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Basics of Fluid Mechanics, 2014a

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542 CHAPTER 13. MULTI–PHASE FLOW<br />

Bubble<br />

Flow<br />

Slug or<br />

Plug Flow<br />

Churn<br />

Flow<br />

Annular<br />

Flow<br />

Dispersed<br />

Flow<br />

Fig. -13.6. Gas and liquid in Flow in verstical tube against the gravity.<br />

is acting in two different directions for these two flow regimes. For the flow against<br />

gravity, the lighter liquid has a buoyancy that acts as an “extra force” to move it faster<br />

and this effect is opposite for the heavier liquid. The opposite is for the flow with gravity.<br />

Thus, there are different flow regimes for these two situations. The main reason<br />

that causes the difference is that the heavier liquid is more dominated by gravity (body<br />

forces) while the lighter liquid is dominated by the pressure driving forces.<br />

Flow Against Gravity<br />

For vertical flow against gravity, the flow cannot start as a stratified flow. The<br />

heavier liquid has to occupy almost the entire cross section before it can flow because<br />

<strong>of</strong> the gravity forces. Thus, the flow starts as a bubble flow. The increase <strong>of</strong> the lighter<br />

liquid flow rate will increase the number <strong>of</strong> bubbles until some bubbles start to collide.<br />

When many bubbles collide, they create a large bubble and the flow is referred to as<br />

slug flow or plug flow (see Figure 13.6). Notice, the different mechanism in creating<br />

the plug flow in horizontal flow compared to the vertical flow.<br />

Further increase <strong>of</strong> lighter liquid flow rate will increase the slug size as more<br />

bubbles collide to create “super slug”; the flow regime is referred as elongated bubble<br />

flow. The flow is less stable as more turbulent flow and several “super slug” or churn<br />

flow appears in more chaotic way, see Figure 13.6. After additional increase <strong>of</strong> “super<br />

slug” , all these “elongated slug” unite to become an annular flow. Again, it can be<br />

noted the difference in the mechanism that create annular flow for vertical and horizontal<br />

flow. Any further increase transforms the outer liquid layer into bubbles in the inner<br />

liquid. Flow <strong>of</strong> near vertical against the gravity in two–phase does not deviate from<br />

vertical. The choking can occur at any point depends on the fluids and temperature<br />

and pressure.

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