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 (φ=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 φ 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 φ 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 φ, the Bejan number decreased with Re augmentation, and at constant Re, increased with raising φ to some extent.