Conversion of mass fraction to mole fraction in FLUENT
 

Hello Friends

I was simulating problem in FLUENT using Species Transport model (No reaction). Two Gases (Deuterium and Oxygen) is coming from bottom. While Other gas Helium is having inlet from sideways. So there is mixture of these Three gases (He, D2 and O2) out of which He is forming bulk. According to guide line of FLUENT while forming mixture i kept Helium at the bottom of table.

Boundary condition are as follow:-
At outlet - Outflow
At Inlet (Sideway) - Mass flow rate of He (2.708e-05 kg/s) and Mass fraction of D2 and O2 are respectively as 0.01 (1%) and 0. Rest is Helium (99%)
At Inlet(Bottom) - Mass flow rate (1.3708e-08) and mass fraction of D2 and O2 are 0.2 and 0.8 respectively.

After Simulation, on post-processing i observed something which i was not able to understand and need help of you guys to comprehend it properly.

At inlet plane (bottom)- I checked the mass fractions and it was as provided (0.2 D2, 0.8 O2 and 0 He). But mole fraction was not as desired. I had observed moles of He at this plane which should not be case. It should be almost zero (mass fraction of He is set to 0). But on contour plot i am observing 86% of Helium. I rechecked mass fraction of He again on this plane and it was 0.

Now how this is possible? If my mass fraction is right, mole fraction should be according to mass fraction. Right ? What i am missing ? There is no extra input for the calculation of moles fraction.

I have tried to give mass flow rate and mole fraction as boundary condition (instead of mass fraction) but problem persist.

Sorry for the long post. Please help me in getting this.