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Basics of Fluid Mechanics, 2014a

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

gas–liquid) flow is an extremely complex three–dimensional transient problem since the<br />

flow conditions in a pipe may vary along its length, over its cross section, and with<br />

time. To simplify the descriptions <strong>of</strong> the problem and yet to retain the important<br />

features <strong>of</strong> the flow, some variables are defined so that the flow can be described as<br />

a one-dimensional flow. This method is the most common and important to analyze<br />

two-phase flow pressure drop and other parameters. Perhaps, the only serious missing<br />

point in this discussion is the change <strong>of</strong> the flow along the distance <strong>of</strong> the tube.<br />

13.6.1 Multi–Phase Averaged Variables Definitions<br />

The total mass flow rate through the tube is the sum <strong>of</strong> the mass flow rates <strong>of</strong> the two<br />

phases<br />

ṁ = ṁ G +ṁ L (13.1)<br />

It is common to define the mass velocity instead <strong>of</strong> the regular velocity because the<br />

“regular” velocity changes along the length <strong>of</strong> the pipe. The gas mass velocity is<br />

G G = ṁG<br />

(13.2)<br />

A<br />

Where A is the entire area <strong>of</strong> the tube. It has to be noted that this mass velocity does<br />

not exist in reality. The liquid mass velocity is<br />

The mass flow <strong>of</strong> the tube is then<br />

G L = ṁL<br />

A<br />

(13.3)<br />

G = ṁ<br />

(13.4)<br />

A<br />

It has to be emphasized that this mass velocity is the actual velocity.<br />

The volumetric flow rate is not constant (since the density is not constant) along<br />

the flow rate and it is defined as<br />

and for the liquid<br />

Q G = G G<br />

ρ G<br />

= U sG (13.5)<br />

Q L = G L<br />

ρ L<br />

(13.6)<br />

For liquid with very high bulk modulus (almost constant density), the volumetric flow<br />

rate can be considered as constant. The total volumetric volume vary along the tube<br />

length and is<br />

Q = Q L + Q G (13.7)

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