Leakage between two compartments with open vent

[Patrik Král]

Hello, 

I have a question. I have two compartments, two zones, leakage vents between them, and each of the compartments has an open vent. The leakage between them does not work, because FDS checks the pressure zones at the beginning of the simulation and  assigns both to the same pressure zone (the outer zone), right? In some cases, this even ends up with an error message.

Example leakage not working (as described above):

&HEAD CHID='LEAKAGE'/
&TIME T_END=100.0/
&DUMP DT_SL3D=0.25/

&MESH ID='MESH', IJK=15,20,20, XB=0.0,1.5,0.0,2.0,0.0,2.0/
&MESH ID='MESH01', IJK=14,20,20, XB=1.5,2.9,0.0,2.0,0.0,2.0/

&ZONE ID='FIRE_ZONE', XB=0.1,1.3,0.1,1.9,0.1,1.9, LEAK_AREA(2)=0.4/
&ZONE ID='INLET_ZONE', XB=1.7,2.9,0.1,1.9,0.1,1.9/

&REAC ID='C/VM2',
      FUEL='REAC_FUEL',
      C=1.0,
      H=2.0,
      O=0.5,
      AUTO_IGNITION_TEMPERATURE=0.0,
      CO_YIELD=0.04,
      SOOT_YIELD=0.07,
      HEAT_OF_COMBUSTION=2.0E+4,
      RADIATIVE_FRACTION=0.35/

&SURF ID='ADIABATIC',
      COLOR='GRAY 80',
      DEFAULT=.TRUE.,
      ADIABATIC=.TRUE./
&SURF ID='LEAKAGE_INLET_FIRE',
      RGB=127,221,255,
      LEAK_PATH=2,1/
&SURF ID='LEAKAGE_FIRE_INLET',
      RGB=127,221,255,
      LEAK_PATH=1,2/
&SURF ID='FIRE',
      COLOR='RED',
      HRRPUA=1000.0,
      TAU_Q=15.0,
      TMP_FRONT=300.0/

&OBST ID='Obstruction', XB=1.4,1.5,0.0,2.0,0.0,2.0/
&OBST ID='Obstruction', XB=0.9,1.3,0.3,0.7,0.0,0.2, SURF_IDS='FIRE','ADIABATIC','ADIABATIC'/

&VENT ID='Vent', SURF_ID='OPEN', XB=2.9,2.9,0.6,1.4,0.8,1.6/
&VENT ID='Vent06', SURF_ID='LEAKAGE_INLET_FIRE', XB=1.5,1.5,0.0,2.0,0.0,0.2/
&VENT ID='Vent07', SURF_ID='LEAKAGE_INLET_FIRE', XB=1.5,1.5,0.0,2.0,1.8,2.0/
&VENT ID='Vent08', SURF_ID='LEAKAGE_FIRE_INLET', XB=1.4,1.4,0.0,2.0,1.8,2.0/
&VENT ID='Vent09', SURF_ID='LEAKAGE_FIRE_INLET', XB=1.4,1.4,0.0,2.0,0.0,0.2/
&VENT ID='Vent13', SURF_ID='OPEN', XB=0.0,0.0,1.4,1.8,1.7,1.9/

&SLCF QUANTITY='PRESSURE ZONE', CELL_CENTERED=.TRUE., XB=0.0,2.9,0.9,1.0,0.0,2.0, FYI='PRESSURE_ZONE'/

&TAIL /

-------------------------------

There is, however, a workaround. If I use a hole with a delay ctrl which, after some time, links the initially sealed zone with the outer zone, the leakage between the two compartments still works even when de facto, both the compartments are then in the same zone (outer zone). 

Example leakage working (as described above):

&HEAD CHID='LEAKAGE'/
&TIME T_END=100.0/
&DUMP DT_SL3D=0.25/

&MESH ID='MESH', IJK=15,20,20, XB=0.0,1.5,0.0,2.0,0.0,2.0/
&MESH ID='MESH01', IJK=14,20,20, XB=1.5,2.9,0.0,2.0,0.0,2.0/
&MESH ID='MESH02', IJK=3,20,20, XB=-0.3,-2.775558E-17,0.0,2.0,0.0,2.0/

&ZONE ID='FIRE_ZONE', XB=0.1,1.3,0.1,1.9,0.1,1.9, LEAK_AREA(2)=0.4/
&ZONE ID='INLET_ZONE', XB=1.7,2.9,0.1,1.9,0.1,1.9/

&REAC ID='C/VM2',
      FUEL='REAC_FUEL',
      C=1.0,
      H=2.0,
      O=0.5,
      AUTO_IGNITION_TEMPERATURE=0.0,
      CO_YIELD=0.04,
      SOOT_YIELD=0.07,
      HEAT_OF_COMBUSTION=2.0E+4,
      RADIATIVE_FRACTION=0.35/

