Localized Leakage Flow Loss Values

[jharbold12]

I have a model to study the pressure build-up during a fire event in an extremely tight compartment. The only leakage available is through gaps in a fire door. I am modeling this using the localized leakage approach as explained in the User Guide. Prior to utilizing the localized leakage approach (no pressure alleviation) the pressure within the compartment quickly rose to several psi, balanced by the heat loss through the large surface area of the compartment. When utilizing the localized leakage approach there is a slight pressure buildup and then complete alleviation indicating that the small gaps which I have defined are more than sufficient to vent all pressure. The flow rates required to do this are somewhere around 2m^3/s of gas, which I believe is unrealistic. I am requesting some clarification of details which I cannot find in the User Guide or the Technical Reference Guide.

1. I have created two vents, applied a SURF to them (not HVAC) relating to the thermal properties of the fire barrier, and made the size of the vent the size of the fire barrier door. I then set the leak area to be the calculated gap around the fire door assembly (36in^2 - for a pair of 8' tall x 4' wide fire doors). I enabled leak enthalpy to account for any heat loss through the gaps. There is a value for Flow Loss with the default as 1.0. I have found an old NIST reference for a C value for door gaps of ~0.6, but it appears that this does not correlate with the given equation within the Technical Reference Guide (equation 11.9). How am I able to utilize a flow loss value to better simulate the rough flow surface through fire door gaps. Without this value it appears that there is uninhibited flow through my defined gap which disallows any pressure buildup over time.
2. Does the defined size of my vent correspond to the leakage or is leakage area solely defined by the leak area parameter?
3. Is this the correct method to model leakage through a door gap? I could model gaps but the mesh required to do so accurately would be much smaller than I have defined currently.

[drjfloyd]

1. If you want to change the default flow coefficient, you can do that. However, a multi psi pressure rise is not possible in reality unless you have a reinforced concrete building. Typical construction will see failures (windows, doors, and walls) at those pressures. Ordinary levels of leakage are generally sufficient to limit pressure rise during a fire to well under a psi of pressure. Take a look at the pressure chapter in the validaiton guide. Other than the PRISME tests (where the rooms were concrete), pressure rises are a few hundred Pascal or ~0.05 psi.

2. The VENT XB tells FDS where to average pressure, temperature, and species to obtain the boundary condiitons for the leakage flow. AREA on HVAC defines the area of the leakage path.

3. This is why the localized leakage approach was developed, to allow one to simulate flow through small openings like door cracks without having to use mm scale grids.