Filtration mechanism inertial impaction

Figure 2.34 Particle capture mechanism in filtration of liquid solutions by depth microfilters. Four capture mechanisms are shown simple sieving electrostatic adsorption inertial impaction and Brownian diffusion...

Filtration is a physical separation whereby particles are removed from the fluid and retained by the filters. Three basic collection mechanisms involving fibers are inertial impaction, interception, and diffusion. In collection by inertial impaction, the particles with large inertia deviate from the gas streamlines around the fiber collector and collide with the fiber collector. In collection by interception, the particles with small inertia nearly follow the streamline around the fiber collector and are partially or completely immersed in the boundary layer region. Subsequently, the particle velocity decreases and the particles graze the barrier and stop on the surface of the collector. Collection by diffusion is very important for fine particles. In this collection mechanism, particles with a zig-zag Brownian motion in the immediate vicinity of the collector are collected on the surface of the collector. The efficiency of collection by diffusion increases with decreasing size of particles and suspension flow rate. There are also several other collection mechanisms such as gravitational sedimentation, induced electrostatic precipitation, and van der Waals deposition their contributions in filtration may also be important in some processes. 

Wet scrubbing uses liquid droplets to remove fine dust in a gas stream. In all types of wet scrubbing, the basic cleaning mechanism involves the attachment of particles to the droplets. The function of the droplets in scrubbers is similar to that of spherical fibers in filtration. Likewise, the primary collection mechanisms in scrubbing are similar to those in filtration, i.e., inertial impaction, interception, and diffusion [e.g., Fan, 1989]. Secondary collection mechanisms include thermophoresis due to temperature gradients, coagulation of particles due to particle electrification, and particle growth due to liquid condensation. 

Usually, the particle filtration occurs via multiple collection mechanism such as sieving, direct interception, inertial impaction, diffusion, and electrostatic collection. In practice, sieving is not an important mechanism... 

It is noticed that for such a filter containing fibres of the same size, it is unavoidable to have the minimum filter efficiency in filtering particles of certain sizes, as shown in Fig. 10.2. For very small particles less than dpi in diameter, the primary filtration mechanism is diffusion. For particles between dpi and dp2, the filter is less efficient, as the particles are too large for a great diffusion effect and too small for a large interception effect. For particles of diameter above dp2, the filter is very efficient again because the interception, along with inertial impaction, effects is predominant in the filtration. 

The combination of the effect of inertial impaction, diffusion, interception, and electrokinetic attraction on the cell particle capture and retention to a fibre is called direct adhesion, and the most important mechanism of leucocyte depletion filtration... 

The liquid droplets in the airflow are removed through the coalescence process in which two or more liquid droplets come into contact with each other to overcome surface tensions of the droplets to coalesce. Nonwoven coalescing filters capture liquid aerosols in individual fibres with depth filtration mechanisms including gravitational settling, inertial impaction, direct interception, and diffusion interactions. After the... 

Nonfixed-pore structure filters depend principally on the filtration mechanisms of inertial impaction and/or diffusional interception to trap particles within the spaces of their internal structure. 

A unique interaction between fluid mechanics and transport exists for filtration processes. Such processes perform better than expected based on the predicted impact of concentration boundary layers. The improvement in performance, a rare occurrence for membrane processes, arises from a combination of hydrodynamic diffusion and inertial lift [51]. Hydrodynamic interactions between particles or colloids that accumulate in the concentration boundary layer lead to shear-induced diffusion away from the membrane surface. Shear-induced diffusion can be significantly larger than molecular diffusion and thereby reduce surface concentrations. For sufficiently large particles at high shear rates, inertial lift becomes the dominant mechanism for particle movement away from the membrane. 

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