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Having problems setting up boundary conditions for paint chamber simulation

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Old   December 3, 2014, 11:26
Default Having problems setting up boundary conditions for paint chamber simulation
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Jon Tao
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Hi everyone and thanks for you help.

I'm working on a project where I have to simulate a spray paint chamber and model the flow velocity and VOC concentration from paint within the chamber. The chamber is essentially a rectangular box with the ends and roof open to air with exhaust fans that suck the air in the chamber out the bottom. The fans are in the ground against the walls along the length of the chamber. The only inflow in the chamber is the spray gun (centered in the chamber), which is spraying a mixture of air and VOC. I specified the VOC as toluene for now. The simulation is set to steady and all fluids are incompressible ideal gases.

The spray speed is specified as well as the mass flow of the VOC, so I have set the spray gun as an inlet velocity for the boundary condition with positive velocity (with the correct mass fraction of VOC). The air going into the fans is also specified so I set those as inlet velocities as well, with negative velocities since the air is leaving the chamber.

The roof and ends of the chamber are set as outflow as the velocity is not specified.

When I run the simulation, the velocity profile looks exactly what I would expect, with high velocity coming out of the spray gun and into the fans and with lower velocities coming in from the roof and ends. But when I look at the concentration, it is the exact opposite of what I would expect. The concentration of the VOC coming out of the spray gun is very low (<1 mass%) and the spray gun is the only source of VOC. When I look at the concentration profile, it looks like the entire chamber is filled with VOC rather than air. I'm thinking that it's because since the roof and ends are set as outflow, but it doesn't know that it should only be taking in 100% air and I don't know how to set that condition.

I have tried setting the roof and ends to inlet vent (against the project specifications) so I could set the concentration to 100% air, but I don't know what the gauge pressure should be and setting pressure to 0 results in an incorrect (or nonexistent) velocity profile.

Please help! Thank you!
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