Two-phase closed thermosiphon is referred to as thermosiphon, also known as gravity heat pipe. Its structure is shown in Figure 1. Gravity heat pipes can be divided into condensation end, adiabatic end and evaporation end from the point of view of heat transfer. The liquid working medium is heated at the evaporating end, evaporates and evaporates through the adiabatic section and then enters the condensing end. In the condensing end, the temperature of the gaseous working medium decreases to release latent heat and form a liquid film on the tube wall. The liquid working medium at the condensation end returns to the evaporation end along the tube wall under the action of gravity, and so on. It can be seen in the cycle process that the gravity heat pipe mainly relies on the boiling evaporation of the working medium at the evaporation end and the exothermic condensation at the condensation end to achieve heat transfer. The same as the principle of ordinary heat pipes, the characteristics of gravity heat pipes are that there is no wick inside the heat pipe, and the condensate returns from the cooling end to the evaporating end without the capillary force generated by the wick, but through the gravity of the condensate itself, so the gravity type heat pipe is simple in structure, convenient in manufacture, low in price, and has good working stability.
Figure 1 - Gravity heat pipe
There are still some limitations in the application of gravity heat pipes at the current stage. Due to the directionality of gravity heat pipes, the evaporation end must be set below the condensation end, and the liquid working medium at the condensation end is used to flow back to the evaporation end along the inner wall of the heat pipe by its own gravity. In addition, the heat transfer effect of the gravity heat pipe is easily affected by the heat transfer limit of its own material, especially when the heat transfer exceeds the heat transfer limit that the heat pipe can withstand, which easily reduces the heat transfer performance of the heat pipe and shortens the service life of the heat pipe. The heat transfer limit of gravity heat pipe mainly includes drying limit, boiling limit and carrying limit. The dry-up limit generally occurs when the liquid filling amount is too small; the boiling limit is caused by the increase of heat flux density, which leads to supercooled boiling in the liquid pool; the carryover limit is mainly caused by the increasing shear force at the gas-liquid interface hindering the backflow of the condensed liquid with the increase of the heat flux density. In order to ensure that the working state of the heat pipe is within the range of these heat transfer limits, the currently generally used methods are as follows: concentrically placing an open-hole bubble suppressing tube in the evaporation end of the gravity heat pipe to inhibit the detachment of the bubbles in this section; an overflow concentric duct is set in the condensation end to reduce the condensation heat resistance of this section; the inner wall of the gravity heat pipe is processed into an axial channel surface to improve the heat transfer coefficient of the thermosiphon.
Zhang Guoqing and others conducted experimental research on the battery cooling system based on gravity heat pipe. The structure of the gravity heat pipe battery cooling system is shown in Figure 2. The system uses acetone as the working fluid, aluminum as the material for making the fins, and the working fluid pipeline and shell material are all copper. Due to the relatively compact structure of the designed system, Compared with the air cooling and liquid cooling solutions of the same battery module, this system has the characteristics of higher technical content, relatively complex process and manufacturing, etc., and the initial investment of the system is also comparable to the traditional air-cooled method, which has more advantages than the complex liquid-cooled system. Moreover, the system has the characteristics of high heat exchange efficiency, remarkable cooling effect and long service life, which can reduce the maintenance and replacement costs of users to a certain extent.
Figure 2 - Structure diagram of gravity heat pipe battery cooling system
As shown in Figure 3, a thermal management system using heat pipes is used. In this system, the evaporating end of the heat pipe is inserted into the bipolar plate of the fuel cell stack, the heat generated inside the battery is mainly transferred to the heat pipe through the pipe wall, and the working medium in the wick of the heat pipe absorbs the heat and evaporates, the evaporating gas flows to the other end due to the small pressure difference and condenses into a liquid to release heat, and the condensate flows back to the evaporating end of the heat pipe due to the gross attraction of the liquid wick. In this way, the working medium inside the heat pipe is circulated to achieve heat transfer, and finally achieve the purpose of lowering the temperature of the fuel cell. The heat transfer inside the heat pipe belongs to the phase change heat transfer of the working medium, and the heat transfer efficiency is high. Due to the small temperature difference in the steam, it has good isothermal properties, which avoids the problem of local high temperature inside the battery. Chinese scholar Sun Zhijian and others have conducted a series of studies on the gravity heat pipe in the battery cooling system, and found that under normal conditions, the bottom area of 9.64×10-4m2 gravity heat pipe battery cooling system can keep the surface temperature of the battery pack below 85 ℃ when the cooling power is 85W. This shows that the gravity heat pipe battery cooling system has good heat dissipation performance and can meet the heat dissipation requirements of the battery under the condition of high heat flux density. At the same time, Sun Zhijian et al. also studied the influence of the change of the inlet wind speed on the heat dissipation system of the gravity heat pipe battery. The results show that the gravity heat pipe battery cooling system can maintain the stability of the battery cooling system when the inlet wind speed and temperature change to a certain extent. This shows that the gravity-based battery cooling system has good heat dissipation performance and can meet the cooling requirements of the battery under the condition of high heat flux density.
Figure 3 - Heat pipe fuel cell management system