Learning Forum

[owdladben]

Hi everyone, I'm trying to use the Boussinesq approximation to model buoyancy driven flow in a classroom however I'm struggling, even after reading the user guide.

I'm trying to model the effect a radiator would have on the spread of a tracer gas within a classroom. I have two case studies I want to look at.

Case Study 1: Room Initialised at 20C, Radiator On with constant Temp 30C, Inlet Ventilation Temp 5.2C, Inlet Tracer Gas Temp 37C

Case Study 2: Room Initialised at 20C, Radiator Off, Inlet Ventilation Temp 15.3C, Inlet Tracer Gas Temp 37C

Some of the assumptions I've made is there is no heat flux from the pupils, walls are insulated (heat flux = 0)...

For these case studies, what is my operating temperature? Also for my material density (air) that I have changed from constant to boussinesq , do I need to change the constant value for density? I noticed when I switched to Boussinesq it went to 0 so I just set this to 1.204kg/m3.

Apologies if this is basic stuff, I'm new to Fluent!

Thanks, Ben.

[owdladben]

I'm sure it makes no difference but for clarity, the tracer gas inlet is 0.7l/s (peak breathing flow rate) while the velocity inlet flow rate is 270l/s

[Rob]

What other inlets/outlets have you got? The two scenarios look sensible, but initially I'd run steady state using species for the exhaled air to see where it goes.
For Bousinesq the operating temperature is the temperature that gives you the correct density. So both air and tracer might be (density of (using air)) 1.225kg/m3 at 15C

[owdladben]

Thanks for the reply, Rob.
So I am already initially running a steady-state model using species transport.
In regards to my inlet and outlets, I have 5 windows which are providing my room with "fresh air" - these have an inlet speed of 0.46m/s. The only other inlet I have is from the student where the tracer gas is being released - this is at 1.3m/s to represent a peak breathing velocity. Overall, my room has a ventilation rate of 4 air changes per hour. Another simplification I used was that my outlet for the room is a large door opening, right now I didn't want to create a larger domain around my classroom to represent the movement of air between outside and inside via the windows.
Does this change anything in regards to my values?
Regards, Ben.

[Rob]

No, that's fine. I just wanted to check you'd got some way for the tracer to escape. You may want to check the HVAC rules, I thought rooms aimed for 6 air changes an hour?

[owdladben]

Well, I thought 4 was low Rob however I based this on the CIBSE recommendation of 10l/s/person. Part of my study will look at increasing this air change rate. So, just a quick one before I become a burden, my operating temperature should I change this to 15C and then the respective material density? Or, should I set this as an average of the temperature within my domain?

[Rob]

I'd set to the mean (or as close as you can without running the model) so the change in temperature is minimised. Remember to adjust the density to the operating temperature.
No burden, we quite like well thought out questions.

[owdladben]

Okay, great. Thank you for your help, Rob.

[Rob]

You're welcome.