1、4839 Smoke Control Through a Double-Skin FaCade Used for Natural Ventilation Wenting Ding Yuji Hacemi ABSTRACT Double-skin faades have been increasingly used, as they open up new possibilities for clients and planners seeking creative new designs that are intelligently adapted to environ- mental con
2、ditions. Natural ventilation is commonly carried out in a building with double-skin faades; however, double- skin faade construction is still not covered by statutory build- ingregulations. Vrtually no information exists on the behavior of this kind of faade in case of aJire. Usually, for a building
3、 with a multistory double-skin faade, smoke from aJire room escaping through the inner faade into the intermediate space between the two skins may accumulute andspread horizontally and/or vertically to other rooms that have openings connected to the intermediate space for thepurpose ofnatural ventil
4、ation. However, considering the similarity of smoke movement and stack ventilation-both driven by stack efect-this paper considers using a double-skin faade for smoke control as well as for natural ventilation. As a result, it is proved that smoke spread can be prevented with suitable arrangement of
5、 open- ings. Therefore, natural ventilation and smoke control can be realized through one system. Reduced-scale model experi- ments and computationalfluid dynamics (CFD) analysis were carried out in this research. INTRODUCTION To reduce global environmental damage and pursue high- quality indoor con
6、ditions in office buildings, the double-skin faade is getting more and more attention, as it provides many possibilities for energy conservation and at the same time creates a good indoor environment. However, double-skin faade construction is still not covered by statutory building regulations. Vir
7、tually no information exists on the behavior of this kind of faade in case of a fire. Usually, for a building with a multistory double-skin faade, smoke from a fire room escaping through the inner faade into the intermediate space between the two skins may accumulate and spread horizon- tally and/or
8、 vertically to other rooms that have openings to the intermediate space for the purpose of natural ventilation. In a naturally ventilated building, there are openings with flow paths throughout the whole building, and when a fire occurs, the flow paths for natural ventilation also become paths for s
9、moke to spread to other, non-fire spaces. This is always pointed out as a weakness ofnaturally ventilated build- ings. However, stack ventilation and smoke movement are both driven by stack effect, which offers us the possibility of using one system to handle these two problems. It has been proven t
10、hat for a naturally ventilated atrium with a solar chim- ney on top of the atrium, natural ventilation and smoke control can be realized through the same system (Ding et ai. 2004; Ding et al. 2003). Similarly, it is also possible to deal with smoke problems and natural ventilation using the same sys
11、tem for buildings with double-skin faades. As long as smoke pres- sure in the double-skin space can be kept lower than that of the occupant space, smoke spread through the openings for natu- ral ventilation will be prevented. PROTOTYPE BUILDING Outline of the Prototype Building A prototype building
12、is proposed for further discussion (Figure 1). It is an eight-story office building with an.atrium space on the north side. Staircase and utility space are contained in the atrium. The south faade of the building is a double-skin faade, and a three-stoy-high thermal storage space called a ?solar chi
13、mney? is considered to be above the double-skin space to strengthen stack effect. The double-skin space is connected with the chimney channel. Wenting Ding is a research associate and Yuji Hasemi is a professor in the Department of Architecture at Waseda University, Tokyo, Japan. 02006 ASHRAE. 181 T
14、hermal storage wall 11 2,01101 22,000 15,000 29,000 Utility space 2,coo I 22,000 15,000 29,000 Figure 1 Outline of the protogpe building (mm); left: plan; right: section. Thermal storage wall Pressure difference against outside( -i Positive to outside, - Negative to outside) Figure 2 Concept of natu
15、ral ventilation. Natural Ventilation in the Prototype Building The south-facing wall of the solar chimney is considered to be a thermal storage wall. Solar radiation passes through the outer glazing of the double-skin faade and the chimney, and air inside the double-skin space and the chimney channe
16、l is warmed. With moderate setting of openings, natural ventila- tion throughout the building is activated by stack effect, as shown in Figure 2. Fresh air is taken in from openings between the atrium and outside, passes through the occupant space, and is discharged into the double-skin space. It is
17、 finally exhausted from the outlets on top of the chimney. As long as the pressure of the double-skin space is lower than that of the atrium, even . Pressure difference against outside( i-: Positive to outside; -: Negative to outside) Figure 3 Concept of smoke control. at the top floor height, natur
