# Heat Pipe Applications

Heat pipes have been applied in many ways since their introduction in 1964 (Vasiliev, 2005; Mochizuki et al., 2011)[1][2]. Depending on their intended use, heat pipes can operate over a temperature range from 4.0 to 3000 K. In all cases, their applications can be divided into three main categories: separation of heat source and sink, temperature equalization, and temperature control. Due to their extremely high thermal conductivity, heat pipes can efficiently transport heat from a concentrated source to a remotely mounted sink. This property can enable dense packing of electronics, for example, without undue regard for heat sink space requirements. Another benefit of the high thermal conductivity is the ability to provide an accurate method of temperature equalization. For example, a heat pipe mounted between two opposing faces of an orbiting platform will enable both faces to maintain constant with equal temperatures, thus minimizing thermal stresses. The temperature control is a result of the capability of heat pipes to transport large quantities of heat very rapidly. This feature enables a source of varying flux to be kept at a constant temperature as long as the heat flux extremes are within the operating range of the heat pipe [3].

## Electronic and Electrical Equipment Cooling

Miniaturization of electronic components is accompanied by increased demands on heat dissipation systems due to the increased density of the components. For example, the digital computer has evolved from a massive system that filled an entire room to a unit which can be stored in a small briefcase. However, the overheating problems associated with the dense packing of heat-generating integrated circuit chips used in the computer (CPU and GPU Cooling) have escalated dramatically. Since the reliability of these and other types of electronic components is sensitive to their operating temperature, steps have been taken to improve heat dissipation by using heat pipes. Other applications to electronic cooling have included rectifiers, thyristors, transistors, traveling wave collectors, audio and RF amplifiers and high density semiconductor packages.

After the introduction of the Pentium processor in 1993, the processor performance and power consumption trend has significantly increased annually. In the year 2000, the heat flux was approximately 10-15 W/cm2, eventually reaching 120-150 W/cm² in 2010. The average power consumption for laptop/notebook computers is currently between 25-50 W, while desktops and servers consume between 80 and 150 W. Regardless of components, power level, or type of computer/processor, as electronics are packed into smaller volumes, it is important to optimize the thermal management components with higher cooling rates.

Figure 1: Remote heat exchanger using a heat pipe embedded in a block for a laptop computer.

Presently, almost all laptop/notebook computers use a heat pipe remote heat exchanger (RHE), where a heat pipe is used to transfer heat from the processor to the heat exchanger. Heat pipes have been an important part of laptop/notebook computer cooling systems and will remain so for the foreseeable future. Figure 1 shows a simplified typical layout of a heat pipe remote heat exchanger for laptop/notebook computers, using the heat pipe embedded inside a metal block (CPU case) over the silicon (die). For simplicity, a single heat sink is shown in Fig. 1 to illustrate the concept of thermal resistance optimization for personal computers. The overall thermal resistance of this system is presented by the following equation:

$Q=Heat\text{ }dissipation\text{ }(W)=\frac{{{T}_{j}}-{{T}_{\infty }}-{{T}_{sys}}}{{{R}_{j-\infty }}}$

where

Rj-∞ = RSi + RTIM + Re + Rc + Ra + RHT-∞

Tj = Junction temperature [°C]

T∞ = Ambient temperature [°C]

Tsys = Impact of other energy sources in the system [°C]

Rj-∞ = Thermal resistance of the CPU die to ambient [°C/W]

RSi = Thermal resistance of silicon (microprocessor package) [°C/W]

RTIM = Thermal resistance through thermal interface material (TIM) [°C/W]

Re, Rc, Ra = The thermal resistances for the evaporator, condenser, and adiabatic section of the heat pipe, respectively.

RHT-∞ = Thermal resistance of heat exchanger to ambient [°C/W]

Figure 2: Pie chart showing component thermal resistances of a heat pipe remote heat exchanger for a laptop PC (Machiroutu et al., 2006).
Figure 3: Various typical CPU thermal resistances versus heat dissipation (Mochizuki et al., 2007)[4].

