One method of enhancing the thermal conductivity, and hence the heat transfer coefficient of a fluid is to add nanoparticles to the fluid creating what is commonly known as a nanofluid. The heat transfer coefficient which is the proportionality between the heat flux and the thermodynamic driving force for the flow of heat shows how effectively heat can be transferred within a system. This coefficient can be passively enhanced by changing flow geometry, system parameters (temperature, velocity, etc), or by enhancing the thermal conductivity of the fluid. In most existing systems, the first two of these are set by system design, meaning that the only method to enhance overall heat transfer is to enhance the heat transfer properties of the fluid.
In this study, we use CuO nanoparticles (40nm) dispersed into water at different loadings to create nanofluids. The heat transfer coefficient enhancement ratio at different fluid temperatures and CuO concentrations were calculated from experimental measurements. The results illustrated that a number of factors including Reynolds number, fluid temperature, and particle concentration are all capable of impacting the enhancement ratio.
To explain the effect of particle concentration, flow rate, and temperature on the hydrodynamic and thermal parameters we developed a CFD model using the Eulerian-Lagrangian approach to study the nature of both laminar and turbulent flow fields in the fluid phase as well as the kinematic and dynamic motion of the dispersed nanoparticles. Information obtained using the CFD model regarding the dynamic motion of the particles should go a long way to explaining the observed trends in the heat transfer coefficient enhancement of a CuO/water nanofluid relative to both temperature and nanoparticle concentration. Our results to date indicate that heat transfer enhancement depends significantly on both the particle motion within the system as well as the position of nanoparticles relative to the tube wall. Interestingly enough, there seems to be little to no dependence of the heat transfer coefficient enhancement on nanoparticle-nanoparticle interactions as has been suggested in previous publications.