A pulsating heat pipe (PHP) is also called an oscillating heat pipe, and its structure is shown in Figure 1. Can be divided into closed loop type and open loop type. The two ends of the closed-loop type tube bundle are connected to form a loop, while the open-loop type heat pipe cannot form a loop due to unidirectional flow. One or several one-way valves are added to the pipeline of the closed-loop pulsating heat pipe to form a derivative type of the closed-loop structure with one-way valve. The one-way valve is used to control the one-way flow of the medium inside the heat pipe. The shape of the pulsating heat pipe is usually serpentine, and a certain amount of working medium is filled in the serpentine pipe. The two ends of the pulsating heat pipe are the heating end and the cooling end elbow, and the middle part is the adiabatic section.
Figure 1 - Basic structure of pulsating heat pipe
The working principle of the pulsating heat pipe is: the liquid working medium is heated and evaporated in a serpentine closed vacuum space composed of metal capillaries in an environment of evaporation temperature lower than normal pressure to generate bubbles; the bubbles rapidly expand and pressurize in the capillary, forming an evaporating end at one end of the heat pipe, pushing the working medium to flow to the condensation end where the temperature is relatively low. In addition, under the action of capillary force and bending force, the vapor generated by heated evaporation and the liquid generated by condensation will form an oscillation state with random distribution of gas and liquid plugs in the entire tube of the pulsating heat pipe. Due to the pressure difference between the cooling end and the heating end and the pressure imbalance between the adjacent capillaries, the working medium oscillates and flows between the heating end and the cooling end in different states, thereby realizing heat transfer in the pulsating heat pipe.
Compared with traditional heat pipes, pulsating heat pipes have the following advantages:
(1) The volume of the pulsating heat pipe is relatively small, the structure is relatively simple, and the cost is relatively low compared to other heat pipes; since the pulsating heat pipe does not require a liquid wick, the structure of the heat pipe itself is relatively simple, and the production cost is also reduced; the oscillating power inside the pulsating heat pipe completely comes from the pulsating heat pipe itself. During the whole working process, it does not need to consume external mechanical work and electrical power, and it completes the self-oscillation process under the action of thermal drive. The operation of the heat pipe is relatively simple.
(2) The heat transfer performance of the pulsating heat pipe is relatively good. It can not only transfer heat through phase change, but also transfer sensible heat through gas-liquid oscillation and convert the heat into work required for oscillation.
(3) The adaptability of the pulsating heat pipe is relatively good, and its shape can be bent arbitrarily. The common pulsating heat pipe is serpentine. The two ends of the heat pipe can be provided with multiple heating ends and cooling ends. The heating and cooling parts of the heat pipe can be selected arbitrarily, and the pulsating heat pipe can work normally regardless of any inclination angle and heating mode. This greatly improves the adaptability of the pulsating heat pipe, which determines that the pulsating heat pipe can be used in many fields. Through the experimental research on the battery heat dissipation system based on the pulsating heat pipe, an experimental platform is built to study the heat transfer characteristics of the battery under different heat generation powers when the pulsating heat pipe placement and the orientation of the battery electrodes are different.
Figure 2 is a schematic diagram of the pulsating heat pipe battery cooling system, in which T1~T17 are thermocouple temperature measurement points. When the pulsating heat pipe is placed in the battery pack, its placement angle will change with the complex working conditions (such as uphill, etc.) when the electric vehicle is actually running. The start-up temperature of the pulsating heat pipe is determined by the target temperature for battery heat dissipation and the target temperature difference. To meet the requirements of battery cooling and heat distribution balance, the start-up temperature of the pulsating heat pipe must be lower than the target temperature, and not higher than the maximum temperature of the battery when the local temperature difference of the battery reaches the target temperature difference. Compared with the conventional cooling system, the pulsating heat pipe battery cooling system has better adaptability because it can choose the placement method more freely, especially when the car is running on a steep slope and other road terrain conditions are complex, it can still maintain a high heat dissipation efficiency.
