IFireSS International Fire Safety Symposium Coimbra, Portugal, 20 th -22 nd April 2015 SCALE MODEL EXPERIMENTS ON SMOKE MOVEMENT IN A TILTED TUNNEL photo Author 1 30 mm 40 mm photo Author 2 30 mm 40 mm photo Author 3 30 mm 40 mm S.I. Tsang Graduate Student The Hong Kong Polytechnic University W.K. Chow Chair Professor The Hong Kong Polytechnic University Gigi C.H. Lui Assistant Professor The Hong Kong Polytechnic University ABSTRACT Smoke movement in tilted tunnel fires were observed in a scale model with a propanol pool fire. A 1/20 scale model tunnel of rectangular cross-section with adjustable horizontal angles was constructed. Propanol was burnt with smoke pellets to generate smoke for visualizing the smoke movement pattern. A light emitting diode torch was used for illustration in taking pictures and videos. Smoke movement pattern in the tunnel at six different inclined angles to the horizontal, with and without smoke barriers were observed. Three smoke barriers were placed along the tunnel and different smoke movement patterns were observed with different barrier depths. Longitudinal ventilation was provided by an axial fan with the ventilation rate adjusted by a transformer. Results indicated that smoke would fall to lower level when the tunnel is tilted at larger angles. Keywords: Scale modeling experiments, smoke movement, tilted tunnel. Corresponding author Department of Building Services Engineering, The Hong Kong Polytechnic University. Hung Hom, Kowloon, Hong Kong, CHINA. Tel.: +852 2766 5843 Fax: + 852 2765 7198. e-mail: beelize@polyu.edu.hk; bewkchow@polyu.edu.hk 543
1. INTRODUCTION Many tunnels were or will be built for vehicles, railway systems and underground subway systems in the Far East [1]. Evacuation with thousands of passengers under a tunnel fire should be studied carefully. Passengers have to travel for a long distance inside tunnels. As specified in the standard NFPA 130 published by the National Fire Protection Association, USA, the maximum egress travel distance of emergency evacuation passage for train tunnels is 762 m [2]. People might even have to travel for a distance over 1000 m in some areas claimed to have difficulties in providing shorter intervals between exits [3]. Performance-based design (PBD) [4,5] was applied to demonstrate that fewer fire safety provisions would be appropriate when comparing with the prescriptive code requirements. Consequently, many PBD projects were just carried out for reducing the costs for fire engineering systems as pointed out [1]. To keep the consultancy fee low, relevant data on safe evacuation with extended travel distances were only worked out from fire simulations using freely downloaded Computational Fluid Dynamics (CFD) software. There were no justification by experiments with full-scale burning tests on such software predictions in most PBD projects as raised by Chow [1,3,5-7]. The design guides and data developed overseas can only be adopted after detailed investigations. Social awareness on safety education of citizens and implementation of government regulations in different areas are not the same. Questions [7,8] on using CFD for PBD projects were raised. The CFD predicted smoke movement in a tilted tunnel in many PBD projects [9] was not justified by any physical experiment, not even by simple scale modelling studies. It is difficult to travel through smoke if the visibility is less than 8 m [1]. Smoke can spread into different parts of a tunnel at a rate much faster than the human travel speed. Consequently, smoke was the major killer in many fires. Therefore, smoke control in a tunnel fire is important for emergency evacuation planning. Longitudinal ventilation is required in tunnels built in the past decade in some places. Smoke should be driven to one side when the longitudinal ventilation provided an air speed higher than a critical value. People can then leave the tunnel from the other end. That longitudinal ventilation design was demonstrated to be workable in a train tunnel in Hong Kong under a small train fire at the early stage of burning [8]. As pointed out before by Chow and associates [10-12] on vehicular and railway tunnels built with an angle θ tilted to the horizontal, there is an acceleration vector gsinθ for smoke movement along the longitudinal axis. Smoke movement pattern of a tilted tunnel will be very different from that of a horizontal tunnel. Smoke control system design would then be very different. The critical velocity of the longitudinal ventilation in a tilted tunnel fire would be very different from that estimated from empirical equation for a horizontal tunnel. Possible scenarios should be justified in PBD projects, at least with scale models, and not just proposing something 544
