How do i impose the outlet mass flow rate?

AndreaVinci · November 25, 2022, 10:05

I have a quick question. If the flow rate at the output of my system is known, how do I impose it? Do i have to use the target mass flow rate in the outlet boundary condition and put the mass flow rate value? And also, if I assign a mass flow inlet why do I get velocity 0 at the inlet?
P.S. I'm using fluent and I'm working on a hvac of a room. Pictures of the geometry are attached to the thread to better explain what I'm doing. Thanks in advance

AlexanderZ · November 28, 2022, 20:29

Quote:

Do i have to use the target mass flow rate in the outlet boundary condition and put the mass flow rate value?

yes
velocity can't be 0 at inlet in cas you've set mass flow rate and pressure, you may see the velocity in initialization tab if you select initialize from inlet

LoGaL · November 29, 2022, 04:00

There’s a mass flow outlet boundary condition.

If you use it, then impose a pressure at the inlet

AndreaVinci · November 29, 2022, 05:01

Quote:

Originally Posted by LoGaL

There’s a mass flow outlet boundary condition.

If you use it, then impose a pressure at the inlet

If i have a mass low outlet then I must have a pressure inlet? did I get it right?

AndreaVinci · November 29, 2022, 05:15

Quote:

Originally Posted by AlexanderZ

yes
velocity can't be 0 at inlet in cas you've set mass flow rate and pressure, you may see the velocity in initialization tab if you select initialize from inlet

So I have to put the target mass flow rate in the outlet bc (which is a pressure outlet) but I must have a velocity inlet. Is that correct?

LoGaL · November 29, 2022, 16:00

No, there's a mass flow rate Boundary condition type. No target mass flow rate. Now you have a pressure outlet, but you can select mass flow outlet option.

AndreaVinci · November 30, 2022, 04:51

Quote:

Originally Posted by LoGaL

No, there's a mass flow rate Boundary condition type. No target mass flow rate. Now you have a pressure outlet, but you can select mass flow outlet option.

Ok, and what type of inlet do I need to use if I have mass flow outlets? Velocity inlet? Pressure inlet? Mass flow inlet?

LoGaL · November 30, 2022, 07:34

Well, somewhere you will need to specify a pressure, don't you think so?

AndreaVinci · November 30, 2022, 07:35

Quote:

Originally Posted by LoGaL

Well, somewhere you will need to specify a pressure, don't you think so?

I don't know, I don't have a pressure in my input datas

LoGaL · November 30, 2022, 07:37

Seems a bit weird that you do not know the pressure of your sistem. Is this incompressible flow?

AndreaVinci · November 30, 2022, 07:41

Quote:

Originally Posted by LoGaL

Seems a bit weird that you do not know the pressure of your sistem. Is this incompressible flow?

I know the mass flow rate at the inlet and I can easily calculate the velocity because I know the inlet area. The pressure at the inlet I guess is the same as the atmosphere pressure

LoGaL · November 30, 2022, 07:43

Ok then impose that pressure at the inlet, and a mass flow at the outlet. Or if you know the pressure at the outlet, you can impose the mass flow at inlet and pressure at outlet.

Somewhere you need to specify a pressure, or your problem doesn't know what is the pressure, so it is ill posed.

AndreaVinci · November 30, 2022, 07:46

Quote:

Originally Posted by LoGaL

Ok then impose that pressure at the inlet, and a mass flow at the outlet. Or if you know the pressure at the outlet, you can impose the mass flow at inlet and pressure at outlet.

Somewhere you need to specify a pressure, or your problem doesn't know what is the pressure, so it is ill posed.

ok thank you

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November 29, 2022, 04:00		#3
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 375 Rep Power: 12	There’s a mass flow outlet boundary condition. If you use it, then impose a pressure at the inlet

November 29, 2022, 16:00		#6
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 375 Rep Power: 12	No, there's a mass flow rate Boundary condition type. No target mass flow rate. Now you have a pressure outlet, but you can select mass flow outlet option.

November 30, 2022, 07:34		#8
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 375 Rep Power: 12	Well, somewhere you will need to specify a pressure, don't you think so?

November 30, 2022, 07:37		#10
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 375 Rep Power: 12	Seems a bit weird that you do not know the pressure of your sistem. Is this incompressible flow?

November 30, 2022, 07:43		#12
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 375 Rep Power: 12	Ok then impose that pressure at the inlet, and a mass flow at the outlet. Or if you know the pressure at the outlet, you can impose the mass flow at inlet and pressure at outlet. Somewhere you need to specify a pressure, or your problem doesn't know what is the pressure, so it is ill posed.