Paper Conference

Proceedings of BauSim Conference 2006: 1st Conference of IBPSA-Germany and Austria

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Bestimmung des durchschnittlichen windinduzierten Luftwechsels in einem Atrium

F. Gerder

Abstract: The air change in normal rooms (office, apartment) has been investigated in the past already extensively and was valued, so that in the planning stage of buildings a good evaluation of the air change to be expected can be carried out. These values are not transferable to atriums with different ventilation requirements. Atriums are often used as connecting passages in new representative office buildings. Nowadays neither secured knowledge about the seasonal course of the air change nor about the appearing middle values of a period exists for atriums. However, the knowledge of this size is necessary to achieve a realistic estimation of the air quality. To ascertain on which effect the air change is to be led back in an atrium, measurements in a real building and extensive simulation calculations were carried out within the scope of a research work [Gerder, 2006]. These investigations have shown that the influence of the wind-induced and the temperature-induced air change do not overlap additiv and in most of the times the wind effect leads to realistic results. Especially in case of natural ventilation the indoor air change is influenced by building geometry, surroundings, wind speed and wind direction. Statistical wind data are available for most locations in Germany. These form therefore a good data basis to the regulation of suitable boundary conditions in the planning stage. With the help of a diagram developed from the results, the average wind-induced natural air change can be determined for a certain building location with knowledge of the annual average of wind direction and wind speed. For the calculation of the wind induced air change the pressure difference between the ventilation openings is necessary. The determination of adequate pressure coefficients is the main problem in this connection. Pressure coefficients which were determined on closed facades lead under circumstances to the fact that the air changes are estimated too high, in particular if both the air inlet and the air outlet are influenced by wind pressure/ suction. In addition, the study proved that only the information of the air change is not enough to characterize the climate in a big space like an atrium, at least the air exchange efficiency should be determined. Beyond that the local age of air and the air exchange index allow a more precise analysis of the indoor air quality. The local air exchange index is influenced by the position of the air inlets and outlets as well as by wind direction and but is nearly independent of the wind speed. With knowledge of the air exchange index and the matching air exchange efficiency the local age of air can be determined in any point in the space for every air change. The flow simulation has turned out suitable planning aid for the investigation of indoor air flows. To reduce the computational calculation time, a combination of fluid dynamics and a calculation according to British Standard Method is sensible. The results can be clearly summarized in a diagram to specify the air change dependent on wind speed and direction specific for building.
Pages: 37 - 38

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