**2. Nano coolants in microchannels for CPU cooling**

In this connection, heat transfer equipment with minimal size can be achieved by adding solid small-sized particles [5]. These particles have high thermal conductivity when compared to conventional coolants like water or ethylene glycol [6]. If the suspended particle size is in the order of microns and millimeter lot of problems like particle settlement, corrosion and more pressure drop needs to be overcome. Because of these drawbacks, milli-size or micron-size particles are not used widely. Choi and Eastman [7] tried suspending nanometer particles in a solution. The nano-sized particle and their low volume fractions prevent particles from clogging and also reduce the pressure drop. Stability is increased and sedimentation of the particle is decreased due to its increased surface area of particles [8]. The efficiency in heat removal is improved since heat transfer takes place at the particle surface. Nanosheets comprising MAXene obtained from Ti3SiC2 MAX phase ternary carbides can be a probable nanocoolant with very efficient thermal management. It is synthesized via shear-induced micromechanical delamination technique. It is also

**109**

*Analysis of Liquid Cooling in Microchannels Using Computational Fluid Dynamics (CFD)*

known as 'rheo–controlled' nanofluid. The thermal conductivity increments are about 45% at 323 K [9]. The corrosion effect of nano coolant accompanying material loss needs to be analyzed. Coolants such as graphene nanoparticles, corrosive water, and ethylene glycol were studied and their effects on parameters like the temperature of the coolant, inlet pressure, and rpm of the pump were recorded [10]. He also confirmed that the corrosion effect was the same for both the base coolant and nanocoolant [11]. The suspended graphene nanoparticles with water as a base solvent and for various volume fractions and measured its convection coefficients for the temperature range of 30 °C to 80 °C. The heat transfer characteristics of hybrid nanofluid (HyNF) flow through the tubular heat exchanger were also studied by [12]. They examined the various parameters like thermal conductivity and heat transfer coefficient under forced convection. The experiments were performed for various nanoparticles ranging from 0.1% to 1.0%volume concentrations added in the base fluid. The results showed that the bulk heat transfer coefficient

was maximum by 48.4% up to 0.7% volume concentration of HyNC.

Labib et al. [13] employed two different base fluids of water and ethylene glycol and analyzed the effect of convective heat transfer mixing of Al2O3 nanoparticles. The CFD results are validated with the experimental data for Al2O3/water nanofluid. The results revealed that Ethylene Glycol base fluid gives better heat transfer enhancement than that of water. The mixture of Al2O3 nanoparticles into CNTs/ water nanofluid is a new concept of combined/hybrid nanofluid that can profitably increase convective heat transfer. Eshgarf and Afrand [14] studied the rheological behavior of COOH functionalized MWCNTs–SiO2/EG–water hybrid nanocoolant for application in cooling systems. They prepared the stable suspension of MWCNTs and SiO2 nanoparticles (50:50 volume%) in a specified amount of a binary mixture of EG–water (50:50 vol.%) for the temperature range of 27.5–50 °C. From the results, it is inferred that the apparent viscosity generally increases with an increase in the solid volume fraction and decreases with increasing temperature. Naqiuddin et al. [15] showed geometrically graded microchannel heat sink for improvements in thermal performances. They showed for a heat load of 2000 W, the average temperature of the microchannel can be reduced to 69.6°Celsius with a temperature variation of 3° Celsius. Microchannel heat sink with micro-scale ribs and grooves for chip cooling is also investigated with CFD studies [16]. The performances of various heat sink parameters like trapezoidal, rectangular, and triangular-shaped microchannel with different channel width and aspect ratio was analyzed. The rectangular microchannel showed the best performance with the aspect ratio among 8.904–11.442. The rectangular microchannel has the lowest thermal resistance, followed by the trapezoid and triangle microchannel. Wang et al. [17] designed a heat sink with micro-scale ribs and grooves for chip cooling. The cooling efficiency is more for rib-grooved microchannel than the conventional smooth rectangular microchannel through experimental and numerical approaches. The Nusselt number of rib-grooved microchannel can be 1.11–1.55 times that of a smooth microchannel. He also studied the apparent friction factor for various rela-

