Local mean age of air: Predictive techniques compared

Abstract

Local mean age, which is a very useful indicator of air change efficiency, may be predicted at design stage by post-processing the results of a computational fluid dynamics (CFD) analysis that has been used to establish the internal air movement characteristics. The local mean age may be determined by a number of procedures. In this paper the use of the zonal method, which is very efficient in terms of computer power, is investigated and compared with alternatives based on the age of air transportation equation and the measurement of contaminant concentration. It is concluded that the effect of turbulent diffusion on air distribution patterns is very slight, with the implication that the zonal method provides an extremely efficient means of assessing the air change efficiency within ventilated buildings. It is also concluded that the type and locations of objects within a space are as important as the method of introducing the air supply in determining the room airflow patterns and local air change efficiency.

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