Particle tracking velocimetry of porous sphere settling under gravity: Preparation of the model porous particle and measurement of drag coefficients

Porous particles are widely used in liquid–solid and gas-liquid-solid reactors, and accurate prediction of drag coefficients ( C D ) for porous particles is essential for designing and optimizing these reactors. The drag coefficients for porous particles, however, have been mainly obtained via theoretical analysis and numerical simulations. Only three experimental studies were carried out to study the drag force on porous spherical particles due to the difficulties in the preparation of model porous spheres with a homogeneous porosity and homogeneous surface properties. In this paper, a method to prepare model porous spheres which have a homogeneous porosity and homogeneous surface properties with a good sphericity for a wider range of relative permeability β has been presented. The details of this method were documented, and it showed that the porous spheres constructed could withstand rigid collisions at the maximum terminal velocity of 1 m/s and keep the structure intact with a negligible quantity of binders. Then gravity-driven settling experiments of a single porous sphere by use of particle tracking velocimetry (PTV) method were carried out to evaluate the feasibility of the use of the prepared porous spheres. Experimental results for 12 porous spheres, with particle Reynolds number ( R e ) ranging from 1 to 108 and relative permeability β from 21.6 to 315.6, agreed well with the previous results in the literature. The porous particles prepared by our proposed method can be potentially used for measuring drag coefficients of the porous permeable spheres in a wider range of particle Reynolds number and permeability and studying the complex interaction between the porous particles and the surrounding fluid.