Inertial particle transport

Figure 3 Schematic filter characteristic of the human respiratory tract for aerosol particles. Three domains can be recognized the domain of deposition decreasing with particle size is solely due to diffusional particle transport, the domain of minimum deposition is due to simultaneous diffusional and gravitational particle transport, and the domain of deposition increasing with particle size due to gravitational and inertial particle transport.

Particle transport and deposition from lurbuieni flows by inertial forces are not well understood and has been the subject of considerable experimental and theoretical study, Correlaiions for rates of particle deposition from lurbuieni pipe flow are discussed in this chapter. The concentrations are a.s.sumed to be sufficiently small to neglect the effects of the particles on the turbulence. Inertial effects can also be used to focus beams of aerosol particles. This effect can be pniduced for suhmicron and even ultrafine particles as described at the end olThe chapter. 

In the branching network of airways, the inspired air is changing its velocity and direction of motion all the time while it is penetrating into the lungs. Particles carried with the air are therefore exposed to inertial forces aU the time. For particles of sufficient mass, these forces result in an inertial displacement and thus in a particle transport toward airway surfaces. This displacement increases with particle... 

Very often inertial deposition in impactors is used to characterize the aerodynamic behavior of aerosol particles. However, much larger inertial forces are applied for particle deposition in impactors than are available for particle deposition in the human respiratory tract. The particle size obtained by this technique is the inertial diameter. This diameter is defined in the same way as the aerodynamic diameter but based on inertial rather than gravitational particle transport. When a particle is not only inertially but also gravitationally transported its inertial diameter is identical with its aerodynamic diameter. 

Mechanisms of Mechanical Particle Transport. When particles do not follow, but diverge from, airflow streamlines and thereby come in contact with airspace surfaces, particle deposition occurs. This diverging from airflow streamlines and particle trajectories is mainly due to mechanisms of mechanical particle transport inertial, gravitational, and diffusional particle transport (Fig. 3). The... 

The trajectory followed by water in a filter mass it is not linear. Water is forced to follow the outlines of the grains that delineate the interstices. These changes in direction are also imposed on particles in suspension being transported by the water. This effect leads to the evacuation of particles in the dead flow zones. Centrifugal action is obtained by inertial force during flow, so the particles with the highest volumetric mass are rejected preferentially. 

Impaction is caused by the inertial mass of the traveling aerosol particles that forces them to move in a straight-line direction even when the flow of the inhaled air transporting them is bent around a curvature. Hence the particles tend to deposit on obstacles placed in the path of their travel. The inertial mass depends on particle size, density, and velocity. The stopping distance S of a particle having mass mP and initial velocity v0iP is defined according to... 

Airborne particles may be delivered to surfaces by wet and dry deposition. Several transport mechanisms, such as turbulent diffusion, precipitation, sedimentation, Brownian diffusion, interception, and inertial migration, influence the dry deposition process of airborne particles. Large particles (dNIOAm) are transported mainly by sedimentation hence, large particulate PAHs tend to be deposited nearer the sources of emission Small particles (dblAm), which behave like gases, are often transported and deposited far from where they originated (Baek et al., 1991 Wu et al., 2005). 

The research of Roy Jackson combines theory and experiment in a distinctive fashion. First, the theory incorporates, in a simple manner, inertial collisions through relations based on kinetic theory, contact friction via the classical treatment of Coulomb, and, in some cases, momentum exchange with the gas. The critical feature is a conservation equation for the pseudo-thermal temperature, the microscopic variable characterizing the state of the particle phase. Second, each of the basic flows relevant to processes or laboratory tests, such as plane shear, chutes, standpipes, hoppers, and transport lines, is addressed and the flow regimes and multiple steady states arising from the nonlinearities (Fig. 6) are explored in detail. Third, the experiments are scaled to explore appropriate ranges of parameter space and observe the multiple steady states (Fig. 7). One of the more striking results is the... 

Particles smaller than 0.1 tim diameter are able to diffuse through the laminar boundary layer by Brownian diffusion, the efficiency of the mechanism increasing as particle size decreases below 0.1 (J-m. In general, rates of Brownian diffusion, which are small even by comparison with molecular diffusion, and do not therefore, represent an efficient process for the transport of sulphur and nitrogen containing particles across the laminar boundary layer. Another mechanism for transport of particles through this layer is inertial impaction. For this process the particle must... 

Eor a given crossflow filtration, the dominant particle back transport mechanism may depend on the shear rate and the particles size [4]. Brownian diffusion is only important for particles smaller than only a few tenths of a micron in diameter with relative low shear, whereas inertial lift is important for particles larger than several tens of microns with higher shear rates. Shear-induced back transport appears to be important for intermediate particle sizes and shear rates. Li et al. [7] reported that the shear-induced mechanism was able to predict fluxes comparable with the critical fluxes identified by the DOTM. 

Three primary mechanisms govern the deposition of aerosols in the respiratory tract inertial impaction, sedimentation, and diffusion (Fig. 7). Early work by Landahl and coworkers showed that both sedimentation and inertial impaction in the mouth, throat, and lungs uniquely depend on the particle aerodynamic diameter [220], Deposition by diffiisional transport is independent of particle density and limited primarily to particles with geometric diameters smaller than 0.5 p,m [221],... 

Unlike diffusion, which is a stochastic process, particle motion in the inertial range is deterministic, except for the very important case of turbulent transport. The calculation of inertial deposition rates Is usually based either on a force balance on a particle or on a direct analysis of the equations of fluid motion in the case of colli Jing spheres. Few simple, exact solutions of the fundamental equations are available, and it is usually necessary to resort to dimensional analysis and/or numerical compulations. For a detailed review of earlier experimental and theoretical studies of the behavior of particles in the inertial range, the reader is referred to Fuchs (1964). 

The general dynamic equation di-scussed in this chapter does not include terms for the inertial transport of particles. Discuss possible methods of incorporating particle tran.spor1 by the inertial mechanism in the GDE. 

Dry deposition refers to transport between and during precipitation events. Particles may deposit by sedimentation, inertial impaction, interception, diffusion,... 

In the previous sections we considered flows with a smooth spatial structure in which the relative dispersion of fluid trajectories is exponential in time and can be characterized by a single timescale, the inverse of the Lyapunov exponent. This is also valid for two-dimensional turbulent flows that have a smooth velocity field in the small-scale enstrophy cascade range (Bennett, 1984). A similar behavior occurs in any dimension at scales below the Kolmogorov scale (the so-called Batchelor or viscous-convective range, see below). In the inertial range of fully developed three-dimensional turbulence, however, the velocity field has a broad range of timescales and they all contribute to the relative dispersion of particle trajectories and affect the transport properties of the flow. 

Fluid Mechanics and Particle Inertia. Particles in a medium experience (1) drag, the resistive force exerted on a particle as it moves in a medium (2) inertial forces, due to the medium flow around the submerged particle or due to the particle path relative to the medium (3) centrifugal or vortex forces, due to the rotational motion of the medium (4) Coriolis forces, due to the linear and rotational motion of the medium (5) turbulent forces, due to the convective transport of the medium and (6) shear gradient, due to the relative movement of medium layers. 

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