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Lack of mass balance with mechnical ventilation #121

Closed GoogleCodeExporter closed 9 years ago

GoogleCodeExporter commented 9 years ago 
What steps will reproduce the problem?
1.Completed a fresh install of CFAST
2.Ran attached input file
3.Opened output file to observe mass imbalance. 

What is the expected output? What do you see instead?
The mass balance for Array1 should sum to zero. There is a flow imbalance of 
about 0.5 kg/s.

What version of the product are you using? On what operating system?
CFAST 6.2.0.291, Windows 7 Enterprise

Please provide any additional information below.

Original issue reported on code.google.com by cout...@asme.org on 2 Oct 2014 at 6:20

Attachments:

GoogleCodeExporter commented 9 years ago

Original comment by cfastdev@gmail.com on 3 Oct 2014 at 4:40

GoogleCodeExporter commented 9 years ago 
[deleted comment]
GoogleCodeExporter commented 9 years ago 
There’s a problem with initialization of pressures for compartments with 
different floor heights.  The pressures get initialized differently inside and 
outside the compartments so that flow is generated when it shouldn’t be there.

This is causing all the odd flows that CFAST is generating for your text case.  
We’re continuing to see how to properly fix the problem. At the moment, the 
best workaround is to reframe the problem so that the floors are all at the 
same height.  I adjusted your sample by “moving” the floors down in the two 
middle rooms and adjusting the vents so they were the same distance from the 
ceiling.  Here’s the output I get for the flow:

Flow Through Vents (kg/s)
To             Through              Upper Layer               Lower Layer       
    Mixing       Mixing       Trace Species (kg)
Compartment    Vent                 Inflow       Outflow      Inflow       
Outflow  To Upper     To Lower     Vented     Filtered
--------------------------------------------------------------------------------
-----------------------------------------------------
Rm_7a          H Comp  2 #1                                            1.20     

               H Outside #1                               1.20         1.264E-04                                                                               

Array1         H Comp  1 #1                               1.20                  

               H Comp  3 #1                                            1.20                                                                                     

Array2         H Comp  2 #1                               1.20                  

               H Comp  4 #1                                            1.20                                                                                     

Exh_1          H Comp  3 #1                               1.20                  

               M Node  1                                               1.20                                                                                     

Outside        H Comp  1 #1                               1.264E-04    1.20     

               M Node  2                                  1.20

Original comment by cfastdev@gmail.com on 10 Oct 2014 at 4:39

Attachments:

GoogleCodeExporter commented 9 years ago 
Mechanical ventilation is not required to see the issue.  See attached test 
case that is just two compartments connected by a vent.  Floor of second 
compartment is 1 m above floor of first compartment.  Not other connections 
except a vent from one compartment to the other.  When mass does balance, there 
is significant flow between compartments. We'll ask Glenn to take a look at the 
test case.

Original comment by cfastdev@gmail.com on 14 Oct 2014 at 2:01

Attachments:

GoogleCodeExporter commented 9 years ago 
Part of the problem seems to be the lack of soffits at the top of the vents for 
compartments 1, 2, and 3.  If you add a 0.1 m soffit, most of the problems go 
away.  We're still looking at a flow inconsistency in compartment 1.

Original comment by cfastdev@gmail.com on 28 Oct 2014 at 7:55

GoogleCodeExporter commented 9 years ago 
This seems to be fixed as of r1687 for CFAST 6.3.  Several bugs were related to 
the issue ( r1608, r1611, r1625, r1647).  Discovered an unrelated bug in trace 
species flow that is still being worked.  See Issue #123.

Original comment by cfastdev@gmail.com on 11 Dec 2014 at 5:39