# Analysis on the Exhaust Air Recirculation of the Ventilation System in Multi-Story Building
###### Abstract  
In South Korea, the installation of a mechanical ventilation system is mandatory for the management of indoor air quality, and various studies concerning the ventilation rate and performance of the ventilation system have been conducted. However, only a few studies have been conducted regarding the recirculation rate of the ventilation system. If the appropriate arrangement of intake and exhaust vents in the ventilation system is not considered, the pollutants emitted from the circulation movement may be recirculated into the indoor environment and cause the degradation of the performance of the ventilation system. Therefore, this study aimed to quantitatively analyze the recirculation rate of pollutants emitted from a kindergarten building with an installed mechanical ventilation system in Seoul, South Korea, using computational fluid dynamics (CFD) analysis, and analyze the effectiveness of the guide panel installed for the prevention of the pollutants' recirculation. The number of cases for the CFD analysis was set to a total of ten based on the ventilation rate in a mechanical ventilation system, external wind direction, and the existence of the guide panel for preventing the recirculation of exhaust air. The maximum recirculation rate of exhaust air without the installation of a guide panel was shown to be 20.0%. The maximum recirculation rate in the case where the external wind speed, direction of wind, and the ventilation rate were assumed to be identical to the other case but the guide panel for preventing the recirculation of exhaust air was assumed to be installed was 7.7%, 12.3% lower compared with the case with maximum recirculation rate.  
ventilation system; computational fluid dynamics; recirculation rate; exhaust air; guide panel 2021 11 May 2021 13 May 2021  
affect the performance of a ventilation system, and only the minimum distance between the intake and exhaust vents is used to arrange the ports uniformly. If the appropriate arrangement of intake and exhaust vents is not considered, the pollutants emitted from the building can be recirculated into the indoor environment via external air movement, as shown in Figure 1, which can degrade the ventilation system's performance. Until recently, most studies have focused on the ventilation rate and performance of the ventilation system [5; 6; 7; 8; 9; 10], but only a few have considered the recirculation of exhaust air.  
The methods for predicting the exhaust air recirculation rate in multistory buildings can generally be divided into the evaluation method through testing or actual measurement using a small-size model, or the computational fluid dynamics (CFD) analysis method via computer simulations [11; 12; 13]. The former offers high reliability in the results because testing equipment is installed on site or in a wind tunnel lab to directly measure the recirculation rate. However, building a wind tunnel test is expensive, and the conditions of various parameters that can occur in reality may not be applied easily [14]. In contrast, CFD analysis evaluates the recirculation rate of exhaust air by simulating an actual situation and applying a suitable physical model. While the measurement equipment in a test or actual measurement process can offer physical data at a specific point at which the equipment was installed, the CFD analysis can offer the data in the whole space. Furthermore, the recirculation rate of exhaust air on each floor can be quantitatively evaluated [15; 16; 17].  
Considering such benefits, this study aimed to analyze the recirculation rate of exhaust air in an actual kindergarten building with a mechanical ventilation system via CFD analysis and use the results as the basic data for designing a ventilation system.