**4. Experimental tests**

To achieve successful results, the experimental tests must reproduce the operation conditions as close as possible to the application for thermal management of thermoelectric cooling. Then, to evaluate the thermal performance of the analyzed passive heat transfer devices, an experimental apparatus and some experimental procedures were used.

## **4.1. Experimental apparatus**

The essential experimental apparatus for the experimental tests, shown in **Figure 13**, is composed of a data logger (*Agilent*™ 34970A with 20 channels), a power supply unit (*Keysight* ™ U8002A), a laptop (*Dell*™), an uninterruptible power supply (*NHS*™), a universal support, and a fan (*Ultrar*™).

For the evaluation of the temperature of the different heat transfer passive devices, K-type thermocouples *Omega Engineering*™ are used. They should be fixed on the outer surface of devices by a thermosensitive adhesive strip *Kapton*™. They should be distributed in the length of the heat pipes and thermosyphon. Thus, there are three thermocouples in the evaporator (*Tevap*,1, *Tevap*,2, and *Tevap*,3), one thermocouple in the adiabatic section (*Tadiab*) e four thermocouples in the condenser (*Tcond*,1, *Tcond*,2, *Tcond*,3, and *Tcond*,4) in passive devices (heat pipes and thermosyphon), as shown in **Figure 14**. For the rod, two thermocouples were fixed in the evaporator

**Figure 13.** Experimental apparatus.

**Figure 12.** Structure sintered copper powder. (a) General view and (b) microscale image.

**Figure 11.** Microgrooves made by wire-EDM. (a) Scheme of microgroove profile and (b) microscale image.

in length, an adiabatic region of 20 mm in length, and a condenser of 100 mm in length. The working fluid used is distilled water with filling ratios related to the evaporator volume based on the best performance of each capillary structure. **Table 1** shows the main characteristics of

The mesh heat pipe used one layer of phosphor bronze screen mesh #100 (**Figure 10a**) as capillary structure. A microscale image of screen mesh #100 is shown in **Figure 10b**. The image was obtained by backscattered electron detector (BSD) for scanning electron microscope (SEM).

The grooved heat pipe shown schematically in **Figure 11a** had 32 microgrooves made by the wire electrical discharge machining (wire-EDM). **Figure 11b** presents the axial microgrooves details with an average diameter of 220 μm by a micro-scale image. The image was obtained by backscattered electron detector (BSD) for scanning electron microscope (SEM). More

the heat transfer passive devices analyzed in this research.

362 Bringing Thermoelectricity into Reality

details about this heat pipe can be found in [24, 28].

More information about this mesh heat pipe can be found in [23].

**5. Data reduction**

culated by:

Defined by:

denser length.

where *Di*

length can be defined by:

**5.2. Uncertainties analysis**

**5.1. Thermal parameters**

*Rth* <sup>=</sup> \_\_\_

the evaporator and the condenser, respectively.

*<sup>k</sup>eff* <sup>=</sup> *<sup>q</sup> <sup>L</sup>eff* \_\_\_\_\_\_\_\_\_\_

*<sup>L</sup>eff* <sup>=</sup> *Levap* \_\_\_\_

The heat transfer cross-sectional area can be defined by:

is the inner diameter of the heat transfer passive device.

*AC* <sup>=</sup> (*πDi*

The thermal performance of the heat pipes and the thermosyphon was analyzed and compared by the operating temperatures (*Top*), the global thermal resistance (*Rth*), and the effective thermal conductivity (*keff*). The analyzed operating temperature is the temperature of the adiabatic region. The global thermal resistance, *Rth*, of a heat pipe and a thermosyphon can be defined as the difficulty of the passive device to transport the heat power and can be cal-

Heat Pipe and Thermosyphon for Thermal Management of Thermoelectric Cooling

Δ*T*

*<sup>q</sup>* <sup>=</sup> (*Tevap* <sup>−</sup> *<sup>T</sup>* \_\_\_\_\_\_\_\_\_ cond)

where, *q* is the heat transfer capability of the device, *Tevap* and *Tcond* are the mean temperature of

The effective thermal conductivity, *keff*, is the property of a certain material to conduct heat.

where, *Leff* is the effective length and *AC* is the heat transfer cross-sectional area. The effective

<sup>2</sup> + *Ladiab* +

2 ) \_\_\_\_\_

where, *Levap* is the evaporator length, *Ladiab* is the adiabatic section length, and *Lcond* is the con-

In general, the experimental uncertainties are associated to the K-type thermocouples, the data logger, and the power supply unit. The experimental measurement uncertainties were analyzed using the uncertainty combination method described in [29] considering the combination of uncertainties of correlated quantities. They are shown in the obtained

*L* \_\_\_\_ *cond*

*AC* <sup>Δ</sup>*<sup>T</sup>* <sup>=</sup> *<sup>q</sup> <sup>L</sup>* \_\_\_\_\_\_\_\_\_\_\_ *eff*

*<sup>q</sup>* (1)

http://dx.doi.org/10.5772/intechopen.76289

365

*AC*(*Tevap* <sup>−</sup> *<sup>T</sup>*cond) (2)

<sup>2</sup> (3)

<sup>4</sup> (4)

**Figure 14.** Thermocouple positions in heat pipes.

(*Tevap*,1 and *Tevap*,2), one thermocouple in the adiabatic section (*Tadiab*) and three thermocouples in the condenser (*Tcond*,1, *Tcond*,2, and *Tcond*,3).

As is already known, for the correct operation of the heat pipe and thermosyphon, a heating system is needed in the evaporator and a cooling system in the condenser. The evaporator can be power dissipation in any kind of resistor (strip, cartridge) or a heat source, as the TEC hot side. The cooling system can consist of forced convection by air, water, or coolant, in most of the cases. The adiabatic section may have variable dimensions (in some cases, it is absent) and should be insulated from the external environment.

Thus, in this research, the heating system of the evaporator is conducted by power dissipation from the passage of an electric current in a nickel-chromium alloy power strip resistor *Omega Engineering*™ with 0.1 mm of thickness and 3.5 mm of width. To ensure that the generated heat by Joule effect is transmitted to the evaporator, an aeronautic thermal insulation and a layer of polyethylene are installed in this region. A fiberglass tape is used in adiabatic section as heat insulation between the support and the passive device. The cooling system using air forced convection consisted of a fan in the condenser region.

#### **4.2. Experimental procedure**

To ensure the best results and the repeatability of experimental tests, the environment temperature was maintained at 20°C ± 0.5°C. A thermal conditioning system *Carrier™* was used for this purpose. A detailed check of the equipment and the heat pipe or thermosyphon (fixing thermocouples, thermal insulation, resistor connection, among others) must be made before each experimental test. The heat pipe or thermosyphon was carefully fixed to the universal support bracket in the adiabatic region in the desired position. The cooling system was turned on in the condenser region and set at a speed of 5 m/s controlled by a potentiometer with a combined error of ±0.2 m/s. The data acquisition system was turned on, collecting the temperatures measured by the K-type thermocouples. The temperatures should be verified according to the environment temperature, and if these were stable and approximately 20°C, finally, the heating system can be turned on and adjusted to the dissipation power desired. The initial load was 5 W and, after approximately 15 min, the thermocouples showed stationary values. If it happened, the thermal load has been increased by 5 W. The load increment was made until the maximum temperature of the device reached the critical temperature (150°C), where the melting of the materials could happen. Data were acquired every 5 s, recorded on the desktop by the software *Agilent™ Benchlink Data Logger 3*.
