Numerical Study on the Performance of Smoke Control System of an. Underground Ring Road

Transcription

1 Numerical Study on the Performance of Smoke Control System of an Underground Ring Road HU Longhua 1, WANG Haobo 1 *, ZHANG Jingyan 2, SU Guofeng 3, HUO Ran 1 & YAO Hong 4 (1 State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei , Anhui, China; 2 Institute of Building Fire Research, China Academy of Building Research, Beijing , China; 3 Center for Public Safety Research, Tsinghua University, Beijing , China; 4 Department of Electronic and Electrical Engineering, Hefei University, Hefei , Anhui, China) Abstract: CFD (Computational Fluid Dynamics) simulations were performed to predict the fire growth and smoke layer development in the underground ring road for the initial design of WUKESONG Culture and Sports Center, as the basketball match stadium for 2008 Olympic Games. Fire Dynamics Simulator version 3.10 of NIST was selected as the tool. A primary design and an improved safe and economical design for the smoke extraction system were verified by comparing the Available Safety Egress Time (ASET) provided by the smoke extraction system with the Required Safety Egress Time (RSET). A solution for the performance-based design of the fire smoke extraction system for such underground ring roads of sports stadium was presented for discussion. Keywords: CFD; smoke control system; performance; underground ring road 1 Introduction 1.1 Background The nineteenth Olympic Games will be held in Beijing in WUKESONG Culture and Sports Center, as one of the sports stadium which will be constructed, will be designed for performing the basketball match. This Center has an underground ring road in the initial design for the purpose of transportation of athletes, ceremony performers, officials and so on. Fire protection systems should be designed for this ring road. The current standard for fire safety design of tall buildings with height up to 24 m in China is the National Standards-Code for fire protection design of tall buildings (GB ), amended in 2005 by the Ministry of Public Security and the Ministry of Construction of People s Republic of China [1]. However, with the fast development of building industry and more and more large, tall and specially designed building appeared, it is facing more and more challenges as GB is essentially a prescriptive code and lacks flexibility. A more rational, economical approach, performance-based fire protection method, appeared and had evolved to meet these needs in recent years [2-5]. For different buildings, the characteristic of fire development is analyzed and the fire protection system will be designed according to these fire characteristics in performance-based fire protection method. The specific case presented in this paper was extracted from the performance-based design of the fire protection systems of for the initial design of the WUKESONG Culture and Sports Center. The primary design of the smoke extraction systems for the north part of its underground ring road was assessed. An improved design, also with consideration of economy, was demonstrated and verified afterward. Although this initial design of the WUKESONG Culture and Sports Center was finally changed and design of underground ring road was abandoned, it is hoped that the result of this paper may serve as an example for discussion of applications of performance-based engineering design for fire protection in such underground ring roads of sports stadiums. 1.2 Evaluation Method In deed, the objectives of performance-based fire safety engineering often cover wide ranges of issues such as property and environmental protection, continuous operation requirements for the machinery and so on [2, 3]. However, as it will accommodate over people during the match time, the main objective for the design of the fire protection systems for WUKESONG Culture and Sports Center is given to the life safety of the building occupants during fire emergencies. The way used to evaluate whether a design matches the objective of life safety or not is whether the Available Safety Egress Time (ASET) is greater than the Required Safety Egress Time (RSET), as a criterion reported in many literatures [2-6]. The RSET will simply be provided for comparison with ASET and the description of the calculation of the RSET will not be reported here. Field modeling, or CFD simulation, which was developed for compartment fire modeling in recent years [7-9], was performed in the present work to simulate the fire growth and smoke movement for the evaluation of ASET. The detecting time and the pre-movement time were set to be 1 min and 3 min, respectively. The egress travel time was calculated to be 12 min by the software package STEPS. So, the total RSET was estimated to be about 960 s (16 min). In determining of the onset of the untenable conditions to evaluate the ASET time, a series of criterions was reported and practically used in the literatures [6]. Accordingly, following criterions were used for the underground ring road fire safety evaluation. (1) The smoke temperature at the head height (2 m) exceeds 60 ; or (2) The optical density of the smoke layer at the head height (2 m) exceeds m, or the visibility at the head height (2 m) is less than 10 m; or the carbon monoxide concentration is greater than 500 ppm at the head height (2 m). 2 General Description of the Underground Ring Road As shown in Fig.1, the gymnasium and the functional rooms together cover an area of m m. The training Supported by China s National Key Technologies R&D Program (NKTRDP) in the 10th five-year plan under Grant No. 2004BA803B03-1 and National Natural Science Foundation of China under Grant No 转载

