2.4.1.1 Instrumentation and calibration

The test rig has to be provided with different measuring devices of temperature, water flow rate, heat (power) input and angular orientation to enable investigating the flow and heat transfer characteristics of the selected thermosyphon. The instruments include:


The instruments are calibrated against standard devices and error analysis and uncertainties of their measurements are evaluated.

## 2.4.1.2 Experimental procedure

The test facility was completed and ready for investigations when all the parts were connected and water circulation system was checked for possible leakages. The operating conditions are set based on the type of the investigation to be carried out. However, in all the cases, the system is allowed to run and stabilize before readings are taken. Preliminary tests are required to determine the time when the system reaches steady state. Certain number of readings are set to be taken for each boundary condition at a set interval of time (usually in seconds). The reading recorded includes the temperatures, flow rates, voltage and current. Various investigations can be carried out using the test rig such as the effects of heat inputs, cooling water flow rate, inclination effects of the pipe, fill ratio, etc. Detailed procedure for each case depends on the type of the investigation to be carried out.

(physics) behind the processes. The processes involved in the CFD modelling of the performance of thermosyphon using volume of fluid (VOF) approach in ANSYS

ii. Meshing of the model: different meshes of different properties (number of

iii. Carrying out a grid independence test: this is done to find out the situation whereby the result is independent of the mesh configuration and to select

iv. Importing the selected meshed file for the investigations into the ANSYS

v. Attaching the user-defined function (UDF); this depends on the modelling

• Setting the thermophysical properties of the materials involved such as thermal conductivity, material properties, density, specific heat

• Validation of the model: to enable validation of the developed model, the boundary conditions and other definitions are made exactly as

• Once the model is validated with the experimental results, then it can

Considerable experimental research works were published on the investigation of the effects of parameters like the geometry, working fluid, fill factor and inclination on the thermosyphon heat pipe performance [21–25]. Hence, apart from the material of the thermosyphon, other important parameters affect its performance,

I.Type of working fluid charged: The common liquid used in thermosyphon is water due to its availability, low cost, safety, etc. Below are some of the

prime requirements for a liquid to be used in heat pipe:

i. Compatibility with wick and wall materials

the configuration which will give less computational time.

• Defining of the solution method and convergence.

• Running the simulation and processing of the results.

vi. Modelling and simulation set up, which includes.

• Defining the boundary conditions.

capacity, viscosity, etc.

those set in the experiment.

be used for further investigations.

2.5 Factors affecting the operations of thermosyphon

ii. Good thermal stability

Fluent can be summarized as follows:

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

Thermosyphon Heat Pipe Technology

Fluent.

such as:

15

approach selected.

i. Generation of the pipe geometry (model).

cells, faces, quality, etc.) are required.

#### 2.4.2 Numerical approach

To enable several investigations on many parameters affecting the performance of thermosyphon with different boundary conditions, numerical approach is usually employed. This is because experimental approach requires more time, energy and huge investment, to investigate many cases under different boundary conditions. There are two numerical approaches that are employed in modelling multiphase flows, namely the Euler-Euler and Euler-Lagrange approaches. In the Euler-Euler approach, the several phases are considered as interpenetrating continua mathematically in which each phase a volume is occupied only without sharing with other phases, while Euler-Lagrange approach utilizes Navier-Stokes equations that are solved for the fluid phase with several numbers of particles tracked in order to solve the dispersed phase. It should be noted that this approach cannot be adopted for applications in which volume fraction is important, especially for the secondary phase. Hence, the Euler-Euler approach is usually used in modelling two-phase closed thermosyphon operations.

Using Euler-Euler approach, three multiphase models are available in ANSYS Fluent:


The mixture model deals with modelling of sedimentation, bubbly flows, particle-laden flows, etc. While applications such as fluidized beds, particle suspension, risers are modelled using Eulerian approach, on the other hand, liquid-gas tracking under steady or transient, free-surface flows, large bubble in liquid are modelled using the VOF approach.

Numerical modelling like computational fluid dynamic analysis (CFD) is an alternative to experimental approach, whereby several studies can be carried out with small investment. In CFD, a set of discretized equations are solved with the help of computer to get an approximate solution [20]. CFD analysis can be carried out on the flow and heat transfer characteristics of a thermosyphon heat pipe in both vertical and inclined orientations using a commercial ANSYS Fluent or any software that can model the simultaneous evaporation and condensation processes taking place in a thermosyphon heat pipe. However, some approaches like volume of fluid (VOF) in ANSYS Fluent require the user to add a user-defined function (UDF) to the modelling process.

The first step in solving any multiphase problem is identifying the suitable multiphase regime which represents the flow needed to be modelled. In this chapter, emphases is put more on the VOF model.

#### 2.4.2.1 Model building

For building a model for simulating the flow and heat transfer characteristics of thermosyphon, a researcher is required to have a good knowledge of the theory

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

boundary condition at a set interval of time (usually in seconds). The reading recorded includes the temperatures, flow rates, voltage and current. Various investigations can be carried out using the test rig such as the effects of heat inputs, cooling water flow rate, inclination effects of the pipe, fill ratio, etc. Detailed procedure for each case depends on the type of the investigation to be carried out.

To enable several investigations on many parameters affecting the performance of thermosyphon with different boundary conditions, numerical approach is usually employed. This is because experimental approach requires more time, energy and huge investment, to investigate many cases under different boundary conditions. There are two numerical approaches that are employed in modelling multiphase flows, namely the Euler-Euler and Euler-Lagrange approaches. In the Euler-Euler approach, the several phases are considered as interpenetrating continua mathematically in which each phase a volume is occupied only without sharing with other phases, while Euler-Lagrange approach utilizes Navier-Stokes equations that are solved for the fluid phase with several numbers of particles tracked in order to solve the dispersed phase. It should be noted that this approach cannot be adopted for applications in which volume fraction is important, especially for the secondary phase. Hence, the Euler-Euler approach is usually used in model-

Using Euler-Euler approach, three multiphase models are available in ANSYS Fluent:

The mixture model deals with modelling of sedimentation, bubbly flows, particle-laden flows, etc. While applications such as fluidized beds, particle suspension, risers are modelled using Eulerian approach, on the other hand, liquid-gas tracking under steady or transient, free-surface flows, large bubble in liquid are

Numerical modelling like computational fluid dynamic analysis (CFD) is an alternative to experimental approach, whereby several studies can be carried out with small investment. In CFD, a set of discretized equations are solved with the help of computer to get an approximate solution [20]. CFD analysis can be carried out on the flow and heat transfer characteristics of a thermosyphon heat pipe in both vertical and inclined orientations using a commercial ANSYS Fluent or any software that can model the simultaneous evaporation and condensation processes taking place in a thermosyphon heat pipe. However, some approaches like volume of fluid (VOF) in ANSYS Fluent require the user to add a user-defined function

The first step in solving any multiphase problem is identifying the suitable multiphase regime which represents the flow needed to be modelled. In this chap-

For building a model for simulating the flow and heat transfer characteristics of thermosyphon, a researcher is required to have a good knowledge of the theory

2.4.2 Numerical approach

Recent Advances in Heat Pipes

a. The Eulerian model

b.The Mixture model

ling two-phase closed thermosyphon operations.

c. The Volume of Fluid (VOF) model

modelled using the VOF approach.

(UDF) to the modelling process.

2.4.2.1 Model building

14

ter, emphases is put more on the VOF model.

(physics) behind the processes. The processes involved in the CFD modelling of the performance of thermosyphon using volume of fluid (VOF) approach in ANSYS Fluent can be summarized as follows:

	- Defining the boundary conditions.
	- Setting the thermophysical properties of the materials involved such as thermal conductivity, material properties, density, specific heat capacity, viscosity, etc.
	- Defining of the solution method and convergence.
	- Running the simulation and processing of the results.
	- Validation of the model: to enable validation of the developed model, the boundary conditions and other definitions are made exactly as those set in the experiment.
	- Once the model is validated with the experimental results, then it can be used for further investigations.

### 2.5 Factors affecting the operations of thermosyphon

Considerable experimental research works were published on the investigation of the effects of parameters like the geometry, working fluid, fill factor and inclination on the thermosyphon heat pipe performance [21–25]. Hence, apart from the material of the thermosyphon, other important parameters affect its performance, such as:

	- i. Compatibility with wick and wall materials
	- ii. Good thermal stability

iii. Wettability of wick and wall materials: it is necessary for the working fluid to wet the wick and the container material, that is contact angle should be zero or very small

500 W, but it decreases when the heat input is above 500 W [18] . But for Abdullahi et al. [19], the performance of the pipe increases as the heat input increases from 20 to 81.69 W, but it tends to decrease as more heat is

supplied, showing the limit of this pipe has been reached under these operating conditions (Figure 9). Hence, the trend of the performance of the thermosyphon (based on the amount of the heat input in the evaporator section) depends on its operating limits. At low heat input, the vapour generated from the evaporator section is small, so there will be significant dry areas in the condenser section; hence, heat transfer is largely by free convection. As the heat is gradually increased, more vapour will rise to the condenser section, there will be high condensation rate on the condenser wall and the dominant heat transfer mechanism will be condensation. But at certain high heat input, thick layer of liquid can be formed on the wall of the pipe causing high thermal resistance and hence lower the heat transfer to the

IV.Inclination angle: since the condenser of thermosyphon must be at the top with the evaporator at the bottom for the condensate to return, this shows that the pipe can be inclined at any angle other than 90°. Regarding the effect of inclination angle on heat pipe performance, conflicting experimental results were reported like angles between 15 and 60° [24], between 40 and 45° [25] and 60° [30] gave the best performance. Others reported higher angles like 90° [31] and 83° [32] as the best performing angles while few reported that inclination angle has no effect [33]. The possible reasons for the contradicting results are the complex nature of the processes taking place in thermosyphon operations and various parameters affecting its performance. Furthermore, those researches are only experimental and considered a small range of inclination angles. With the contradictory experimental results in the literature and lack of, or limited, numerical studies on the effect of inclination, Abdullahi et al. [19] addressed these issues through the development of a CFD model that studied the effects of inclination angles (10–90°) and experimentally validated the model. Experimental and numerical results showed that increasing the inclination angle will improve the thermosyphon heat pipe performance to reach its maximum value at 90°, but this effect decreases as the heat input

V.Flow rate of cooling water: the rate at which cooling water is passing in the water jacket around the condenser of a thermosyphon affects its performance.

Performance of thermosyphon aligned vertically at different heat inputs [17, 19].

cooling water, hence reduction of performance.

Thermosyphon Heat Pipe Technology

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

increases [19] (Figure 10).

Figure 9.

17


The selection of the working fluid must be based on thermodynamic considerations which are concerned with the various limitations to heat flow occurring within the heat pipe, like viscous, sonic, capillary, entrainment and nucleate boiling levels.

Some common liquids used in heat pipe include water, acetone, ethanol, ammonia, nitrogen and methanol. However, recent researches have shown potentials of using other liquids alone or mixed with water like nanofluids [26–28].

II.Quantity of the working fluid charged: the quantity of the liquid charged in relation to the volume of the evaporator, called fill ratio, FR or liquid ratio, plays a vital role in the performance of thermosyphon. Fill ratio is defined as the ratio of volume of the working fluid in an unheated pipe, Vliq, to the volume of the evaporator, Ve:

$$FR = {}^{V\_{liq}}/\_{V\_e} = \frac{4V\_{liq}}{\pi D^2 l\_e} \tag{5}$$

The quantity of the fluid to be charged has to be properly selected, which depends on the intended applications, as insufficient amount of fluid causes dry out while excessive amount reduces performance and increases the cost of the pipe. FR of a thermosyphon should be between 40 and 60% for vertical pipes and between 60 and 80% for inclined pipes [4, 29] . For example, Emami et al. [30] and Asgar [18] obtained 45 and 50% as best FR respectively.

III.Heat input: The amount of heat supplied in the evaporator affects the performance of the thermosyphon depending on other factors such as size, fill ratio, its geometry and operating limits. Experimental results have shown that the performance of the thermosyphon increases with the increase in heat input up to their operating limits. It increases with increase between 350 and

iii. Wettability of wick and wall materials: it is necessary for the working fluid to wet the wick and the container material, that is contact angle

iv. High latent heat: a high latent heat of vaporisation is desirable in

hence to maintain low pressure drops within the heat pipe

order to transfer large amounts of heat with minimum fluid flow, and

v. High thermal conductivity: the thermal conductivity of the working fluid should preferably be high in order to minimize the radial temperature gradient and to reduce the possibility of nucleate boiling

vi. Low liquid and vapour viscosities: the resistance to fluid flow will be minimized by choosing fluids with low values of vapor and liquid

tension is desirable in order to enable the heat pipe to operate against

vii. High surface tension: in heat pipe design, a high value of surface

gravity and to generate a high capillary driving force

The selection of the working fluid must be based on thermodynamic considerations which are concerned with the various limitations to heat flow occurring within the heat pipe, like viscous, sonic, capillary, entrainment and nucleate boiling

Some common liquids used in heat pipe include water, acetone, ethanol, ammonia, nitrogen and methanol. However, recent researches have shown potentials of

II.Quantity of the working fluid charged: the quantity of the liquid charged in relation to the volume of the evaporator, called fill ratio, FR or liquid ratio, plays a vital role in the performance of thermosyphon. Fill ratio is defined as the ratio of volume of the working fluid in an unheated pipe, Vliq, to the

FR <sup>¼</sup> Vliq <sup>=</sup>Ve <sup>¼</sup> <sup>4</sup>Vliq

The quantity of the fluid to be charged has to be properly selected, which depends on the intended applications, as insufficient amount of fluid causes dry out while excessive amount reduces performance and increases the cost of the pipe. FR of a thermosyphon should be between 40 and 60% for vertical pipes and between 60 and 80% for inclined pipes [4, 29] . For example, Emami et al. [30] and Asgar

III.Heat input: The amount of heat supplied in the evaporator affects the performance of the thermosyphon depending on other factors such as size, fill ratio, its geometry and operating limits. Experimental results have shown that the performance of the thermosyphon increases with the increase in heat input up to their operating limits. It increases with increase between 350 and

πD<sup>2</sup> le (5)

using other liquids alone or mixed with water like nanofluids [26–28].

should be zero or very small

at the wick or wall surface

viii. Acceptable freezing or pour point

viscosities

Recent Advances in Heat Pipes

volume of the evaporator, Ve:

[18] obtained 45 and 50% as best FR respectively.

levels.

16

500 W, but it decreases when the heat input is above 500 W [18] . But for Abdullahi et al. [19], the performance of the pipe increases as the heat input increases from 20 to 81.69 W, but it tends to decrease as more heat is supplied, showing the limit of this pipe has been reached under these operating conditions (Figure 9). Hence, the trend of the performance of the thermosyphon (based on the amount of the heat input in the evaporator section) depends on its operating limits. At low heat input, the vapour generated from the evaporator section is small, so there will be significant dry areas in the condenser section; hence, heat transfer is largely by free convection. As the heat is gradually increased, more vapour will rise to the condenser section, there will be high condensation rate on the condenser wall and the dominant heat transfer mechanism will be condensation. But at certain high heat input, thick layer of liquid can be formed on the wall of the pipe causing high thermal resistance and hence lower the heat transfer to the cooling water, hence reduction of performance.


Figure 9. Performance of thermosyphon aligned vertically at different heat inputs [17, 19].

Figure 10. Variation of the thermosyphon performance with inclination angle at different heat inputs [17, 19].

As the rate of the heat removal from the vapour increases, more condensate returns to the evaporator for another cycle. The effect of cooling water flow rate at constant heat input was investigated on the performance of thermosyphon heat pipe [19]. The heat input was fixed at 101 W while five different flow rates ranging from 0.00156 to 0.00611 kg/s were investigated. Temperature and the flow rate readings were recorded for each run and the effects of the cooling water flow rate were evaluated based on the overall thermal resistance, rate of heat transfer to the cooling water, outlet temperature of cooling water, performance of the thermosyphon, etc. The results from such work have shown that the performance of the pipe in terms of heat transfer to the cooling water increases with the increase in the cooling water flow rate. This is due to the mass flow of the cooling water which results in the enhancement of the rate of heat transfer from the pipe wall to the cooling water and subsequent increase in the efficiency.

Receiver in solar collector (solar systems): thermosyphon is proved to be a good choice as a receiver for solar concentration systems due to its advantages stated

Several parameters affect the operation of thermosyphon such as fill ratio, working fluid, inclination, geometry, heat input, cooling water flow rate, etc. Experimental and numerical (CFD) studies are usually carried out to enable the investigation of the effects of some of these parameters on the performance of thermosyphon heat pipe for use in various engineering applications. Investigations on the effects of heat input, fill ratio, flow rate of cooling water on the temperature distributions on the wall of the pipe, overall thermal resistance and overall performance of the pipe at vertical orientation were shown to be possible both experimentally and using CFD. Also, the effect of inclination angle of thermosyphon on those parameters was successfully added in the Fluent. Hence, the chapter has shown that volume of fluid (VOF) model's approach in ANSYS together with UDF and other software can fully simulate the complex evaporation and condensation processes taking place in thermosyphon for both vertical and inclined orientations.

[36, 37] as shown in Figures 3 and 11.

Thermosyphon Heat Pipe Technology

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

Developed compound parabolic collector with thermosyphon as receiver [17].

3. Conclusions

19

Figure 11.

### 2.6 Applications of thermosyphon

In addition to the general advantages of heat pipes, thermosyphon type is found to be highly durable, reliable and cost-effective, which make them useful for various applications, such as:

I.Solar heating of building [16].


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

Figure 11. Developed compound parabolic collector with thermosyphon as receiver [17].

Receiver in solar collector (solar systems): thermosyphon is proved to be a good choice as a receiver for solar concentration systems due to its advantages stated [36, 37] as shown in Figures 3 and 11.
