# horizontal pipe calculation

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 January 3, 2010, 06:29 horizontal pipe calculation #1 Senior Member   Join Date: Nov 2009 Posts: 125 Rep Power: 9 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? __________________ Thank you for your kind attention. Kind regards, mactech001 Currently using: ANSYS v13

 January 4, 2010, 19:50 #2 Member   Tristan Burton Join Date: Mar 2009 Posts: 43 Rep Power: 10 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

January 5, 2010, 00:07
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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?
Attached Images
 velContour01.jpg (29.4 KB, 28 views)
__________________
Thank you for your kind attention.

Kind regards,
mactech001
Currently using: ANSYS v13

 January 5, 2010, 13:32 #4 Member   Tristan Burton Join Date: Mar 2009 Posts: 43 Rep Power: 10 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

 January 5, 2010, 22:03 #5 Senior Member   Join Date: Nov 2009 Posts: 125 Rep Power: 9 Tristan, thanks for your reply and pointing out my dumb mistake..... now i can move on to another problem. __________________ Thank you for your kind attention. Kind regards, mactech001 Currently using: ANSYS v13

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