without any experimental evidence [9]. Further, vertical barriers (or downstands) are commonly installed to restrict smoke movement along the ceiling. Smoke movement in a tilted tunnel with smoke barriers of different depths should also be studied. In this paper, smoke movement in tilted tunnel fires were observed in a scale model. A 1/20 scale model tunnel with adjustable horizontal angles was constructed for the experiment. Propanol was burnt with smoke pellets to generate smoke for visualizing the smoke movement pattern. Smoke movement pattern in the tunnel at six different inclined angles to the horizontal, with and without smoke barriers were observed and reported. Different smoke movement patterns were also observed with different barrier depths. Longitudinal ventilation was provided by an axial fan with varied ventilation rates. 2. SCALING EFFECTS The real tunnel is scaled down to form a 1/20 model using the Froude scaling. The length scales L m and L f (in mm) are taken to be the heights of the model and the full-scale tunnels respectively. The heat release rates Q m and Q f (in kw) in the model and full-scale tunnels, the velocities V m and V f (in m/s) in the model and full-scale tunnels are scaled by [13,14]: Q Q m f L L m f 5 2 (1) V V m f L L m f 1 2 (2) A design fire of burning a small passenger car with Q f of 2 MW [12] was selected, though the value might be too low [1,3]. Appling Froude scaling to the 1/20 scale model, Q m can be calculated by: 5 2 1 Lm 6 Q 210 W 1118W (3) m 20 L f Value of Q m used in the model is therefore about 1.1 kw. 545
3. SCALE MODEL A 1/20 scale model of tunnel of rectangular shape with a cross-sectional area of 105000 mm 2 was used. Its dimensions were mm (H) x 375 mm (W) x mm (L) as shown in Fig. 1. It was made up of 6 mm thick transparent acrylic sheets for the observation of the smoke movement. Different tunnel inclined angles, barrier depths and ventilation conditions could be adjusted in the experiments to study the critical velocities under different fires. Barriers of different depths were made and installed on the ceiling of the tunnel to observe the smoke movement patterns just under and passing through the barriers. B C Fan A Thermocouples 375 A B C (a) Schematic drawing Pool fire Fan (b) Longitudinal view Figure 1: The tunnel model 546
A small propanol pool fire was used to induce air movement inside the tunnel. Smoke pellets were used to generate smoke as tracer to visualize the smoke movement. The amount of this clean fuel was prepared for maintaining adequate long burning duration and for smoke visualization. Since the mass loss rate was not steady, the amount of fuel required could not be estimated by simply multiplying the average mass loss rate and burning time. By trials, it was found that 1.8 g of propanol would allow 5 minutes of burning. The tunnel model was adjusted to incline at six different angles from 0 o to 30 o with and without smoke barriers. Rectangular smoke barriers of depths D being 50 mm, 100 mm and 150 mm were constructed. They were placed in three positions A, B and C. Each of them was separated by 425 mm along the mm tunnel as shown in Fig. 1 and Fig. 2. The barriers were well sealed to the ceiling of the tunnel model to minimize smoke leakage. In this experiment, longitudinal ventilation was provided by an axial fan with the ventilation rate adjusted by a transformer. A Light Emitting Diode (LED) torch was used for illustration of the smoke movement patterns and a high resolution anti-vibration camera was used for taking pictures and video. A total of 18 K-type thermocouples were used to measure the smoke layer temperatures. 425 425 425 425 425 425 425 425 Pool fire D Pool fire D Fan 4. EXPERIMENTS (a) No fan (b) With fan Figure 2: Cross-sectional view of setup I There are four stages of experiments to investigate the effects of the smoke barrier and the longitudinal ventilation. The following result is the behavior of smoke just under and passing through the barriers. Stage 1: In this stage, the angle of the tunnel is tilted (0 o, 5 o, 10 o, 15 o, 20 o and 30 o ) under 4 barrier conditions, i.e. with no barriers and smoke barriers with depths 50 mm, 100 mm and 150 mm. The separation between each barrier along the tunnel model is 425 mm. The aim of this stage is to investigate how the tilted angle and the depth of the smoke barriers affect the smoke pattern. During this stage, the upper hot smoke layer temperature is recorded by the thermocouples and 547
