EnergyPlus: how to model external Virtual Srf/Air Wall

asked 2018-01-23 07:59:55 -0600

MG
31 ●1

updated 2018-01-23 09:34:21 -0600

Hello, I'm trying to model a vertical vented cavity in EnergyPlus 8.8 using various thermal zones and an airflow network. I would like to use the first thermal zone (zone 1 in figure) as a "fake" zone to control the inlet volume. That is, the real inlet grid is located on top of zone 1 and the actual cavity starts from zone 2.

Figure:https://drive.google.com/file/d/1ai11...

My question is about the vertical wall in zone 1 (highlighted in blue in figure). I would like that vertical surface to be essentially inexistent, it should transmit short and longwave radiation and not participate in convective/conductive exchange between zone 1 and the exterior. I have already set that surface to be always open for the airflow model. The best answer would be to use an IRT (Infrared Transparent) material, but it can only be applied to internal surfaces beween two zones: this should be an external surface instead. Other options could be to use something like a Virtual surface or an Air Wall but I don't know how to model them directly in EnergyPlus (editing the IDF).

Any suggestion is welcome, thank you very much.

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In the physical reality that you are attempting to model, what parts of zone 1 exist? Is it only the back surface (aligned with the building facade)? Are the bottom surface and side surfaces physically present? The inlet grid (top surface of zone1 / bottom surface of zone 2) is obviously physically present. I am wondering if you need to worry about modeling the surface behaviors at zone 1 - could you instead enforce an inlet condition at the zone 1 node? For example, set zone 1 temp equal to outside air temp?

Molly Curtz ( 2018-01-24 11:16:12 -0600 )edit

Thank you very much for your answer. In zone 1 the bottom, side and back surfaces are all physically present, only the blue one should be "virtual". I don't need to model the blue surface behaviour itself, so your idea could be very interesting. Do you have any suggestion about how to set up custom inlet conditions in zone1?

MG ( 2018-01-26 03:42:40 -0600 )edit

My concept was get the OSA temp at the Zone 1 node. You could try to do that by either increasing the air change rate in Zone 1 such that the air change volume will overwhelm any other effects, for example conductive exchange between zone 1 and exterior between the virtual surface, or maybe by using EMS to set conditions at Zone 1? I don't know if you are worried about say radiation from the back wall to the air mass in zone 1. If you want a more detailed approach - maybe it is better to try to model the "virtual surface" as a window, so it will transmit light, and adjust the properties

Molly Curtz ( 2018-01-26 13:05:22 -0600 )edit

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answered 2018-01-23 12:55:45 -0600

Avi
4320 ●2 ●11

I believe you want to use SurfaceProperty:ExteriorNaturalVentedCavity which does exactly what you are aiming to do. You can read about it in the Engineering Reference and in the Input Output Reference.

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Thanks for your answer. Yes, I have already tried that, but I want to create my own specific model in this case. Anyway, my point is not how to model the vented cavity; rather, I am wondering how to set up the material of the external wall (the blue one in figure). As said before, I need it to be something similar to a IRT surface / Air Wall / Virtual surface, but for external surfaces (separating a thermal zone from the outside, just to be clear). Do you think this is possible in EnergyPlus?

MG ( 2018-01-24 07:41:51 -0600 )edit

@MG aren't Material's properties enough? You can control Thickness {m}, Conductivity {W/m-K}, Density {kg/m3}, Specific Heat {J/kg-K}, Thermal Absorptance, Solar Absorptance, Visible Absorptance