Re: atmospheric dispersion/dilution.......question

[Stewart Farber <farbersa@optonline.net>]

Hi:

I would suggest that increasing dilution flow volume increases the stack plume 

exit velocity which does potentially change the effective stack height, not the 

effective wind speed. If in your dispersion model, you are able to take credit 

for the plume being able to break through the envelope around a building [ie: no 

building wake effect applied to the release] then there will be somewhat greater 

dilution before the plume would touch down and you would see greater Chi/Q 

dilution factors. However, if your model does not allow you to take credit for an 

elevated release when the stack gas exit velocity to mean wind speed ratio 

exceeds some value [often the case for short stacks on buildings], then there is 

likely no credit for an elevated release and you get no credit for the plume 

behaving as an elevated release in the cases of lower wind speeds, before plume 

touchdown would occur.



 

Stewart Farber

4/29/03 7:23:31 AM, Dave Brown <david.brown@nist.gov> wrote:



>

>

>

>  Date:   Tue, 29 Apr 2003 07:23:31 -0400

>  From:   Dave Brown <david.brown@nist.gov>

>  Subject:atmospheric dispersion/dilution.......question

>  To:     radsafe@list.vanderbilt.edu

>

>

>

>  I have a question related to an observation of some calculations I've done

>  recently regarding atmospheric diffusion of effluent releases.  

>

>  Consider a case where there is a fixed release rate, Ci/sec, at a fixed

>  stack flow rate, cfm.  There is also the option of a large dilution fan that

>  uses outside air to dilute the effluent concentration going out the stack by

>  say a factor of 500.  In this case, although the effluent concentration has

>  significantly decreased, the effective Q or Ci/sec remains the same.  My

>  observation is that the only thing this dilution fan accomplishes in the

>  calculations is to increase the effective stack height thus changing the

>  vertical dispersion coefficient and the effective wind speed. This does not

>  have a proportionally significant affect on the resulting downwind

>  concentrations. It would seem that this dilution fan should have more impact

>  on the downwind dilution....but it doesn't. Anyone have a good explanation/

>  justification for this...? My initial thoughts are that in the scope of

>  atmospheric dilution , the dilution fan is small in comparison. This is just

>  an observation though, I'm looking for a more technical explanation

>

>

>  Regards,

>

>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

>  Dave Brown, CHP

>  National Institute of Standards and Technology

>  100 Bureau Drive, Stop 3543

>  Bldg 235 Rm B104

>  Gaithersburg, MD  20899-3543

>

>  301-975-5810 - office

>  301-921-9847 - fax

>  david.brown@nist.gov

>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

>  *The content of this message has not been endorsed by my employer*







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