Liquid droplet trajectory

Spray regime (or drop regime, Fig. 14-20c). At high gas velocities and low liquid loads, the liquid pool on the tray floor is shallow and easily atomized by the high-velocity gas. The dispersion becomes a turbulent cloud of liquid droplets of various sizes that reside at high elevations above the tray and follow free trajectories. Some droplets are entrained to the tray above, while others fall back into the liquid pools and become reatomized. In contrast to the liquid-continuous froth and emulsion regimes, the phases are reversed in the spray regime here the gas is the continuous phase, while the liquid is the dispersed phase. 

In the second part, flow in the vapor space of the separator, where the gas phase is a continuous phase, was modeled. An Eulerian-Lagrangian approach was used to simulate trajectories of the liquid droplets since the volume fraction of the dispersed liquid phase is quite small. The grid used for the vapor space is shown in Fig. 9.20. The simulated gas volume fraction distribution near the gas-liquid interface and corresponding gas flow in the vapor space are shown in Fig. 9.22. The gas volume fraction distribution and the gas velocity obtained from the model of the bottom portion of the loop reactor were used to specify boundary conditions for the vapor space model. In addition to the gas escaping from the gas-liquid interface, it is necessary to estimate the amount of liquid thrown into the vapor space by the vapor bubbles erupting at the... 

Two methods are used to prevent tray liquid blowing. The simplest is probably the picket-fence exit weir (see Fig. 13). In this arrangement, a high exit weir interrupts the trajectory of flying liquid droplets. The slots control liquid depth on the tray. 

Figure 26 a shows the trajectory of 10 pm particles which tend to follow the stream lines of the flow. As they enter the torch, they move vertically downwards until they reach the fire ball. At this point they just skim over its surface and move radially outwards where they are entrained by the sheath gas flow in the space between the fire ball and the quartz tube. Being so small, they are drawn into the fire ball by electromagnetic pumping. As the particles are exposed to the high temperatures encountered they vaporize very quickly. (Note that an open circle indicates a solid particle, a shaded circle represents a liquid droplet and a partially shaded circle stands for a partially melted particle.)... 

When a liquid flows out of a small tube at a very low flow rate, it forms a pendant droplet at its tip. As the droplet grows it effectively blocks the flow of gas in the annulus which surrounds it. Pressure difference between the upstream and downstream sides of the pendant droplet, in addition to viscous fluid drag, provides the force that strips the liquid droplet from its tube-ejecting it from the generator. Since, typically, the droplet occupies a large fraction of the hole area, a rather short length of hole is sufficient to establish a very stable exit trajectory. 

Atomization breakup is mainly characterized by a liquid high-speed jet which disperses into a fine spray of many single droplets directly behind the nozzle exit. A further characteristic is the arising spray cone so that the droplet trajectory is not inevitably in line with the nozzle. The ejected droplets usually exhibit a droplet size distribution rather than a constant droplet volume. The actuation is continuous and very strong which leads to very high velocities inside the nozzle. 

The main calculation algorithm is shown in Fig. 4. In the first three steps, the import and transformation of particle trajectories and the fluid profile are performed. Afterwards the iterative part of the algorithm is started. In step 4, new liquid droplets are generated in the system. Number of droplets Ndrop, which are generated per one time step At, was calculated as ... 

The solution of the gas flow and temperature fields in the nearnozzle region (as described in the previous subsection), along with process parameters, thermophysical properties, and atomizer geometry parameters, were used as inputs for this liquid metal breakup model to calculate the liquid film and sheet characteristics, primary and secondary breakup, as well as droplet dynamics and cooling. The trajectories and temperatures of droplets were calculated until the onset of secondary breakup, the onset of solidification, or the attainment of the computational domain boundary. This procedure was repeated for all droplet size classes. Finally, the droplets were numerically sieved and the droplet size distribution was determined. 

Figure 5.5. (a) Calculated trajectories of liquid metal sheet and droplets near a close-coupled atomizer (Atomization gas Ar, Ma = 1 at nozzle exit, Metal Ni, 7° = 1877 K, rii(JmL = 3.74). (Reprinted with permission from Ref. 325.) (b) High-speed video imaging of actual gas atomization process with a close-coupled atomizer. (Courtesy of Mr. Paul Martiniano and Dr. Paul Follansbee, General Electric Corporate R D, Schenectady, NY, USA.)... 

The droplets carried above the froth may return to the froth or, for very small droplets, are entrained to the tray above. When the froth level approaches the tray above, as at high rates of vapor flow, even the large droplets cannot complete their trajectories and thus impact the tray above, possibly moving through the perforations as entrained liquid. Such a phenomenon drastically reduces tray efficiency. 

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