Breakup mechanism

Droplet Breakup—High Turbulence This is the dominant breakup mechanism for many process applications. Breakup results from local variations in turbulent pressure that distort the droplet... 

Two-Fluid (Pneumatic) Atomizers This general category includes such diverse apphcations as venturi atomizers and reac tor-effluent quench systems in addition to two-fluid spray nozzles. Depending on the manner in which the two fluids meet, several of the breakup mechanisms may be apphcable, but the final one is high-level turbulent rupture. 

Illustration Satellite formation in capillary breakup. The distribution of drops produced upon disintegration of a thread at rest is a unique function of the viscosity ratio. Tjahjadi et al. (1992) showed through inspection of experiments and numerical simulations that up to 19 satellite drops between the two larger mother drops could be formed. The number of satellite drops decreased as the viscosity ratio was increased. In low-viscosity systems p < 0(0.1)] the breakup mechanism is self-repeating Every pinch-off results in the formation of a rounded surface and a conical one the conical surface then becomes bulbous and a neck forms near the end, which again pinches off and the process repeats (Fig. 21). There is excellent agreement between numerical simulations and the experimental results (Fig. 21). 

Janssen, J. M. H., and Meijer, H. E. H., Droplet breakup mechanisms stepwise equilibrium versus transient dispersion. J. Rheol. 37(4), 597-608 (1993). 

The phenomenon of liquid j et breakup has been the subj ect of theoretical and experimental investigations for more than one century. [37H41][115][21°][219H230] Reviews of liquid jet breakup mechanisms have been made by Tanasawa and Toyoda, 411 McCarthy and Molloy,[230] and Reitz and Bracco,12291 among others. 

As described previously, in the atomization sub-model, 232 droplet parcels are injected with a size equal to the nozzle exit diameter. The subsequent breakups of the parcels and the resultant droplets are calculated with a breakup model that assumes that droplet breakup times and sizes are proportional to wave growth rates and wavelengths obtained from the liquid jet stability analysis. Other breakup mechanisms considered in the sub-model include the Kelvin-Helmholtz instability, Rayleigh-Taylor instability, 206 and boundary layer stripping mechanisms. The TAB model 310 is also included for modeling liquid breakup. 

The temperature of a liquid metal stream discharged from the delivery tube prior to primary breakup can be calculated by integrating the energy equation in time. The cooling rate can be estimated from a cylinder cooling relation for the liquid jet-ligament breakup mechanism (with free-fall atomizers), or from a laminar flat plate boundary layer relation for the liquid film-sheet breakup mechanism (with close-coupled atomizers). 

Wieringa, J.A. Dieren, F. van Janssen, J.J.M. Agterof, W.G.M., 1996, Droplet breakup mechanisms during emulsification in colloid mills at high dispersed phase volume fraction, Chemical Engineering Research Design, 74, 554-562. 

Droplet Breakup—High Turbulence This is the dominant breakup mechanism for many process applications. Breakup results from local variations in turbulent pressure that distort the droplet shape. Hinze [Am. Inst. Chem. Eng.., 1, 289-295 (1953)] applied turbulence theory to obtain the form of Eq. (14-190) and took liquid-liquid data to define the coefficient ... 

Next we examine the breakup mechanism of immiscible droplets in a continuous phase and that of liquid filaments (30). 

A long, stationary droplet or "thread" of one fluid in another can break up into a string of smaller droplets, and this breakup mechanism has been further treated for slowly moving bubbles by Flumerfelt and co-workers (69,70). In tubes, thread breakup produces bubbles whose lengths are on the order of four times the diameter of the unbroken thread. The incorporation of this mechanism into a constricted tube model for dispersion flow in porous media is described in the chapter by Prieditis and Flumerfelt. 

A heterogeneous pore structure with varying aspect ratio would increase the frequency of breakup and coalescence, which should increase the observed mobile ganglia size distribution. However, the basic flow mechanism should remain unchanged. Also the relative importance of snap-off as a breakup mechanism would be increased relative to dynamic splitting. Here too a detailed study seems desirable. 

In addition to this thread breakup mechanism, gas fingers and large bubbles can also experience bubble snap-off when passing through narrow pore constrictions (20,21). Although snap-off phenomena can be quite complex (21-24), the static analysis of Roof (20) indicates that the resulting bubble diameters are at least twice the pore constriction diameter. 

---