Abstract
Heating, ventilating, and air‐conditioning (HVAC) systems usually supply air, which is a mixture of fresh air from the outdoor environment, and return air from rooms via the ventilation ductwork. This air reduces the heat load and cost impact of air conditioning using outdoor air. This recirculation of room air in air‐conditioning systems is reasonable in terms of energy saving; however, the deterioration of air quality might be a concern because of the recirculation of contaminated room air. Here, we numerically investigate the effect of pollutant recirculation/return on the formation of concentration distributions of local pollutants in indoor environments when the mixing ratio of recirculated air in the HVAC system changes. We discuss the detailed structure of the formation mechanism of local pollutant concentration distributions using various indices for indoor ventilation efficiency in simplified room models. Among the indices, visitation frequency and net escape probability are the ones that directly assist in evaluating the recirculation/return characteristics of indoor pollutants. As a result, when the proportion of air that is recirculated becomes large, the number of pollutants returning to a target local domain, the visitation frequency, increases exponentially, and the net escape probability—which directly expresses the probability of pollutant discharged from the target domain—is close to zero.
Highlights
The rate of outdoor airflow required to maintain a certain level of indoor air quality is generally determined using two factors: the indoor threshold concentration, which is used as an index/guideline for ventilation design, and the amount of pollutants generated in the target indoor space
The indoor pollutant concentration field in the presence of “return air” in the HVAC system is determined by the uniform pollutant concentration owing to the existence of return air and the non-uniform concentration distribution formed in the room by heterogeneous pollutant sources and the non-linear flow pattern produced when there is zero return air
When considering indoor ventilation efficiency, especially when discussing the formation of a concentration field focusing on the recirculation of pollutants by an HVAC system with return air, comprehensive evaluations of the formation structure of the pollutant concentration distribution and transport mechanism are essential because the proportion of return air has a dominant impact on indoor pollutant concentration levels
Summary
The rate of outdoor airflow required to maintain a certain level of indoor air quality is generally determined using two factors: the indoor threshold concentration, which is used as an index/guideline for ventilation design, and the amount of pollutants generated in the target indoor space. When the proportion of return air is zero, i.e., all fresh air/outdoor air is being conditioned, a non-uniform concentration distribution is formed by pollutants that are heterogeneously distributed, and a non- uniform flow pattern is formed in the room. The indoor pollutant concentration field in the presence of “return air” in the HVAC system is determined by the uniform pollutant concentration owing to the existence of return air and the non-uniform concentration distribution formed in the room by heterogeneous pollutant sources and the non-linear flow pattern produced when there is zero return air. When considering indoor ventilation efficiency, especially when discussing the formation of a concentration field focusing on the recirculation of pollutants by an HVAC system with return air, comprehensive evaluations of the formation structure of the pollutant concentration distribution and transport mechanism are essential because the proportion of return air has a dominant impact on indoor pollutant concentration levels
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