Surface vapor flux

The simulated weather changes that produced the variations in surface vapor flux rate shown in Figure 4.11 began with a soil sample with about 50% pore moisture saturation in a chamber under an artificial atmosphere. A quantity of DNT was introduced beneath the surface, at a depth typical of a landmine, about 3.5 cm. The atmosphere in the chamber was controlled to 50% relative... 

Figure 4.11 Effect of wetting and drying on surface vapor flux of 2,4-DNT.

In the sputtering process, each surface atomic layer is removed consecutively. If there is no diffusion in the target, the composition of the vapor flux leaving the surface is the same as the composition of the bulk of the material being sputtered, even though the composition of the surface may be different from the bulk. This allows the sputter deposition of alloy compositions, which can not be thermally vaporized as the alloy because of the greatly differing vapor pressures of the alloy constituents. 

Fig. 7-7 (a) Average annual precipitation (P) and evaporation (E) per unit area versus latitude. Arrows represent the sense of the required water vapor flux in the atmosphere, (b) Incoming solar energy (top of atmosphere and surface) and outgoing terrestrial energy versus latitude. 

Gaertner (1965) studied nucleate pool boiling on a horizontal surface in a water pool under atmospheric pressure. He increased the surface heat flux gradually. The vapor structures on the surface progressed from discrete bubbles to vapor columns and vapor mushrooms, and finally to vapor patches (dryout). The observed pictures of vapor mushroom and vapor patch are also sketched in Figure 5.3. 

Example 6.3 Consider Example 6.2 for a shallow pool of methanol with its bottom surface maintained at 25 °C. Assume that natural convection occurs in the liquid with an effective convective heat transfer coefficient in the liquid taken as 10 W/m2 K. Find the surface temperature, surface vapor mass fraction and the evaporation flux for this pool. 

Taylor and Spencer (1990) pointed out that a laminar layer can be regarded as the limiting distance above the soil surface to which the smallest eddies of the overland turbulent flow can penetrate. Therefore, above this layer, transport takes place through eddy diffusion, and the corresponding vapor flux can be described by... 

A direct implication of this experiment becomes apparent It is quite possible to determine favorable and unfavorable times for vapor flux sampling, based on moisture events. Furthermore, it may sometimes be possible to artificially enhance sampling by appropriate local introduction of moisture.7 Otherwise, it may prove more effective, in some environments, to sample surface soil particles rather than vapor. 

Traditionally, water is used as the test substance for determining v,a. Its air-water partition constant at 25°C is A)a/w = 2.3 x 10 5, which is much smaller than Kac cal of Eq. 20-4. Thus, the exchange of water vapor at the air-water interface is solely controlled by physical phenomena in the air above the water surface. The flux of water into air (evaporation) is given by (see Eqs. 20-6, 20-7, 20-9a) ... 

Vapor transport differs from surface diffusional transport, where the flux is always in the surface plane. For both surface diffusion and vapor transport, the diffusion potential at the surface is proportional to the local value of 7sk if the surface free energy is isotropic. For surface diffusion, the interface normal velocity is related to a derivative (i.e., the divergence of the flux). Also, the total volume is conserved during surface diffusion. For vapor transport, the interface normal velocity is directly proportional to the vapor flux, and the total number of atoms is not necessarily conserved. 

Failing to incorporate soil-gas advection induced by barometric pumping into gas-phase subsurface transport models may, under certain conditions, under predict contaminant flux to the atmosphere. As previously described, Smith et al. (1996) compared TCE vapor fluxes measured with a chamber device to TCE in groundwater being removed by a pump-and-treat system and discharge into a surface-water receiving body at the same site. These researchers found VOC removal rates by flux to the atmosphere comparable in magnitude to both of the other attenuation pathways. 

The droplet current / calculated by nucleation models represents a limit of initial new phase production. The initiation of condensed phase takes place rapidly once a critical supersaturation is achieved in a vapor. The phase change occurs in seconds or less, normally limited only by vapor diffusion to the surface. In many circumstances, we are concerned with the evolution of the particle size distribution well after the formation of new particles or the addition of new condensate to nuclei. When the growth or evaporation of particles is limited by vapor diffusion or molecular transport, the growth law is expressed in terms of vapor flux equation, given by Maxwell s theory, or... 

The stages of heat transfer in AGMD (Figure 19.7) include heat flux from the feed boundary layer to the membrane surface, vapor permeation through the membrane, and the diffusion in air gap, then condensation at the cold surface and finally heat transfer to the cooling water. 

FIGURE 15.31 Stages in subcooled pool boiling as the surface heat flux is increased, (a) no bubble formation (natural convection heat transfer) (6) bubbles grow but do not detach (c) bubbles increase in number and some detach to condense in the bulk liquid (d) bubbles become so numerous that they coalesce to form a continuous vapor layer on the heated surface (from Hewitt et al. [13], with permission. Copyright CRC Press, Boca Raton, FL). 

A more detailed treatment using a logarithmic driving force for vapor flux and the concept of the humidity potential coefficient O while accounting for the influence of the moisture vapor flux on the transfer of heat to the surface, namely, the Ackermann correction Luikov number Lu, which is essentially the ratio of the Prandtl number Pr to the Schmidt number Sc, has also been introduced. 

The Decreasing Drying-Rate Period Once the surface attained the hygroscopic range, the vapor pressure becomes smaller than the saturated vapor pressure (Figure 40.18). Consequently, the external vapor flux is reduced and the heat flux supplied to the medium is... 

Figure 55.17 shows the evolution of the surface moisture content under different intermittent temperature drying. On the onset of each tempering period, the surface moisture content inaeases. The increment is particularly prominent for the second tempering period. For the first tempering period, the product surface is still moisture saturated aud vapor flux continues to be transferred from the surface to the air. Once the surface becomes partially dried, internal moisture moves to the surface and an immediate increment in surface moisture is observed. There may be an inaeased moisture... 

There are several possible mechanisms to explain the enhancement of absorption during surface renewal. The Marangoni Effect results from the fact that dilute solutions of water C10 %) exhibit abnormalities in regard to surface tension. Jones and Ray [8] have observed that absorption of ions at the surface continues until a specific number of sites are occupied. The concentration of these sites is about 5 per 10 surface molecules. If liquid vapor is continually condensed on the drop surface, new surface for sites is being formed at a rate fixed by condensation. A second mechanism for enhancement, Stefan flow, is a trapping of the gas molecules into the liquid phase by the condensing vapor flux. The Stefan flow flux can be expressed as [9] ... 

However, if a heat flux resulting in surface vaporization is supplied through the wall... 

Prototype components for divertor elements, which can reliably remove the predicted power flux densities of about 10 MW/m, have been produced and successfully tested (Samm 2008). However, the impact of transient effects such as disruptions and ELMs impose a further very serious challenge for the integrity of divertor components (Federici et al. 2001). These events may cause surface damage and high erosion losses due to surface melting and melt layer losses, surface vaporization, cracking, and spallation. A disruption is the usually unplanned sudden termination of the plasma current that may occur if operational limits of plasma confinement are exceeded. One but not the only example of such an operational limit is the density limit. A disruption always leads to the sudden loss of plasma confinement and the... 

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