The thermal resistances of both the evaporator and condenser sections of the heat pipe are much higher than the adiabatic sections. Figure 2 (Machiroutu et al., 2006) shows a simple pie chart with fractional distributions of various thermal resistances for a heat pipe remote heat exchanger technology for laptop computers. Machiroutu et al. (2006) also showed that the overall thermal resistances presented in Fig. 2 can be reduced by 17% by reducing the thermal resistances of the evaporator and condenser sections. Mochizuki et al. (2007)[4] presented the various typical CPU thermal resistances versus heat dissipation (Fig. 3) by assuming a junction temperature of 100°C, outside ambient temperature of 35°C, and a system temperature rise of 10°C. Mochizuki et al. (2006) [5]presented a number of potential future cooling technologies for higher power cooling chips, including vapor chambers and loop heat pipes (LHPs) for desktop computers and servers. Figure 4 (Mochizuki et al., 2006)[5]fins are soldered directly to the vapor chamber and proposed to replace the solid copper metal interface heat spreaders (IHS) with a two-phase micro-channel vapor chamber. Wu et al. (2011) proposed a cold energy storage system using wickless thermosyphons for cooling large scale data centers (8800 kW).

Figure 4: Thermal solution integration for desktop and servers (Mochizuki et al., 2006)[5].

Annular heat pipes are proposed for use in the fusing units of copy machines by some copy machine manufacturers as heat drums for fast thermal response and uniform fusing (Jalilvand et al., 2006). The proposed annular heat pipe is composed of two unequal diameters, where the evaporation and condensation is separated in the radial direction so that the heat is transferred in the radial direction. While miniaturization is not a significant factor in large-scale electrical equipment, the application of heat pipes to this area is important due to the increase in efficiency when the components operate at lower temperatures (Oslejsek and Polasek, 1976; Giessler et al., 1987; Momose et al., 1987; Alonso and Perez, 1990)[6][7][8].

Electric motors have been designed which incorporate heat pipes around the periphery of the rotor for cooling during operation. Another design replaced the solid motor shaft with an on-axis rotating heat pipe with an internal taper for cooling. These designs removed heat from the interior of the electric motor so that the bearings operated at a lower temperature, which increased efficiency. Also, since the resistance of the electric windings decreases with temperature, less power was required to maintain a particular load. Similarly, circuit breakers and electric transformers designed with heat pipe cooling benefit from the decrease in internal temperature.

Underground power transmission cables that are placed close to hot water or steam pipes are sometimes subjected to limitations in the maximum permissible current due to thermal interference. Therefore, it is advantageous to place maintenance-free cooling systems using heat pipes at these locations. Several heat pipes are placed at the hot spot along the direction of the cable, which are bent so that the condenser sections are above ground or in the soil away from the heated section (Iwata et al., 1984)[9]. The two methods of removing heat, either by natural convection to the atmosphere or by conduction to the soil, provide sufficient cooling so that higher current loads are capable of being provided to the user.

## Energy Systems

With home heating costs increasing, more attention has been focused on the use of heat pipes to collect solar energy (Bienert, 1973; Roberts, 1978)[10][11]. A relatively simple design incorporates a bank of inclined thermosyphons exposed to the south side of a residence. Solar energy is absorbed and transported into the living space, where it is convected to the interior air or stored in a water tank for later use. During the night, the thermosyphons essentially act as thermal diodes, since the only way heat can be transferred from the interior to the outside is by axial conduction through the pipe walls. A similar design can be used for desalinating sea water using solar energy. In this application, however, a heat pipe would be positioned at the focal point of a trough-shaped parabolic reflector in order to generate the high temperatures and heat fluxes necessary for desalination. Singh et al. (2011)[12] presented the design and characteristics of various energy conservation systems and renewable energy systems utilizing heat pipes as the thermal control mechanism. A wide range of energy systems including data center cooling, agricultural products cold storage, bakery waste heat recovery and automotive dashboard cooling were discussed. It was argued that zero emission and economical advantages can be achieved by using thermosyphon and capillary pumped loop.

Concentrating photovoltaic (PV) systems use low-cost optical systems such as the Fresnel lens, a mini-reflecting mirror that can concentrate solar intensity from 200 to 1000 suns. The concentrated solar energy delivered from the solar cell is from 20 to 100W/cm². Part of the energy is directly converted to electricity, while the remainder is removed as waste heat. Heat pipe cooling systems (Fig. 5) were developed to passively remove the high heat flux at the PV cell and reject it to the ambient by natural convection (Akbarzadeh and Wadowski, 1996; Gi and Maezawa, 2006; Anderson et al., 2008)[13][14][15].

Figure 5: Solar house which has an energy collection mechanism (Gi and Maezawa, 2006)[14].

Heat transfer devices and methodologies for novel thermal energy storage systems (TES) for various applications including concentrating solar power (CSP) generation systems using latent heat phase change materials (PCMs) are in great demand in the energy field. However, latent heat thermal energy storage PCMs, despite their great potential, suffer from low thermal conductivity. Faghri (1990; 1991)[16][17] invented two methods (Figs. 6 and 7) to significantly increase the thermal conductivity of PCMs by embedding micro or conventional heat pipes in the PCM for applications in thermal energy storage systems and heat exchangers.

