Coagulation inertial

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. 

The second method for aerosol coagulation in turbulent flows arises because of inertial differences between particles of different sizes. The particles accelerate to different velocities by the turbulence depending on their size, and they may then collide with each other. This mechanism is unimportant for a monodisperse aerosol. For a polydisperse aerosol of unspecified size distribution, Levich (1962) has shown that the agglomeration rate is proportional to the basic velocity of the turbulent flow raised to the 9/4 power, indicating that the agglomeration rate increases very rapidly with the turbulent velocity. Since very small particles are rapidly accelerated, this mechanism also decreases in importance as the particle size becomes very small, being most important for particles whose sizes exceed 10-6 to 10"4 cm in diameter. In all cases brownian diffusion predominates when particles are less than 10-6 cm in diameter. 

Aerosol particles used for inhalation deposit within the lower respiratory tract mainly by inertial impaction, sedimentation, and diffusion. Loose fractal aerosols were found to settle slower and therefore had more time to increase gravitational coagulation with other floes leading to much more rapid particle growth. This will increase the chance of the aerosol floes settling on the airway walls before reaching the end of the airways. 

Turbulence may also lead to coagulation as a re.sull of inertial effects. When particles of different sizes (masses) are present in the same accelerating eddy, a relative motion is induced between the particles that may lead to collision. Again the scale of the particle motion is confined to distances < X. The mean. square relative velocity between the particles can be approximated using the force balance for Stokesian particles (Chapter 4) ... 

Ttirhulem Shear Coagulation 206 Turbulent Inertial Coagulation 206 Limitations on the Analysis 207 Comparison of Collision Mechanisms 208 Equation of Coagulation Continuous Distribution Function 208... 

Most of the methods used for aerosol degradation are based on intensifying the processes of coagulation, coalescence, adhesion of aerosol particles on different surfaces (on solid walls of filters, or water drops, as in artificial irrigation), and sedimentation (by changing the velocity and direction of aerosol streams during the inertial settling e.g. in so called cyclones). 

Principles of basic importance during the separation are based on the effect of inertial and electric forces, gravity, and the resistance of a fluid medium. Besides this, the diffusion, the interception principle and other effects find application. In particular types of industrial separators, a combination of these principles is usually used and, in addition, the particle motion may be affected by further transport phenomena, such as thermophoresis, diffusion and coagulation. The separation process is naturally affected by characteristics of the particles to be separated, by the technical operating parameters of the gas to be purified, and also by certain characteristics of the separator itself. Thus, the separation process can hardly be exactly described mathematically, and most separators are designed on the basis of experimental data rather than pure theoretical principles. 

In a turbulent flow there are two chief mechanisms of drop coagulation [2], that of turbulent diffusion and that of inertia. The inertial mechanism is based on the assumption that turbulent pulsations do not completely entrain the drop. As a result, relative velocities attained by drops due to turbulent pulsations depend on their masses. The difference in the pulsation velocities of drops of various radii causes their approach and leads to an increase of collision probability. The mechanism of turbulent diffusion is based on an assumption of full entrainment of drops by turbulent pulsations with scales, playing the chief role in the mechanism of approach of drops. Since drops move chaotically under the action of turbulent pulsations, their motion is similar to the phenomenon of diffusion and can be characterized by a coefficient of turbulent diffusion. 

Consider now coagulation caused by the inertial mechanism. In this case, consideration of molecular and hydrodynamic forces results in the following coagulation frequency [7] ... 

Note that Eq. (15.48) applies for drops of greatly differing sizes. For the inertial mechanism of coagulation, this case is of greatest interest, because for drops of commensurable size the basic mechanism of coagulation is turbulent diffusion. 

Let us restrict ourselves to the case of fast coagulation, assuming that each collision of bubbles results in their coalescence. The study of mutual approach of bubbles in a laminar flow is based on the analysis of trajectories of their relative motion. The equations of non-inertial motion of a bubble of radius a relative to a bubble of radius h in the quasi-stationary approximation are ... 

---