2. Nanofluids

The cooling process is the process of heat transfer from a heat source and removed to the environment (heat sink) at the lower temperature. In forced convection heat transfer occurs at high-temperature to low-temperature fluids, which are separated by pipe walls. Increased heat dissipation is done by increasing the convection surface area, i.e., using fins. This method has been abandoned because of the higher the dimensions, and the more massive the equipment consequently the price becomes expensive. The second way is to increase the fluid flow rate, but it will have an impact on the greater use of energy, which causes the system to be inefficient.

Figure 1. Comparison of thermal conductivity for different materials.

Performance Evaluation Criterion of Nanofluid http://dx.doi.org/10.5772/intechopen.74610 279

Figure 2. Dimensionless physical properties of nanofluids in comparison to those of pure water.

The new method to increase heat transfer is to improve thermal properties of fluids especially heat conductivity. This way is by adding solid particles with high thermal conductivity into the cooling fluid with low thermal conductivity as shown in Figure 1.

Started by Maxwell as the pioneer [8], adding solid particles of micro size into the fluid is expected to improve thermal properties of fluids, especially heat conductivity. Because the particle size is large enough, the particles quickly agglomerate and cause clogging the channel. Later, Choi [6] introduced the term nanofluid defined as colloids made of a base fluid and nanoparticle size (1–100 nm). The properties of the fluid increase especially the heat conductivity (k), viscosity (μ), and density (r) increase in proportion to the increased particle volume concentration, whereas the specific heat (Cp) decreases proportionally to the increase in volume concentration. The effect of nanoparticle concentration on the physical properties of nanofluid has been studied by describing the variation in the ratio of physical properties of nanofluid to pure water as a function of nanoparticle volume concentration [9]. Figure 2 shows the effect of nanoparticles Al2O<sup>3</sup> of nanoparticle concentration on the physical properties of nanofluid, i.e., viscosity, density, and thermal conductivity, which have increased, while the specific heat is slightly decreased compared to the base fluid.

Figure 1. Comparison of thermal conductivity for different materials.

and the use of the magnetic field. Over the past few decades, the effect of high electric fields on the rate of heat transfer is widely known as electrohydrodynamics (EHD), [4] and the effect of the magnetic field on magnetic iron oxide particle (Fe3O4) numerically has been investigated using control volume finite element method (CVFEM) [5]. However, when the available space is limited by the process, it is interesting to use the device with the same size or smaller with better performance. It can be achieved by modifying the cooling with higher thermal conductivity to enhance the heat transfer coefficient when compared with that of conventional fluids for the

The objective of this chapter is to how to decrease the size of the thermal system or to increase their transferred thermal power using nanofluid. Nanofluids are colloidal suspensions of nanoparticles which are engineered to have the thermal conductivity higher than that of the base fluid and which can be used for this purpose [6, 7]. However, together with thermal conductivity enhancement, the viscosity is increased, and the gain in transferred heat is paid regarding pumping power. There is a competition between heat transfer rate and pumper

The cooling process is the process of heat transfer from a heat source and removed to the environment (heat sink) at the lower temperature. In forced convection heat transfer occurs at high-temperature to low-temperature fluids, which are separated by pipe walls. Increased heat dissipation is done by increasing the convection surface area, i.e., using fins. This method has been abandoned because of the higher the dimensions, and the more massive the equipment consequently the price becomes expensive. The second way is to increase the fluid flow rate, but it will have an impact on the greater use of energy, which causes the system to be inefficient. The new method to increase heat transfer is to improve thermal properties of fluids especially heat conductivity. This way is by adding solid particles with high thermal conductivity into the

