OPENING DESIGN TO IMPROVE THE NATURAL VENTILATION PERFORMANCE OF HIGH-RISE RESIDENTIAL BUILDINGS

Transcription

1 OPENING DESIGN TO IMPROVE THE NATURAL VENTILATION PERFORMANCE OF HIGH-RISE RESIDENTIAL BUILDINGS Cho Rong Kim 1, Ga Young Cho 1, Sun Woo Lee 1, Myoung Souk Yeo 2, and Kwang Woo Kim 2 1 Department of Architecture, Graduate School of Seoul National University, Seoul, Korea 2 Department of Architecture, College of Engineering, Seoul National University, Seoul, Korea ABSTRACT Increase in high-rise residential building has changed the envelope of residential building to aluminum curtain wall which requires the use of the single-sided ventilation to be used instead of the cross ventilation. It causes dissatisfaction mainly with indoor air velocity and ventilation volume. Therefore, the objective of this study is to improve the natural ventilation performance of high-rise residential building by opening design. To improve the natural ventilation performance in single skin façade, the effect of different types of window will be evaluated. Furthermore, evaluation will be performed to evaluate the improvement when double skin façade is applied. This evaluation will be continued to know how to plan openings of double skin façade, such as inlets, outlets and outside façade windows. The evaluations will be carried out using a computational fluid dynamics (CFD) simulation program. KEYWORDS Double-skin facade, natural ventilation, high-rise residential building, CFD INTRODUCTION Recently the high-rise residential buildings have increased in Korea. Generally, existing residential buildings has balcony with aluminum chassis as envelope. However, aluminum curtain wall, the typical envelope of high-rise office buildings, has been applied to high-rise residential buildings without considering the needs of residential building. Moreover, high-rise buildings have center core structure such that some residential units face exterior environment with only one side. This causes single-side ventilation to be used instead of cross ventilation and occupant to dissatisfy with ventilation. According to survey (Cho et al. 2007), the main dissatisfaction with ventilation is that occupants cannot feel air flow even when they open the window. For this reason, occupants feel a lack of ventilation even though it exceeds the standard ventilation rate for indoor air quality. Furthermore, ventilation volume is not enough for free cooling, hence cooling energy increases. Current envelope of high-rise residential buildings needs to be improved, but the opening area and depth are restricted because of safety. A solution is using new type of windows like PAF type. Also, double-skin façade is applicable since it is known to be an effective way to reduce cooling energy through natural ventilation. The objective of this study is to improve the natural ventilation performance of high-rise residential buildings through opening design. For this purpose, current state of typical envelope of high-rise residential buildings should be understood first. To improve natural ventilation performance, the effect of different type of window will be evaluated. Furthermore, evaluation will be performed to know the improvement when double skin façade is applied. Evaluation will be continued to know how to design openings of double skin façade, such as inlets, outlets and external windows. Corresponding Author: Tel: , Fax: address: snukkw@snu.ac,kr

2 Ventilation can be categorized into buoyancy-driven ventilation and wind-driven ventilation. This study ACH 10.5 ACH Indoor (living room) ACH mm Outdoor Wind direction Wind direction [degree] Figure 1. Unit plan of high-rise residential building Figure 2. Ventilation volume (SSF with top-hinged out swinging window) focuses on wind-driven ventilation because natural ventilation is mostly used in spring and fall at which time there is little temperature difference between indoor and outdoor and there is the higher wind speed in high-rise building which makes wind-driven ventilation be superior to buoyancy-driven ventilation. METHOD OF SIMULATION Analysis model This study was performed by conducting simulations to understand the effect of various envelopes. CFD models of single and double skin façade were used. Comprehensive CFD models including turbulent flow are presented. The commercial CFD tool, STAR-CD with the standard k-ε turbulent model was used. Evaluation model Evaluation models are based on a high-rise residential building in Korea. The floor plan of selected unit and dimensions of evaluation model are shown in Figure 1. The evaluation model is a living room and it uses single-side ventilation. The façade is combined with two open modules which are each 900mm wide and two closed modules which are each 1800mm wide. To evaluate wind-driven ventilation, wind speed and direction were set for boundary condition. Mean wind speed was 2.5m/s in May, when natural ventilation was used most actively. To consider wind speed difference according to building height, local wind speeds at 100m height for upper floors and 10m height for lower floor are calculated with following equation. So, 5.3m/s and 2.5m/s are set for boundary condition. α α δ H U H = U (1) H δ U δ H α H : mean wind speed at height H : wind boundary layer thickness at local building terrain : exponent for local building terrain : wall height above ground on upwind building face

