Metal foam hydrodynamics: Flow regimes from pre-Darcy to turbulent

Metal foam is a relatively new class of porous media. The internal morphology of the foam is composed of connected cells each having many ligaments that form a web. In addition, metal foam has very high porosity (often greater than 90%) and a large surface area density. These properties are exploited in many applications, e.g., heat exchanger, reactors and filters. Flow regimes, and transition from one to another, are critical for understanding energy dissipation mechanisms for flow through the foam, and for heat transfer or reaction rates. While this topic is well studied in traditional porous media, e.g., packed beds, it is not well understood for foam-like porous media such as metal, graphite and polymeric foams. Pressure drop parameters such as permeability and form/inertial drag coefficients are very divergent for metal foam. The choice of an appropriate characteristic length for metal foam has also varied among researchers. In the current study a large set (88 points) of carefully-obtained experimental data for pressure drop of water flow in aluminium foam having 20 pores per inch and a porosity of 87.6% was collected. The range of flow Reynolds number covered all known flow regimes in porous media from pre-Darcy to turbulent. Flow regimes and transition between them were identified and compared to their counterparts in traditional porous media and to what is available for metal foam. The current data correlated very well using the friction factor based on the square root of permeability (measured in the Darcy regime) and the Reynolds number based on the same characteristic length. It is shown that the same foam exhibits different values of its permeability and Forchheimer coefficient in different flow regimes. The finding of this study can help in numerical and analytical work concerning flow and heat transfer in foam-like porous media.