[TRNSYS-users] Modelling a greenhouse thermal screen by using aninternal window in Type 56

[knut.erik.enerstvedt at niva.no]

Dear Matt,

Thanks for replying to my posting.

I have done some testing of my approach to modelling the thermal screen, 
and sometimes I get this warning message: 

"Small internal timesteps (<0.1% of simulation timestep) used for humidity 
calculations due to large coupling air flows between zones!"

This occured when I ran a simulation with the screen suddenly opened at 
some time, which means that the screen went from fully closed to fully 
opened from one time step to the next. Then the air exchange between the 
zones is large due to large temperature/density gradients. The warning 
message did not appear when the screen was fully opened all the time. I 
think maybe I can solve the problem by implementing gradual opening of the 
screen. Do you have any other suggestions?

Also, about the air exchange: Can I use the air exchange rate (absolute 
value, of course) from the equation as coupling air flow into both 
adjacent zones, regardless of flow direction? It seems to me that the 
coupling air flow should be the same from both sides to maintain air 
balance. Am I correct?

Thanks again for your help.


Best regards,
Knut Erik 





From:
"Matt Duffy" <duffy at tess-inc.com>
To:
knut.erik.enerstvedt at niva.no, trnsys-users at engr.wisc.edu
Date:
10/29/2009 12:29 PM
Subject:
Re: [TRNSYS-users] Modelling a greenhouse thermal screen by using 
aninternal window in Type 56




Dear Knut Erik,

Your approach sounds very good. Yes, you may set the thermal properites of 
the thermal screen in the Window TypeManager where you mentioned. For more 
information about the reflection coefficient, see page 59 of the TRNBuild 
manual, Volume6. I don't really see any other red flags in your approach, 
and I do think taking the absolute value of your air flow calculation 
would be sufficient to maintain the correct balance.

Best regards,

Matt Duffy

----- Original Message -----
From: "" <knut.erik.enerstvedt at niva.no> 
Sent: Mon, October 26, 2009 10:04
Subject: [TRNSYS-users] Modelling a greenhouse thermal screen by using 
aninternal window in Type 56


Hello TRNSYS users, 

I am modelling a horizontal thermal screen in a greenhouse by using an 
internal (ADJACENT) window between to zones in Type 56, one below and one 
above the window (screen). The "window" is a virtual window with no 
glazing and no frame to let all the radiation through to the lower zone 
when the screen is open. I have defined an internal shading factor 
controlled by an input variable to model the effects when the screen is 
opened or closed. In the Window Type Manager, I wonder about the following 
settings: 

Optional Properties of Shading Devices: 

Additional Thermal resistance: Can I use this to set the thermal 
properties of the screen? Will it be effective only when the shading 
factor is greater than zero? 
 Reflection Coefficient of Internal Device: How should I set this? 

Convective Heat Transfer Coefficient of Window: Should I use 0 (direct 
contact) when the screen is open and 11 (inside value) when it is fully 
closed? 

 I also have a question about coupling of air flow between the upper and 
lower zones. I use an equation based on the degree of screen opening, 
temperature difference and density difference to calculate air flowrate 
across the screen. The direction of flow depends on the level of 
temperature and/or density in the two zones. How can I ensure air balance 
between the adjacent zones? Can I take the absolute value of this air 
flowrate as air flow inputs into BOTH zones, that is the air flow from 
lower to upper zone equals the air flow from upper to lower zone? 

All replies are higly appreciated. 

Thanks, 
Knut Erik
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