7. Heat transfer analysis of the heat exchanger

To enhance the heat transfer rate between the hot and the cold fluid flows, heat exchangers are commonly applied in air cooling systems. Relations between the hot and the cold side temperatures as well as the optimum heat transfer surface area can be calculated by applying energy balances to both the hot and the cold sides of the TE modules over a differential area [101].

The heat transfer on the hot side of the TE devices can be increased using cross air flow or counter air flow [102]. The COP of TE devices could be improved by minimizing the difference in temperature between their hot and cold faces while applying appropriate electrical power [100]. A TE system used for cooling or warming airflow with a high COP of 1.5 was reported [93]. To achieve such a relatively high COP, the temperature difference between the hot and the cold faces of the TE modules was maintained at 5C.

Other favorable working strategies using different heat transfer methods such as liquid cooling with phase change materials were also reported [103]. Theoretical and experimental studies were conducted to examine the performance characteristics of TE water cooling system for electronic cooling applications under small heat loads [11]. A TE liquid chiller was developed with 430 ml capacity and a COP ranged between 0.2 and 0.8 for a temperature of 5–15C below ambient [104]. A cylindrical, water-cooled heat sink for TE air conditioners was designed and characterized [105]. In this context, a thermosyphon with phase change was developed to improve the thermal resistance of the heat exchanger at the hot side of the TE by 36% [12]. This increased the COP of a TE module by 26% at an ambient temperature of 20C and 36.5% at 30C. Using evaporative cooling the COP of TE air-conditioning system was improved by 20.9% [106].

TE devices, as electronic components, do not allow direct contact with coolant. Therefore, instead of pumping coolant directly through TE coolers, channels plate liquid cooling system is used. A channels plate block is a heat conductive metal, such as aluminum or copper, which is filled with channels. The base of the water block is a flat metal surface that is placed directly on top of the hot side of the TE module being cooled using thermal paste to improve transferring the heat between the two surfaces. When the TE hot side heats the block, the liquid coolant absorbs the heat as it flows through all the channels, which will be dissipated through a radiator. The same system can be applied at the cold side for the transfer of the cool due to high thermal resistance between the cold side of the TE and the space being cooled.

resistance; consideration of optimum values for voltage and current of each module; verification of thermal efficiency of each module and calculations of temperature difference, maximum cooling capacity according to the measurement results, figure of merit and values of

Heat sink performance at the hot side is more important than heat sink at the cold side because the heat flux density at hot side is higher. Allocation of the heat transfer area or heat transfer coefficients between hot and cold sides is particularly important. For given hot and cold side fluid temperatures, there exists an optimum cooling capacity which leads to maximum COP

The COP of TE devices could be improved by minimizing the difference in temperature between their hot and cold faces [100]. The hot side of the TE cooler exhibits very high power densities that demands sophisticated cooling infrastructure with high pumping power.

To enhance the heat transfer rate between the hot and the cold fluid flows, heat exchangers are commonly applied in air cooling systems. Relations between the hot and the cold side temperatures as well as the optimum heat transfer surface area can be calculated by applying energy balances to both the hot and the cold sides of the TE modules over a differential area [101].

The heat transfer on the hot side of the TE devices can be increased using cross air flow or counter air flow [102]. The COP of TE devices could be improved by minimizing the difference in temperature between their hot and cold faces while applying appropriate electrical power [100]. A TE system used for cooling or warming airflow with a high COP of 1.5 was reported [93]. To achieve such a relatively high COP, the temperature difference between the hot and the

Other favorable working strategies using different heat transfer methods such as liquid cooling with phase change materials were also reported [103]. Theoretical and experimental studies were conducted to examine the performance characteristics of TE water cooling system for electronic cooling applications under small heat loads [11]. A TE liquid chiller was developed with 430 ml capacity and a COP ranged between 0.2 and 0.8 for a temperature of 5–15C below ambient [104]. A cylindrical, water-cooled heat sink for TE air conditioners was designed and characterized [105]. In this context, a thermosyphon with phase change was developed to improve the thermal resistance of the heat exchanger at the hot side of the TE by 36% [12]. This increased the COP of a TE module by 26% at an ambient temperature of 20C and 36.5% at 30C. Using evaporative cooling the COP of TE air-conditioning system was improved by 20.9% [106].

TE devices, as electronic components, do not allow direct contact with coolant. Therefore, instead of pumping coolant directly through TE coolers, channels plate liquid cooling system is used. A channels plate block is a heat conductive metal, such as aluminum or copper, which is filled with channels. The base of the water block is a flat metal surface that is placed directly on top of the hot side of the TE module being cooled using thermal paste to improve

7. Heat transfer analysis of the heat exchanger

cold faces of the TE modules was maintained at 5C.

internal resistance [99].

256 Bringing Thermoelectricity into Reality

[64, 80, 92].

