Species wall boundary conditions for mass transfer

  • Last Post 20 November 2019
LukeM posted this 18 November 2019


I am simulating single-phase flow through a pipe. I'm trying to add mass transfer to my simulations, but I encountered some unexpected issues with the boundary conditions. 

I'm blowing air through my pipe, and I'm trying to simulate pure benzene on the walls of the pipe diffusing into the airflow. I set the wall mass fraction of benzene to 1.0. But when I ran my simulation, it seemed like the air was being replaced with benzene, rather than benzene being added to the gas flow. The image below shows the mass fraction of benzene moving through the pipe (benzene enters the pipe in the last 2/3rds of the pipe). The mass fraction of benzene leaving the pipe is nearly 1. Where did the air go? 

Mass Fraction of Benzene in Pipe

The density in my pipe is also changing through the pipe.

Density through pipe

I'm wondering if my choice of a volume-weighted-mixing-law is causing the change in density. But why is the air disappearing? 

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abenhadj posted this 19 November 2019

I would rather provide a flux for the component. You might add a source term in the first cell layer to the wall. Check the order of the species in the mixture panel.

Best regards, Amine

LukeM posted this 19 November 2019

That's a good idea. Unfortunately, the flux is the value I'm trying to measure. I'm trying to measure the amount of benzene that enters the air by the end of the pipe. Sorry, I probably should have mentioned that in my original post. Is there any other way besides defining the flux? 

abenhadj posted this 19 November 2019

Source term. Setting the value of the specie at wall is not equivalent to having a source of that specie through the wall.

Best regards, Amine

LukeM posted this 19 November 2019

But what value do I give the source term? I'm trying to measure the amount of benzene entering the system, so I can't specify the amount coming through the wall. 

abenhadj posted this 20 November 2019

The value you want to add as source into the domain: if you do not know that than you need to make assumptions.

You want to measure the amount coming in: then you need to define the walls as inlets.

Best regards, Amine