Interzone airflow between adjacent zones with open walls and dedicated AHUs

asked 2022-04-04 08:12:44 -0500

Mariam
65 ●1 ●6

updated 2022-04-07 02:09:59 -0500

I am simulating a large theater with multiple levels (vertically) and with an opening in the roof that makes about 30% of the roof area. The theater is served by multiple AHUs. I have divided the large space into subspaces (Zones) based on AHUs' serving areas, such that each Zone is served by an AHU. I created opening surfaces between the adjacent zones (can be horizontal and vertical based on the Zone location relative to adjacent ones). I want to model the interflow between the zones - given mechanical ventilation - that closely represents the real life scenario. I have experimented with both AirFlowNetwork model and the ZoneMixing and the results of the two are different. I read that AirFlowNetwork model is more complex and detailed, yet it does not work/provide accurate results for some scenarios (e.g., Air circulation and/or air temperature stratification within a thermal zone.). I could not decide which is representative, sufficient, and reliable to use for my case. Given my limited expertise with Energy+, I would like to ask if I should use AirFlowNetwork model or ZoneMixing will be sufficient for this application. Additionally, I would appreciate any feedback, ideas, or suggestion regarding my request.

* UPDATE *

Based on @Jason DeGraw's answer, I would like to ask if my strategy to approach this is reasonable. If ZoneMixing is used, I will need to include ZoneMixing objects in both directions in order to insure mass balance, correct? For example, assuming that Zone 1 and Zone 2 only interact which each other, I should have the following because ZoneMixing statement only affects the energy balance of the “receiving” zone and does not produce any effect on the “source” zone (Source: https://bigladdersoftware.com/epx/doc...). Hence, complementary mixing statements of zones with air exchange should be included while having Delta Temperature set to 0 in order for mixing to occur regardless of the relative zone temperatures.

ZoneMixing,
   Zone2_To_Zone1 ,          !- Name
   Zone1 ,                          !- Zone Name
   On 24/7 ,                               !- Schedule Name
   Flow/Zone ,                             !- Design Flow Rate Calculation Method
   10.00000 ,                                !- Design Level
   ,                                       !- Volume Flow Rate per Area {m3/s/m2}
   ,                                       !- Volume Flow Rate Per Person {m3/s/person}
   ,                                       !- Air Changes per Hour {ACH}
   Zone2 ,                          !- Source Zone Name
   0.0;                                     !- Delta Temperature

ZoneMixing,
   Zone1_To_Zone2 ,          !- Name
   Zone2 ,                          !- Zone Name
   On 24/7 ,                               !- Schedule Name
   Flow/Zone ,                             !- Design Flow Rate Calculation Method
   10.00000 ,                                !- Design Level
   ,                                       !- Volume Flow Rate per Area {m3/s/m2}
   ,                                       !- Volume Flow Rate Per Person {m3/s/person}
   ,                                       !- Air Changes per Hour {ACH}
   Zone1 ,                          !- Source Zone Name
   0.0;                                     !- Delta Temperature

Under the assumptions that (1) the HVAC system design insures thermal comfort, and (2) the paths of air exchange between zone are independent of the AHU's supply-return air paths, I am thinking of using the air velocity thermal comfort requirements (which is I think is a maximum of 2 m/s) knowing the cross-section area between the zones and air density to set the maximum air exchange rates between the zones (i.e., Design Level). Kindly let me ... (more)

answered 2022-04-05 11:39:09 -0500

Jason DeGraw

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

updated 2022-04-11 20:37:35 -0500

Based upon the description of what you're trying to model and your goal (close to representative of the real life scenario), you may be using the wrong tool. EnergyPlus can't simulate the air movement inside large volumes in the way that would be very accurate, but it there are possibly some ways to alleviate the problems. If you know how the air moves within the theater volume, either from observation or from more detailed simulation (computational fluid dynamics), then it is possible to set up a model that mimics that (with ZoneMixing, AirflowNetwork, or maybe the RoomAir model) but it is difficult to get EnergyPlus to compute those air movements for you. AirflowNetwork implements a pressure network that computes bulk airflows given proper inputs, but those inputs would very difficult to obtain for this case and would require either measurement or more detailed simulation. ZoneMixing allows the specifications of the flow, but I'm not sure where you would get that information without either measurement or more detailed simulation (unless you are less concerned about the details).

If you just want to make sure that the air in the theater is pretty well mixed, then ZoneMixing will do that for you (but you'll need to figure out the flow rates). For more detailed results than that would allow, you'll need a more detailed tool than EnergyPlus.

UPDATE

If you have done the CFD, then you should be able to post-process those results to get the total flows across the surfaces and some estimate of the pressures involved. This can be used to develop an AirflowNetwork that will approximate the movement of air for the conditions in your CFD simulation. If it works and produces the same flow pattern that the CFD does then that is reasonable and would allow for some variation of the conditions and still produce acceptable results. However, that's going to be a lot more work than than using ZoneMixing and/or ZoneCrossMixing and probably not really all that much better. For those all that's needed will be the flows from the CFD. Setting the flows from thermal comfort requirements is not a good idea, so I advise strongly against that. Use the flows from the CFD, otherwise you're just creating a set of well-mixed zones that could be treated as one zone.

As to whether this is all reasonable or a good strategy or not, that depends on how comfortable you are with the limitations. For larger volumes, gradients are just important and there's not much that can be done about it. If you can get results that make sense, then it is a defensible strategy as long as it is acceptable that you are mimicking the internal flow patterns instead of predicting them.

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Thank you Jason for your reply. I have already conducted a CFD analysis to identify air paths. I do not have enough information or data to model AirFlowNetworks. While for ZoneMixing, I will have to figure out realistic air exchange flow rates as no sensing devices are available to provide this information. I would appreciate it if you could give me your feedback on my strategy to model this problem as per my latest update based on your answer.

Mariam ( 2022-04-06 03:05:46 -0500 )edit

@Mariam If you've done the CFD, processing those results should give you the the information needed to create an AirflowNetwork or calculate the flow rates for ZoneMixing. I'll update my answer to reflect that.

Jason DeGraw ( 2022-04-07 16:02:06 -0500 )edit

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