Replies: 4 comments 6 replies
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The area of the VENT has no effect on localized leakage flow rate. I am not familiar with Contam parameters. You might check its userguide to understand what these terms are. |
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Please reread the User's Guide. 
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Thanks, I have a question regarding FDS about supply versus extract and an opening. I noticed that when modeling a room with extract versus supply at the same flow rate, the pressure drop for the extract case is significantly higher than for the supply case. Could there be a reason for this? I understand that the discharge coefficient should be the same for both cases, so I wanted to check if my understanding of the pressure drop is correct.. |
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If you have a compartment with a fire then the air you are supplying will be at ambient temperature and the air you are exhaust is going to be at some higher temperature and have lower density. |
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Its just a room with an opening and modelled as supply vs extract at same flow rates with no fire, yet pressure drop is different |
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provide a simple pair of input files that demonstrate the issue. |
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&HEAD CHID='Supply_vs_door_opening_3m3s_'/ &MESH ID='MESH-merged-01', IJK=40,18,30, XB=-0.2,3.8,-0.2,1.6,0.0,3.0/ &DEVC ID='Gas 11', QUANTITY='PRESSURE', XYZ=8.4,0.6,1.4/ &MATL ID='GYPSUM PLASTER', &SURF ID='Surface01', &OBST ID='Obstruction', XB=-0.2,0.0,-8.326673E-17,1.4,0.0,3.0/ &HOLE ID='Hole', XB=8.6,8.8,0.4,0.8,-0.01,2.0/ &VENT ID='Mesh Vent: MESH-merged-03 [XMAX]', SURF_ID='OPEN', XB=11.9,11.9,-0.2,1.6,0.0,3.0/ &SLCF QUANTITY='PRESSURE', ID='Slice01', PBZ=1.0/ &TAIL / 2nd simulation with extract &HEAD CHID='Extract_vs_door_opening_3m3s_'/ &MESH ID='MESH-merged-01', IJK=40,18,30, XB=-0.2,3.8,-0.2,1.6,0.0,3.0/ &DEVC ID='Gas 11', QUANTITY='PRESSURE', XYZ=8.4,0.6,1.4/ &MATL ID='GYPSUM PLASTER', &SURF ID='Extraction01', &OBST ID='Obstruction', XB=-0.2,0.0,-8.326673E-17,1.4,0.0,3.0/ &HOLE ID='Hole', XB=8.6,8.8,0.4,0.8,-0.01,2.0/ &VENT ID='Mesh Vent: MESH-merged-03 [XMAX]', SURF_ID='OPEN', XB=11.9,11.9,-0.2,1.6,0.0,3.0/ &SLCF QUANTITY='PRESSURE', ID='Slice01', PBZ=1.0/ &TAIL / |
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I think this is a combination of grid resolution and the fact that the two flow fields are not the inverse of each other. |
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But isn't it expected that the pressure drop across the opening should be same whether extract or supply. The cd should be constant, just the flow direction is varied |
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There is not a one size fits all Cd. There is a dependence on the specific flow conditions at the orifice. In the supply case you have a jet of air directed at the opening which is about the same size as the opening. That jet does not have to turn to flow out the door. In the extract case the air is not a jet coming at the door. It is being drawn in from all the cells around the door. Some flow must turn to go through the door. The two flow fields are not the inverse of each other and pressure drop is a function of the flow field. All CFD models have numerical error and that error increases with grid size. |
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I have a question regarding the FDS leakage flow model. Does the above formula also apply to the localized approach? Would increasing the vent size have any effect, or is the flow governed solely by the leak area? Additionally, what does the flow loss value represent?
Secondly, when using CONTAM, there are additional inputs for leakage modeling, such as the flow exponent and pressure difference. How do these parameters relate to the way leakage is modeled in FDS?
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