3 U δ α : eorological station hourly wind speed, measured at height H : wind boundary layer thickness at eorological station : exponent for the eorological station H : height of anemoer at eorological station Unlike typical residential buildings facing south, high-rise residential units are possible to face all direction and natural ventilation performance varies with wind direction. Therefore, boundary conditions for wind directions are 0, 45 and 90 from normal of façade (Figure 1) Evaluation case Typical envelope of current high-rise residential building is the single-skin façade (SSF) and it has two top-hinged out swinging windows. (Figure 3(a)) and the opening depth of window is 150mm. PAF window which has double effective opening area than top-hinged out swinging window while it has the same depth of opening is applied to improve natural ventilation performance in single-skin façade. (Figure 3(b)) Double-skin façade (DSF) evaluation model is developed to adjust residential building. Double-skin façade is the system consisting of an external facade, an intermediate space and an inner façade. The external façade generally has two types of opening which are called the inlet and outlet. Evaluation model of general double-skin façade is shown Figure 3(c). However, when wind-driven ventilation is superior to temperature-driven ventilation, inlet and outlet cannot guarantee that the ventilation volume will increase because there is possibility that outdoor air flows inside of inlet and outlet, not indoor. Therefore, external windows were added to improve natural ventilation performance in cases (d) and (e). The depth of intermediate space was 0.3m to minimize reduction of living area. The inner façade Table 1. Feeling of comfort according to air velocity Air velocity Thermal comfort Less than 0.25m/s Being ignorant of wind 0.25~0.50m/s Feeling pleasantness 0.50~1.00m/s Becoming aware of wind 1.00~1.50m/s Cooling effect by wind More than 1.50m/s Feeling unpleasantness Single skin façade (a) top-hinged out swinging (b) PAF Double skin façade (c) without external window (d) with external window (top-hinged out swinging) Figure 3. Diagram of simulation cases (e) with external window (PAF)

4 Ventilation volume [m 3 /h] Figure 4. Air velocity measuring points Figure 5. Ventilation volume (SSF) has two openings and the type of opening is sliding which provide better maintenance and larger effective opening area. Evaluation criteria Natural ventilation performance can be evaluated based on whether there is adequate outdoor air and indoor air velocity is comfortable. Figure 2 shows the ventilation volume according to wind direction in 2.5m/s wind when current typical envelope is applied. The results exceed the standard of ventilation rate for indoor air quality in all wind directions. The adequate ventilation rate for free cooling is higher than for indoor air quality but its criterion depends on characteristic of indoor situation. This study compares the evaluation cases with ventilation volume of current typical envelope. As the ventilation volume increases according to wind speed (Choi et al. 2006), the comparison will be made in 2.5m/s wind which provides lower ventilation volume. Indoor air velocity degrades the comfort of occupants. (Table 1) Indoor air velocity should be less than 1.5m/s. The recommended velocity is higher than 0.25m/s, allowing occupants to feel airflow. Measuring points are set to understand the distribution of indoor air velocity. (Figure 4) the points are placed at the center of room and the center of each opening. SIMULATION RESULTS Ventilation volumes when SSF is applied are shown in Figure 5. Except when wind direction is 0 degree, ventilation volume of PAF type is lower even though effective opening area is bigger. This is shown by the streamline of air movement. (Table 2) Wind flows through the gap between the façade and glass pulled out instead of flowing in. Therefore, PAF type is ineffective in increasing the ventilation PAF (y=2.25) PAF (y=0) PAF (y=-2.25) top-hinged out swinging (y=2.25) top-hinged out swinging (y=0) top-hinged out swinging (y=-2.25) Air velocity [m/s] Distance from envelope [m] Figure 6. Indoor air velocity distribution (SSF 2.5m/s) Figure 7. Indoor air velocity distribution (SSF 5.3m/s)

5 Table 2. streamline of air movement SSF (top-hinged out swinging) SSF (PAF) DSF (general) DSF (top-hinged out swinging) DSF (PAF)

