Evaporative flux

The relation between the columnar inclination angle (P) and the angle of incidence (a of the evaporation flux was found to be 2tan/5 = tana (45). The so-called tangent rule is not always vafld in a variety of experimental situations. 

Although fluxes of precipitation and river discharge can be quite accurately determined on a local scale, large portions of the globe, especially the oceans and Antarctica, are essentially ungauged, requiring extensive extrapolation of existing data. Evaporation fluxes are even less well known, since calculation requires... 

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. 

Figure 16. Schematic illustration of envelopes of gas species i, in this case Mg, surrounding a volatilizing molten chondrule in space. The size of the gas envelope is a function of ambient background pressure P by virtue of the effect that pressure has on the gas molecule diffusivity D,. The diffusion coefficient can be calculated from the kinetic theory of gases, as shown. The level of isotopic fractionation associated with volatilization of the molten chondrule depends upon the balance between the evaporative flux J vap and the condensation flux Tom When the fluxes are equal (i.e., when = 0), there is no mass-dependent isotope fractionation associated with volatilization. This will be the case when the local partial pressure P, approaches the saturation partial pressure P,...

Since initially the concentration of MCF in air, C,a, is zero, the evaporation flux calculated from Eq. 20-6 is (A = 0.3 m2, surface area of puddle) ... 

Ultimate pressure. This refers to the lowest pressure that can be reached in a system with negligible gas sources. For cryocondensation, a situation is reached where the condensation flux density is equal to the evaporation flux density. If Ac < v4vcssc,wan, this corresponds to the ultimate pressure giving ... 

Let us check this result with some data measured at Lake Zurich. This lake has an area of 68 km2 and average depth of 50 m. There are two sources of DCB to the lake sewage, which delivers 62 kg/ year, and flow from upstream, which delivers 25 kg/year. The downstream flow removes 27 kg/year from the lake. There is no accumulation of DCB in the lake s sediment. What is the evaporative flux of... 

In visualizing the molecidar configurations, we. sometimes observed that vapor molecules colliding with tlie lic uid surface apparently drove other molecules out of the liquid. This kind of correlation between 1 he condensation flux and the evaporation flux, or molecular e.xchange. may have a significant effect on the rate of condensation. Following... 

The evaporative flux from a solid or liquid surface is given by the Hertz-Knudsen equation (see Hirth and Pound, 1963)... 

We can illustrate some of the effects associated with evaporation and condensation by considering a commonly invoked special case, which we will call free evaporation, and which arises when the pressure of the evaporating species in the gas is assumed to be negligible compared to their saturation vapor pressure. Equation (2) can then be used to represent the evaporative flux of elements or isotopes, and the flux will fractionate... 

Here F and Fj are the incident and re-evaporated fluxes of active atoms or molecules the fluxes Fj are related to the partial pressures Pj by the equations... 

The Smoluchowski equation for the discrete size distribution (Chapter 7) can be generalized to take into account evaporation from the particles using the evaporative flux terms that appear in nucleation theory... 

Figure 1.26 shows the release time-courses of four flavors included in RM-Jl-CD at 50°C and 31%-75% RH (Furuta et al., 2008). D-limonene and L-menthol were more stable than AITC and EB under various RH conditions. At 75% RH, except for L-menthol, the flavors initially released markedly, nonetheless the release slackened off noticeably after 24 h. This might be caused by the occurrence of collapse of the powders, in which case the powder surface was covered with a hydrated layer, resulting in a decrease in the flavor evaporation flux, as mentioned in Section I.4.2.2. 

Water is being evaporated into a countercurrent air sireem at atmospheric pressure flowing at a rate of 2.2 lbm air/min (1 kg/min) in a vertical wetted-wall column 1.2 in. (0.03 m) in diameter. If the water flow rate is about 0.22 Ib /min (0,1 kg/min). estimate the evaporative flux at a point in the columu where the water tent parent re is 40°C and the mole fraction of HjO in the bulk gas is 0.01. 

Equation (13.7) is called the condensation equation and describes mathematically the rate of change of a particle size distribution n(v,t) due to the condensation or evaporation flux / ( , /), neglecting other processes that may influence the distribution shape (sources, removal, coagulation, nucleation, etc). A series of alternative forms of the condensation equation can be written depending on the form of the size distribution used or the expression for the condensation flux. For example, if the mass of a particle is used as the independent variable, one can show that the condensation equation takes the form... 

These data can be evaluated using a modified Knudsen equation where the evaporative flux Q is expressed by... 

It is often more important to define change in concentration rather than the evaporative flux and, since the actual mass of solute evaporating can be expressed... 

With fctot having units cm s and concentration in g or mol cm, the flux would be expressed in g or mol cm s In many cases, Ca is essentially zero and the evaporative flux would be a first order function of the bulk water concentration, C. The overall mass transfer velocity, can be perceived as the mean velocity with which the molecules cross the boundary and as demonstrated earlier, the evaporative half-life would be given by... 

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