Modelling and assessing ventilation efficiency in an imperfectly mixed ventilated air space.

To ensure indoor air quality an efficient ventilation system should provide fresh air in those parts of a room where it is required. To assess whether the ventilation system fblfils the main objective, different definitions of local ventilation efficiency (the local mean age of air, the local ventilation rate, the local purging flow rate and the local air exchange rate) are reported in literature. Tracer gas techniques (step up method, step down method and pulse method) and CFD-models are mostly used in research to identify the 3-D distribution of these ventilation parameters in a ventilated air space. In our laboratory the 'local volumetric concentration of fresh air flow rate' [m³/s.m³] was introduced as a new ventilation parameter. A total amount of 90 step up experiments was performed in a laboratory test installation with a volume of 9 m3 [3x2x 1.5m] to model the 3-D distribution of this ventilation parameter in relation to different values of the ventilation rate (120-300 m³/h) and the heat supply (0-400 Watt) as control inputs. In the main part of the paper some results are given of the modelled 3-D distribution of the 'volumetric concentration of fresh air flow rate' in the test installation. This distribution of fresh air flow rate is assessed in relation to the air flow pattern which is visualised by smoke experiments and quantified by image analysis and the Archimedes number.