Forced convective heat transfer of nanofluids in microchannels

https://doi.org/10.1016/j.ijheatmasstransfer.2008.03.033Get rights and content

Abstract

Convective heat transfer coefficient and friction factor of nanofluids in rectangular microchannels were measured. An integrated microsystem consisting of a single microchannel on one side, and two localized heaters and five polysilicon temperature sensors along the channel on the other side were fabricated. Aluminum dioxide (Al2O3) with diameter of 170 nm nanofluids with various particle volume fractions were used in experiments to investigate the effect of the volume fraction of the nanoparticles to the convective heat transfer and fluid flow in microchannels. The convective heat transfer coefficient of the Al2O3 nanofluid in laminar flow regime was measured to be increased up to 32% compared to the distilled water at a volume fraction of 1.8 volume percent without major friction loss. The Nusselt number measured increases with increasing the Reynolds number in laminar flow regime. The measured Nusselt number which turned out to be less than 0.5 was successfully correlated with Reynolds number and Prandtl number based on the thermal conductivity of nanofluids.

Introduction

Convective heat transfer in microchannel has been proved to be a very effective method for the thermal control micro electronic device [1], [2], [3], [4]. In fact, a considerable heat flux of the order of 107 W/m2 can be removed in microchannels with a surface temperature below 71 °C [1]. Another approach to enhance the convective heat transfer coefficient in the microchannel may be utilizing nanofluids as working fluids. This can be possible 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 [5], [6], [7].

Various models have been proposed for explaining the mechanism of thermal conductivity enhancement with increasing temperature and decreasing nanoparticle sizes [8], [9], [10], [11]. Through an order-of-magnitude analysis, Prasher et al. [12], [13] have proposed that the localized convection due to the Brownian motion of nanoparticles is a key mechanism for the enhancement of the thermal conductivity of nanofluids. Further, the relationship between Brownian motion and enhanced thermal conductivity of nanoparticles was observed by optical visualization of the particles and measurement of the thermal conductivity [14].

Less extensive study on the measurement of the convective heat transfer coefficient with nanofluids has been done. Pak and Cho [15] measured the convective heat transfer coefficient with nanoparticles of γ-Al2O3 and TiO2 dispersed in water. Their experimental results have revealed that the heat transfer coefficients of the nanofluids increase with increasing the volume fraction of nanoparticles and the Reynolds number. Their heat transfer data showed Nusselt numbers up to 30% higher than predicted by the pure liquid correlation. Lee and Choi [16] have found that the nanofluid in the microchannel heat exchanger dramatically enhances cooling rate compared with the conventional water cooled and liquid-nitrogen-cooled microchannel heat exchangers. Xuan and Li [17] measured the convective heat transfer coefficient and friction factor of Cu–water nanofluids of the turbulent flow in a brass tube of the inner diameter of 10 mm. Their experimental results have shown that the nanofluid enhances remarkably the heat transfer coefficients without substantial increase in the friction factor. A correlation for the convective heat transfer coefficient of nanofluid was also suggested by considering the microconvection and microdiffusion of the suspended nanoparticles. Convective heat transfer coefficients of nanofluids, made of γ-Al2O3 nanoparticles in deionized water along a copper tube of 4.5 mm diameter in laminar flow regime were measured by Wen and Ding [18]. They obtained significant enhancement in the entrance region and some decrease along the axial direction to approach a constant value. Recently, Buongiorno [19] suggested that a reduction of viscosity within and consequent thinning of the laminar sublayer leads to abnormal increase in the convective heat transfer coefficient in turbulent flow regime.

In this study, convective heat transfer coefficient and friction factor of Al2O3 nanofluids in rectangular microchannels were measured in laminar flow regime. The measured Nusselt number which turned out to be less than 0.5 was successfully correlated to the Reynolds number and Prandtl number based on the thermal conductivity of nanofluids.

Section snippets

Device design and fabrication

A silicon wafer polished on both sides (p-type 〈100〉 single crystal with 525 μm (±5μm) thickness) was used to fabricate an integrated microsystem consisting of a single channel on one side and two localized heaters and five polysilicon temperature sensors along the channel on the other side. A 17 mm × 33 mm die for the device was used. The fabrication of the device began with the thermal growth of a 0.5 μm oxide layer on the front side of the silicon wafer. A 0.540 μm polysilicon film was deposited on

Experimental apparatus

A schematic of experimental apparatus is shown in Fig. 3. The apparatus consists of (a) working fluid handling system, (b) microchannel test chip, and (c) data acquisition system. The working fluid handling system includes a high-pressure gas source, a pressurized fluid reservoir, micro filter, a pressure gauge, and a volumetric graduate. Nanofluids with suspending Al2O3 nanoparticles (Sigma) in base fluids were used as working fluids. The size of nanoparticles suspended in working fluids was

Sensor calibration

Although the polysilicon line heater on SiO2 layer is a good sensor to measure the temperature [20], [21], the resistance values from the sensors could change depending on the degree of phosphorus doping, the sensor dimensions fabricated and the condition of wire bonding. Thus, all the temperature sensors were calibrated before measurements. The resistance values were considerably different depending on the temperature among sensors, even though the temperature was linearly related to the

Results and discussions

Several particle volume fractions of 0.6%, 1.2% and 1.8% were used in this experiment. The Reynolds numbers tested are in the range between 5 and 300. Fig. 5 shows the local convective heat transfer coefficients for nanofluids with various volume fractions of nanoparticle and pure water in 50 × 50 μm2 channel at Reynolds numbers of 14.0 and 80.0. Appreciable increase more than 32% in the convective heat transfer coefficient for the nanofluids with 1.8 volume percent of Al2O3 nanoparticle compared

Conclusions

The convective heat transfer coefficient and the friction coefficient of nanofluids of Al2O3 with diameter of 170 nm in microchannels were measured. Appreciable enhancement of the convective heat transfer coefficient of the nanofluids with the base fluid of water and a mixture of water and ethylene glycol at the volume fraction of 1.8 volume percent was obtained without major friction loss. It has been found that the Nusselt number increases with increasing the Reynolds number in laminar flow

Acknowledgements

Two (Jung-Yeul Jung and Hoo-Suk Oh) of authors have been supported by the Second Stage of BK21 program. This research was supported by the Chung-Ang University Research Grantsin 2008.

References (27)

  • S. Lee et al.

    Measuring thermal conductivity of fluid containing oxide nanoparticles

    ASME J. Heat Transfer

    (1999)
  • J.A. Eastman et al.

    Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles

    Appl. Phys. Lett.

    (2001)
  • S.P. Jang et al.

    Role of Brownian motion in the enhanced thermal conductivity of nanofluids

    Appl. Phys. Lett.

    (2004)
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