Mixed convection heat transfer in a CuO–water filled trapezoidal enclosure, effects of various constant and variable properties of the nanofluid

• New correlations are developed for thermophysical properties of CuO–water nanofluid.
• Impacts of thermophysical models on mixed convection heat transfer are analyzed.
• Model of Vajjha and Das and proposed correlations are recommended for simulation.
• Results for different models converge as nanoparticles volume fraction increases.

Different models for the thermophysical properties of the CuO–water nanofluid have been proposed in recent years. In the more sophisticated variable-property models, the thermophysical properties of the nanofluid are considered to be functions of the temperature and the volume fraction of the nanoparticles; while, in the constant-property models, they depend on the nanoparticles volume-fraction only. In this study, a new variable-property model is proposed for the thermophysical properties of the CuO–water nanofluid based on the experimental and the theoretical results available in the literature. The impacts of using the newly generated as well as the existing models on the flow and temperature fields during numerical simulation of mixed convection heat transfer in a trapezoidal enclosure filled with the CuO–water nanofluid are investigated. The simulation results are presented in terms of the average Nusselt number and the entropy generation within the enclosure for a wide range of Richardson numbers and volume fractions of the nanoparticles. In general, more heat transfer enhancements and higher entropy generations are observed employing the variable-property models which consider the effect of the Brownian motion as compared to using the constant-property Maxwell–Brinkman model. Furthermore, the results indicate that the effective thermal conductivity of the nanofluid for a variable-property model plays a pre-eminent role in the heat transfer and the entropy generation inside the enclosure. However, the differences between the average Nusselt number and the entropy generation obtained using the different considered variable-property models decrease with increasing the nanoparticles volume fraction.