Figure 2 - Schematic diagram of pulsating heat pipe battery cooling system
A pulsating heat pipe is used to dissipate heat from the square battery. When the pulsating heat pipe is placed vertically and the battery electrode is facing down (the bottom), the temperature changes on the battery surface and each temperature measurement point of the heat pipe are shown in Figure 3. The discharge (heating) time was 800s, and the heat generation power increased from 20W to 35W. At the end of the discharge, the corresponding maximum surface temperatures of the battery were 54.85℃, 62.90℃, 69.44℃, and 76.37℃, respectively. Compared with 63.35℃, 71.96℃, 82.50℃ and 90.17℃ under natural cooling, the maximum temperature decreased by 8.50℃, 9.06℃, 13.06℃ and 13.80℃, respectively. Under each power, the time it takes for the maximum temperature to reach the target temperature of 50°C is 584s, 400s, 326s and 258s, which are shorter than the time for the maximum temperature to reach the target temperature during natural cooling. When the heat generation power is 20W, continue to heat and record the temperature change at each point. When heated to about 700s, the temperature at point T1 begins to oscillate slightly, indicating that because the local temperature of the battery surface reaches the evaporation temperature of the working medium in the tube, the pulsating heat pipe begins to vibrate locally; after 1000s, the temperature measured at the steaming end and the condensing end of the pulsating heat pipe oscillated obviously; when the heat generation power was 25W, the obvious oscillation appeared at about 800s. When the heat generation power is 30W and 35W, after the corresponding 400s and 300s, the temperature fluctuations at the steaming end and the condensing end of the pulsating heat pipe are larger, indicating that the oscillation in the tube is more severe at this time. It is not difficult to see that since the starting temperature of the pulsating heat pipe is close to the target temperature, the starting time of the pulsating heat pipe is all later than the time required for the maximum temperature of the battery surface to reach the target temperature. During the heat generation time of 800s, when the heat generation power is 20W and 25W, the pulsating heat pipe basically does not start. At this time, the heat transfer of the heat pipe is mainly carried out by sensible heat, and the battery is dissipated by the heat conduction capacity of the copper pipe itself; when the heat generation power is 30W and 35W, after the maximum battery temperature exceeds the target temperature, the pulsating heat pipe starts successively. The heat transfer before the heat pipe starts is mainly carried out by sensible heat, and the heat transfer when the heat pipe starts is mainly carried out by latent heat, and most of the heat is transferred from the battery to the cooling water through the phase change of the working fluid in the tube. Therefore, in the design of the power battery cooling system based on the pulsating heat pipe, to ensure that the maximum battery temperature does not exceed the target temperature at the end of the maximum allowable discharge current discharge, the starting temperature of the pulsating heat pipe must be lower than the target temperature.
Figure 3 - Pulse heat pipe placed vertically battery temperature change
When the pulsating heat pipe is placed in the battery pack, its placement angle will change with the complex working conditions (such as uphill, etc.) when the electric vehicle is actually running. Due to the good adaptability of the pulsating heat pipe, the condensing end can be combined with the whole vehicle to dissipate heat through the top windward (vertical placement) or the side windward (horizontal or inclined placement). Keeping the battery electrodes facing down, when the pulsating heat pipe is placed horizontally, the heat generation power changes from 20W to 35W. The temperature changes on the battery surface and each part of the heat pipe are shown in Figure 4.
Figure 4 - The temperature change of the battery placed horizontally with the pulsating heat pipe
When the heat generation power is 20W, 25W and 30W respectively, with the increase of heating time, the surface temperature of the battery gradually increases, and the maximum temperature of the battery surface corresponding to each power is 54.21℃, 61.50℃ and 67.89℃ when it reaches 800s. Compared with when the heat pipe is placed vertically, the maximum temperatures differ by 0.64℃, 1.40℃ and 1.55℃, respectively. According to the change of the temperature of each part of the heat pipe under the corresponding power, it can be seen that when the heat generation power is less than 30W, the heating to about 800s, the pulsating heat pipe is in the state of no vibration or local vibration; when the heat generation power is greater than 30W, the maximum temperatures at 800s corresponding to each power are 70.36℃, 74.93℃ and 79.25℃, respectively, and the pulsating heat pipe is fully activated. At this time, it can be seen from the change of battery surface temperature that when the heating power is 35W, 40W and 45W respectively, the corresponding heating time is in the range of 550~800s, 500~800s and 550~800s, the temperature increase of the battery becomes smooth, the overall trend tends to be stable, and the trend of local temperature difference increases is not obvious. To ensure that the maximum temperature of the battery surface does not exceed 50℃, the temperature of the stable zone can be controlled within the target temperature by adjusting the starting temperature of the pulsating heat pipe.
