The application of an air curtain range hood in reducing human exposure to cooking pollutants

Highlights

• The effect of air curtain range hood in reducing human exposure was assessed through orthogonal experiment.

• The application of air curtain could decrease the turbulence intensity and improve local airflow organization.

• The impact of human walking on adding human exposure could be relieved by using air curtain.

• Relationship between intake fraction and contaminant removal efficiency was discussed.

Abstract

The function of air curtain range hood in improving human exposure was investigated by conducting experiment and CFD simulation. The intake fraction was estimated to reflect the effect of the air curtain. Factors affecting human exposure, including the exhaust rate, heat source intensity, air supply angle, air supply velocity, and slot width of the air curtain, were incorporated according to an orthogonal design. To comprehend the roles that the air curtain performed, the characteristics of flow and concentration field were captured through visualization experiment and CFD method. Furthermore, the effect of using an air curtain to resist the impact of human walking on human exposure was examined. The exhaust rate, heat source intensity, and air supply angle were significant factors that affect human exposure with a significance level of 0.01. Compared with the traditional range counterpart, air curtain range hood could decrease the intake fraction by factor of 1.5–5. Air curtain could reduce turbulent intensity around the range hood and improve local airflow organization. Meanwhile, air curtain would considerably protect walking humans from exposure to pollutants. The outcome of this study could help us understand the effect of air curtain range hood with a comprehensive perspective.

Introduction

The rigorous lockdown policy for preventing the spread of the 2019 novel coronavirus (COVID-19) in many countries boosted the frequency of household cooking activities. Researchers conducted field tests to illuminate the condition of personal exposure to cooking-related particulate matter (PM) during the COVID-19 lockdown in China [1]. A 17.4 $\mu g/m^3$ increment of average daily PM_2.5 concentration was observed and revealed a growing risk of human exposure. In addition to the particulate matter [[2], [3], [4]], the gaseous pollutants, such as carbon oxides (CO_x) [5,6], nitrogen oxides (NO_x) [6], and volatile organic compounds (VOCs) [5,7,8], were also common ingredients of cooking pollutants [9]. Even potentially toxic ingredients including polycyclic aromatic hydrocarbons (PAHs) [10,11], black carbon [2] and unsaturated aldehydes [12] also existed in cooking pollutants. Epidemiologic studies verified an association between human exposure to cooking pollutants and potential health hazards under certain conditions. Long-term observation studies revealed that human exposure to cooking pollutants was related to enhanced risks of lung cancer [13], respiratory infections [14], bronchitis [15], immune diseases [16], and cardiovascular diseases [17]. Furthermore, from the short-term perspective research, lung function was significantly impaired by a two-day exposure to typical Chinese cooking pollutants [18]. A toxicological mechanism research also revealed the effect of cooking pollutants on human health [19]. Cell viability would recede while the human bronchial epithelial was exposed into cooking pollutants [2]. Effective ventilation of the residential kitchen is therefore vital to avoid the health hazards of cooking pollutants.

Ventilation systems are widely applied to discharge the cooking pollutants and prevent human exposure in residential kitchens. In practice, however, human exposure conditions in kitchen environments are unsatisfactory. Liu et al. reviewed air quality in a Chinese-style residential kitchen and found that the concentrations of CO, CO₂, TVOCs, and PM exceeded the limits of the Chinese national standard (GB/T 18883-2002) in certain conditions [20]. On-site survey by Wag et al. in severe cold regions of northeast China confirmed that the winter concentration of formaldehyde and VOCs exceeded the standard (GB/T 18883-2002) by 2.5 and 2 times, respectively [21]. Similar research showed that the PM_2.5 concentration can reach 1000 $\mu g/m^3$ and last 10–20 min during cooking in an old residential community in Shanghai [22]. These studies remind us to further improve the performance of ventilation systems and decrease human exposure in residential kitchens. On the one hand, a high-performance range hood was pursued by researchers and could directly eliminate cooking pollutants. Researchers concentrated on methods such as changing the range hood shape [23,24], adding side [25,26] or guide panels [27], and altering the modulating space locations of hoods [28,29] to enhance their capture performance. Enhancement of the capture performance was accompanied by a complicated configuration or specific condition that difficult to use. Augmenting the exhaust rate was also considered an effective method to achieve a performance improvement of a range hood [30,31]. The capture efficiency of a range hood could reach 95 % while the exhaust rate was 550 m³/h [32]. Nevertheless, the 2D-PIV experiment conducted by Chen et al. showed that the thermal plume would expand and spillage would occur from the edge of the range hood even with an exhaust rate of up to 600 m³/h [33]. Furthermore, the practical exhaust rate of a range hood generally diverges from the rated exhaust rate because of the limitation of the make-up airflow rate [21]. The real exhaust rate of a range hood was merely 25 % of the rated exhaust rate of 900 m³/h with the kitchen door and windows closed, thereby leading to a deficient capture efficiency of less than 75 % [5]. These investigations implied a limited capacity of the range hood to maintain nethermore low level of human exposure.

