Total air age: an extension of the air age concept

Air age is an important index to evaluate indoor air quality in ventilated rooms. Current definition of air age is limited to indoor part, i.e., the air age at the inlets of air supply diffusers is assumed to be zero. This work extends the air age concept to the so-called “total air age” by considering the impact of air delivery process on air age. The air age in duct is solved using one-dimensional flow characteristic, which includes the mixing process between airflows with different ages. It is then coupled with the room air age which is solved using computational fluid dynamics technique. An iterative algorithm is proposed to calculate total air age in general ventilation systems and a direct method is further derived for systems with only one air handling unit. Two examples are given to demonstrate the calculation of total air age and the distributions of total air age are compared with those of traditional air age.