Flow Properties and Passive Scalar Transport in a Model Room With Relevance to Ventilation Efficiency

This paper describes an experimental investigation of air flow patterns and related passive scalar transport inside a model room with emphasis on the statistical time-scales of the dispersion process and the local ventilation effectiveness. Time-averaged and instantaneous structures of the turbulent flow were examined by local measurements inside a negatively pressurized cubic enclosure of 2 m side dimension and ventilated through single-supply and exhaust registers at an extract flowrate of 1.81 m³/min with a corresponding air change of 13.6 per hour. Sulphur hexafluoride was introduced at the centre of the enclosure and used to mark the flow. The distributions of its time-averaged concentration inside the room were used to deduce the degree of mixing and passive scalar transport.

The measurements of the time-averaged flow field showed that the arrangement with no sources of forced internal mixing resulted in the ventilating flow occurring in a channel connecting the supply and exhaust. Outside this jet-like main stream and particularly in corners, the flow stagnated with high concentrations of the tracer gas in these regions.

Time variations of the instantaneous local concentration values at different locations inside the room for a process of purging following steady injection were used to estimate the probability density function of the local residence age whose shape provided further experimental evidence of the prolonged dilution due to pollutant fluid which entered stagnation zones. The results also showed that the statistical time-scales of the dispersion and the mixing efficiency are independent of the level of the steady pollutant generation.

The ratio of the mean age to its standard deviation is recommended as a local mixedness factor and comparison with the mixing efficiency defined on the basis of integral time-scales and the exponential decay model revealed that it represents better the local variations, which is essential to assessing local indoor air quality. The extent of validity of the exponential decay model was also evaluated by comparing the time constant of this model with model-independent integral time-scales and it is shown that the underlying assumptions of the exponential decay model precludes the representation of spatial gradients.