Leakage Modelling with Pressure Zones

Posted: January 27th, 2016 in News

Updated Information Available

PyroSim 2017.2 now supports pressure zone and localized leakage in the user interface. The post Leakage Modelling using Pressure Zones – Updated 2018-06 updates and replaces the contents of this post.

Some users have asked about modeling leakage using PyroSim, so this post will give two examples of how this can be done. Leakage refers to the air that escapes through small gaps (i.e at the top of a door) as a compartment is pressurized by a fire. The leakage area can be smaller than the mesh size, so the gaps cannot be modeled directly and a different approach is required. 

Leakage is discussed in Section 9.3.2 of the FDS User Guide. For our examples, we will use the same approach as in the door_crack FDS verification problem. To download the FDS input file, click this link (door_crack.fds), select the file, then click Raw to display only the file contents. You can copy the raw contents of the file and paste directly into PyroSim.

Pressure Zone Leakage

As described in the FDS manual, “The pressure zone leakage approach is intended to capture the bulk leakage that occurs through walls.” A pressure zone is a sealed compartment within the computational domain.  Pressure zone leakage uses the pressure difference between the “background” pressure of two zones to calculate the leakage flow. It is assumed the flow is small and that flow into a zone will be at that zone’s temperature (no heat transport from the zone from which the leak is occurring).

Pressure zone leakage is defined in three steps:

  1. Define the zones.
  2. Define a leak surface.
  3. Apply the leak surface to a solid with boundaries in both zones.

Pressure zone leakage is supported in the PyroSim graphical interface. The following steps were used to create the model shown below.

  1. Define Zone 1 (draw with the tool or double-click Zones in the tree). This is an enclosed zone bounded by INERT surfaces and an obstruction. Specify the leak area.
  2. Create a Leak surface. The Surface Type is Air Leak. Specify the Leak Path between Zone1 and Zone0 (the default outer zone).
  3. Create an obstruction that uses the Leak surface.

In this example, we supply air at a rate of 1 kg/s through a 1 m2 vent. The leakage area is 0.1 m2. Then air flows through the obstruction to the OPEN boundary.

Figure 1: Pressure zone model. Air is supplied to Zone 1, leaks to Zone 0 and flows out the OPEN vent.
Figure 1: Pressure zone model. Air is supplied to Zone 1, leaks to Zone 0 and flows out the OPEN vent.

Since the flow rate is known, the expected pressure drop can be calculated using equation 9.6 in the FDS User Guide:

 

leakage equation

The calculation matches the pressure for Zone 1 shown below.

Figure 2: Plot of pressure in Zone 1. It reaches a steady state with leakage to Zone 0.
Figure 2: Plot of pressure in Zone 1. It reaches a steady state with leakage to Zone 0.

Localized Leakage

As described in the FDS manual, “The local leakage approach is intended to represent leakage through a specific crack. For example, a cracked open door might have a opening that is too small to resolve with the grid. One would; however, still want to capture the fact that hot gases could escape the top of the crack and cold gases enter the bottom.”

At the present time, PyroSim does not support local leakage in the graphical interface, so we place the data in the Additional Records section of the input file. We use the following steps:

  1. Define Zone 1. You can leave the zone Leak Area as zero.
  2. Create the obstruction between the zones.
  3. Add the following VENT and HVAC lines to the Additional Records section of input.
Figure 3: Add these lines to the Additional Records section. They define vents and a leakage flow between the zones.
Figure 3: Add these lines to the Additional Records section. They define vents and a leakage flow between the zones.

The results for this case are similar to those described above, but the flow is local to the vent locations.

Localized leakage provides the option to calculate the gas temperature of the leaking air. In the following example, the Zone 1 supply air is at 500 C, and two local leakage vents are used: one at the top of the door and one at the bottom.

To make this model, we used the LEAK_ENTHALPY=.TRUE. option and added the following lines in Additional Records.

Figure 4: Additional records for two leakage paths that preserve thermal energy.
Figure 4: Additional records for two leakage paths that preserve thermal energy.

At first, the hot air in Zone 1 rises and the flow into Zone 0 has hot air only at the top. When Zone 1 becomes filled with hot air, the air at the bottom vent is also hot.

Figure 5: Temperature when hot air is supplied and model uses local leakage. Hot air leaks through top vent.
Figure 5: Temperature when hot air is supplied and model uses local leakage. Hot air leaks through top vent.

Download the PyroSim input files here: