Settling Velocity of Porous Spherical Particles

An experimental study was conducted to better understand the settling velocity of porous spherical particles. Tests of the settlement of porous particles with porosity, $\epsilon$, ranging from 0.53 to 0.86 in glycerol–water mixtures in a range of Reynolds number from 1.9 to 24,470 were conducted. The study utilized particle tracking and image velocimetry (PTV and PIV) to digitize the settlement trajectory in order to provide additional insights on the flow field around the particle. Particle properties, namely mass, porosity, and permeability, and fluid properties, namely density and viscosity, can potentially affect the terminal settling velocity. Experimental results reveal that flow permeation through the porous particle significantly increases with increasing porosity when the latter exceeds 0.77, but the drag coefficient decreases with increasing porosity at the same Reynolds number when the latter is less than approximately 100. The terminal settling velocity of porous and solid spherical particles with equal mass in air and in liquids (settling medium) is compared independently. Concurrent permeability effects on pressure drag reduction and frictional drag increment leading to the respective increase and decrease of the terminal settling velocity of porous particles are discussed. Flow visualization around a falling particle shows that compared with solid particles, porous particles cause earlier flow separation and the formation of a wider wake at $Re<100$ but narrower wake at $Re>100$. Examination of the flow field through PIV analysis reveals that increasing porosity increases the velocity and weakens the turbulence in the recirculating (wake) region.