Chapter 4 - Thermophysical and rheological properties of unitary and hybrid nanofluids

Abstract

Understanding the thermophysical properties of the working fluids in a heat transfer system is essential for improving the thermal performance of heat transfer equipment. Clarification of the complicated and entangled mechanisms of the thermophysical properties and the operating conditions need detailed experimental and modeling efforts. Nanofluids, although showing the desired improvements in the thermophysical properties, are more sensitive to operating conditions compared to traditional working fluid. The working fluid can be a single fluid, its mixtures, as well as a nanosuspension. Ethylene glycol, ethanol, water, motor oil, ammonia, and halogenated hydrocarbons are some instances of working fluids in thermal systems. This book chapter reveals the thermophysical and rheological properties of unitary and hybrid nanofluids. The behavior of nanofluids used as working fluid in a thermal system affects the thermophysical and rheological properties of the flow within the system, as well as the thermal performance of the overall system. Nanofluids are complex dispersed suspensions, and the stability, chemical and thermal compatibility of nanoparticles must be ensured during the preparation procedures and during the operation conditions. This chapter includes thermal conductivity, heat capacity, density, and viscosity alteration of nanofluids under different operating conditions such as nanoparticle type, size, temperature, and concentration. The application of nanofluids is very promising and can be a good candidate for new-generation working fluids in industrial applications. Recently the application of hybrid nanofluids has become more popular since they enhance thermophysical properties according to the selection of compatible two nanoparticle types compared to unitary nanofluids. The thermal conductivity, the viscosity and the density of the nanofluids rise with increasing concentration and alter with base fluid and nanoparticle type. Temperature increase augments the thermal conductivity but it reduces the viscosity and density of the nanoblend. Specific heat capacity of the nanofluid decreases with increasing concentration and it is directly proportional to the temperature. This study contains a thorough digest of recent novel works of researchers in the nanofluid field and aims to model and encourage the research in the academic community, as well as to lead to the more efficient use of thermal applications, industrial processes, and new technologies.