Penetration of particles

Indoor air quality has been a public health concern for several decades now. Indoor air quality is affected both by infiltration of outdoor air in buildings and indoor sources such as smoking, gas cooking, and use of consumer products [6]. Penetration of particles into indoor environments depends on particle size, air exchange rates, and other factors. Consideration of indoor sources is important because they may be associated with significant health effects, e.g., environmental tobacco smoke. Presence of indoor sources may further complicate assessment of the impact of outdoor air on indoor air. In this chapter we separately describe the impact of indoor sources and outdoor air on indoor pollution, because health effects of outdoor and indoor generated particles may differ as their composition differs [7]. 

Figure 2.9 shows part of the results Elperin et al. obtained for the axial relative concentrations profile of silica gel particles. The major feature of this distribution is that the concentration sharply increases towards the impingement plane and the value near the impingement plane can be as high as 20-28 times the concentration in the feeding stream. This is mainly accounted for by the penetration of particles to and fro... 

The FSCBG aerial spray computer program is the result of more than a decade of refinement and verification of spray dispersion models used by the USDA Forest Service and the U. S. Army for predicting the drift, deposition and canopy penetration of particles and drops downwind from aircraft releases. This paper describes the mathematical framework of the models and selected applications of the models to military and Forest Service projects. 

Thatcher, T.L. and D.W. Layton (1995). Deposition, resuspension, and penetration of particles within a residence, Atmos. Environ., 29, 1487-1497. 

Aerodynamic diameter Impactor Inertial penetration of particle through slip stream as air flow is curved around impactor plate +... 

Penetration of particles into fluid Flotation tests Penetration time Sediment height Critical solid surface energy distribution References R. Ayala, Ph.D. thesis. Chemical Engineering, Carnegie Mellon University, 1985. Fuerstaneau et al. Colloids and Surfaces, 60, 127 (1991). Vargha-Butler et al., in Interfacial Phenomena in Coal Technology, Botsaris Glazman (eds.). Chap. 2, 1989. 

The resulting model is shown on the RHS of Fig.4.5-1. Clearly each vessel represents a state in a Markov process vessel 4 is the collector of the particles from which only the carrying stream is leaving at flow rate 2Qi. Q12, Q21, Q23 and Q 32 are recycle streams. This is due to the penetration of particles from one stream into the other where the impingement zone is designated by 2 in Fig.4.5-1 and is simulated as a perfectly mixed reactor. The effect of penetration is emphasized by the fact that movement of particles to vessel 4 is only possible from vessels 1 and 3. Whenever a particle reaches vessel 4, it remains there, i.e. the vessel is a trapping state. 

As indicated before, a possible increase in the mean residence time tm of the particles might be anticipated due to penetration of particles into the opposed stream, consequently undergoing multiple circulation followed by damped oscillations. This behavior is controlled in the above model by the quantity R, i.e. the recycle stream, and is demonstrated as follows for extreme cases. 

The major characteristic of impinging streams is the penetration of particles from one stream into the opposite one through the impingement plane (Fig.4.5), thus, increasing their mean residence time in the reactor as well as their relative velocity with respect to the air. The scheme in Fig.4.5-4 demonstrates this effect in the following way. Reactors 1 to 10 simulate the impingement zone of the particles and in each reactor particles reside for some time. A pulse of particles introduced in reactor 1 may be divided into three streams. One stream occupies reactors 3 to 5, the other, reactors 6 to 8, and the third one will occupy reactors 9 and 10. Eventually, the pulse accumulates in reactor 12 while passing reactor 11. 

The former is specified by the manufacturer, detoniiiied according to the penetration of particles of sodium chloride with mass median diameters of 0.6 pm and need not be verified as part of the validation program. 

As in dust collection, where surface coatings have been available for many years, the treatments are designed to present a microporous structure to the slurry which effectively restricts the penetration of particles to all but a few micrometres in depth. Consequently, a filter cake quickly forms on the surface of the coating and by restricting the particles to the surface, the same cake can be easily discharged at the end of the filtration cycle. Unlike the coatings in dust collection, however, the microporous structure in liquid applications has to withstand much higher pressures. Failure to do so will result in structural collapse and premature pressurisation of the filter. 

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