[ranxiaoran]

Hello everyone! When I want to choose combustion model in Supersonic combustion, I find many paper using Finite-Rate model. So why they use this model but not Eddy-Dissipation model? I thought the Eddy-Dissipation model is the most complex model so there are more details.

[LoGaL]

Eddy dissipation model is among the cheapest models. It’s not garbage in some situations but in general it is not accurate

[ranxiaoran]

Originally Posted by LoGaL

Eddy dissipation model is among the cheapest models. It’s not garbage in some situations but in general it is not accurate

So what about the Eddy concept model? Is that better than finite-rate or not?
thank you for your reply!

[LuckyTran]

Finite Rate is a laminar flame model and doesn't model any turbulence-chemistry interaction. It works really well when there isn't much turbulence.

Eddy dissipation at least has some model for the turbulence-chemistry interaction. EDC is a more advanced form of EDM for turbulent flames. The problem with EDM and EDC is that they assume the flame burns only as fast as the turbulent mixing allows and cannot proceed as fast as the chemical kinetics might otherwise allow.

So depending on the environment around the flame, one of the models tends to perform really poorly.

[ranxiaoran]

Originally Posted by LuckyTran

Finite Rate is a laminar flame model and doesn't model any turbulence-chemistry interaction. It works really well when there isn't much turbulence.

Eddy dissipation at least has some model for the turbulence-chemistry interaction. EDC is a more advanced form of EDM for turbulent flames. The problem with EDM and EDC is that they assume the flame burns only as fast as the turbulent mixing allows and cannot proceed as fast as the chemical kinetics might otherwise allow.

So depending on the environment around the flame, one of the models tends to perform really poorly.

Thank you for your reply! I understand it now!

[LoGaL]

I know in the EDC you can incorporate finite rate chemistry.


Just understand that in combustion there are two time scales: the mixing time between fuel and oxidant, and the reaction time.
The mixing time is almost alwais the dominant between the two. Once mixed, the reaction takes place very fast. So in some cases, it is possible to assume that the burn time is simply mixing dependent. This is what eddy dissipation model does.

In my experience, with EDM you can get something reasonable (not good) in low mach number, high temperature combustion chambers of standard aero-engines, though you accept that you will overpredict the flame temperature. I don't think it is the same story with supersonic combustion.

[ranxiaoran]

Originally Posted by LoGaL

I know in the EDC you can incorporate finite rate chemistry.


Just understand that in combustion there are two time scales: the mixing time between fuel and oxidant, and the reaction time.
The mixing time is almost alwais the dominant between the two. Once mixed, the reaction takes place very fast. So in some cases, it is possible to assume that the burn time is simply mixing dependent. This is what eddy dissipation model does.

In my experience, with EDM you can get something reasonable (not good) in low mach number, high temperature combustion chambers of standard aero-engines, though you accept that you will overpredict the flame temperature. I don't think it is the same story with supersonic combustion.

Yes, I find many papers say that they use the finite rate model but there are no paper use the EDC or EDM. Now I think maybe that is because the burning time and mixing is strongly coupled in the supersonic simulation.
I understand now and thank you for your reply!