&CTRL ID='invert', FUNCTION_TYPE='ALL', LATCH=.FALSE., INITIAL_STATE=.TRUE., INPUT_ID='TIMER->OUT'/
&DEVC ID='TIMER->OUT', QUANTITY='TIME', XYZ=0.0,0.0,0.0, SETPOINT=5.0/

&SURF ID='ADIABATIC',
      COLOR='GRAY 80',
      DEFAULT=.TRUE.,
      ADIABATIC=.TRUE./
&SURF ID='LEAKAGE_INLET_FIRE',
      RGB=127,221,255,
      LEAK_PATH=2,1/
&SURF ID='LEAKAGE_FIRE_INLET',
      RGB=127,221,255,
      LEAK_PATH=1,2/
&SURF ID='FIRE',
      COLOR='RED',
      HRRPUA=1000.0,
      TAU_Q=15.0,
      TMP_FRONT=300.0/

&OBST ID='Obstruction', XB=1.4,1.5,0.0,2.0,0.0,2.0/
&OBST ID='Obstruction', XB=0.9,1.3,0.3,0.7,0.0,0.2, SURF_IDS='FIRE','ADIABATIC','ADIABATIC'/
&OBST ID='Obstruction', XB=0.0,0.0,1.4,1.8,1.7,1.9, CTRL_ID='invert'/
&OBST ID='Obstruction', XB=-2.775558E-17,-2.775558E-17,1.4,1.8,1.7,1.9, CTRL_ID='invert'/
&OBST ID='Obstruction', XB=0.0,0.0,1.4,1.8,0.0,1.7/
&OBST ID='Obstruction', XB=0.0,0.0,0.0,1.4,0.0,2.0/
&OBST ID='Obstruction', XB=0.0,0.0,1.8,2.0,0.0,2.0/
&OBST ID='Obstruction', XB=0.0,0.0,1.4,1.8,1.9,2.0/
&OBST ID='Obstruction', XB=-2.775558E-17,-2.775558E-17,1.4,1.8,0.0,1.7/
&OBST ID='Obstruction', XB=-2.775558E-17,-2.775558E-17,0.0,1.4,0.0,2.0/
&OBST ID='Obstruction', XB=-2.775558E-17,-2.775558E-17,1.8,2.0,0.0,2.0/
&OBST ID='Obstruction', XB=-2.775558E-17,-2.775558E-17,1.4,1.8,1.9,2.0/

&VENT ID='Vent', SURF_ID='OPEN', XB=2.9,2.9,0.6,1.4,0.8,1.6/
&VENT ID='Vent06', SURF_ID='LEAKAGE_INLET_FIRE', XB=1.5,1.5,0.0,2.0,0.0,0.2/
&VENT ID='Vent07', SURF_ID='LEAKAGE_INLET_FIRE', XB=1.5,1.5,0.0,2.0,1.8,2.0/
&VENT ID='Vent08', SURF_ID='LEAKAGE_FIRE_INLET', XB=1.4,1.4,0.0,2.0,1.8,2.0/
&VENT ID='Vent09', SURF_ID='LEAKAGE_FIRE_INLET', XB=1.4,1.4,0.0,2.0,0.0,0.2/
&VENT ID='Vent14', SURF_ID='OPEN', XB=-0.3,-0.3,0.0,2.0,0.0,2.0/

&SLCF QUANTITY='PRESSURE ZONE', CELL_CENTERED=.TRUE., XB=-0.3,2.9,0.9,1.0,0.0,2.0, FYI='PRESSURE_ZONE'/

&TAIL /

-------------------------------

Now, why is there the limitation that the compartments must not be in the "same zone" (understand they both must not have open vents at the same time at the beginning of the simulation) when with a delay, it actually works? Could FDS analyse the zones before it applies the open vents, so this limitation would not be there? 

Particularly in big models, the compartments might not be thoroughly sealed, therefore the zoning might not work, and a pressure test could be quite time consuming. (Although, this would not solve possible gaps between the two particular compartments, but only to compartments linked to the outer zone and not connected to one another)

Thank you for your reply.

Kind regards

Patrik

[dr_jfloyd]

There are two types of leakage in FDS. 

1) Zone leakage where the leakage is due to the difference in the background pressure rise between two different zones. This leakage requires that two different zones be present as it only uses the background pressure. If you start the simulation with two zones and then break the barrier between the zones, then this leakage will stop as both zones will wind up with the same pressure. This leakage is intended to capture the bulk leakage through all the surfaces of a compartment. In this case using the perturbation pressure does not have meaning as one is not defining specific flow paths but rather broad surface areas where leaks occur. 

2) Localized leakage where the leakage is due to the pressure difference between two VENTs. In this case the pressure is the combination of the background pressure and the local perturbation pressure. This leakage does not require different pressure zones. It is intended to represent known leakage paths such as an undercut beneath a door.