Figure 6: Thermal energy storage system (Faghri, 1990)[16].
Figure 7: Micro heat pipe exchanger (Faghri, 1991)[16].

The effect of heat pipes to improve the performance of latent heat thermal energy storage systems has been investigated experimentally and theoretically (Shabgard et al., 2010; Robak et al., 2011; Nithyanandam and Pitchumani, 2012)[18][19][20]. The experimental results of Robak et al. (2011)[19] showed 60% and more than 100% increase in the heat transfer rates during melting and solidification, respectively, compared to the fin-assisted storage systems as well as non-fin, non-heat pipe configurations. Shabgard et al. (2010)[18] developed a thermal network modeling approach for computationally-efficient simulation of such heat pipe-assisted latent heat thermal energy storage systems. The modeling results also showed substantial heat transfer enhancements during charging and discharging of latent heat thermal energy storage systems equipped with heat pipes. The developed thermal network model was later extended to simulate a large scale heat pipe-assisted cascaded latent heat thermal energy storage system for CSP applications [21].

Fuel Cells are versatile energy conversion devices with numerous potential applications: large electrical plants, stationary electricity generation, vehicle propulsion and small portable power (Faghri and Guo, 2005; 2008)[22][23]. In recent years, proton exchange membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs), solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs) have become the major types of fuel cell technologies that draw the most attention for commercialization. Heat pipe technology is beneficial for fuel cell development in two main ways: using heat pipes in fuel cells as thermal management components and using the heat pipe concept in the fuel cell systems to achieve passive high-effective fluids-thermal management. Most fuel cell systems produce electrical energy at high efficiencies that may range from 40% to 60% based on the lower heating value (LHV) of the fuel. For example, a fuel cell operating at 1.0 kW and 50% efficiency generates 1.0 kW of waste heat. This heat may be dissipated by convection, conduction, radiation or phase change. The heat generated in a fuel cell stack may be dumped to the atmosphere, but often, it is used in other system components that require heat. In some cases, the heat is used to run a thermodynamic cycle for additional power generation. Heat pipes can be utilized in fuel cell systems for thermal management purposes which allow for effective use of the fuel cell byproduct, heat, leading to a substantial increase in heat transfer and overall system efficiency (Faghri and Guo, 2005; 2008)[22][23].

The matter of heat generation in fuel cell stacks presents challenges for thermal management. Stacks operating at 40% to 60% efficiency generate heat at the same rate to more than twice the rate of electric generation. Due to changes in mass concentration, temperature gradients, and in some cases, phase change throughout the stack, the heat generation is not uniform. This non-uniform heat generation further increases thermal gradients in the stack. Increasing the mobility of the heat is a challenge that, if met, leads to three main benefits: (1) The risk of stack failure due to overheating is reduced, (2) The stack operates more closely to its design temperature, resulting in better power density and efficiency, and (3) The heat can be reused, perhaps for reactant preheating, prevaporization, combined cycle operation, or cogeneration. For example, the heat pipe embedded with a bipolar plate (Faghri and Guo, 2008)[23] is an innovative approach that would increase heat transfer in fuel cell stacks while requiring significantly smaller thermal gradients and much smaller volumes and weights than alternative methods. As conceptualized in Fig. 8, a bipolar plate is fabricated with holes, into which micro heat pipes are inserted and bonded. The micro heat pipes that are shown in Fig. 8 are embedded in an axial and transverse direction.

Figure 8: Bipolar plate integrated heat pipe (Faghri and Guo, 2008)[23].

Another method for thermal control in the fuel cell stack is presented with the integrated bipolar plate flat heat pipe (Faghri and Guo, 2008)[23]. For example, as conceptualized in Fig. 9, a carbon bipolar plate contains a flat heat pipe. A challenge for developing this component is sealing the heat pipe. Due to the permeability of the carbon, two halves of the bipolar plates are coated with a liner--silver activated brazing alloy (ABA). Also in the enclosure is a porous wick of metal foam, felt, or porous carbon. The two halves are then sealed together, perhaps with brazing in an inert gas. The structure of the interior is designed to allow good electrical conductivity, thermal conductivity to the wick, and structural support, while the working fluid is typically at a pressure different than the surroundings.

Figure 9: Micro heat pipe embedded bipolar plate (Faghri and Guo, 2008).

