Acceleration of droplets

Originally the electrospray interface, like the thermospray interface, was limited to use with very low flow rates of mobile phase from capillary or microbore HPLC columns or capillary electrophoretic separations. The acceleration of droplet evaporation by... 

The mean droplet size from the entrainment is 3.9 mm, which is rather large. The subsequent disintegration due to the relative motion between the droplets and gas flow may be possible. When the free stream gas velocity and Weber munber of 12 to 2 are used, the droplet stability criterion gives a diameter of about 0.6 to 0.1 mm. The very high entrainment rate shown above will certainly slow down the gas flow, specially in the boundary layer region, since the entrainment process and subsequent acceleration of droplets require considerable momentum transfer from the gas to liquid. When the onset of entrainment gas velocity is used as a mean gas velocity in the droplet boundary layer, the criterion gives the droplet the size of 2.4 to 0.4 nun. Thus it is expected that the size of droplets is in the range of 0.4 to 3.9 mm. 

Droplet Dispersion. The primary feature of the dispersed flow regime is that the spray contains generally spherical droplets. In most practical sprays, the volume fraction of the Hquid droplets in the dispersed region is relatively small compared with the continuous gas phase. Depending on the gas-phase conditions, Hquid droplets can encounter acceleration, deceleration, coUision, coalescence, evaporation, and secondary breakup during thein evolution. Through droplet and gas-phase interaction, turbulence plays a significant role in the redistribution of droplets and spray characteristics. 

The gas phase exists as a continuum in the central core of the conduit, and the liquid phase exists as a continuous film along the tube wall and as droplets in the gas phase. New droplets are continually being formed at the gas-liquid interface. The motion of the gas phase accelerates the droplets to a velocity approaching that of the gas phase. Droplets in the central core... 

In internal mixing atomization (for example centrifugal-pneumatic atomization), 159] the liquid metal and gas enter the swirl jet atomizer tangentially under pressure (Fig. 2.13)J159] The two fluids rotate, form a mixture, and accelerate in the confuser. Due to the strong centrifugal force, the liquid metal forms a film at the nozzle exit even without the presence of the gas. With the applied gas, the liquid film is atomized into a fine dispersion of droplets outside the nozzle. 

Subjected to steady acceleration, a droplet is flattened gradually. When a critical relative velocity is reached, the flattened droplet is blown out into a hollow bag anchored to a nearly circular rim which contains at least 70% of the mass of the original droplet. Surface tension force is sufficient to allow the bag shape to develop. The bag, with a concave surface to the gas flow, is stretched and swept off in the downstream direction. The rupture of the bag produces a cloud of very fine droplets presumably via a perforation mode, and the rim breaks up into relatively larger droplets, although all droplets are at least an order of magnitude smaller than the initial droplet size. This is referred to as bag breakup (Fig. 3.10)T2861... 

The spreading behavior of droplets on a non-flat surface is not only dependent on inertia and viscous effects, but also significantly influenced by an additional normal stress introduced by the curved surface. This stress leads to the acceleration-deceleration effect, or the hindering effect depending on the dimensionless roughness spacing, and causes the breakup and ejection of liquid. Increasing impact velocity, droplet diameter, liquid density, and/or... 

It had been shown that acceleration of the vaporization rate of liquid droplets can be achieved only in a small proportion by increasing the droplet heating rate or by enriching the fuel with oxidants [10]. It is possible, however, to induce an early droplet breakup by incorporating additives that promote droplet explosion in the existing fuel, such as organic azides [9] or, as shown in a following section, some of the hydrocarbon compounds studied in this work. Some of the HED... 

FIGURE 2. Continuous phase liquid holdup profiles (CPLHP) for different centrifugal accelerations for droplet size R = 5jc10 /w, surface concentration r=5jcl0 il g//n, ionic strength m =0.1M,thickness of adsorbed protein layer L, = 12xl(T /n and zeta potential = 12mV. 

A coalescer works in the same way as a demister, except that it is used to accelerate the removal of droplets of a heavier liquid from a flowing lighter liquid. An ordinary coalescer is shown in Fig. 26.5. This coalescer was used to remove entrained caustic from a flowing isobutane stream. The liquid isobutane would impact the coalescer pad at a velocity of 1 to 2 ft/min. The droplets of caustic, which have a higher surface tension than isobutane, would adhere to the surface of the coalescer fibers. As the caustic droplets grew bigger and heavier, they would drain down the fibers of the pad, and into the boot. 

Data predictions for droplets moving freely in turbulent gas streams are confounded by the problem of ballistics of droplets. Until the droplet is essentially accelerated or decelerated to the gas stream velocity, Reynolds number, thus Nusselt number, and thus X are changing constantly, and precise calculations require very small steps. The drag coefficient is of considerable importance. El Wakil, Uyehara, and Myers (117) em-... 

Containers that are not properly sealed are prone to accelerated loss of continuous phase molecules due to evaporation. This in turn increases oil droplet concentration, which may subsequently accelerate emulsion breakdown because of the increase in the frequency of droplet collisions. 

Liquid-liquid mixing has been widely used in chemical industries. The state of dispersion is determined by the balance of the break-up and coalescence of droplets. In the case of liquid-liquid mixing, the breakup of the droplet is accelerated in the impeller region. Although the droplet size distribution in the operation has been expressed by various PSD functions, the PSD function that is utilized the most is the normal PSD function. However, there is no physical background to apply the normal PSD function to the droplet size distribution. Additionally, when the droplet size distribution is expressed by various PSD functions, it becomes difficult to discuss the relationship between the parameters in PSD and operation conditions. This is one of the obstacles for developing particle technology. 

According to Moilliet, Collie, and Black (14), a first approximation shows that the empirical constant k must be equal to (pg/2)1/2V where p is again the density of the liquid, g the acceleration of gravity, and V the volume of the droplet. However, this value for k gives dimensionally incorrect results, and correct dimensionality is obtained by setting k = (pg/2) V . ... 

In this equation D is the difference between the density of the droplet and that of the air, g is the acceleration of gravity, that is to say the velocity acquired by a freely falling body in one second, and s is the viscosity of the air. 

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