A comparative parametric study on single-phase Al2O3–water nanofluid exchanging heat with a phase-changing fluid

• A model for a nanofluid-to-phase-changing fluid heat exchanger was developed.
• A comparative analysis of heat transfer rate and pumping power is proposed.
• Increased particle concentration contributes significantly to pumping power.
• Viscosity enhancement counteracts thermal conductivity enhancement.
• Other thermal resistances and increased friction attenuate higher Nusselt number advantages.

For their enhanced thermophysical properties and heat transfer characteristics, considerable attention has been recently given to nanofluids. Most studies of the heat transfer performance of nanofluids, as compared to their base fluid, have been carried out so far with focus on the enhancements on their thermophysical properties, like thermal conductivity and viscosity, and on the heat transfer coefficient enhancements. However, one of the main applications of nanofluids as heat transfer fluids would be in heat exchangers, where thermal resistances from the other fluid, as well as increased friction, would attenuate the advantages from their high thermal conductivity and eventually high Nusselt number. The present study provides a general model for the analysis of a heat exchanger in which a nanofluid exchanges heat with a phase-changing fluid, emulating, for example, an evaporator, part of a vapour-compression heat pump system. A comparative analysis of the heat transfer rate and pumping power is carried out for a water-based Al2O3–water nanofluid substituting pure water. For the purpose of the comparison, non-dimensional enhancement factors, for pressure drop and heat exchange rate, related to pumping power and thermal capacity, respectively, were defined. It was observed that increased nanoparticle concentration contributes significantly to the augmentation of pumping power. Furthermore, the thermal conductivity enhancement is counteracted by other properties of the fluid, equally affected by the increase of particle concentration, and by the presence of other thermal resistances of the heat exchanger.