To improve the heat transfer coefficient, we mainly consider enhancing the heat transfer coefficients of the tube side and shell side. Many researchers have conducted extensive research in this area and achieved significant results. This paper mainly discusses the enhanced heat transfer of the tube side in shell-and-tube heat exchangers — changing tube geometry or inserting objects inside tubes, introducing research progress of various enhanced heat transfer tubes such as spirally fluted tubes, transversely corrugated tubes, spiral flat tubes, tube inserts, finned tubes, converging-diverging tubes, and three-dimensional internally ribbed tubes.
1. Spirally Fluted Tube
Spirally fluted tubes are special tubes with convex and concave features on the tube wall. The spiral grooves on the tube wall can significantly improve the heat transfer coefficient inside and outside the tube in both phase-change and non-phase-change heat transfer, playing a role in bilateral enhancement. According to the types of spiral grooves processed on the outer surface of the plain tube, spirally fluted tubes are divided into single-start and multi-start types, with main structural parameters including groove depth e, groove pitch p, and groove helix angle β. From 1970 to 1980, the United States, the United Kingdom, and Japan conducted extensive research on spirally fluted tubes[1]. South China University of Technology, Beijing Institute of Technology, and Chongqing University also conducted experimental studies on spirally fluted tubes, all achieving significant results. Moreover, research has shown that single-start spirally fluted tubes perform better than multi-start types. Currently, whether in terms of heat transfer, flow resistance, fouling performance, or in convective heat transfer without phase change and condensation heat transfer with phase change, research on the enhanced heat transfer of spirally fluted tubes has reached a relatively high level from theory to practice. Further combined with the development of computer hardware and software, the simulation of heat transfer of spirally fluted tubes in different applications to find general correlations and optimize the structural dimensions of spirally fluted tubes will be the direction of future research.
2. Transversely Corrugated Tube
In 1974, the former Soviet Union first proposed the transversely corrugated tube. It is a type of tube made from an ordinary round tube blank, with grooves rolled on the outer wall perpendicular to the axis, while forming a circle of raised ring ribs inside the tube. The enhancement mechanism is: when the fluid inside the tube passes through the transverse ring ribs, axial vortices form near the tube wall, increasing the disturbance of the boundary layer, which is conducive to heat transfer through the boundary layer. When the vortex is about to dissipate, the fluid flows through the next transverse rib, continuously generating axial vortices, thus maintaining a continuous and stable enhancement effect. Transversely corrugated tubes are mainly used to enhance the heat transfer of single-phase fluid inside the tube. Research by South China University of Technology found that at the same flow velocity, the flow resistance of transversely corrugated tubes is smaller than that of single-start spirally fluted tubes.
3. Spiral Flat Tube
The spiral flat tube is a type of heat exchange tube first proposed by the Swiss company Allares and later improved by the American company Brown. Due to the unique structure of the tube, the fluid inside the tube is in a spiral flow, promoting the degree of turbulence. Liang Longhu[3] conducted experimental research, showing that the film heat transfer coefficient inside the spiral flat tube is usually significantly higher than that of an ordinary round tube, being more obvious at low Reynolds numbers, reaching 2-3 times; as the Reynolds number increases, the heat transfer coefficient can usually be increased by more than 50%.
4. Tube Inserts
There are many types of tube inserts, mainly including: twisted tapes, helical coils, helical strips, helical fins, twisted tapes, and static mixers. The enhanced heat transfer mechanisms of various inserts can generally be divided into the following four types:
(1) Forming swirling flow;
(2) Disrupting the boundary layer;
(3) Displacement effect between the central fluid and the fluid near the tube wall;
(4) Generating secondary flow. The advantage of tube inserts is that they can be used to retrofit old heat exchanger equipment to improve their heat transfer performance. While enhancing heat transfer, they can also achieve the purpose of removing fouling. The British company CalGavin developed an insert called Heatex, which consists of a group of circular elements extending to the tube wall, which can improve the tube-side heat transfer efficiency by 2-15 times[4]. The company also developed a garland-type insert called Hitran Matrix Elements, which can greatly increase the heat transfer coefficient without increasing the pressure drop. For liquid applications, it can increase the tube-side heat transfer efficiency of a shell-and-tube heat exchanger by 25 times; for gas applications, it can increase the corresponding value by 5 times. In addition, compared with normal flow velocity, this insert increases the anti-fouling ability of the heat exchange tube by 8-10 times[5].
5. Internally Finned Tube
Internally finned tubes are processed using special welding processes and equipment, and the heat transfer process of the fluid inside the tube is single-phase forced convection heat transfer. Its main characteristic is to improve the heat transfer performance of the heat exchanger by expanding the heat transfer area inside the heat transfer tube and enhancing the heat transfer inside the tube. In 1971, the United States first proposed internally finned tubes, and in the 1990s, developed an internally helically finned tube for efficiently enhancing phase-change heat transfer inside the tube. Japan, the former Soviet Union, and other countries also conducted extensive research work. In the early 1980s, research by Hitachi Cable Co., Ltd. showed that using left-right staggered helical internally finned tubes to enhance the heat transfer of single-phase fluid can increase the internal heat transfer coefficient to about 2.8 times that of a plain tube[6].
6. Converging-Diverging Tube
The converging-diverging tube is composed of alternating multiple converging sections and diverging sections. The fluid is caused to become turbulent under the action of this tube structure, thereby improving heat transfer efficiency. Research on the application of converging-diverging tubes to single-phase flow has been extensively conducted. South China University of Technology proposed a modified converging-diverging tube, which reduces the expansion section in each converging-diverging unit to a very small size and connects them using convex arcs, concave arcs, and straight lines. At the same time, experimental research on the natural convection boiling heat transfer characteristics of this improved tube was also conducted, showing that the natural convection boiling heat transfer performance of the improved converging-diverging tube is superior to that of the ordinary converging-diverging tube[7]. Chen Ying[8,9] conducted experiments and simulation calculations, showing that this improved converging-diverging tube has a good enhanced heat transfer effect.
7. Three-Dimensional Internally Ribbed Tube
The three-dimensional internally ribbed tube is an efficient enhanced heat transfer element processed on the inner wall of an ordinary round tube using special tools and certain methods. The fluid inside the tube is subjected to the action of three-dimensional ribs, which reduces the thickness of the thermal boundary layer, thereby improving the convective heat transfer film coefficient. In certain flue gas tube convective heat transfer applications, the three-dimensional internally ribbed tube has a unique self-cleaning function. Li Qingfang[10] found through experiments that the convective heat transfer coefficient between the flue gas and the three-dimensional internally ribbed tube can reach 3.2 times that of a plain tube, which is better than other enhanced tubes such as threaded tubes.
