I need some advice with my boundary conditions (picture attached)

JuPa · January 18, 2018, 04:56

I have a system with three inlets and one outlet.

The outlet is 10 L/min (I know what it is in kg/s and in m/s)
Flow rate @ inlet 1 is unknown
Flow rate @ inlet 2 is unknown
Inlet 2 has a filter. I know the pressure drop of the filter.
Flow rate @ inlet 3 is unknown

I am usually a CFX user. In CFX I would model inlet 1, 2 and 3 as an opening boundary condition:

Does Fluent have this feature?
How would you recommend I model my boundaries? Vent maybe?
Is there a way to tell Fluent I have a filter at the entrance of inlet 2? Would it need to be a new cell zone condition and model it as a porous media?

LuckyTran · January 18, 2018, 13:38

You need to supply the inlet pressures of the 3 inlets. Otherwise the problem has many solutions.

For the filter. Do you mean that you know the loss curve? For a given filter (i.e. a given loss curve), if you know the pressure drop then that implies you already know the flowrate across the filter.

You shouldn't need to use the porous media as a cell zone model. Several BC's and interface options are available, i.e the porous interface, or even the fan BC allows you to input a loss coefficient or pressure jump. Or you could just give inlet 2 a lower inlet pressure.

JuPa · January 19, 2018, 05:03

Thanks for your response LuckyTran.

Just to be clear - there is a suction pump at outlet 1 sucking at 10 L/min. Air is drawn through the inlets from atmosphere.

Quote:

You need to supply the inlet pressures of the 3 inlets.

Air is being sucked into the inlets from atmosphere. I.e. 101325 Pa.

Since the operating pressure is atmospheric, I set the inlet pressure to 0 Pa.

Quote:

For the filter. Do you mean that you know the loss curve?

Apologies for my ambiguity. Yes - I have the loss curve and not the actual pressure drop.

JuPa · January 23, 2018, 09:04

Hi, could anyone offer any further thoughts on this?

Thanks

DEd · January 26, 2018, 16:24

One thought (except use total pressure inlet) is that you model a part of the space outside the inlet of the pipes in order to get correct contraction of the flow.

JuPa · January 29, 2018, 05:15

Thanks DEd. I'll try that and see if I get better results.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Radiation in semi-transparent media with surface-to-surface model?	mpeppels	CFX	11	August 22, 2019 07:30
Wrong multiphase flow at rotating interface	Sanyo	CFX	14	February 7, 2017 17:19
Velocity vector in impeller passage	ngoc_tran_bao	CFX	24	May 3, 2016 21:16
Difficulty In Setting Boundary Conditions	Moinul Haque	CFX	4	November 25, 2014 17:30
Error finding variable "THERMX"	sunilpatil	CFX	8	April 26, 2013 07:00

January 18, 2018, 13:38		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	You need to supply the inlet pressures of the 3 inlets. Otherwise the problem has many solutions. For the filter. Do you mean that you know the loss curve? For a given filter (i.e. a given loss curve), if you know the pressure drop then that implies you already know the flowrate across the filter. You shouldn't need to use the porous media as a cell zone model. Several BC's and interface options are available, i.e the porous interface, or even the fan BC allows you to input a loss coefficient or pressure jump. Or you could just give inlet 2 a lower inlet pressure.

January 23, 2018, 09:04		#4
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	Hi, could anyone offer any further thoughts on this? Thanks

January 26, 2018, 16:24		#5
DEd Member Daniel Edebro Join Date: Feb 2016 Location: Gothenburg Posts: 41 Rep Power: 10	One thought (except use total pressure inlet) is that you model a part of the space outside the inlet of the pipes in order to get correct contraction of the flow.

January 29, 2018, 05:15		#6
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	Thanks DEd. I'll try that and see if I get better results.