Simulation Study on the Spreading of Fire-induced Smoke in Natural-ventilated Double Skin Facade Buildings

Jingjing Shao1(), Siegfried K. Yeboah2, Tiesheng, Zhu1, Yilin, Li3

1 School of Architecture and Transportation Engineering, Ningbo University of Technology, Ningbo, China

Jingjing.Shao@nbut.edu.cn, https://orcid.org/0000-0002-7093-762X

2 Faculty of Science and Engineering, The University of Nottingham Ningbo China, Ningbo, China

3 School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai, China

Abstract. Double Skin Facade (DSF) is getting more and more popular these days due to its energy saving potential and aesthetics. This particularity design brings certain difficulties to the fire protection of the building. Especially for natural-ventilated DSFs, the smoke flow between floors and DSF is more complicated depending on ventilation strategies. In order to gain more understanding and to acquire reliable data for future design, an naturally ventilated DSF building, Center of Sustainable Energy and Technology in Ningbo, was studied numerically. Computational Fluid Dynamic (CFD) analysis was carried out focusing on the characteristics of smoke distribution. Results showed that even though CSET building met the national fire safety requirement, it still failed to prevent fire spreading, or ensure evacuation safety of occupancy. Simulations also suggested that opening up the DSF during fire would actually be more effective at preventing fire from spreading as well as ensuring occupancy evacuation.

Keywords: Simulation study, Smoke control, Double skin facade.

Introduction

In recent years, the double-skin facade (DSF) has been widely applied in modern buildings due to its aesthetics and energy-saving capacity [1]. DSF serves as a kind of envelope structure of buildings, which composes inner glass facade, outer glass facade and an air cavity in between (Fig. 1). DSF can be 1) internal ventilated through openings on inner glass facade, 2) external ventilated through openings on outer glass facade and 3) non-ventilated.

Fig. 1. General assembling of DSF

This particularity design brings certain difficulties to the fire protection design of the building. Yet how to effectively design a fire protection system for DSF buildings has become a huge problem. Despite of the latest releases of Code for Fire Protection Design of Buildings [2] and Technical Standard for Smoke Management Systems in Buildings [3], how to design an reliable smoke control systems for natural-ventilated DSF is still unclear. It is mainly because of the lack of understanding of how fire and smoke would spread during fire. Even though there have been some studies trying to address this problem through modelling or experiments [4-6], these studies were only limited to non-ventilated type of DSF, who had no openings on both sides of glasses. However, most of the DSF buildings in China are actually ventilated, for instance the zero carbon building in Tsinghua University, Beijing SOHO Residences, etc. For ventilated DSFs, the openings between floors and DSF make smoke control more complicated.

In order to gain more understanding and to acquire reliable data for future design, the current study would take the ventilated DSF of Center of Sustainable Energy Technologies (CSET) in Ningbo as an example. A 2-D model has been built to evaluate the fire-induced smoke flow characteristics inside a building using validated numerical models.

Method

Case Study

The building under evaluation is the Center of Sustainable Energy Technologies (CSET) located at Ningbo China. The building incorporates a south-facing double skin facade which tilts forward from both the top and the base of the wall. During winter period the opening at the top of the DSF is closed to allow fresh air to enter the building through openings in the base of the inner glass. The return air supply is evacuated through the top opening in the light-well as shown in Fig. 2a. In summer, the DSF serves as a thermal buffer for reducing heat gain into the building and for removing the extract ventilation form the building as shown in Fig. 2b [7]. A detailed schematic diagram of ventilation opening on inner glass facade is shown in Fig. 3.

Fig. 2. Ventilation strategies of DSF

Fig. 3. Schematic diagram of ventilation opening on inner glass

Fire Design Strategy

Being a multi-story building, CSET is designed with a minimal fire protection protocol. There is no sprinkler system or hydrant system in place. The most potential cause of fire is electrical fire.

Natural ventilation is adopted as the smoke management strategy. Once the fire is detected, the openings to light well will be open, allowing smoke to travel up and leave building from top. All the openings on DSF will be closed automatically, as well as any HVAC systems.

There are two exit points inside the building, on F1 and F3 respectively. Occupants on F4 and F5 need to get to F3 first before leave the building, through the evacuation staircase which shares walls with light well. F2 occupants needs to use this set of staircase to get to F1 for evacuation.

Numerical Model and Simulation

ANSYS FLUENT 16.0 software was used to simulate the smoke distribution in the cavity of the DSF. It is assumed that fire started in the middle of F3. Simulation was run three times with three different scenarios as shown in Table 1.

Table 1. Simulation Scenarios

Opening to DSF	Opening on top of DSF	Opening to Light well
Base Case	Close	Close	Open
Scenario 1	Open	Close	Open
Scenario 2	Open	Open	Open

As recommended by previous studies [7], for the benefit of saving computational resources and simulation time as well as minimizing errors, a simplified two-dimensional section of the building was modeled with a 0.5-2mm grid size range. The meshing has been shown in Fig. 4.

