Simulation and Optimization of Metal-foam Tube Banks for Heat Transfer Enhancement of Exhaust Heat Exchangers

Abstract

An essential issue of waste heat recovery (WHR) from engine’s exhaust gas, which could be effectively purified by modern after-treatment techniques, is the development of high-efficient, compact and lightweight high-temperature gas heat exchanger. Due to its multi-functional advantages, open-cell metal foam shows promising potentials in gas heat transfer enhancement. Thus, it is meaningful to explore the feasibility of this novel structure’s application on engine exhaust gas, by replacing bulky structures outside tubes with metal foam layers. In this paper, the heat transfer enhancement of forced convection in metal foam-wrapped tube banks was numerically studied. The Darcy-Forchheimer-Brinkman momentum model considering viscous loss and inertial loss, and the local thermal non-equilibrium energy model which considers the interstitial heat transfer between fluid and solid were employed for the porous zone. Effects of foam thickness and free stream velocity were analyzed to examine the heat transfer and pressure drop characteristics. A comparative analysis was also conducted with a bare tube bank. Results showed that the heat transfer performance of metal foam tube bank can enhance 1.8 to 2.0 times, measured in terms of the area goodness factor. In an array with fixed foam thickness, the better heat transfer performance is obtained at lower free stream velocities. It was also found that, within the designated fluid velocity range, tube banks with thicker foam layers show higher heat transfer rates and larger pressure drops as well. However, there exists an optimal dimensionless thickness of foam layer at 0.15 which gives the highest area goodness factor for the configuration considered in this study.