<br><font size=2 face="sans-serif">Dear Matt,</font>
<br>
<br><font size=2 face="sans-serif">Thanks for replying to my posting.</font>
<br>
<br><font size=2 face="sans-serif">I have done some testing of my approach
to modelling the thermal screen, and sometimes I get this warning message:
</font>
<br>
<br><font size=2 face="sans-serif">"Small internal timesteps (<0.1%
of simulation timestep) used for humidity calculations due to large coupling
air flows between zones!"</font>
<br>
<br><font size=2 face="sans-serif">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?</font>
<br>
<br><font size=2 face="sans-serif">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?</font>
<br>
<br><font size=2 face="sans-serif">Thanks again for your help.</font>
<br>
<br>
<br><font size=2 face="sans-serif">Best regards,</font>
<br><font size=2 face="sans-serif">Knut Erik </font>
<br>
<br>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">From:</font>
<td><font size=1 face="sans-serif">"Matt Duffy" <duffy@tess-inc.com></font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">To:</font>
<td><font size=1 face="sans-serif">knut.erik.enerstvedt@niva.no, trnsys-users@engr.wisc.edu</font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Date:</font>
<td><font size=1 face="sans-serif">10/29/2009 12:29 PM</font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Subject:</font>
<td><font size=1 face="sans-serif">Re: [TRNSYS-users] Modelling a greenhouse
thermal screen by using aninternal window in Type 56</font></table>
<br>
<hr noshade>
<br>
<br>
<br><font size=3><br>
Dear Knut Erik,<br>
<br>
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.<br>
<br>
Best regards,<br>
<br>
Matt Duffy<br>
</font>
<br><font size=3><b>----- Original Message -----<br>
From:</b> "" <knut.erik.enerstvedt@niva.no> <b><br>
Sent:</b> Mon, October 26, 2009 10:04<b><br>
Subject:</b> [TRNSYS-users] Modelling a greenhouse thermal screen by using
aninternal window in Type 56<br>
</font>
<br><font size=2 face="sans-serif"><br>
Hello TRNSYS users,</font><font size=3> <br>
</font><font size=2 face="sans-serif"><br>
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:</font><font size=3>
<br>
</font><font size=2 face="sans-serif"><br>
Optional Properties of Shading Devices:</font><font size=3> <br>
</font><font size=2 face="sans-serif"><br>
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?</font><font size=3> </font><font size=2 face="sans-serif"><br>
Reflection Coefficient of Internal Device: How should I set this?</font><font size=3>
<br>
</font><font size=2 face="sans-serif"><br>
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?</font><font size=3> <br>
</font><font size=2 face="sans-serif"><br>
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?</font><font size=3> <br>
</font><font size=2 face="sans-serif"><br>
All replies are higly appreciated.</font><font size=3> <br>
</font><font size=2 face="sans-serif"><br>
Thanks,</font><font size=3> </font><font size=2 face="sans-serif"><br>
Knut Erik<br>
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