Influences of vent location on the removal of gaseous contaminants and indoor thermal environment

https://doi.org/10.1016/j.jobe.2020.101679Get rights and content

Highlights

  • Buoyancy-driven exchange flow through a large vent is investigated.

  • The indoor density distribution is obtained using a light attenuation method.

  • The vent location affects the exchange flow rate and the density distribution.

  • Models are proposed to estimate the exchange flow rates.

  • The applications in contaminants removal and natural ventilation are presented.

Abstract

Heat sources and gaseous contaminants widely exist in modern buildings. Buoyancy-driven exchange flow through a large opening, such as a window, a doorway or an atrium, is ubiquitous in our daily life. Analogous experiments were carried out in a water tank to investigate the effect of vent location on the characteristics of the exchange flow. The model tank filled with coloured brine solution was immersed in a large reservoir filled with fresh water. The transient distribution of the reduced gravity is measured using a light attenuation technique. For the exchange flow through a vertical opening, the reduced gravity in the chamber is vertically stratified instead of uniformly distributed. Moreover, the density distribution was influenced by the location of the opening. The buoyancy-driven flow through a horizontal opening is highly oscillatory and the resulting density distribution is relatively uniform. Different models were proposed to estimate the ventilation flow rate through a vertical opening or a horizontal one. Hopefully, this study could provide useful advice with regards to the ventilation design and contaminant removal in buildings.

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 g'/g0' 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, 0<Δρ/ρ<0.071, 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)

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