Thread breakup

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 key factor in the commercialization of surfactant-based mobility control will be the ability to create and control dispersions at distances far from the injection well (TJ ). Capillary snap-off is often considered to be the most important mechanism for dispersion formation, because it is the only mechanism that can form dispersions directly when none are present (39,40). The only alternative to snap-off is either leave-behind, or else injection of a dispersion, followed by adequate rates of thread breakup and division to maintain the injected lamellae. 

First, if lubricated fingers of the gas phase become sufficiently long and the displacement rate sufficiently small, the fingers can breakup into a set of small bubbles in the same way a long, stationary fluid thread breaks when suspended in a second immiscible phase (17-19). In tubes, such thread breakup produces bubbles on the order of 4 times the undisturbed thread diameter. 

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. 

Some of the results of the Janssen model are quite interesting. It was found that a high viscosity of the dispersed phase promotes a finer dispersion due to the delay of thread breakup and coalescence. In general, lower viscosities of either phase result in coarser morphology. Highly viscous systems cannot be dispersed finer than 0.1 micron since coalescence starts to dominate as the drop size reduces much below 1 micron. 

Although knowledge of the interfacial tension in polymer/polymer systems can provide important information on the interfacial stmcture between polymers and, thus, can help the understanding of polymer compatibility and adhesion, reliable measurements of surface and interfacial tension were not reported until 1965 for surface tension [135,138] and 1969 for interfacial tension [127,154] because of the experimental difficulties involved due to the high polymer viscosities. Chappelar [145] obtained some preliminary values of the interfacial tension between molten polymer pairs using a thread breakup technique. The systems examined included nylon with polystyrene, nylon with polyethylene (PE), and poly(ethylene tere-phthalate) with PE the values are probably only qualitatively significant [174]. 

Until now, the effect is not explicitly described in literature. Based on experiences during spray experiments with the LamRot, the question of whether the cross-wind flow leads to an increase in mean drop size and in span value of the drop size distribution has to be answered. In case of confirmation, the results of the similarity trials on the thread breakup in the field of gravity could help to find better design of the gas distributor for a spray drying process with the laminar operating rotary atomizer. The span value of the dried product is the main objective as it is the crucial quality feature of the spray drying process. The optimized process could be applied to produce high qualitative products, e.g. for pharmaceuticals, chemicals, or food materials. 

This paragraph presents the investigation on the effect of the cross-wind flow oti breakup of stretched and reshaping threads. As an introduction, a short literature review about the thread breakup is given. Afterward, the similarity test rig and the experimental methods are explained. Finally, the results for the breakup length, the mean drop size, and the drop size distribution are discussed. 

Fig. 22.3 Influence of the cross-wind flow on the wave number and the growth rate of waves at surface tension-driven thread breakup. Larger wave number corresponds to shorter critical wavelengths resulting in smaller drops. On the other hand, the growth rate increases with increasing gas-Weber number. This leads to shorter breakup time and breakup length [11]...

Fig. 22.4 Experimental test rig for the similarity trials of laminar thread breakup under the influence of cross-wind flow. The liquid is pumped (i) from the collective tank (2) to the buffer tank (i) where pulsations are damped. Different capillary (4) configurations were used within this work including different diameters and inclination angles a. The capillary is mounted in the gas flow channel (5) made by PMMA. An axial fan (6) providing under-pressure controls the air velocity. Behind the flow, a light panel is installed and in front of it a CCD camera is placed [33]...

In addition, similar experiments were done as performed by Kitamura and Takahashi [11, 32], The drop size firom threads without stretching was determined by high flow rate through the capillary. The thread breakup was superimposed by a cross-wind flow. By the comparison of the drop volume and the volume of the disintegrated thread segment, the breakup wavelength is calculated with (22.10). 

The span value for different process conditions is plotted in Fig. 22.12. The results from completely filled vertically oriented capillary are used for comparison at different dimensionless viscosities and flow rates. For thread breakup without cross-wind flow, the span values are very small (0.19 < span < 0.37). The course of the span value vs. gas-Weber number is equal for the given process conditions. For increasing gas-Weber number below Wgg<0.3, the span value increases substantially. Further on, the span value increases with a nearly constant rate [33],... 

For validation of the similarity trials, the drop size during spraying with the LamRot and the thread breakup experiments was compared. The prediction in (22.11) serves as comparison at similar gas-Weber number as well as the dimensionless viscosity and flow rate. In Fig. 22.13, the mean drop size is plotted. The prediction reproduces the mean drop size with sufficient agreement. 

Replicate trials show a reliable agreement and indicate that the results are valid. The experimentally based optimal flow rate is 4(X) < Vswirl,opt,exp < 600m /h. Higher swirl flow rate leads to an increase in relative velocity and causes disturbances on the thread breakup. The small difference between the optimum theoretical flow rate and the flow rate during experiments is probably caused by friction losses in the swirl chamber. 

Kamplade, J., Kiiesters, A., Mack, T., et al. (Submitted). Similarity trials on the thread breakup from open channel flow at laminar rotary atomizer. Chemical Engineering and Technology (Submitted for publication). 

Although the above equations apply to Newtonian systems, it was shown experimentally that they also apply to situations encountered in polymer blending (Elmendop, 1991). The absence of the effect of viscoelasticity on polymer thread breakup is attributed to the very low deformation rates of the final stages of breakup (10 to 10 s ). At these rates it can be assumed that most polymers exhibit Newtonian behavior with viscosities equal to their zero-shear values. Examples for the use of the above equations follow. 

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