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Minimum PDF table enthalpy exceed in xxx cells

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Old   January 30, 2014, 09:09
Default Minimum PDF table enthalpy exceed in xxx cells
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Hi everyone,
I am working on pulverized coal combustion. When i solved the case , i encountered" Minimum PDF table enthalpy exceed in xxx cells ". How can i fix it ?
Aspect ratio is about 25 , i suppose it is a little high value , is there any relation between aspect ratio and solving?

Last edited by charismatrix; February 4, 2014 at 17:53.
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Old   February 3, 2014, 16:14
Default same problem in my Non premixed model
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Hello charismatrix,

i have the same problem in my simulation. I am working in species- non premixed combustion model. After few iterations, i have this message '' Minimum PDF table enthalpy exceeded in xxx cells on zone'' and the number of cells continue to increase until the divergence.
Did you find the solution of this problem? I often use this combustion model with Fluent V14.5 but it is the first time I encounter this problem with the new version 15.
Thanks
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Old   February 3, 2014, 17:54
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Hello Dina,
I couldn't find any solution to this problem. I solved the same case in Fluent v12 ( I exported the mesh and prepared a case in v12) , I encountered " minimum PDF table temperature in xxx cells on zone xxx". I suppose these problems are same, only different expressions are used in different versions. I think we should search the problem as " minimum PDF table temperature in xxx cells on zone xxx " , because few people have v15, so few encountered the problem as " Minimum PDF table enthalpy exceed in xxx cells ".
If you find a solution to this , please tell me.
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Old   February 5, 2014, 15:18
Default quality mesh
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Quote:
Originally Posted by charismatrix View Post
Hello Dina,
I couldn't find any solution to this problem. I solved the same case in Fluent v12 ( I exported the mesh and prepared a case in v12) , I encountered " minimum PDF table temperature in xxx cells on zone xxx". I suppose these problems are same, only different expressions are used in different versions. I think we should search the problem as " minimum PDF table temperature in xxx cells on zone xxx " , because few people have v15, so few encountered the problem as " Minimum PDF table enthalpy exceed in xxx cells ".
If you find a solution to this , please tell me.

these are my quality mesh:

Mesh Quality:
Orthogonal Quality ranges from 0 to 1, where values close to 0 correspond to low quality.
Minimum Orthogonal Quality = 1.53746e-01
Maximum Aspect Ratio = 4.08620e+01

I think it is not so bad.
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Old   February 6, 2014, 00:57
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Nasharuddin MJ
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hi there, try:

1. reduce relaxation factor.
2. reduce maximum fraction heat loss. refer Minimum PDF Table Temperature
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Old   August 7, 2014, 08:26
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Hello everyone,

I was trying to simulate an IC engine problem. The moment ignition takes place, continuity equation reaches upto like e+05 By reducing the urf, i managed to get the continuity to e-02. According to a tutorial, i had set residual level for continuity to 0.1, hence the solution was converging. Later a warning appeared as "maximum PDF temperature in xxx cells ". when i generated the PDF table, for hydrogen the maximum temperature generated in PDF table is 2530 K (approx). Is it that the temperature should not exceed this limit? But if it is so, then the results wont match the experimental ones. Kindly help me.

Also, this is my first post. Pardon me if I go wrong somewhere.
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Old   August 7, 2014, 09:01
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Hi everyone,
I solved this problem. Solution is simple. Follow these steps;
On command window write;
"define/models/species>non-premixed-combustion-expert" (push enter)
(Here, some functions will appear)
Probability Density Function for PDF Table (0 for double-delta 1 for beta) [1]
(push enter)
Second order interpolation for PDF Table (else 4th order) [yes]
(push enter)
Maximum Temperature to Adiabatic Flame Temperature Ratio [1]
(push enter)
Maximum Heat Loss Fraction [0.6669999957084656]
(push enter)
Maximum Heat Gain Fraction [0.25]
(push enter)
Enable checking of PDF table temperature limits? [yes] "This is the reason !"
(write "no" and push enter)
Equilibrium Calculations at Inlets:
0:full-equilibrium
1:frozen-composition
[1]
(push enter)


It must be solved. Let me know good news
Good luck...

---------------------------------------------
Istanbul Technical University - Mechanical Engineering Faculty
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Last edited by charismatrix; August 7, 2014 at 15:13.
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Old   August 7, 2014, 09:03
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Hey thanks a lot, this was really a quick reply. I will let you know after solving..
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Old   August 8, 2014, 00:47
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hi there,

if you turn off the checking PDF temperature limit, the warning goes off. But I'm afraid, at certain number of cell the temperature might goes too high or too low and you never know about it. U can check it using contour. It might affecting your simulation. The reason for this checking PDF temperature is to give you early warning regarding temperature with respect to PDF range.

In my opinion:

1. Check material properties and BC. I've experience it before. Since i don't have the luxury to set up a testing rig , i always play around with material properties and BC setting. Some case with extreme temperature condition, these setting does play it role!

2. Double check your PDF table. mistake during PDF generation could lead to wrong temperature range. Especially when converting to daf and LHV.

nevertheless, it just my opinion. i solve my problem by adjusting the material properties, BC and fine tuning my PDF. If you guy have other ideas. It will be great if we can share it .
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