Effect of a recirculated air curtain with incomplete coverage of room width on the protection zone in ventilated room
Introduction
Indoor spaces are easily threatened by chemical or biological pollutants [[1], [2], [3]]. Several infection incidents, such as SARS [4], H1N1 influenza [5], and COVID-19 [[6], [7], [8]], have increased people's concern regarding protecting the indoor environment from contamination. Too high indoor dust concentration is also prone to explosion accidents [9]. Ventilation is a key measure to reduce airborne contaminants. In addition to the traditional mixing ventilation, a series of advanced ventilation and air distribution patterns, such as displacement ventilation [10], under-floor air distribution [11], impinging jet ventilation [12], stratum ventilation [13], and personalized ventilation [14], have been proposed to improve the effectiveness in removing contaminants. Excellent air distribution patterns considerably eliminate indoor contaminants to better protect the occupied space. However, the common air distributions have little effect on the short-distance transmission of airborne contaminants. The contaminants released from the source are directly dispersed to the nearby personal areas, resulting in high exposure levels. When the released contaminants have strong initial air momentum, such as during sneezing and coughing, they threaten larger distances [15]. It is important to prevent high contaminations from directly invading the occupied subzone to improve the zonal protection capacity.
As an aerodynamic barrier, air curtain has been applied in certain fields. It can separate heat, moisture, and contaminants between zones remarkably. Navaz et al. [16] analyzed the entrainment of warm air by an air curtain in an open refrigerated display case. They concluded that lowering the Reynolds number of the air curtain reduced the entrainment rate; however, a high momentum was still required to enforce the integrity of the air curtain. Giráldez et al. [17] studied the heat and moisture insulation in a refrigerated chamber using non-recirculating, recirculating, and twin-jet air curtains. They found that the twin-jets provided a moderate efficiency improvement compared to the single jet. Recirculating air curtains produced a significant increase in the sealing efficiency. Goubran et al. [18] investigated the flow characteristics of building entrances equipped with air curtains. Through the developed infiltration characteristic curves, two flow conditions, i.e., the optimum and the inflow breakthrough conditions, were confirmed. The higher curtain supply speed resulted in the better air curtain performance. Jung et al. [19] analyzed the performance of an air curtain to block the contaminated adverse airflow in a tunnel considering the installed angle of the slit nozzle and discharge air velocity at the nozzle outlet. The installed angle of 0 deg could not block the adverse airflow due to the tunnel pressure difference. A greater discharge air velocity was needed for the increase of pressure difference. Luo et al. [20] proposed to install an air curtain system to isolate the local pit environment from a large-space exhibition hall. The isolation efficiency could achieve more than 80%. The penetration of air pollutants and dusts into the pit was prevented. Kairo et al. [21] studied the efficiency of an air curtain in preventing insects from entering a building. They found that a discharge air velocity of 7.5 m/s might prevent a strong flyer with high kinetic energy from entering a building. The results of various studies have yielded the optimal air curtain parameters for effective containment of heat, moisture, and contaminants in areas outside the protected space.
