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 mactech001 January 3, 2010 06:29

horizontal pipe calculation

Dear all,

i'm trying to calculate horizontal pipe pressure drop at the inlet and outlet. the pipe size is D=25mm in diameter and L=100mm long. Fluid density is rho=1046.84kgm^-3 and viscosity is u = 2.8 centipoise = .0028 Nsm^-2.

meshing is done in WB with Sweep method and body element size of 1mm.

From CFX calculation, pressure difference was:
areaAve(pabs)@inlet-areaAve(pabs)@outlet = 12 Pa.

i tried to check using Darcy-Weisbach equation, del_p=lambda*L/D*v^2/(2g)
where lambda (using equation of Blasius) = 0.3164*Re^(-.25); Re=v*D*rho/u came about 3801.52

but del_p=0.0136 Pa

Why is my analytical and CFX results not the same please?? Can someone point me to the correct direction please?

 Tristan January 4, 2010 19:50

What are you using for boundary conditions at the inlet and the outlet? Do you end up with a similar velocity profile at both? If not, then you aren't simulating fully developed pipe flow and you won't get agreement with analytical calculations due to entrance length effects dominating your calculation.

Tristan

 mactech001 January 5, 2010 00:07

1 Attachment(s)

my inlet BC: inlet, mass flow rate 0.209 kg/s, temperature: 363K
outlet BC: opening, opening relative pressure: 0 Pa, temperature: 363K

also attached is velocity contour at the cross-section along the pipe length. is this paramter sufficient to show a fully developed flow please? or should i also look at other parameters please?

 Tristan January 5, 2010 13:32

It looks like the flow is still developing i.e. boundary layers are thicker at the end of the domain than at the start of the domain. Given that your domain is only 4 pipe diameters long this is not surprising since you would expect the entrance length region to be 10-20 pipe diameters long. Your other problem is the units in your analytical calculation. I get deltaP=0.00136 m from your formula which corresponds to 13.96 Pa (multiply through by rho*g).

Tristan

 mactech001 January 5, 2010 22:03

Tristan, thanks for your reply and pointing out my dumb mistake..... now i can move on to another problem.

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