problem in combustion-eddy dissipation model?

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  • Last Post 26 November 2018
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sooraj546 posted this 20 November 2018

I have done a very simple combustion of a fuel mixture using the following CH4, O2, H2, H2O, CO, CO2, N2 and C2H6 as the fuel+ air with 21% o2. using ANSYS-FLUENT-Eddy Dissipation Model I have calculated STOICHIOMETRICALLY the amount of O2 required for complete combustion (above fuel mixture composition given so calculated accordingly) and supplied it, so temperature and every thing is coming fine but when excess amount of O2 is supplied the the temperature increases further consuming all the EXCESS O2 eg if i supply 20 % excess O2 it should reflect in exhaust unconsumed rt but its not happening, rather a huge temperature increase is happening compared to the stoichiometric one. Kindly help me out guys , why is this happening ??? THANKS A LOT IN ADVANCE........

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abenhadj posted this 20 November 2018

Perhaps it is more convenient a to use a model which reacts more sensitive to chemistry then a one which is only steered by mixing and turbulence. 

Best regards, Amine

sooraj546 posted this 20 November 2018

THANKS A LOT FOR REPLYING

Sir i have tried with EDM, EDM-finite rate in both same results are almost same that means mixing is the rate limiting right AND chemical rates are much faster than mixing. But EDC is not working for my problem .Moreover when i try finite rate only after many iterations divergence comes.

Could you plz give me a valid reason why in EDM its consuming oxygen above stoichiometry,  (I know its mixing controlled)

abenhadj posted this 20 November 2018

No intermediate species and dissociation effects and temperature might be then over-predicted. I will go for more detailed reaction mechanism.

Best regards, Amine

sooraj546 posted this 20 November 2018

 

 

No intermediate species and dissociation effects and temperature might be then over-predicted. I will go for more detailed reaction mechanism." like what sir eg:finite rate , but i am facing divergence issue there

 

I havent understood your answer properly could u plz elaborate

 

abenhadj posted this 20 November 2018

Detailed reaction mechanism for your case + other combustion model. 

Best regards, Amine

sooraj546 posted this 20 November 2018

Sir i have tried EDM-finite rate ALSO

both cases same results are coming

THANKS A LOT FOR REPLYING

abenhadj posted this 20 November 2018

How many reactions are then included? 

Best regards, Amine

seeta gunti posted this 20 November 2018

Hello,

When you are modeling with EDM or Finite rate models, how many reactions are given in the reaction panel?

As Amine has suggested go for detailed mechanism ( eg: GRI mechanism, or skeleton mechanism etc)with Finite rate model.  With EDM, you may not solve the detailed mechanism and EDM always over predicts the predictions. 

Make sure that you have detail reaction mechanism for CO formation.

Regards,

Seeta

 

sooraj546 posted this 20 November 2018

THE REACTIONS GIVEN ARE 

CH4 + 1.5O2 = CO + 2H2O

C2H6 + 2.5O2 = 2CO + 3H2O

CO + 0.5O2 = CO2

H2 + 0.5O2 = H20

In total four reactions where included.
Can you suggest me a model which i should go for .available models

rwoolhou posted this 20 November 2018

Just checking my understanding of the problem, my combustion chemistry is OK, but I'm not an expert on the combustion modelling side of things.

 

With excess oxygen the mean outlet temperature is higher than and you don't have enough oxygen in the outlet?  Please confirm you're checking both of these values using mass weighted average surface reports. 

For the excess oxygen did you increase the inlet air flow or the oxygen concentration? 

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sooraj546 posted this 20 November 2018

sir,

The problem description

i have calculated the amount of O2 required for completely  converting  ch4 c2h6 co etc in eq below to co2 and h20. Its experimental results with this much amount of o2 supplied are available. Simulation results perfectly matching with the temperature  pressure values at the outlet of exp results.  etc model used (eddy dissipation model) which considering the mixing rate as the rate limiting step. 

