Development of Freeze Resistant Heat Pipe With Water as Working Fluid for Automotive Applications

The most widely used combination of container material and working fluid for a heat pipe is copper and water since it allows transporting high heat flux with small tube diameter over long distances. Water has favorable thermodynamic properties such as appropriate viscosity and surface tension for good capillary pressure and high latent heat capacity to facilitate handling of high heat fluxs.

In recent times, heat pipe has been used in variety of applications ranging from electronics cooling, energy conservation, health care, automotive and aviation industry. Out of these, automotive and aviation industry applications have some specific requirements related to the operating condition of the heat pipe. These applications require the heat pipes to sustain the subzero temperatures for longer time or in cyclic manner without damaging the heat pipe container.

It is a well-known phenomenon that water expands 8 to 9% of its volume on freezing which could exert enormous force on the storage container when frozen and can lead to rupture of the container. Various freeze protection techniques have traditionally been used in domestic and automotive applications for many years. Some of the freeze protection techniques include recirculation of water, drain down or drain back [1], electric strip heating [2], use of some compressible inserts to accommodate the expansion of water [3], and use of certain antifreeze fluids [4].

Out of all the above mentioned techniques only compressible inserts and use of antifreeze fluids are promising since the other systems would not allow the heat pipe to be a fully passive heat transfer device.

Few studies have been conducted on using the compressible inserts in heat exchangers, thermosyphons and heat pipes to protect against the frost damage due to repeated exposure to subzero temperatures. Inventor Riley [5] has suggested in his patent the use of flattened compressible rubber tubes as a means for protecting solar water heating equipment against frost damage. In his patent, he has claimed that the rubber inserts would compress and accommodate for the expansion of water upon freezing and avoid any frost damage to the copper tube. Ernst and Sanzi [6] have adapted a similar idea to Riley but have used it in the heat pipes instead of normal pipes carrying water. Ernst and Sanzi have suggested a use of spiral structured metal mesh to be inserted in the heat pipe that can compress like a spring and accommodate for the expansion of water upon freezing. Eastman [7] has suggested a flexible high lift wick with a controlled quantity of working fluid such that it is completely retained inside the wick all the time as a solution against frost damage. Reed [8] has suggested an pressurized inflatable insert within the evaporator section of the heat pipe to prevent it from rupturing. None of these patents claims have been transformed into a commercial product due to the difficulty in manufacturing of heat pipes with the compressible inserts in actual applications.

Use of antifreeze in the heat pipes have not got enough attention from the research community in the past. Most of the investigation on antifreeze topic have been concentrated on developing auxiliary systems to avoid freezing of heat pipe working fluids [9] rather than making working fluid freeze sustainable, which is one of the main objective of current investigation.

Imura [10], have been one of the leading researcher who claimed the use of antifreeze fluid mixed with water as working fluid in heat pipes to protect them against frost damage.

In his patent, Imura have suggested the use of glycols or antifreeze agents with water as a working fluid in heat pipes to reduce the risk of freezing damage. He claimed that using glycol with water in certain percentage mixture will avoid the freezing of water into a solid crystal and instead will be solidified as crystal slurry, which would not create any mechanical stresses on the heat pipe container wall. However, Imura does not mention the precise concentration of antifreeze fluid and water mixture that is suitable for frost resistant heat pipes. His work also lacked in providing the information on life testing of the heat pipes for evaluating the long term thermal performance and any degradation in performance due to the generation of any non-condensable gases. The presence of non-condensable gases in the evacuated heat pipe disturbs the stability of heat pipe and make it inoperative after few working cycles. Apart from that heat pipes in this claim have been subjected to only single freezing cycle at −20 °C for 20 h of holding time which are insufficient to validate the freeze protection ability of such mixtures for application in automobiles and aviation, where the repeatability of freezing is more often. In similar investigation, Sumberg [11] proposed to mix ethanol with water to protect heat pipes from freeze damage. In his work, wider range of ethanol concentrations from 1 to 7.5% by volume were found to produce weak crystals of solid during freezing that could breaks up and expand without rupturing the wall of the heat pipes. However, there is no detailed investigation outlined on thermal performance and long term reliability of such combination in literature.

