Inflow boundary condition for HVAC simulation

siw · January 24, 2014, 05:45

Hi,

I'm modelling the airflow into a room through a wall installed filter from an outside atmosphere. Downstream of the room is a fan that draws the air through the system, the fan is not included in the model but the mass flow rate is set to drive the air.

A large hemispherical domain was used to represent the atmosphere, reference pressure is 1atm and relative pressure in the atmosphere domain was 0 Pa, this with the outlet mass flow is a stable set-up. However, I found that the atmosphere boundary was mostly outflow and had a high pressure region in the centre of the hemisphere. But the mass flow imbalance (out - in) was tending to zero. But the outflow in the atmos. domain just doesn't seem right to me.

So I changed it to an inlet but CFX forced the boundary to be 100% walls so the mass flow imbalance was -3.0 kg/s, i.e. evacuating the room - clearly wrong.

I'm avoiding setting an inlet at the filter face and having the atmosphere domain to allow the airflow to do what it needs before entering the filter.

I cannot find any other HVAC models to include an atmosphere domain, only those setting a velocity at an inlet (e.g. open window face).

Any recommendations for the best set-up? Thanks.

singer1812 · January 24, 2014, 09:22

Opening BC is correct.

You might not have the BC far enough away from the fan. Sounds like the fan is drawing in air from the opening in essentially a straight line.

This may or may not be acceptable. What are you looking to view? Air distribution in room? If you are using a momentum source for the fan, then this will not be effected by your setup.

Are you looking for DP across fan? Then you might be ok, but you might need to move the opening back some.

siw · January 24, 2014, 10:27

Maybe I was not clear enough in my description but the airflow is from left to right in the picture. The fan is not included about I know the mass flow rate that the fan drawn through the room from the outside. The model consists of 3 domains (atmosphere, filter and room).

singer1812 · January 24, 2014, 10:50

I understood the left to right.

I had the fan in the filter area, but that is no issue.

The condition on the BC seems to indicate it is too close (If you want it to be more quiessent at the BC).

Once again it boils down to what you want to see out of this problem. Is the filter modeled as a porous media with directional loss? Are you trying to get DP across the filter? Are you trying to get air flow pattern set up around the room? What is your intent?

siw · January 24, 2014, 11:45

The atmosphere boundary is far from the filter relative to the filter and room sizes. The filter is a fluid domain with a momentum loss term and turbulence sink terms included. The aim is the get the correct pressure loss acros the filter and airflow behavior within the room - there's stuff in the room that I've omitted in the picture.

singer1812 · January 24, 2014, 14:06

Can you show pic of P and Vs in domain? I am not sure you have an issue.

Did you put up monitor points? I would suggest monitor P and each side of filter, and inlet and outlet. Make sure these have flat lined out prior to stopping the run.

Troubling is the high pressure at the inlet. Are you getting this results using conservative values of Tot Pressure, then that might be ok, and just is showing you are drawing air in a straight line fashion. Straight line velocity might not be real and can be caused by insufficient mesh resolution or BC being too near filter.

Not sure how you modeled the filter. Is it a directional P loss, momentum sink, or what? Are you sure you got that right (you arent trying to accelerate the flow back out again, I have accidentally done this before)?

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January 24, 2014, 09:22		#2
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20	Opening BC is correct. You might not have the BC far enough away from the fan. Sounds like the fan is drawing in air from the opening in essentially a straight line. This may or may not be acceptable. What are you looking to view? Air distribution in room? If you are using a momentum source for the fan, then this will not be effected by your setup. Are you looking for DP across fan? Then you might be ok, but you might need to move the opening back some.

January 24, 2014, 10:27		#3
siw Senior Member Stuart Join Date: Jul 2009 Location: Portsmouth, England Posts: 733 Rep Power: 25	Maybe I was not clear enough in my description but the airflow is from left to right in the picture. The fan is not included about I know the mass flow rate that the fan drawn through the room from the outside. The model consists of 3 domains (atmosphere, filter and room).

January 24, 2014, 10:50		#4
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20	I understood the left to right. I had the fan in the filter area, but that is no issue. The condition on the BC seems to indicate it is too close (If you want it to be more quiessent at the BC). Once again it boils down to what you want to see out of this problem. Is the filter modeled as a porous media with directional loss? Are you trying to get DP across the filter? Are you trying to get air flow pattern set up around the room? What is your intent?

January 24, 2014, 11:45		#5
siw Senior Member Stuart Join Date: Jul 2009 Location: Portsmouth, England Posts: 733 Rep Power: 25	The atmosphere boundary is far from the filter relative to the filter and room sizes. The filter is a fluid domain with a momentum loss term and turbulence sink terms included. The aim is the get the correct pressure loss acros the filter and airflow behavior within the room - there's stuff in the room that I've omitted in the picture.

January 24, 2014, 14:06		#6
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 20	Can you show pic of P and Vs in domain? I am not sure you have an issue. Did you put up monitor points? I would suggest monitor P and each side of filter, and inlet and outlet. Make sure these have flat lined out prior to stopping the run. Troubling is the high pressure at the inlet. Are you getting this results using conservative values of Tot Pressure, then that might be ok, and just is showing you are drawing air in a straight line fashion. Straight line velocity might not be real and can be caused by insufficient mesh resolution or BC being too near filter. Not sure how you modeled the filter. Is it a directional P loss, momentum sink, or what? Are you sure you got that right (you arent trying to accelerate the flow back out again, I have accidentally done this before)?