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sree January 10, 2007 04:34

combustion - activation energy
 
Hello every one

i am trying to run one of the case which is having experimental results, but i am not getting the result.

it is a simple can combustor. with natural gas ( methane ) as fuel. i have used finerate/eddy dissipation model for combustion.

in fluent we have activation energy and pre exponential factor for each step of reaction ( i am using 2 step reactions ) , how do we know that they arr correct values, is there any process/ document stating about these values for each step of reactions??

thanking you


James Willie January 10, 2007 10:28

Re: combustion - activation energy
 
Hi,

Is your flow laminar or turbulent? I guess it will be turbulent. If so, then the finite rate/eddy dissipation model works by determining the arrhenius rate and the turbulent mixing rate and picking the smaller of the two.

Before iginition, it is almost always going to be the arrhenius but once ignition occurs, the reverse is true. Why? Arrhenius rate is higly temp. sensitive and so it shoots up after ignition and becomes almost always bigger than the turbulent mixing rate.

The pre-exponential factor and activation energy were determined for a stoichiometric methane/air mixture and you need not bother too much about this if you are using the combined model....why? the rate is always going to be eddy dissipation model once ignition occurs. It is also advisalble to not play with the parameters A and B in the eddy dissipation model unless you are pretty sure.

If you are running only the finite rate chemistry model, then depending on your equivalence ratio, the preexponential factor will change. The activation energy for a given reaction stays the same....except with the addition of a catalyst?

Check the papers by Charlie Westbrooke titled:

Charles K. Westbrook and **Frederick L. Dryer. "Simplified Reaction Mechanisms for the Oxidation of Hydrocarbon Fuels in Flames". Combustion Science and Technology, 1981, Vol. 27, pp. 31-43.

Charles K. Westbrook and Frederick L. Dryer. "Chemical Kinetic Modelling of Hydrocarbon Combustion". Prog. Energy Combustion Science, 1984, Vol. 10, pp. 1-57.

The activation energy and pre-exponential factors were determined from these papers and implemented in Fluent.

Good luck!

James


sree January 11, 2007 03:36

Re: combustion - activation energy
 
Hello

first of all thank you for your reply and detailed description. I have understood that there should not be any change in the final temperature distribution of the combustion problem for eddy dissipation model and finite rate/eddy dissipation model. (Before iginition, it is almost always going to be the arrhenius but once ignition occurs, the reverse is true. Why? Arrhenius rate is higly temp. sensitive and so it shoots up after ignition and becomes almost always bigger than the turbulent mixing rate. ) But i am observing much difference in the temperature distribution and the maximum temperature region is shited to other location in my case. If you allow me to interact through your mail id, i can send you the detailed pictures of temperature distributions of both the models. My work is regarding the spaldings experiment.


James Willie January 11, 2007 03:54

Re: combustion - activation energy
 
Hello,

For steady RANS, the temperature at the combustor outlet should not changed once you have convergence and the residuals become steady....that is true.

If you look at the expression for the reaction rate for the arrhenius case, it is exponential dependent on the temp and so if temp is low (no ignition), this value is very small and less than the turbulent mixing rate. So the arrhenius rate becomes the rate of the reaction. After the ignition, the temperature goes up and so the reaction rate for arrhenius becomes higher than the turbulent mixing rate and hence the turbulent mixing rate becomes the rate of the reaction.

Are you running steady RANS or what? are your residuals dropping steadily? What value have you calculated for the adiabatic flame temperature?

The area weighted average of the adiabatic flame temperature at the combsutor outlet should be almost equal to that calculated.......if this is true and your residuals become steady, then you can be sure of convergence.

Hope it helps...my email address is willie@idg.rwth-aachen.de


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