Investigation of heat transfer in metal-foam tubes

Abstract

Nowadays metallic foams attract increasing attention in making compact heat exchangers due to its advanced thermal properties. In this project, the performance of forced convection and flow boiling heat transfer in metal-foam tubes has been investigated experimentally and theoretically. Compared to those of plain tubes, the research results reveal that the use of metal foam can significantly enhance heat transfer capabilities due to its strong fluid mixing capability and the high surfacearea density. In present study, the heat transfer performance and flow resistance of R134a vapour flow in metal-foam tubes have been measured. The results show that both the microstructure of metal foams and tube/foam attaching methods (contact thermal resistance) can significantly affect the heat transfer performance. Compared to plain tubes, the use of metal foams could improve the heat transfer by 5-15 times. To examine the effect of different tube/foam combining methods on heat transfer, a group of metal-foam tubes with different attaching methods have been tested experimentally. Since no experimental result for two-phase heat transfer in metal foams can be found in the open literature despite their potential wide range applications, the boiling flow and heat transfer in horizontal metal-foam tubes are experimentally investigated. The results show that the two-phase flow resistance and heat transfer both increase as the pore size tends to be smaller for a given porosity. The boiling heat transfer will be enhanced by increasing the vapour quality for high mass flow rates, but it is not always true for low mass flow rates. This different heat transfer behaviour can be attributed to different flow patterns occurring inside the metal-foam tubes. The two phase flow pattern can be indirectly inferred from the cross-sectional wall surface temperature fluctuations and the temperature difference between the wall surface and refrigerant fluid. In addition to the experimental research, analytical and numerical investigations have been conducted to predict the heat transfer performance of forced convection and flow boiling heat transfer in these tubes. The effect of contact thermal resistance between the tube wall and metal foam structures was considered in the model. The results show that the overall thermal resistance of a metal-foam tube is a combination of the resistance of the metal-foam structure and the resistance between solid phase and fluid phase. The increase of relative density and the decrease of pore size of metal foams can reduce these resistances respectively. The thesis also reports the investigations of the effects of these parameters on enhancing the overall heat transfer performance. This was carried out through a detailed parametric analysis. The results show that the thermal performance of a metal-foam heat exchanger can be superior to that of conventional finned tube heat exchangers.