[sasanghomi]

Dear CFD community,

I am trying to model chimney effect via CFX. I have modelled the atmosphere and the flue gas ducting and a heat source has been located inside the ducting parts in order to increase air temperature. It is strange to me that the activation and deactivation if Buoyancy effect in the fluid domain has no effect on the mass flowrate of air inside the stack. In other word, the chimney effect does not work properly. Any idea will be appreciate.

[ghorrocks]

What is "The chimney effect"? Do you mean that the heat of a chimney causes the air to rise and exit the chimney at the top?

You have got something wrong in the setup. Have you defined the gravity vector?

If you cannot find what is wrong then post your output file and we will have a look.

[sasanghomi]

Thank you Glenn for your reply.
Chimney Effect: The temperature of gas inside the chimney is higher than ambient and it leads to density change. So, buoyancy force drive the fluid flow inside the stack.

I have activated the buoyancy effect and g=-9.8 m/s2

Please give me hints if you have any.

the output file has been attached.

Please be informed that some expressions have not been used in the simulation.

[ghorrocks]

You have a zero reference pressure, and you have defined the inlet and outlet to have close to 1 atm pressure, but with a small pressure difference between them. I presume there is a small altitude difference between the two boundaries to create this pressure difference.

Have you read the documentation on how reference pressure works for simulations with a gravity vector? It is important to understand this as I suspect you have set this up incorrectly.

You probably need to set your reference pressure to 1 [atm] (or whatever your reference pressure is), and your inlet and outlet should be 0 [Pa] as the hydrostatic pressure difference due to height is already taken care of - see my previous paragraph.