Study on Train Obstruction Effect on Smoke Control near Tunnel Cross-Passage

Transcription

1 Study on Train Obstruction Effect on Smoke Control near Tunnel Cross-Passage Hou Y. S., Li Y. F.*, Li J. M. Beijing University of Technology, College of Architecture and Civil Engineering, Beijing, China *Corresponding author ABSTRACT Theoretical analysis and numerical simulation are carried out to investigate the critical velocity in a tunnel cross-passage. Considering the effect of train blocking, several cases are analyzed by FDS software to verify whether the critical velocity can control smoke in the cross-passage or not. Result indicates that the distance of the train head away from the cross-passage as 1/2 power of the tunnel velocity. During the train cross the cross-passage, the value velocity of the cross door increase gradually and then decrease when the middle of the train stocking in the door of cross-passage. The maximum value of velocity is greater than the critical velocity calculate by Froude model. In other cases the air velocity fails to prevent smoke propagation from accident side of the tunnel and the gas full of the cross-passage cause risk of evacuation. KEYWORDS: Tunnel cross-passage, critical velocity, train, locking, smoke movement. NOMENCLATURE c p Q c specific heat (kj/(kg K)) convective heat release rate (kw) A t area of main tunnel (m 2 ) A d area of cross-passage tunnel (m 2 ) H d height of cross-passage tunnel (m) