Aerodynamic shear

Some quantitative studies1498115011 on droplet size distribution in water atomization of melts showed that the mean droplet size increases with metal flow rate and reduces with water flow rate, water velocity, or water pressure. From detailed experimental studies on the water atomization of steel, Grandzol and Tallmadge15011 observed that water velocity is a fundamental variable influencing the mean droplet size, and further, it is the velocity component normal to the molten metal stream Uw sin , rather than parallel to the metal stream, that governs the mean droplet size. This may be attributed to the hypothesis that water atomization is an impact and shattering process, while gas atomization is predominantly an aerodynamic shear process. 

The aspirator s pumping mechanism is quite sophisticated, and it all begins by water streaming past the water jet nozzle. By decreasing the internal diameter at the point where the water leaves the nozzle, there is an increase in water speed passing this point. Because the MFP of the gas and vapor molecules within the aspirator is much less than the pump dimensions, aerodynamic shear causes air movement in the desired direction. This air movement will occur regardless of whether there is direct contact of the air with the water or not. Additionally, because the boundary between the rushing water is quite turbulent, air is physi-... 

As we have already seen, the state of soluble as well as insoluble monolayers can deviate from a equilibrium state defined at constant temperature, pressure, bulk and surface concentrations. A deviation from the equilibrium state of the corresponding adsorption layer can be triggered by vertical and lateral concentration gradients due to adsorption/desorption processes or by hydrodynamic or aerodynamic shear stresses, as shown in Fig. 3.1. 

Fig. 3.1. Action of hydrodynamic or aerodynamic shear stresses at adsorption layers of insoluble surfactants...

The pressure (p) and shear stress (tj distributions over an airfoil-shaped body are shown schematically in Figure 1. The pressure and shear stress distributions exerted on the body surface by the moving fluids are the two hands of nature that reach out and grab the body, exerting a net force on the body—the aerodynamic force. 

This treatment removed 92% of the Pu contamination, as measured by the activity of the 241 Am daughter product of241 Pu. It was even more effective in reducing the subsequent rate of re-suspension by the wind, which was 99% lower than before the treatment. As discussed later (Section 6.13), re-suspension depends strongly on the shearing stress exerted by the wind on the ground, and this was less after the treatment because the aerodynamic roughness of the surface was reduced. 

The shearing stress, r, exerted by the wind on the ground entails a downwards flux of momentum. In the aerodynamic boundary layer above the surface, the momentum is transferred by the action of eddy diffusion on the velocity gradient. The friction velocity is defined by w = t/pa and is a measure of the intensity of the turbulent transfer. Near to a rough surface, the production of turbulance by mechanical forces... 

These deductions lead to important simplifications that permit an analytic solution to the flow field. The restriction to tall canopies in the theory of Finnigan and Belcher, 2004 [189] can be interpreted as a requirement that almost all the momentum flux is absorbed as aerodynamic drag on the foliage and not as shear stress on the underlying surface. 

The aerodynamic diameter is one of the most common equivalent diameters. It can be defined as the diameter of a unit den.sity sphere with the same terminal settling velocity as the particle being measured. The aerodynamic diameter is commonly used to describe the mt)lioii of particles in collection devices such as cyclone separators and impactors. However, in shear flows, the motion of irregular particles may not be characterized accurately by the equivalent diameter alone because of the complex rotational and translational motion of inegular particles compared with spheres. That is, the path of the irregular particle may not follow that of a particle of the same aerodynamic diameter. It is of course possible that there may be a. sphere of a certain diameter and unit density that deposits at the same point this could be an average point of deposition because of the effects of turbulence or the. stochastic behavior of irregular particles. 

In gas dynamic atomization, a liquid stream is broken-up into small entities which are dried very quickly. Because the formation of droplets occurs in aerodynamic suspension, the material experiences no shear and the liquid temperature does not rise above the local dew point, despite high gas temperatures. Since drying and subsequent cooling are rapid, organic materials do not have time to oxidize, degrade, or experience any other damage. Food powders often exhibit better flavor, texture, and instant characteristics than comparable powders from other spray dryers. Because a low pressure stream of slurry is pumped and dispensed, the system can also handle corrosive and abrasive products easily. Control over particle size is normally better. Fig. 7.82 depicts SEM photographs of some typical products. 

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