Mehdi Bahiraei

Irreversibilities caused by heat transfer and friction are evaluated for a non-Newtonian hybrid nanofluid flow in a double-tube minichannel heat exchanger by calculating entropy generation rates. The nanofluids are prepared by dispersing two different nanoparticles, namely Tetra Methyl Ammonium Hydroxide (TMAH) coated Fe3O4 nanoparticles and Gum Arabic (GA) coated Carbon Nanotubes (CNTs). Variable thermophysical properties are employed such that thermal conductivity is considered dependent on temperature and concentration, while viscosity is dependent on temperature, concentration and shear rate. The results show that heat transfer is the main cause in entropy generation at low concentrations while friction is the main factor at high concentrations. By increasing water temperature at the annulus side, the nanofluid thermal entropy generation increases while the nanofluid frictional entropy generation decreases. For low magnetite concentration, maximum thermal entropy generation occurs at the lowest CNT concentration while for high magnetite concentration, it happens at the highest CNT concentration. By increasing the Reynolds number, minimum point of thermal entropy generation moves toward smaller magnetite concentrations. In addition, at low magnetite concentration, total entropy generation rate possesses a minimum (optimal) point with respect to CNT concentration while an ascending trend is observed at high magnetite concentrations