METHODOLOGY FOR NATURAL VENTILATION DESIGN FOR HIGH-RISE BUILDINGS IN HOT AND HUMID CLIMATE

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1 The 2005 World Sustainable Building Conference, METHODOLOGY FOR NATURAL VENTILATION DESIGN FOR HIGH-RISE BUILDINGS IN HOT AND HUMID CLIMATE P C Wong 1 D Prasad 2 M Behnia 3 1 Faculty of Built Environment, University of New South Wales, Australia, jwongpc@hotmail.com 2 Centre for Sustainable Built Environment (CSBE), Faculty of Built Environment, University of New South Wales, Australia 3 The University of Sydney, Australia Keywords: thermal comfort, double-skin façade, computational fluid dynamic, natural ventilation Summary The research attempts to look into the viability of double-skin façade in providing natural ventilation for the high-rise office buildings in hot and humid environment. The behaviour of airflow patterns induced by wind and thermal forces through the double-skin façade into the interior office space and their effects onto the thermal comfort within the space are analysed with the use of computational fluid dynamic simulations and to identify the possible window periods for natural ventilation to be introduced to the office space. 1. Introduction Extensive research has been carried in defining what is thermal comfort and the parameters that affecting it. All those findings had confirmed the importance of human factors and human influence towards the creation of a thermally comfortable indoor environment (Fanger 1970 and Ruck 1989). In more recent experimental studies concerning the effects of some human factors on the comfort conditions in particular geographical location, Dear, Leow and Foo (1991) found that people working in naturally ventilated buildings in hot and humid country could accept a temperature value of up to 3 0 C warmer than Fanger s values. This with other similar findings especially the newly published ASHRAE standard (2004) have given the opportunity in introducing natural ventilation for commercial buildings in the hot and humid region. 1.1 Double-skin Façade and Thermal Comfort A number of interesting investigations and findings are reported in the literature pertaining to passive ventilation in buildings and the thermal performance of double-skin facades. Even though most of the researches are done mainly in temperate countries conditions but they have revealed close link between natural ventilation design and the function of double-skin façade. Grabe et al. (2001) developed a simulation algorithm to investigate the temperature behaviour and the flow characteristics of double facades with natural convection through solar radiation. Similar works on natural convection ventilation also reported by Ziskind et al. (2002, 2003), Bansal et al. (1994), Hamdy and Fikry (1998), and Priyadarsini et al. (2003). Most of them are using the idea of stack effect or the solar chimney concept and found that passive ventilation in summer is possible even for multi-storey buildings. In particular Priyadarsini et al. (2003) have concluded the energy efficiency of stack system used in residential of a hot and humid climate region. Li Y and Delsante (2001) went a step further to investigate the effects of natural ventilation caused by wind and thermal forces in a single zone building with two openings. Ventilation graphs are plotted using the air change parameters (thermal air change, wind air change and the heat loss air change) for design purposes. Gratia and Herde (2004) attempted to look at the impact of double-skin façade facing southern direction in a temperate climatic condition. Thermal analysis using simulation software of different seasons of a year was done for a low-rise office building with and without double-skin façade. It was found that significant energy saving is possible if natural ventilation could be exploited through the use of double-skin façade. This paper attempts to bridge the gap of looking into the possibilities of natural ventilation in high-rise office buildings specifically in the hot and humid climate region with the use of double-skin façade. The unique façade construction is thought to be able to act as a stack in providing required ventilation for the thermal comfort of the internal space. Airflow effects induced by wind and thermal forces onto a single office module

2 The 2005 World Sustainable Building Conference, constructed are to be observed for the first stage before a complex multi-storey office with all the thermal comfort parameters included are to be analysed. Therefore it is the intention of this paper to report on the findings of the first stage of the problem at hand. 2. Methodology 2.1 Computational Fluid Dynamic Simulation Computational Fluid Dynamic (CFD) has become a useful tool for designers in the study of indoor and outdoor environment conditions in building designs. The parameters such as air velocity and relative humidity solved by CFD are critical for designing an acceptable indoor comfort environment. CFD technique has been applied with considerable success in building design and the advantages in analysing ventilation performance have been reported by Murakami (1992). Papakonstantinou et al. (2000) has demonstrated that numerical solutions for ventilation problems can be obtained quickly and in good agreement with the experimental measurements. 2.2 Validation of The CFD Software A virtual prototyping simulation software called Airpak (2002) is used in this research to model the complex energy transfer through the component layers of the multiplayer façade through the optimisation of the appropriate opening sizes on the glazing, the width of the intermediate space and the ventilation rate through the internal office space. The validation of the software has been carried out by comparing the experimental and simulation results from another commercial simulation software called FloVent which was carried out by Manz H (2003). The measured hourly outdoor air temperature shown in Figure 1 are used for piecewise linear interpolation for the transient simulations. The simulation model for the validation is shown in Figure 2 and one of the comparison results are shown in Figure 3 below. Series 1 are the measured surface temperatures for the inner pane in the experimental results and Series 2 are the simulation results from Airpak. Both of the results are compared and analyzed and it was found that the variation is within 5% of the acceptable error tolerance Air Temp (C) Outdoor Temp Time (h) Figure 1 Measured hourly outdoor air temperature Figure 2 Simulation model for the validation

