MIXING OF A POINT-SOURCE INDOOR POLLUTANT BY FORCED-CONVECTION

The mixing rate of pollutants emitted from indoor sources influences t he effectiveness of pollutant removal by building ventilation and the potential variability of exposure for a given release scenario. Quanti tative information is scant on the mixing rate and the factors that go vern it. We present mixing data for an instantaneously released tracer gas, carbon monoxide, in a sealed, unoccupied room under a range of f orced airflow conditions, in which the flow is induced by blowers. The resulting mixing times, from 2 to 42 minutes, are related to the mech anical power of the air jets produced by the blowers. Mixing times are found to correlate well with the inverse of the cube root of power, i n accordance with theoretical predictions and experimental observation s for mixing in chemical reactors. The exposure index, defined as the time-averaged concentration at a point relative to the time-averaged c oncentration for the room as a whole, is presented for three experimen tal conditions, yielding quantitative information on the appropriatene ss of the well-mixed hypothesis under various flow conditions. In gene ral, the exposure period following instantaneous release of a point-so urce pollutant must be much greater than the mixing time for the assum ption of uniform mixing to hold. The correlation between mixing time a nd power input is used to predict the mixing time from the mixing acti on of a supply air jet for a typical ventilation scenario. The predict ed mixing time, tau(mix) similar to 7 min, is substantially lower than the time scale for removal by ventilation, tau(vent) similar to 48 mi n. Under these conditions, complete mixing of an instantaneous release , point-source pollutant would be approximately attained within the in terior space well before the pollutant would be thoroughly removed by ventilation.