Slug-flow unstable

If the air mass flow rate is reduced even further, it is found that the particles are conveyed gently and in the form of slugs (at point 4 in Fig. 1). Therefore, there are two boundaries (i.e. Boundaries 1 and 2, see Fig. 1) separating the slug-flow, unstable-zone and dilute-phase regimes. It has been observed that the pressure fluctuation near Boundary 1 is different from that near Boimdary 2 [3]. Typical examples obtained under horizontal flow conditions are shown in Fig. 2. 

As more air was added to the channel, the slug flow became unstable, the slug bubble broke down, and eventually the churn flow occurred in the channel. As shown in Fig. 5.3d, the most significant feature of flow characteristics in the churn flow is that the pressure oscillated at a relatively high amplitude, since the gas plug and liquid bridge flowed through the test section alternatively. 

Unstable Slug Flow Froth, Turbulent, Semi-Annular) in Vertical Tubes... 

For air and water, in a one inch tube, the superficial gas velocity limits for the film suspension region were about 31 ft./sec. to 41 ft./sec. (see Fig. 10 also). The value of 31 ft./sec. agrees with the value to be expected for the critical gas flooding velocity at zero liquid flow from an extrapolation of the results of Nicklin and Davidson. Hence, the approach of the latter workers in their analysis of unstable slug flow would seem to be valid for net liquid flows down to zero. 

Churn flow If the velocity of a two-phase mixture in slug flow is increased, the large slugs of gas will tend to become unstable, with the possibility of breakup. The result is the destruction of the slug flow pattern, with an oscillating characteristic being established. 

Churn flow The flow is highly unstable with the fluid traveling up and down in an oscillatory fashion but with a net upward flow. This flow pattern is in fact an intermediate regime between the slug flow and annular flow regimes. 

This paper presents results from a theoretical and experimental investigation into the unstable zone between dilute-phase and low-velocity slug-flow. The new boundary model predictions compare well with the experimental data. 

An investigation into the boundary of low-velocity slug-flow pneumatic conveying has been conducted experimentally and theoretically. The mechanism for the formation of the imstable zone has been developed. Based on mass, force and momentum balances and the unstable zone mechanism, a theoretical model for the prediction of the unstable zone boundary has... 

Pressure drop prediction of low-velocity slug-flow materials in the unstable zone... 

The theory developed for low-velocity slug-flow [5] still can be used to predict pressure drop caused by long slugs in the unstable zone as long as kw can be determined accurately. 

Fig. 6 Example of pneumatic conveying at the unstable/low-velocity slug-flow boundary.

In the first step, the influence of ultrasound on the slug flow of a liquid-liquid two-phase system in a channel was monitored with a high-speed camera. The observable influence is mainly due to bubbles with vibrating surfaces that are formed due to the ultrasound irradiation. Thus, unstable emulsions are generated within the segments, which themselves remain. In order to quantify the influence of ultrasound, the hydrolysis of p-nitrophenyl acetate (42) was utilized. Microstructures of varying material (PEEK, glass) with different... 

The transition of slug to annular or slug to stratified flow can be interpreted as a flooding condition. This approach describes the condition in which a wave, formed on the liquid film, may become unstable and increase indefinitely until it forms a slug (Bankoff and Lee, 1983). 

At low liquid rates, the onset of instability occurs at a constant value of the total superficial velocity, and is predictable from holdup and flooding data for wetted wall columns. As liquid flow rates increase, Nicklin and Davidson predict that unstable flow begins at lower values of the gas flow rate. For high liquid flow rates, however, the slug length becomes important, and the unstable flow will begin at higher values of gas flow rate. Therefore, a definite liquid flow rate exists at which an unstable flow pattern appears with a minimum gas flow rate. 

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