In addition to the aforementioned means of upgrading the capture performance of a range hood, studies have focused on methods of promoting airflow organization in kitchen spaces to mitigate human exposure. Kim et al. proposed a novel ventilation system with a ceiling air supply inlet and exhaust outlet that decreased the pollutant concentration during a short time compared with ventilating kitchen with a range hood alone [34]. A ventilation system based on an impinging jet and the Coanda effect was presented by Li et al. who found that the contaminant and capture efficiency of this new ventilation system increased by nearly 20 % compared with that of a mixed air supply system [35]. Furthermore, the local airflow organization surrounding control zone of the range hood was optimized due to human exposed to it immediately. Chen et al. investigated the performance of range hood with a fresh air supply inlet appended on its cabinet [36]. The peak concentration of PM in the human breath zone was decreased by 20 % compared with the range hood without a fresh air supply. Meanwhile, an upward air curtain located on the countertop around the stove was introduced and its effect on relieving human exposure was detected. Huang and Chen et al. developed and ameliorated an upward air curtain range hood with an air curtain parallel to the edge of the counter [[37], [38], [39], [40], [41]]. An axis fan with a slot outlet was employed and the recirculated indoor air was applied to form an upward air curtain. The performance of this kind of range hood was assessed through tracer gas experiment, and the experimental results detected minimal pollutants in the front of range hood with properly designed air supply velocity and angle. Cao et al. applied an air curtain to make up air, optimized the local airflow organization, and found that the individual exposure level was mitigated by 2–3 orders of magnitude compared with making up air through an open window [42]. Similar studies by Zhou et al. confirmed that air curtain with optimized air supply parameter could improve the capture efficiency of range hood [[43], [44], [45]]. Liu et al. presented air curtain with an air conditioning system to enhance the indoor air quality and thermal environment amid humans [46]. The capture efficiency of this system could exceed 97 %. However, the air curtain systems mentioned above have complicated auxiliary configurations and adverse impacts on cooking procedures. Thus, a downward air curtain which could integrate with range hood named air curtain range hood was developed. Liu et al. surveyed the performance of range hood with downward air curtain and guide plate [27]. The results showed air curtain could prevent cooking pollutants dispersing into a kitchen environment. Nevertheless, the effect of air curtain range hood on improving local airflow organization and relieving individual exposure was rarely discussed.

Studies proved that the performance of range hood would be affected by disturbing airflow [47,48]. Walking by motion was commonly appeared while cooking process in residential kitchen. Researchers substituted human body by sheet plate to simulate human walking and investigated the effect of human walking on performance of range hood [38,39,41]. The results depicted that the cooking pollutant was dragged from control zone of range hood to kitchen environment. And the upward air curtain could defense the influence of human walking in certain condition. To accurately replicate the scene of a human walking, the verified CFD model with dynamic mesh method was conducted in our previous study [49]. The wake induced by human walking would have an intense impact on the thermal plume, and the capture efficiency of range hood would decrease by 50 %. However, the human exposure condition with a walking human and the effect of air curtain range hood on resisting it should be further investigated.

Previous research on air curtain range hoods mainly concentrated on the improvement of its capture performance and seldom highlighted the assessment of human exposure. Meanwhile, the melioration of the local airflow organization because of the application of an air curtain should be further investigated. The effect of walking on the individual exposure of humans and the function of air curtain to resist its influence also should be investigated. Therefore, the orthogonal experiment and CFD simulation were conducted in this study to illuminate the aforementioned problems. Tracer gas experiment and visualization experiment were implemented. The intake fraction was selected to estimate the condition of human exposure. Five key factors, i.e., the exhaust rate, heat source intensity, air supply velocity, air supply angle of the air curtain, and air curtain slot width, were considered at a steady state without human walking. The effect of human walking on the airflow characteristics and intake fraction were investigated during transient state. The outcome of this study could be helpful for reducing human exposure in residential kitchens and promoting the use of air curtain range hoods.