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RE: [EnergyPlus_Support] I wonder a part of AFN algorithm

Wonjun:

The air density is a function of pressure, temperature and humidity ratio
and is calculated using the following equation in the AirflowNetwork model
of EnergyPlus:

      rhoair = pb/(287.d0*(tdb+KelvinConv)*(1.d0+1.6077687d0*w))

where 
	pb = absolute outdoor pressure + indoor gauge pressure
	tdb = dry bulb temperature in C
	w = humidity ratio

Hope this helps.

Lixing



-----Original Message-----
From: EnergyPlus_Support@xxxxxxxxxxxxxxx
[mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Wonjun
Sent: Tuesday, August 02, 2011 9:51 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] I wonder a part of AFN algorithm

Hello fellows 
I wonder a part of AFN algorithm, especially the consideration of stack
effect. I have read many text and E+ engineering references, but can't find
the answer of my question. 

As you know, bascially the air mass flow is based on the power law equation
like below 
air mass flow(M)=C*(dP)^n

the pressure difference of two consecutive nodes (dP) in AFN is (Static P1 +
Dynamic P1 + Stack P1) - (Static P2 + Dynamic P2 + Stack P2).
But generally this kind of Bernoulli's Equation assumes density as the
constant. 
If so, Stack P (=rho*g*h) is just the function of height. 
Because the density and gravity acceleration are constant.

If so, how that equation considers pressure difference from temperature
difference? 

I think AFN consider stack effect in this way but can't sure right or not(I
can't find the detailed assumption from Engineering reference). 
If I apply treat the air as an Ideal Gas 
Density(rho)=P/RT
Then the density is function of Temperature like this (Because R is constant
and P would be 101.325kPa)
or from the linear assumption
rho=2.7567-0.0053T
then the Stack pressure rho*g*h can consider the pressure difference from
temperature diffrerence also.

I look foward to get an answer of my question.  
Thanks in advance. 

Best regards. 




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

Primary EnergyPlus support is found at:
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http://groups.yahoo.com/group/EnergyPlus_Support/

Attachments are currently allowed but be mindful that not everyone has a high speed connection.  Limit attachments to small files.

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