Age-of-Air

The age of air is the mean time taken for air molecules arriving within the domain to travel to the point of interest.

The calculation of the local mean age-of-air in DesignBuilder CFD is made by solving an additional partial differential equation. The equation is derived from the contaminant concentration equation (a ‘passive scalar’ equation because it does not react with the velocity field). The assumption for the calculation of age-of-air is that the production of contaminants throughout the room is uniform:

This equation uses the results from the velocity distribution and the distribution of turbulent viscosities throughout the domain. Because the distribution of theta does not affect the velocity field, the age-of-air calculations are conducted after the main simulation.

Local Air Change Effectiveness (ACE)

ASHRAE states that “One common definition of air change effectiveness is the ratio of a time constant to an age of air”. The equation used by DesignBuilder CFD for ACE is:

ACE compliance criteria refers to ASHRAE Standard 129. This standard describes a method for measuring the ACE of mechanically ventilated spaces and buildings. The standard gives the following equation for the time constant,

where:

τ_N = nominal time constant,

m = exhaust air stream index,

Q_ex = exhaust air stream flow rate,

A_ex = age of air at the exhaust.