Fanno Flow: I just don't get it 1

[SilverRule]

Hi guys,

I just don't seem to get this topic fundamentally and also how to actually apply it. Just focusing on compressible flow by the way.

Wikipedia starts by saying "Fanno flow is the adiabatic flow through a constant area duct where the effect of friction is considered."

Basically, I don't see what makes this a unique and separate category. It doesn't sound special at all. Constant area is the simplest assumption and is the default case while analyzing any typical generic pipe flow. As for friction, where is friction NOT considered? Friction is like the base default phenomenon that is fundamental in all flows. Pretty much all real life pipe flow calculations consider the effect of friction by default. Why does this have to be mentioned specifically? If you DON'T consider friction, that's when you would highlight that because that wouldn't be normal. So do they mean something different by it?

The fact that it's adiabatic is the only thing special about it.

The other part that is completely unintuitive about it is this concept of frictional choking (as opposed to sonic choking) that comes up in relation to this topic. Like this statement: "for a flow with an upstream Mach number less than 1.0, acceleration occurs and the flow can become choked in a sufficiently long duct".

From all of my experience messing with fluid flow software and understanding of theory, it should be the opposite. Meaning...for a given pressure differential, the shorter the pipe the more likely it is that there will be choking at the endpoint. If you have a longer pipe, the fluid can move at a lower velocity lowering the chance that it will reach Mach = 1.

What am I not understanding?

 

[JOHN T SMALL]

My "go-to" document for compressible (and incompressible) flow theory and practical problem solving is Crane Technical Paper No. 410, "Flow of Fluids Through Valves Fittings and Pipe". I found it to be so usefull I even bought a copy!
Here you will find clarity on the Mach nr issue.

 

[pierreick]

Hi,
Consider this resource to support your work:

http://www.engsoft.co.kr/download_e/steam_flow_e.htm

Good luck
Pierre

 

[Latexman]

Maybe more analysis of Gino Girolamo Fanno's model would help. I don't know which reference is the best one. Best one to me means, the one that gives an accurate, historical description of Gino's work, puts it into perspective at the time he did the work, and shows in detail the practical utilization of the work that Gino, and others that followed him, have developed. I don't think there is any one reference that does all this, to be honest. I studied flow in constant-area ducts with friction from Volume 1 of The Dynamics and Thermodynamics of Compressible Fluid Flow by Ascher H. Shapiro.

At the time, Gino's model gave people a graphical method for looking at adiabatic flow problems. This increased their understanding. Kind of like the McCabe-Theile method did for me in multi-stage distillation. In addition to the Fanno Flow model helping solve adiabatic flow problems in pipes, when used with a Rayleigh Line, it also helps explain graphically certain peculiar features of normal shock waves.

I do agree with you, the individual assumptions contained in Gino's model are not so special. I do think Gino was special though, to combine all these asumptions into a model that closely estimates many practical applications. The sum of the assumptions and the molding of it into a useful model is special.

Good Luck,
Latexman

 

[SilverRule]

Maybe more analysis of Gino Girolamo Fanno's model would help. I don't know which reference is the best one. Best one to me means, the one that gives an accurate, historical description of Gino's work, puts it into perspective at the time he did the work, and shows in detail the practical utilization of the work that Gino, and others that followed him, have developed. I don't think there is any one reference that does all this, to be honest. I studied flow in constant-area ducts with friction from Volume 1 of The Dynamics and Thermodynamics of Compressible Fluid Flow by Ascher H. Shapiro.

At the time, Gino's model gave people a graphical method for looking at adiabatic flow problems. This increased their understanding. Kind of like the McCabe-Theile method did for me in multi-stage distillation. In addition to the Fanno Flow model helping solve adiabatic flow problems in pipes, when used with a Rayleigh Line, it also helps explain graphically certain peculiar features of normal shock waves.

I do agree with you, the individual assumptions contained in Gino's model are not so special. I do think Gino was special though, to combine all these asumptions into a model that closely estimates many practical applications. The sum of the assumptions and the molding of it into a useful model is special.

Good Luck,
Latexman

Then I guess the main confusion is my second question which it doesn't look like you touched. That one completely flips my understanding upside down.

 

[JOHN T SMALL]

From Crane:
"The maximum velocity of a compressible fluid in a pipe is limited by the speed of sound in the fluid. Since pressure decreases and velocity increases as fluid proceeds downstream in a pipe of uniform cross section, the maximum velocity occurs in the downstream end of the pipe. lf the pressure drop is sufficiently high, the exit velocity will reach the speed of sound.
lnvestigation of the complete theoretical analysis of adiabatic flow has led to a basis for establishing correction factors, which may be applied to the Darcy equation for this condition of flow. These correction factors compensate for the changes in fluid properties due to expansion and are identified as Y net expansion factors; given on page A-23."
Years ago compressible flow simulators (for flare networks for example) would become unstable around M 1, which impelled me to make a rather simple spreadsheet based on the modified Darcy equation.
Perhaps you can try this yourself in order to test your understanding

 

[katmar]

Long pipes are likely to have higher pressure drops than short ones. This results in larger density drops and larger increases in velocity.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Latexman]

The other part that is completely unintuitive about it is this concept of frictional choking (as opposed to sonic choking) that comes up in relation to this topic. Like this statement: "Meaning...for a given pressure differential, the shorter the pipe the more likely it is that there will be choking at the endpoint. If you have a longer pipe, the fluid can move at a lower velocity lowering the chance that it will reach Mach = 1.".

From all of my experience messing with fluid flow software and understanding of theory, it should be the opposite. Meaning...for a given pressure differential, the shorter the pipe the more likely it is that there will be choking at the endpoint. If you have a longer pipe, the fluid can move at a lower velocity lowering the chance that it will reach Mach = 1.

What am I not understanding?

Your statements, "Meaning...for a given pressure differential, the shorter the pipe the more likely it is that there will be choking at the endpoint. If you have a longer pipe, the fluid can move at a lower velocity lowering the chance that it will reach Mach = 1.", are true, but you have imposed a constant ΔP condition. The Wikipedia statement, "for a flow with an upstream Mach number less than 1.0, acceleration occurs and the flow can become choked in a sufficiently long duct", implies the flow is held constant as more and more pipe is added until the endpoint chokes.

Apples and oranges. Both are true.

Good Luck,
Latexman

 

[SilverRule]

Your statements, "Meaning...for a given pressure differential, the shorter the pipe the more likely it is that there will be choking at the endpoint. If you have a longer pipe, the fluid can move at a lower velocity lowering the chance that it will reach Mach = 1.", are true, but you have imposed a constant ΔP condition. The Wikipedia statement, "for a flow with an upstream Mach number less than 1.0, acceleration occurs and the flow can become choked in a sufficiently long duct", implies the flow is held constant as more and more pipe is added until the endpoint chokes.

Apples and oranges. Both are true.

Good Luck,
Latexman

Mannnn that gets me every time!! haha. Thank you so much! I appreciate it.