Thermal performance investigation of metal foam heat exchanger for micro-gas turbine

Abstract

This study presents the experimental monitoring of heat transfer performance of metal foams in transient state. Micro gas turbines require a compact recuperator with high effectiveness to achieve higher thermal efficiency. Porous media such as metal and ceramic foams are characterised by high surface-to-volume ratio. They are known to increase heat transfer and potentially can be incorporated in recuperators. Their structure is ideal for thermal management of compact and lightweight applications. The idea is to combine excellent thermal properties of metal foams with turbine gases heat fluxes exploitation, in order to elevate the temperature of a different working fluid such as water or compressed air before it enters the combustor. A novel facility was designed and developed for monitoring heat transfer mechanisms that occur in metal foams. A copper cylinder is filled with metal foam which is heated in transient state by a sudden switch from cold to hot water flow whereas a cold stream cools the device in a crossflow configuration. The study demonstrates a method based on computerisation of true-colour analysis of digital images for surface temperature visualisation using thermochromic liquid crystals (TLC). Results of temperature as well as local and mean heat transfer coefficient were obtained showing that the hot flow inside the foam was more dominant in heat transfer than the cold flow in the empty channel. The method is promising for the evaluation of transient phenomena in a tube that is filled with porous media.