In this research, we have thoroughly examined the different mass concentrations of antifreeze fluid based on ethylene glycol and water mixture, and suggested the best suited concentration that can be used in a frost resistant heat pipes. Ethylene glycol has been considered in this study due to its superior thermophysical properties as compared to other candidate fluids, and its ability to form stable azeotropic mixture with water. Extensive testing on the performance of these heat pipes have been done and its comparison with pure water heat pipe is presented. Accelerated life cycle testing is carried out on these heat pipes and the effect of antifreeze fluid on the reliability of heat pipe is discussed. The work proposes a potential additive to water to extend its working and storage range as heat pipe working fluid, and specifically addresses the topics of thermal performance and long term reliability of such working fluid.

It is a very well-known fact that water expands upon freezing. The density of water reaches to its maximum level of 999.72 kg/m³ at 4 °C and as the water changes phase from liquid to solid the density actually reduces down to 916.2 kg/m³ at 0 °C [12]. Water expands by 8 to 9% by its volume on freezing due to the crystallization of water into an open hexagonal form that occupies larger space. Ice being a solid, it is highly incompressible in its state and exerts extreme forces on the container during freezing. Freezing of water inside the pipe or a tube can result in very high mechanical stresses on the container wall. Repetitive nature of this stress on the container wall would result in mechanical damage such as bulging of wall or bursting of pipe. This phenomenon of expansion of water upon freezing has been a problem for residential and industrial plumbing situated in adverse climatic conditions [13,14]. Similarly this phenomenon also affects the other applications using metal tubes carrying water and operating below subzero temperatures. Varieties of innovative methods have been suggested by many researchers over the last century to overcome the damage due to freezing. A few patents can be found in the literature that claim to provide a protection against the freezing of working fluid using the compressible inserts inside the heat pipes [3,5–8].

Antifreeze is an additive that is used with a working fluid to lower its freezing temperature as well as alter the freezing crystallization bonds. Water has the most favorable properties to be the preferred working fluid for the heat pipes in most of the applications. It has been proven with long reliability testing that distilled or de-ionized water is much compatible with copper. Most of the antifreeze additives are form from an alcohol or glycols. Some of the common examples of the antifreeze alcohols are Ethanol, Ethylene Glycol, Propylene Glycol, and Glycerol. Table 1 lists some antifreeze agents that are commonly used for most commercial applications. Few physical properties of interest are also listed in Table 1.

All the above mentioned antifreeze additive are readily soluble in water and form a homogenous mixture at any concentrations. Adding the antifreeze additive to water would alter its freezing and boiling temperature. Above table shows the latent heat of vaporization of pure additive, freezing point and boiling point at 10% concentration of mixture of the additive with water. Fig. 1 presents finding from the open tests that were conducted by mixing high concentrations of ethylene glycol in water to observe formation of soft slurry.

Following the results of Imura [10], ethylene glycol was selected as an antifreeze additive to evaluate the appropriate concentration ratio to avoid the freezing damage in case of a heat pipe subjected to −40 °C temperature and 250 freezing cycles. Thermal performance testing was done on these heat pipes with antifreeze working fluid and the thermal performance degradation was evaluated.

Ethylene glycol was selected as an antifreeze additive and the initial freezing test was performed to evaluate the appropriate concentration of additive and water mixture that can be used in the heat pipe to protect it from the freezing damage. In this case, 6 mm outer diameter copper tube with the thickness of 0.2 mm and with circumferential grooves was selected for testing of the antifreeze additive mixture. 150 mm length and composite fiber wick structure was selected for the heat pipe. According to the patent claims by Imura [10], the concentration of additive with water was varied from 0.5 to 10% by mass to avoid the freezing damage to the heat pipe container. Addition of antifreeze additive in very low percentage to the water will not lower the freezing point much, but it helps to reduce the strength of the crystal bonds of the ice and results in formation of ice slurry instead of a hard crystal ice. Imura's patent claim uses the freezing condition of −20 °C temperature and single cycle of freezing and holding time of 20 h. Since these conditions do not satisfy the requirements in the automotive and aviation applications, they were modified to cyclic heating and freezing shock test for minimum of 250 cycles.

Heating and freezing shock machine with cycle as shown in the Fig. 2 was used for the initial freezing test to verify the minimum concentration of ethylene glycol and water mixture required to keep the heat pipe container undamaged. Heating and freezing shock machine was programed for temperature rise from −40 °C to 90 °C and temperature drop from 90 °C to −40 °C in 5 min respectively. Holding time was programed to be 55 min at both 90 °C and −40 °C with each heating and freezing cycle being 2 h long. Initial test samples were subjected to 250 cycles of shock heating and freezing to validate the appropriate amount of ethylene glycol concentration that is sufficient to provide a protection against the repetitive freezing damage.