Passive DMFC technology uses various capillary approaches to manage methanol and water without the need for a complex micro-fluidic subsystem (Fig. 10) (Guo and Faghri, 2006a; 2006b; Faghri and Guo, 2005; 2009)[24][25][22][26]. At the core of this new technology is a unique passive system that uses the heat pipe concept for fuel delivery. Furthermore, the fuel cell is designed for both passive water management and effective carbon dioxide removal. The passive components that are critical to the fuel cell design are the fuel delivery, and air-breathing and water recirculation systems. The passive fuel delivery system stores pure methanol, which can be mixed with water in situ without the use of a pumping system, and can be passively supplied to the fuel cell anode at an optimal concentration. Since water is needed in the anode for the methanol reaction to occur, the water created in the cathode can be passively supplied to the anode. This water recirculation, in conjunction with passive methanol fuel delivery, can dramatically extend the operation time of the fuel cell per refueling. The passive mass transfer concept (wick structure) developed in heat pipe technology is an effective approach for mass transfer management in various fuel cell technologies. The proposed DMFC technologies developed were operated passively, without moving parts, which resulted in a highly reliable system. Due to their significantly longer charging life, passive miniature DMFC system are seriously being considered for replacing the battery for applications such as cell phones, digital cameras and laptops.

Figure 10: Passive direction methanol fuel cell using heat pipe concept (Faghri and Guo, 2008)[23].

## Aerospace and Avionics

Heat pipes are very attractive components in the area of spacecraft cooling and temperature stabilization due to their low weight penalty, zero maintenance, and reliability. Structural isothermalization is an important problem with respect to orbiting astronomy experiments due to the possible warpage from solar heating. During orbit, an observatory is fixed on a single point such as a star. Therefore, one side of the spacecraft will be subjected to intense solar radiation, while the other is exposed to deep space. Heat pipes have been used to transport the heat from the side irradiated by the sun to the cold side in order to equalize the temperature of the structure. Heat pipes are also being used to dissipate heat generated by electronic components in satellites. Early experiments of heat pipes for aerospace applications were conducted in sounding rockets which provided six to eight minutes of 0-g conditions. In 1974, ten separate heat pipe experiments were flown in the International Heat Pipe Experiment (McIntosh et al., 1976)[29]. Also in 1974, heat pipe experiments were conducted aboard the Applications Technology Satellite-6 (Kirkpatrick and Brennan, 1976)[30], in which an ammonia heat pipe with a spiral artery wick was used as a thermal diode. With the use of the space shuttle, flight testing of prototype heat pipe designs continued at a much larger scale. In 1983, on the eighth space shuttle flight, a 6-ft. monogroove heat pipe with Freon 21 as the working fluid was flight tested as a heat pipe radiator (Rankin, 1984)[31]. The Space Station Heat Pipe Advanced Radiator Element, which consisted of a 50-ft. long high capacity monogroove heat pipe encased in a radiator panel, was flown on the space shuttle in 1989 (Brown et al., 1990)[32], and during a 1991 shuttle flight, two heat pipe radiator panels were separately flight tested (Brown et al., 1991)[33]. Heat pipe thermal buses were proposed which facilitate a connection between heat-generating components and external radiators (Morgownik and Savage, 1987; Amidieu et al., 1987; Peck and Fleischman, 1987). The components may be designed with a clamping device which can be directly attached to the heat pipe thermal bus at various points in the spacecraft. In 1992, two different axially grooved oxygen heat pipes were tested in a Hitchhiker Canister experiment that was flown aboard the Shuttle Discovery (STS-53) by NASA and the Air Force to determine startup behavior and transport capabilities in micro gravity (Brennan et al., 1993)[34].

An advanced capillary structure which combined re-entrant and a large number of micro grooves for the heat pipe evaporator was investigated in microgravity conditions during the 2005 FOTON-M2 mission of the European space agency (Schulze et al., 2007)[35]. Swanson (2007)[36] presented the NASA thermal technical challenges & opportunities for the new age of space exploration with emphasis on heat pipes and two phase thermal loops.

