Question-and-Answer Resource for the Building Energy Modeling Community
Get s tarted with the Help page

# Skylight air mass flow coefficient

Hello,all! I have a question about the simulation of open skylight. I looked it up in the I/O Reference, it is written like this:

The AirflowNetwork model does not have a model for bi-directional flow through large horizontal openings in exterior surfaces. For this reason, AirflowNetwork:MultiZone:Component:Detailed-Opening should not be used for exterior horizontal openings. The best modeling technique in this case is to put an AirflowNetwork:MultiZone:Surface:Crack object in a horizontal surface and use a large air mass flow coefficient. Crack flow is assumed to be uni-directional in any given timestep (but can reverse flow direction from timestep to timestep).


What I want to know is the range of the air mass flow coefficient, and how much should I set this value to indicate that the skylight is open. Or where can I find the relevant reference? Thanks a lot!

edit retag close merge delete

Hello dear,

Has anyone received any response or defined any strategy on this subject?

( 2020-12-18 10:32:23 -0500 )edit

Sort by » oldest newest most voted

The general strategy here is to set up the AirflowNetwork objects so that the airflow through them at a given pressure difference matches a given value. The ideal case would be to have an experimental measurement of airflow.

The documentation is advising the use of the AirflowNetwork:MultiZone:Surface:Crack object because other objects aren't appropriate in this case. Don't get too caught up in the "crack" in the name, you'll want to think of it more like flow through an orifice (see, for example, https://en.wikipedia.org/wiki/Orifice_plate) where the flow rate is proportional to the square root of the pressure difference. If you lack specific information, a place to start is using 0.5 as the exponent for the crack object.

To set the flow coefficient, if you don't have a measurement, then the next best thing would be to find something in the literature. Then there's some algebra to do to get the flow coefficient that's needed. I've answered a question on that process here, so I won't repeat all that. Follow that procedure with n=0.5.

more