18、al ventilation of all the occupant spaces can be realized. Smoke Control in the Prototype Building When a fire breaks out in any room, smoke may leak into the double-skin space or atrium space. For the purpose of natu- ral ventilation, openings are located between the outside and the atrium, the atr
19、ium and occupant space, occupant space and double-skin space, and double-skin space and the outside. As the openings between the outside and the atrium serve as fresh air supply openings, during a fire smoke flows into the double- skin space. It accumulates in the chimney space and at the same time
20、is exhausted from the openings on top of the chim- ney. It is predicted that the smoke layer will descend below the occupant space due to the small volume of the solar chimney. 182 ASHRAE Transactions: Research However, as long as the pressure of the double-skin space can be kept lower than that of
21、the atrium space, even at the top floor height, as shown in Figure 3, smoke accumulated in the double-skin space will not invade the occupant space. Conditions for obtaining moderate natural ventilation in the occupant space and preventing smoke propagation to the occupant space will be examined. Re
22、duced-scale model experiments and computational fluid dynamics (CFD) analy- sis are discussed. MODEL EXPERIMENTS Actually, it is almost impossible to preserve the Grashof number in the reduced-scale model and the full-scale building. However, it is found that even for free convection, as long as the
23、 turbulent intensity of the flow is over some value of the Grashof number, the basic characteristic of the flow becomes independent of the Grashof number. For the naturally venti- lated space, most of the flow region can be regarded as such a state (Shoda 1981). Therefore, substituting the turbulent
24、 viscosity vf, which is proportional to the product of the local velocity U and length 1 (Murakami 1980) for the Grashof number, Scale Modeling Between the reduced-scale model and the full-scale prototype building, the following scale modeling is taken into VtK u1 (6) account. When considering smoke
25、 movement, the Froude number is most commonly used. It can be thought of as the (Gr), K e. (7) u ratio of the inertia force to buoyancy. The turbulent Grashof number becomes in accord with the Froude number. (1) Considering ATM = AT, then U = characteristic velocity, mis g = acceleration due to grav
26、ity, mis2 I = characteristic length, m ing relations can be obtained: r r Considering the same ambient environment, the follow- The modeling mentioned above can be used to establish relations between the reduced-scale model and the full-scale 1 prototype building. Description of the Experimental Mod
27、el (2) ATM = AT, where AT = temperature rise, K (3) Considering manual operability of the experiments and the similarity of the basic characteristic of the flow in the reduced-scale model and the full-scale prototype building, the experimental model is reproduced as 1/25 of the full-scale prototype
28、building. Figures 4 and 5 show an outline and a photograph of the experimental model. (4) Q = heat release rate, kW Experimental Conditions A4 = model F = full-scale. To realize preferable airflow throughout the building, the When considering free convection, the Grashof number is arrangement Of Ope
29、nings be Planned always used, which is the ratio of the buoyancy to viscous (Figure 6). Openings between the outside and the atrium allow force. fresh air to flow into the atrium. Doors between the atrium and occupant space are considered openings for ventilation, and their area is assumed as 2 m2 o
30、n each floor. Openings between discharged into the double-skin space and finally exhausted where from openings on top of the chimney. Experiments with g = acceleration due to gravity, m/s2 several arrangements of openings were carried out (Table 1) = thermal expansion coefficient, 1K for both natura
31、l ventilation and smoke movement. Tempera- AT = temperature rise, K tures in the double-skin space, occupant space, and atrium and 1 = characteristic length, m pressure distribution in the double-skin space are measured in V = viscosity, m2/s. all of the experiments (Figure 6). Gr = e (5) V occupant
32、 space and double-skin space allow aidsmoke to be ASHRAE Transactions: Research 183 Extenor 5 mm thick insulator -c -Interior Panel heater / 20 mm thick styofoam + A16n8 , 20 mm thick Styrofoam 8 20 mm thick Styrofoam 7F 20 mm thick Styofoam 6F 2OmmthickStyOfOam 5 3 / 20 mm thick Styofoam , 20 mm th
33、ick Styrofoam 4F SF ,20 mm duck Styrofoam 2 20 mm thick Styofoam 1 880 200 32cm x 4cm x2 16 cm x 2cm x 8F 8 cm x 4cm x 8F 4 cm x 4cm x 8F (8 m2 x 2) (2m2 x 8F= 16m2) (2 mz x 8F= 16mZ) (i m2 x 8F=8m2) B g L a 1 mm left: A-A section; right: plan. Pressure difference Temperature Figure 6 Arrangement of
