Boundary Condition at outlet

nabidinhomessi · December 15, 2015, 10:05

I am trying to simulate a double pipe heat transfer of fluid problem in FLUENT and want to find the pressure difference from the result. I know the mass flow rate at the two inlet and the temp of the two working fluid.Now as i don't know the outlet pressure which outlet condition should i use for my problem>????

Experts plz help i am in a mess

davidwilcox · December 15, 2015, 23:15

you could try using the outflow boundary condition. This is where the pressure is extrapolated. What solver are you using?

nabidinhomessi · December 16, 2015, 03:01

I am using pressure based solver.I have already tried outflow condition for both the outlet.But it works when the mass flow rate of both the inlet are same.When i try to simulate using two different mass flow rate in the inlets solution doesn't converge!!!!Can you help?

davidwilcox · December 17, 2015, 00:51

Wait. you have two outlets? what is the weightings that you used? Have you tried reducing the relaxation factors? But before you adjust those factors, make sure you read about the weightings on the outflow condition if you have two outlets. A quick google search on "outflow flow rate weighting" will bring you there.

nabidinhomessi · December 17, 2015, 04:28

Yeah i have two outlets. It is a double pipe heat exchanger ,so there are two inlet and two outlet for the two fluids .... I set 1 for both the outflow weighing plz tell me about the relaxation factor i tried reducing some but nothing changes.

davidwilcox · December 17, 2015, 09:22

please read the attached link when using outflow:
http://aerojet.engr.ucdavis.edu/flue...ug/node246.htm

adjust your weighting factor accordingly

davidwilcox · December 17, 2015, 09:26

Also check the quality of your mesh if the tweaking the relaxation factors do not work. I had a problem once when the quality of some cells were below 0.05. I improved the quality and it went smoothly

nabidinhomessi · December 17, 2015, 11:52

I think my outflow conditions are good with outflow weighting 1 .Can you suggest me how can i improve mesh quality.I used simple mapped face meshing.And oh one important thing :

Actually when i use same mass flow rate for both inlets the solution converges within 150 iteration BUT problem arises with even slight difference in mass flow rate, say mass flow rate for one inlet=.245 and another inlet=.24 then the whole scenario changes ,It doesn't converge.

davidwilcox · December 18, 2015, 01:21

I normally check and improve my mesh quality in ICEM. What i normally do is run a check first on determinants to make sure I do not have the negative volume issue, then i do a check on quality. I check which areas of the domain have bad quality and improve that area either by refining or shifting some vertices around (this is in blocking).
When you said it did not converge, did you mean it diverged? Or did you mean the residuals do no go below your convergence criteria?

nabidinhomessi · December 18, 2015, 01:37

The residuals oscillate in a straight line above the converged criteria(10e-03). What you say about my inlet bc? Is there a problem in mass flow inlets and outflow conditions?

davidwilcox · December 18, 2015, 01:58

I cannot see anything wrong with your inlet BC. Try setting the under relaxation factor for Pressure to 0.1 (This is the limit by default setting in Fluent).

nabidinhomessi · December 18, 2015, 04:02

I tried using 0.1 for pressure relaxation nothing changes though > First i set mass flow rate for both the inlet to 0.245 it works fine it converges within 100 iteration. But when i set 0.245 for one inlet and 0.25 for other inlets the residuals don't fall below the criteria it keeps going on a straight line way above the converged criteria.

December 15, 2015, 10:05	Boundary Condition at outlet	#1
nabidinhomessi New Member Nabid Aunjum Hossain Join Date: Nov 2015 Posts: 14 Rep Power: 10	I am trying to simulate a double pipe heat transfer of fluid problem in FLUENT and want to find the pressure difference from the result. I know the mass flow rate at the two inlet and the temp of the two working fluid.Now as i don't know the outlet pressure which outlet condition should i use for my problem>???? Experts plz help i am in a mess

December 17, 2015, 00:51		#4
davidwilcox Senior Member david Join Date: Oct 2012 Posts: 142 Rep Power: 13	Wait. you have two outlets? what is the weightings that you used? Have you tried reducing the relaxation factors? But before you adjust those factors, make sure you read about the weightings on the outflow condition if you have two outlets. A quick google search on "outflow flow rate weighting" will bring you there. nabidinhomessi likes this.

