Researches On Fluid And Heat Transfer In A Double Pipe Heat Exchanger With Metal Foams

Metal foams are a class of porous material with both functional and structural properties, which is developed in recent decades. The metal foams are functional materials made of metal matrix and continuous or discontinuous gas with novel mechanical, thermal, electrical and acoustic properties. Metal foams can be divided described as open- or closed-cell. Open-cell metal foams have the following characteristics in the enhanced heat transfer technology: (1) lightweight, offering high strength and rigidity; (2) high internal surface area, so when fluid flows through metal foams, it can get greatly contact areas. The rate of heat transfer is enhanced by conducting the heat to the material struts.(3) Normal foam ligaments in the flowing direction results in boundary layer disruption and mixing and strengths the degree of fluid turbulence. (4) All open-cells are connected, so all surface area can be fully used. From above on, open-cell metal foams can be used to manufacture lighter, more compact, more efficient heat exchanger.Porous metal foams are still in the research, development stage and have not yet been widely applied as a new functional material. In this paper the fluid flow and heat transfer of tube heat exchanger filled with metal foams was researched. The main research work and its results include:The first part, theoretical analysis for the basic parameters of metal foams was worked. The results show the flow parameters for aluminum foams. This part is experiment and numerical simulation's precondition.The second part, an experimental research device and a data acquisition system were designed. Hot air flows in the inside square pipe, and cool water flows in the outside tube. AL foams were filled in the inner pipe in the double-pipe heat exchanger.The third part, built three-dimensional model of the tube heat exchanger for the experimental test section by Gambit, and simulated analysis for fluid flow and heat transfer property in the tube heat exchanger by Fluent.The theoretical analysis, experimental investigation and numerical simulation results are as follows:The first part shows that numerical simulations are consistent with the experiments. Both pressure drop and heat transfer rate increase with increasing the flow rate or the pore density. The filled Al foam enhances the heat transfer greatly but also raises the pressure drop. Comprehensively considering pressure drop and the performance of heat transfer, the heat exchanger filled with the Al foam is much better than that without any foam.The second part simulated the water-water double-pipe heat exchanger with metal foam filled in the both pipes. Porosity, Reynolds number, diameter, the thermal conductivity of base material relationship with fluid flow and heat transfer in metal foams were analyzed. Results show both pressure drop and heat transfer rate increase with increasing porosity or Reynolds number. For pressure: when Reynolds based on permeability less than 20, the pressure drop over the metal foams is dominated by permeability, when it more than 20, the pressure drop is dominated by inertial coefficient; the pressure drop of water through two pipes of different diameters or different thermal conductivity of base material but the same porosity is almost identical indicating that the pressure drop is mainly caused by the solid structure of the metal foam rather than the pipe wall. For heat transfer: when the Reynolds number more than 10,000, with the Reynolds number increases, the overall heat transfer coefficient was significantly increase and stabilize, especially for high porosity of the foam metal; With the increase of diameter or heat transfer coefficient of base metal , overall heat transfer coefficient is also significantly increased.The third part, overall performance of double-pipe heat exchanger with metal foam was compared by introducing the thermal resistance and pumping-power, and the performance factors (I ) show the overall performance is improved significantly.The results can provide a reliable basis for the fluid flow and heat transfer character in tube heat exchanger filled with metal foams. The results also have very important significance for process intensification technology and optimal design of efficient heat exchanger.