Second law analysis of nanoparticle shape effects on the behavior of water-alumina nanofluid in shell-and-tube heat exchangers with trapezoidal oblique baffles

Abstract

This study addresses the critical problem of optimizing heat exchanger performance through nanofluid design by investigating the influence of nanoparticle shape in shell-and-tube heat exchangers with trapezoidal oblique baffles on entropy generation. A key focus is the differentiation between thermal and frictional entropy components, aiming to provide a clearer understanding of how these factors interact and influence energy efficiency. Water-alumina nanofluid with brick, platelet, blade, oblate spheroid, and cylinder nanoparticles was studied at a fixed Reynolds number of 5000 for the hot fluid, while water as the cold fluid was tested at Reynolds numbers of 5000, 10000, 15000, and 20000. Global thermal entropy generation of the hot fluid increased with Reynolds number, with platelet nanoparticles showing a 4.94% increase from 5000 to 20000. Conversely, frictional entropy generation decreased, and nanoparticle shape significantly impacted entropy generation, with platelet nanoparticles generating 63% more entropy than oblate spheroids, which exhibited the lowest entropy generation. Additionally, platelet-shaped nanoparticles exhibited the highest relative viscosity (1.432) and pressure drop, whereas oblate spheroid particles showed the lowest viscosity (1.036). The Bejan number remained high for both fluids (0.975–0.986), confirming the dominance of thermal irreversibility. For the cold fluid, total entropy generation decreased by up to 56% as Reynolds number increased from 5000 to 20000, with oblate spheroid nanoparticles inducing the highest total entropy generation and platelet particles the lowest. The results highlight nanoparticle morphology as a key design parameter for nanofluid-based heat exchangers, offering valuable insights for optimizing nanofluid formulations and baffle-assisted designs based on second-law performance.