Investigating non-Newtonian nanofluid flow in a narrow annulus based on second law of thermodynamics

The present research attempts to study the convective heat transfer and analysis of the second law of thermodynamics for the non-Newtonian nanofluid flow containing TiO2 nanoparticles within a narrow annulus. The base fluid is solution of 0.5 wt% Carboxymethyl Cellulose (CMC) in water. The employed nanofluid is of the pseudo-plastic behavior, and power law model is utilized for apparent viscosity. The convective heat transfer coefficient increases by increasing Reynolds number and nanoparticle concentration. In higher concentrations, due to intensified shear-thinning behavior, and consequently, flatter velocity profile, convective heat transfer increases with a greater rate by an increase in concentration. Thermal, frictional and total entropy generation rates, both local and integrated, are assessed at different Reynolds numbers, concentrations and heat fluxes. The results show that increasing the concentration and Reynolds number leads to a decrease in thermal entropy generation rate and an increase in frictional entropy generation rate. The profile of thermal entropy generation rate at the annulus cross section is flatter at higher Reynolds numbers; while the same occurs for frictional entropy generation rate at lower Reynolds numbers. The lower the Reynolds number, the more evident the effect of heat flux change on total entropy generation rate. At low heat fluxes, the total entropy generation rate increases by increasing the concentration. However, at high heat fluxes, the total entropy generation rate decreases by concentration increment that can be beneficial from the second law viewpoint. Moreover, the effect of changing the value of heat flux on Bejan number is greater at higher concentrations.