Initial freezing test was performed on 8 heat pipe samples to verify the quantity of ethylene glycol concentration suitable to sustain the freezing damage. Samples were kept in the bottom heat mode position such that all the working fluid is saturated in the evaporator section of the heat pipe. Sample number 1 is pure water sample for the baseline comparison between the remaining samples. It was observed that sample 2 and 3 with 0.7% and 1% concentration of ethylene glycol and water by weight respectively could not sustain the cyclic freezing and thaw. Both the samples were ruptured similar to the pure water sample.

Sample 4, 5, 6, and 7 have ethylene glycol concentration of 1.5%, 2%, 2.5%, and 3% respectively in water by weight. All the four samples experienced expansion/bulging of the container tube as shown in Table 2. Sample 7 with 3% ethylene glycol experienced the minimum expansion from the original outer diameter of 6 mm to the expanded diameter of 6.53 mm.

It is seen from the initial freezing test that the mixture of ethylene glycol and water with concentration of 0.7% to 3% by weight could not assist in reducing the risk of freezing damage due to expansion of water below subzero temperatures under repeated cyclic conditions.

Initial freezing test shows that the sample 8 with ethylene glycol concentration of 3.5% by weight in water could sustain the freezing test without any damage or expansion/bulging to the heat pipe container tube. After 250 cycles of heating and freezing shock with the specifications mentioned in Fig. 2, the dimensions of the sample 8 heat pipe were unchanged.

Degassing of the working fluid is necessary before charging it into the heat pipe and vacuum sealing the container. It is established in the earlier section that 3.5% concentration of ethylene glycol in water by mass was the appropriate amount to avoid the freezing damage to the heat pipe. De-ionized water and ethylene glycol was mixed in the glass jar using the magnetic stirrer to have a homogenous mixture. This mixture of water and ethylene glycol was kept in a vacuum chamber for 5 min to remove any dissolved gasses.

Copper tube of diameter 6 mm and wall thickness of 0.2 mm with internal groves was cut to a length of 165 mm, such that the final length of the heat pipes will be 150 mm. Composite fiber wick that consists of copper fibers with 50 μm diameter and spiral or spring to keep the fibers along the inner wall circumference of the heat pipe was used as a wick structure in these heat pipes. Figure 3 present the structure of composite wick. It should be noted that composite wick based on copper fiber provides superior performance as compared to mesh wick due to better flow properties including pore size, permeability and wick porosity. When compared to powder wick, composite wick would provide similar performance for shorter lengths (<150 mm) however performance of power wick would dominate at longer lengths due to high capillary pressure provided by fine powder wicks. After inserting the fiber wick along with spiral, heat pipe was swaged from one end and welded to close the bottom end of the heat pipe. The other end of the heat pipe was swaged to a smaller size diameter to form nozzle to charge the working fluid from this end and then crimp the nozzle after final charging. Working fluid was charged in the heat pipe using a vacuum chamber connected to the charging station. Vacuum was maintained in the chamber depending on the quantity of final fluid charge to be retained in the heat pipe. Heat pipe was weighed using a precision scale before and after charging the heat pipe with de-ionized water and ethylene glycol mixture. Measuring the weight of heat pipe before and after the charging let us know the quantity of working fluid being added to each of the heat pipes.

An automated process was used to create vacuum in the heat pipe using the hot oil bath and solenoid controlled valves. A solenoid operated valve is used to close the nozzle of the heat pipe and a vertical robotic rail was used to insert the bottom of the heat pipe in the hot silicon oil bath maintained at 140 °C. Heating time and nozzle opening time is optimized depending on the size of the pipe, internal wick structure, amount of initial charge of working fluid and the amount of final charge to be maintained.

Samples with different filling ratio in the range of 10 to 32% by volume were examined to determine effect on the performance of the heat pipe (Table 3).