CH4 + 1.5O2 = CO + 2H2O

C2H6 + 2.5O2 = 2CO + 3H2O

CO + 0.5O2 = CO2

H2 + 0.5O2 = H20

But in order to study the effect of excess o2 in the mechanism. I simply increase the velocity in the air-inlet say from 100 to 130 m/s so flow rate increases thus more o2 is going in right. But in this case also it consume all o2 and the temperature shoots up as well.

how can it consume more than that is required stoichiometrically. Since eddy dissipation model is ,mixing controlled i used finite rate/eddy dissipation which uses min of chemical reaction rate or the mixing as the rate for turbulent chemistry interaction. Here also i am getting the same results i.e. excess o2 consumed and temperature increases.

 

I am checking the species composition and temperature using mass weighted average of surface reports at outlet. all case oxygen mole fraction is mainted at 0.21

rwoolhou posted this 20 November 2018

OK. Check the composition on a plane about 2 cells into the domain from the outlet, and check there's no backflow. You should see the same results, I'm just checking nothing weird is happening at the boundary. 

sooraj546 posted this 20 November 2018

Yes sir i have checked it, its completely fine

 

 

sooraj546 posted this 22 November 2018

Kindly someone reply plzzzz...............

abenhadj posted this 22 November 2018

1/Expection is that with the excess of oxidizer the reaction will be "quicker" and intense leading to higher temperature

2/Still believe that the model you are using would result in higher temperatures. That is what I expect from using 1-or few steps reactions mechanisms.

3/You can enable the increased or adjusted Cp's (according to  ifrf) to act against higher spots. More information about the heat capacities might be found in the User's Guide with title Defining Properties for the Mixture and Its Constituent Species.

You can type this in your console:

(set-ifrf-cp-polynomials 'the_name_of_your_mixture_material)

4/I would encourage using a more detailed mechanism and EDC model. 

 

Best regards, Amine

sooraj546 posted this 22 November 2018

My model is very a complicated furnace and using EDC is very computationally expensive still i tried with this 1 step mechanism and then also all the excess o2 gets used up and much more higher temperature was the result .

I tried with EDM-finite rate where the rate limiting step will be the minimum of chemical reaction kinetics or mixing rate etc. results came very near to that of the EDM. 

 

Anyways sir i will try with a detailed combustion mechanism and let u know 

 

THANKS A LOT FOR REPLYING

sooraj546 posted this 22 November 2018

One more request sir, Do u no from where can i get the chemkin file for detailed methane combustion. So that i can c if my results are varying accordingly

 

abenhadj posted this 23 November 2018

You can check some university repositories if they have some. Look after GRI-Mech for natrual gas.

Good Luck!

Best regards, Amine

sooraj546 posted this 23 November 2018

Thanks a lot for that reply sir I already got chemkin file but the problem is i have a CO has source term and if I use pdf approach or chemkin approach the usage of CO SOURCE TERM IN another domain is not possible of individual source Possible as that option disappers

abenhadj posted this 25 November 2018

I do not understand the problem. Please post a screenshot please of the issue related with kinetic mechanisms (not PDF..)

Best regards, Amine

sooraj546 posted this 26 November 2018

Thanks a lot for replying...................

sir my problem are,

problem 1.

I have a gas species (CO) , energy source term and a VR rate for a reaction as well.

so when i change to Eddy Dissipation Concept there will be no option for including VR rates as shown below

what should i do about it, So EDC is just completely mixing controlled as EDM it just includes the more detailed calculation of fine structures in eddy etc. ?

 

problem 2.

when i try to calculate using non premixed - PDF approach as shown below  

the option for entering the species in my case CO gas vanishes i.e. i cant enter any gas species source terms but energy species can be entered as usual. i have given the pic below? 

Kindly reply sir.............. are there any sort of reviews for my problems ?

abenhadj posted this 26 November 2018

For problem 1 do you need the UDF to enter the CO mass? Mass source as usual via cell zones.

For problem 2 that is clear as the mass reactions are backed from lookup tables (that is why I wrote not share things related to PdF)

Best regards, Amine

sooraj546 posted this 26 November 2018

In problem 1

Sir i am entering co mas through boundary condition but in some region of the furnace, i need to enter the co source vis cell zone as its essential for my calculation. All my reaction proceeds by consuming CO 

ok thanks a lot for replying 

abenhadj posted this 26 November 2018

Yes and this is possible with all species reaction models (not with PDF based and not with flamelet based models). 

Best regards, Amine

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