3 The 2005 World Sustainable Building Conference, 30 Inner Pane Surface Temp (C) Series1 Series Time (h) Figure 3 Comparison between the measured and CFD results 2.3 The CFD Models The single office module in 3D is constructed with the geometrical dimensions of 3.5m x 5.0m x 2.6m height (Figure 4). Numerous of simulation runs have been carried out for the benchmarking purposes in which a typical curtain walling office module was observed and a simplified nomogram has been established to define the initial parameters for thermal comfort in the tropic region. These results are compared with the simulation runs from the office module with double-skin façade construction. The simplified double-skin façade of the office module has openings on each of the external and internal panes with 6mm thick glass used at the external pane and 6/12/6mm double glazed used for the inner pane (Figure 5). Internal heat sources of two computers, four ceiling lights and two persons are introduced in the office space for thermal comfort analysis. The office module has two vents at the rear wall to introduce cross ventilation from the internal a/c space across the internal office space. Figure 4 Standard curtain walling for office module

4 The 2005 World Sustainable Building Conference, Figure 5 Double-skin facade for office module 2.3 The CFD Simulation In view of the complexity of the problem at hand, the modeling of the computer model has been broken down into different levels. The initial simulation was concentrated onto a single office space within a high-rise office building. The commercial office spaces could be grouped under three different sizes, namely small (~20m 2 ), medium (~50m 2 ) and large (>100m 2 ). This paper is focusing on the first office group, which is the small office space. The simulations are performed under steady state condition using k-epsilon turbulent model. The simulated wind speeds of 0m/s to 3.0m/s are used to model expected ground level wind velocities with ambient temperature of 30 0 C and relative humidity of 60% to 100%. The external temperature at the rear wall is set at 30 0 C to simulate a corridor area open to the external space. Only wind direction which perpendicular to the double-skin façade has been looked at for this stage. The upwind distance from the outer pane of the double-skin façade is set at 3 times the length of the office module. 3. Discussion 3.1 The Analysis Of The Simulation Results For the first stage of the analysis which this paper is going to report on, all simulations, be it the benchmarking cases or the double-skin scenarios, generated a cross ventilation effects from the internal naturally ventilated space across the office and discharged out through the internal pane opening into the intermediate space of the double façade. The strength of the cross ventilation will mainly depends on the airflow resistances within the intermediate space and the internal office space, together with the pressure differences between them. The magnitude of the internal ventilation will depend on the summation of the airflow resistances and in turn control by the smallest cross section area of the opening within the space.

5 The locations of the glass openings on the outer pane of the double-skin façade in relation to the inner pane will have effect onto the indoor thermal and airflow velocity. It was found that the higher the opening is located from the floor level it will generate a stronger stack effect within the air gap which in turn will pull more air out from the office space through the vents at the rear wall. The temperature generated within the office space is much desirable and closer to human comfort requirement. The airflow pattern created will be a good cross ventilation effect with cool air coming into the office space from the vents and right across and above the internal space and discharged out through the high level opening at the inner pane. This has lead to the selection of specific type of the double-skin construction, namely the Multi-storey Façade, which will create the strongest stack effect to pull maximum amount of air from the internal office space (Oesterle 2001). 3.2 Formulation Of The Nomogram The 2005 World Sustainable Building Conference, The results obtained from the benchmarking simulations, which is a typical curtain walling system façade, are compared to the results from the proposed prototype double-skin façade. The nomogram is formed by three axis which represent the three important parameters in thermal comfort analysis, temperature, air velocity and relative humidity. Boundaries of thermal comfort are plotted onto the nomograms from the analysis of the simulation results and the they are compared to see whether there are any advantages for using double-skin construction for an office building in the tropical climate. Figure 6 has shown that there are positive points in using the double-skin construction, as the shaded area for the double-skin façade is larger than the normal curtain walling construction (left side nomogram), even though this finding is only represent the low level results for the high-rise office building in study. Figure 6 Nomograms for benchmarking (left side) and double-skin facade 4. Conclusion This paper has found that a high level single opening at the outer pane of the double-skin façade will create a desirable cross ventilation airflow pattern at the internal office space. It was also found that double-skin façade did improve the thermal comfort of an internal office space by reducing the temperature from C to C, with the external wind velocity to be around 1.5m/s. The internal temperatures are still considered a bit high (as the model constructed for this paper is only considering the low level of a high-rise office building) but the situation will be expected to improve when multi-storey spaces are linked together in a highrise building when the stack effect of the air gap will increase tremendously. The results could be improved by using wind turbine at the top of the façade to increase the airflow velocity at the intermediate space to give effective airflow speed within the internal space. This will be looked at and analysed further in the coming development of the research. References Airpak User s Manual. 2002, Fluent Inc. and ICEM-CFD Engineering. ASHRAE. 2004, ANSI/ASHRAE Standard Thermal Environmental Conditions for Human Occupancy, Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.

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