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