Learning Forum

[abtharpe42]

Let's say I want to use a stoichiometric fully premixed mixture of hydrogen and air, with the hydrogen mass fraction in the fuel coloumn being 1 and the oxidizer coloumn containing oxygen and nitrogen at 0.233 and 0.767 respectively. The stoichimetric mixture fraction is about 0.0285, so for the Fully Premixed FGM setup, I entered 0.0285 in the blank for "Mixture Fraction Value for Flamelet Generation" and then generated the tables. 

After that, I input entered 0.0285 in the "Mean Mixture Fraction" blank for my velocity inlet of 5 m/s. After running initially with cold flow and eventually patching a progress variable of 1 over the whole 2d pipe domain, I obtained these results:

What am I missing with this procedure that's preventing me from achieving convergence?

[Vijay Narayan]

Hi Alex,

When we use fully premixed model, by default the number of grid points in mixture fraction space is 1 (by default). Hence, i dont think we need to increase the default value to 32.

After you completed the cold flow run first, you would have the entire pipe filled with premixed mixture and you are igniting the entire system by patching PV=1, so technically you would see the entire domain sitting at high temperature.

What is your objective here ? are you trying to see the flame propagation. If so, you could explore a trasient simulation, and patch a small region at the start with PV=1 and then observe the flame propagating forward.

 

[abtharpe42]

Hey, Vijay. In the few days since this post I've ironed out my struggles with this model. My convergence issues were due to setting a bad outlet mixture fraction and not the matching inlet mixture fraction. My initial lack of understanding of how the Fully Premixed FGM works were why my residuals were bad. As for why the mixture fraction grid points are set to 32 and not 1, I don't know. It was set to that when I went back to take screenshots.

[Vijay Narayan]

Hi,

Sorry for the delayed response. It's good to hear that you were able to sort out the issues.