Modelling an intake system
 

Dear All,

I need to model a kind of an air intake system. I have a common duct where the air is sucked from ambient and at the other end is the duct divided into two smaller ducts. In the two smaller ducts there is a suction fan installed in each duct. A system like a “Y”
I know the desired mass flow through the system. What kind of boundaries should I use, especially for the outlet (where the fans are).
Would it be correct to use an inlet vent, where the air is sucked from ambient
Exhaust fan at the outlet of the two small ducts and then use a constant pressure jump. And adjusting the pressure jump until the desired mass flow is obtained.
The duct is not a symmetric “Y” so I would expect an uneven mass flow in the two “legs”, can I predict that if I use the pressure jump at the outlet

Thanks in advance
S

If your flow is incompressible <0.3mach use velocity boundaries at the fan side.

You can solve this different ways, easiest would be to blow the air through your system instead of suction at two locations.
Just use a velocity in at your orifice and see the flow distribution at the outlet sides. This should give you a good idea on how to dimension your 2 fans

Thanks for your reply.
Just before the fan (in real world) then must the flow see a pressure rise through the fan, how do I get the effect into the calculation if I don't use the exhaust fan outlet boundary
S

I need you to be more precise about your geometry, are the fans located inside the duct and is the duct continuing before and after the fan?
Also make sure to extend your boundaries for better convergence.

What is your actual goal of this CFD? to see the flow pattern?
If only you need to dimension the fans i would use other tools.
If your goal is flowrate X through orifice in then I would take a calculator, find the ratio of the 2 areas of branch 1 and 2 and from this get the flow rate distribution with respecty to total flow rate.

NO CFD required.