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Basics of Fluid Mechanics, 2014a

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11.7. FANNO FLOW 463<br />

11.7.8 The Practical Questions and Examples <strong>of</strong> Subsonic branch<br />

The Fanno is applicable also when the flow isn’t choke 34 . In this case, several questions<br />

appear for the subsonic branch. This is the area shown in Figure (11.25) in beginning<br />

for between points 0 and a. This kind <strong>of</strong> questions made <strong>of</strong> pair given information to<br />

find the conditions <strong>of</strong> the flow, as oppose to only one piece <strong>of</strong> information given in<br />

choked flow. There many combinations that can appear in this situation but there are<br />

several more physical and practical that will be discussed here.<br />

11.7.9 Subsonic Fanno Flow for Given 4 fL<br />

D<br />

This pair <strong>of</strong> parameters is the most<br />

natural to examine because, in most<br />

cases, this information is the only M 1 4fL<br />

provided information.<br />

pipe<br />

(<br />

4 fL<br />

D<br />

) For a given<br />

, neither the entrance<br />

Mach number nor the exit Mach<br />

number are given (sometimes the<br />

entrance Mach number is given see<br />

the next section). There is no known<br />

exact analytical solution. There are<br />

and Pressure Ratio<br />

M 2 4fL Δ P 1 D<br />

P 2 D P = P ∗<br />

M =1<br />

hypothetical section<br />

Fig. -11.33. Unchoked flow calculations showing the<br />

hypothetical “full” tube when choked<br />

two possible approaches to solve this problem: one, by building a representative function<br />

and find a root (or roots) <strong>of</strong> this representative function. Two, the problem can be<br />

solved by an iterative procedure. The first approach require using root finding method<br />

and either method <strong>of</strong> spline method or the half method or the combination <strong>of</strong> the two. In<br />

the past, this book advocated the integrative method. Recently, this author investigate<br />

proposed an improved method.<br />

This method is based on the entrance Mach number as the base. The idea based<br />

on the idea that the pressure ratio can be drawn as a function <strong>of</strong> the entrance Mach<br />

number. One <strong>of</strong> difficulties lays in the determination the boundaries <strong>of</strong> the entrance<br />

Mach number. The maximum entrance Mach number is chocking Mach number. The<br />

. The equations are<br />

numbers by perturbation method and the analytical solution<br />

lower possible Mach number is zero which creates very large 4 fL<br />

D<br />

solve for these large 4 fL<br />

is<br />

D<br />

M 1 =<br />

√<br />

[ ] 2 P2<br />

1 −<br />

P 0<br />

k 4 fL<br />

D<br />

(11.206)<br />

Equation (11.206) is suggested to be used up to M 1 < 0.02. To have small overlapping<br />

zone the lower boundary is M 1 < 0.01.<br />

34 These questions were raised from many who didn’t find any book that discuss these practical<br />

aspects and send the questions to this author.

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