Boundary Condition for Mechanical Ventilation System

[Anonymous]

Hi,

I am fairly new to simulation world. I need to model two of my projects where i need help regarding material assignment and boundary condition. The scenario is following:

Image 1 : One side Exhaust fan , opposite side Natural Opening and also jet fan to attain air speed

Query :

1. Should I assign the exhaust fan material as internal fan or just a solid material ? Should i model the exhaust fan exactly same as real one?

2. Should I assign both flow and pressure to exhaust fan?

3. Should I assign both pressure and airflow to the opening?

4. The jet fan modeling procedure I have already seen in another video of smoke system modeling. My question is in this case should i assign any pressure or velocity to the fan along with the flow? Should i assign the hollow section of the jet fan as solid material?

Image 2 : One side Exhaust fan , opposite side Supply air fan

Query :

1. Should I assign the exhaust fan material as internal fan or just a solid material ? Should i model the exhaust fan exactly same as real one?

2. Should I assign both flow and pressure to exhaust fan?

3. Should I assign both pressure and airflow to the Supply air fan?

Will be very thankful if anybody can give proper guidance.

[matt.bemis]

Hi @Anonymous,

Thanks for reaching out to us on the forums! This shouldn't be too hard to figure out. If you have the flow data for the fan (constant or fan curve), and the fan is on the inside of the domain, not at an exterior wall, then use an internal fan and do not apply any boundary conditions. The internal fan will handle the flow- not need for boundary conditions. 

If the fan is at the exterior wall/face of the domain, why not just skip using the fan and use a flow boundary condition? That would accomplish the same thing.

Thanks,

[Anonymous]

Hi Matt,

Thanks for your feedback. So what i understand from your reply is :

1. Jet fan inside the boundary : Hollow Casing - Solid , Circular Rotor - Internal Fan + Airflow with direction

2. Exhaust Fan at boundary Wall : No modeling required, used as surface , Airflow with direction + zero pressure

3. Free inlet at boundary wall : used as surface , no airflow , zero pressure

Please correct me if I am wrong.

I will run the simulation based on these conditions and share the result if any confusion arise.

Thanks a lot again.

[Amal.C]

Hi @Anonymous,

Please allow me here to answer your questions 🙂 

Jet fan inside the boundary : Hollow Casing - Solid , Circular Rotor - Internal Fan + Airflow with direction: If the fans are inside your model, then you need to use the internal fan material. Before assigining this material, please make sure to model the geometry of your fan as described in the article How to model an axial fan for use in CFD. 

Aside setting the flow direction, in the material editor of the internal fan material, you will need to give:

• the flow rate and choose the variation method (constant, fan curve, velocity profile)
• the rotational speed if available

Exhaust Fan at boundary Wall : No modeling required, used as surface , Airflow with direction + zero pressure

Replace the fan at the boundary with a surface where you apply the appropriate flow rate (please do not add a pressure boundary condition there)

Free inlet at boundary wall : used as surface , no airflow , zero pressure

Well done here 🙂 

Does this help?

Thanks,

Amal