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February 3, 2010, 06:04
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Convergency of solution
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#1 |
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Member
Panos
Join Date: Nov 2009
Posts: 33
Rep Power: 17  |
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 |
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February 3, 2010, 06:31
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#2 |
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New Member
Khor Yee Shin
Join Date: Jan 2010
Location: Glasgow UK
Posts: 6
Rep Power: 16 |
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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February 3, 2010, 07:16
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#3 |
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Member
Panos
Join Date: Nov 2009
Posts: 33
Rep Power: 17 |
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 |
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February 3, 2010, 07:27
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#4 |
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Senior Member
Svetlin Filipov
Join Date: Mar 2009
Location: United Kingdom
Posts: 176
Rep Power: 17 |
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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February 3, 2010, 10:17
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#5 |
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Member
Panos
Join Date: Nov 2009
Posts: 33
Rep Power: 17 |
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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February 3, 2010, 10:29
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#6 |
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Senior Member
Svetlin Filipov
Join Date: Mar 2009
Location: United Kingdom
Posts: 176
Rep Power: 17 |
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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February 3, 2010, 11:34
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#7 |
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Member
Panos
Join Date: Nov 2009
Posts: 33
Rep Power: 17 |
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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February 4, 2010, 03:30
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#8 |
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Senior Member
Svetlin Filipov
Join Date: Mar 2009
Location: United Kingdom
Posts: 176
Rep Power: 17 |
I admit, I can not understand the task without a scheme of the task...
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February 4, 2010, 05:03
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#9 |
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Member
Panos
Join Date: Nov 2009
Posts: 33
Rep Power: 17 |
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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February 4, 2010, 05:05
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#10 |
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Member
Panos
Join Date: Nov 2009
Posts: 33
Rep Power: 17 |
Here is the scheme
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| Tags |
| convergence, flow, fluent, heat |
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