Learning Forum

[singh.112]

I am trying to model partially premixed combustion for my burner using FGM with premixed flamelet under Partially premixed combustion modelling section in fluent. 

In my case, both air and fuel streams are separate to each other. Before generation of flamelets, in the boundary section, I have defined mole fraction of the fuel (CH4) as 1 and air is defined as O2 0.21 and N2 0.79.  In the boundary conditions section in the fuel stream I have defined mixture fraction value for the decided equivalence ratio value as suggested in fluent user manual. 

Till this points everything is okay. But as soon as I check the contours after running few iterations after initialization, I can see fuel is present in the air steream eventhoough in the boundary section I have defined fuel and air completely separate. This thing is persistent through out the run. I have attached user manual settings screen shot and the burner schematic to have a better understanding of the case setup.

[singh.112]

[Essence]

Hello,

Can you share the panels of the BCs and Partially premixed models?

[singh.112]

Please check all the attachments as requested. 

 

[singh.112]

Please let me know if you need some more details on this. 

 

[Rob]

From the image I can't see any fuel in the air section. Can you post some more diagnostic images? 

[singh.112]

Please check the attachments. 

In the previous attachments laso you can see in the FGM settings in boundary section, mole fraction of H2 is devided in both air and fuel. 

Waiting for you reply. 

[Rob]

What's the air flow speed look like? With a volume fraction as posted above diffusion could be a factor. How is the convergence looking? 

[singh.112]

Air flow rate is significant. It is just that at inlet of air it self fuel species are available. Which is very odd to me. Unless FLUENT by default does it if we select premixed flamelet generation in chemkin physical space. Because when generate flamelet considering progress variable space I do not face this issue. Also with diffusion flamelet approach this issue does not arises. I hope this helps. 

 

Convergence is fine. There is no issue with issue with convergence or any other thing. Even the temperature distribution looks as it should be. But the flame is behaving like fully premxied instead of partially premixed as it should be. 

 

Could you please help me to confirm if my assumption is right that fluent by default redistribute fuel species on the inlet because of the selection of premixed flamelet generation under chemkin physical space ? 

 

[Rob]

If the system is not premixed why are you turning on a premixed model? 

[singh.112]

Thanks, Rob, for your response. 

 It is recommended that the premixed flamelet approach be used for the partially premixed combustion model. It is mentioned in the fluent user manual. 

Using a premixed flamelet is correct or not, I can only check once I go to the validation part. 

But my question here is whether the fluent person is supposed to behave like this or not in premixed flamelet condition.

I have observed one more thing. This problem only appears when I am generating flamelets in physical space. When I consider progress variable space to generate premixed flamelets this issue does not come. 

Could you please confirm why only with chemkin physical space flamelet generation this is issue is arrising? 

 

 

[singh.112]

 

Hi Rob, 

I think the isssue lies over here. 

I have selected fully premixed option which is takes only single mixture fraction. This is forcing the settings to be changed as fully premixed conidtion. 

I will try with selecting this "fully premixed option" and see how does it go.

Thanks for your kind support. 

Manish.