34、 openings and measuring points. Table 1. Experimental Arrangement of Openings Openings Case 4 cm x 4 cm x 8F (i m2 x SF = 8 mZ) 8 cm x 4 cm x 8F It7 m2 x RF= Ihm2) I 32 cm x 4cm x 2 Figure 5 Photograph of the experimental model. 1 84 * Model Size (Full-Scale Size F = floor ASHRAE Transactions: Resea
35、rch O2468 -0.2 -0.1 o 0.1 0.2 I a i - O2468 -02 -01 o 01 0.2 Case A8D8 -.l IJ Case A16D8 Case A16D16 O2468 -0.2 -0.1 o 0.1 0.2 Temperature rise in the double-skin space (T) Pressure difference (Pa) Figure 7 Comparison of CFD and experimental results (natural ventilation). In the case of the natural
36、ventilation experiments, to simu- late the temperature rise of the double-skin faade and blinds due to absorption of sunshine, panel heaters are used. Although temperature rise occurs on both surfaces, glazing and blinds, in the experiments temperature rise of the glazing is considered small and onl
37、y temperature rise of the blinds is reproduced. Panel heaters are attached to the interior surface of the outer faade for easy operation. Temperature rise of the thermal storage wall is also reproduced by panel heaters. Insu- lators are used as exterior surfaces to reduce heat loss from double-skin
38、space and the chimney channel. Considering the thermal properties of the blinds and the thermal storage wall, temperature rise of the blinds is set 10C higher than outside and that of the thermal storage wall is set 20C higher than outside. Of course, temperature rise will be dependent on actual cli
39、mate conditions. During the experiments, incense sticks were used to judge airflow direction near the openings. In the case of the smoke movement experiments, consid- ering that a fire occurs on the first floor, ethanol is used as a fire source and put in a shallow metal dish with a diameter of 4.5
40、cm. The dish is put on the bottom of the base floor and in the center position (Figure 4). A smoke stick is used to visu- alize smoke movement. Assuming an initial real fire with a heat release rate of 3 MW, it will be reproduced as about 960 W in the experiments according to the similarity law of E
41、quation 4. In each experiment, 15 mL of ethanol is burned and the time-averaged heat release rate can be calculated as ASHRAE Transactions: Research 185 P VAH, 4 = 7, (9) where 4 = time-averaged heat release rate, kW; P V = volume of burned ethanol, m3; AHc = effective heat of combustion of ethanol,
42、 Hikg; and t = burning time, s. According to the results of the experiments, the time-averaged heat release rate of the fire source is about 880 W, which equals a real fire of around 2.7 MW, according to the similarity law of Equation 4. = density of ethanol, kg/m3; NUMERICAL MODELING Although airfl
43、ow conditions can be grasped through the model experiments to some degree, there are still many unknown factors existing in the experiments due to limitations of the testing instruments and model scale. Therefore, numer- ical calculation is also conducted to contrast with the experi- mental results.
44、 The numerical simulations for this research are done using a general three-dimensional computational fluid dynamics (CFD) model. The indoor zero-equation model and standard K-E model are used for reproducing phenomena of natural ventilation and smoke movement, respectively. The computational domain
45、 is basically divided into a grid of 0.03 x 0.03 x 0.03 m control volumes. The thermal property of all the walls and floors is set as that of the experimental model. Opening conditions are also set as that of the experiments (Table 1). In the case of the smoke movement experiments, a cylin- drical v
46、olumetric heat source is used to reproduce the fire source, whose diameter is set as 4.5 cm and height is set as (McCafiey 1979) Zf= 0.08Q2I5, where Q is the heat release rate of the fire source in the exper- iments, which is set as the calculated value according to Equa- tion 9, based on the experi
47、mental results. COMPARISON OF CFD ANALYSIS AND EXPERIMENTAL RESULTS All of the results show that the temperature rise of the simulation is somewhat higher than that of the experiments. One of the reasons for this difference may be low airtightness of the experimental model. For the simulation model,
48、 on the other hand, no heat will be lost due to existing gaps. In the experiments, pressure differences between the outside and the double-skin space are measured at four points. Pressure differ- ences between the outside and the atrium are also illustrated in the figures according to the simulation
49、 results. Natural Ventilation Openings on top of the chimney in Case A16D8 are two times those of Case A8D8, which leads to a small temperature rise in the double-skin and chimney channels. Pressure differ- ence from the bottom to the top of the double-skin space decreases gradually, which means the driving force for venti- lation of the occupant space also decreases. Therefore, airflow of the upper floors should be specially confirmed. As the experimental results show, airflow direction at openings between the atrium and occupant space on the eighth floor
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