The
visitation frequency (VF) proposed by Csanady [11] basically and
developed by Sandberg conceptually in ventilation index represents
how many times a contaminant particle enters/returns a
target domain. The average value of the VF in a target domain
indicates the ventilation efficiency for contaminant particles
removed from the domain, taking into consideration return and
recirculation of contaminant particles. The local purging flow rate
(L-PFR) introduced by Sandberg and Sjöberg [4] as a measure of
ventilation effectiveness for indoor air represents the net airflow
rate required to purge or remove a contaminant from a local
domain. L-PFR is the airflow rate that determines the average
concentration of the contaminant in a local domain. In contrast
to the flow rate obtained by integrating the velocity crossing a
surface, L-PFR is simply defined by the amount of a contaminant
generated and the average concentration of that contaminant in
the local domain within a room [12].