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1. Heat pipe performance requirements
As an energy storage device element, power battery is one of the core components of electric vehicles. The power battery will generate a lot of heat during the charging and discharging process, which will cause the local temperature of the battery to be too high, which will affect the battery performance and even lead to a sharp reduction in the battery life. Therefore, the optimal design of the thermal management system of the power battery is the guarantee for the good operation of the battery, which greatly affects the development and popularization of the power battery. Conventional heat dissipation methods (such as natural air cooling and forced air cooling methods) have been unable to meet the needs of high heat flow density heat dissipation of power batteries. It is necessary to develop new cooling methods to meet the requirements of high heat flow density heat dissipation of battery modules. The development of liquid cooling technology is very effective for the heat dissipation of the battery, which can quickly transfer most of the heat generated by the battery to the outside world. The development and application of heat pipe technology solves problems such as difficulty in heat dissipation in power battery systems. The heat pipe cooling system is a heat conductor that relies on the phase change of the working liquid in the heat pipe for efficient heat transfer. Heat pipes have become one of the important technologies for efficient heat dissipation of electronic and electrical equipment due to their good heat flux density variability, thermal conductivity, excellent constant temperature characteristics and environmental adaptability. The heat pipe is a heat transfer element that realizes heat transfer by the phase change of its internal working liquid. It has the following basic characteristics:
(1) Good thermal conductivity. The heat transfer inside the heat pipe mainly depends on the gas and liquid phase change of the working liquid, the internal thermal resistance is very small, and it has good thermal conductivity.
(2) Good isothermal performance. The steam inside the heat pipe is in a saturated state, and the saturated temperature determines the pressure of the saturated steam. The pressure drop generated by the saturated steam flowing from the evaporation stage to the condensation stage is very small, and the temperature drop is relatively small, so the heat pipe has good isothermal performance.
(3) Heat flux density variability. The heating area of the evaporation end and the cooling end can be determined according to the actual situation, so the input heat flux density to the heat pipe can be changed. For example, when the heat pipe inputs heat in a relatively small heating area, it can output heat in a larger cooling area, and vice versa, which changes the heat flux density and makes the heat pipe more flexible in application.
(4) The reversibility of the direction of heat flow. For a horizontally placed cored heat pipe, since the internal circulation power is capillary force, either end of the heat pipe can be used as the evaporation end or the condensation end, and the two ends can be interchanged, so the flow direction can be interchanged.
(5) Thermal diode and thermal switch performance. Heat pipes can be made into thermal diodes or thermal switches. The thermal diode only allows the heat flow to flow in one direction, and the heat only conducts heat conduction in one direction, and is not allowed to flow in the opposite direction. If the heat flow flows in the opposite direction, its thermal conductivity will be seriously reduced; the thermal switch is that when the temperature of the heat source is higher than a certain temperature, the heat pipe starts to work, and when the temperature of the heat source is lower than this temperature, the heat pipe does not transfer heat.
(6) Constant temperature characteristics. A heat pipe with constant temperature characteristics is a controllable heat pipe. Usually, the thermal resistance inside the heat pipe does not change with the increase of heat. Therefore, when the heat input by the heating pipe changes, the temperature of the heat pipe will also change. With the development of science and technology, people have designed controllable heat pipes, which can make the thermal resistance of the condensing end change with the change of heat. When the heat increases, the thermal resistance of the condensing end will decrease; conversely, as the heat decreases, the thermal resistance on the condensing end increases. In this way, when the heat of the heat pipe changes, the temperature of the steam changes very little, so as to control the temperature.
(7) Adaptability to the environment. The shape of the heat pipe can be changed with the shape of the external equipment, and its heat transfer performance will not change greatly. The heat pipe can set the evaporation end and the cooling end according to the specific environment. The environmental adaptability of the heat pipe can make it applicable to many fields, and it has been used in space equipment for a long time.
