Drop size measurement

Fig. 9.3 Sauter mean diameter < 32 calculated from drop size measurements at single nozzles of liquid systems (a) toluene (dispersed phase d) water (continuous phase c) and (b) butanol d) water (c), is dependent on the mean velocity Vjv of the dispersed phase in the nozzle. (From Ref. 5.)... 

Drop size measurements were made on water sensitive papers exposed inside cooling towers at various levels. Results are given for samples taken under eliminators, over eliminators and high in towers. Design and operational factors which affect the values are discussed. Droplet removal efficiencies are given for conventional louvre eliminators and for louvre eliminators modified with plastic meshes. 

Magnesium Oxide Method for Drop Size Measurement. Droplet size may be determined from the impression made by a drop when collected on a coated slide. A layer of MgO has been used commonly as a collecting medium. Drops form a crater in the MgO on impaction. Based on past cahbrations (3, 4), the-diameter of this crater, as determined from microscopic observation, can then be related to droplet size. 

T. Helpio, and A. Kankkunen Measurements of the effect of black liquOT firing temperature on atomization performance. Part 2 Drop size measurements. Helsinki Technical University, Laboratory of Energy Engineering and Environmental Protection, Research report (1994). 

Example 2 Liquid-Liquid Stirred Tank for Drop Size Measurements... 

AU experimentalists who work with immiscible liquid-liquid systems must be concerned about the cleanliness of the system. Liquid-liquid experiments, particularly drop size measurements, are notoriously sensitive to changes in the cleanliness of the fluids, the vessel, and the mixers. The concern is that any trace of impurity, be it surfactant or greasy fingerprint, can alter the interfacial tension, alter the dynamics of the interface (repressing coalescence), or if preferentially wetted by the dispersed phase, act as a center for coalescence. 

In all cases fast, accurate methods are advantageous. However, because of problems such as coalescence, not all techniques or instruments are suitable for studies involving drops. Following a critical review of methods that had been used for drop size measurement, AzzopardF concluded that non-intrusive optical techniques can be most suited for such applications. 

A laser diffraction spectroscope (Spraytec, Malvern, Herrenberg, Germany) was adjusted, unless otherwise specified, 25 cm underneath the nozzle orifice. Spray drop size measurements were performed for single-phase sprays, taking the varying of the refractive index of different liquids into account. The emulsions were... 

Fig. 21.18 Measuring procedure to obtain local spray drop sizes measuring the spray drop size distributions all over the spray cone, invert the integral data into local data at a certain distance from the nozzle orifice...

In Fig. 21.29, a comparison of the highly viscous liquids is displayed. As discussed before, for high shear viscosity liquids, the influence of the gas pressure diminishes. The spray drop size measurements of the PVP K30 solution conform very well to the measurements of the maltodextrin solution. Surprisingly, the deviation in surface tension (mentioned in Fig. 21.7) seems to have no significant influence on the atomization process with atomizer geometry EAL... 

Lin et al. [70, 71] have modeled the effect of surface roughness on the dependence of contact angles on drop size. Using two geometric models, concentric rings of cones and concentric conical crevices, they find that the effects of roughness may obscure the influence of line tension on the drop size variation of contact angle. Conversely, the presence of line tension may account for some of the drop size dependence of measured hysteresis. 

Drops coalesce because of coUisions and drainage of Hquid trapped between colliding drops. Therefore, coalescence frequency can be defined as the product of coUision frequency and efficiency per coUision. The coUision frequency depends on number of drops and flow parameters such as shear rate and fluid forces. The coUision efficiency is a function of Hquid drainage rate, surface forces, and attractive forces such as van der Waal s. Because dispersed phase drop size depends on physical properties which are sometimes difficult to measure, it becomes necessary to carry out laboratory experiments to define the process mixing requirements. A suitable mixing system can then be designed based on satisfying these requirements. 

The prediction of drop sizes in liquid-liquid systems is difficult. Most of the studies have used very pure fluids as two of the immiscible liquids, and in industrial practice there almost always are other chemicals that are surface-active to some degree and make the pre-dic tion of absolute drop sizes veiy difficult. In addition, techniques to measure drop sizes in experimental studies have all types of experimental and interpretation variations and difficulties so that many of the equations and correlations in the literature give contradictoiy results under similar conditions. Experimental difficulties include dispersion and coalescence effects, difficulty of measuring ac tual drop size, the effect of visual or photographic studies on where in the tank you can make these obseiwations, and the difficulty of using probes that measure bubble size or bubble area by hght or other sample transmission techniques which are veiy sensitive to the concentration of the dispersed phase and often are used in veiy dilute solutions. 

In backlight or shadowgraph technique, the light comes from behind and is directed toward the camera. This technique is very useful in measuring bubble or drop size distributions (Figure 15.3). 

The moving-drop method [2] employs a column of one liquid phase through which drops of a second liquid either rise or fall. The drops are produced at a nozzle situated at one end of the column and collected at the other end. The contact time and size of the drop are measurable. Three regimes of mass transport need to be considered drop formation, free rise (or fall) and drop coalescence. The solution in the liquid column phase or drop phase (after contact) may be analyzed to determine the total mass transferred, which may be related to the interfacial reaction only after mass transfer rates have been determined. 

Contact angle measurements were obtained using a goniometer, measuring the advancing angle from 2 to 20 microliter drop sizes, of purified water upon polymer films at room temperature. Films were cast on metal plates and allowed to dry slowly from chloroform solutions. Several spots were measured on each film and the results averaged. 

K2. Keily, D. P., Measurement of drop size distribution and liquid water content in natural clouds, Dept, of Meteorol., Mass. Inst, of Tech., Contr. No. AF19(628)-259, NASA Rept. No. N64-30005 (1964). 

In the moving drop technique (also described in Chapters 7 and 9), a drop of the organic or aqueous phase is produced at the end of a vertical column filled with the other phase. The drop travels along the tube, during which extraction occurs across the drop surface. By measuring the time of traveling, the drop size, and from the volume of collected drops, it is possible to evaluate the rate of extraction (see Chapter 9 for a detailed discussion of drop behavior and mass transfer). 

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