Nanofluids are a class of heat transport fluids created by suspending nano-scaled metallic or nonmetallic particles into a base fluid. Some experimental investigations have revealed that the nanofluids have remarkably higher thermal conductivities than those of conventional pure fluids and are more suited for practical application than the existing techniques of heat transfer enhancement using millimeter and/or micrometer-sized particles in fluids. Use of nanoparticles reduces pressure drop, system wear, and overall mass of the system leading to a reduction in costs over existing enhancement techniques. The enhancement of the heat transfer coefficient is determined experimentally using a CuO based nanofluids by dispersing CuO nanoparticles (40nm) with different particles loadings (0.25%, 1% and 2%wt) into water. The heat transfer coefficient enhancement ratio at different concentrations of CuO in water were calculated at 40 and 70°C. The experimental results illustrated that various factors including flow rate (Re number), particle concentration and temperature are all capable of impacting the enhancement ratio. To further explain the impact of these variable on the hydrodynamic and thermal parameters of a nanofluid, we developed a CFD model using an Eulerian-Lagrangian approach to study the nature of both the laminar and turbulent flow fields of the fluid phase as well as kinematic and dynamic motion of the dispersed nanoparticles. The main goal is to provide additional information about the fluid and particle dynamic to explain the observed behavior in the experimentally observed trends of the heat transfer coefficient enhancement of CuO/water relative to both nanoparticle concentrations and fluid flowrates. Our results indicate that heat transfer enhancement significantly depends on particle motion within the system and is highly dependent upon the position of nanoparticles relative to the tube wall.
The simulation results revealed that in some cases under the laminar flow regime, particles are able to move through thermal boundary layer into the bulk flow of the fluid. When compared with experimental results, it was found that heat transfer coefficient was highest in these cases. Based on both the experimental and theoretical results under turbulent flow, particles add little to the mixing and turbulence within the fluid resulting in negligible if any enhancements. Based on our results, laminar flow should be chosen so that the heat transfer coefficient can be effectively enhanced in these systems as the highest enhancements in this work were observed under these conditions. However, it is very important that the nanoparticles pass thermal boundary layer to provide the highest heat transfer enhancement. To ensure that this is the case, simulations of the particle trajectories should be performed based on the exact geometry of the system to ensure that the correct particle concentrations and fluid temperature are used.
