Learning Forum

[abdallahbh]

Hello,nI am trying to model the process of air purification through nitrogen and oxygen adsorption on certain zeolites using Fluent. I have defined 4 UDFs to determine the adsorption equilibrium, mass transfer rate, the mass source term for O2 and the mixture and an energy source term. nTo model the process, I am using the 4 cycle Skarstrom PSA approach by initializing the inlet as a mass flow inlet with a set flowrate, temperature and pressure, and the outlet as a wall. When the bed reaches a pressure of 4 bar, I change the outlet to a pressure outlet for the flow to exit. After defining the outlet as a pressure outlet, I need to set a neumann boundary condition where the change in molar fraction with respect to flow direction is 0. I am having trouble setting this boundary condition, as the model is based on compressible ideal gas and I can't set the outlet as an outflow.nConsequently, my concentration at the outlet after changing the BC to pressure outlet drops drastically, which should not happen.nIs there anyway to solve this issue? I can link any UDF needed and any BC set in the model if needed.nAny help is appreciated.n

[Amine Ben Hadj Ali]

Either via volumetric source term at the cells near the outlet or via a profile where your output will be for sure a mass fraction but you can provide it as a result of diffusion equation.n

[abdallahbh]

I tried providing a udf, which equates the value of the mass fraction at the wall to the mass fraction of the cells near the wall, however continuity does't converge after I input this profile to the outletnn

[Rob]

If you're removing species at a certain rate near a wall, are you removing the same mass of material? If you look at the flow field how does this mass flux compare to the rest of the flow? Ie are you taking too much mass out of the domain?n

[abdallahbh]

Mass source at the wall should be 0. Mass is removed based on pressure, temperature and molar fraction in each cell. I will check how the mass flux compares nownn