard value, effectively reducing the thermal jet surface temperature characteristics. The maximum temperature difference for the Str 1 discharge port is 0.071 K. With respect to the perimeter of the discharge port, the perimeter of the circle discharge port is 62.832 mm, whereas the perimeter of the Str 1 oval discharge port is 241.080 mm, which is about 3.5 times the length of the perimeter of the circle discharge port. In the case of the same discharge flow, changing the discharge radius ratio can effectively increase the mixing heat exchange interface between the thermal jet and the environmental water and can improve the mixing heat exchange effect.