Influence of sample thickness and measurement set-up on the experimental evaluation of permeability of metallic foams

The pressure drop measured when a fluid flow through a porous medium is presumed a linear function of the medium thickness along the flow direction. Recent works have, however, reported that such linear dependence is not valid for highly porous foams. The deviations observed were attributed to the relative contribution of entrance and exit effects on the total pressure drop. In this work, the effect of the measurement set-up and of the specimen thickness on the pressure drop was investigated. Permeability measurements were carried out with nickel-chromium foams. Two different set-ups for holding the samples were tested. Pressure drop vs. velocity curves were obtained with water at room temperature. The results obtained showed that the normalized pressure drop was affected by the specimen thickness and the set-ups used. The size of the annular section supporting the specimens has a clear influence on the magnitude of the deviation from the expected linear behavior between the pressure drop and the medium thickness. This effect has been attributed to the fluid diffusion and flow expansion into the annular region covered by the specimen support. The magnitude of this effect is more important as the thickness of the specimens increased. No significant entrance or exit effect was observed when the supporting annular section was minimized. The dispersion of permeability constants caused by this effect was determined and discussed.