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-   -   Eddy Dissipation Model (http://www.cfd-online.com/Forums/cfx/22293-eddy-dissipation-model.html)

Enots March 14, 2006 12:45

Eddy Dissipation Model
 
Hello,

I want to make combustion simulations of some CxHy. But I really don't know how to set the coefficients A and B, the "mixing rate limit", chemical timescale and extinction temperature. Only the combustion of methane is explained in the manual. Does anybody have experiences with the EDModell? Which coefficient does what? How can I get the best values for these coefficients?

Enots March 16, 2006 13:58

Re: Eddy Dissipation Model
 
The most common coefficients for gas phase combustion is A=4 and B=0.5. But why is B not recommended for multi-step combustion? How can I calculate the chemical timescale and mixing rate limit? Does nobody ever use combustion with the Eddy Dissipation Modell?

Jenny March 18, 2006 07:55

Re: Eddy Dissipation Model
 
Because it will not work very good. B is a so called product limiter. (Look in the wiki for combustion). If more the one step the products of the first, will be consumed be the second step. This will have the consequence that the first reaction will slow down and extinguish.

Concerning your first questions: The constants A and B are the model constants. As the EDM is physcically not very meaningful you will tune everything with this constants.

Enots March 24, 2006 05:47

Re: Eddy Dissipation Model
 
Thank you very much for your reply Jenny. My problem is that i tried many variations of these parameters and always the solver stops cause of too high temperatures. Do you know something of the other tree parameters like mixturing limit and so on? Thank you in advance.

Jenny March 28, 2006 02:08

Re: Eddy Dissipation Model
 
In most cases (not obligatory) the highest temperature is the adiabatic flame temperature. However if the temperature is above this value you should check your specific heat capacity and enthalpy definition.

Enots March 28, 2006 02:36

Re: Eddy Dissipation Model
 
Hi Jenny, I'm using the specific heat capacity and enthalphy which are given in Nasa Format by CFX. The wall has a constant Temperature. The reaction mechanism is a two step so I disabled coefficient B. I slowed down the coefficient A to 0.1. Now the temperature is ok but not the hole fuel is getting burnt although there are 3500K. The second problem: The total pressure is rising up to 100 000 atm(bar) near the wall for free slip and for no slip. If you have an idea please tell it to me.


Jenny March 28, 2006 03:15

Re: Eddy Dissipation Model
 
I would not expect that the EDM causes this problem. You should check your setup from the beginning. What kind of problem do you try to solve? Burner? IC engine? Something else?

Enots March 28, 2006 03:34

Re: Eddy Dissipation Model
 
It's a rocket combustion chamber. I'm burning Jet-A/O2. Simple cylindrical geometry. One side in then burning and on the other side out. k-epsilon turbulence model. It seems so simple but.....

Jenny March 29, 2006 03:03

Re: Eddy Dissipation Model
 
Is your case steady or transient?

Enots March 29, 2006 06:19

Re: Eddy Dissipation Model
 
Steady.....

Jenny March 29, 2006 10:21

Re: Eddy Dissipation Model
 
Whats your start solution? Completly unreacted domain?

aulu August 12, 2011 04:11

Hallo.

I have got a question concerning the eddy-dissipation model. I have read that it's possible to modell 2-step-reactions with EDM. But if I look for the mass fraction of the product of the second reaction it's always 0. My reactions are as follows:
CH4 + 1.5 O2 -> CO +2H2O
CO + 0.5 O2 -> CO2.
I tried several values for A and B for both reactions. Has somebody an idea which values should be taken for which reaction to get CO2 as product?

Thanks in advance!


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