Thermal properties and convective heat transfer of phase changing paraffin nanofluids

Fluids containing micro-sized solid-liquid phase changing particles have been proposed to be promising candidates as future heat transfer fluids. In addition, smaller nano-sized particles have been claimed to enhance the heat transfer performance of fluids even if the phase change is not exploited. For the first time, we conduct full scale convection heat transfer measurements combining these two consepts. Three water-based paraffin mixture nanofluids with particle mass fractions of 5–10% are prepared and measured with an annular tube heat exchanger with Reynolds numbers varying in the range of 700–11000. In addition, the fluids are characterized: latent heats, specific heats, viscosities, thermal conductivities, densities and particle size distributions are all determined experimentally. In agreement with previous studies of solid-particle nanofluids and nanoemulsions, also the phase changing nanofluids are found to exhibit Nusselt numbers clearly higher (up to ∼60% in the turbulent regime) than water when compared on the basis of equal Reynolds numbers. However, the differences in Prandtl numbers are shown to explain these deviations in Nusselt numbers. Indeed, the well-known Gnielinski correlation is able to explain the results and thus, significant anomalies in the convection heat transfer caused by neither the melting of the phase change material nor the presence of the nanoparticles are observed. However, the nanofluids have systematically slightly higher Nusselt numbers than the correlation would predict, but the deviations are within the accuracy of the correlation (10%). When compared by using equal pumping powers, the nanofluids exhibit heat transfer performance poorer than that of water. The positive impact of the latent heat is outweighed by the negative effects of the increased viscosity and decreased specific heat.