Modeling Transport Mechanism in Nanofluids

Nanofluid connotes a colloidal suspension with dispersed nano-sized particles. Experiments in the past decade reveal that nanofluids have a significantly higher thermal conductivity in comparison to that of the base fluid. Recently, a number of phenomenological models have been proposed to explain the anomalous heat transfer enhancement seen in nanofluids. In this paper, we make a systematic survey of experimental and modeling endeavors with nanofluids. We find that most of the models are phenomenological in nature and addresses only a part of the problem. In some models, inconsistencies are very conspicuous. We also note that the effectiveness of nanofluids depends not just on the thermal conductivity but also on other properties such as viscosity and specific heat. The concomitant changes in other transport properties such viscosity and diffusion are largely overlooked by the scientific community to date. To explain the property changes in a coherent way, we propose molecular dynamics simulations. We show that all the transport coefficients can be evaluated using the linear response theory. Finally, we discuss the enhanced phonon transport mechanism in low-dimensional quantum systems such as nano-tubes and quantum dots. We believe that such nano-structures are optimal for enhancing the heat transfer in nano-fluids. shear rate, shear thinning and thickening can 1.0 Introduction: Fine dispersion of solid particles in a liquid is generally known as a colloidal suspension. A colloid can be a solid, liquid or gaseous substance that is insoluble yet remains in suspension, and dispersed in another fluid medium. In contrast, dissolved solids are ionic and not colloidal. Colloids are ubiquitous. They are found in nature such as in living cells and are also common in many chemical agents. Typically, the solvent or more precisely, the base medium is water and the particles are macro molecules or large agglomeration of molecules. Traditionally and not surprisingly, colloids are investigated for chemical and rheological properties. Colloids display very interesting shear behavior. Depending on the be observed. Thinning refers to a decrease in effective viscosity while thickening leads to the opposite effect. The study of heat transport in solid dispersions is relatively recent. Ahuja [1] showed that sub-micron polystyrene suspensions in aqueous glycerine increased the heat transfer by a factor of two under laminar flow conditions. At the same time, negligible differences were seen for the pressure drop even with a high particle volume fraction of 9%. Even with such impressive data, dispersions were not considered very useful as a heat transfer enhancement technique. A major drawback stems from the size of the particles which are typically in the micron range. They pose significant corrosion and erosion hazards in engineering systems. 2.57: Nano-to-Macro Transport Processes Page 1