The heat generation power is increased from 20W to 45W, and the local temperature difference change on the battery surface when the pulsating heat pipe is placed vertically is shown in Figure 5. In the first 200s, the pulsating heat pipes were not fully activated, and the heat transfer of the heat pipes was mainly carried out in the form of sensible heat. When the local temperature difference of the battery reaches 5℃, the corresponding time under each heat generation power is 148s, 110s, 72s, 58s, 56s and 32s respectively. Compared with air natural convection cooling, when pulsating heat pipes are used for heat dissipation, the time for the local temperature difference of the battery to reach the target temperature difference under each heat generating power is extended by about 30s, which is significantly shorter than the discharge time of 800s or 600s. Therefore, starting from the balance of heat distribution, when designing the battery cooling system, to ensure that the local temperature difference on the battery surface does not exceed the target temperature difference at the end of discharge, the pulsating heat pipe must be activated before the local temperature difference on the battery surface reaches the target temperature difference.
Figure 5 - Local temperature difference variation on the surface of the battery placed vertically by the pulsating heat pipe
When the battery electrodes face upward and the pulsating heat pipes are placed vertically and horizontally, the local temperature difference on the battery surface under different heat generation powers is shown in Figure 6. When the pulsating heat pipe is placed vertically, the time for the local temperature difference on the battery surface to reach 5℃ under the power of 20~45W is 1572s, 748s, 572s, 118s, 82s and 68s respectively; when the pulsating heat pipe is placed horizontally, the time for the local temperature difference on the battery surface to reach 5℃ under the power of 20~45W is 1158s, 710s, 560s, 110s, 108s and 72s, respectively. When the heat generation power is 20W, the time for the local temperature difference on the battery surface to reach 5℃ is significantly earlier than when the pulsating heat pipe is placed horizontally and vertically; at 25W, in the range of 900~1200s, when the heat pipe is placed vertically, the local temperature difference on the battery surface slowly oscillates and rises, while when the heat pipe is placed horizontally, the local temperature difference on the battery surface increases significantly; at other powers, the local temperature difference on the battery surface is slightly higher when the pulsating heat pipe is placed horizontally. The main reason is that when the pulsating heat pipe is placed horizontally, especially during the local vibration of the heat pipe, the backflow resistance of the working medium is relatively large, resulting in a slight increase in the local temperature difference of the battery.
Figure 6 - Local temperature difference change on the battery surface when the battery electrode faces upwards
The following conclusions can be drawn from the above experiments of the battery cooling system based on the pulsating heat pipe.
(1) To ensure that the maximum battery temperature does not exceed the target temperature at the end of the maximum allowable discharge current discharge, the starting temperature of the pulsating heat pipe must be lower than the target temperature.
(2) When designing the battery cooling system, to ensure that the local temperature difference on the battery surface does not exceed the target temperature difference at the end of the discharge, the pulsating heat pipe must be started before the local temperature difference on the battery surface reaches the target temperature difference.
(3) When the pulsating heat pipe is placed horizontally, the backflow resistance of the working medium is relatively large during the local start-up of the heat pipe, and local burn-out phenomenon will occur, resulting in a slight increase in the local temperature difference on the battery surface. This phenomenon can be avoided when the pulsating heat pipe is placed vertically .
Researchers also have in-depth research on pulsating heat pipes. Swanepoel designed a pulsating heat pipe and applied this pulsating heat pipe to the thermal management of a lead-acid battery (optima spirocell, 12V, 65A·h) and the thermal management of hybrid electric vehicles. Among the control components, the heat transfer performance of different wall materials and medium in the pulsating heat pipe is analyzed. The lead-acid battery pack for heat dissipation of HEV with pulsating heat pipe is placed at the rear of the car, and combined with the flowing air during driving, the heat dissipation is enhanced, mainly to study the feasibility of applying pulsating heat pipe to HEV. Through numerical simulation and experimental verification, only when the diameter of the pulsating heat pipe is less than 2.5m, the working medium in the pulsating heat pipe can use ammonia gas, and the structure of the pulsating heat pipe can only be applied to the thermal management of the battery through a reasonable design.