**3.2 Testing facility for thermosyphon performance**

108 Heat Exchangers – Basics Design Applications

Figure 6 shows an outline of the facility for loading working fluid. It consists of an array of 2 resistors of the band type with an electrical power of 250 W each. The supply of electricity to the resistance was provided by a voltage variable autotransformer of 1.4 kW and power was

Before starting the loading procedure of fluid inside the thermosyphon, the tube was cleaned with soapy water and rinse with distilled water followed by methyl alcohol and

First to fill the thermosyphons with the required filling rate, it must be known how quickly the fluid evaporates in the loading process. To carry out this process a known mass of water was introduced, then the evaporator section was heated and some time was allowed to

In this research seven tests of loading were performed at different times. The testing time was measured when the steam started to leave the thermosyphon from the needle valve installed on the top, until closing the valve. After valve was closed and the device was cooled off, the working fluid was removed to check how much fluid was ejected from

The loading process consisted of heating for some time the thermosyphon filled with a little more than the desired amount of fluid. This in order to evaporate the difference, and so fill all the volume with water vapor, so that it forces the non-condensable gases to escape through the needle valve installed on the top of the thermosyphon, along with water vapor excess. Knowing the rate of evaporation of the fluid, the loading process time depends on

To check the amount of fluid evacuated during the loading time, the steam was captured and taken through a condenser to a container where the mass ejected can be measured by a

calculated from data obtained by a voltmeter and an ammeter, both digital.

finally dried by applying heat. After that the process of loading fluid began.

**3.1 Installation for loading of working fluid** 

evaporate this water mass.

thermosyphon during the loading process.

the amount of fluid is desired to leave inside the thermosyphon.

Fig. 6. Outline of the facility for loading working fluid.

Figure 7 shows a schematic of the experimental setup. It consists of a thermosyphon installed in a wind tunnel. On the section of the evaporator are placed, three electrical resistances of 250 W and 110 V AC. The power source is the same as in the previous installation.

Heat is removed from condenser section by a flow of cooling air. The test section is located on the suction side of the tunnel; the air is forced by an axial ventilator with a 1.12 kW motor. To measure the air temperatures at the inlet and outlet of the test section type "K" thermocouples were used. Moreover, seven K-type thermocouples were placed on the outer surface of the tube in order to measure the temperature distribution along the thermosyphon. The record of these temperatures was achieved by a Cole Parmer data acquisition system.

Fig. 7. Schematic of the experimental setup for thermosyphon performance testing.

Thermocouples were placed from the bottom of the evaporator in the positions of 5, 18, 35.5, 52, 65, 76.5 and 88 cm. These thermocouples were attached firmly to the surface and small sections of the fins were removed to provide a better placement.

The supply of heat to the evaporator section is calculated using the following relationship:

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery 111

*meva* -3.26829 [g/min] *b* 45.72 [g] *y* 0.32 [g] *meva* 0.06 [g/min] *b* 0.35 [g]

Table 4. Evaporation rate *meva* , intercept *b*, standard deviation *y* and uncertainties in the

To study the temperature distribution along the thermosyphon, temperatures were recorded simultaneously at three points in the evaporator section, at one point in the adiabatic section

Figure 9 shows the temperature variation over time during the loading process. An applied power of 242 W and an ambient temperature of 20 °C were the test conditions. It was obtained a loading rate of 20% with a fluid excess of 5%. During the loading process may be present operation limits of the thermosyphon because this loading process is similar to the

**4.2 Temperature distribution on the outer surface of the thermosyphon tube** 

Fig. 9. Temperature rise versus time along the thermosyphon during loading.

The data shown below were obtained in tests of thermosyphons loaded with five different rates. After the tests, from each thermosyphon was extracted and measured the amount of water contained in them in order to determine their loading rate. The obtained loading rates

**4.3 Transported heat, efficiency and isothermal behavior** 

were: 28.82%, 24.81%, 18.91%, 15.06% and 10.17%.

estimated values of the evaporation rate *meva* and intercept *b*.

operation of a semi-open thermosyphon (Zhu et al., 2004).

and three more points in the condenser.

$$Q\_{sup} = \frac{\text{U}^2}{\text{R}} = \text{U}\text{I}\tag{25}$$

On the other hand, the heat dissipated by the condenser is obtained by the following relationship:

$$Q\_{ext} = \dot{m}\_a c\_{pa} \left( T\_{out} - T\_{in} \right) \tag{26}$$

The experimental tests consisted of the systematic variation of the supply of heat flow to the evaporator, keeping constant the amount of working fluid, the electric motor speed and the length of heating. Once the temperatures are stable throughout the thermosyphon this is considered a test point. Thus the data of temperature distribution and the heat dissipated is obtained for a loading volume of working fluid. Table 3 shows the parameters that were varied for these tests.