A heat pipe laser mirror has been fabricated in order to test the feasibility of this technology compared to water-cooled or uncooled mirrors for high power lasers (Barthelemy et al., 1978)[37]. Presently, uncooled mirrors are limited to only a few seconds of use prior to distortion from thermal overheating. Water-cooled mirrors have longer service times, but are subjected to high internal pressures causing distortion, which must be removed by polishing under pressure. A copper-water heat pipe mirror was constructed, which was not affected by the problems associated with conventional mirrors due to the heat pipe action. The experiments conducted used a 10 kW carbon dioxide laser, and it was found that heat pipes can be used with success if the heat pipe is sufficiently preheated. Thermal diodes have been proposed for use in cooling low-temperature sensors, such as an infrared detector in low subsolar earth orbits (Williams, 1978)[38]. This type of heat pipe was proposed due to its characteristic of being able to cool the instrument during normal operation, but effectively insulating it when exposed to an external heat flux. One type of thermal diode uses a liquid reservoir at the evaporator end of the heat pipe, which does not communicate with the wick structure. During normal operation, the reservoir is empty. If the condenser is subjected to an external heat flux, however, the working fluid condenses in the reservoir, causing the wick to dry out. This results in the heat pipe becoming an insulator, because heat can only be conducted axially through the thin pipe wall. Heat pipes have also been qualified and/or used for thermal control applications in avionic systems including aircrafts with more electric architectures. Radioisotope Stirling systems are proposed to replace the radioisotope thermoelectric generating systems as a long-lasting electricity generation solution in space missions due to their higher efficiency (Thieme and Schreiber, 2003)[39]. In the current radioisotope Stirling systems if the Stilrling engine stops, the heat removal from the system would be ceased and the insulation will be spoiled to prevent damage to the clad fuel, but the mission will also be ended. Alkali-metal variable conductance heat pipes are proposed and tested to allow multiple stops and restarts of the Stirling engine (Tarau and Anderson, 2010)[40]. In the proposed design, the evaporator of the heat pipe is connected to the heat generation module. During the normal operation, the heat is transferred from the heat generation unit to the heater head of the Stirling engine by evaporation and condensation of the sodium working fluid. When the Stirling engine stops, the temperature and pressure of the heat pipe working fluid increases. The higher pressure inside the heat pipe compresses the non-condensable gas and opens up a radiator through which the heat is dissipated and the system temperature stabilizes. Once the Stirling engine restarts, the temperature and pressure drop and the radiator is covered by the non-condensable gas to prevent unwanted heat rejection.

## Heat Exchangers and Heat Pumps

Increases in the cost of energy have promoted the use of heat pipe technology in industrial applications. Due to their high heat transfer capabilities with no external power requirements, heat pipes are being used in heat exchangers for various applications. In the power industry, heat pipe heat exchangers are used as primary air heaters on new and retrofit boilers. The major advantages of heat pipe heat exchangers compared to conventional heat exchangers are that they are nearly isothermal and can be built with better seals to reduce leakage. Heat pipe air heaters should also be cheaper than conventional tubular heat exchangers, as they are smaller and can be shipped in a small number of modules. Heat pipe heat exchangers can serve as compact waste heat recovery systems which require no power, a low pressure drop and are easy to install on existing lines. Heat pipe heat exchangers can be categorized into gas-gas, gas-liquid, and liquid-liquid type heat units. Among these three, gas-gas heat pipe heat exchangers have the widest application in industry. A gas-gas heat pipe exchanger consists of a group of externally finned heat pipes which reclaim waste heat (Hassan and Accensi, 1973; Holmes and Field, 1986)[41][42]. These units eliminate cross-contamination due to the solid wall between the hot and cold gas streams. Also, the heat pipe design is totally reversible (heat can be transferred in either direction). Gas-gas energy recovery units typically fall into three categories: heat recovery in air-conditioning systems (low temperatures), recovery of excess process heat for space heating (moderate temperature), and recovery of waste heat from high temperature exhaust streams for reuse in the process (preheating of combustion air, for example). The units for these applications vary in size and construction depending on the specific application, but many commercial models are now available that implement this heat pipe design. Gas-liquid heat pipe exchangers are less commonly available than gas-gas models due to the fact that the present design of waste heat boilers is very efficient. In the past, exhaust heat from boilers was simply dispersed to the atmosphere. Figure 11 shows schematics for waste heat recovery with liquid-gas and gas-gas heat exchangers. Faghri (1993a)[43] invented an innovative design for a centrifugal heat pipe vapor absorption heat pump (Fig. 12). The heat pipes in this heat pump system are disk-shaped, with one face partially or completely being the evaporator and the opposite face partially or completely being the condenser (Faghri, 1994)[44]. The wick is designed such that the centrifugal force aids in the delivery of the condensate to the evaporator. This design will significantly improve the heat and mass transfer characteristics of the rotating components of the vapor-absorption heat pump by increasing the heat pumping capacity that can be packaged in a given value. This results in a more efficient and compact vapor-absorption heat pump system.

