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Basics of Fluid Mechanics, 2014a

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11.7. FANNO FLOW 451<br />

11.7.5.1 Maximum Length for the Supersonic Flow<br />

It has to be noted and recognized that as opposed to subsonic branch the supersonic<br />

branch has a limited length. It also must be recognized that there is a maximum<br />

length for which only supersonic flow can exist 27 . These results were obtained from the<br />

mathematical derivations but were verified by numerous experiments 28 . The maximum<br />

length <strong>of</strong> the supersonic can be evaluated when M = ∞ as follows:<br />

4fL<br />

D<br />

4 fL max<br />

D<br />

= 1 − M 2<br />

kM 2 + k +1 k+1<br />

2k ln 2 M 2<br />

2 ( 1+ k−1<br />

2 M 2) =<br />

−∞<br />

(M →∞) ∼<br />

k ×∞ + k +1 (k +1)∞<br />

ln<br />

2 k (k − 1)∞<br />

= −1<br />

k + k +1 (k +1)<br />

ln<br />

2 k (k − 1)<br />

= 4fL<br />

D<br />

(M →∞,k =1.4) = 0.8215<br />

4 fL max<br />

D<br />

= 4fL<br />

D<br />

(M →∞,k =1.4) = 0.8215 (11.203)<br />

The maximum length <strong>of</strong> the supersonic flow is limited by the above number. From the<br />

above analysis, it can be observed that no matter how high the entrance Mach number<br />

will be the tube length is limited and depends only on specific heat ratio, k.<br />

11.7.6 Working Conditions<br />

It has to be recognized that there are two regimes<br />

that can occur in Fanno flow model one <strong>of</strong> subsonic<br />

flow and the other supersonic flow. Even the flow<br />

in the tube starts as a supersonic in parts <strong>of</strong> the<br />

tube can be transformed into the subsonic branch.<br />

A shock wave can occur and some portions <strong>of</strong> the<br />

tube will be in a subsonic flow pattern.<br />

The discussion has to differentiate between<br />

two ways <strong>of</strong> feeding the tube: converging nozzle<br />

or a converging-diverging nozzle. Three parameters,<br />

the dimensionless friction, 4 fL , the entrance<br />

D<br />

Ì ¼<br />

Ì<br />

½<br />

<br />

<br />

¡× ¼ <br />

Ä ½<br />

<br />

½<br />

¡× ¼ <br />

Ä ¾<br />

<br />

<br />

<br />

<br />

ÄÖÖ µ Ä<br />

<br />

Fig. -11.23. The effects <strong>of</strong> increase <strong>of</strong><br />

on the Fanno line.<br />

4 fL<br />

D<br />

Mach number, M 1 , and the pressure ratio, P 2 /P 1<br />

are controlling the flow. Only a combination <strong>of</strong> these two parameters is truly independent.<br />

However, all the three parameters can be varied and some are discussed separately<br />

here.<br />

27 Many in the industry have difficulties in understanding this concept. The author seeks for a nice<br />

explanation <strong>of</strong> this concept for non–fluid mechanics engineers. This solicitation is about how to explain<br />

this issue to non-engineers or engineer without a proper background.<br />

28 If you have experiments demonstrating this point, please provide to the undersign so they can be<br />

added to this book. Many <strong>of</strong> the pictures in the literature carry copyright statements and thus can be<br />

presented here.<br />

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