Pressure gradients during uniform fluid flow in porous media are traditionally assumed to be linear. Thus pressure loss across a sample of porous medium is assumed directly proportional to the thickness of the sample. In this study, measurements of pressure gradients inside coarse granular (2 – 18 mm particle size) porous media during steady gas flow were carried out. The results showed that pressure variation with distance in the porous media were nonlinear near the inlet (where pressure gradients were higher) but became linear at greater distances (with a lower gradient). This indicates that pressure loss in porous media consists of two components: (1) a linear pressure gradient and (2) an initial pressure loss near the inlet. This initial pressure loss is also known from hydraulics in tubes as a minor loss and is associated with abrupt changes in the flow field such as narrowings and bends. The results further indicated that the minor loss depends on particle size and particle size distribution in a manner similar to that of the linear pressure gradient. There is, thus a close relation between these two components. In porous media the minor loss is not instantaneous at the inlet point but happens over some distance starting upstream from the inlet and ending some distance downstream.
Water, Air and Soil Pollution, 2013, Vol 224, Issue 1666
Granular porous media; Fluid flow; Fluid pressure loss; Minor losses; Pressure gradient; Particle size distribution