Learning Forum

[shisha3301]

I have a question regarding the pressure values. Initially I ran a simulation with no energy equation and then one with an energy equation with defined temperature boundary conditions and got the same pressure values. Then I added a species model with a humidity for air and water vapour but the pressure values changed. So why did the pressure values stay the same when I activated the temperature equation with temperature boundary conditions, but when I added species model to simulate humidity the pressure values changed.

[Rob]

Why would the pressure change with temperature?

[shisha3301]

No it didn't change but it changed when I applied species model for relative humidity, so why did it change with the application of species.

[Rob]

How much vapour did you add? Remember density is set in the materials panel, either at the species or mixture level. If you didn't change any defaults then you may not get the result you expect.

[shisha3301]

Yes I left the settings as the default. Just used a mixture of water vapour and air and applied mass fraction value for relative humidity.

[shisha3301]

Relative humidity was 30 percent at inlet and 100 percent at wall and outlet

[Rob]

And the vapour mass fraction? What density model did you use?

[shisha3301]

at inlet 0.00438 and temperature of 20c at wall 33.5c and 0.03101 and at outlet 37c and 0.04001.
density model of mixture is incompressible ideal gas

[Rob]

How well converged are the results? I'd expect to see a slight difference but it may be fairly small between the cases.

[shisha3301]

yeah the results are converged. I plotted a countour plot of density and the values were not constant long the domain. Should it be constant or change and does that affect the pressure value

[Rob]

Now check the area average pressure on inlet & outlet: I assume you're using a mass flow boundary?

[shisha3301]

no, a velocity outlet and a pressure inlet

[Rob]

Check the mass flow in the system between the cases too.

[shisha3301]