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Basics of Fluid Mechanics, 2014a

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

first estimate the waviness <strong>of</strong> the<br />

surface can be neglected. The estimation<br />

<strong>of</strong> the pressure difference<br />

under the assumption <strong>of</strong> equal<br />

shear stress can be applied. In<br />

the same fashion the pressure difference<br />

under the assumption the<br />

equal velocity can be calculated.<br />

The actual pressure difference can<br />

be between these two assumptions<br />

but not must be between them. This model and its assumptions are too simplistic<br />

and the actual pressure difference is larger. However, this explanation is to show<br />

magnitudes and trends and hence it provided here.<br />

To calculate the required pressure that cause the liquid to dry, the total balance is<br />

needed. The control volume include the gas and liquid volumes. Figure 13.16 describes<br />

the general forces that acts on the control volume. There are two forces that act<br />

against the gravity and two forces with the gravity. The gravity force on the gas can<br />

be neglected in most cases. The gravity force on the liquid is the liquid volume times<br />

the liquid volume as<br />

F gL = ρg<br />

The total momentum balance is (see Figure 13.16)<br />

F gL +<br />

V olme/w<br />

{}}{<br />

hL (13.92)<br />

A/w A/w force due to pressure<br />

{}}{ {}}{<br />

{ }} {<br />

L τ w = L τ w + D ΔP (13.93)<br />

G L<br />

Substituting the different terms into (13.93) result in<br />

( 2ΔP (D − h)<br />

ρgLh+ L<br />

+ 2 ghρ )<br />

L<br />

= L ghρ L<br />

+ D ΔP (13.94)<br />

L<br />

3<br />

3<br />

Simplifying equation (13.94) results in<br />

or<br />

4 ρgLh<br />

3<br />

ΔP =<br />

=(2h − D)ΔP (13.95)<br />

4 ρgLh<br />

3(2h − D)<br />

(13.96)<br />

This analysis shows far more reaching conclusion that initial anticipation expected.<br />

The interface between the two liquid flowing together is wavy. Unless the derivations<br />

or assumptions are wrong, this analysis equation (13.96) indicates that when D>2 h<br />

is a special case (extend open channel flow).

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