Thermodynamic assessment of water-alumina nanofluids as secondary working fluids in refrigeration systems aiming at the external irreversibilities of the cycle.

This paper is aimed at investigating the suitability of nanofluids as secondary working fluids in refrigeration systems with the focus on the thermodynamic losses that take place externally in the condenser and the evaporator, i.e. heat transfer across a finite temperature difference and viscous pressure drop. In order to address the trade-off effects related to heat transfer enhancement and pumping power penalty, a mathematical model for the thermodynamic optimization of condensers and evaporators was employed considering the thermophysical properties of water-alumina nanofluids. Numerical simulations were carried out to assess the effects of key heat and fluid flow parameters on the rate of entropy generation. The global minimum was achieved for pure water, suggesting that, in the case that the heat exchangers are optimized to minimize the external entropy production, the water-alumina nanofluids might not be the best choice as secondary heat transfer fluids in refrigeration systems. The effects on the condensing and evaporating temperatures and, therefore, on the internal irreversibilities of the cycle are still to be investigated.