mass conservation with localised leakage

[marietta-13]

Hi,
I've been trying to model a mechanical extract ventilation system for a stair/lobby and been trying to understand why the mass conservation plots don't match up within the lobby. The mass conservation in the adjacent spaces is fine.

In the lobby, I have the following:

• Vent for Extract fan. Measuring the mass flow rate at the boundary
• Openings for doors to the fire compartment and stair. Measuring the mass flow rate at the gas phase
• Localised leakage to represent leakage through a closed door gap. Measuring the mass flow rate at the gas phase at the vent on the side of the lobby

I would expect to see the M Flow at extract fan = M Flow at doors+ M flow from leakage, but there is a slight error (10%). When simplifying the model, I believe this is caused by the localised leakage (if there is no leakage, M Flow at extract fan and the M Flow at doors perfectly matches up).

Can you confirm if this is likely to be the case, and if so is there any way to get an accurate value for the mass flow rate across the localised leakage?

[mcgratta]

Leakage is implemented as an HVAC vent. There is a table in the User's Guide listing output for HVAC networks.

[drjfloyd]

Based on your post it sounds like you want to measure mass flux at the wall (purple arrow) but are using a gas phase measurment (orange). When measuring away from the wall, you have the possiblity for air from cells around the leakage vent to be entrained into the leakage flow (green arrows). As Kevin noted you can output the actual mass flow in the virtual duct being used for each leakage path.

[marietta-13]

Great - thanks both for you help!