Two-phase mixed convection heat transfer and entropy generation analysis of a non-Newtonian nanofluid inside a cavity with internal rotating heater and cooler

In this study, mixed convection inside a square cavity filled with Cu-water nanofluid is simulated using the Eulerian two-phase mixture model. The base fluid is considered to be non-Newtonian and obeys from the power law. Two rotating cylinders are placed inside the heat exchanger and heat is transferred through the nanofluid from the hot cylinder to the cold one. In addition to the natural convection heat transfer inside the heat exchanger, rotation of the internal cylinders can provide forced convection heat transfer. The direction of rotation of cylinders can strengthen or weaken the natural convection effects in different regions, hence, four cases with various rotation directions for the cylinders are considered. The effects of changes in the value of imposed angular velocity (in terms of Richardson number), non-Newtonian power-law index, Rayleigh number and nanofluid volume fraction on both the heat transfer and entropy generation are discussed. The results have been illustrated that the shear-thinning or shear-thickening behavior of the non-Newtonian nanofluid can noticeably change the effect of forced convection on the heat transfer efficiency. Besides, regarding the values of Rayleigh number, Richardson number and non-Newtonian power-law index, the addition of nanoparticles into the non-Newtonian base fluid might adversely affect the heat transfer efficiency. In addition, to investigate irreversibilities, total entropy generation is discussed. It is indicated that shear-thinning nanofluids result in higher values of total entropy generation.