As shown in Figure 1, the sintered heat pipe is composed of a tube shell, a liquid wick and an end cap. Pump the inside of the tube to a negative pressure of 1.3×10-1~1.3×10-4Pa and then fill it with an appropriate amount of working liquid, so that the capillary porous material of the wick close to the inner wall of the tube is filled with liquid and then sealed. When the evaporating end is heated, the liquid in the capillary wick evaporates and vaporizes, and the vapor flows to the condensing end under a small pressure difference to release heat to form a liquid, and the liquid flows back to the evaporating end along the porous material by capillary force, and in this way, the heat is transferred from one end of the micro heat pipe to the other end. During this cycle, the working medium transmits a large amount of heat, and its heat transfer efficiency is hundreds or even thousands of times that of the copper rod.
Figure 1 - Schematic diagram of the structure of the sintered heat pipe
Because the sintered heat pipe adopts the liquid-absorbing core structure, the process of returning the liquid working medium to the evaporation end is completed under the capillary force, without relying on the action of gravity. This avoids the problem that the liquid working medium of the heat pipe is difficult to return to the evaporation end under microgravity conditions. After the liquid absorbing core structure is adopted, the circulation process of the liquid working medium is faster, which is conducive to the rapid transfer and diffusion of heat, and improves the heat transfer efficiency of the heat pipe. In recent years, with the continuous deepening of research and the continuous improvement of the processing level of sintered heat pipes, sintered heat pipes have been well used in many extreme environmental conditions.
At present, the main problem faced by sintered heat pipes is that the structure and materials of the heat pipes are difficult to meet the needs of high heat flux environment conditions, especially in the heat transfer process of sintered heat pipes, the liquid working medium in the evaporation end boils and vaporizes, the thickness of the vacuum chamber increases, the working mass on the upper part of the liquid absorbing core decreases, and local drying occurs, the actual phase change heat transfer area decreases, and the thermal resistance of the evaporation end increases. When the working quality is large, the liquid absorbent core is filled with liquid working medium, which effectively slows down the local drying phenomenon. The temperature difference at the condensing end is very different under different working masses: when the working mass is less, the input power and the thickness of the vacuum chamber have little effect on the temperature difference at the condensing end; when there are more working masses, the temperature difference of the condensation end increases with the decrease of the thickness of the vacuum chamber. Therefore, rationally arranging the structure of the heat pipe and selecting an appropriate amount of work quality to improve the heat transfer efficiency of the heat pipe are still the focus of future research.
The schematic diagram of the battery cooling system based on the flat sintered heat pipe is shown in Figure 2. By building an experimental platform, the heat transfer and distribution in the battery are studied. Four copper heat pipes are evenly distributed on the surfaces of two adjacent batteries, and their evaporation ends are bonded to the batteries by thermally conductive silica gel (ZC-801) to reduce the contact thermal resistance between them. Among them, T1~T22 are thermocouple temperature measurement points, which are used to measure the temperature of different positions in the heat pipe battery cooling system. In order to increase the contact area between the heat pipe and the battery surface, the evaporation end of the heat pipe is generally processed into a flat shape, and water is used as the working medium in the pipe. The condensing end of the heat pipe is cooled by a constant temperature water tank. Through experiments, it is found that the local temperature difference of the battery increases with the decrease of the inclination angle of the heat pipe. When the heat pipe is installed vertically, the local temperature difference of the battery is not greatly affected by the slope of the road surface. Even under variable power and periodic operating conditions, the heat dissipation of the heat pipe can still maintain the uniformity of the battery heat distribution. In addition, the use of sintered heat pipes as the heat dissipation system of the power battery can adapt to the operation of the power battery pack under the condition of high heat flux density under high current conditions. At this time, the heat distribution inside the battery can still maintain a certain uniformity and stability. The sintered heat pipe has many of the above advantages, which make it play an increasingly important role in the thermal management system of the hybrid electric vehicle battery.
Figure 2 - Schematic diagram of the experimental setup of the square battery cooling system based on the flat sintered heat pipe
As shown in Figure 3, for the square battery with different heating power, the heat pipe is placed vertically, the change of the heat pipe, the water inlet and outlet and the ambient temperature with time. It can be seen from the experimental results that with the continuous increase of the heating power, the temperature of the evaporating end and the condensing end of the heat pipe is constantly rising, and the heat pipe has started to start at about 30℃. The temperature difference between the heat pipe temperatures T11 and T21 on the left and right sides also increases with the increase of heating power; the main reason for this may be that the degree of vacuum is not absolutely the same during heat pipe processing. As the heating power increases, the difference in steam pressure in the pipe gradually increases. In addition, since the cooling water enters from the left to the right, the greater the heating power, the greater the temperature difference between the inlet and the outlet of the cooling water, and the difference in the cooling capacity between the left and right sides may also reduce the heat transfer capacity of the heat pipe.
