Heat Transfer Enhancement Using Nanofluids for Thermal Energy Applications /

Abstract

Heat transfer is basic need for energy production for many energy applications. This is accomplished by thermal fluids that exchange heat with a heat source or another fluid for either thermal energy or electrical energy production. The use of conventional thermal fluids faces problems of low convective heat transfer and thermal conductivity. Nanofluids are a kind thermal fluid with the potential of revolutionizing heat transfer in energy systems. Having increased convective heat transfer and thermal conductivity than conventional fluids, they offer environmental benefits together with low energy costs. There is a strong applicability of nanofluids in commercial and domestic applications such as automotive, energy production like solar and nuclear energy. A nanofluid is a dispersion of nanometer sized particles in a conventional fluid (base fluid) such as water. Because of their sub microscopic nature, they easily mix with the fluid simultaneously increasing thermal conductivity of the fluid due to their metallic nature. Thus, they offer advantages of higher thermal conductivity of solids and rapid flow of fluids. This has led to increased research in this area and many theoretical, experimental, and simulation models have arised complicating an accurate assessment of heat transfer involved. First an extensive and detailed literature review of both experimental along with simulation study highlighting the differences and similarities of approaches as well as results. The analysis revealed the domination of forced convection in applications and thus formed the basis of this study. A full three dimensional (3D) CFD analysis for forced hydrodynamically and thermally developing laminar nanofluid flow in pipes is applied in CFD code ANSYS CFX using different comparison criteria. Effects of concentration and diameter of nanoparticle, heat flux, inlet temperature, Re number, as well as the type of nanofluid itself (based on type of nanoparticle and base fluid) on heat transfer is investigated. Understanding of the results revealed that concentration is the dominant factor for enhancing heat transfer. A concentration of 1%-5% increased the convective heat transfer by more than 5% for nanofluids like alumina (Al2O3-water). This is found to be extremely beneficial in pipes whose wall temperature decrease requiring less heat to cool them. Based on the differences in heat transfer for temperature dependent and independent models, it is found that the application determines the type of thermophysical and convective models to be used.