Fig. 4. Meshing of CSET model

For the turbulence model, the Standard k-epsilon (k-ε) model was applied since it is one of the most frequently used models in fluid dynamics and also the most common turbulence model for fluid flow simulations. The fire-induced smoke is described with Species Transport Model with a mixture of Carbon Monoxide (CO) and air. The PISO calculation method was used the solve the problem. The governing equations are expressed as:

(1)

(2)

Where, P, μm and ρg are static pressure, molecular viscosity and gravitational body force, respectively.

Results and Discussion

Base case

With the original design, once fire is detected, DSF would be sealed completely in order to prevent smoke from spreading to DSF. It meets with the suggestions from most of researchers in this field [4-6, 8, 9].

As Fig. 5 presented, smoke entered the light-well soon after fire and started to travel downwards. Smoke penetrated into lower floors in 60s. By then, the temperature of smoke was already over 500˚C, it was highly likely to cause another fire.

Instead of discharged to outside, smoke stayed inside the light well. As the smoke could not be discharged effectively, it started to spread into evacuation stairs just beside the light well, causing difficulties for evacuation. The current smoke control strategy was not suitable at all as the aim of smoke control system is to ensure the safety evacuation of occupancy.

Fig. 5. Smoke spreading in winter mode-Sealed DSF

Scenario 1

In order to reduce the risk of smoke hazard in CSET, two approaches have been proposed and analyzed with CFD.

In this scenario, light well was still the only exhaust point for smoke, but openings to DSF would be open while the top of DSF remain closed. It was expected that fire induced smoke entering DSF would be cooled down enough by cold fresh air and finally leave building from bottom of DSF.

Once ignition, it only took 40s for smoke to take over the whole floor, and another 5s to enter DSF and light-well. As the simulation results suggested (Fig. 6), most of the fire induced smoke entered DSF and mixed with cold fresh air, causing temperature to drop significantly, from 677˚C to 477 ˚C. Under the influence of buoyant force, smoke spread to F4 in 60s and F5 in 70s, causing fire to expand. It was also noticed that smoke travelled back into building space before they could be exhausted through light well.

Simulation results suggested that opening DSF during fire would reduce the amount of smoke inside the light well. It might be a good thing in this particular case, as the evacuation staircase is located just next to the well.

Fig. 6. Smoke spreading-Scenario 1

Scenario 2

In this scenario, openings on the side and top of DSF would be open during fire, as well as the path of light well. Results are shown in Fig. 7 and Fig. 8.

Fig. 7 showed the smoke distribution under scenario 2. Compared with base case, much less smoke was found in light-well. As a result, it was less likely to invade into the evacuation staircase. But additional smoke prevention strategies should be applied to prevent smoke re-entering from opening on light-well. Fig. 7 also suggested much less smoke in DSF, and most of which was exhausted from top of DSF in 7s.

Fig. 7. Smoke spreading- Scenario 2

Fig. 8 compared the temperature distribution inside DSF. There was little temperature variation on the external glass facade, while that of inner facade varied dramatically ranging from 677˚C to 70˚C. It is mainly because smoke inside DSF were mostly attached to inner glass facade. Such a big temperature variation posed a great threat to the integrity of glass facade.

Fig. 8. Temperature distribution- Scenario 2

It was worth to point out that there also existed a large temperature difference over the two sides of glass. The surface exposed to smoke can be as high as 677˚C, while the other surface not exposed to smoke was only 70˚C. Such a big temperature difference, glass would broke in 95s as estimated by Su [10]. Once the glass was broken, fire will enter DSF directly threatening the whole structure of building. So further enhancement on fire resistance of glass should be explored.

Conclusions

In this paper, the mixed ventilated DSF of CSET building were simulated. It was found that the current fire prevention design is not sufficient. Contrary to general cognition, sealing the DSF off may not be the best option for CSET building, as smoke would invade the evacuation staircase next to the light-well. So two different solutions have been proposed and analyzed. Scenario 1 was to open the inner glass openings, allowing smoke to get into the DSF, hoping it would leave from bottom. However, it only led to the spreading of fire. Scenario 2 was to open all the openings of DSF for smoke discharge. This method was proved to be effective. DSF provided another path for smoke, leaving much less smoke into light-well, it was less likely to invade into the evacuation staircase. But additional smoke prevention strategies should be applied as smoke re-enters the building from opening on light-well. The cold fresh air in DSF also helped to cool smoke down as it travelled up, reducing the risk of fire expanding. Yet to prevent glass from breaking due to high temperature difference, further enhancement on fire resistance of glass was necessary and would be the focus of future studies.

Acknowledgments. The project is supported by Zhejiang Provincial Natural Science Foundation (Number LQ19E080004).

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