2 gymnasium covers an area of 72 m 36 m. The width designed for the north, west and east part of the ring road is 13.2 m while that for the south part is 7.5 m. 2 branch roads with width of 8 m located at the middle and south end of the west part and east part of the ring road each. The branch roads will connect the underground ring road with an upward tunnel leading to the ground level. 7 upward stairs were designed for evacuation of occupants from the underground level in case of emergency. The clear height of the underground ring road is 10 m. The downward distance of all beams in this ring road is 2.4 m. The space between m is partly filled by some pipes, machines and so on. Stair # m Stair # m Stair # m 8m 123.6m Stair # 4 Stair # 5 Stair # 6 Stair # 7 72 m 8m 36 m 7.5 m Fig. 1 Plan view of the underground ring road According to GB , an air supply system should be designed in this underground ring road to cooperate with the smoke extraction system in case of a fire and the air supply rate should not be less than 50% of smoke extraction rate [1]. But the fact is that it seems to be very hard to find any more free space to accommodate another big complex system. Performance-based fire protection engineering was brought forward to find an economical and safety-ensured way to solve this problem. The process of performance-based design of smoke extraction system for the north part of this underground ring road will be reported later in this paper. 3 Model Application The software package, Fire Dynamics Simulator (FDS), based on Large Eddy Simulation (LES) with a postprocessing visualization tool, SMOKEVIEW, developed by National Institute of Standards and Technology (NIST), USA is now a practical tool for simulating fires [10]. The set of the Navier-Stokes equations for low-speed, fire-driven fluid flow was solved numerically with LES. It is second-order accurate in space and time differences. Fire Dynamics Simulator version 3.10 was selected to model the fire growth and smoke movement in this underground ring road. Eight domains were defined to build the structure of the ring road and another one domain was defined to give finer grids to specify the fire source. Girds in the fire domain was specified as 5 grids per meter and that in the other 8 domains were specified as 2 grids per meter to save computing time. The 7 upward stairs were specified as cuboids with a full size opening at the top ceilings. And as it is unknown currently that how many space will be filled by the pipes, machines and so on, an assumption was made that 50% of the space from height 5 m upwards to the soffits of the beams was filled by obstructions. Their surfaces were specified as CONCRETE. Simulations were performed in a computer with Pentium IV 2.4 GHz CPU and 2 GB RAM. 4 Design Fire Traffic fires are the most possible to occur in an underground road. The most possible fires that will occur in this underground ring road are car fires, truck fires and bus fires. The experimental data describing the heat release rate history of such traffic fires were firstly reviewed to give more appropriate fire scenario design for the fire simulation. Some full-scale tests to understand the fire behavior in tunnels had been carried out formerly. Experiments reported by Heselden in 1976 indicated that the heat release rates of burning car, van and lorry/coach were 3 MW, 10 MW and 20 MW [8]. The World Road Association (PIARC) had also recommended fires for tunnel design, which are widely used for tunnel design in many countries, were 5 MW for passenger car and 20 MW for bus/truck fire. A full-scale burning car test was also conducted in Karlsruhe University in Germany as reported in The maximum heat release rate measured in this test was about 4.5 MW [11]. Typically, t-2 fire was used to characterise the fire growth stage in an enclosure. The fire growth can be widely represented as follows. t 2 Q = ( ) t 819 g

3 Fires are usually categorized into Ultra-fast, fast, medium and slow corresponding to the t g values (75 s, 150 s, 300 s and 600 s, respectively). Experimental data revealed that the ultra-fast fires are associated with plastic fuels or oil pool fuels while fast fires with papery boxes and medium fires with cotton polyester innerspring mattress [12]. 25 heat release rate (MW) fire designed time (s) Fig. 2 Fire scenario designed for simulation According to the information above, following fire was designed for simulation to give safety-proof and some conservative evaluation for the performance of smoke extraction system in the ring road: An ultra-fast fire with steady heat release rate of 20 MW was selected as shown in Fig.2. The dimensions of the fire source were 10 m (L) 2 m (W) 2 m (H). And in order to give conservative prediction of visibility by FDS, the REACTION was set to be Crude Oil to have more soot-yield for combustion. The CO production ratio was default set by FDS. 5 Preliminary Design and CFD Verification Preliminary, following technics were applied for the design of fire smoke extraction system for this underground ring road. (1) Totally, 11 smoke compartments were designed by smoke curtains with height of 5 m to block the smoke spread. However, the beams downward with elevations m should also act partly as the smoke barriers. (2) Only natural fresh air supply will be used to cooperate with the smoke extraction system as there is no more space to inhabit a big-complex air supply system. Fresh air will be naturally supplied from the stairs and the branch road into the underground ring road. (3) 2 mechanical fans with a volume flux of m 3 /h each were designed for smoke extraction for the north part of this underground ring road. The distribution of the smoke extraction fans, ducts and opening are schematically shown in Fig.3. Fig. 3 Preliminary design of the smoke extraction system for the underground ring road (north part) (4) When a fire occurs, only the openings in the fire-affected smoke compartment will be opened for smoke extraction to give the maximum smoke extraction volume flux and efficiency for the fire-affected compartment. If openings in 2 compartments were opened, the extraction volume flux for each compartment would be half of the total value, and be 1/N of that when opening in N compartment open. CFD simulation for verification was carried out by FDS version Fire was assumed to occur in smoke compartment #1. Only the openings in this smoke compartment would be opened to give a smoke extraction volume flux of m 3 /h for this 820