The thermal performance of the antifreeze heat pipes was tested by using simulator with electric heating. The heat load simulator was in the form of a square copper block of 4 cm² (20 mm × 20 mm) area embedded with two electric resistance heating cartridges. To allow uniform heating of the heat pipe evaporator, a semicircular groove is made on the copper block to fit the 6 mm diameter heat pipe. Thermal grease is used to achieve maximum surface contact between the electric heater block and the heat pipe. A 24 V DC power supply with voltage regulator was used to control the rate of heat input to the test setup. Such power supply fitted with power meter was able to provide current accuracy of ±0.1 A and voltage accuracy of ±0.01 V.

Here, $Te$ is the average of the external temperature measuring thermocouples attached at the heat pipe evaporator, and $Tc$ is the average of the external temperature measuring thermocouples attached at the heat pipe condenser section.

Figure 5 shows the variation in the thermal resistances of heat pipe samples with working fluid as a mixture of ethylene glycol and water (3.5% mass concentration) with respect to change in the rate of heat transfer. Working fluid charge in the heat pipe samples varies in the range of 10 to 32% by volume. The thermal resistance is measured immediately after manufacturing the heat pipe samples in a horizontal orientation. Initial testing results for ethylene glycol and water heat pipes show a similar trend like pure water heat pipes. Working fluid charging ratio has a significant impact on the performance of the heat pipe depending upon the orientation and working conditions. Charging ratio in the range of 10 to 25% by volume has shown that the thermal resistance varies from 0.45 °C/W to 0.2 °C/W with increase in the rate of heat transfer through the heat pipe. While for the charging ratio in the range of 25 to 32% by volume, the thermal resistance varies from 1.1 °C/W to 0.5 °C/W. This trend is similar to the typical heat pipes with pure water as working fluid, where the larger quantities of charging ratio causes flooding of working fluid at the evaporator at lower rate of heat transfer and hence results in the higher values of thermal resistance.

Figure 6 illustrates the comparison between the thermal resistance for samples of ethylene glycol and water heat pipe with the pure water heat pipes. Out of all the available samples of ethylene glycol plus water heat pipes, only four are chosen with optimum charging ratio for comparison with pure water and copper heat pipes. Thermal resistance of a pure water heat pipe remains fairly constant (0.2 °C/W–0.25 °C/W) with increase in the rate of heat transfer when charged with optimum quantity of working fluid. However it can be observed that the optimally charged ethylene glycol and water heat pipe has higher thermal resistance at lower rate of heat transfer and goes on decreasing as the rate of heat transfer increases. This variation in the thermal resistance of the ethylene glycol and water heat pipe could be attributed to the slightly higher saturation temperature of homogenous mixture of ethylene glycol and water (3.5% concentration by mass)[15].

Each heat pipe is subjected to cyclic freezing and heating for 250 times to confirm the frost resistant properties of the actual heat pipes with working fluid using mixture of ethylene glycol as additive in the water at 3.5% concentration by mass. Thermal resistance of 4 heat pipe samples is measured again after the cyclic freezing and thaw test to check for any degradation in the thermal performance of the heat pipes. In Fig. 8, the thermal resistance of the heat pipe 1, 2, 3, and 4 are illustrated. Thermal resistance measurements are repeated to ensure the repeatability and the error bars are included in the plot. It can be observed that the thermal resistance of ethylene glycol mix water heat pipe before and after the freezing test falls within or very close to the error range of the measurement for all the heat pipes under test. Thermal resistance of ethylene glycol (3.5% mass concentration) and water heat pipe with 6 mm diameter and 150 mm length is within the range of 0.45 to 0.22 °C/W for rate of heat transfer varying from 12 to 50 W. Evaporator length of the heat pipe during these experiments was fixed to 30 mm. These heat pipes can carry heat flux in the range of 2.12 to 8.84 kW/m² with evaporator temperature varying between 42 and 84 °C.

Non-condensable gases affect the performance of the heat pipe by occupying the volume inside the condenser and reducing its heat transfer capacity. Generation of non-condensable gases in the heat pipe is one of the major hurdles in successful implementation of some new working fluid and different container materials. Water is considered as a superior working fluid [16] for operation in the temperature range of 350 to 500 K.

Compatibility of new working fluid and container material can be tested by using the accelerated life testing method as presented in this research. Non-condensable gases are generated in the heat pipe due to reaction between the working fluid and container material at working temperatures over the extended period of time. Accelerated life testing method involves simulating working conditions with elevated working temperature and holding the heat pipe at higher temperature for smaller period of time. This process accelerates any reaction that can be caused between the container material and working fluid.