Started by Maxwell as the pioneer [8], adding solid particles of micro size into the fluid is expected to improve thermal properties of fluids, especially heat conductivity. Because the particle size is large enough, the particles quickly agglomerate and cause clogging the channel. Later, Choi [6] introduced the term nanofluid defined as colloids made of a base fluid and nanoparticle size (1–100 nm). The properties of the fluid increase especially the heat conductivity (k), viscosity (μ), and density (r) increase in proportion to the increased particle volume concentration, whereas the specific heat (Cp) decreases proportionally to the increase in volume concentration. The effect of nanoparticle concentration on the physical properties of nanofluid has been studied by describing the variation in the ratio of physical properties of nanofluid to pure water as a function of nanoparticle volume concentration [9]. Figure 2 shows the effect of nanoparticles Al2O<sup>3</sup> of nanoparticle concentration on the physical properties of nanofluid, i.e., viscosity, density, and thermal conductivity, which have increased, while the

cooling fluid with low thermal conductivity as shown in Figure 1.

specific heat is slightly decreased compared to the base fluid.

same geometry.

278 Microfluidics and Nanofluidics

power.

2. Nanofluids

Figure 2. Dimensionless physical properties of nanofluids in comparison to those of pure water.

At first, the researchers only investigated the effect of particle volume concentration on thermal conductivity enhancement. Three possible approaches have been pursuing the study of nanofluid: experimental, empirical, and numerical.

3. Thermal properties of nanofluid

conductivity affect viscosity value.

viscosity.

3.1. Thermal conductivity of nanofluid

balls into a liquid known as the Maxwell model.

and kp is the thermal conductivity of the nanoparticles.

Thermophysical property, especially the thermal conductivity, is a vital issue in nanofluid heat transfer phenomena. Prediction of thermal conductivity has been a severe challenge until now because many parameters affect the thermal conductivity values. Temperature, type of the base fluid, nanoparticle material, shape, size, volumetric fraction, production, and mixing methods may significantly change the thermal conductivity. The literature research on thermal conductivity of nanofluids is a guide to understand how different parameters affect the value and what kind of thermal conductivity model is selected for the calculation heat transfer enhancement.

Secondly, the viscosity is also crucial in nanofluid heat transfer performance, as the usage of nanofluid viscosity also increases. Prediction of the viscosity of nanofluids is also a challenging topic because of its increase in the pumping power. The similar parameters that affect thermal

Researchers have widely studied nanofluid thermal conductivity. Investigation the increase of analytical thermal conductivity a solid-liquid mixture by adding solid particles of micro-size

knf <sup>¼</sup> kp <sup>þ</sup> <sup>2</sup>kb <sup>þ</sup> <sup>2</sup><sup>φ</sup> kp � kb

kp þ 2kb � φ kp � kb

where φ is the volume fraction of the nanofluid, kb is the thermal conductivity of the base fluid,

Hamilton and Crosser [13] proposed a model for nonspherical particles by introducing a shape

knf <sup>¼</sup> kp <sup>þ</sup> ð Þ <sup>n</sup> � <sup>1</sup> kb � ð Þ <sup>n</sup> � <sup>1</sup> <sup>φ</sup> kb � kp

kp þ ð Þ n � 1 kb � φ kb � kp

A modified Maxwell's model was proposed by Xuan and Li [14] by considering the Brownian motion of the particles in the base fluid for the thermal conductivity enhancement given as

� �φ

kb þ

rφCp 2

ffiffiffiffiffiffiffiffiffiffiffiffi kBT 3πrcμ

s

� �φ

where kB is the Boltzmann constant, rc is the apparent radius of the cluster, and μ is a dynamic

Figure 3 shows the variation of knf =kb as a function of alumina volume fraction with and without CTAB along with its best fits with and without interfacial resistance. The knf =kb value

factor n given by n = 3/φ. The thermal conductivity is expressed as follows:

knf <sup>¼</sup> kp <sup>þ</sup> <sup>2</sup>kb <sup>þ</sup> <sup>2</sup> kb � kp

kp þ 2kb � kb � kp

� �

� �

� � kb (1)

Performance Evaluation Criterion of Nanofluid http://dx.doi.org/10.5772/intechopen.74610 281

� � kb (2)

(3)
