Thermosyphon Heat Pipe Technology

Bala Abdullahi, Raya K. Al-dadah and Sa'ad Mahmoud

#### Abstract

Heat pipes play vital roles in increasing heat transfer performance of many engineering systems such as solar collectors and this leads to an increase in their usage. Investigation on the performance of heat pipes under different operation conditions and inclination angles is required for effective utilization. In this chapter, a general overview on the construction, operation, advantages, and classifications of heat pipes is presented. Particular attention is given to the heat pipe without wick material in the inner diameter (thermosyphon). Intensive discussions are presented on the construction, operations, advantages and applications of thermosyphon heat pipe. The experimental and numerical approaches on the performance evaluation and characterization of thermosyphon are discussed. A detailed procedure on how experimental work is carried out on thermosyphon is discussed including instrumentation and calibration of the devices. Modelling and simulation of the performance of thermosyphon are discussed, including the model set-up procedure. Factors affecting the performance of thermosyphon such as fill ratio, working fluid, heat input, inclination angles, are analysed based on the overall thermal resistance and thermosyphon performance. Current researches on the effects of major factors affecting the operation of thermosyphon are presented, as well as their current development and various applications in engineering systems.

Keywords: thermosyphon, evaporator, condenser, thermal resistance, inclination angle

### 1. Introduction

The world's needs for effective heat transfer devices/mechanisms are increasing so as to minimize heat losses, minimize systems cost, enhance heat removal and transportation as well as to increase lifespan of some devices. In some instances, heat is required to be removed from a system (like solar photovoltaic, electrical devices, turbine blades, etc.) in order to keep it at a certain operation temperature, while in other cases, it is required to be transferred to a certain region to keep it at high temperature. Some elements/metals such as copper and aluminium are found to be good conductors of heat as they transfer heat effectively from one region to another. Their ability to transfer heat effectively is due to their molecular arrangements and type of bonds between their molecules. Various systems such as aircraft, electronics, heat exchangers, solar collectors, etc. require effective means of heat transfer. One of the devices recognized as effective means of heat transfer is heat pipe, whose idea was introduced by Graugler in 1942, but its first unit was invented by Grover in 1962;

then, its important properties were studied and identified, and its development started [1]. Hence, with the growing need for efficient heat transfer devices, interest in the use of heat pipes for various applications is increasing due to the roles they play in improving the thermal performance of solar collectors and heat exchangers particularly in energy savings and increasing efficiency of the systems.

Heat pipe is an efficient two-phase heat transfer device which uses latent heat of fluids to transfer energy from one place to another by means of simultaneous evaporation and condensation in a sealed container. It consists of evaporator and condenser sections with or without adiabatic section in between them. Depending on the type, heat pipe may have wick materials on its internal surface where the simultaneous evaporation and condensation take place in the wick structure. In such types of heat pipe, evaporator section can be placed at the top, since the wick structure can return the condensate from the condenser section against gravity. Hence, in a wick heat pipe, the condensed liquid is returned to the evaporator by capillary effects with the assistance of the wick materials as shown in Figure 1.

However, many applications do not require inserting wick material on the inner surface of the pipe, because the condenser section can be placed at the top, so that the condensed liquid returns to the evaporator by gravity. This type of wickless heat pipe is called thermosyphon as shown in Figure 2 Hence, for thermosyphon, the condenser must be above the evaporator, while for the wick heat pipe, the capillary forces in the wick ensure the condensate returns to the evaporator regardless of its position.

#### 1.1 Working principles of heat pipe

Heat pipes consist of sealed vessel usually made from aluminium or copper with or without wick material lined on the inner surface and working fluid charged under a vacuum condition. It is made up of two main sections: evaporator, where the working fluid absorbs heat, and condenser, where the working fluid rejects heat (Figures 1 and 2). As heat is added to the working fluid in the evaporator section, it evaporates into vapour when it reaches its saturation temperature. It rises to the condenser with the assistance of buoyancy force and due to the vapour pressure difference between the two sections. The liquid condenses by giving out its

enthalpy to the cooling water in the condenser section and returns back to the

Heat pipes offer advantages over other heat transfer devices used for various applications in engineering systems. The technology has undergone rapid development due to their operational advantages [3]. Some of these advantages include:

i. High thermal conductivity: In terms of heat transfer, heat pipes are better than the best conductor; hence, they are referred to as 'superconductors'.

evaporator for another cycle.

Thermosyphon Heat Pipe Technology

DOI: http://dx.doi.org/10.5772/intechopen.85410

Operation of thermosyphon [2].

Figure 2.

1.2 Advantages of heat pipe

ii. Light weight.

vii. Low cost.

7

iii. Efficient heat transfer.

iv. Flexibility in design.

v. Isothermal operation.

1.3 Classifications of heat pipe

vi. Tolerance to freezing, shock and vibration.

There are different types of heat pipes, classified based on [4]:

Figure 1. Operation of wick heat pipe [2].

Thermosyphon Heat Pipe Technology DOI: http://dx.doi.org/10.5772/intechopen.85410

then, its important properties were studied and identified, and its development started [1]. Hence, with the growing need for efficient heat transfer devices, interest in the use of heat pipes for various applications is increasing due to the roles they play in improving the thermal performance of solar collectors and heat exchangers partic-

fluids to transfer energy from one place to another by means of simultaneous evaporation and condensation in a sealed container. It consists of evaporator and condenser sections with or without adiabatic section in between them. Depending on the type, heat pipe may have wick materials on its internal surface where the simultaneous evaporation and condensation take place in the wick structure. In such types of heat pipe, evaporator section can be placed at the top, since the wick structure can return the condensate from the condenser section against gravity. Hence, in a wick heat pipe, the condensed liquid is returned to the evaporator by capillary effects with the assistance of the wick materials as shown in Figure 1. However, many applications do not require inserting wick material on the inner surface of the pipe, because the condenser section can be placed at the top, so that the condensed liquid returns to the evaporator by gravity. This type of wickless heat pipe is called thermosyphon as shown in Figure 2 Hence, for thermosyphon, the condenser must be above the evaporator, while for the wick heat pipe, the capillary forces in the wick ensure the condensate returns to the evaporator regardless of its position.

Heat pipe is an efficient two-phase heat transfer device which uses latent heat of

Heat pipes consist of sealed vessel usually made from aluminium or copper with

or without wick material lined on the inner surface and working fluid charged under a vacuum condition. It is made up of two main sections: evaporator, where the working fluid absorbs heat, and condenser, where the working fluid rejects heat (Figures 1 and 2). As heat is added to the working fluid in the evaporator section, it evaporates into vapour when it reaches its saturation temperature. It rises to the condenser with the assistance of buoyancy force and due to the vapour pressure difference between the two sections. The liquid condenses by giving out its

ularly in energy savings and increasing efficiency of the systems.

1.1 Working principles of heat pipe

Recent Advances in Heat Pipes

Figure 1.

6

Operation of wick heat pipe [2].

Figure 2. Operation of thermosyphon [2].

enthalpy to the cooling water in the condenser section and returns back to the evaporator for another cycle.

## 1.2 Advantages of heat pipe

Heat pipes offer advantages over other heat transfer devices used for various applications in engineering systems. The technology has undergone rapid development due to their operational advantages [3]. Some of these advantages include:

