2.4 Measurement of the performance of thermosyphon

The performance of thermosyphon under different conditions is evaluated based on the overall thermal resistance Rth, given by:

$$R\_{th} = \frac{T\_{ae} - T\_{ac}}{Q\_{in}} \tag{2}$$

where Tae and Tac are respectively the average temperatures on the evaporator and condenser while Qin is the heat supplied to the evaporator.

However, the performance of the thermosyphon can also be calculated as the ratio of the heat transfer to the cooling water to the heat input as [18]:

$$
\eta = \left. \mathbb{Q}\_{\text{out}} \right|\_{\mathbb{Q}\_{\text{in}}} \tag{3}
$$

while the current was read from the power regulator. The evaporator section is also insulated with 25-mm-thick pipe insulator to reduce the heat loss to the ambient environment (Figure 8). For measuring the temperature distribution along the pipe, 12 surface thermocouples were placed at different locations on the test pipe; 4 on the evaporator wall (at 0.02, 0.07, 0.12 and 0.17 m from the tip of the

evaporator) and 2 on the condenser wall at 0.25 and 0.35 m as shown in the figures. The electric wires were wrapped in such way that they are not directly on the thermocouples so as to not affect their readings. Two probe thermocouples were installed at the inlet and outlet of the manifold to measure the temperatures of the cooling water. Three other thermocouples were used on the water jacket and one on the insulator to measure the effectiveness of the insulation and the jacket. All the

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 instru-

The instruments are calibrated against standard devices and error analysis and

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

readings were sent to Pico TC-08 data loggers connected to a PC.

Picture of the heat transfer characterization of thermosyphon test rig [17, 19].

□ Thermocouples, both surface and probe types.

uncertainties of their measurements are evaluated.

□ Angular measurement instrument such as protractor

□ Electric power regulator (or hot water supply in some cases).

2.4.1.1 Instrumentation and calibration

Thermosyphon Heat Pipe Technology

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

ments include:

13

Figure 8.

□ Flow meter.

□ Data logger.

2.4.1.2 Experimental procedure

The rate of heat transfer to the cooling water, Qout, can be evaluated by:

$$\mathbf{Q}\_{out} = \dot{m}\mathbf{C}\_p(T\_{out} - T\_{in}) \tag{4}$$

where Tin and Tout are respectively the inlet and outlet temperatures of the cooling water, while m: and Cp are the mass flow rate, kg/s and the specific heat capacity of water, kJ/kg-K respectively.

Two approaches are usually employed in the performance characterization of thermosyphon, namely:


#### 2.4.1 Experimental study on the performance of thermosyphon

The thermosyphon heat pipe can be experimentally characterized and the effects of some parameters on its performance evaluated. Figures 7 and 8 show a schematic diagram and picture of a typical test rig for the performance characterization of thermosyphon constructed at the University of Birmingham, UK, for analyzing the performance of a two-phase closed thermosyphon. It consists of a 0.4-m-long two-phase closed thermosyphon heat pipe, heating coil, water jacket and other instrumentations.

The heat can be supplied by hot water circulating around the evaporator or by electric power supply. In Figures 6 and 7, the evaporator section is wrapped evenly with electric wire with electric energy supplied and controlled by TSx1820P Programmable DC PSU 18 V/20A power regulator to provide the heat required for boiling the working fluid inside the pipe. A multimeter is used for measuring the voltage input which is connected close to the pipe to account for the voltage drop

Figure 7. Schematic diagram of the experimental test rig for thermosyphon characterization [17, 19].

where Tae and Tac are respectively the average temperatures on the evaporator

However, the performance of the thermosyphon can also be calculated as the

The rate of heat transfer to the cooling water, Qout, can be evaluated by:

where Tin and Tout are respectively the inlet and outlet temperatures of the cooling water, while m: and Cp are the mass flow rate, kg/s and the specific heat

Two approaches are usually employed in the performance characterization of

The thermosyphon heat pipe can be experimentally characterized and the effects of some parameters on its performance evaluated. Figures 7 and 8 show a schematic diagram and picture of a typical test rig for the performance characterization of thermosyphon constructed at the University of Birmingham, UK, for analyzing the performance of a two-phase closed thermosyphon. It consists of a 0.4-m-long two-phase closed thermosyphon heat pipe, heating coil, water jacket and other

The heat can be supplied by hot water circulating around the evaporator or by electric power supply. In Figures 6 and 7, the evaporator section is wrapped evenly with electric wire with electric energy supplied and controlled by TSx1820P Programmable DC PSU 18 V/20A power regulator to provide the heat required for boiling the working fluid inside the pipe. A multimeter is used for measuring the voltage input which is connected close to the pipe to account for the voltage drop

Schematic diagram of the experimental test rig for thermosyphon characterization [17, 19].

η ¼ Qout=Qin (3)

Qout ¼ mC\_ <sup>p</sup>ð Þ Tout � Tin (4)

and condenser while Qin is the heat supplied to the evaporator.

2.4.1 Experimental study on the performance of thermosyphon

capacity of water, kJ/kg-K respectively.

thermosyphon, namely:

Recent Advances in Heat Pipes

• Experimental

• Numerical

instrumentations.

Figure 7.

12

ratio of the heat transfer to the cooling water to the heat input as [18]:

Figure 8. Picture of the heat transfer characterization of thermosyphon test rig [17, 19].

while the current was read from the power regulator. The evaporator section is also insulated with 25-mm-thick pipe insulator to reduce the heat loss to the ambient environment (Figure 8). For measuring the temperature distribution along the pipe, 12 surface thermocouples were placed at different locations on the test pipe; 4 on the evaporator wall (at 0.02, 0.07, 0.12 and 0.17 m from the tip of the evaporator) and 2 on the condenser wall at 0.25 and 0.35 m as shown in the figures. The electric wires were wrapped in such way that they are not directly on the thermocouples so as to not affect their readings. Two probe thermocouples were installed at the inlet and outlet of the manifold to measure the temperatures of the cooling water. Three other thermocouples were used on the water jacket and one on the insulator to measure the effectiveness of the insulation and the jacket. All the readings were sent to Pico TC-08 data loggers connected to a PC.
