Comprehensive evaluation of the entropy generation in oval twisted double-pipe heat exchanger using non-Newtonian nanofluid using two-phase mixture model

The aim of this study is to investigate various aspects influencing the entropy production in a twisted double-pipe heat exchanger using two-phase mixture model. A combination of different percentages of CuO nanoparticles (phi=0-3%) in 0.5 wt% Carboxymethyl Cellulose (CMC)-water as a non-Newtonian nanofluid (at 298 K) in the inner tube and water was used as the heating fluid at 308 K at the opposite direction. The simulation outcomes indicated that rising the Re from 500 to 2000 for nanofluid, increased thermal and frictional entropy. On the other hand, raising the phi from 0 to 3% sufficiently lowered the thermal and frictional entropy, which was much more significant in frictional entropy generation rate. For Re=2000, the increase in phi from 0% to 3% caused a decrement in thermal, frictional, and total entropy of 9.2%, 15.3% and 11.8%, respectively. The effect of twisting pitch (2-6 mm) changes on both types of entropy generation also showed that the higher twist pitch resulted in the greater entropy generation, mainly for thermal entropy (24.2%). The results also showed that for all phi, the Bejan number decreased with Re augmentation, and at constant Re, increased with raising phi to some extent.