For nanofluid synthesis, there are many key factors to be considered [18]. Stable nanofluid [19] can be formed by one and two-step methods [20]. Both one-step and two-step methods can produce nanoparticles in suspension and also agglomeration

*DOI: http://dx.doi.org/10.5772/intechopen.96248*

**2.1 CPU cooling with hybrid nanofluid**

tive rib heights of 0.6, 0.73, and 0.85 μm.

**2.2 Synthesis of nanofluid**

*Analysis of Liquid Cooling in Microchannels Using Computational Fluid Dynamics (CFD) DOI: http://dx.doi.org/10.5772/intechopen.96248*

known as 'rheo–controlled' nanofluid. The thermal conductivity increments are about 45% at 323 K [9]. The corrosion effect of nano coolant accompanying material loss needs to be analyzed. Coolants such as graphene nanoparticles, corrosive water, and ethylene glycol were studied and their effects on parameters like the temperature of the coolant, inlet pressure, and rpm of the pump were recorded [10]. He also confirmed that the corrosion effect was the same for both the base coolant and nanocoolant [11]. The suspended graphene nanoparticles with water as a base solvent and for various volume fractions and measured its convection coefficients for the temperature range of 30 °C to 80 °C. The heat transfer characteristics of hybrid nanofluid (HyNF) flow through the tubular heat exchanger were also studied by [12]. They examined the various parameters like thermal conductivity and heat transfer coefficient under forced convection. The experiments were performed for various nanoparticles ranging from 0.1% to 1.0%volume concentrations added in the base fluid. The results showed that the bulk heat transfer coefficient was maximum by 48.4% up to 0.7% volume concentration of HyNC.

#### **2.1 CPU cooling with hybrid nanofluid**

*Heat Transfer - Design, Experimentation and Applications*

is cooled externally by employing a fan [3].

mentioned in [2].

use liquid fluid to gain the generated heat by convective means and deliver it to the external surroundings. Also, convective systems have the potential to be designed for various configurations. As proved in [1], bulk heat transfer is more significant than the thermal properties for most applications because bulk transport is the main reason for heating or cooling. This micron-sized particle, when used in low volume fractions prevents particles clogging and also reduces the pressure drop. PC coolers are helpful for this reason using appropriate working liquids coolant. They will greatly enhance the system's cooling performance. Heat sinks are equipped for simultaneous cooling of several chips and also processor supply circuits, graphic cards, and CPU. In employing a heat sink very low flow rates of coolants are required and thereby the noise generations are also minimal as

This liquid cooling is categorized into active and passive cooling. The principle of an active liquid cooling system for computers is the same as that used in combustion engines, with the coolant being water which is circulated by a pump. This Heat sink made of microchannels is mounted on the CPU and sometimes on the components like GPU and Northbridge and cooled typically a radiator. The radiator itself

Both active liquid cooling systems and passive liquid cooling systems are used depending upon the requirements. The passive system often discards a fan or a water pump, hence the reliability of the system can be theoretically increased and or making it quieter than active systems. However, they are much less efficient in discarding the heat and thus also need to have much more coolant and thus a much

Although liquid cooling under forced convection enables higher heat dissipation rates, air cooling is the most common technique for heat removal. The primary advantage of air cooling is its easy operability with less noise. Forced air-cooling processes may be further classified into serial and parallel flow systems. In a serial system, the air stream is passed over successive rows of modules or boards, so that each row is treated by the same air that has been preheated by the previous row. The power and airflow requirements are the key factors in serial airflow resulting in an extensive air temperature rise across the machine. Parallel airflow systems can be used to reduce the temperature rise in the cooling air. In this system, the printed circuits or modules are all equipped with a parallel air supply. In this method, each

