# Mass Diffusion: Transient and Steady State BC

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 November 12, 2008, 23:23 Mass Diffusion: Transient and Steady State BC #1 rval Guest   Posts: n/a Hi all, I am simulating mass diffusion through a porous media, so I created an additional variable (smoke) and used diffusive transport equation. I want to track smoke concentration (kg/m^3) at the outlet and plot it against time until the concentration reaches inlet concentration levels. After reading through CFX 11.0 Tutorial 4, I learned that I need to do a steady state simulations and use the results for initial condition in the transient simulation. However, I am a little confused about the boundary conditions in both steady state and transient simulations. Here's the BC I used Steady State Inlet Normal Speed = 0.05 [m s^-1] Additional Variable Value = 0 [kg m^-3] Outlet Relative Pressure = 0 [Pa] Additional Variable Value = 0 [kg m^-3] Transient Inlet Normal Speed = 0.05 [m s^-1] Additional Variable Value = 1 [kg m^-3] Outlet Relative Pressure = 0 [Pa] Additional Variable Value = 0 [kg m^-3] The result shows outlet concentration is very low in the order of 10^-5 kg/m^3. Am I making a mistake in assigning BCs? Can someone advice me on inlet and outlet BC ( speed and additional value) in both steady state and transient simulation. Sorry for such a long message. Please let me know if I can provide any other details. Thanks

 November 15, 2008, 16:16 Re: Mass Diffusion: Transient and Steady State BC #2 Joao Guest   Posts: n/a There is no problem with your results, the concentration in outlet is very low. Do you have an analitical solution for this problem? I have one and the concentration in the outlet is 1 and in outlet is 0. In the simulation and Peclet's number = 0,01, the concetration is 1 (inlet) and 1,53E-06 (outlet). So your results are not bad.

 November 15, 2008, 16:18 Re: Mass Diffusion: Transient and Steady State BC #3 Joao Guest   Posts: n/a There is no problem with your results, the concentration in outlet is very low. Do you have an analitical solution for this problem? I have one and the concentration in the inlet is 1 and in outlet is 0. In the simulation and Peclet's number = 0,01, the concetration is 1 (inlet) and 1,53E-06 (outlet). So your results are not bad. Sorry for my bad english.

 November 19, 2008, 01:52 Re: Mass Diffusion: Transient and Steady State BC #4 rval Guest   Posts: n/a Thanks Joao. Think I understand now.

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