Concentric droplet mixing

The high potential and small radius of curvature at the end of the capillary tube create a strong electric field that causes the emerging liquid to leave the end of the capillary as a mist of fine droplets mixed with vapor. This process is nebulization and occurs at atmospheric pressure. Nebulization can be assisted by use of a gas flow concentric with and past the end of the capillary tube. 

Fig. 17. Comparison of volumetric dispersed-phase concentration distribution for various levels of droplet mixing for second-order reaction in the continuous phase, = 0.006 [after Zeitlin and Tavlarides (Z3)].

Kakuichi first dealt with a very important analysis of the distribution potential in small systems [17]. If theconcentration of NaCl in W and the concentration of TBATPB in NB are constants, a similar effect of volume ratio (Fnb/1V) on tho equilibrium potential and distribution ratio of TBA, TPB , Na, and CP is shown (Fig. 17). When the size of droplets is too small, the surface of the double layer is large enough in comparison with its volume, the electroneutrality condition may not be obeyed, and unusual behaviour of the system can be found. As pointed out by Kakuichi the system mentioned is very important for understanding the processes taking place in a small droplet mixed in a water environment. This important system will be investigated in more detail later. 

Sprays. Aerosol spray emulsions are of the water-in-oil type. The preferred propellant is a hydrocarbon or mixed hydrocarbon—hydrofluorocarbon. About 25 to 30% propellent, miscible with the oil, remains in the external phase of the emulsion. When this system is dispensed, the propellant vaporizes, leaving behind droplets of the w/o emulsion (Fig. 2b). A vapor tap valve, which tends to produce finely dispersed particles, is employed. Because the propellant and the product concentrate tend to separate on standing, products formulated using this system, such as pesticides and room deodorants, must be shaken before use. 

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

An impeller with a high fluid head is one with high peripheral velocity and discharge velocity. Such impellers are useful for (I) rapid reduction of concentration differences in the impeller discharge stream (rapid mixing), (2) production of large interfacial area and small droplets in gas-hquid and immiscible-liquid systems, (3) sohds deagglomeration, and (4) promotion of mass transfer between phases. 

When a liquid is dispersed into droplets the surface area is increased, which enhances the rates of heat and mass transfer. For a particular liquid dispersed at constant concentration in air the MIE varies with approximately the cube of surface average droplet diameter, hence the MIE decreases by a factor of about 8 when the surface average diameter D is halved (A-5-1.4.4). Ease of ignition is greatly enhanced for finely divided mists with D less than about 20 /rm, whose MIE approaches that of the vapor. Below 10 /rm a high flash point liquid mist (tetrahydronaphthalene) was found to behave like vapor while above about 40/rm the droplets tended to burn individually [ 142]. Since liquid mists must partially evaporate and mix with air before they ignite, the ease with which a liquid evaporates also affects MIE (Eigure 5-1.4.4). 

The coalescence-redispersion (CRD) model was originally proposed by Curl (1963). It is based on imagining a chemical reactor as a number population of droplets that behave as individual batch reactors. These droplets coalesce (mix) in pairs at random, homogenize their concentration and redisperse. The mixing parameter in this model is the average number of collisions that a droplet undergoes. 

Here vq is the measured tangential velocity profile at time t and (ve,steady) is the value at steady-state. Both intensity indices have a value of unity at f = 0, and approach zero as t approaches infinity. Figure 4.5.15 shows the variation of the intensity indices with average strain, for an outer cylinder velocity of 0.05 cm s 1. These plots indicate that the mixing process occurs in two stages, where the velocity profile develops only after the droplet concentration profile is essentially uniform. It can be seen that 1 decays to zero at approximately 100 strain units, whereas Iv shows that the steady-state velocity profile is reached only when y ps 400. From Figure 4.5.14 it can be seen that when y = 115, flow is detected... 

Fig. 4.5.15 Variation of the velocity difference intensity index ( ) and concentration mixing intensity index ( ) with average strain, for bulk droplet volume fraction of 0.4. Lines are shown to guide the eye.

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