Figure 11: Waste heat recovery using heat pipe heat exchangers.
Figure 12: Schematic of centrifugal heat pipe heat pump vapor-absorption cycle (Faghri, 1993a)[43].

## Gas Turbine Engines and the Automotive Industry

The temperature limitation is one of the most crucial limiting factors related to the efficiency of a gas turbine aircraft engine or power gas turbine. An increased turbine inlet temperature decreases both the specific fuel and air consumption, while increasing efficiency. This desire for a high turbine inlet temperature, however, is often in conflict with materials available to withstand the high temperature. As a result, innovative cooling systems for hot-gas-path components are required. Among the hot-gas-path components, the first-stage rotor blade and nozzle guide vane require the most challenging cooling consideration. In addition to improving energy utilization efficiency, effective cooling could also drastically improve the reliability of high-speed rotating components. It has been observed that the creep life of turbine blades is reduced by half with every 10 to 15°C rise in metal temperature. Therefore, the temperature of the turbine blade must be kept within certain tolerable limits. The primary cooling technology in use today for turbine blades and nozzle guide vanes (NGVs) is the film cooling technology. Since 1995, a number of efforts have been initiated to utilize the concepts of miniature, radially rotating, high-temperature heat pipes, for gas turbine blades and disk cooling. This also includes a series of experimental and numerical studies (Cao, 1996; 1997; 2010; Zuo et al., 1998)[45][46][47][48]. High temperature heat pipe cooling is a promising cooling technology for gas turbine hot components, such as first stage rotor blades, nozzle guide vanes, and rotor disks. This has the potential to significantly reduce the temperature of these hot components, allowing for a much higher gas turbine inlet temperature, while also reducing the consumption of high-pressure compressor air. A Stirling engine heated by sodium heat pipes has been constructed and tested (Alleau et al., 1984; 1987; Meijer and Khalili, 1990)[49]Alleau, T., Bricard, A., and Thouvenin, A., 1987, "Stirling Engine Coupled with a Sodium Boiler," Proceedings of the 6th International Heat Pipe Conference, Grenoble, France, 748-752. </ref>[50][51]. This engine operates at highest efficiency when the thermal energy is supplied to it at a very constant temperature and high heat fluxes. Helium is heated to increase its pressure, which is used to drive pistons and a crankshaft for the generation of power from thermal energy. The sodium heat pipes deliver heat from a molten salt heat storage system to the gaseous helium which is used to drive the engine. The coupling of a Stirling cycle engine with sodium heat pipes can also be used in the direct conversion of solar energy.

Nguyen et al. (1992)[52] proposed a thermosyphon Rankine engine for power generation using solar and other available waste heat energy sources. The basis of the engine (Fig. 13) is the modification of a heat pipe to incorporate a turbine and thereby, making the system into a Rankine cycle engine to convert thermal energy to electrical energy.

Another potential application of heat pipes has been toward the cooling of vehicle brake systems (Maezawa et al., 1981)[53]. Normally, the brake pads and rotors of a conventional automobile brake system suffer greatly from the effects of elevated temperatures. Thermal cycling induces stresses and causes increased amounts of corrosion, which shorten the working lives of the rotors.

High temperatures at the rotor-pad interfaces cause embrittlement of the brake pad material, which also reduces the service life. Rotating and stationary heat pipes have been proposed to cool both the rotor and brake pads, which would result in lower, more uniform temperatures. The use of this development could also increase the service life of brake pads so that ‘lifetime’ pads, lasting the entire life of the automobile, may become a reality. Vibration/shock-tolerant capillary two-phase loops are developed to meet the cooling requirements of the high performance electronics of future military vehicles which undergo intense shocks during operation (Tang and Park, 2008)[54]. The shock tolerant feature is achieved by utilizing an in-situ sintered wick which also reduces the manufacturing costs.

## Production Tools

An important application of heat pipes is in the field of die casting and injection molding (Winship, 1974; Reay, 1977)[55][56]. The most obvious use of heat pipes in this field is the removal of heat during the solidification process; however, heat pipes are also useful in minimizing thermal shock in the dies. Die casting involves introducing a material in molten form into a closed die. The material is cooled until it solidifies, the part is removed, and the process repeated. An important consideration is the time required for the material to cool into solid form, so most dies are water cooled. It is often difficult to cool inaccessible parts of the die, however, so heat pipes are used to cool these sections. These heat pipes can also be used to preheat the die to assure the continuous flow of molten material due to the reversible nature of heat pipes.

