Hi @chris,
I wanted to examine the air temperature stratification in an atrium (air temperature at select points across the atrium height). Is it possible for the Adaptive Map component to output the Air Temperature on these points similarly to how it outputs the Operative Temperature?
Thank you in advance!
Just to clarify, I need to use a stratification model that can output the Air Temperature at different grid points across the atrium height. So if this component considers the Air Temperature to be constant, could you please advise on the appropriate HB modelling method to use? Thanks!
EnergyPlus doesn’t model air temperature differences within individual Rooms/Zones and so neither do the thermal maps. Granted, EnergyPlus does a relatively decent job accounting for heat transfer that results from stratification (for example, using less resistive thermal coefficients for RoofCeilings than for Floors) and the EnergyPlus AFN can model buoyancy driven flow from room to room (as long as the flow is uni-directional as you might find between an atrium and its adjacent rooms). The thermal maps will also interpolate air temperature values across air boundaries so that you don’t get a harsh transition of air temperatures across a boundary that doesn’t actually exist.
I think I answered your question about the recommended say of doing this here:
Hi @chris, thank you for the explanations, much appreciated. I think I was not very clear in describing my problem:
I am currently interested in air stratification (vertical), only in a single zone that looks like in the image below, having one opening (blue outline) and a considerable height (12m). I am interested in reading the air temperature at the nodes you see, which are placed at different heights, just using constant air volume for ventilation, and no heating or cooling. So my focus is on air stratification within a single zone.
Could you please advise as to the most appropriate method to derive those Air Temperatures, and how to select a constant air volume system (with fixed supply air temperature). Again, thank you for your help and sorry if I confused you before!
As I said, the EnergyPlus AFN does not do this. So, if you are tying to estimate the 1-2 C difference that you might find from stratification like this, you’ll probably need to do some CFD, or maybe edit the IDF to use some of these other Rom Airflow models (eg. the Mundt model), or do some hand calcs.
For hand calcs, this PhD thesis put together a simplified formula from several CFD simulations using inputs of internal heat gains vs incoming flow rates:
https://dspace.mit.edu/handle/1721.1/74907
The formulas in section 8.3.2 could probably give you an estimate of the 1-2C differences you’d find from stratification.
But it’s important to keep in mind that this 1-2C difference from stratification is likely an order of magnitude smaller than the radiant temperature differences that you have in an atrium with an all-glass façade like that. So just remember not to mistake the trees for the forest.
Thanks for sharing the knowledge and resources!
Yes, as per the hand calcs, it seems that the air temperature differential is low.
I digged into the dissertation and into the forum, and it seems that you have actually implemented a method of it in HB Legacy, which involves natural ventilation (I used the E+ WindAndStackOpenArea model). The image below shows a result when using this HB Legacy workflow. So I now have 2 more questions:
1.Are there any limitations in this HB Legacy workflow with respect to the building physics that you would think are significant or worth noting?
2.Is there a plan for implementating this workflow in LBT 1.4.0 in the future?
I knew that this would come back to haunt me. Yes, the legacy thermal maps tried to use that model from that PhD thesis but it’s important to highlight that the model was only developed for single-story naturally ventilated rooms where the only inlet is an open window and the only outlet is close to the ceiling on the far side of the room from window.
The cases for which people tried to use this model in Legacy (like displacement ventilation systems or large naturally ventilated atriums with many inlets and outlets) were just too beyond the original use case for the results to be clearly more accurate than EnergyPlus’s default assumption that Room air temperatures are well-mixed. The stratification pattern is just going to be very different depending on where the inlet(s) and outlet(s) are and it’s rare to have them organized as they were in the PhD paper unless you are only studying a single-story naturally ventilated room. It also goes without saying that any celling fans or mechanical systems (except displacement ventilation) would actively combat the stratification and would make it virtually non-existent.
So, most of the time you are better off using the well-mixed assumption of EnergyPlus and there’s a good chance that this is closer to reality than what you see in the Legacy thermal maps. Or, if you know exactly what is going to drive the stratification in your model (as you would if you were designing a displacement ventilation system), you could use one of the suggestions that I noted above, either using the Mundt model in your EnergyPlus simulation, doing some CFD, or doing some hand calcs.
Thanks Chris!
Thank you for your explanations, I understand that this is not a one-method-for-all-problems, and that as a model it resembles a single-zone naturally ventilated space more so than other scenaria. I wanted to test this for inter-software comparisons and for learning purposes, and your remarks under this thread were really helpful, much appreciated as always!!
