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Total pressure drop in laminar channel flow (calculation of friction factor)

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Old   October 24, 2021, 12:16
Default Total pressure drop in laminar channel flow (calculation of friction factor)
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HOU Xiao
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Hello everyone,

I'm calculating the total pressure drop at Re = 2000, for water at 300K in a pip D = 0.1 m, L = 2 m. And this is a verification for the other studies.

The problem is, the theoretical result for friction factor is around f = 64/Re = 0.032, but the result is much higher (f = 0.0729).

Could you help me to verify my settings as follows?

------------------------------------------------------------

Re = 2000, density: 998.2 kg/m^3, viscosity: 0.001003 kg/(m s)
As a result:
hydraulic-diameter: 0.1 m (for pip D = 0.1 m)
inlet-velocity: 0.0201 m/s
sandgrain roughness height = 0

------------------------------------------------------------

Results:

Area-Weighted Average Total Pressure: inlet: 0.49894214 Pa
outlet: 0.20497435 Pa
friction-factor: f = 0.0729 >> 0.032

------------------------------------------------------------

Did I make a mistake of common sence or there are something special to be noticed?
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Old   October 24, 2021, 17:48
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64/Re is for fully developed flow.


Are you using streamwise periodic BC's or is this with a velocity inlet and outlet type calculation? You might need to take into account developing effects.
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Old   October 2, 2023, 01:27
Question the result is not matching with theoretical calculation
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I am facing the same issue.

Simple fluid Flow problem
Problem Description:
Pipe diameter: 0.0478 m
density: 998 kg/m^3
Velocity: 0.036 m/s


theoretical calculation:
Re: 1500
Friction factor f=64/Re
delta P= 0.57 Pa

Fluent Analysis:
Inlet BC: Velocity inlet
outlet BC: Pressure outlet 0 Pa (atmospheric condition)
Model: laminar

Result:
Pressure at inlet: 1.23 Pa
delta P = pressure inlet - pressure outlet = 1.23-0 = 1.23 Pa which is 115% higher than theoretical value.


Need help

Thanks in advance
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Old   October 2, 2023, 01:57
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A velocity inlet is not a fully developed flow. The theoretical value for this case is NOT 64/Re. You can find in engineering handbooks the apparent friction factor for developing cases. Use the right formula! It should not match 64/Re. If anyone matches 64/Re for a developing flow then either they are a liar or they have messed up.


If you want to match 64/Re then use periodic BC's so that you do have a fully developed flow, or apply the fully developed profile at the inlet, or make the pipe super long so that entrance effects can be ignored.
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Old   October 5, 2023, 06:26
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Thanks for your response.

Can you help me how to apply fully developed profile as velocity inlet ?

Thanks in advance
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