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May 13, 2013, 03:22 
When to stop the iteration?

#1 
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House xu
Join Date: Aug 2010
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Hello,
I have one similified model for microchannel heat exchanger. Please find the attachment of detailed discription. only about 0.8 million meshes but quality is bad (several sharp angles near tangency area). Although I'm proficient in Gambit and know the skills to improve the volume mesh quality, I consider my problem as a common case and can be calculated without diverging issue. So for saving time, I gave up to improve my meshes in all 6 options(1 baseline and 5 modified). I choose SSTKW with lowRE correlation, and reduce relaxing factors. and what's important, I deactive energy equation for firstround iteration, and all 6 option got perfact results for flow and turbulence equations, converged at about 500 steps. Then in secondround iteration, I deactive flow and turbulence while active energy equation. As usual, I reduce the convergence residual of energy equation to 1e8, and add a monitor for massaveraged temperature at outlet. The monitored temperature seems change quickly in first 510 steps while became almost flat when residual is about 2e7, then, every 56 step, the temperature will still increased by 0.01 degree C. So I try to resuming the iteration, the residual curve of energy start rising. and then, temperature is out of limitation, that's a signal of divergence. I found this problem happened in all the 6 options, since the final difference of monitored temperature of each option is smaller then 0.30.5 degree C. So in my simulation the results will influenced by whole iteration steps, I am afraid it can't reflect the actual phenomenon. 

May 13, 2013, 03:49 

#2 
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Join Date: Aug 2011
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There is no option other than improving your mesh. What's the temperate difference between the inlet and outlet? What is the Reynolds number based on channel width? What are the discretization schemes? You should try structured mesh for simple geometry.


May 13, 2013, 04:08 
Re

#3 
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House xu
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Reynolds number is about 250.
Temperature of air inlet is 35 deg C. Temperature of heating wall is 50 deg C. Air outlet temperature based on current CFD results is about 47 deg C. SIMPLE method and secondorder upstream discretization is adopted except pressure (Standard) 

May 13, 2013, 05:17 

#4 
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The flow should be laminar. I did not compute but a 0.30.5K temperate different at the different results in large log mean temperature difference, right? The pressure difference is a much more useful indicator for mesh independence. Please find the pressure difference for all six cases.


May 13, 2013, 07:35 

#5 
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OJ
Join Date: Apr 2012
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You have a conjugate heat transfer, along with notsosimple flow passage why do you say you know the simulation is a simple case? Shouldn't the mesh independence study, along with the quality improvement, be on the top of your todo list?
OJ 

May 20, 2013, 00:25 

#6  
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House xu
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Quote:
Yes, to adopt laminate flow model with viscous heating, the calculation can be easily converged, and I found the results of flow field is very close to the one with previous sstkw model. anyway, I will use laminate flow model later in this kind of problem since its calculation effiency. To stop the calculation when considering heat transfer I now set the energy residue to default 1e6. 

May 20, 2013, 00:34 

#7  
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House xu
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Quote:
Mesh sensitivity and quality is really important for a new simulation, thanks for your suggestion. 

May 20, 2013, 00:50 

#8  
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House xu
Join Date: Aug 2010
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My secondround iteration will do some modifications on the model.
I will consider the tubeinside refrigerant flow which is twophase. So their will be two kinds of fluid, one is air, the other is twophase refrigerant(FREON). Do you know how to do the definition in FLUENT? I found if twophase model is activated, the material option in fluid cell boundary condition would be missing. That means the outside air can not be defined. Can I define it as threephase flow with air, liquid refrigerant, gas refrigerant. Quote:


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