Heat Pipe Working Principles Essay

Introduction

A heat pipe is a device of extremely high thermal conductance. It is in many ways similar to the thermosiphon except that it is capable of transporting the working fluid condensate against gravity. The vapour-liquid phase-change device moves heat from a hot reservoir to a cold reservoir by use of capillary forces generated by a wick and a working fluid. A heat pipe is made up of three sections: the evaporator section, the adiabatic section, and the condenser section. The heat to be transferred heats the pipe at one end (the evaporator section) causing the working liquid to evaporate. The vapour is then transmitted, through the adiabatic section, to the condenser section of the heat pipe where the vapour liquefies and emits its latent heat to the cold surrounding. Upon condensation, the liquid is returned to the evaporator by capillary action in the wick. However, many other return methods can be used depending on the type of heat pipe. These methods include centripetal force, electrokinetic forces, magnetic forces, and osmotic forces. The continuous flow of vapour from the evaporator to the condenser and the movement of the condensate from the condenser to the evaporator ensures a continuous heat transfer by the pipe.

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The Process of Cooling by Heat Pipe

The cooling process starts at the evaporator section of the heat pipe. When heat is transferred from the external source, through the walls of the pipe, and into the evaporator, it heats up working fluid. The heat energy, in the form of latent heat of vaporisation, is used during evaporation. The vapour flows towards the condenser where it is converted back to liquid and, as a result, gives up its latent heat through the condenser layer and wall to the external cold reservoir. The process repeats itself severally until the source of heat is cooled down. The thermal conductance of the heat pipe allows the device to transfer the heat from the enclosed chamber to a cold reservoir and thus cooling down the sink.

Application of heat pipes

Heat pipes have found numerous applications since its invention. Some of its application include refrigeration, satellite thermal control, electronic cooling, engine cooling, and air conditioning. The working fluid and the material used for the pipe's wall are varied depending on the application of the pipe.

Refrigeration

Heat pipes are used for cooling the interior compartment of refrigerators. The heat pipe transfer heat from these enclosed containers to a colder sink so as to achieve the cooling process discussed earlier. In order to maintain the cool conditions, the interior compartment is insulated from the rest of the surrounding. Such system of refrigeration is used in food preservation among many other purposes. The performance and the heat conductance of the heat pipe are much better compared to plain pipes and metal fins. In another application, heat pipes are used in chilled food display cabinets. These retail cabinets depend solely on convective heat transfer. The food on display is cooled by cold air coming from within the cabinet. An axial flow fan directs the cold air into the display shelves as a jet. The cool air is achieved by passing it over the refrigerant evaporator.

Satellite Thermal Control

Unlike other applications, this application does not involve a high operating temperature. In the 1960s, Daverall and Kemme came up with a heat pipe for satellite use which applied water as its working fluid. Variable conductance heat pipe (VCHP) were made again for a satellite. For this type of heat pipe, an artery is incorporated to provide a low-pressure drop path for transporting the liquid from the condenser to the evaporator from where it re-enters the heat pipe circumference through fine pore wick provided around the walls of the heat pipe. The main purpose of a heat pipe in space technology is to provide for satellite thermal control. When the device was used for the first time in 1968, the wall material was made up of 6061 T-6 aluminium alloy while the wick material used was 120-mesh aluminium. Freon 11 was used as the working fluid. The heat pipe worked to minimise the temperature variation between the various transponders in the satellite. In another application, cryogenic heat pipe, which uses nitrogen as its working fluid, is used for cooling detectors in satellite infrared scanning system.

Electronic cooling

Cooling of electronic components is another main application of heat pipe. While the level of performance continues to increase in electronic devices, there is a continuation reduction in their size. As a result, the system is vulnerable to produce excessive heat which might lead to underperformance or failure. The traditional forced air convection has become inadequate to maintain the temperature within the working conditions. The use of a heat pipe in thermal management is believed to be the solution. It is cheaper and ensures these higher power electronic devices are effectively cooled down. Heat pipes are used in mobile devices including laptop and mobile devices. The high power requirement despite the reduced size requires an efficient heat removal system. Tabular heat pipe, flat heat pipes, micro heat pipes and arrays, loop heat pipes, pulsating heat pipe, and direct contact systems, are the examples of the type of heat pipes that can be applied in the electronics industry.

