Butterfly 'heatTransfer' Recipe Input Clarification

[edpmay]

Hi all,

I’ve been working with Butterfly for a little bit for the first time and I have a clarification question I was hoping someone could help me with?

My intent with this model was to be able to look at an internal air temperature stratification simulation with the effect of downdraft due to a cold window surface influencing the result. I have one air input (supply air register with 30°C / 1.0 m/s air) coming into the space and an ‘open door’ at the rear wall for air exit/outflow with pressure set to 0. I was (I think) able to set up the model correctly and it all seems to be working ok so far.

My question is in regards to the 'heatTransfer’ solver and in particular the ‘temperature’ variable input. I see in the component notes that this is set to 300k by default, but the space I am trying to model is supposed to have an interior set point of 20°C interior air (293.15 K) with 22°C wall/floor/ceiling surfaces, so I am wondering if this ‘temperature’ variable is the appropriate place to input that default/starting air temp? I couldn’t quite understand from the reference materials:

OpenFoam.com/ExtendedCodeGuide

what this ‘T_ref’ variable is for? Though it is clear that it has a large effect on the output / results for the simulation.

Below is a quick view with the default (26.85 °C) input:

and then with 20 °C used as this ‘T_ref’ input (all other settings stay the same):

Just wondering about it and if I am understanding the setup here correctly and what this variable is supposed to be used for? Any thoughts / references or input are much appreciated!

oh, and just for reference in case it matters, this is using:

• Rhino V5 SR13 64-bit (5.13.60913.21340, 09/13/2016)
• Grasshopper 0.9.0076
• Butterfly Ver 0.0.04 / Component Nov 22, 2017

thanks for a great tool!
-Ed

[OlivierDambron]

hi @edpmay

I believe this discussion on github will answer your question https://github.com/ladybug-tools/butterfly/issues/203 .

Your results differ given the same number of Times ( in controlDict), your starting point was farther from the default Temperature , thus making it longer to converge.

What I usually do is a weighted average of surface temperature over their respective areas.

Also note that there is no Radiation model implemented yet in butterfly. For small temperature gradients up to 3-5 degC (if I’m not mistaken) this shouldnt cause issues, though larger than that the distribution of temperature will not be reliable. Have a look at openFoam tutorials with radiation models implemented and you could readapt the butterfly case manually.

Olivier

[edpmay]

Thanks Olivier, that is all extremely helpful and yes it does clear that up for me.

I will take a look at the radiation modules and see if it seems like something I can implement on this. The intent behind this model was to look at some seasonably high dT values for the glass surface (4-10°C dT) so likely would be impactful.

thank you very much,
-Ed