Temperature Dependency of Ceramic Nanofluids Shows Classical Behavior

Abstract

P RESENTLY, nanofluids are among the most intensively investigated options to enhance heat transfer [1–3]. Here, we understand nanofluids as liquids inwhich particleswith sizes ranging from about 10 to 200 nm are suspended in a base fluid. Particle materials are either pure metals (Au, Ag, Cu, etc.), oxide ceramics (Al2O3, CuO, SiO2, etc.), or nonmetals (e.g., diamond and carbon nanotubes). The base fluid is mainly water, but several studies with ethylene glycol, toluene, mineral engine oil, and others were also carried out (see, e.g., [4,5]). The general expectation of nanofluids is that their thermal conductivity is significantly higher than that of the base fluid. However, two main streams of results are currently reported in the literature. Although a large number of publications (e.g., [1–3,6,7]) support an extraordinary enhancement going beyond the classical theory by Maxwell [8], several more recent papers show an agreement with the classical view [9–11]. The situation is obviously especially difficult because, so far, a satisfactory theory does not exist, and a sufficient number of experiments have not been carried out that could explain all phenomena observed. However, it must be assumed that a nonlinear dependency exists of the thermal conductivity of a nanofluid and the particle size [12], the volume fraction [6], the particle shape [13], and the temperature. Here, we will focus on one given particle size and one given particle shape and investigate the dependency on the temperature alone. This strategy limits the number of relevant parameters and allows to draw firm conclusions. For the practical application of nanofluids, the knowledge of thermophysical properties such as viscosity, density, thermal conductivity, and their variationswith temperature is essential. However, it is still not currently possible to calculate these properties from scratch knowing only the thermodynamic properties of basefluid and particles, volume fraction, and geometry of nanoparticles and other physical and chemical properties that constitute the characteristics of a nanofluid. Therefore, it is still a matter of measurements to provide thermophysical properties as the base for heat transfer and other applications. The scope of the present work is to experimentally determine the thermal conductivity of three water-based nanofluids with ceramic particles in a temperature range between 20 and 60 C. For that purpose, a special static measurement devicewas developed. Numerical simulations and preliminary experiments were undertaken to make sure that no disturbing effects like convection and sedimentation contaminate the measurements and that the probe is properly calibrated. However, the study focuses primarily on the question of whether or not the thermal conductivity over the mentioned temperature range shows classical behavior, as proposed by Eapen et al. [10].