# 3D Numerical Simulation and Performance Analysis of CO2 Vortex Tubes
## 3 Results and Discussion
### Effect of Inlet Pressure and Hot-End Tube Length on Temperature  
Figures 6 and 7 show the effect of inlet pressure and hot-end tube length on hot and cold outlet temperature. The inlet temperature was set at 277 K. As the inlet pressure rose, the cooling outlet temperature declined continuously, reaching a minimum temperature of 245 K at 1 MPa, with a larger temperature drop from 0.2 to 0.6 MPa, and gradually leveling off at 0.6 to 1.0 MPa. In the comparison of two swirl flow pipes with lengths 125 and 175 mm, the cold end temperature of the 175 mm pipe (with the longer hot-end tube) was lower than that of the 125 mm swirl flow pipe (with the shorter hot-end tube). The hot-end outlet temperature increased more than that of the cold end outlet temperature. The inlet pressure of the swirl flow pipe is the driving force that makes the internal airflows rotate and separate. The effect of the inlet pressure is greater than the effect of the tube length on the cold outlet of the swirl flow pipe. As shown in Figure 6, the cold outlet temperature of the swirl flow pipe (with length 175 mm) reaches the lowest temperature point of 244.5 K at 0.9 MPa. Therefore, it can be concluded that there will be a minimum cold outlet temperature as the pressure gradually increases. As can also be seen in Figure 6, when the length of 175 mm swirl flow pipe at 0.9 MPa, the hot-end temperature will also fluctuate, after which the hot-end temperature continues to rise as the pressure increases. separation of hot and cold streams in the swirl flow pipe are in the hot-end pipe area. The outer side of the tube wall distributes the hot airflow with high cyclonic velocity, and the core side distributes the cold airflow with low velocity. The inlet pressure affects the inlet velocity of the airflow, which in turn affects the separation of the hot and cold airflow in the swirl flow pipe. The current experimental research on swirl flow pipes is mainly reflected in the inlet pressure, and the temperature changes inside the swirl flow pipe are analyzed through numerical simulation results. The minimum temperature of the cold outlet of the swirl flow pipe appears to increase and then decreases with the increasing pressure of the inlet port. The simulation results show that when the compressed gas enters the swirl flow pipe at a pressure of 0.8 MPa, the minimum temperature of the 125 mm swirl flow pipe is 252.4 K, and the minimum temperature of the 175 mm swirl flow pipe is 246.0 K. At this time, the swirl flow pipe reaches the optimum heat balance, mainly because the inlet pressure rises and the tangential inlet velocity increases. The increased pressure gradient inside the tube enhances the shear work degree of the rotating flow from the inside to the outside and results in temperature reduction, but when the pressure increases to 1 MPa, the effect of vortex control of low temperature is decreased. The maximum temperature of the swirl flow pipe increases continuously with growing inlet pressure. The thermal temperature effect increases less when the inlet pressure is small, such as 0.2, 0.3, 0.4 MPa, at 0.6, 0.8, 1.0 MPa. The thermal temperature increase of the swirl flow pipe gradually increases; the 175 mm pipe increases more than that of the 125 mm pipe. Figure 9 shows the radial temperature distribution at the inlet of the swirl flow pipe. From the simulation results, it can be seen that the radial temperature is symmetrically distributed, with the higher temperature at the outer tube wall and the lower temperature at the inner axial region, which also confirms the existence of temperature differences between the internal and external fluids of the swirl flow pipe, which carry out energy transfers. The higher the inlet pressure, the higher the temperature difference. An increase in the length of the hot-end tube leads to an increase in the temperature of the flow outside the swirl flow pipe.