Accessibility: A New Concept to Evaluate Ventilation Performance in a Finite Period of Time

A new concept of ‘accessibility’ is presented that considers the influence of supply air or of a contaminant inside ventilated spaces within a finite period of time. Two new indices—accessibility of supply air (ASA) and accessibility of contaminant source (ACS)—are defined to evaluate the relative effect of spread of supply air or contaminant in a given period of time. The two new indices in a two-dimensional ventilated room have been numerically analysed to demonstrate the characteristics of the new concept. It is shown that accessibility increases with the increase of time. For emergency ventilation, people usually care about accessibility in a short period of time, which is usually less than twice the nominal time constant in the ventilated room. The analysis indicates that accessibility over such a period of time is very different from that over an ‘infinite’ period, i.e. the steady state case. The ASA and ACS scale influence the supply of air or contaminant in each zone quantitatively and thus they can be used to guide ventilation strategy, especially when an emergency occurs.