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The distribution of airflow in a residential building varies with the periodic operation of a heating, ventilating and airconditioning (HVAC) system. Depending on the HVAC fan operation, mixing airflow (fan ON) or stratified airflow (fan OFF) occurs in the space. The objectives of this study are 1) to examine the time needed for room air to stabilize after a central ventilation fan turns ON/OFF and 2) to evaluate how the difference in distribution of gaseous and particulate pollutants in the space depends on fan operation mode. In the study, experiments measured the spatial distribution of airflow and pollutant concentrations in a full-scale environmental chamber. The measured data then provided the basis to establish a reliable computational fluid dynamics (CFD) model, which investigated the concentrations of gaseous and particulate pollutants depending on the mechanical fan operation mode. The results indicate that the transition between mixing flow and stratified flow occurs in a time scale of seconds, implying that the airflow in residential buildings is primarily mixing or stratified flow. The results show little spatial variation of tracer gas and particles in the mixing flow regime, whereas larger temporal and spatial variations are present in stratified flow. Additionally, the variations in particle concentrations are higher than those in gaseous concentrations. In certain areas of the room the particle concentration with stratified flow is up to thirty times higher than that with mixing flow, implying a high potential of exposure to particles when the fan is OFF.

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Citation: ASHRAE Transactions, vol. 114, pt. 2, Salt Lake City 2008