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Natural ventilation using ZoneVentilation:WindandStackOpenArea

asked 2016-10-31 10:44:30 -0500

anaijazi gravatar image

updated 2016-10-31 14:51:21 -0500

Hi,

I made a simple shoebox model to check the modeled air flow using the ZoneVentilation:WindandStackOpenArea. The model is a single zone, 10 m x 10 m x 3m for a zone area of 100 m2 and a zone volume of 300 m3. There is one window on the south elevation that is 50% of the facade, for a window area of 15 m2. The operable fraction is constant and set to 1. For this test, I set the minimum indoor and outdoor temperatures to -100 C and the maximum indoor and outdoor temperatures to 100 C (i.e. no natural ventilation controls). The maximum wind speed is 40 m/s and is not reached by any of the weather files I tried. There is also no zone infiltration for this exercise.

According to the InputOutputReference entry on the ZoneVentilation:WindandStackOpenArea object, the volumetric airflow rate is a product of the opening effectiveness (when set to autocalculate ranges from 0.3 to 0.55 depending on wind direction), the opening area, the opening area fraction, and the local wind speed.

From the hourly output for Environment:Site Wind Speed and Environment:Site Wind Direction I calculated the opening effectiveness and the volumetric flow rate from the formula referenced to in the InputOutputReference for a window area of 15 m2 and an effective angle of 180 (for the south elevation). Will call this the predicted volumetric flow rate.

I compared this prediction to the output from ZONE:Zone Ventilation Air Change Rate which I converted to a volumetric flow by multiplying by the zone volume and dividing by 3600 s/hr. Will call this the modeled volumetric flow rate.

I find that the predicted volumetric flow rate is 5-10x higher than the modeled volumetric flow rate in three different weather files (Lisbon, Portugal, Boston, Massachusetts, and Miami, Florida).

Would love some expert insight into why there is a discrepancy - am I really comparing apples-to-apples here?

Thanks, Arfa

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Did you calculate the predicted volumetric flow rate at one time step or a period of time? The wind speed and wind direction may vary hour by hour. Some old weather files came from the local airport weather forecast which reported once in three hours.

TianZhichao gravatar image TianZhichao  ( 2016-10-31 19:58:58 -0500 )edit

I calculated the predicted volumetric flow rate for each hourly time step using hourly outputs for wind speed and wind direction.

anaijazi gravatar image anaijazi  ( 2016-10-31 22:15:46 -0500 )edit

Check your calculation process once more, because it is less likely E+ was wrong. You can also use the more sophisticated method AirflowNetwork to model the natural ventilation and it is easy for box building.

TianZhichao gravatar image TianZhichao  ( 2016-11-01 21:26:22 -0500 )edit

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answered 2016-11-02 08:39:11 -0500

Archmage gravatar image

I would speculate that you didn't adjust wind speed for height above ground. The weather file wind data is usually for a 10 m height above ground. When apply this model, EnergyPlus will adjust this speed for each zone in the model based on the height of the zone's centroid, 1.5 m in your case. The setting of Terrain in the Building object has an impact on this adjustment -- see the section in Engineering Reference called Local Wind Speed Calculations

You also don't mention how you dealt with the stack part of this model, so that could be a factor in your comparison.

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Adjusting for local wind speed resolved much of the discrepancy. Thanks for the information! I did not include an object for calculation of stack effect

anaijazi gravatar image anaijazi  ( 2016-11-03 11:09:40 -0500 )edit

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Asked: 2016-10-31 10:44:30 -0500

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Last updated: Nov 02 '16