Droplet population

There are a variety of ways to describe the droplet population. Figures 14-88 and 14-90 illustrate one of the most common methods, the plot of cumulative volume against droplet size on log-normal graph paper. This satisfies the restraint of not extrapolating to a negative drop size. Its other advantages are that it is easy to plot, the results are easy to visualize, and it yields a nearly straight line at lower drop sizes. 

Cloud albedo is calculated to be sensitive to the droplet population. Twomey (1977) showed theoretically that albedo is enhanced by the addition of particles to the atmosphere. 

It should be pointed out that another possible source of nucleation is the interface between the two phases under consideration. In some of the early droplet works, the authors found that the greatest supercooling needed for the crystallization of a certain droplet population was dependent on the superficial characteristics of the droplets [60,62,66]. 

In the case of polymer blends, the fractionated crystallization phenomenon that has been widely reported for many polymer systems can not be attributed to simple size effects. For instance, in Fig. 1, one could argue that the different exotherms originated in the crystallization of different droplet populations that have diverse average diameters. This cannot be the case, since the droplet distribution is monomodal and a smooth variation in heat... 

Gurciullo, C. S., and S. N. Pandis, Effect of Composition Variations in Cloud Droplet Populations on Aqueous-Phase Chemistry, J. Geophys. Res., 102, 9375-9385 (1997). 

The ability to separate the removal rates due to air bubbles from drop aggregation/coalcscencc for each oil drop size permitted a detailed study of the system variables. These variables and their ranges of variation are shown in Table I. Note that the first-order removal rate constants were independent of residence time and oil droplet population in the feed and effluent. The variables which may influence the rate constants are air flowrate, temperature, NaCI concentration, bubble diameter, cationic polymer concentration, and oil drop diameter. 

M. Simon, S. A. Schmidt, H.-J. Bart, The Droplet Populance Model -Estimation of Breakage and Coalescence. Chem. Ing. Techn,... 

In more realistic situations there is a certain probability of the emulsion droplets coalescing with the bulk oil phase or a part of the bulk oil becoming emulsified. The physics of such complex fiow conditions is not well understood at present. The starting point of describing such a fiow would be to treat it as a normal two-phase flow and use the concept of relative permeability and a model for the rheological properties of the emulsion phase. To account for the material exchange between the bulk phase and the emulsion phase, some form of droplet population balance model will be needed. 

Droplet size distributions obtained with any means mentioned here are relatively well represented by a Rosin-Rammler distribution with an exponent of approximately 2. This means that approximately 80 percent of the droplet population mass is in the range of 0.39 to 1.82 times the median droplet size. 

N0 typically ranges between 50 and 500 cm-3. rm ranges between 5 and 25 pm, and values of P = 2 and 5 have been used for tropospheric clouds. Consider rm = 20 pm and an optical pathlength of 1 m. Calculate and plot (a) the size distribution and (b) the transmission of light through a droplet population with this modified gamma distribution for both p = 2 and 5. For the plot of T(X), consider Mj ranging from 0 to 800 cm-3. Identify the sensitivity of the number concentration at which T = 0.5 to the two values of p. [The size distribution can be plotted as n(r)/No.]... 

The discussion above concerns total solute concentration. For individual species, because their aerosol concentrations are also generally size-dependent, there is an additional reason for size-dependent droplet concentrations. Concentrations of some major aerosol species measured in small and large droplets in a cloud are shown in Figure 17.19. The droplet population in these measurements was separated into only two samples with significant overlap and therefore the concentration deviations shown probably underestimate the actual differences. 

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