Core-annular flow patterns

Figure 9. Idealised clustering and core-annular flow patterns and the corresponding plots of intermittency index versus radius (Brereton and Grace, 1992b).

Annular flow pattern is characterized by a thin water film, which flows along the channel wall with the nitrogen comprising the central core. Unlike annular macro-... 

Annular flow In annular flow, a thin wavy liquid film flows along the wall with a mist of gas and entrained liquid in the core. This flow pattern is observed at high velocities and low liquid fraction. 

In annular flow, liquid flows as a thin film along the pipe wall and gas flows in the core. Some liquid is entrained as droplets in the gas core. At veiy high gas velocities, nearly all the liquid is entrained as small droplets. Inis pattern is called spray, dispersed, or mist flow. 

Figure 5.3e shows the situation when the air velocity was increased to Ugs = 20 m/s. It is seen from this figure that the liquid bridges in churn flow disappeared and a liquid film formed at the side walls of the channel with a continuous gas core, in which a certain amount of liquid droplets existed. The pressure flucmations in this case became relatively weaker in comparison with the case of the churn flow. The flow pattern displayed in Fig. 5.3f indicates that as the air velocity became high enough, such as Ugs = 85 m/s, the liquid droplets entrained in the gas core disappeared and the flow became a pure annular flow. It is also observed from Fig. 5.3f that the flow fluctuation in this flow regime became weaker than that for the case shown in Fig. 5.3e, where Ugs = 20 m/s. 

With increasing superficial gas velocity the gas core with a thin liquid film was observed. The flow pattern, displayed in Fig. 5.14c (the second, third and fourth channels from the top), indicates that as the air velocity increased, the liquid droplets entrained in the gas core disappeared such that the flow became annular. 

Annular flow. In annular flow there is a continuous liquid in an annulus along the wall and a continuous gas/vapor phase in the core. The gas core may contain entrained droplets—dispersed mist—while the discontinuous gas phase appears as bubbles in the annulus. This flow pattern occurs at high void fractions and high flow velocities. A special case of annular flow is that where there is a gas/vapor film along the wall and a liquid core in the center. This type is called inverse annular flow and appears only in subcooled stable film boiling (see Sec. 3.4.6.3)... 

In the bulk boiling section, which appears when the bubble boundary layers have developed so as to fill the core and all of the liquid has reached saturation, there are two important flow patterns bubbly flow and annular flow. In bubbly flow, the bubbles are evenly distributed in the saturated liquid. The superheated... 

The microreactor contained a single channel with two feeds for gas and liquid [309,273]. The gas flows were set so high that an annular-flow regime was reached with a central gas core surrounded by a liquid film wetting the channel. This flow pattern has a very high interface and low liquid-side resistance due to the thin film. Formic acid was used as a solvent. 

A schematic of the flow pattern used to represent annular flow is shown in Figure 10. A preliminary model for pressure drops in the annular flow regime for the circular tubes under consideration here was reported in Garimella et al. [27], followed by the more detailed model [28] described below. For the development of this model the following assumptions were made steady flow, equal pressure gradients in the liquid and gas core at any cross section, uniform thickness of the liquid film and no entrainment of the liquid in the gas core. The measured pressure drops were used to compute the Darcy form of the interfacial friction factor to represent the interfacial shear stress as follows ... 

The wispy-annular flow, Fig. 4.44 d, consists of a relatively thick liquid film at the wall, even though the liquid fraction in the vapour or gas core of the flow is still large. The film is interspersed with small bubbles, and the liquid phase in the core flow is mainly made up of large drops that sometimes coalesce into liquid strands. This type of flow pattern is normally observed when the mass flux is large. 

A pattern that frequently appears is annular flow, Fig. 4.44 e. It is characterised by the fact that the main portion of the liquid mass is at the wall and the gas or vapour phase, that is interspersed with drops, flows in the core of the tube at a significantly higher velocity. 

Annular flow The liquid phase flows along the pipe or channel walls, as a more or less continuous stream, with the gas phase acting as a core. The gas phase may carry droplets of liquid that may be generated by the breakup of waves on the surface of the liquid film. Some liquid drops may fall back into the liquid phase, so that there may be a continuous liquid interchange between the continuous liquid phase and the gas phase. Furthermore, the liquid may contain entrained gas bubbles. The pattern detail will depend very strongly on the flow conditions in the system. Hewitt and Hall Taylor describe a subpattern of annu-... 

Liquid circulation is developed in a bubble column or airlift reactors because of the introduction of gas, and it affects the performance of the reactor. As shown in Figure 2, if a gas is injected in the center of the bubble column, in the core region (r < R ) the liquid rises with the bubbles and the liquid velocity decreases with distance from the column center. In the outer annular region (r > R ), liquid flows downward [16]. Between these two sections there is a transition point (r = R ) at which the velocity is zero. In the case of high flow rate condition, the transition point occurs at around R = 0.7R. It is important to estimate the extent of the induced liquid phase mixing. While experimental study of the liquid circulation has been carried out by a number of investigators, theoretical analysis of this problem is rather limited. Some models based on the pressure balance or the energy balance have been proposed for the liquid phase flow patterns [1]. 

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