4 fire-affected compartment. The smoke extraction system would be activated by HEAT detectors with RTI=50 and ACTIVATION_TEMPERATURE=68 installed at this smoke compartment. Along with the growth of the fire, the HEAT detector installed activates at 234 s and the smoke extraction system begins to operate. The contours of smoke temperature, visibility and CO concentration in the north part of this underground ring road at 600 s (Fig.4). It can be seen from Fig.4 (b) and 4 (d) that the visibility at height of 2 m in the north part of the underground ring road was most less than 10 m. Dangerous situation appeared due to the visibility criterion at 600 s, although it seemed that the temperature criterion and CO concentration criterion at 2 m height had not reached at this time. Noting that the value of RSET is 960 s, the preliminary design of the smoke extraction system failed to guarantee life safety when emergency of a fire occurred in north part of the underground ring road. (a) temperature contours at the same x-z cross-sectional plan (b) visibility contours at the same x-z cross-sectional plan (c) CO concentration contours at the same x-z cross-sectional plan (d) visibility contours at x-y cross-sectional plan for z=2.0m Fig. 4 Predicted temperature, visibility and CO concentration contours under the preliminary design of smoke extraction system (at 600 s) 6 Improved Design and CFD Verification As the preliminary design failed to provide safety, the common way is to increase the extraction volume flux. But another problem encountered was that there was not more space to accommodate larger size ducts to give more extraction volume flux. So, in order not to make large changes to the preliminary design to save investment, an economical improved design was brought forward, mainly on the management of the operation of the smoke extraction system; (a) ~ (c): the same with preliminary design; (d): when a fire was detected in the north part of the underground ring road, all of the openings in this north part, including smoke compartment #11, #1 and #2, would be opened with the start-up of the 2 smoke extraction fans. In order to give even smoke extraction flux to each opening, 3 openings in the smoke compartment #1 would change to connect with the ducts in the smoke compartment #11 as shown in Fig.5. Openings #11 #1 #2 Fan1 (160,000m 3 /h) #10 #3 Fan2 (160,000m 3 /h) #9 # 4 Ducts # 8 # 5 Fig. 5 Improved design of the smoke extraction system for the underground ring road (north part) The contours of smoke temperature, visibility and CO concentration in the north part of this underground ring road at s are shown in Fig.6. It can be seen that the contours of temperature 30, visibility 10 m and CO concentration 100 ppm are all higher than that in Fig.4 for preliminary design. Although Fig.6 gives the situation after double of time compared with Fig.4, the smoke layer interface height is bigger than 2 m in s. The ASET is larger than RSET and the successful evacuation can be ensured for the improved design. 821

5 (a) temperature contours at the same x-z cross-sectional plan (b) visibility contours at the same x-z cross-sectional plan (c) CO concentration contours at the same x-z cross-sectional plan Fig. 6 Predicted temperature, visibility and CO concentration contours under the improved design of smoke extraction system (at s) 7 Conclusions A case of performance-based design of the fire smoke extraction system for an underground ring road was presented. Based on CFD simulation results, the performance of the fire smoke extraction systems was evaluated. Safe evacuation in case of the fire was also achieved by improved management of the operation of the smoke extraction system with keeping the capacity of the smoke extraction fan, rather than traditionally by enhancing the capacity of the smoke extraction fan with a much larger investment according to the current prescriptive building code. An economical and safety-ensured design was found. The way demonstrated here may be referred to when designing fire smoke extraction systems for similar underground roads. References [1] The Ministry of Public Security of P.R.China. Code for Fire Protection Design of Tall Buildings. GB , 2001 [2] Buchanan A H. Fire engineering for a performance-based code. Fire Safety J, 1994, 23: 1-16 [3] Buchanan A H. Implementation of performance-based fire codes. Fire Safety J, 1999, 32: [4] He Y P, Wang J, Wu Z K, et al. Smoke venting and fire safety in an industrial warehouse. Fire Safety J, 2002, 37: [5] Hu L H, Li Y Z, Huo R, et al. Smoke filling simulation in a boarding-arrival passage of an airport terminal using multi-cell concept. J of Fire Sciences, 2005, 23(1): [6] Hadjisophocleous G V, Benichou N. Performance criteria used in fire safety design. Automation in Construction, 1999, 8: [7] Novozhilov V. Computational fluid dynamics modeling of compartment fires. Progress in Energy and Combustion Science, 2001, 27: [8] Chow W K, Jojo S M. Numerical studies on performance evaluation of tunnel ventilation safety systems. Tunnelling and Underground Space Technology, 2003, 18: [9] Chow W K, Hung W Y. On the fire safety for internal voids in highrise buildings. Building and Environment, 2003, 38: [10] McGrattan K B, Forney G P, Floyd J E, et al. Fire Dynamics Simulator (Version 3) User s Guide. Gaithersburg, MD., [11] Cheng Y P, John R. Experimental research of motorcar fire. Journal of China University of Mining & Technology, 2002, 31 (6): [12] Babrauskas V. Burning rate. In: SFPE Handbook of Fire Protection Engineering. Quincy, MA: Fire Protection Association,