In this connection, heat transfer equipment with minimal size can be achieved by adding solid small-sized particles [5]. These particles have high thermal conductivity when compared to conventional coolants like water or ethylene glycol [6]. If the suspended particle size is in the order of microns and millimeter lot of problems like particle settlement, corrosion and more pressure drop needs to be overcome. Because of these drawbacks, milli-size or micron-size particles are not used widely. Choi and Eastman [7] tried suspending nanometer particles in a solution. The nano-sized particle and their low volume fractions prevent particles from clogging and also reduce the pressure drop. Stability is increased and sedimentation of the particle is decreased due to its increased surface area of particles [8]. The efficiency in heat removal is improved since heat transfer takes place at the particle surface. Nanosheets comprising MAXene obtained from Ti3SiC2 MAX phase ternary carbides can be a probable nanocoolant with very efficient thermal management. It is synthesized via shear-induced micromechanical delamination technique. It is also

bigger coolant reservoir (giving more time to the coolant to cool down).

board or module is supplied with a fresh supply of cooling air [4].

**2. Nano coolants in microchannels for CPU cooling**

**108**

Labib et al. [13] employed two different base fluids of water and ethylene glycol and analyzed the effect of convective heat transfer mixing of Al2O3 nanoparticles. The CFD results are validated with the experimental data for Al2O3/water nanofluid. The results revealed that Ethylene Glycol base fluid gives better heat transfer enhancement than that of water. The mixture of Al2O3 nanoparticles into CNTs/ water nanofluid is a new concept of combined/hybrid nanofluid that can profitably increase convective heat transfer. Eshgarf and Afrand [14] studied the rheological behavior of COOH functionalized MWCNTs–SiO2/EG–water hybrid nanocoolant for application in cooling systems. They prepared the stable suspension of MWCNTs and SiO2 nanoparticles (50:50 volume%) in a specified amount of a binary mixture of EG–water (50:50 vol.%) for the temperature range of 27.5–50 °C. From the results, it is inferred that the apparent viscosity generally increases with an increase in the solid volume fraction and decreases with increasing temperature.

Naqiuddin et al. [15] showed geometrically graded microchannel heat sink for improvements in thermal performances. They showed for a heat load of 2000 W, the average temperature of the microchannel can be reduced to 69.6°Celsius with a temperature variation of 3° Celsius. Microchannel heat sink with micro-scale ribs and grooves for chip cooling is also investigated with CFD studies [16]. The performances of various heat sink parameters like trapezoidal, rectangular, and triangular-shaped microchannel with different channel width and aspect ratio was analyzed. The rectangular microchannel showed the best performance with the aspect ratio among 8.904–11.442. The rectangular microchannel has the lowest thermal resistance, followed by the trapezoid and triangle microchannel. Wang et al. [17] designed a heat sink with micro-scale ribs and grooves for chip cooling. The cooling efficiency is more for rib-grooved microchannel than the conventional smooth rectangular microchannel through experimental and numerical approaches. The Nusselt number of rib-grooved microchannel can be 1.11–1.55 times that of a smooth microchannel. He also studied the apparent friction factor for various relative rib heights of 0.6, 0.73, and 0.85 μm.

#### **2.2 Synthesis of nanofluid**

For nanofluid synthesis, there are many key factors to be considered [18]. Stable nanofluid [19] can be formed by one and two-step methods [20]. Both one-step and two-step methods can produce nanoparticles in suspension and also agglomeration

of particles. Thus, it is required to synthesis a suspension of non-agglomerated and also well-monodispersed nanoparticles in the liquid are the key steps for the enhancement of the thermal properties of nanofluid.

To withstand the operating temperature it should high thermal stability

i.For the homogeneity of the medium, its dispersion should be uniform

ii.Chemical compatibility and ease of chemical manipulation.