Recent research has been exploring the effects of separating subzones by installing air curtains inside the building rooms to better prevent direct exposure to the contaminant source. Shih et al. [22] proposed to use an air curtain to restrain the pollutant dispersion for ensuring personnel safety in a contaminated cleanroom. The sealing performance was optimized by analyzing the effects of ejection velocity, angle, and installation height. Cao et al. [15,[23], [24], [25]] carried out systematic studies on the effectiveness of an air curtain in subzone protection with the air curtain installed inside a room. A downward plane jet was arranged in the middle of the room to deliver fresh air to prevent direct lateral dispersion of contaminants. The containment effects of air curtain on the tracer gas source, exhaled polluted air, and coughed jet source were analyzed in detail. The air curtain exhibited a superior protective effect when the air supply momentum was appropriate. If the contaminant source has no initial airflow momentum, a lower air supply velocity between 1 and 1.5 m/s can effectively inhibit the diffusion of contaminants [23]. Zhao et al. [26] studied the use of air curtain in a large building enclosure to separate room airborne pollutants. The effects of supply velocity, width of air curtain, and enclosure height on the sealing performance were investigated. Jha et al. [27,28] conducted small scale waterbath experiments to investigate the effectiveness of an air curtain to prevent the contaminant transport with the wake of a moving person from a dirty to a clean zone in a corridor. The contaminant transport could be reduced by 40% by using the air curtain. The buoyancy-driven flow had a negligible effect on the infiltration flux. Most of the recent research on subzone protection in rooms focused on the air curtain with a fresh air supply. This method essentially delivers a portion of the total fresh air volume downward to establish an air curtain, which is a new approach with respect to traditional air distribution patterns. However, similar to using traditional air curtain at the doors of the buildings, it is a feasible measure to directly install a recirculated air curtain in the ventilated room. In this case, the indoor airflow field is determined by the air curtain jet, air jet from each air supply inlet, and layout of the exhaust air outlet. In addition, because the supply air for the air curtain comes from the indoor polluted air, it is unclear whether the recirculated air curtain can significantly reduce the contaminant concentration in the protection zone. Shen et al. [29] studied the potential of using a recirculated air curtain to reduce the heat transfer to the target zone and save energy. A curtain effectiveness of up to 69.3% could be achieved, and the maximum temperature difference between the occupied and unoccupied zones was as high as 7.4 °C. Ye et al. [30] proposed to use an air curtain to separate the patient from doctor in a consulting ward. An air curtain, with a high efficiency filter, was installed in the consulting desk to provide clean air supply for the air curtain. The air curtain in their work was essentially equivalent to fresh air. There are few studies on the prevention of contaminant dispersion using recirculated air curtain installed in a room. Moreover, unlike installing an air curtain at the door, because the length or width of a room is much larger than the width of a door, particularly for large space buildings, it is unrealistic to install a complete air curtain along the entire length. Therefore, it is more practical to install an incomplete air curtain that covers only a portion of the length or width of the room. It is worth studying whether the contaminant from the source zone can be effectively prevented under the condition of an incomplete air curtain.
In this study, an incomplete and recirculated air curtain for preventing contaminant dispersion was numerically investigated. The impacts of the air curtain length, air extract position in the air curtain, and position of the contaminant source were primarily analyzed.
Section snippets
Case design
A typical office room was built, as shown in Fig. 1.
The dimensions of the room were 5 m (length) × 4 m (width) × 3 m (height). The room was divided into two subzones, the protection zone and the source zone. A gaseous contaminant with the properties of CO2 was released at a rate of 0.2 g/s in the source zone. The contaminant source was set to an internal cell zone through which the airflow could pass. The size of the source was 0.1 × 0.1 × 0.5 m. The main consideration of the source parameter
Effect of length of recirculated air curtain
The airflow fields at the middle plane (Z = 0 m) for different air curtain lengths are illustrated in Fig. 5.
When there was no air curtain, the two air supply jets were fully developed in their subzones. When the air curtain was installed, the airflow field varied significantly. For the air curtain lengths of 0.5 and 1 m, the short air curtain did not significantly influence the airflow characteristics in the source zone; however, those in the protection zone was affected and a vortex was
Conclusion
The aerodynamic barrier effect of an incomplete and recirculated air curtain on a steady contamination was investigated numerically. The impacts of the air curtain length, air extract position in the air curtain, and position of the contaminant source were analyzed. The main conclusions are as follows:
- (1)
In the ventilated space, adding an incomplete and recirculated air curtain with an appropriate length further reduced the pollution in the protection zone based on traditional ventilation
CRediT authorship contribution statement
Yu Liu: Writing – original draft, Validation, Methodology, Investigation, Data curation. Kaiying Qiu: Validation, Methodology, Investigation, Data curation. Xiaoliang Shao: Writing – review & editing, Writing – original draft, Supervision, Methodology, Formal analysis, Data curation, Conceptualization. Penglei Shi: Investigation, Formal analysis, Data curation. Yemin Liu: Formal analysis, Data curation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study is supported by the National Natural Science Foundation of China (Grant No. 51878043), Beijing Natural Science Foundation (Grant No. 8212012), Special Fund of Beijing Key Laboratory of Indoor Air Quality Evaluation and Control (Grant No. BZ0344KF21-01), and Fundamental Research Funds for the Central Universities (Grant No. FRF-TP-20-001A3).
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