Filling of gas in a Cylinder (Time dependent mass fractions)
 

Hello,

A cylinder currently having air at atmospheric pressure is to be filled with nitrogen so that at t=0 MF.air=1, MF.nitrogen=0 and at t=5min it is vice versa. How can I model that? (MF=mass fraction)

I can create a mixture with variable composition but how to change mass fractions with time in a transient analysis?

Best Regards,
Abid

Your description seems incomplete.

If the cylinder is full of Air at t=0, and you are only injecting Nitrogen, it will always have Air and Nitrogen; therefore, Air.mf will always be greater than 0, and N2.mf will never be 1.

Sorry it was left to mention that air is also being sucked out at the same time this reducing MF.air and as nitrogen is being pumped in thus increasing MF.nitrogen. At the end, air air sucked out and nitrogen is pressurized till 2bar.

This is a straight forward multicomponent mixture simulation. You just start with the cylinder initial condition being air.mf = 1, nitrogen.mf = 0 and you inject fluid with air.mf=0, nitrogen.mf=1. With some suction modelled the cylinder will then have the nitrogen.mf increase over time.

Make sure you have done the CFX tutorials so you understand how to activate the models.