Sudden Pipe Expansion Flow

hammad0252 · December 23, 2018, 11:04

Hi, I am trying to model the flow in a pipe with expansion in it.

The issue is with the calculation of Pressure drop. The value obtained through simulation comes to be around 6500 Pa, while the theoretical one, calculated by Borda-Carnot Equation is 1400 Pa. The velocity change is accurate, as in it decreases from 12 m/s to 0.12 m/s in accordance with the area ratio but the pressure drop is way off.

I am using k-epsilon turbulent model with velocity inlet and Static Pressure at outlet. Images of the mesh are displayed below.

ghorrocks · December 23, 2018, 19:36

FAQ: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F

In your case I can see straight away that your mesh is never going give accurate results. The problems include:
* To coarse in the streamwise direction
* Massive jump in element size from the outer inflation layers

And don't forget that the Borda-Carnot equation has an empirical factor in it, and empirical factors can be in error by 50% (or more in some cases). I would not use an empirical equation like this as a validation case for a CFD model.

hammad0252 · December 24, 2018, 04:23

So I made the mesh finer in the streamwise direction and changed the inflation layer to smooth transition setting.

The results haven't changed much, change in pressure drop is about 3% between the two meshes. I am inclined to consider that the analysis is alright and the problem may just be the comparison method.

Gert-Jan · December 27, 2018, 16:37

How do you define "Pressure drop"?

evcelica · December 27, 2018, 18:43

It seems to me that maybe you are including the pressure drop from the 0.1 meter length of 0.01m radius pipe? And the empirical correlation does not?

That would be a little over 5000 Pa difference, which is the difference you are seeing.

I would extend the inlet, so you have fully developed flow, then subtract our the portion of the pressure drop which came from the tube itself, and not the expansion.

Make a plot of pressure and total pressure vs length, and you should be able to see the constant slope develop after it is fully developed. Do not take this dP/dL into account.

December 23, 2018, 11:04	Sudden Pipe Expansion Flow	#1
hammad0252 New Member Hammad Iftikhar Join Date: Nov 2015 Posts: 9 Rep Power: 11	Hi, I am trying to model the flow in a pipe with expansion in it. The issue is with the calculation of Pressure drop. The value obtained through simulation comes to be around 6500 Pa, while the theoretical one, calculated by Borda-Carnot Equation is 1400 Pa. The velocity change is accurate, as in it decreases from 12 m/s to 0.12 m/s in accordance with the area ratio but the pressure drop is way off. I am using k-epsilon turbulent model with velocity inlet and Static Pressure at outlet. Images of the mesh are displayed below.

December 23, 2018, 19:36		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	FAQ: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F In your case I can see straight away that your mesh is never going give accurate results. The problems include: * To coarse in the streamwise direction * Massive jump in element size from the outer inflation layers And don't forget that the Borda-Carnot equation has an empirical factor in it, and empirical factors can be in error by 50% (or more in some cases). I would not use an empirical equation like this as a validation case for a CFD model. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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December 24, 2018, 04:23		#3
hammad0252 New Member Hammad Iftikhar Join Date: Nov 2015 Posts: 9 Rep Power: 11	So I made the mesh finer in the streamwise direction and changed the inflation layer to smooth transition setting. The results haven't changed much, change in pressure drop is about 3% between the two meshes. I am inclined to consider that the analysis is alright and the problem may just be the comparison method.

December 27, 2018, 16:37		#4
Gert-Jan Senior Member Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,928 Rep Power: 28	How do you define "Pressure drop"?

December 27, 2018, 18:43		#5
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	It seems to me that maybe you are including the pressure drop from the 0.1 meter length of 0.01m radius pipe? And the empirical correlation does not? That would be a little over 5000 Pa difference, which is the difference you are seeing. I would extend the inlet, so you have fully developed flow, then subtract our the portion of the pressure drop which came from the tube itself, and not the expansion. Make a plot of pressure and total pressure vs length, and you should be able to see the constant slope develop after it is fully developed. Do not take this dP/dL into account.