Table 3. Test parameters.

#### **4. Results and discussion**

#### **4.1 Thermosyphon behavior during loading of the working fluid**

Figure 8 shows the time and mass of water leaved in thermosyphon after the loading tests applying 272 W to evaporate the fluid. By applying a least squares method to approximate the data it was obtained an evaporation rate equal to 3.2683 g/min. Figure 8 shows the line fitted to the data obtained experimentally, which has a negative slope. Table 4 presents the values of standard deviation in the final internal mass measurement and uncertainties in the estimated value of the interceptor and the value of the evaporation rate, calculated by the least squares method.

Fig. 8. Final internal mass experimental values approximation to a line.

2

*<sup>R</sup>* (25)

*Q mc T T ext a a out in <sup>p</sup>* (26)

*U Q UI*

On the other hand, the heat dissipated by the condenser is obtained by the following

The experimental tests consisted of the systematic variation of the supply of heat flow to the evaporator, keeping constant the amount of working fluid, the electric motor speed and the length of heating. Once the temperatures are stable throughout the thermosyphon this is considered a test point. Thus the data of temperature distribution and the heat dissipated is obtained for a loading volume of working fluid. Table 3 shows the parameters that were

> Loading rates (Ψ) 10%, 15%, 20%, 25%, 30% Heat supply 800 W < Q < 2000 W Mass flow of cooling air 0.175, 0.247, 0.380 kg/s

Figure 8 shows the time and mass of water leaved in thermosyphon after the loading tests applying 272 W to evaporate the fluid. By applying a least squares method to approximate the data it was obtained an evaporation rate equal to 3.2683 g/min. Figure 8 shows the line fitted to the data obtained experimentally, which has a negative slope. Table 4 presents the values of standard deviation in the final internal mass measurement and uncertainties in the estimated value of the interceptor and the value of the evaporation rate, calculated by the

> 2 3 4 5 6 7 8 9 10 Time (min)

**4.1 Thermosyphon behavior during loading of the working fluid** 

*sup*

relationship:

varied for these tests.

Table 3. Test parameters.

least squares method.

10

Fig. 8. Final internal mass experimental values approximation to a line.

15

20

25

30

35

40

**4. Results and discussion** 


Table 4. Evaporation rate *meva* , intercept *b*, standard deviation *y* and uncertainties in the estimated values of the evaporation rate *meva* and intercept *b*.

#### **4.2 Temperature distribution on the outer surface of the thermosyphon tube**

To study the temperature distribution along the thermosyphon, temperatures were recorded simultaneously at three points in the evaporator section, at one point in the adiabatic section and three more points in the condenser.

Figure 9 shows the temperature variation over time during the loading process. An applied power of 242 W and an ambient temperature of 20 °C were the test conditions. It was obtained a loading rate of 20% with a fluid excess of 5%. During the loading process may be present operation limits of the thermosyphon because this loading process is similar to the operation of a semi-open thermosyphon (Zhu et al., 2004).

Fig. 9. Temperature rise versus time along the thermosyphon during loading.

#### **4.3 Transported heat, efficiency and isothermal behavior**

The data shown below were obtained in tests of thermosyphons loaded with five different rates. After the tests, from each thermosyphon was extracted and measured the amount of water contained in them in order to determine their loading rate. The obtained loading rates were: 28.82%, 24.81%, 18.91%, 15.06% and 10.17%.

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery 113

In Figure 11 it can be seen that as the heat flow supplied into the evaporation area increases, the ability to transfer heat of the thermosyphons decreases. Also, it can be seen in this plot that the thermosyphon loaded with 18.91% is the one that has a greater capacity to transport

Fig. 12. Temperature profiles versus amounts of delivered heat (cooling air flow: 0.38 kg/s).

heat.

From the results of this experiment can be extracted the heat transport capacity, efficiency and the temperature profile along the thermosyphons. Due to the restriction in the scope of this chapter there are presented below only the results for a cooling air mass flow of (0.38 kg/s).

Figure 10 shows the heat transported by the thermosyphons, loaded with different percentages of working fluid, with respect to operating temperature (thermocouple installed on adiabatic zone, *Tad*). As shown in Figure 10, the thermosyphon with a load of 18.91% show the greatest amount of heat extracted in comparison to the rest of the thermosyphons. Therefore, in can be concluded that the thermosyphon loaded with this amount of fluid is the one that had a higher heat transfer for this flow of cooling air. On the other hand, the thermosyphons loaded with 28.82% and 10.17%, are the thermosyphons which have the lowest amounts of heat extracted.

