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Two Phase Flow in pipelines

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Reasons for Two Phase formation

In pipelines when velocity of liquid is increased

sufficiently or a portion of the pipeline is elevated

(as in the case of siphon) , it causes the static

pressure in the pipeline to decrease.

When this pressure becomes less than the vapor

pressure of liquid vaporization of liquid occurs.

It leads to the formation of two phases(liquid/vapor ) in pipeline

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Types of flow pattern

1.Segregated flow

(i) Stratified

ii) Wavy iii)Annular

2. Intermittent

i )Plug flow ii) Slug flow

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3. Distributed flow

i)Bubble flow

ii) Mist flow

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Stratified flow

At low gas velocities ,complete separation of

two phases occurs.

The gas goes to the top and the liquid to the

bottom of the tube ,separated by an

undistributed horizontal interface .

Hence the liquid and gas are fully stratified in

this regime

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Wavy Flow

Increasing the gas velocity in a stratified flow

leads to the formation of waves on the

interface These waves travel in the direction

of flow.

However this crest do not reach the top of the

tube .

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Annular Flow

At even larger flow rates ,the liquid forms a

continuous annular film around the perimeter

of the tube ,the film is thicker at the bottom

than the top.

The interface between the liquid annulus and

the vapor core is distributed by small

amplitude waves and the droplets may bedispersed in the gas core .

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Intermittent flow

Further increasing the gas velocity ,these

interfacial waves become large enough to wash

the top of the tube .

This regime is characterized by large amplitudewaves intermittently washing the top of the tube

with smaller amplitude waves in between .

Large amplitude waves often contain entrainedbubbles . The top wall is continuously wetted by

large amplitude waves

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Intermittent flow is also a composite of plug

flow and slug flow regimes . These are of

following two types :

I ) Plug flow

Ii)Slug flow

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Plug Flow

This flow regime has liquid plugs that are

separated by elongated gas bubbles.

The diameters of the elongated bubbles are

smaller than the tube such that the liquid

phase is continuous along the bottom of the

tube below the elongated bubbles. (D

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Slug flow

At higher velocities ,the diameter of the

elongated bubbles become similar to the

channel height .(D=H)

The liquid slugs separating such elongated

bubbles can also be described as large

amplitude waves.

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Mist Flow

At very high gas velocities ,all the liquid may

be stripped from the wall and entrained as

small droplets in the new continuous phase .

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