# Structural Designing of Supersonic Swirling Devices Based on Computational Fluid Dynamics Theory
## 1 Introduction  
Liquefied natural gas (LNG) is a liquid product formed by the separation of preprocessed, qualified natural gas after compression and cooling to its boiling point. LNG is an advanced clean energy source, and under standard conditions, its volume is approximately \(1/625\) times that of the same amount of gaseous natural gas [1, 2]. Supersonic swirl separation is the most widely used technique for the purification and liquefaction of natural gas [3, 4, 5, 6]. A typical supersonic swirling separator consists of swirling vanes, a supersonic nozzle, a swirling separation section, and a diffuser, as shown in Figure 1[7]. This technique combines the expansion and cooling of gases to perform centrifugal separation in a single compact device. The technique is highly advantageous because of its small footprint, simple structure, high efficiency, low energy consumption, environmental friendliness, and absence of any rotating part in the device. Thus, it has significant applicationprospects [8]. The liquefaction efficiency of supersonic cyclonic separators is closely related to their structural designs. At present, the structural design of supersonic swirling separators mainly focuses on the profile, location, and structure of the swirling generators and the cross-section of the flow channels. The nozzle design consists of four parts: the inlet stabilization section, de Laval nozzle, flow-conditioning straight tube section, and diffuser. The de Laval nozzle consists of three regions: convergent section, throat, and divergent section. Commonly used methods to design the convergent section include the Witoszynski curve, Batchelor-Shaw curve, bicubic curve, quintic curve, and area ratio methods [9]. In the design of supersonic separator tubes, the Witoszynski curve and area ratio are the most frequently used methods, as they yield good results. The commonly used methods to design the divergent section include the characteristic line method, Foelsch method, Lao method, area ratio method, and equal-slope method. Because the area ratio method is based on gas dynamics theory, it has good application performance; therefore, it is a widely used method in the design of divergent sections. The equal-slope method is another widely used method because its design and calculation are simple, and profiles are easy to fabricate and have good practical application performance. [PERSON] et al. [10] studied the relationship between the length and the outlet diameter of the divergent section, and found that the separation tube should be installed at the back of the de Laval tube. The diffuser compresses the remaining natural gas that has not been liquefied by increasing its pressure and temperature and transports it into the natural gas pipeline. Based on both simulation and experimental studies conducted on the above design methods, researchers have shown that the Witoszynski curve design is the most suitable for convergent sections, and both the area ratio and equal-slope methods are suitable for divergent sections [11]. In addition, it was found that the geometric design of each tube section had a significant effect on the cooling and separation effectiveness of the device, and that the system was required to be designed based on parameters such as the flow, pressure, and temperature of the object to be processed. However, owing to the assumptions and empirical values associated with the design methods currently used, there is a difference between the actual operational performance and the design performance of supersonic swirling separators. Therefore, a design method that is more accurate is required. With the rapid development of computer technology, the field of rotational flow mechanics has achieved significant progress and maturity. Innovations in computer technology have not only provided more powerful computational and analytical tools for the study of rotational flow mechanics but have also driven comprehensive development in the areas of theory, experimentation, and application within this field [12; 13].