Fig. 10. Transported heat versus the operating temperature (cooling air flow: 0.38 kg/s).

Fig. 11. Transported heat versus supplied heat (cooling air flow: 0.38 kg/s).

From the results of this experiment can be extracted the heat transport capacity, efficiency and the temperature profile along the thermosyphons. Due to the restriction in the scope of this chapter there are presented below only the results for a cooling air mass flow of

Figure 10 shows the heat transported by the thermosyphons, loaded with different percentages of working fluid, with respect to operating temperature (thermocouple installed on adiabatic zone, *Tad*). As shown in Figure 10, the thermosyphon with a load of 18.91% show the greatest amount of heat extracted in comparison to the rest of the thermosyphons. Therefore, in can be concluded that the thermosyphon loaded with this amount of fluid is the one that had a higher heat transfer for this flow of cooling air. On the other hand, the thermosyphons loaded with 28.82% and 10.17%, are the thermosyphons which have the

Fig. 10. Transported heat versus the operating temperature (cooling air flow: 0.38 kg/s).

Fig. 11. Transported heat versus supplied heat (cooling air flow: 0.38 kg/s).

(0.38 kg/s).

lowest amounts of heat extracted.

*Q*ext (W) In Figure 11 it can be seen that as the heat flow supplied into the evaporation area increases, the ability to transfer heat of the thermosyphons decreases. Also, it can be seen in this plot that the thermosyphon loaded with 18.91% is the one that has a greater capacity to transport heat.

Fig. 12. Temperature profiles versus amounts of delivered heat (cooling air flow: 0.38 kg/s).

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery 115

**7. Nomenclature** 

*a* – width of the gas-gas heat exchanger, [m]; *cp* – heat capacity at constant pressure, [J/kg K];

*h* – convective heat transfer coefficient, [W/m2 K];

*Leva* – characteristic length of evaporation, [m];

*D* – binary diffusion coefficient, [m2/s];

*hm* – mass transfer coefficient, [m/s];

*k* – thermal conductivity, [W/m K];

*E* – modulus of elasticity, [Pa];

g – gravitational force, [m/s2]; *H* – specific enthalpy, [kJ/kg];

*A* – area, [m2];

*d* – diameter, [m];

Gr – Grashof number;

*I* – electric current, [A];

*m* – mass flow, [kg/s];

*R* – electric resistance, [Ω]; Ra – Rayleigh number;

*s* – specific entropy, [kJ/kg ºC]; *T* – temperature, [oC, K];

*t* – wall thickness of container, [m];

*u* – specific internal energy, [kJ/kg];

– thermal diffusivity, [m2/s];

– kinematic viscosity, [m2/s];

– specific volume, [kg/m3];

– density, [kg/m3].

– relative humidity;

0 – normal conditions;

*β* – volumetric thermal expansion coefficient, [K-1];

*l* – length, [m];

*m* – mass, [kg]; Nu – Nusselt number; Pr – Prandtl number; *p* – pressure, [Pa]; *Q* – heat flow, [W]

*r* – radius, [m];

*U* – voltaje, [V];

x – quality; *Ψ*– loading rate;

Subscripts:

*a* – air;

*V*– volumen, [m3]; *v* – velocity, [m/s];

 – stress, [N /m2]; *S* – entropy, [kJ/ºC]

Figure 12 shows the temperature profiles along the thermosyphons for different loading rates. In the plots of Figure 12 it can be seen that profiles with higher temperatures are of the thermosyphons loaded with rates of 24.81%, 18.91% and 15.06%. This means that these thermosyphons reached higher temperatures, compared to the other two thermosyphons, under the same conditions of heat supply and cooling air flow. Moreover, these plots show that the lowest average temperature differences between condenser and evaporator are presented in the thermosyphon loaded with 10.17% in the range of heat input of 500 W to 800 W.

Also, it can be noted that for this thermosyphon it was presented a relative increase of temperature at the bottom of the evaporator for each value of the heat supplied, specifically for the heat supply 2000 W, this indicates the start of drying in the area of evaporation.

In the plots of Figure 12 it can be observed that at the top of the condenser (thermocouple Tc1) there is a higher temperature than in the middle of the condensation zone (low thermocouple Tc2). For this flow of cooling air, the rate of heat extraction is higher compared to the steam generation in the evaporator, so in this region drying occurs causing a rapid condensation on the top of the thermosyphon.
