A Comment on the Utility of Nanofluids: Interactive Influence of Nanoparticle Size and Amount at Varying Temperatures

Abstract

Interactive influence of nanoparticle diameter, dp and volumetric fraction, φ on major thermophysical characteristics of relative thermal conductivity, kr and dynamic viscosity, μr of nanofluids as a primary function of temperature, T is determined to assess the utility of nanofluids. In the case study common base fluids of water (W) and ethylene glycol (EG) are used. Spherical shape Ag and Al2O3 with a selected dp range of (20–100 nm) are used in the covered φ range of (0.25–5%). Influence of T is set by considering T = 293 K and T = 323 K. The referred data ranges are applied for the calculation of kr and μr of nanofluids Ag–W, EG and Al2O3–W, EG in reference to a calculation procedure provided previously by the authors. The calculated magnitudes of kr and μr are expressed as a function of dp, φ and T. The results reveal that kr and μr rise dramatically as dp decrease from 40 nm to 20 nm and φ increase from 2% to 5% at T = 293 K and T = 323 K. The calculations confirm the relevant literature that the required pumping power increase is associated with low dp and high φ. Therefore, here exists limiting magnitudes of dp, φ as a function of T for the effective utilization of nanoparticles in base fluids. In order to generalize the fact non-dimensional parameters of Prandtl Number, Pr and Reynolds Number, Re should be referred due to the definitions of each depend on thermophysical characteristics and the cited dp, φ and T. The calculations herein have the validity range of Re and Pr of base fluids as 0.0002–0.032 and 3.58–210.30, respectively.