Evaporation coefficient

FIG. 11-20 General range of long-tiihe vertical- (LT 0 evaporator coefficients, = ( F — 32)/l,8i to convert British thermal units per hour-square foot-degrees Fahrenheit to joules per square meter-second-kelvins, multiply hy 5,6783,... 

For the majority of metals, the evaporation coefficient is found to be unity, but, as mentioned before, the coefficient of many non-metallic elements with a complex vaporization mechanism such as... 

Finally, it is to be expected that the evaporation coefficient of a very stable compound, such as alumina, which has a large heat of sublimation resulting from the decomposition into the elements, will be low. Since the heat of evaporation must be drawn from the surface, in die case of a substance widr a low thermal conductivity such as an oxide, the resultant cooling of the surface may lead to a temperature gradient in and immediately below the surface. This will lower die evaporation rate compared to that which is calculated from the apparent, bulk, temperature of the evaporating sample as observed by optical pyromeuy, and thus lead to an apparently low free surface vaporization coefficient. This is probably die case in the evaporation of alumina in a vacuum. 

Figure 16 Evaporation coefficients for ice. (From Ref. 4. The references in the legend are identified in Ref. 4.)...

Offringa, J.C.A., de Kruif, C.G., Van Ekeren, P.J., Jacobs, M.H.G. (1983) Measurement of the evaporation coefficient and saturation vapor pressure of fraras -diphenylethene using a temperature-controlled vacuum quartz-crystal microbalance. J. Chem. Ther-modyn. 15, 681-690. 

General Mass Burning Considerations and the Evaporation Coefficient... 

Three parameters are generally evaluated the mass burning rate (evaporation), the flame position above the fuel surface, and the flame temperature. The most important parameter is the mass burning rate, for it permits the evaluation of the so-called evaporation coefficient, which is most readily measured experimentally. 

The use of the term evaporation coefficient comes about from mass and heat transfer experiments without combustion—that is, evaporation, as generally... 

In order to obtain the solution desired, a value of Ts is assumed, the vapor pressure of A is determined from tables, and mAs is calculated from Eq. (6.98). This value of mAs and the assumed value of Ts are inserted in Eq. (6.97). If this equation is satisfied, the correct Ts is chosen. If not, one must reiterate. When the correct value of Ts and mAs are found, BT or BM are determined for the given initial conditions Tx or mAco. For fuel combustion problems, mAcc is usually zero however, for evaporation, say of water, there is humidity in the atmosphere and this humidity must be represented as mAco. Once BT and BM are determined, the mass evaporation rate is determined from Eq. (6.87) for a fixed droplet size. It is, of course, much preferable to know the evaporation coefficient (5 from which the total evaporation time can be determined. Once B is known, the evaporation coefficient can be determined readily, as will be shown later. 

The previous developments also can be used to determine the burning rate, or evaporation coefficient, of a single droplet of fuel burning in a quiescent atmosphere. In this case, the mass fraction of the fuel, which is always considered to be the condensed phase, will be designated mr, and the mass fraction of the oxidizer ma. m0 is the oxidant mass fraction exclusive of inerts and i is... 

It is now possible to calculate the burning rate of a droplet under the quasisteady conditions outlined and to estimate, as well, the flame temperature and position however, the only means to estimate the burning time of an actual droplet is to calculate the evaporation coefficient for burning, f3. From the mass burning results obtained, f3 may be readily determined. For a liquid droplet, the relation... 

Richardson, Hightower and Pigg (1986) used a quadrupole mounted in a vacuum chamber to measure the vapor pressure of sulfuric acid as a function of temperature in the range 263 T < 303 K. They operated well into the free-molecule regime and assumed an evaporation coefficient of unity. Richardson et al. correlated their data by means of the equation... 

Fig. 3. Relative evaporation coefficient a of relevant elements in a system (impurities, reduc-...

Note that this suggests that the term (a/HRT), the evaporation coefficient for dissolved gases moving from the liquid to the gas phase, is analogous to a, which reflects the efficiency of uptake across the inter-... 

The Temperature-Dependence of) the Evaporation Coefficient for Evaluating Cooling Towers... 

---