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December 15, 2015, 23:15		#2
davidwilcox Senior Member david Join Date: Oct 2012 Posts: 142 Rep Power: 13	you could try using the outflow boundary condition. This is where the pressure is extrapolated. What solver are you using?

December 16, 2015, 03:01		#3
nabidinhomessi New Member Nabid Aunjum Hossain Join Date: Nov 2015 Posts: 14 Rep Power: 10	I am using pressure based solver.I have already tried outflow condition for both the outlet.But it works when the mass flow rate of both the inlet are same.When i try to simulate using two different mass flow rate in the inlets solution doesn't converge!!!!Can you help?

December 17, 2015, 04:28		#5
nabidinhomessi New Member Nabid Aunjum Hossain Join Date: Nov 2015 Posts: 14 Rep Power: 10	Yeah i have two outlets. It is a double pipe heat exchanger ,so there are two inlet and two outlet for the two fluids .... I set 1 for both the outflow weighing plz tell me about the relaxation factor i tried reducing some but nothing changes.

December 17, 2015, 09:22		#6
davidwilcox Senior Member david Join Date: Oct 2012 Posts: 142 Rep Power: 13	please read the attached link when using outflow: http://aerojet.engr.ucdavis.edu/flue...ug/node246.htm adjust your weighting factor accordingly

December 17, 2015, 09:26		#7
davidwilcox Senior Member david Join Date: Oct 2012 Posts: 142 Rep Power: 13	Also check the quality of your mesh if the tweaking the relaxation factors do not work. I had a problem once when the quality of some cells were below 0.05. I improved the quality and it went smoothly

December 17, 2015, 11:52		#8
nabidinhomessi New Member Nabid Aunjum Hossain Join Date: Nov 2015 Posts: 14 Rep Power: 10	I think my outflow conditions are good with outflow weighting 1 .Can you suggest me how can i improve mesh quality.I used simple mapped face meshing.And oh one important thing : Actually when i use same mass flow rate for both inlets the solution converges within 150 iteration BUT problem arises with even slight difference in mass flow rate, say mass flow rate for one inlet=.245 and another inlet=.24 then the whole scenario changes ,It doesn't converge.

December 18, 2015, 01:21		#9
davidwilcox Senior Member david Join Date: Oct 2012 Posts: 142 Rep Power: 13	I normally check and improve my mesh quality in ICEM. What i normally do is run a check first on determinants to make sure I do not have the negative volume issue, then i do a check on quality. I check which areas of the domain have bad quality and improve that area either by refining or shifting some vertices around (this is in blocking). When you said it did not converge, did you mean it diverged? Or did you mean the residuals do no go below your convergence criteria?

December 18, 2015, 01:37		#10
nabidinhomessi New Member Nabid Aunjum Hossain Join Date: Nov 2015 Posts: 14 Rep Power: 10	The residuals oscillate in a straight line above the converged criteria(10e-03). What you say about my inlet bc? Is there a problem in mass flow inlets and outflow conditions?

December 18, 2015, 01:58		#11
davidwilcox Senior Member david Join Date: Oct 2012 Posts: 142 Rep Power: 13	I cannot see anything wrong with your inlet BC. Try setting the under relaxation factor for Pressure to 0.1 (This is the limit by default setting in Fluent).

December 18, 2015, 04:02		#12
nabidinhomessi New Member Nabid Aunjum Hossain Join Date: Nov 2015 Posts: 14 Rep Power: 10	I tried using 0.1 for pressure relaxation nothing changes though > First i set mass flow rate for both the inlet to 0.245 it works fine it converges within 100 iteration. But when i set 0.245 for one inlet and 0.25 for other inlets the residuals don't fall below the criteria it keeps going on a straight line way above the converged criteria.