[jamesmaca]

Hi all,

I am modelling coal combustion using the PDF mixture fraction approach however as my job solves I have the following message appear at most iterations...

Minimum PDF table temperature in 1023 cells on zone 10

...the number of cells varies however it never seems to reduce down to zero. I have tried changing the number of enthalpy points and mean mixture fraction points when creating the PDF table however at present this does not seem to make any difference. Also the air entering my kiln is at 337K and ~1000K while I have set the minimum temperature as 298K

Does anyone know what this message means....is it a problem and if so how do I fix it??

Thanks for anyones help.

[cguy]

in the text interface,

define/models/species

look for the expert option and decrease

Maximum Heat Loss Fraction

and then regenerate your table...

[fabiandiazmateus]

I had this problem in my simulation:

Minimum pdf table temperature in XXX cells

I solved it by changing the default values of viscosity and thermal conductivity that come with fluent. I used the next equations:

Thermal conduc (W/m-K) = -1.049352E-12x3 - 8.863118E-09x2 + 8.105885E-05x + 7.183288E-04

Viscosity (Kg/m-s) = 1.139977E-15x3 - 1.051178E-11x2 + 4.567342E-08x + 4.878605E-06

Where x = temperature (K)

[burritolover]

Hello,

I am having the same problem and have tried to follow your advice but have gotten stuck trying to find the Maximum Heat Loss Fraction in the TUI command window. The following shows how far I have gotten. If you could assist me I would greatly appreciate it.

>
adapt/ file/ report/
close-fluent mesh/ solve/
define/ parallel/ surface/
display/ plot/ views/

> define

/define>
beta-feature-access materials/ profiles/
boundary-conditions/ mesh-interfaces/ solution-strategy/
custom-field-functions/ mixing-planes/ turbo/
dynamic-mesh/ models/ units
enable-mesh-morpher-optimizer? operating-conditions/ user-defined/
injections/ parameters/

/define> models

/define/models>
acoustics/ heat-exchanger/ steady?
addon-module multiphase/ unsteady-1st-order?
crevice-model? radiation/ unsteady-2nd-order-bounded?
dpm/ solidification-melting? unsteady-2nd-order?
energy? solver/ viscous/
eulerian-wallfilm/ species/

/define/models> species

/define/models/species>
diffusion-energy-source? off? thermal-diffusion?
inlet-diffusion? save-gradients? volumetric-reactions?
multicomponent-diffusion? species-transport?

/define/models/species>

[fabiandiazmateus]

The problem with the minimum pdf table temperature can be solved by lowering the under relaxation factor for energy even below 0.9. It's going to take more iterations to assure a low energy unbalance of the model but sometimes it is necessary.

[burritolover]

Hello,

Thanks for all the replies. I was finally able to find the location where I could lower the Maximum Heat Loss Fraction! You were correct it is under "non-premixed-combustion-expert". Originally it was set to 0.66, I set it to 0.4 and ran the simulation again. Unfortunately this did not help fix the error. I still have "minimum pdf table temp" error occuring on every iteration and to many cells.
I have also tried reducing the under relaxation factor for Energy to 0.6 and this did not help either.
I have increased the mesh as well as increased the inlet temperature to 600K for the fuel and the oxide to no avail.
I have noticed that my P1 residuals are acting strangely though. They are far below the other residuals.

Any thoughts?

[cguy]

You really want all scalar, including energy and P1, URFs near 1.
If your case diverges, try 0.99 for both energy and P1. Again,
you need to re-generate the PDF after you lower the heat loss
parameter. I would ignore this message if only a few cells are
hitting the lower limit. But, you can lower the heat loss to 0.1
if it still bothers you.

[NiBa]

Originally Posted by burritolover

Hello,

Thanks for all the replies. I was finally able to find the location where I could lower the Maximum Heat Loss Fraction! You were correct it is under "non-premixed-combustion-expert". Originally it was set to 0.66, I set it to 0.4 and ran the simulation again. Unfortunately this did not help fix the error. I still have "minimum pdf table temp" error occuring on every iteration and to many cells.
I have also tried reducing the under relaxation factor for Energy to 0.6 and this did not help either.
I have increased the mesh as well as increased the inlet temperature to 600K for the fuel and the oxide to no avail.
I have noticed that my P1 residuals are acting strangely though. They are far below the other residuals.

Any thoughts?

Where is the tab non premixed combustion expert? i couldn't find it at all. i try to reduce the maximum heat loss fraction because i have the same problem.

