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Basics of Fluid Mechanics, 2014a

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13.6. MULTI–PHASE FLOW VARIABLES DEFINITIONS 545<br />

Ratio <strong>of</strong> the gas flow rate to the total flow rate is called the ’quality’ or the “dryness<br />

fraction” and is given by<br />

X = ṁG<br />

ṁ = G G<br />

G<br />

(13.8)<br />

In a similar fashion, the value <strong>of</strong> (1 − X) is referred to as the “wetness fraction.” The<br />

last two factions remain constant along the tube length as long the gas and liquid<br />

masses remain constant. The ratio <strong>of</strong> the gas flow cross sectional area to the total<br />

cross sectional area is referred as the void fraction and defined as<br />

α = A G<br />

(13.9)<br />

A<br />

This fraction is vary along tube length since the gas density is not constant along the<br />

tube length. The liquid fraction or liquid holdup is<br />

L H =1− α = A L<br />

(13.10)<br />

A<br />

It must be noted that Liquid holdup, L H is not constant for the same reasons the void<br />

fraction is not constant.<br />

The actual velocities depend on the other phase since the actual cross section the<br />

phase flows is dependent on the other phase. Thus, a superficial velocity is commonly<br />

defined in which if only one phase is using the entire tube. The gas superficial velocity<br />

is therefore defined as<br />

U sG = G G<br />

ρ G<br />

The liquid superficial velocity is<br />

= X ṁ<br />

ρ G A = Q G (13.11)<br />

U sL = G L<br />

ρ L<br />

=<br />

(1 − X) ṁ<br />

ρ L A<br />

= Q L (13.12)<br />

Since U sL = Q L and similarly for the gas then<br />

U m = U sG + U sL (13.13)<br />

Where U m is the averaged velocity. It can be noticed that U m is not constant along<br />

the tube.<br />

The average superficial velocity <strong>of</strong> the gas and liquid are different. Thus, the ratio<br />

<strong>of</strong> these velocities is referred to as the slip velocity and is defined as the following<br />

SLP = U G<br />

U L<br />

(13.14)<br />

Slip ratio is usually greater than unity. Also, it can be noted that the slip velocity is not<br />

constant along the tube.

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