## Medicine and Human Body Temperature Control

Another application is of heat pipes is related to human physiology. A surgical probe incorporating a cryogenic heat pipe is being used to destroy tumors in the human body (Basiulis, 1976)[57]. This type of surgery, where the tissue is frozen instead rather than irradiated, is beneficial because the surrounding tissue sustains practically no damage. Also, the surgery results in very little bleeding or pain. The cryoprobe is a hand-held device with a reservoir of liquid nitrogen and a 12-inch heat pipe extension, which is maintained at approximately 77 K for about one-half hour. Fletcher and Peterson (1997)[58] invented a catheter using micro heat pipes that provide precise temperature control for treating diseased tissue.

Figure 13: Experimental thermosyphon Rankine engine (Nguyen et al., 1992)[52].

Another application related to human physiology concerns the control of body temperature (Faghri et al., 1989b; Faghri, 1993b)[59][60]. In regions or occupations where humans are exposed to extreme temperatures, such as workers in polar regions or foundry workers, adverse health effects from these environments are evident. Frostbite on the extremities in cold regions and heat exhaustion in warm climates are very serious problems which must be handled with extreme care. These problems can be avoided with the use of gloves, socks, and suits in which heat pipes are placed in order to transfer heat either to or from parts of the body (Fig. 14). In cold climates, heat pipes could bring heat from the torso to the extremities such as the fingers, toes, and ears to prevent frostbite. Figure 15 shows a conceptual design for cold weather handwear with heat pipes, where body heat is transferred from the forearm to the fingers. In very hot environments, such as those experienced by fire fighters, a cold suit employing heat pipes could be developed which would be lighter and less bulky than the suits presently worn. This type of suit would also be beneficial in the respect that the wearer would be kept cooler, resulting in more time available for extracting people from a burning building, for instance.

Figure 14: Temperature regulation system for the human body using heat pipes (Faghri, 1993b).
Figure 15: Models of cold weather handware with heat pipes (Faghri et al., 1989b)[59].

Similar garments or blankets using heat pipes (Faghri, 1993b)[60] could also be applied to medical patients whose own body temperature regulation system is impaired or not functioning. Patients with spinal cord injuries or people with Multiple Sclerosis often become overheated during hot weather because of the inability of the nervous system to control their body temperature. A heat pipe suit equipped with an external heat exchanger could significantly improve the quality of life of these patients by allowing them to be more exposed to the elements. Likewise, patients who are bedridden often become overly warm or cold. In some cases, a blanket with a single-phase fluid loop is placed on the patient for heating or cooling. Since this type of blanket is quite heavy due to the fluid loop, a heat pipe blanket has been proposed, which would decrease the weight and improve the heat transfer and temperature uniformity.

It has been found experimentally that when the temperature of the cauterizing forceps used in surgical operations increase above 80°C, the tissue under operation tends to stick to the forceps. Small-diameter heat pipes are employed to precisely control the forceps temperature by removing heat from the forceps tip as well as the tissue (Bilski and Broadbent, 2010)[61]. The heat generated during cauterization evaporates the working fluid of the heat pipe. The vapor moves towards the cold section of the heat pipe where it dissipates its thermal energy content and condenses. Capillary action then returns the condensed liquid to the forceps tip. This technology represents the smallest known mass-produced heat pipe assembly (with a diameter of 2.34 mm) and makes it possible to passively control the temperature of forceps tips with diameters ranging from 0.5 to 4 mm.

## Ovens and Furnaces

One of the first applications of heat pipes (actually, two-phase closed thermosyphons) was in baking ovens (Anonymous, 1867)[62]. Normal flame-heated baking ovens warmed the firebrick in the firebox, which then conducted the heat to the baking chamber so that the baked food was not contaminated by the combustion products. With the advent of the heat pipe, heat was transferred to the baking chamber by the evaporation and condensation of the working fluid within the array of tubes. This arrangement was not only more efficient, saving up to 25 percent of t( he fuel normally needed, but also resulted in a more uniform oven temperature. A similar concept is the gas-fired restaurant griddle, which is a flat plate heat pipe separating the gas fire from the cooking surface (Basiulis, 1976)[57]. This provides a surface with an extremely uniform temperature, even with heavy food loading. It also has the benefit of a fast warm-up time from a cold start, and the efficiency is comparable to the above-mentioned baking oven. Other food processing equipment has been designed using heat pipe technology such as braisers, kettles, saucepans, and deep fat fryers (Lamp, 1978)[63].

