Slug-dispersed flow

To create fine dispersion in microcapillaries, a micromixer (e.g., caterpillar mixer) needs to be attached upstream. At elevated flow velocity the static internals create dispersion, and as a result, part of the continuous phase flows in the form of small droplets in the dispersed phase. 

This flow regime is observed when the flow velocity is further increased in the microchannel with structured internals. Very fine droplets of one phase into the other are created. 

The flow regime transition in liquid-liquid flow could be explained by applying the dimensionless numbers. The flow patterns in liquid-liquid systems depend on the volume fraction of dispersed phase e ) and hydraulic diameter of the microchannel in addition to the dispersed phase Reynolds number 

The investigation of volume fraction of dispersed phase in the microchannel is not trivial. Equating volume fraction (e ,) to volumetric flow fraction (Pjy = Vjy/iVjy + Vq), a new group, Rej df le ), is introduced to characterize the flow pattern of liquid-liquid systems in capillaries [40]. The following criteria were obtained for the toluene/water system  

As the criterion possesses dimension, it is advised to use SI units. It is important to note that the above criterion is valid for microchaimels without structured internals. 

---

A flow-pattern map comprises dispersed flow, annular flow, slug-dispersed flow and slug-annular flow [278]. The highest specific interface measured amounts to 16 000 m2/m3. A porous surface structure (100 cm2) in the reaction channel can be generated by a sulfurhexafluoride plasma etch process with silicon nitride masking [278],... 

The mass transfer performance of a caterpillar micromixer under slug-dispersed flow regime is given in Table 7.10. The fine dispersion results in very high specific interfacial area leading to as high as 2.25 s. ... 

Figure 5.16c indicates that as the channel size was reduced to Jh = 0.866 mm, the dispersed bubbly flow pattern vanished from the flow regime map. Figure 5.16a-c indicates that the slug-churn flow transition line shifted to the right, as the channel size was reduced. Similar trends were also found in small circular tubes by the... 

A hydrodynamic characterization of the micro reactor is given in [12], A flow-pattem map reveals the existence of dispersed flow, annular flow, slug-dispersed... 

Second, a peak-intensity ultrasound echo can be used to detect the gas-liquid interface, but in this case the aim is the development of a flow meter capable of estimating the ratio of component phases accurately and in real time. Our results are promising for the estimation of the liquid flow rate of gas-liquid two-phase flow further research will produce valuable data that will allow the estimation of flow rates for the two phases simultaneously. The results presented here show the liquid flow rate estimated by the peak echo intensity method can provide an accurate estimate of the actual liquid flow rate. This method can be applied to pure liquid as well as to a two-phase flow where the void fraction is as high as 50%. The flows tested are of the stratified, elongated bubble, and slug flow types. Other types of flow such as wave flow and dispersive flow were not tested the present experimental setup does not provide the gas and liquid flow rates needed to achieve such flows. 

Chum flow or slug flow-annular (dispersed) flow transition. The flow becomes annular when the gas flow rates are enhanced to a certain point. The high gas flow rate causes a wavy interface of the liquid film. As a consequence, parts of the waves will enter the gas core as entrained drops. This results in an upward flow direction of the liquid, due both to interfacial shear and to drag on the waves and drag on the droplets. Annular flow can therefore exist only when the gas velocity is sufficient to lift the droplets in the gas core. The minimum gas velocity required to suspend a drop is determined from the balance between the gravity and drag forces on the drop. 

F. 2.1 Flow configurations obtained in miciochannels a Drop flow, b Plug (slug) flow, c Plug-drop flow, d Deformed interface flow, e Aimular flow, f Parallel flow, g Plug-dispersed flow, h Dispersed flow (Kashid et al. 2011)... 

The flow regimes observed in liquid-liquid flow in microchannels such as drop, slug, slug-drop, deformed interface, annular, parallel, and dispersed flow are depicted in Figure 7.9. 

Table 7.10 Mass transfer literature on slug-dispersed and dispersed flow. ...

Here the bubble velocity corresponds to the velocity of the slug of the dispersed flow, which was assumed to be identical with the twophase flow velocity... 

As is evident from the Baker diagram/ liquid and vapor velocities as well as quality determine the type of flow that will predominate. At relatively low vapor flow rates, stratified flow may exist with very little interaction between the two phases. As the vapor velocity is increased, wavy flow, then slug flow, annular flow, and finally mist or dispersed flow become successively... 

---