Contradictory Evidence for the Role of Temperature and Particle Size in Nanofluid Thermal Conductivity
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Contradictory Evidence for the Role of Temperature and Particle Size in Nanofluid Thermal Conductivity Rebecca J. Christianson1, Jessica Townsend1 1 Franklin W. Olin College of Engineering, Olin Way, Needham, MA 02492, U.S.A. ABSTRACT The prospects for increased cooling capacity from the use of nanofluid coolants has created a tremendous amount of interest. However, in the years since the initial thermal conductivity measurements of nanoparticle suspensions were reported, there has been much inconsistency in data published in the literature. The International Nanofluids Benchmarking Exercise was a significant step towards creating a reliable set of data on the thermal conductivity enhancement of stable nanofluids, however there remain many unanswered questions. Most significant, perhaps, is the contradictory results on the effects of particle size and temperature. In the past year alone it is possible to find published reports on nominally identical samples claiming precisely opposing trends in thermal conductivity with decreasing particle size at room temperature. Some studies also claim an increasing enhancement at higher temperatures, sometimes linking this to small particle sizes. In this work we review the literature claims for particle size and temperature results, the theories used to support those claims, as well as presenting new data with the aim of resolving the dispute and identifying the origins of the evidence for contradictory claims. . INTRODUCTION At present time, many engineering applications are limited primarily by the ability to dissipate the heat produced. Many researchers are therefore engaged in the search for ways to improve cooling. One of the primary hurdles to overcome in improving flow cooling performance has always been the relatively low thermal conductivity of most practicable liquids. Since Maxwell [1] it has been known that high thermal conductivity solid particles, when added to a liquid, will improve the thermal conductivity of suspension over that of the base fluid. However, the settling out of these particles degrades the thermal performance and can clog cooling channels. With the advent of the technology to make consistent, nanometer-sized particles this route to improved thermal performance has become more feasible, since the Brownian agitation for particles of this size is sufficient to keep the particles suspended against the influence of gravity. In the 1990’s SUS Choi [2] was the first author to report on the thermal conductivity of suspensions of nanometer-sized particles and coin the term ‘nanofluid’. His group reported that the observed thermal conductivity for these suspensions greatly exceeded the predictions of classical effective medium theory. Following their publication, many other authors also reported anomalously high thermal conductivity for various nanoparticle suspensions [3]. However, other authors have found little or no anomalous increase in the thermal properties of nanofluids [4]. In recent years, consensus [5] has been building that for stable suspensio
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