Best way to model interzone airflow through Air Walls with AFN?

asked 2020-09-02 17:23:39 -0600

saeranv
407 ●4 ●8

updated 2021-04-05 09:49:32 -0600

This can be further broken down into two questions, first, what is the best way to model interzone airflow through Air Walls with the EnergyPlus AirflowNetwork, and secondly, (assuming cracks are a good way to achieve interzone air flow) how can I derive an appropriate flow coefficient for this scenario?

1. What is the best way to model interzone airflow through Air Walls with the AFN?

The EnergyPlus 'Tips and Tricks' section on Air wall, Open air connection suggests using a large vertical opening between zones with the AFN, which seems to realistically capture both the volume and two-way flow of air between two zones open to each other. However I have since learned that EnergyPlus does not support using the SimpleOpening with BuildingSurfaces (only with FenestrationSurfaces). Our team would like to use the Air Wall, rather then a FenestrationSurface, to capture radiant exchange between zones.

The next best strategy seems to be to model the airflow using an extremely leaky crack. There seems to be some precedent for this, referenced in the DetailedOpening Input/Output documentation, which suggests using an AFN Crack with a "large air mass flow coefficient" for large horizontal openings, since regular horizontal openings won't model two-way flow in exterior surfaces. While this method won't capture the two-way flow that occurs in the AFN SimpleOpening, its seems like I should be able to reproduce the volume (with a correctly chosen flow coefficient), while maintaining the radiant exchange properties of the Air Wall.

2. How to derive a Cq?

If the second strategy seems reasonable, the second question is: what is the best approach to derive a mass flow coefficient that would replicate the air flow quantity of a large-ish vertical opening? My instinct is to try and approximate the mass flow coefficient from the formula used for the surface EffectiveLeakageArea?

The derivation I have in mind is as follows:

The crack formula is: 

m = Cq * dP^n 

where: 

m_crack = mass flow rate kg/s 

Cq = mass flow coefficient 

dP = pressure difference across crack 

n = mass flow exponent

The ELA formula is: image description

So to derive the Cq from the ELA formula:

m_crack = m_ela  

Cq * dP^n = ELA * Cd * sqrt(2 * density) * dP_r^(0.5 - n) * dP^n 

The dP^n cancels out, I assume n = 0.5 (for turbulent flow) which makes dP_r(0.5-n) = 1, and rearrange to get: 

Cq = ELA * Cd * sqrt(2 * density)

So I plug in a large opening area for the ELA parameter, and some assumption for air density to get the Cq for the interzone crack. I realize I can use the Surface:ELA object to achieve the same result, but we've already programmed infiltration into our software using the AFN Crack method, so would like to avoid introducing a new EP object, if possible.

Please let me know if this seems reasonable, of if there's a better way of modeling interzone air mixing with an Air Wall. Thanks in advance.

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answered 2020-09-03 15:31:59 -0600

Jason DeGraw

2208 ●6 ●10 https://github.com/jasondegraw

Good questions. Here's what I would do:

This one, I think, is a shortcoming of E+. Air walls are a new addition, and I think there is a good argument for at least allowing the SimpleOpening object to be used. If you put in an issue on the E+ repo it can be looked into. I'd argue that this is potentially a bug, and those can often be dealt with quicker. The next best thing (for now at least) would be to use one of the power law objects. Those are usually used for smaller openings, but can also be used for large openings if you are careful and two way flow is not called for. With larger temperature gradients, one-way flow is not very realistic.
It's possible to convert between the various power law forms as you are doing, but be careful with the reference conditions. The crack form does includes corrections to account for non-reference conditions (at reference conditions the values are 1), so keep that in mind if you are doing anything with the reference conditions. I'd think it would be easier to just use the ELA object (as you mention), but this sort of approach will also be OK.

Basically, what you're doing is probably OK in a lot of situations. The hard part is going to be telling when it is not OK. There is some literature on this, if that's of interest I can see if I can find some of that. I'd definitely run some simulations and see if the airflow results are reasonable.

Comments

Thanks for the answer!

It would be great to capture the two-way flow, and radiant exchange through an Air wall. Happy to add an issue to the E+ repo for you guys to look over.
Yes good point, I notice there is a reference temperature parameter in the EP crack equation.

saeranv ( 2020-09-03 16:24:53 -0600 )edit

And I'm definitely interested in further literature on the subject. Thus far, my key sources have been:

Sherman, M.H., and D.T. Grimsrud. Measurement of Infiltration Using Fan pressurization and Weather Data. Energy and Environment Division, Lawrence Berkely Laboratory. University of California. 1980.
Hutcheon, Neil B., and Gustav O.P Handegord. Building Science for a Cold Climate.
Standard Test Method for Determining Air Leakage Rate by Fan Pressurization. ASTM International. Designation: E 779-03. 2003.
EP documentation.

saeranv ( 2020-09-03 16:27:51 -0600 )edit

Section 2.2.2 of COMIS Fundamentals is "Air Flow through Large Openings", and while its references are older there are a lot there. The detailed opening model described there is included in EnergyPlus. The CONTAM (and AIRNET) documentation give descriptions of the simpler model that is included in EnergyPlus. I believe CONTAM now uses a computed condition to determine if two-way flow is warranted rather than what is done in E+. In any case, starting with the the references in these documents I think you'll find a lot.