Recently, heat transfer in mini channels within heat exchangers is drawing substantial attention trying to improve their performance. The proper selection of channel dimensions and nonuniform distribution of the channels can improve the cooling power [107]. Therefore, thermal and hydrodynamic characteristics of channels need to be examined and developed. A TE system using liquid cooling for electronic application using micro-channel heat sink was proposed and its experimental analysis performance was investigated [108]. The effect of channel width, coolant flow rate and heat sink material on the heat transfer rate was also examined [76].

Although micro-channel heat exchangers are able to dissipate higher heat flux densities, the slow flow rate creates a large increase in the temperature alongside the direction of the coolant flow in both channel material and the coolant. Surface roughness also participates in the heat transfer characteristics and the drop of pressure of coolant flow in a channel. Many studies clearly reported that the roughness has an effect on the flow of the coolant and heat transfer characteristics, in addition to the laminar and turbulent transition [109, 110]. Micro channel heat exchangers with different designs and coolants were manufactured and tested and the experimental results confirmed the superiority of this cooling technique [111, 112].

Heat removal through parallel channels involves a complex combination of convection, conduction and coolant flow. In a rectangular channel plate with width W, height H and length L, taking the advantage of the symmetry of the channels, a unit cell containing only one channel with the surrounding metal is chosen. The results obtained can easily be applied to the whole plate. Heat transport in the unit cell is a conjugate problem that mixes heat conduction in the metal and convective heat transfer to the coolant. The dissipated heat in the surrounding regions conducts to the channel side walls, which is then absorbed, through convection, by the coolant and carried away by the circulation.

These parameters can be summarized by stating them as thermal resistances. Conductive resistance, Rcond, is determined by thermal characteristics of aluminum that conducts the dissipated heat in the region surrounding the sidewalls of the channel. Convection resistance, Rconv, is a result of the convection from sidewalls of the channel to the coolant. Heat resistance, Rheat, is a result of heating up of coolant in the downstream direction as the flow is pushed toward the channel exits. These can be expressed as:

$$R\_{conv} = \frac{1}{h\,A} \tag{13}$$

where A is the channel surface area. Assuming that heat is transmitted from all the sidewalls, the surface area will be:

$$A = 2\operatorname{L}(W + H) \tag{14}$$

here h is the convective heat transfer coefficient:

$$h = \frac{N\_u K\_f}{D\_h} \tag{15}$$

8. Conclusions

Author details

Raghied M. Atta

References

tion. 2017;74:682-688

4028-4040

Address all correspondence to: ratta@taibahu.edu.sa

Engineering College, Taibah University, Madinah, Saudi Arabia

In this chapter, a short review of technologies related to the TE cooling was presented. The new methodologies of system design and system analysis have enabled the design of highperformance TE cooling systems. This includes the use of the basic physical properties of TE modules and the flow equations to identify the TE cooling design parameters to maximize the COP of the TE cooling systems. To minimize the energy demands in TE cooling systems and increase their energy effectiveness, solar TE cooling technologies such as active building envelope, solar thermoelectric coolers are suggested to be used in zero-energy environments.

Thermoelectric Cooling

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http://dx.doi.org/10.5772/intechopen.75791

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Conversion, 10–12 March 1982; Arlington, TX. USA: IEEE. pp. 136-141

International Journal of Energy Research. 2008;32:1316-1328

where Kf is coolant thermal conductivity, Nu is the Nusselt number calculated with the Dittus-Boelter equation [113],

$$N\_u = \, 0.023 \, P\_r^{0.4} \, R\_e^{0.8} \tag{16}$$

in which Pr is Prandtl number and Re is Reynolds number. Dh, the hydraulic diameter, is defined as:

$$D\_h = \frac{4\text{ (cross sectional area)}}{\text{perimeter}} = \frac{4\text{ }\text{V} \text{H}}{2\text{ }(\text{W} + \text{H})} \tag{17}$$

Hence the convective can be expressed as:

$$R\_{conv} = \frac{D\_h}{2 \text{ N}\_u \text{ K}\_f \text{L} (W + H)} \tag{18}$$

The heat resistances can be expressed as:

$$R\_{heat} = \frac{1}{\mathcal{C}\_p \rho\_c f} \tag{19}$$

where Cp is the coolant specific heat and r<sup>c</sup> is coolant density. f is the volumetric flow rate for each channel which is defined as:

$$f = \text{coulomb velocity}^{\*} \text{ cross sectional area} = \upsilon \,\,\text{W} \,\, H \tag{20}$$

The coolant viscosity and thermal conductivity vary according to the temperature [114]. The conductive resistances can be expressed as:

$$R\_{cond} = \frac{\mathcal{W}}{k \,\, L \,\, H} \tag{21}$$

where k is the thermal conductivity of the channels plates material.

For fluid dynamical and thermal phenomena that occur in the channels with corrugated walls, different heat transfer characteristics can be observed. Generally, the wall corrugation enlarges the surface of the channels and creates turbulence. However, most studies stated that the rise in temperature of the walls along the direction of the flow is almost linear [115–117].

Recently, heat sinks with nano-fluid have shown potential to achieve lower thermal resistance [118, 119]. In addition, cooling technologies based on heat removal from the heat sinks using synthetic jet [120], either single-phase or two-phase flow, are noticeable.