The ethylene glycol and water heat pipes were subjected to continuous 130 °C temperature for 500 h using an electrically heated oven.

Figure 9 shows the simple setup that is used to test the non-condensable gases that could be generated inside the heat pipes after it undergoes accelerated life testing. Bottom heat orientation of the heat pipe will result in accumulation of any gases, formed as result of chemical reactions inside heat pipe, in top section of the condenser. The fixed length of evaporator of heat pipe is immersed in a temperature controlled hot water bath. Here, the temperature is measured at three different locations with fixed distance from the top of the heat pipe over the condenser section, as shown in Fig. 8. By comparing temperatures of thermocouples on condenser section to each other, and to evaporator section, non-condensables could be precisely detected inside heat pipe system. Condenser temperature is observed over the period of accelerated life testing of heat pipes to estimate the extent of generation of non-condensable gases. Temperature of the hot water bath was maintained at 60 °C during the testing.

Figure 10 illustrates the variation in the evaporator, condenser and ambient temperature of the sample heat pipes as they are subjected to the elevated temperatures for accelerated life testing. Temperature performance tests were conducted on each heat pipe three times during the span of life cycle testing. Heat pipe performance was recorded before subjecting the heat pipe to elevated temperature, after completion of 320 h and finally after the completion of 500 h. It was observed that the evaporator temperature of the heat pipe always remained constant and very close to 60 °C. This is because the evaporator of each heat pipe is inserted in to the temperature controlled water bath where the water temperature is always maintained at 60 °C. However, the temperature measured at three different locations over the condenser of the heat pipe samples as shown in Fig. 10 appears to have reduced after the 500 h of accelerated life testing. Initial observation of reduction in the condenser temperature of the heat pipes point toward the degradation in performance of the heat pipes and the most probable reason for this would be generation of non-condensable gases inside the heat pipe. However after observing the variation in the ambient air temperature, the trend of condenser temperature is better understood. Heat pipes under testing have the condenser exposed to ambient air and the natural convection heat transfer happens over the heat pipe condenser area. The decrease in the ambient temperature corresponds to the reduction in the condenser temperature of the heat pipe after 500 h of life testing. Ambient temperature varies in between 1 and 2 °C while testing of the heat pipe samples. A similar trend is observed in the variation of the condenser temperature of the heat pipe. This shows that the heat pipes have not degraded in their performance over the accelerated life testing of 500 h exposure to 130 °C environment.

As an outcome of this investigation, we have qualified concept of binary mixture as working fluid based on water and ethylene glycol (3.5% by mass) combination. As shown in Table 4, in this investigation, we have qualified diameter six heat pipes with 150 mm length for both thermal performance and long term reliability.

In this article we have presented the:

• Development of freeze resistant copper heat pipe with binary working fluid for automotive and aviation application that can sustain the cyclic freezing and thaw conditions without any structural damage.

• A binary antifreeze fluid based on ethylene glycol is added to water in small mass concentrations to avoid the formation of solid crystal ice when subjected to sub-zero temperatures.

• A minimum mass concentration of ethylene glycol to water is established for avoiding any kind of mechanical damage to the copper container (bulging or cracking) by cyclic freezing and thawing (−40 to +90 °C) 250 cycles.

• It is observed that 3.5% of ethylene glycol by mass concentration to water would be an appropriate mixture to avoid any damage to the copper pipe under cyclic freezing and thaw conditions.

• Further the heat pipes were subjected to accelerated life testing for continuous 500 hours at elevated temperature of 130 °C. There was no observable degradation in the thermal performance of the heat pipe after being subjected to accelerated life testing.

For implementation of this binary mixture into the commercial use of heat pipes, further research on long term reliability is required. In this research, only 6 mm diameter copper tube was used as a container. It is recommended that the similar mass concentration must be tested with different sizes of copper tubes with cyclic freezing and thaw for any signs of container bulging. In addition, extensive accelerated life testing is recommended to further confirm that there is no formation of non-condensable gases, when subjecting heat pipes to high temperature working condition for extended period of time.

Author would like to acknowledge the opportunity provided to work in research facility of Fujikura Ltd., Tokyo, Japan for the period of 6 months to undertake this research. Author would also like to acknowledge the support extended by the technical staff at Fujikura Ltd.

The authors attest that all data for this study are included in the paper.