Evaporation constant

Comprehensive treatment of the evaporation constant X is given by Spalding 109), and a comprehensive examination of Nusselt numbers for droplets is presented by Ranz and Marshall 102). The first work in the field is that of Froessling 29), after whom the appropriate equations are named. According to the combined treatments of Spalding and of Ranz and Marshall, when diffusion is controlling at relatively low vaporization rates... 

The slope, — d(2ri) /d0, of the straight lines which characterize combustion of various compounds as well as the lighter petroleum fractions during the evaporation stage has been termed the evaporation constant —i.e., n2 = rr -o — 0/4 and e = 2M/(tp°ti) (7-11, 4 )- Experimentally determined values of this constant are presented in Table IV. The burning lifetimes of fuel drops initially at the boiling point—i.e., with no preheat period—can be calculated by dividing the square of the initial drop diameter by the evaporation constant. 

Evaporation constants have been calculated from the published data of Hall and Diederichsen (22) and are given also in Table IV. From the luminous tracks on their photographic records these investigators estimated the burning times of drops projected... 

Included in Table IV are drop evaporation constants determined for some commercially important fuels. To conserve space the data of Kobayasi 27) are tabulated here in the form of averaged values at various temperature levels. 

EFFECT OF LIQUID PHASE COMPOSITION ON BURNING RATE. Many of the evaporation constants for drops of pure organic compounds burning in air at 20° C. are in the range 0.0070 to 0.0110 sq. cm. per second (Table IV). Values for some of the lighter petroleum fractions also fall in this range. 

Kiley (35) attempted to burn liquid monopropellants in nitrogen by the method of suspended droplets and by the technique of a porous Alundum sphere (7 mm. in diameter). These experiments did not yield stable combustion in an inert gaseous atmosphere. For combustion in air he reports the evaporation constants for hydrazine and nitromethane (Table IV). 

Goldsmith (16) subjected suspended droplets to an upward stream of air. Effective evaporation constants obtained from a plot of the square of the droplet diameter vs. time are presented in Table VII. Forced convection produced burning rates as high as 36% above the corresponding natural convection values. 

Air Velocity. Effective Evapn. Constant, Increase in Evaporation Constant,... 

This expression agrees with the empirical relationship given in equations (32) and (33) and can be used in equation (36) to determine the evaporation constant. Equation (58) shows that the principal dependence of rh on the properties of the fluid is expressed by m depends logarith-... 

These results are in agreement with experiment Theoretical and experimental values of the evaporation constant K are roughly of the order of 10" cm /s. 

Thus, the limited validity of the BET equation is due to the shortcomings in the model itself rather than to our lack of knowledge of the various parameters such as the number of layers, the variation of heat of adsorption E, or the evaporation constant b, etc. in the higher layers. 

The heat flux related to water evaporation in the CCL, q ap, is obtained by multiplying Qyap by the CCL thickness Icl- Unfortunately, the evaporation constant Kyap is poorly known, which makes numerical evaluation of q ap rather unreh-able. Figure 1.11 shows the dimensionless heat flux due to water evaporation in the CCL, normalized with respect to this flux at lOO C, assuming = 0, that is, fast water vapor removal. As can be seen, the heat flux qyap is a strong function of the cell temperature. Evaporation is an important mechanism of cell/stack cooling, and Figure 1.11 provides another argument in favor of cell operation at an elevated temperature. 

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