why does fluent use species mass fraction as default inlet condition?

Weiqiang Liu · May 11, 2020, 11:19

hi all,

I am trying to set species inlet condition of a combustor according to fuel equivalence ratio mentioned in the paper.

I am wondering why does fluent use mass fraction as default inlet condition instead of mole fraction?

Because it would be more convenient to use mole fraction of species to set equivalence ratio of fuels.

Best regards

Weiqiang

vinerm · May 11, 2020, 11:43

Fluent provides option to use mole fraction instead of mass fraction; if you are using old version then it will only have mass fraction. It's primarily for legacy reason but also because primary users of Fluent in the industry do not use mole fractions but mass fractions.

Weiqiang Liu · May 11, 2020, 22:48

Quote:

Originally Posted by vinerm

Fluent provides option to use mole fraction instead of mass fraction; if you are using old version then it will only have mass fraction. It's primarily for legacy reason but also because primary users of Fluent in the industry do not use mole fractions but mass fractions.

Hi Vinerm,

Thanks very much for your answer!

Best regards

Weiqiang

LuckyTran · May 12, 2020, 05:14

The governing equations for the species transport equation are written in mass fraction. Since that's the primitive variable that the solver operates on, naturally it should default to mass fraction.

It's way easier to build transport equations this way valid for compressible flows because the advective flux is the mass-flux.

Weiqiang Liu · May 12, 2020, 05:24

Quote:

Originally Posted by LuckyTran

The governing equations for the species transport equation are written in mass fraction. Since that's the primitive variable that the solver operates on, naturally it should default to mass fraction.

It's way easier to build transport equations this way valid for compressible flows because the advective flux is the mass-flux.

yes, thanks very much for your answer. it enlightened me a lot.

Best regards

Weiqiang

May 11, 2020, 11:19	why does fluent use species mass fraction as default inlet condition?	#1
Weiqiang Liu Senior Member Weiqiang Liu Join Date: Feb 2018 Posts: 278 Rep Power: 9	hi all, I am trying to set species inlet condition of a combustor according to fuel equivalence ratio mentioned in the paper. I am wondering why does fluent use mass fraction as default inlet condition instead of mole fraction? Because it would be more convenient to use mole fraction of species to set equivalence ratio of fuels. Best regards Weiqiang

May 11, 2020, 11:43	Mole Fractions	#2
vinerm Senior Member Vinerm Join Date: Jun 2009 Location: Nederland Posts: 2,946 Blog Entries: 1 Rep Power: 36	Fluent provides option to use mole fraction instead of mass fraction; if you are using old version then it will only have mass fraction. It's primarily for legacy reason but also because primary users of Fluent in the industry do not use mole fractions but mass fractions. __________________ Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority.

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May 12, 2020, 05:14		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	The governing equations for the species transport equation are written in mass fraction. Since that's the primitive variable that the solver operates on, naturally it should default to mass fraction. It's way easier to build transport equations this way valid for compressible flows because the advective flux is the mass-flux.