Figure 3 - Temperature change of heat pipe under different heat generation power
The heat dissipation and soaking characteristics of the heat pipe to the battery are studied through the experiment of the square battery heat dissipation system of the flat sintered heat pipe, and it is found that for the flat sintered heat pipe to dissipate heat from the battery, the effective heat dissipation capacity and the effective heat soaking capacity of the heat pipe must be considered at the same time through the control of the temperature rise of the battery and the local temperature difference. The so-called effective heat dissipation capacity is the critical heat dissipation capacity when the maximum temperature of the battery is controlled by the heat pipe to the target temperature, and the effective heat dissipation capacity is the critical heat dissipation capacity when the heat pipe is used to control the local temperature difference of the battery to the target temperature difference. The local temperature difference of the battery increases with the decrease of the inclination angle of the heat pipe. When the heat pipe is installed vertically, the local temperature difference of the battery is not greatly affected by the slope of the road surface. Under variable power and periodic working conditions, the heat dissipation of the heat pipe can still maintain the uniformity of the heat distribution of the battery. As shown in Figure 4, it is found through experiments that due to the different driving conditions of electric vehicles, such as uphill, downhill, and inclined roads, when the heat pipe in the battery pack is fixedly installed, its inclination angle to the horizontal plane will also change accordingly. When the heating power is 30W, the local temperature difference on the battery surface changes when the angle between the length of the heat pipe and the horizontal plane is different. When the included angle is 90°, that is, when the heat pipe is placed vertically, the local temperature difference of the battery does not exceed 5℃; when the inclination angle decreases to 45°, the heat pipe is placed obliquely, and the local temperature difference of the battery exceeds 5℃ for the first time at 440s; but after that until 900s, the local temperature difference fluctuated around 5℃; when the heat pipe was placed vertically, the local temperature difference exceeded 5℃ again at 424s and continued to increase. When the heat pipe is placed vertically, the working medium at the condensation end is cooled and flows back to the evaporation end through the dual action of gravity reflux and capillary force of the capillary core. With the decrease of the inclination angle of the heat pipe, the resistance encountered by the reflux of the liquid working medium in the direction of gravity increases; when the heat pipe is placed horizontally, the condensed working medium mainly flows back by capillary force. If the heat pipe is installed vertically, when the inclination angle of the heat pipe is 45°, the gradient of electric vehicles is 100%, while the current gradient of electric vehicles is generally less than or equal to 30%, and the maximum slope of the four-level highway designed by the highway does not exceed 9%, even if it is an outside highway, it generally does not exceed 25%. Therefore, when the heat pipe is installed vertically, the influence of the road gradient on the heat dissipation capacity of the heat pipe is very small, that is, the heat transfer resistance caused by the difference of road gradient can be ignored.
Figure 4 - The local temperature difference varies with the placement angle of the heat pipe
According to the change of the local temperature difference on the battery surface with time under different heating powers, the heating is stopped when the local temperature difference is higher than the target temperature difference. With the continuous increase of heating power, when the local temperature difference reaches 5℃, the required time is 30s, 42s, 78s and 144s, respectively. If the battery heat generation power exceeds 30W in the acceleration, climbing or overspeed conditions of the electric vehicle, the designed flat sintered heat pipe can be used for the corresponding time under each power, which can effectively control the temperature rise and heat balance of the battery.
When the heat pipe is placed vertically, as shown in Figure 5(a), it is the change of the maximum temperature and local temperature difference of the battery when the heating power is gradually decreased from 30 to 5 W. After the battery is heated at 30W, the maximum temperature of the battery changes gently and the local temperature difference is relatively stable under the successively decreasing heating power. As shown in Figure 5(b), it is the change of the maximum temperature and local temperature difference of the battery when the battery is heated in a random cycle of 30W. The four heating times were 542s, 458s, 650s, and 600s, respectively, and the shelving time interval between two consecutive heatings did not exceed 300s. Therefore, within the range of the effective heat dissipation and heat soaking capability of the heat pipe, the heat distribution of the battery is more uniform and stable after the battery is started with a large current and during the cycle.
Figure 5 - Local temperature difference under different cycle conditions
According to the difference in the surface structure of the square battery and the cylindrical battery, a battery heat dissipation experimental platform based on the flat sintered heat pipe was built, and the heat dissipation and heat dissipation characteristics of the heat pipe to the battery were studied. The main conclusions are as follows:
For the heat dissipation of the battery by the flat sintered heat pipe, the control of the temperature rise of the battery and the local temperature difference must simultaneously consider the effective heat dissipation capacity and the effective heat dissipation capacity of the heat pipe; the local temperature difference of the battery increases with the decrease of the inclination angle of the heat pipe. When the heat pipe is installed vertically, the local temperature difference of the battery is little affected by the slope of the road surface; under variable power and periodic working conditions, the heat dissipation of the heat pipe can still maintain the uniformity of the heat distribution of the battery.