# 3D Numerical Simulation and Performance Analysis of CO2 Vortex Tubes
## 1 Introduction
Of course, in addition to the relatively large impact of the inlet pressure on the capability of the swirl flow pipe, the internal flow state of the swirl flow pipe can also be significantly changed by different compressed gases. [PERSON] [14] examined the effect of three test fluids, air, N\({}_{2}\), and CO\({}_{2}\), on the performance of the swirl flow pipe and found that the optimal thermodynamic performance of the swirl flow pipe occurred when the hot end of the tube was 175 mm and the diameter was 10 mm, and the cold end temperature of the swirl flow pipe strengthened with increasing inlet pressure. Finally, the best performance of the swirl flow pipe was found when using CO\({}_{2}\). However, the effects of swirl flow pipe length and inlet pressure on energy separation need to be studied more deeply through numerical simulation methods. [PERSON] [15; 16] built a 3D numerical model in the fluid simulation software Ansys 19.0 based on the physical swirl flow pipe dimensions, with CO\({}_{2}\) as the working fluid and the boundary conditions set to pressure inlet and pressure outlet. The simulation results showed that the temperature at the cold end of the swirl flow pipe decreased and that the temperature at the hot end increased with increasing inlet pressure. The above experiments and simulations demonstrated that the use of CO\({}_{2}\) was the optimal working fluid in the swirl flow pipe, but in the numerical simulations of Sorin, since there were only three pressure gradients (0.55 MPa, 0.85 MPa, 1.3 MPa), no clear conclusions were drawn as to how the pressure affected the performance of the swirl flow pipe. Another essential factor in evaluating the performance of a swirl flow pipe is its structural dimension parameters [17]. [PERSON] [18] showed the effect of swirl flow pipes with a tube diameter of 4 mm and tube lengths of 135 mm, 270 mm, and 350 mm on the internal fluid flow characteristics. The results showed that as the distance from the thermal outlet of the cyclone to the inlet increased, the return point of the cold flow was further away from the inlet, and the temperature difference between the hot and cold flows became larger resulting in a greater separation effect. [PERSON] [19] analyzed swirl flow pipes with a diameter (D) of 11.4 mm, cooling mass fraction of about \(\alpha=0.3\), tube lengths (L) of 91, 106, 120, 230, and 350 mm, and obtained the maximum temperature difference and optimal L/D ratio by experiments and computational fluid dynamics (CFD). The results showed that the best performance was obtained when the aspect ratio was 9.3 (L = 106 mm). From the above study, it can be concluded that the structural parameters of the pipe have an effect on the cold and heat separation, especially the length of the hot end of the tube. However, there is no specific mention in the literature as to what the reason for this phenomenon is, and it is necessary to continue to study the energy separation mechanisms of the swirl flow pipe by numerical simulations combined with experimental verification. In this study, the accuracy of the numerical simulation data was verified with the experimental results of [PERSON] [14]. To explore the effects of different aspect ratios and inlets on the flow state inside the swirl flow pipe, three-dimensional numerical simulations were carried out. CO\({}_{2}\) was chosen as the working medium, and the 3D numerical model was designed according to the real size. The flow law of cold and hot airflow separation in a swirl flow pipe was analyzed by axial velocity, radial velocity, axial pressure, radial pressure, and internal temperature distribution of the swirl flow pipe.