[TRNSYS-users] TRNBLD - active layer - Thickness of adjacent layers

Tue Dec 13 04:29:33 PST 2011

Dear Leen, dear TRNSYS users,

Thanks for your quick reply.

However, I'm not really satisfied with your suggestion, as I want to understand the reason behind it.

If you are right, I violated the requirement "outside diameter over pipe distance < 0.2". But, this requirement is completely independent from the layer thickness of the adjacent layer, isn't it?

On the other side, if both adjacent layers must be conform to the minimum thickness of 0.3 times pipe spacing, I would have caused a violation of this requirement by setting the pipe spacing to 24 cm and the layer thickness to 8mm. But for this, I didn't receive any warning.

Last but not least, there are oppositional specifications for the required ratio between thickness of adjacent layer(s) and pipe spacing in the TRNBLD user manual. On page 5-186 the ratio should be greater than 0.3, on page 5-187 the ratio should be equal or smaller than 0.3 and on page 5-59 the ratio should be equal or greater than 0.3. So, nearly all possibilities are mentioned. I'm a bit confused.

Looking forward to some light into the dark,

Kind regards,

Johannes Schrade

Johannes Schrade
Abteilung Wärmetechnik

Fraunhofer-Institut für Bauphysik
Nobelstr. 12
70569 Stuttgart

johannes.schrade at ibp.fraunhofer.de <mailto:johannes.schrade at ibp.fraunhofer.de> 
Telefon 0711 / 970-3416
Telefax 0711 / 970-3399
www.ibp.fraunhofer.de/wt/ <http://www.ibp.fraunhofer.de/wt/> 

Johannes Schrade
Department Heat Technology 

Fraunhofer Institute for Building Physics
Nobelstr. 12
70569 Stuttgart, Germany

johannes.schrade at ibp.fraunhofer.de <mailto:johannes.schrade at ibp.fraunhofer.de> 
Phone +49 711 / 970-3416
Fax +49 711 / 970-3399
www.ibp.fraunhofer.de/wt/ <http://www.ibp.fraunhofer.de/wt/> 

Von: leen peeters [mailto:l.f.r.peeters at gmail.com] 
Gesendet: Montag, 12. Dezember 2011 22:09
An: Schrade, Johannes
Cc: trnsys-users at cae.wisc.edu
Betreff: Re: [TRNSYS-users] TRNBLD - active layer - Thickness of adjacent layers

Johannes,

as soon as you decrease the distance between the pipes, you come closer to the limits set regarding the modeling of the heat transfer in the direction of the water flow (direction z in the mathematical model). The stability of that model requires that you achieve a certain boundary condition, which is a.o. function of the thermal resistance of the water-pipe-inter pipe distance. In order to ensure these boundary conditions are satisfied the ratio of pipe outside diameter over pipe distance < 0.2 AND the ratios of layer thickness over pipe distance > 0.3. The 0.024-value you received for the minimum layer thickness comes from this limitation. I assume the extra measure is invoked once you come close to the 0.2-limit.

As soon as you take 0.1 for the pipe distance, you can again select 0.008 m for layer thickness ...

Playing around with conductivity or capacity does not help, as the measure seems to be implemented through the above limits only ....

Leen

On Mon, Dec 12, 2011 at 1:48 PM, Schrade, Johannes <Johannes.Schrade at ibp.fraunhofer.de> wrote:

Dear TRNSYS users,

I have some problems with the active layer module of TRNBLD, which I used for modeling a floor heating system.

The active layers, which should be displayed in the model, are defined as following:

Layer 1:

·         Pipe spacing: 24 cm

·         Pipe outside diameter: 16 mm

·         Pipe wall thickness: 2 mm

Layer 2:

·         Pipe spacing: 8 cm

·         Pipe outside diameter: 16 mm

·         Pipe wall thickness: 2 mm

By specifying the thicknesses of the layers adjacent to the active layers, the following curiosity appeared.

For case 1, I set the thickness of the layer above the active layer to 73 mm, which is greater than 0.3 times the pipe spacing, and I set the layer below to 0.5 times the pipe outside diameter (requirements taken from manual TRNSYS 17, 5-187, table). This step had worked pretty good and the simulation was running without any problems.

In a second step, I revised the model and included the second active layer into the TRNBLD model. As the floor screed should be identical for both cases, I tried to define the thicknesses of the adjacent layers the same way as I did it in case 1. But when I tried to reduce the thickness of the adjacent layer below, the following warning appeared: "thickness must be in the range of 0.024 to 1.0".

In order to understand this warning I had a closer look to the manual of TRNBLD. On page 5-58 (TRNSYS 17) I found a remark that both layers adjacent to the active layer must be greater than 0.3 times pipe spacing. This would explain, why the warning appears, but I do not understand, why the first case should be unproblematic, as the required minimum thickness in this case should be 72mm and why this is not mentioned in the mathematical description.

Does anyone has an idea, what could be the reason behind that, or might this be a known or unknown bug? If this requirement dependent on the pipe spacing should be really used for both layers, this would mean that extremely thick floor screeds of +15cm are necessary to model a floor heating system with a pipe spacing of 24 cm. This sounds absolutely unrealistic. 

Hoping for some help,

Kind regards,

Johannes Schrade

Johannes Schrade
Abteilung Wärmetechnik

Fraunhofer-Institut für Bauphysik
Nobelstr. 12
70569 Stuttgart

johannes.schrade at ibp.fraunhofer.de <mailto:johannes.schrade at ibp.fraunhofer.de> 
Telefon 0711 / 970-3416
Telefax 0711 / 970-3399
www.ibp.fraunhofer.de/wt/ <http://www.ibp.fraunhofer.de/wt/> 

Johannes Schrade
Department Heat Technology 

Fraunhofer Institute for Building Physics
Nobelstr. 12
70569 Stuttgart, Germany

johannes.schrade at ibp.fraunhofer.de <mailto:johannes.schrade at ibp.fraunhofer.de> 
Phone +49 711 / 970-3416 <tel:%2B49%20711%20%2F%20970-3416> 
Fax +49 711 / 970-3399 <tel:%2B49%20711%20%2F%20970-3399> 
www.ibp.fraunhofer.de/wt/ <http://www.ibp.fraunhofer.de/wt/> 

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