TUrbulent airflow

From Eq. (8.2) it is obvious that the Stokes number Stk and thus the deposition efficiency by impaction increase with increasing particle size and airflow velocity. Impaction occurs most frequently in the upper respiratory tract (pharynx, larynx, and main trachea), where particles larger than 5 p,m are trapped because of their size and the fast and turbulent airflow exerted. Also in the upper tracheobronchial region, impaction is the most prominent mechanism (Hinds 1998). 

To maximize drug particle dispersion, mechanical means may be introduced into the flow path to generate a turbulent airflow that exerts... 

When air is forced through such a powder bed by the patient inhaling, the bed dilates. The turbulent airflow within the device detaches the drug particles from the carrier particles within the device itself the drag particles are then carried on the airstream into the lungs. Those carrier particles that escape from the device are largely deposited in the oropharynx of the patient. 

On a broader scale, pharmaceutical manufacturers were beginning to utilize absolute filtration as a primary engineering control in the maintenance of large, carefully controlled clean spaces in the batch production of quality-controlled parenteral products. In this application, LAF was supplied directly to production lines and extended critical worksurfaces within defined, non-turbulent entrance and exit planes as parallel or columnated airflow (misnamed laminar flow ). This highly controlled laminar airstream was supplied to the critical worksurface, in addition to conventionally supplied turbulent airflow to the general space, provided through terminal diffusers for filtration of the balance of room air. In this manner, the stepped control of all critical, as well as support areas was achieved. 

Carruthers, D.J., Hunt, J.C.R., and Weng, W.-S. (1988) A computational model of stratified turbulent airflow over hills, FLOWSTAR I. In Computer Techniques in Environmental Studies (ed. P. Zanetti), Springer-Verlag, 481-492. 

Finnigan, J J., and Brunet, Y. (1995) Turbulent airflow in forests on flat and hilly terrain, in Wind and Trees, 3 0, Eds. M.P. Coutts and J. Grace. Cambridge University Press. UK. 

Hunt, J.C.R. (1996) Atmospheric diffusion from a steady source in a turbulent airflow at low mean wind speeds, Note to UK Atmospheric Dispersion Modelling Working Group (National Radiological Protection Board), Report R292,19-22. 

Yamada, T. (1982) A numerical model study of turbulent airflow in and above a forest canopy, J. of Meteorological Society of Japan Vol. 60, No. 1,439 154. 

Typically, in a room operating with turbulent airflow, the air should be introduced from ceiling diffusers and extracted from the room at low level to help give a flushing effect in the room. 

Sheldon started his Ph.D. studies at a time when the field of aerosol science was in its early stages of development. Working with H.F. Johnstone, he focused on how particles in turbulent airflow are deposited on the walls of pipes and ducts. Sheldon made important contributions right from the start he introduced the notion of a stopping distance of a particle injected into stagnant air, and then used this concept to predict particle motion through the viscous boundary layer to the surface. His thesis work laid the foundation for much of the later work on deposition of particles in industrial systems as well as dry deposition from the ambient atmosphere, where turbulent eddies impart velocities normal to the mean flow and enable particles to reach the surface. 

Evaporation is accompanied by local cooling and condensation by local warming. The addition of mas s to droplets and the conduction of latent heat away from the surface of the droplet or particle are, as has been pointed out by Pritchard (1987), the determining processes of particle growth. Under conditions of turbulent airflow, conduction of heat away from the surface of droplets or particles is enhanced and hygroscopic growth rates exceed those found under still conditions or under conditions of laminar airflow. The implications of this will be considered later. 

FIGURE 5.32 Local Nusselt numbers NuJiHi for thermally developing and hydrodynamically developing turbulent airflow (Pr = 0.7) in a smooth equilateral triangular duct [176]. 

The first analytical study to predict the performance of tubes with straight inner fins for turbulent airflow was conducted by Patankar et al. [118]. The mixing length in the turbulence model was set up so that just one constant was required from experimental data. Expansion of analytical efforts to fluids of higher Prandtl number, tubes with practical contours, and tubes with spiraling fins is still desirable. It would be particularly significant if the analysis could predict with a reasonable expenditure of computer time the optimum fin parameters for a specified fluid, flow rate, etc. 

In the air- or air-jet texturing process, flat, fully drawn multifilament yarns are fed slack into a stream of air (Fig. 1.6a). The air stream creates turbulence in which random loops are formed. The overfeeding of yarn is essential so that an excess of yarn length is available within the air-jet texturizing unit. The turbulent airflow disrupts and rearranges the filaments so that the loops formed are interlocked with sufficient interfiber friction to lead to a stable form. Since texturing is accomplished with air, not heat, this method can be used on all different types of materials, including nonthermoplastic fibers such as rayon. ... 

Later works by Miles [91-94]and Benjamin [84] showed that the evolution of wind stress component in phase with the wave slope arises from interaction of the surface perturbations and the mean turbulent airflow characterized by boundary-layer type velocity profiles. Their models are considered quasi-laminar, as the perturbations in turbulent Reynolds stresses induced by the surface perturbations were ignored. 

In order to adequately flush the room with clean air the exhaust should not be positioned near the inlet grids in the ceiling. Special inlet grids (diffusers) should generate turbulent airflow. In air class B exhaust at floor level is mostly preferred. A hollow wall construction offers the possibility of incorporating air ducts (exhaust channels) within the wall. These may make, however, other provisions on that particular part of the wall impossible. 

A sample three-dimensional simulation of the triple bifurcation is shown in Fig. 44. Here, turbulence airflow condition is assumed. It is seen that the simulation results are comparable with Ihe earlier simulation of Mazaheri and Ahmadi. ... 

Zahmatkesh, I., Abouali, O., and Ahmadi, G. (2006). Numerical Simulation of Turbulent Airflow and Particle Deposition in Human Upper Oral Airway. FEDSM2006-98309, ASME Second Joint U.S.-European Fluids Engineering Summer Meeting, Miami, FL. 

Under most exposure conditions the dry deposition velocity will be the combined effect of both resistances [46]. At well-stirred and highly turbulent airflow conditions, however, 0 and the dry deposition velocity solely depends on surface... 

An evaluation of airflow patterns revealed that insufficient bullet-trap exhaust caused the unbalanced ventilation system to deliver turbulent airflow at the firing line. 

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