Natural ventilation using ZoneVentilation:WindandStackOpenArea
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
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.
I calculated the predicted volumetric flow rate for each hourly time step using hourly outputs for wind speed and wind direction.
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.