Slurry particle inertia

The design of a cross-flow filter system employs an inertial filter principle that allows the permeate or filtrate to flow radially through the porous media at a relatively low face velocity compared to that of the mainstream slurry flow in the axial direction, as shown schematically in Figure 15.1.9 Particles entrained in the high-velocity axial flow field are prevented from entering the porous media by the ballistic effect of particle inertia. It has been suggested that submicron particles penetrate the filter medium and form a dynamic membrane or submicron layer, as shown in... 

Thin L-shaped probes are commonly used to measure solids concentration profile in slurry pipelines (28-33), However, serious sampling errors arise as a result of particle inertia. To illustrate the effect of particle inertia on the performance of L-shaped probes, consider the fiuid streamlines ahead (upstream) of a sampling probe located at the center of a pipe, as shown in Figure 2. The probe has zero thickness, and its axis coincides with that of the pipe. The fluid ahead of the sampler contains particles of different sizes and densities. Figure 2A shows the fluid streamlines for sampling with a velocity equal to the upstream local velocity (isokinetic sampling). Of course, the probe does not disturb the flow field ahead of the sampler, and consequently, sample solids concentration and composition equal those upstream of the probe. 

Joosten et al.51 explained the data based on the increase in the apparent viscosity of the slurry by the addition of solids. The volumetric mass-transfer coefficients, as a function of the relative viscosity of slurry obtained by them, are shown in Fig. 9-16. These data show that as the density of the solids decreased the value kLaL decreased faster with the increase in the relative viscosity. These data also show that for particle sizes < 250 pm, the suspended solid particles do not significantly affect the gas-liquid volumetric mass-transfer coefficient when the apparent viscosity of the slurry is not higher than four times that in the liquid. At high solids concentration, bubble coalescence and subsequent reduction in gas holdup can be the major cause in the reduction of fcLaL. The data show that kLoL in a three-phase slurry depends on the difference in density between the solids and liquids. The greater inertia of the heavier particles may create a stronger disturbance at the gas-liquid interface and thus affect the value of kL. 

Nasr-El-Din and Shook (58) studied solids distribution in a vertical pipe downstream of a 90° elbow. They tested sand-water slurries of various solid concentrations and particle sizes. The slurry flows were turbulent, and the particle Stokes number (inertia parameter) based on the pipe diameter and bulk velocity varied from 0.5 to 3. The solids distribution downstream of the elbow was found to be a function of the radius of curvature of the elbow, solids concentration, and particle size. 

Figure 4.9 shows AcoustoSizer measurements of the dynamic mobility of a 100 nm diameter silica slurry. The ratio of particle radius to double layer thickness, Ka, is around ten in this case. From these measurements it can be seen that the mobility magnitude and arguments have a similar form to the systems with surface conductance in the previous section, that is, the mobility has a phase lead and the magnitudes increase with frequency. In this case there are no inertia forces to slow the particles down, for the ratio of inertia to viscous forces ... 

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