Re: [EnergyPlus_Support] questions how COMIS calculates infiltration

[Frederick Winkelmann <FCWinkelmann@xxxxxxx>]

Ian -- see answers below.

Fred Winkelmann

idoebber wrote:

> I am trying to understand how COMIS calculates the pressure difference across
> a crack/opening.  I understand that the pressure difference across an crack on
> an exterior wall is due to stack (temperature) and wind effects.
>
> Stack Effects:
> In the summer time when the air is hotter outside, assuming 0 wind speed, the
> outside air pressure should be greater then the inside air pressure thereby
> causing infiltration into the zone.  Conversely, in the winter time with
> colder outside air, assuming 0 wind speed, exfiltration should occur since the
> inside pressure is greater then the outside pressure.  When I did a test run
> on a specific crack in the zone, infiltration occured for both the summer and
> winter.  What am I assuming that is wrong?

COMIS assumes (like other air flow programs) that the net air flow into a zone--i.e., the sum of incoming and outgoing
flows--is zero. So any infiltration must be balanced by exfiltration, whether it is summer or winter. For example, assuming
you only have one zone (so there is no interzone air flow), infiltration through cracks has to be balanced by exfiltration
through other cracks. If there is interzone flow, you might have infiltration through cracks in a zone being balanced by air
flow from that zone to an adjacent zone.

>
>
> Wind Effects:
> In a brief design day caculation, I had the wind coming from the south hitting
> directly on a southern wall of my building.  I thought the surface pressure on
> that particular wall should increase with a positive Cp value.  When I
> outputted the surface-averaged wind pressure on the outside surface, the
> values that I got were negative.  I thought these outputed surface-average
> wind pressure was the added pressure on the exterior wall on top of the
> atmospheric pressure of the stagnant outside air.  If this is true, shouldn't
> a wind directly on a surface have a positive surface-average wind pressure.

Yes, I would think the Cp in this case should be positive. Please send us your input file and we'll check your results.

>
>
> Finally is there any documentation on how COMIS determines the final pressure
> difference based upon adding together stack and wind effects.  I want to
> understand the relative importance of the two.

A good technical description of COMIS calculations can be found at http://epb1.lbl.gov/comis/users.html#publications --
where you can download Lawrence Berkeley National Laboratory Report LBNL-42182:
COMIS- An International Multizone Air Flow and Contaminant Transport Model

>
>
> Thanks
> Ian
>
>
> To unsubscribe from this group, send an email to:
> EnergyPlus_Support-unsubscribe@xxxxxxxxxxxxxxx
>
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
>
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
>
> Attachments are not allowed -- please post any files to the appropriate folder in the Files are of the Support Web Site.
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/


------------------------ Yahoo! Groups Sponsor ---------------------~-->
Rent DVDs Online - Over 14,500 titles.
No Late Fees & Free Shipping.
Try Netflix for FREE!
http://us.click.yahoo.com/Tq9otC/XP.FAA/3jkFAA/dkFolB/TM
---------------------------------------------------------------------~->

To unsubscribe from this group, send an email to:
EnergyPlus_Support-unsubscribe@xxxxxxxxxxxxxxx

The primary EnergyPlus web site is found at:
http://www.energyplus.gov

The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/

Attachments are not allowed -- please post any files to the appropriate folder in the Files are of the Support Web Site.




 

Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/