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.

中文翻译：

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室内环境中存在回流和再循环气流的局部区域污染物回流特性

供暖、通风和空调 (HVAC) 系统通常供应空气，它是来自室外环境的新鲜空气和通过通风管道从房间返回的空气的混合物。这种空气减少了使用室外空气进行空调的热负荷和成本影响。空调系统中室内空气的这种再循环在节能方面是合理的；然而，由于受污染的室内空气的再循环，空气质量的恶化可能是一个问题。在这里，我们数值研究了当 HVAC 系统中再循环空气的混合比发生变化时，污染物再循环/回流对室内环境中局部污染物浓度分布形成的影响。我们讨论了在简化房间模型中使用各种室内通风效率指标的局部污染物浓度分布形成机制的详细结构。其中，访问频率和净逃逸概率是直接辅助评价室内污染物再循环/回流特性的指标。结果，当再循环的空气比例变大时，污染物返回目标局部域的数量、访问频率呈指数增长，净逃逸概率——直接表示污染物从目标排出的概率。域——接近于零。访问频率和净逃逸概率是直接帮助评估室内污染物再循环/返回特性的因素。结果，当再循环的空气比例变大时，污染物返回目标局部域的数量、访问频率呈指数增长，净逃逸概率——直接表示污染物从目标排出的概率。域——接近于零。访问频率和净逃逸概率是直接帮助评估室内污染物再循环/返回特性的因素。结果，当再循环的空气比例变大时，污染物返回目标局部域的数量、访问频率呈指数增长，净逃逸概率——直接表示污染物从目标排出的概率。域——接近于零。