Nanoparticles are a class of materials that exhibit unique physical and chemical properties compared to those of the same material at the bulk scale. One method of enhancing the thermal conductivity, and hence the heat transfer coefficient, of a fluid is to add nanoparticles to the fluid creating a so called nanofluid. The term “nanofluids” was coined by researchers at Argonne National Laboratory and refers to a two-phase mixture composed of a continuous liquid phase and dispersed nanoparticles in suspension. Nowadays, nanofluids are considered to be the next-generation heat transfer fluids as they offer exciting new possibilities to enhance heat transfer performance compared to pure liquids.

Heat transfer coefficient of a fluid is the proportionality coefficient between the heat flux that is a heat flow per unit area and the thermodynamic driving force for the flow of heat. It shows how effective heat can be transferred within a system and can be passively enhanced by changing flow geometry, boundary conditions, or by enhancing thermal conductivity of the fluid. In most existing systems, since the first two of these are set by design, the only method to enhance heat transfer is to enhance heat transfer properties of the fluid.

In this study, we used three sizes of Cu nanoparticles with different particle loadings and dispersed them into PAO formulated motor oil to create nanofluids. Measurements of heat transfer coefficients and other fluid properties were performed. Both base and nanofluids appeared to behave like Newtonian fluids and an up to 25% enhancement of the heat transfer coefficient was observed in the laminar flow regime. The heat transfer coefficient is shown to increase with increasing Reynolds number. However, as fluid temperature increases, the heat transfer coefficient decreased. Various factors including Reynolds number, fluid temperature, nanoparticle size, and nanoparticle loadings are all capable of impacting the enhancement ratio. A consistent downward trend of enhancement ratio with respect to Reynolds number was observed for the nanofluids discussed in this work. Future studies in the turbulent flow regime are needed to confirm this trend. Finally, a theoretical model to predict the heat transfer coefficient of nanofluids is developed based on previously published correlations the results of which are in excellent agreement with the experimental data.