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Old   February 3, 2010, 06:04
Default Convergency of solution
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Panos
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Hello fluent users
I simulate flow and heat transfer for air stream in a cylinder with heated walls. I try solve the problem and even when my solution seem converged, the outlet massweighted static temperature is not stable as you can see on the diagram below.
After 2000 iterrations i use descretization second order upwind for energy as it is recommended by the users manual and i reduce the under-relaxation factor for energy to 0,8.
The mass flow report seems satisfying
Mass Flow Rate (kg/s)
-------------------------------- --------------------
airin 0.0020031099
outlet -0.00200311
---------------- --------------------
Net -1.2356829e-10

I want your opinion if my simulation is converged.
Thanks
Attached Images
File Type: jpg scaled residuals.jpg (36.9 KB, 31 views)
File Type: jpg massweighted average.jpg (29.9 KB, 25 views)
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Old   February 3, 2010, 06:31
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Khor Yee Shin
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Convergency is not an easy issue. While the residuals are decreasing (which is a good sign for convergence), value may keep fluctuating. It pretty much depends on your requirement for precision and accuracy. For my case, even when the residual of continuity fell below 1e-05, the values are still changing slightly.
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Old   February 3, 2010, 07:16
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Hello Albert
I understand what you say and i try to find some ways to have correct conclusions from simulation. I stopped iterrating after recieved the above diagrams.. although the static temperature of the outlet keeps changing...
What do you think?
Mass balance seems ok
Mass Flow Rate (kg/s)
-------------------------------- --------------------
airin 0.0020031099
outlet -0.0020031099
---------------- --------------------
Net -2.2983516e-12

Can anybody recommend me any other way to check accuracy and convergency??
Thanks in advance
Attached Images
File Type: jpg Mass weighted average Temp.jpg (32.3 KB, 5 views)
File Type: jpg Scaled residuals.jpg (36.6 KB, 9 views)
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Old   February 3, 2010, 07:27
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Svetlin Philipov
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Try to find experimental data.... if you do not have such - try to refine the mesh and reach grid independent solution...check mass balance and energy too.... take a look at the flow pattern (if you have experience).... if everything is OK - take it for the last solution...
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Old   February 3, 2010, 10:17
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All was fine but when i checked the heat transfer i took the following imbalance
Total Heat Transfer Rate (w)
-------------------------------- --------------------
airin -5.7922
downres -365.65987
outlet -275.32605
res1 240.02551
res2 93.855843
res3 25.469884
res4 6.5671688
res5 1.4115112
res6 5.2419963
upres 80.329595
wall1 54.51353
wall2 26.473086
---------------- --------------------
Net -112.88999
By iterrating more the imbalance reduced...so i will keep iterrating until it goes close to zero...
I hope thats right
Thanks again
Suggestions are welcome
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Old   February 3, 2010, 10:29
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Svetlin Philipov
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For me it is not obvious what these boundaries are but it is strange to have negative energy at the inlet!!!!
As far as energy is a product of mass flow rate and enthalpy it is not real to have positive flow rate and finally receive negative energy?!?!?!!?!

may be I could not understand the problem....
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Old   February 3, 2010, 11:34
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Philipov i assume that the report refers to the thermal energy that is consumed by the air that enters the furnace. Cold air might ''consume'' energy to rise its temperature..
The problem refers to a cylinder with heated(res1,res2,res3,res4,res5,res6) and non heated walls(upres, wall1,2, downres). The air comes from airin (velocity inlet) that is located axisymetrically at the top of the cylinder (its a annulus of 63,6mm^2 crossection) and the air goes out at the bottom of the cylinder (the outlet diameter is equal to the cylinder diameter and r= 6,7cm)
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Old   February 4, 2010, 03:30
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I admit, I can not understand the task without a scheme of the task...
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Old   February 4, 2010, 05:03
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Hello
I send you an unprepared scheme of the process. The air flow is stable and can controled from 10 to 200 lit/min. The cylinder sides has stable temperature which is known by experiment measurements. The outlet boundary type is Pressure Outlet
I hope it will help you to understand my case
Thanks
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Old   February 4, 2010, 05:05
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Here is the scheme
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