Nanofluid Flow in Microchannel Heat Sinks

One of the methods for enhancing heat transfer in the microfluidic cooling devices is the application of additives to the working fluid. Therefore, one possible route is increase the thermal conductivity of the working fluids. Nanofluids, i.e., dilute suspensions of nanoparticles in liquids (water, ethylene glycol, and oil), have been found to possess enhanced thermophysical properties, and it can enhance the convective heat transfer coefficient since has higher thermal conductivities and viscosities than the base fluids. Nanofluid consists of a base fluid such as water and nano scale metallic or non-metallic particles. The commonly used nanoparticles are metals – e.g., Cu, Au, Ag, Fe, Al, and Zn; metal oxides – e.g., Al2O3, CuO, Fe2O3, Fe3O4, SiO2, TiO2, and ZrO2; carbide ceramics (SiC

and TiC), nitride ceramics (AlN and SiN); carbon materials – e.g., carbon nanotubes, graphite, and diamond (Tullius et al., 2011 and Sundara et al., 2017). Nanoparticle suspensions in liquids have received great attention in recent years because the nanofluids having unprecedented stability of suspended nanoparticles were proven to be having anomalous thermal conductivity even with small volume fraction of the nanoparticles (Jung et al., 2009).

Lee and Mudawar (2007) experimentally investigated the heat transfer coefficient in the straight rectangular microchannels heat sink by using a nanofluid as the cooling fluid containing small solid particles of Aluminium oxide (Al2O3) mixed with water, where two

volumetric concentrations of Al2O3 particles, 1% and 2%, were tested in their study. Twenty-

one parallel rectangular microchannels were etched on the copper plate with width and length of 1 cm and 4.48 cm, respectively, while the cross-section area of the microchannel was 215

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µm wide by 821 µm deep. Operating conditions for their study were as follows: Re = 140– 941, total power = 100–300 W, water inlet temperature = 30 oC, inlet pressure = 1.17–1.36 bar, and outlet pressure = 1.12 bar. Both single- and two-phase convective heat transfer results were obtained.

Their measurements showed that the high thermal conductivity of nanoparticles significantly enhanced the heat transfer coefficient especially in the microchannel entrance region; while in the downstream fully developed region the enhancement was weaker, proving that nanoparticles have an appreciable effect on thermal boundary layer development. They found that there are 13% increase in heat transfer coefficient for 2% Al2O3 in water. Also, they

concluded that increasing nanoparticle concentration increases single-phase pressure drop compared to that of water alone as the coolant at the same Reynolds number. However, increasing nanoparticle concentration does not have a clear influence on the friction factor. They noticed that increasing the heat flux has a very weak effect on the heat transfer coefficient for pure water, but an appreciable effect for the nanofluid and this effect was increased as the volumetric concentrations of Al2O3 within the nanofluid was increased. For

single-phase turbulent flow regime, the heat transfer enhancement by using nanofluids becomes weak compared with that of single-phase laminar flow regime, because the heat transfer coefficient has a weak thermal conductivity dependence at high flow rate as well as decreased specific heat.

In the case of two-phase flow cooling, they proposed not to use nanoparticles in the microchannel heat sinks. Despite of small size of these particles, it is caused catastrophic failure for cooling system once boiling commenced by depositing into relatively large clusters near the channel exit due to localized evaporation. They filled the entire microchannels and prevented coolant from entering the heat sink.

Jung et al. (2009) measured the friction factor and convective heat transfer coefficient of Al2O3water nanofluids with diameter of 170 nm in straight rectangular microchannels.

Various particle volume fractions of nanofluids were examined in the experiments to investigate the effect of the volume fraction of nanoparticles (𝜑) on the fluid flow and convective heat transfer in microchannels. With a 1.8% volume fraction of Al2O3 in water, in

the laminar flow regime, they found that the measured convective heat transfer coefficient increased up to 32% over that of the distilled water, with an acceptable increase for friction loss. With considering the volume fraction of nanoparticles, the measured data of the Nusselt number (𝑁𝑢 ) for the various nanofluids of laminar flow regime in microchannels was correlated by the following equation:

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𝑁𝑢 = 0.014 𝜑0.095 𝑅𝑒0.4 𝑃𝑟0.6 (2.15)

Experiments were performed by Ho et al. (2010) to investigate the influence of using Al2O3water nanofluid of 1 and 2 vol.% as the coolant on the forced convective cooling

performance of a copper microchannel heat sink. The heat sink fabricated consisted of 25 parallel rectangular microchannels, and each microchannel had a length of 50 mm and a cross- sectional area of 283 µm in width by 800 µm in height. With the Reynolds number ranging from 226 to 1676, the hydrothermal performance of the nanofluid-cooled microchannel heat sink has been investigated in terms of the friction factor, the average heat transfer coefficient, the thermal resistance, and the maximum wall temperature. For the largest flow rate tested for the nanofluid of 1 vol.%, the measured data showed that the nanofluid cooled microchannel outperformed the water cooled microchannel, having significantly higher heat transfer rates (the average heat transfer coefficient increases by about 70%) and thereby marked reductions in the thermal resistance (reduced by about 25%) as well as in the maximum wall temperature were found. Despite the nanofluid-cooled heat sink markedly enhancing heat transfer rate due to the presence of the nanoparticles in water, the nanofluid of 1 or 2 vol.% Al2O3 flowing through the heat sink appeared to give only slightly increase

in the friction factor.

Mohammed et al. (2011a) carried out numerical simulations on laminar nanofluids flow and heat transfer characteristics in triangular shaped microchannel heat sink made from aluminum. The performance of microchannel heat sink was examined by using water as a base fluid with different types of nanofluids such as Al2O3, Ag, CuO, diamond, SiO2, and

TiO2 as the coolants with nanoparticle volume fraction of 2%. Based on their results, they

found that diamond nanoparticles dispersed in water is preferable to attain overall heat transfer enhancement. In the other hand, Ag nanoparticles dispersed in water is recommended to achieve low pressure drop and low wall shear stress, compared with pure water.

Naphon and Nakharintrthe (2013) investigated experimentally the pressure drop and heat transfer characteristics of nanofluids cooling (Titanium Dioxide (TiO2) particles with

deionized water) in a mini-rectangular fin heat sink. The aluminium heat sinks with three different channel heights of 1, 1.5 and 2 mm were fabricated and the effects of the nanofluids inlet temperature, coolant Reynolds number and heat flux on the pressure drop and heat transfer characteristics of mini-rectangular fin heat sink were examined. It was found that average heat transfer rates obtained from nanofluids as coolant are higher than those for the deionized water as a coolant at the same operating conditions. Also, they showed that the heat flux has an insignificant effect on the pressure drop of the nanofluids, where pressure drop decreased slightly as heat flux increases and this due to nanofluids viscosity.

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Fig. 2.6: 3-D schematic view of micro pin fin heat sink suggested by Peles et al. (2005).

Although adding nanoparticles to a base fluid can influence the cooling process positively, there are still challenges. Overall, these fluids leave sedimentation of particles, fouling, erosion, high pressure drop, and may even clog the channel over time.