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-   -   PDF approach (urgent!!!) (http://www.cfd-online.com/Forums/fluent/32191-pdf-approach-urgent.html)

anand September 23, 2003 14:50

PDF approach (urgent!!!)
 
hello, my problem is combustion (PDF approach)in a duct that contains a bluff body. At the center of the bluff body, fuel jet will enter the duct. I am using Fluent for this model. Since there is different inlets for fuel and air with different velocities, I want to know how to give the fuel inlet without using injections.

Thanks in advance.. waiting for a quick reply

Ale September 24, 2003 09:25

Re: PDF approach (urgent!!!)
 
You should be able to define two different velocity inlets for the fuel jet and the air stream (you have to do this when building your grid), so that you can define different velocities for the two inlets.

Regards,

Ale

anand September 25, 2003 14:30

Re: PDF approach (urgent!!!)
 
thank you sir, i have builded the grid accordingly. but my problem is how to give fuel inlet,bcos only one fluid (air)available in the material panel.i want to use prepdf combustion modelling(without using injections). regdas anand

Ale September 26, 2003 03:21

Re: PDF approach (urgent!!!)
 
When you use the PDF approach, you have to build the look-up table by prePDF and generate a .pdf file. When you have loaded this file, a new material (pdf-mixture) is available. Then you can set different boundary conditions for the two inlets of your domain (the air stream and the fuel jet): you have to set the different velocities and the mixture fraction and variance. Usually mixture fraction = 1 for the fuel and 0 for the oxidizer.

I hope this helps.

Regards, Ale

anand September 26, 2003 06:52

Re: PDF approach (urgent!!!)
 
thank you sir, i will try this sir. in my case fuel is ch4/h2. how to give this. sir i have onemore doubt regarding the mixture variance. i understood the mixture fraction . i don't know about variance . can you plz explain me. thank you anand

anand September 26, 2003 07:30

Re: PDF approach (urgent!!!)
 
Hi Sir, I have created the pdf lookup table. and i have activated non-premixed combustion and i read the pdf look table. than i have given mixture fraction 1 for fuel and zero for air .i have given two different velocities(fuel and air). than i iterated . solution is diverging and giving the folloeing msg. Error: divergence detected in AMG solver: enthalpy Error Object: ()

can you plz check and tell me sir regards anand

Ale September 26, 2003 09:05

Re: PDF approach (urgent!!!)
 
Hi,

the fuel can have any composition, you just have to define it in prePDF before computing the look-up table.

What kind of table did you choose? Adiabatic or non-adiabatic? Did you receive any warning or error message during the computation of the look-up table? I think you could stard with the adiabatic table (so that you don't have to solve the energy equation).

Regards,

Ale

anand September 26, 2003 12:10

Re: PDF approach (urgent!!!)
 
basically system is not adiabatic. now i will use that adabatic. and i have doubt on mean mixture fraction for fuel. how much i have to give. regards anand

Ale September 26, 2003 12:32

Re: PDF approach (urgent!!!)
 
The b.c. for fuel mixture fraction at the fuel inlet is one, if you have defined for the fuel its real composition.

Regards,

Ale

anand September 27, 2003 00:29

Re: PDF approach (urgent!!!)
 
thank you sir. regards anand


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