Influences of vent location on the removal of gaseous contaminants and indoor thermal environment
Introduction
Thermal environment and indoor gaseous contaminants could directly affect the working efficiency and health of building occupants [1]. Natural ventilation is an efficient and energy-saving method in diluting contaminants and improving indoor air quality [2]. Therefore, it is becoming increasingly popular in recent years. Both wind and buoyancy can be the driven forces for natural ventilation. Because wind generally is unsteady and unpredictable, it is very difficult to quantify the characteristics of wind-driven natural ventilation. Indoor air temperature is different with ambient temperature as a result of the lighting, office equipment and human activities in buildings [3]. Therefore, buoyancy might be the steady dominant driving force for natural ventilation [4,5]. Since the establishment of the ‘emptying filling box’ model [6], extensive studies have concentrated on buoyancy-driven ventilation. In many practical situations, indoor and outdoor air are connected by a doorway or a window. Many residential apartments and office rooms can be characterized as a single-zone building with a single-sided large vent [7]. In addition to a doorway or a window, the single opening also can be a horizontal atrium or stairwell. It has been concluded that the location of the exhaust vent in the impinging jet ventilation system could significantly influence indoor temperature stratification and thermal comfort [8]. In practice, the vent for natural ventilation might also be placed in different locations. It is of practical importance to study the influence of the vent location on buoyancy-driven exchange flow.
Buoyancy-driven flow through a vertical opening has been previously studied. Linden et al. [6] investigated mixing ventilation in an enclosure with a vertical vent close to the ceiling. It is generally assumed that the indoor temperature is approximately uniformly distributed in classical mixing ventilation. Wang et al. [4] numerically investigated indoor thermal profiles when the vent located at approximately half height of a sidewall. The results indicated that the indoor temperature was more stratified than that of classical mixing ventilation, which might imply the vent location could influence the indoor temperature stratification. Sash windows are widely used in modern buildings, the window panes can be pushed to the bottom or the top of the window. The locations of the window panes may influence ventilation performance and the indoor temperature stratification. If the exchange flow is established through a vent close to the floor, for example a doorway, the resulting indoor temperature distribution could be totally different from that in classical mixing ventilation [9,10]. It is meaningful to investigate the effect of the vent location on the density stratification of the indoor air. Because the flow through a large opening is bidirectional, there is an exchange interface dividing the inflow and outflow regions in the vertical opening. The exchange interface locates at the middle of the opening if the mixing and dissipation are neglected. It is useful to evaluate whether there is a significant deviation between the ideal exchange interface location and the practical exchange interface location.
The mechanism of buoyancy-driven flows through a horizontal ceiling vent is different from that through a vertical opening [11]. The buoyancy-driven flow through a horizontal opening is not stable but oscillatory. Epstein [12] proposed four flow regimes by varying the aspect ratio of the horizontal opening. In some circumstances, there may exist a pressure difference at both sides of the horizontal opening as a result of thermal expansion. Chow et al. [13] studied the exchange flow through a horizontal vent induced by both buoyancy force and pressure difference. A characteristic flow parameter was introduced to determine the dominating driving force of the exchange flow. Du et al. [14] experimentally investigated the effect of the boundary heat transfer on the exchange flow rate through a horizontal opening. Due to the complexity of the flow, it is normally considered that a horizontal vent is less efficient than a vertical vent in terms of ventilation flow rate. In the case of an exchange flow through a horizontal vent, the density distribution in the entire chamber has not been experimentally presented. It is necessary to further study the ventilation efficiency and indoor temperature. This study will also make comparisons between horizontal opening and vertical opening in terms of indoor temperature stratification and ventilation flow rate.
In order to visually observe the evolution of the indoor density under different vents, an appropriate research method should be carefully selected. Brine-water technique has been widely used to study the characteristics of buoyancy-driven ventilation [6,[15], [16], [17]]. Visualization of the flow is easily achieved by adding some dye in the brine solution. More importantly, the evolution of the density in the entire flow field could be measured by introducing a light attenuation technique [[18], [19], [20], [21]]. Therefore, the brine-water modelling combined with a light attenuation technique is an appropriate method to achieve the aforementioned aims of the present study.