6 volume. However, the difference in ventilation volume according to wind direction is reduced and it could be an advantage for applying PAF type. The maximum velocity appears in 45 degree wind direction so it is indicated in Figure 6 and Figure 7. Air velocity is decreased dramatically according to room depth and it is less than 0.25m/s in most part of the room in 2.5m/s wind. The result shows that inflow air velocity may not cause discomfort when using top-hinged out swinging type but occupant cannot feel airflow at the area deeper than 0.4m from envelope. Indoor air velocity is slightly increased when using PAF type. So wind causing discomfort appears in front of the window. But, from 0.5m depth, air velocity is similar to top-hinged out swinging window and does not influence the comfort of occupants. At 30th floor, pleasant airflow occurs at the leeward side of the room but higher airflow than 1.5m/s appear near envelope. When DSF is applied, ventilation volume increases due to the increase in opening area. However, when wind flows parallel to the general double-skin façade (90 degree), ventilation volume is less than single skin façade top-hinged out swinging windows applied. (Figure 8) The streamline in general DSF shows outdoor air comes in only in 45 degree wind direction. This confirms the necessity of external windows to increase ventilation volume and indoor air circulation. Comparing DSF without external windows, ventilation volume increases from 40% to 200% when external windows are applied. Noticeably, PAF type windows are more effective in increasing the ventilation volume. Applying external windows, inlets and outlets together, various pressures by wind is distributed in facade so ventilation is enhanced. Thus, inflow air occurs in DSF which has PAF windows unlike PAF windows in SSF. Maximum indoor velocity appears at 45 degree. Indoor air velocity should be checked because above 1m/s airflow appears near internal façade. In general DSF, indoor air velocity is decreased. It is due to the fact that airflow does not come into indoor space. When external window is added, indoor air velocity near the external façade was found to be above 1.5m/s which causes discomfort. However, it appears in intermediate space and so Figure 8. Ventilation volume (DSF) Figure 9. Indoor air velocity distribution (DSF y=-2.25) Figure 10. Indoor air velocity distribution (DSF y=0) Figure 11. Indoor air velocity distribution (DSF y=2.25)

7 occupants feel comfort in indoor space even in 5.3m/s wind. (Figure 8) Lastly, air velocity in the room area slightly increases than SSF with PAF windows so occupants feel more comfort. CONCLUSIONS This study was aimed to find a way to improve the natural ventilation in high-rise residential building by opening design. The effect of the types of window and the effect of various openings in double-skin façade were evaluated through simulations. Based on the simulations results, the following conclusions may be drawn. 1) Inlet and outlets of general double-skin façade could not provide improved ventilation volume than current envelope. Adding external windows helps to increase ventilation rate and indoor air circulation. 2) Top-hinged out swinging window is more effective than PAF window to increase ventilation rate in SSF. However, PAF window is better to use as external windows in DSF. 3) In SSF, PAF window provides more comfortable indoor air velocity in most part of the room compared to top-hinged out swinging window causes occupants cannot feel air movement. But, both types cause discomfort near the envelope. In DSF, air velocity which causes discomfort appears only in intermediate space and comfortable air velocity appears in the room. REFERENCES 1. G. Y. Cho, C. R. Kim, S. W Lee, C. S. Park, M. S. Yeo and K. W. Kim (2007), An Analysis of Envelope to Improve Environmental Performance of High-rise Residential Building, Proceeding of Clima 2007 Wellbeing Indoors, B02I S. H. Park, G. Y. Cho, S. W. Lee, J. H. Jo, M. S. Yeo and K. W. Kim (2006) A Study on the Envelope to Improve Interior Environment Performance of High-rise Residential Building, Proceedings of the Korean Housing Association 2006, Guohui Gan (2000) Effective depth of fresh air distribution in rooms with single-sided natural ventilation, Energy and Buildings, Vol. 31, S. T. No and K. S. Kim, A Study on the Characteristics of Natural Airflow Through Single-sided Openings with Variable Position and Geory, Journal of Architectural Institute of Korea, Vol. 21, No. 8, T. H. Choi, M. S. Jeon, J. H. Lee, T. Y. Kim and S. B. Leigh, Indoor Airflow of High-Rise Apartment with Different Types of Box-Windows, Proceedings of Korean Air-Conditioning and Refrigeration Engineering, ASHRAE Handbook Fundamental 2005