Second-Law Analysis of Transient Heat Transfer Enhancement in Tubes Equipped with Elliptical-Cut Twisted Tape Using Hybrid Nanofluids under Mixed Convection

الملخص

Abstract

This study numerically investigates entropy generation during transient heat transfer in a vertical receiver tube fitted with elliptical-cut twisted tape inserts, with particular emphasis on the thermodynamic impact of forced and mixed convection conditions. Simulations are performed using STAR-CCM+, employing an Al₂O₃–Cu/water hybrid nanofluid as the working medium to enhance thermal transport and capture realistic solar receiver behavior. The analysis focuses on the distribution of entropy production arising from both heat transfer and viscous dissipation under unsteady flow conditions influenced by buoyancy effects. Results indicate that entropy generation is strongly dependent on the convection regime, where the inclusion of buoyancy in mixed convection reduces thermal irreversibility by improving temperature uniformity and weakening local gradients along the tube surface. In contrast, forced convection exhibits higher entropy levels due to stronger thermal non-uniformity. Although the twisted tape induces localized increases in frictional entropy generation due to intensified velocity gradients, its overall contribution remains secondary compared with thermal effects. A comparison between the two regimes shows that mixed convection achieves a notable reduction in total entropy generation, reaching approximately 58.2%, highlighting its superior thermodynamic performance. The findings demonstrate that coupling hybrid nanofluids with swirl-inducing inserts under buoyancy-assisted conditions is an effective strategy for reducing irreversibility and improving the efficiency of solar thermal receiver systems.

Keywords: Hybrid nanosuspension; Entropy generation (local/global); Bejan parameter; Time-dependent heat transfer; Elliptical-cut swirl tape; Thermodynamic irreversibility