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۱٫ INTRODUCTION

During the last two decades, there has been a monumental thorough search for more efficient cooling equipment for electronics and for more compact and reliable thermal management systems. Among different available electronic cooling devices, plate fin heat sinks are the most widely used because of several major advantages such as their simple design and low manufacturing costs. But there are cases where we need to sacrifice simplicity of heat transfer devices for their higher efficiency. To attain this goal, slight but efficacious modifications should be made, such as adding some pins to the currently available plate fin heat sinks. This paper investigates the effect of adding circular pins to the base plate of a plate fin heat sink by using nanofluids. In recent years, nanofluids have been proved to be an ideal candidate for enhancing heat transfer (Choi, 1995; Lee et al., 1999). Various studies have been conducted on the performance of convective heat transfer of nanofluids (Wen and Ding; Heris and Etemad, 2006; Hwang et al., 2009). They have concluded that nanofluids provide heat transfer enhancement in comparison with their corresponding base fluids. Mansour et al. (2011) studied the mixed convection of a water– Al2O3 mixture inside an inclined tube and concluded that higher volume fractions of particles clearly induced a decrease in the Nusselt number in the horizontal position. Ho et al. (2010) conducted experiments to investigate forced convective cooling performance of a copper plate fin microchannel heat sink with the Al2O3– water nanofluid as a coolant. Their results showed that the nanofluid-cooled heat sink outperforms a water-cooled one, having a significantly higher average heat transfer coefficient and thereby markedly lower thermal resistance and wall temperature. Pantzali et al. (2009) performed experimental and numerical analyses of the effect produced by using CuO–water nanofluids in a miniature plate heat exchanger (PHE) with a modulated surface. In their study, heat transfer enhancement was more pronounced at lower cooling liquid flow rates. More recently, Zhou et al. (2012) investigated experimentally the convective heat transfer and friction characteristics of a silver nanofluid in a micro-pin-finned heat sink. They stated that the volume fraction of silver nanoparticles significantly affected the convection heat transfer coefficient of the micro-pin-finned heat sink, and the thermal resistance of the nanofluid was lower than that of deionized water. Also, Duangthongsuk et al. (2012) presented an experimental study on the heat transfer and pressure drop characteristics of 1.0, 2.0, and 3.0 wt.% Al2O3–water nanofluids flowing through an aluminum rectangular microchannel heat sink (MCHS). The results indicated that the heat transfer performance of MCHS increased with increasing Reynolds number as well as particle concentrations. They reached a maximal 15%- increase in the heat transfer coefficient using nanofluids. In another effort, Ijam et al. (2012) studied a laminar flow of Al2O3–water and TiO2–water nanofluids with different volume fractions of nanoparticles as coolants for a copper minichannel heat sink. The result showed that the adding of Al2O3 and TiO2 nanoparticles to water at a volume fraction of 4% enhanced the thermal conductivity by 11.98% and 9.97%, respectively. Finally, Selvakumar and Suresh (2012) prepared CuO/water nanofluids with volume fractions of 0.1% and 0.2%. They used a thin channeled copper water block of overall dimension 55 × ۵۵ × ۱۹ mm for their study. The convective heat transfer coefficient of the water block was found to increase with the volume flow rate and nanoparticle volume fraction, and the maximum rise (29.63%) in the convective heat transfer coefficient was observed for the 0.2% volume fraction compared to deionized water. In their work, a correlation was proposed for Nusselt number which fits the experimental Nusselt number within 7.5%. To the knowledge of the present authors, there has not been any previous work concerning the cooling capability of alumina–water nanofluids in a plate pin-finned heat sink, especially with lower particle volume fractions. Thus, the main objective of the present work was to analyze experimentally the performance of Al2O3– water nanofluids at small particle volume concentrations for the purpose of heat dissipation in a novel heat sink and to compare the results with the traditional plate fin heat sink.

۲٫ PREPARATION AND PROPERTIES OF NANOFLUIDS

Al2O3 spherical nanoparticles (purchased from Wacker, Germany) with an averaged particle size of 18 nm and 99.9% purity were dispersed in distilled water, as the base fluid, to form Al2O3–water nanofluids. The nanofluids were synthesized by the two-step method, without any surfactant in order not to affect the viscosity and thermal conductivity (k) of suspensions. The desired volume fractions of alumina–water nanofluids were prepared by mixing appropriate quantities of nanoparticles with the base fluid, and then sonicated in an ultrasonic bath (Hielscher UP400S, H40sonotrode) for at least 90 min. The nanofluids used in the current study stayed stable for a period of 72 h without any visible settlement. Four volume fractions of the nanofluids, Φ = ۰٫۵, ۱, ۱٫۵, and 2 vol.%, were prepared for the experiment.

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