Air conditioning

Heat pipes are important tools to the course of energy conservation and environmental protection. A heat pipe heat exchanger has the capability of recovering heat from hot exhaust gases, which can be used for air conditioning. Also, heat pipes can be used in air conditioning to transfer heat into and out of a passive cooling system. It transfers heat between the air and the passive cooling systems. This is achieved when cool air is used to freeze the PCM at night while the day heat, which is extracted from the room air, is used to melt the PCM. In order to attain the transition temperature range, the system is made up of a single module, which consists of PCM that has half of the heat pipe embedded while the other half remains exposed to air. Both sections of the pipe are installed with finned heat exchangers. Additionally, the heat pipes are designed for a reversible operation that means the direction of heat flow changes from day to night and vice versa. The system has a fan which pulls air across the heat pipes. During the night, the air is drawn from outside through a motorised window vent. During the day, the window vent is normally closed, and the air is drawn directly from the room through an open flap.

Aside from the abovementioned applications, heat pipes can also be relevant in many other activities. Heat pipes can be used in cooling engines, nuclear reactor, chemical reactors. The devices are also applicable to snow melting and deicing. Furthermore, the food industry has adopted the technology in many operations including cooking, cooling and defrosting meat,

Construction and Material

A standard heat pipe is made up of an evaporator section and a condenser section. Depending on the external geometrical requirement, an adiabatic section can be included. The primary purpose of the adiabatic section is to separate the evaporator and the condenser. Viewing from the cross-section, a heat pipe is made up of a wall, the wick structure and the vapour space. A heat pipe is more like a hollow pipe with wick lining on its wall. A vapour space is available in the middle to allow for the movement of vapour to the condenser. The wick is usually saturated with the working fluid. Some heat pipes such as the rotating heat pipes do not use wicks as their return methods. Instead, the centripetal force is used. Therefore, instead of wicks, the wickless walls are tapered to allow the condensate to return to the evaporator through centripetal force.

Material selection depends on the performance required and the type of the heat pipe. Since its invention, materials including glass, copper, nickel, stainless steel, molybdenum and TZM molybdenum have been used as wall materials. 6067 T-6 aluminium alloy is also an effective wall material, especially in satellite thermal control. Some of the wick material that can be used include wire mesh such as the 120-mesh aluminium. There is a wide range of working fluid that can be used in heat pipes. They include water, cesium, sodium, lithium, and bismuth.

Revolving Heat Pipes

It has many similarities to the rotating heat pipe. However, in other instances, revolving heat pipes are defined as heat pipes with the axis of rotation being parallel to (but offset from) their axis. It is a sealed hollow shaft with a slight internal taper along its axial length and contains a fixed amount of working fluid. It is similar to the standard heat pipe (capillary heat pipe) except that it is wickless and applies centrifugal force as its return method. Just like a capillary heat pipe, it has three longitudinal sections: evaporator, adiabatic and condenser section. Although it is not mandatory, wall tapering is incorporated help in directing the condensate back to the evaporator as the pipe rotates about its longitudinal axis. Revolving heat pipes are desirable especially when more than one heat pipe need to be used to increase the total heat transfer. Since the heat pipe is a distance away from the axis of rotation, numerous pipes can be used without encountering the heat transfer limitation for a single heat pipe. Furthermore, this type of heat pipe does not experience the capillary pumping limitation as in the case of conventional capillary heat pipes. As a result, the transport capability of a revolving heat pipe way much superior as compared to wicked heat pipes. The rotating heat pipe can be sub-divided into two: the radial rotating heat pipe and the axial rotating heat pipe.

Working Principles of a Revolving Heat Pipes

The operation principle of a revolving heat pipe is in many ways similar to that of a conventional capillary heat pipe. However, unlike wicked heat pipes, a revolving heat pipe utilises centrifugal force to transfer the condensate back to the evaporator. An external heat source heats the wall of the evaporator section. With the increase in temperature, the working fluid evaporates and moves to the condenser section of the heat pipe in vapour form. The vapour carries with it the latent heat of vaporisation that is released at the condenser side of the pipe during the process of condensation. Heat is transferred during the vaporization and condensation process. Once the vapour is condensed back to liquid, the condensate is transported back to the evaporator due to the rotation of the heat pipe about its axis. The rotation produces a centrifugal force with a component along the pipe's wall. The resultant force will cause the liquefied working fluid to flow back to the evaporator region. The rotating condenser component maintains high performance by the effective removal of the working fluid from the cooler surface by the pumping effect.

Mathematical Model

The model on the working of a revolving heat pipe is based on the following assumptions:

• The gravity force is neglected
• Only one-dimensional liquid flow is considered
• The fluid flow is laminar and Newtonian
• The liquid velocity is zero on the wall, and viscous stress is zero on the liquid/ vapour interface.
• Vapour pressure remains the same and nearly equal to saturation pressure.

Formation of Vapor and Liquid Films and their Role in Heat Transfer

The rotation of the heat pipe has a significant influence on the basic heat transfer and mass transfer mechanism involved in its operation. RHP can be divided into various components including rotating boiler, rotating condenser, two-phase annular flow transfer pipe and a centrifugal pump. According to Dani...