the processes of the experiments are recorded by camera. The cross-sectional view of this setup I is shown in Fig. 2a. Stage 2: In this stage, everything is the same as Stage 1 but ventilation by an axial fan is introduced. It aims to investigate how the ventilation influences the smoke pattern. The critical velocity which is the minimum velocity for preventing the back layering is found. The cross-sectional view of this setup is shown in Fig. 2b. Stage 3 In this stage, the setup is similar to that in Stage 1. However, the barrier separation is increased to investigate the effects of the separation of the barriers. The cross-sectional view of this setup II is shown in Fig. 3a. Stage 4 In this stage, the setup is similar to Stage 3 but ventilation is incorporated. It is for investigating whether there is any change in the ventilation rate or how much it changes if the separation of the barrier is increased. The cross-sectional view of this setup is shown in Fig. 3b. 425 A 850 C 425 A C 425 850 425 D barrier Pool fire D Pool fire Fan (a) No fan (b) With fan Figure 3: Cross-sectional view of setup II 5. RESULTS From the experimental results of many tests mentioned in the above stages, the tunnel arrangement without tilting an angle to the horizontal and another one with a tilted angle at 5 o, with smoke barrier depth of 50 mm and with no ventilation being provided in setup I were selected to be discussed in more detail. The transient smoke movement pattern observed at about every 4 seconds in each case is shown in Fig. 4 and Fig. 5 respectively. As shown in Fig. 4, when the plume rises up and reaches the ceiling, it spreads along the ceiling slowly. When it passes through the smoke barriers, disturbance and turbulence occur. After passing through the third barrier, it cannot further move on and it falls down. Back layering occurs. It is because the bombardment of the smoke and the barrier and the heat exchange between the smoke layer and the ambient air cause the loss of buoyancy force on the smoke. 548
(a) At about 0 s (b) At about 4 s (c) At about 8 s (d) At about 12 s (e) At about 16 s (f) At about 20 s Figure 4: Transient smoke pattern in horizontal tunnel with 50 mm barriers 549
For tunnel tilted at 5 o with smoke barrier depth of 50 mm and no ventilation is provided, the results are shown in Fig. 5. When the tunnel is tilted, the smoke passes along the ceiling quickly. When it passes through the barrier, it has a vigorous disturbance and turbulence. Similar to the previous case, back-layering occurs after the smoke passes through the third barrier. (a) At about 0 s (d) At about 12 s (b) At about 4 s (e) At about 16 s (c) At about 8 s (f) At about 20 s Figure 5: Transient smoke pattern in 5 o tilted tunnel with 50 mm barriers 6. CONCLUSIONS The difference in smoke movement pattern between the non-tilted and tilted tunnel is shown in Fig. 6. Firstly, smoke spreads along the ceiling more quickly in the tilted tunnel than in the nontilted tunnel. This is due to the gravitational acceleration of the smoke in the tilted tunnel. Secondly, the disturbance of the smoke just passing through the barriers in the tilted tunnel is more vigorous. This may be due to the higher velocity of the smoke in the tilted tunnel. It has less time to change the direction of the flow so that greater interference is produced. With the bombardment between the barriers and smoke layer, heat is lost from the hot smoke to the barriers. Thirdly, in both cases, back-layering occurs when the smoke approaches the end of the tunnel. 550
There are two reasons accounting for this phenomenon. One is the entrainment of the smoke layer and the ambient air that causes significant heat loss. The other is the heat loss between smoke and the ceiling, also the bombardment between smoke and the barriers. Finally, it is found that the back-layering occurs earlier in the tilted tunnel. This is because the heat loss rate of the tilted tunnel is faster than that of the non-tilted tunnel, therefore, buoyancy loss occurs earlier in the tilted tunnel. (a) Non-titled tunnel (b) 5 o titled tunnel Figure 6: Transient smoke pattern of the non-tilted and tilted tunnels 7. AKNOWLEDGMENTS The work described in this paper was supported by the Construction Industry Institute (Hong Kong) / PolyU Innovation Fund for the project Assessment of a fire model in simulating combustion for construction projects with fire engineering approach. 8. REFERENCES [1] Chow W.K. Fire safety concerns for subway systems in Hong Kong, Fire Safety Asia Conference (FiSAC) 2011, Suntec, Singapore, 12-14 October 2011. [2] National Fire Protection Association (NFPA) - Standard for Fixed Guideway Transit and Passenger Rail Systems, NFPA 130, USA, 2010. 551
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