On the effect of provision of balconies on natural ventilation and thermal comfort in high-rise residential buildings
Introduction
Natural ventilation is proven to be an effective low-cost solution for space conditioning, especially in cooling dominant climates [1], [2]. Being a passive solution, building energy consumption and associated negative environmental effects can be reduced by implementation of natural ventilation. Furthermore, building occupants in subtropical climates have a tendency to live in naturally ventilated buildings rather than fully air-conditioned spaces [3].
In addition to the external weather conditions as the main driving force, architectural design features play an important role in natural ventilation performance and indoor airflow behaviour. Design parameters that alter the internal airflow include type, size and placement of the openings, internal layout, height and orientation of a building, and façade features such as balconies [4], [5], [6], [7], [8].
Private outdoor spaces such as balconies are perceived as one the most desired features in subtropical climates that can be used for a different range of activities [3], [9], [10]. Balconies act as buffer spaces between indoor and outdoor that not only reduce the occupants' exposure to the pollutions [11] but also result in significant heating and cooling load reduction [12]. In addition, balconies can reduce noise level –the commonly stated limitation of natural ventilation-by acting as an acoustic protection device [13].
From a natural ventilation point of view, the addition of a balcony alters the pressure distribution on a building façade and consequently affects the ventilative forces [14]. Chand et al. [14] carried out a wind tunnel experiment on a five-storey building with mounted balconies to study this impact. Their results demonstrated an alteration in pressure distribution on the windward side and no significant change on the leeward side. While Chand et al.'s study focused on pressure distribution on the façade of a case model without openings, their experimental data was later used for CFD validation and subsequent evaluation of the effect of balcony provision on indoor ventilation performance [15], and thermal comfort [16]. The results indicated that mass flow rate increases and average velocity decreases in the case of single-sided ventilation, while no significant change was observed under cross ventilation mode [15]. Thermal comfort status was also reported with no change [16]. Prianto and Depecker [17], [18] adopted a numerical method to investigate the effect of balcony, internal divisions, and openings on indoor velocity and thermal comfort in a two-storey dwelling. They found that both balconies and openings play an important role in the modification of indoor velocity and thermal comfort condition.
While these studies have been concerned with the effect of balconies on natural ventilation, they were all based on simple geometries, and the combined effect of balcony features (i.e. balcony type and depth) with other determinant parameters such as ventilation mode and incident wind direction are not adequately investigated. The objective of this study, therefore, is to investigate the impact of these parameters on natural ventilation and indoor thermal conditions. Accordingly, full-scale measurements were carried out in a residential unit located in a high-rise residential building in Brisbane, Australia. The collected data was then used for validation of a CFD model and good agreement between the data and the simulation results were obtained. Two ventilation modes (single-sided and cross ventilation), two balcony types (semi-enclosed and open balcony), four balcony depths (10%, 20%, 30%, and 40%), and four wind directions (0°, 45°, 90°, and 180°) were defined as variables. From that 70 case studies were formulated to investigate the separate and combined effect of these variables. The validated CFD model was then used for calculation of air velocity in the case studies.
Average velocity was used as a criterion to evaluate the effect of the variables on overall ventilation performance. Average velocity is linearly correlated with qualities such as airflow rate and air change per hour [19], and is also a determinant in thermal comfort calculations. Therefore, average velocity can be used as a good indicator of ventilation performance. Acquired average velocity along with typical meteorological data for Brisbane were further used in calculations of SET* index for thermal comfort evaluation of the occupied zone.
Section snippets
Field measurement
Field measurements were carried out in a unit located on the fifth floor of a 36-storey residential building located at Brisbane Central Business District (CBD), Australia. The building is oriented 35° toward the west and the case study unit is located at the eastern end of the building. Fig. 1 shows the case study building and its surroundings where the case study building is indicated in red boundary.
The case study layout consists of two balconies at two opposite sides of the living area
Results summary
Indoor air flow of the case studies was obtained using the validated CFD model. Average velocity in the living area volume was extracted from the results and corresponding SET* values were calculated. Results are presented using average velocity and thermal comfort. The obtained velocity results are summarised in Fig. 7 and are categorised based on the investigated variables. The presented results for each variable (x-axis) is cumulative results from all the simulated cases where the parameter
Conclusion
In-situ full-scale measurements of air velocity were conducted in a high-rise residential apartment. The collected data was used to validate a CFD model from which a detailed investigation of the separate and combined effect of the balcony type and depth, ventilation mode, and the wind angle on indoor ventilation was performed. Various case studies were formulated based on two balcony types, four balcony depths, two ventilation modes and four wind angles. Average velocity and SET* index were
Limitations and future work
The main aim of this study is to provide comparative results about the effect of different balcony attributes on natural ventilation performance of high-rise residential units. Similar to any other study, this study has some limitations that need to be addressed in future research. The current study is carried out with an isothermal assumption and buoyancy-driven ventilation is neglected due to dominant effect of wind. In future studies, buoyancy-driven ventilation can be considered to evaluate
Acknowledgements
Computational and data visualisation resources used in this work were provided by the HPC and Research Support Group, Queensland University of Technology, Brisbane, Australia. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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