Free-fall droplet

To estimate the available interaction time in a free-fall droplet experiment the steady-state balance between gravity and Stokes drag can be analyzed. A free-falling droplet will be subject to both gravitational and drag forces. Assuming that... 

Similar criterion has been obtained by Taylor. 2051 Hinze[270] estimated Wecri, to be 22 for a free-fall droplet, and 13 for a low-viscosity liquid droplet exposed suddenly to a high-velocity air stream. The latter value is comparable to those for water, methyl alcohol, mercury, and a low-viscosity silicone oil obtained by other investigators. 1275H277]... 

Single-droplet breakup at very high velocicty (L/velocity) . This governs drop size in free fall as well as breakup when droplets impinge on solid surfaces. 

Beard and Pruppacher have examined the free-falling velocity for small water droplets in saturated air. These results could be correlated as follows... 

Basic Breakup Modes. Starting from Lenard s investigation of large free-falling drops in still air,12671 drop/droplet breakup has been a subject of extensive theoretical and experimental studies[268] 12851 for a century. Various experimental methods have been developed and used to study droplet breakup, including free fall in towers and stairwells, suspension in vertical wind tunnels keeping droplets stationary, and in shock tubes with supersonic velocities, etc. These theoretical and experimental studies revealed that droplet breakup under the action of aerodynamic forces may occur in various modes, depending on the flow pattern around the droplet, and the physical properties of the gas and liquid involved, i.e., density, viscosity, and interfacial tension. 

In addition to interfacial area, the time of contact between the two phases affects thermal efficiency. This is best illustrated by the example given in Figure 5.1. Suppose we have a column Z feet tall and we gravity feed water in the form of droplets at a rate of 1 drop/sec. If there is no initial velocity imparted to the drops, then each drop descends through the column in accordance with the free-fall law ... 

Fill greatly increases the holdup time of liquid in the tower. Properly arranged fill will never allow liquid droplets to reach their free-fall velocity. Recall our simple example of water droplets falling freely in a column. At the feed, the water droplets have zero initial velocity, but by the time they arrive at the end of the column they have reached their terminal-fall velocity. By interrupting the flow with slats, each time a droplet strikes the fill it is as if it is released at the top of the column in Figure 5.1 with a zero initial velocity. Consequently, droplets never reach their terminal free-fall velocity and contact time is tremendously increased. 

As the oil flows within its designated cross-section area through the horizontal vessel, free water droplets form and begin to drop at a terminal velocity rate. The Vessize program calculates this terminal velocity VTWO. As discussed previously in the oil dehydration section, the Stokes law settling equation is used for the water droplet fall rate VTWO. 

In the commercial application, the drop tube method, as mentioned above, is suitable for mass production. The detailed investigations, such as temperature measurement of each small droplet and in situ observation of microstructure formation are not easy to attain because each droplet is in free fall. Here, the levitation method, where an Si droplet with a diameter of mm can be levitated by electromagnetic force using an electro-magnetic levitator (EML), as shown in Fig. 8.5, is a powerful investigation technique because the controlled droplet position enables us to measure the surface temperature of the droplet by pyrometer and to observe the crystallization behavior in situ by a high-speed video camera (HSV) [16-18]. 

Finally, another advantage of the semisolid process is described, based on simple heat transfer calculation during free fall. In the case of the sample superheated above Tm, the falling distance is more than 10 m, since the sample is undercooled due to suppression of nucleation. However, for the semisolid process, where each droplet includes a tiny solid particle before ejection, the falling distance required for complete crystallization is considerably reduced to 3 mm for /s >0.1 because of the lack of undercooling. This information is useful in terms of capital investment. 

---