Modelling an Open-Cell Foam Heat Exchanger
Metal foams are interesting materials with many potential uses. They are characterized by a cellular structure represented by a metal (or a metal alloy) and gas voids inside. A common metallic cellular material is a solid sponge, also known as open-cell metal foam. Due to their intrinsic high porosity and large specific surface area [1], these materials are considered to have very promising properties to improve efficiency and minimize the required weight and volume of novel industrial heat exchangers. However, the complexity of the convective heat transfer process demands a preliminary and hugely wide experimental activity to design foamed components with a good quality for energy transferring systems. The development of computational models, among others [2,3,4], can help to reduce experimental works and costs, although the task is very challenging. In this work we use COMSOL Multiphysics® 6.2 to model the fluid flow and heat transfer in a three-dimensional heat exchanger prototype (Fig.1). In the device a 40 PPI (pores per linear inch) open-cell foam (Fig.2), manufactured by ERG Aerospace Co. (Oakland, CA, USA) from a C10100 copper alloy, has been connected to three cylindrical tubes carrying hot water, to improve the heat transfer with a cold air stream. We start our modelling work by generating the exact geometry of the heat exchanger in SolidWorks® and importing the resulting CAD into COMSOL Multiphysics®, by using the capabilities of the CAD Import Module. In the model, the heat is transferred by convection and diffusive mechanisms in the fluids, while heat conduction is set in the solid regions of the system. The weakly compressible air flow through the copper cellular material and the water flow in the tubes are both assumed as laminar and coupled to the heat transfer mechanisms. We set up the Conjugate Heat Transfer physics from the Heat Transfer module complemented with the CFD module. In the computational model, the copper foam is represented as a porous media in the Local Thermal Nonequilibrium interface, using the properties given by its manufacturer and expressing the interstitial convective heat transfer coefficient as a General Configuration. The numerical results of the simulations are compared with experimental measurements obtained in a laboratory for the same heat exchanger prototype. These results show that the computational model developed with COMSOL Multiphysics® is effective for modelling the conjugate heat transfer process of the three-dimensional heat exchanger with open-cell sponge. The computational results prove that the energy transfer of the exchanger is enhanced taking advantage of the copper sponge's high porosity and large specific surface area. As an instance, the temperature isosurfaces of Fig. 3 point out that the cold air flow is quite heated when it flows through the copper sponge connected to the water tubes of the heat exchanger.