Section snippets
Experiments
Fig. 1 shows the experimental setup. The dimensions of the model tank were 40 (length) × 40 (width) × 30 (height) cm. There were four rectangular openings of dimensions 8 × 8 cm on the tank. Three vertical openings were located on the right side wall and the horizontal one was located at the bottom. The model was immersed in a large reservoir of dimensions 300 (length) × 100 (width) × 80 (height) cm. Initially, the large reservoir was filled with freshwater, whereas the model tank was filled
Experimental observations and exchange flow patterns
Fig. 3(a–e) show the transient distribution of in the tank in Exp. V1. The exchange flow through opening 1 is established immediately after the plug is removed. Due to the negative buoyancy, dense fluid in the tank flows out through the lower part of the opening. Meanwhile, a return flow enters the tank through the upper part of the opening. Therefore, there is an exchange interface at the opening. As indicated by Fig. 3(a), the flow experiences a short fluctuation period due to the
Conclusions
A series of experiments are carried out in a water tank to study the characteristics of the buoyancy-driven flow through a large opening. Possible implications with regards to contaminants removal and building ventilation are analyzed. The density difference, , is caused by the salinity difference. According to the scaling law, the experimental results can be used to understand the buoyancy-driven flow induced by a temperature difference of 0–20 °C. To avoid confusion, the
CRediT authorship contribution statement
Dong Yang: Writing - original draft, Methodology. Song Dong: Data curation, Investigation. Tao Du: Writing - review & editing, Funding acquisition. Wenhui Ji: Visualization, Project administration.
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
The authors acknowledge the support from the National Natural Science Foundation of China (NSFC) under Grant No. 51806022 and No. 51976017, the Project No. cstc2019jcyj-bshX0079 sponsored by Natural Science Foundation of Chongqing, China, the Project No. XmT2018037 funded by Chongqing Special Postdoctoral Science Foundation, the Project No. 2018T110945 funded by the China Postdoctoral Science Foundation. Tao Du would like to acknowledge the China Postdoctoral Council for the financial support
References (27)
- et al.
Experimental assessment of the impact of natural ventilation on indoor air quality and thermal comfort conditions of educational buildings in the Eastern Mediterranean region during the heating period
J. Build. Eng.
(2019) - et al.
Impact of an external boundary wall on indoor flow field and natural cross-ventilation in an isolated family house using numerical simulations
J. Build. Eng.
(2017) - et al.
Predicting single-sided airflow rates based on primary school experimental study, Build
Environ. Times
(2016) - et al.
Assessment of single-sided natural ventilation driven by buoyancy forces through variable window configurations
Energ. Build.
(2017) - et al.
Experimental study on the cooling performance of solar-assisted natural ventilation in a large building in a warm and humid climate
J. Build. Eng.
(2018) - et al.
Wind-induced single-sided natural ventilation in buildings near a long street canyon: CFD evaluation of street configuration and envelope design
J. Wind Eng. Ind. Aerod.
(2018) - et al.
Evaluation of thermal comfort, IAQ and energy consumption in an impinging jet ventilation (IJV) system with/without ceiling exhaust
J. Build. Eng.
(2018) - et al.
Transient buoyancy-driven ventilation: Part 1. Modelling advection
Build. Environ.
(2011) - et al.
On ventilation of a heated room through a single doorway
Build. Environ.
(2004) - et al.
Buoyancy and inertial force on oscillations of thermal-induced convective flow across a vent
Build. Environ.
(2011)
Multiple patterns of heat and mass flow induced by the competition of forced longitudinal ventilation and stack effect in sloping tunnels
Int. J. Therm. Sci.
Transient evolution and backlayering of buoyancy-driven contaminants in a narrow inclined space
Build. Environ.
On the backlayering flow of the buoyant contaminants in a tunnel with forced longitudinal ventilation
Build. Environ.
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