Convective heat transfer optimization in a circular tube based on local exergy destruction minimization

Abstract

In this study, the equilibrium equation of available potential, which reveals the relation of available potential and local exergy destruction rate, is determined, and the expressions of available potential and local exergy destruction rate are given. To improve heat transfer enhancement and reduce increase amplitude of flow resistance, a method termed as fluid-based heat transfer enhancement is proposed relative to surface-based heat transfer enhancement. An optimal mathematical model by constructing Lagrange function with exergy destruction corresponding to irreversibility loss of heat transfer process and fluid power consumption to flow loss of fluid is adopted to validate this method. To obtain the optimal flow structure in a tube, the tube flow is divided into two parts: core flow and boundary flow. For reducing the irreversibility loss in the core flow, we take fluid exergy destruction as optimization objective with prescribed fluid power consumption. For reducing the flow resistance in the boundary flow, we take fluid power consumption as optimization objective with prescribed fluid exergy destruction. The optimization equations for the convective heat transfer in laminar flow are derived, which are solved numerically. The longitudinal swirling flows in the tube are found at different parameters. In the optimized flow, heat transfer is enhanced greatly while accompanied with a little increase of flow resistance. Comprehensive performance, the ratio of increases in heat transfer and flow resistance, reaches at 3.65 after optimization.