High temperature furnaces have been developed for heat-treating, sintering, and other applications (Finlay et al., 1976; Brost et al., 1990)[64][65]. These furnaces are actually annular heat pipes with liquid-metal working fluids. The isothermal behavior of the interior of the annular heat pipe makes it ideal for use as a furnace. The temperature gradient across the length is very small, which is always a significant problem with conventional furnaces. Due to the operation of the heat pipe, only a small portion of the outer surface of the annular heat pipe needs to be heated. Upon insertion of a cold charge, the area in contact with the cold charge becomes the condenser. Annular heat pipe furnaces are also being used in the calibration of temperature measurement devices (Chengsheng et al., 1984; Bassani et al., 1990)[66][67]. Thermocouples, RTD's and thermistors have been calibrated using a variable conductance heat pipe furnace; the temperature of which was measured with a traceable platinum RTD. Optical pyrometers, which measure surface temperature from the emitted radiation, have also been calibrated with heat pipe blackbody sources.

## Permafrost Stabilization

Permafrost stabilization is an important matter in designing the foundations of buildings or other structures in arctic regions. During the summer months, heat is conducted into the permafrost, which tends to melt the water trapped in the soil. Normally, this thawing would allow the foundation to sink or shift, thereby damaging the building. To avoid this problem, engineers responsible for the Trans-Alaskan pipeline placed heat pipes around the pilings of the pipeline supports (Fig. 16) (Waters, 1976)[68]. The heat from the soil adjacent to the pilings was dissipated to the ambient by the heat pipes, which maintained a permafrost bulb around the pilings throughout the year.

Mashiko et al. (1989) [69] developed an artificial permafrost storage facility in Hokkaido, Japan (Fig. 17), where the evaporator of the heat pipe is buried underground and the condenser is exposed to the ambient. The heat pipe transfers the ground heat to the ambient in winter which freezes the soil. When the temperature rises in the spring, the heat is not transmitted from the ambient to the ground (thermal diode), so the frozen soil is kept throughout the year. The main feature of this system is that no power is required, and the temperature is kept constant for a long period of time.

Figure 16: Application of heat pipes for permafrost stabilization of structural supports on the Trans-Alaskan pipeline.
Figure 17: Cold energy storage using heat pipes.

## Manufacturing

High temperature heat pipes have been proposed for use in the manufacturing of glass bottles (Brost et al., 1973)[70]. The glass bottle forming procedure starts by periodically dipping a steel piston into a steel form filled with molten glass. This forms a hollow glass tube, which is later blown into its final shape. The initial glass temperature is around 1100°C and the surface temperature of the piston needs to be kept around 600°C. At higher piston temperatures, the glass will stick to the piston, and at lower temperatures, the glass viscosity increases, causing insufficient deformation during the forming process. Insufficient deformation is the cause of thin-walled bottles which contribute to the waste rate. A stainless-steel/potassium heat pipe was proposed and tested, and it was found that the heat pipe could be kept nearly isothermal. This resulted in a higher dipping frequency and a reduced amount of glass bottle waste.

## Transportation Systems and Deicing

Several innovations using heat pipe technology have been proposed which could improve the safety and reliability of transportation systems. Heat pipes have been used to melt the ice and snow on roadways, bridges, and aircraft runways by transporting geothermal heat stored in the ground to the pavement (Suelau et al., 1976; Bartsch et al., 1987)[71][72]. Shiraishi et al. (1992)[73] presented several newly developed snow removal and deicing methods using heat pipes such as prevention of snow damage to support wires for telephone poles, a snow melting system for pavement, and a large scale snow melting system for roads in Japan. Corrugated, long heat pipes are utilized for these applications.

People working on ships at sea during the winter months are plagued by the problems associated with icing. Anything exposed to the elements, such as the decks and handrails, are constantly icing over during the winter. The usual method of removing the ice is to use an axe or hatchet, which is extremely dangerous when the seas are rough. The potential for severe cuts or falling overboard is always present. To solve this problem, heat pipes have been incorporated into the decks and handrails to transport waste heat from the engine (Matsuda et al., 1981)[74]. This heat constantly melts the ice before large deposits are formed. Another similar deicing problem addressed by the use of heat pipes concerns navigation buoys (Larkin and Dubuc, 1976)[75]. Off the east coast of Canada, navigation buoys are frequently subjected to ocean spray and freezing temperatures, which results in the buoys capsizing due to the weight of the ice. Researchers have constructed a prototype buoy which was, in essence, a large ammonia thermosyphon heated from the sea. It was experimentally determined that the superstructure of the thermosyphon was kept free from ice, but that the auxiliary float seriously iced up.

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