[Weiqiang Liu]

Hi all,

I am doing a micro-combustor simulation with Fluent. I read a lot of paper about how to set the inlet boundary condition.

all paper mentioned the procedure is to set inlet velocity and then keep the inlet mixture equivalence ratio constant, like 1.

I am wondering how to set the absolute value of fuel fraction in the mixture.

For example, the mole fraction of methane is 0.05 and the mole fraction of oxygen is 0.1, in another case the mole fraction of methane is 0.1 and the oxygen fraction is 0.2. In both cases , the equivalence ratio is 1. However, in most papers, those authors just mention equivalence ratio instead of absolute value of fuel mole fraction. Can anybody help me with this problem?

Thanks all.

Weiqiang.

[LuckyTran]

You misunderstand that equivalence ratio is a boundary condition. There's lots of reasons why people report equivalence ratio in their papers but equivalence ratio is just a hypothetical property which can have an arbitrary definition.

If you use any species transport equations, the transported variable is the mass fraction of each species. Of course mass fraction and mole fraction differ by just the molecular weight. At boundaries where you apply boundary conditions, you have to specify the constraint on the transported variable (which is mass fraction, and not equivalence ratio).

When you don't transport all the species, you sometimes use a simplified model and consider only two streams (oxidizer & fuel). Instead of individual mass fractions of each species, the transported variable is now a mass mixture fraction (mass of total stream that is fuel). Here again you also specify the fuel mass mixture fraction at the boundaries (at the inlet for example) and not the equivalence ratio.


So to answer your question you don't need to figure out how to specify equivalence ratio at the inlet, that would be wrong. You should be specifying the mole/mass fractions. What people write in papers is not always what they actually do.

[Weiqiang Liu]

Hello,
Sorry to reply this late. I really appreciate your such detailed answer. I can understand the boundary condition of species should be mass or mole fraction of the whole flow. However, I am trying to reproduce some results in literatures where they define their inlet mixture by equivalence ratio.

Obviously, if I don’t know the mass or mole fraction of inlet mixture and I just keep the equivalence ratio the same with the literature , I will get very different results.

So how can I know the mass/mole fraction of the inlet mixture? Or Authors normally would mention this in their paper about mass/mole fraction?

Thanks very much!!

Weiqiang.

[LuckyTran]

Normally knowing the equivalence ratio would be enough information so that you can convert to mass/mole fractions. Why isn't it for your case? Did the authors not tell you what the mixture was? I.e. what is defined as the fuel and what is defined as the oxidizer? Usually whatever is not fuel, is considered the oxidizer stream (e.g. air). In chemistry, feel species are chemically inert.

[Weiqiang Liu]

for example, the mole fraction of methane is 0.05 , the mole fraction of oxygen is o.1. in other case, the mole fraction of methane is 0.1 and the mole fraction of oxygen is 0.2. In both cases, the equivalence ratio is 1. how can I know which case it is ?

Thanks

[LuckyTran]

Originally Posted by Weiqiang Liu

for example, the mole fraction of methane is 0.05 , the mole fraction of oxygen is o.1. in other case, the mole fraction of methane is 0.1 and the mole fraction of oxygen is 0.2. In both cases, the equivalence ratio is 1. how can I know which case it is ?

Thanks

Right...

So if the mole fraction of methane is 0.05 and the mole fraction of oxygen is 0.1, 0.05+0.1=0.15. Where is the missing 85% of the mixture? And in your second example, 0.1+0.2=0.3, 70% is unaccounted for.

What is the mixture!? If this is the only information you have in the paper, and it's not obvious from context, then I guess you are shit out of luck. You can write a letter to the authors I guess, or to the journal editor and complain why this information is not front-loaded.

[Weiqiang Liu]

ok thanks. Maybe I did not read the paper very carefully. Thanks for your reply

[Weiqiang Liu]

Originally Posted by LuckyTran

Right...

So if the mole fraction of methane is 0.05 and the mole fraction of oxygen is 0.1, 0.05+0.1=0.15. Where is the missing 85% of the mixture? And in your second example, 0.1+0.2=0.3, 70% is unaccounted for.

What is the mixture!? If this is the only information you have in the paper, and it's not obvious from context, then I guess you are shit out of luck. You can write a letter to the authors I guess, or to the journal editor and complain why this information is not front-loaded.

Maybe I did not read the paper very carefully. Thanks very much for your reply