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Introduction
The new Bannerman Road school in Bristol has been designed with a high proportion of glazing to allow as much natural light to penetrate into the occupied areas as possible. The downside of this strategy is the increased risk of over-heating in summer.
Flow Analysis were commissioned to predict the conditions within the school, with reference to a target dry bulb temperature of 26 degrees for external conditions of 21 degrees. If possible, it was hoped that the use of mechanical ventilation could be avoided.
The assessment was carried out using both dynamic thermal simulation and computational fluid dynamics. The former was used to obtain a picture of the hour-by-hour annual temperature variation, and the latter was used to examine the air flow in detail to assess the effectiveness of the proposed openings.
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Dynamic Thermal Simulation
A model of the entire school was built in the dynamic thermal simulation software. Appropriate building fabrics were taken from the software database and time-varying heat loads were added for occupants and equipment gains. Appropriate schedules were used for occupancy and aperture opening. Finally, a Bristol weather data set was chosen to drive the simulation over a one-year time period.
Attention was focussed on the school hall and a typical classroom. An increased number of zones were used for the hall to increase the resolution. It was found that the resultant temperature in the hall and the classroom exceeded 25 degrees for 58 hours of the year and 45 hours of the year respectively. This is well within the CIBSE Guide(A) "Comfort criteria for naturally ventilated buildings".
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CFD Study - School Hall
Computational fluid dynamics (CFD) can provide a snapshot of the air flow and temperature at a particular time-instant. In this case, a time -instant with weather conditions close to the design conditions was chosen. This was a mid-summer day with high solar radiation and external air temperature of 21.1 degrees.
The above animation illustrates the paths taken by the air as it enters the hall through the various openings. The pathlines from the 2 internal doors (at the bottom and bottom-right on the figure) tend to find the shortest route to exit through the roof. However, the pathlines from the remaining external openings are more tortuous and illustrate a greater amount of mixing.
The rate of ventilation is very high (37 air changes per hour), and so the air temperature in the hall is mostly close to ambient. The resultant temperatures in the occupied zone (illustrated by the horizontal cut in the figure to the left), are just below the design conditions of 26 degrees.
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CFD Study - School Classroom
The same time-instant was chosen for the CFD model of a typical classroom. The air enters the classroom through an external door and two windows, and on the opposite side, through one internal door. The only exit is via a roof vent.
The animation to the right shows the path taken by the air as it enters the classroom through the various openings. It was found that there is only a weak flow of air through the internal door (at the far end), and this makes its way rapidly upwards to the roof vent. In contrast the air entering through the two windows takes a longer route illustrating a higher degree of mixing.
Overall, the resultant temperature in the classroom was also below the design limits in the occupied zone.
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