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June 30, 2011, 06:33 
mass fraction of species

#1 
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Join Date: Feb 2011
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Hello everybody,
I have got a understanding problem concerning the mass fraction of a species in a mixture at Fluent. I looked up the definition of mass fraction for the case of Fluent: “Mass fraction of speciesn (in the Species... category) is the mass of a species per unit mass of the mixture (e.g., kg of species in 1 kg of the mixture).” I have a geometry with one inlet and two outlets. According to the definition of mass fraction I expected the mass fraction at the inlet to be the sum of the mass fraction at the two outlets, but unfortunately my results differ from that expectance: In my case the mass fraction of the inlet and the mass fraction of each outlet are equal (that means the sum of the mass fraction of the outlets is twice as large as the mass fraction at the inlet). I am really confused! Can somebody help me to understand this? Thanks a million in advance! Lilly 

June 30, 2011, 13:16 

#2 
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Micael Boulet
Join Date: Mar 2009
Location: Quebec, Canada
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You expect that the mass going in is the same as the mass going out. This is correct, but it is wrong to expect the mass fraction at the inlet to be the sum of the mass fraction at the outlets. It could be totally normal that they equal, as you experience. Look further into the concept of mass fraction, you will understand.
 edit: I mean: ...the mass going in... (corrected) Last edited by Micael; July 5, 2011 at 17:31. 

July 4, 2011, 08:14 

#3 
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Byron Smith R J
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Location: India
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hai
Mass fraction in a steady, unreacting flow is independent of quantity. for example if you pass air through inlet and the mass flow rate is divided into two at the outlet, the mass fractions of oxygen and nitrogen remain same both at inlet and outlet. mass fractions change only when reactions or additional species inlets are induced in the flow. then mass fractions are calculated using the species transport equations. hope it helps in clearing your doubt 

July 25, 2011, 05:00 

#4 
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Join Date: Feb 2011
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Thank you Micael and byronsmith for your help (and sorry for the late reply)!
I thought about what you were writing and I finally found my error in reasoning. Thanks a million! 

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