2. Compatibility and service life of heat pipes
The compatibility of the heat pipe means that within the expected working life of the heat pipe, there is no significant physical and chemical reaction between the working liquid in the pipe and the shell, or there are slight changes but no effect on the working performance of the heat pipe. Ensuring the good compatibility between the heat pipe shell and the working liquid is one of the important performance indicators of the heat pipe. Only heat pipes with good compatibility can guarantee stable heat transfer performance, long working life and the possibility of being used in certain industries. Carbon steel water heat pipe solves the problem of heat pipe compatibility through chemical treatment, which makes this high-performance, long-life and low-cost heat pipe widely popularized and used in the industry.
There are many factors that affect the life of the heat pipe. In conclusion, the main forms of incompatibility of the heat pipe are the following three aspects, namely the generation of non-condensable gas, the deterioration of the thermal properties of the working liquid, and the corrosion and dissolution of the shell material.
(1) Produce non-condensable gas. Due to the chemical reaction or electrochemical reaction between the shell material of the heat pipe and the working fluid in the tube, a gas that is insoluble in the working liquid is produced. At this time, when the heat pipe is working, the non-condensable gas flows to the condensation end together with the gas at the evaporation end and accumulates and does not liquefy at the condensation end, resulting in a decrease in the effective heat transfer area at the condensation end of the heat pipe, an increase in thermal resistance, and a sharp decrease in heat transfer performance.
(2) The thermal properties of the working fluid deteriorate. The deterioration of the thermal properties of the working liquid means that some working fluids will gradually decompose at a certain temperature and generate new chemical substances, which chemically react with the shell material of the heat pipe, causing the shell to dissolve and become thinner. Such substances are mostly organic materials, such as toluene and hydrocarbons. The deterioration of the thermal properties of the working liquid will seriously affect the heat transfer performance of the heat pipe.
(3) Corrosion and dissolution of the shell material. The corrosion and dissolution of the shell and tube materials are caused by the flow of the working medium and the uneven temperature distribution of the heat pipe when the heat pipe is working. The corrosion of the tube and shell material will cause a series of problems such as the deterioration of the heat transfer performance of the heat pipe, the increase in the flow resistance of the working medium, and the shortening of the service life. Corrosion and dissolution of tube and shell materials are prone to occur in high-temperature heat pipes made of alkali metals.
3. Working conditions of heat pipes
The heat pipe needs to meet certain conditions during normal operation. Figure 1 shows the shape of the vapor-liquid interface inside the heat pipe, the steam mass flow, pressure, and the relationship between the tube wall temperature Twc and the steam temperature Tv in the tube with time. From the evaporation end to the condensation end of the heat pipe, the static pressure difference between the vapor phase and the liquid phase at the vapor-liquid interface maintains a linear relationship with the capillary pressure difference there.
The necessary condition for the heat pipe to work is
ΔPc≥ΔPl+ΔPv+ΔPg
Among them, ΔPc is the driving force of the working liquid flow inside the heat pipe, used to overcome the vapor pressure drop ΔPv from the evaporation end to the condensation end in the heat pipe, the pressure drop ΔPl of the condensed liquid flowing back from the condensation end to the evaporation end and the pressure drop ΔPg of the internal fluid caused by gravity. The specific situation of ΔPg depends on the working environment of the heat pipe, which may be a positive value or a negative value.
Although the heat transfer ability of the heat pipe is relatively strong in reality, it does not mean that the heat pipe can increase its heat load infinitely, and many factors restrict the heat transfer rate of the heat pipe. When the heat pipe reaches its limit, the heat transfer cannot continue to increase. For example, when the heat pipe reaches a certain limit, the evaporative end of the heat pipe dries up and overheats, and the circulation of the working fluid is interrupted. And for some other limit, when it reaches the limit, the flow rate of steam will not increase unless the operating temperature is changed. The limits that limit the heat transfer of heat pipes include capillary force limit, sound speed limit, carrying limit, boiling limit and fluid viscosity limit. These heat transfer limits depend on the shape and size of the heat pipe, the structure of the internal wick, the working medium and the working environment.