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N2 flow in air

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Old   February 20, 2015, 08:15
Post N2 flow in air
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Simone Spanu
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I'm new here, so hi to everybody !
I'm writing because i have a problem in fitting my simulation to experimental data.
The simulation is about a tunnel with a conveyor belt at its bottom and three inlets on its top. At the beginning the tunnel is full of air and N2 is flowing inside of it from each inlet. I set it up as a multicomponent simulation, creating a variable composition mixture of air ideal gas and N2 ideal gas. On the inlets i set mass flow rate and N2 mass fraction equal to 1. At the beginning and at the end of the tunnel i set an opening condition (N2 mass fraction equal to 0 and atmospheric pressure). Then i ran it for 4 s with a time step of 0.02 s.
With this configuration I achived N2 mass fraction of about 0.77 in the tunnel and Air mass fraction of 0.23, from which i calculated O2 residual (about 5%).
From the tests i have that O2 residuals at the tunnel center is between 1% and 2% wich means i have a much higher N2 concentration there!
I tried to modify some parameters but the simulation result doesn't get better.
I was thus wodering if my model is correct and if there's a way to improve it.

Many thanks.
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Old   February 20, 2015, 08:38
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If you are interested in O2 mass fraction, or concentration, does it make sense to consider Air as a pure substance with fixed composition? Is the N2 in the Air different than the one being injected that is not allowed to mix ? Similarly, is the Air composition in the test accurately known before N2 is injected ?

I would have setup the case as variable composition made of O2, N2 and Ar (if you think it matters). Setup the initial condition for the domain to the known composition of Air (say 78%N2, 21%O2, 1%Ar +/- tolerance), and perform the calculation.

Then, you can compare your results to the tests. I would check for sensitivity to the initial conditions, is your Air O2 rich, or O2 deficient ? You are looking at differences of 1->2% between tests and simulations w/o stating the deviation on the tests.

Hope the above helps,
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air, cfx, gas, multicomponent, nitrogen

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