[SJSW]

"Maximum Heat Loss Fraction" is here:

/define/models/species> non-premixed-combustion-expert
Probability Density Function for PDF Table (0 for double-delta 1 for beta) [1]
Interpolation method for PDF Table (0 for linear 2 for polynomial) [2]
Maximum Temperature to Adiabatic Flame Temperature Ratio [1]
Maximum Heat Loss Fraction [0.667] 0.5
Maximum Heat Gain Fraction [0.25]
Enable checking of PDF table temperature limits? [yes]
Equilibrium Calculations at Inlets:
0:full-equilibrium
1:frozen-composition
[1]

--
What should I do when two error message "Maximum PDF table temperature in 4778 cells on zone 150097" and "Minimum PDF table temperature in 514 cells on zone 150097 " appear at the same time?

[Ahmad134]

I also had the same problem. The issue was resolved when I used secondary air stream. RFL value for secondary air should be=1.0
http://aerojet.engr.ucdavis.edu/flue...mit-tempadjust
You can take a look on the user guide as well. Any further suggestions will be appreciated.

[peppino]

Originally Posted by burritolover

Hello,

Thanks for all the replies. I was finally able to find the location where I could lower the Maximum Heat Loss Fraction! You were correct it is under "non-premixed-combustion-expert". Originally it was set to 0.66, I set it to 0.4 and ran the simulation again. Unfortunately this did not help fix the error. I still have "minimum pdf table temp" error occuring on every iteration and to many cells.
I have also tried reducing the under relaxation factor for Energy to 0.6 and this did not help either.
I have increased the mesh as well as increased the inlet temperature to 600K for the fuel and the oxide to no avail.
I have noticed that my P1 residuals are acting strangely though. They are far below the other residuals.

Any thoughts?

I have experienced the "Minimum PDF table enthalpy exceeded" problem and it was solved changing the Maximum Heat Loss Fraction to a higher value than the default, i.e. I changed it from 0.66 to 0.9.
seems reasonable given the name of the parameter. to lower it as suggested earlier in the thread did not work

[hvgedam]

Yes...for the minimum pdf enthalpy warning, maximum heat loss fraction should be increased to 1 or above.

The maximum pdf enthalpy warning, can be resolved by increasing the maximum heat gain fraction to 1 or above.

As these fractions are increased, the max mean temperature is increased and min temperature decreases in the pdf table. This can checked from "display pdf table".

[wwa]

Originally Posted by SJSW

"Maximum Heat Loss Fraction" is here:

/define/models/species> non-premixed-combustion-expert
Probability Density Function for PDF Table (0 for double-delta 1 for beta) [1]
Interpolation method for PDF Table (0 for linear 2 for polynomial) [2]
Maximum Temperature to Adiabatic Flame Temperature Ratio [1]
Maximum Heat Loss Fraction [0.667] 0.5
Maximum Heat Gain Fraction [0.25]
Enable checking of PDF table temperature limits? [yes]
Equilibrium Calculations at Inlets:
0:full-equilibrium
1:frozen-composition
[1]

--
What should I do when two error message "Maximum PDF table temperature in 4778 cells on zone 150097" and "Minimum PDF table temperature in 514 cells on zone 150097 " appear at the same time?

I recommend checking mesh quality in that zone. Other than that, see if you can use other boundary conditions around that zone such as constant temperature for example.

[wwa]

Originally Posted by peppino

I have experienced the "Minimum PDF table enthalpy exceeded" problem and it was solved changing the Maximum Heat Loss Fraction to a higher value than the default, i.e. I changed it from 0.66 to 0.9.
seems reasonable given the name of the parameter. to lower it as suggested earlier in the thread did not work

I agree with peppino. Increasing the value worked for me.
The question is does it actually help or it just makes the warning message disappear?

[Milan2013]

Originally Posted by peppino

I have experienced the "Minimum PDF table enthalpy exceeded" problem and it was solved changing the Maximum Heat Loss Fraction to a higher value than the default, i.e. I changed it from 0.66 to 0.9.
seems reasonable given the name of the parameter. to lower it as suggested earlier in the thread did not work

How is this parameter (maximum heat loss fraction) exactly defined? Thanks

[LuckyTran]

The range of heat loss is defined when you generate the PDF table. In the TUI type:

/define/models/species/non-premixed-combustion-expert

and then follow all the prompts. If you're using a different combustion model then use the appropriate command.

Eventually you'll reach a prompt for:
Maximum Heat Loss Fraction
and
Maximum Heat Gain Fraction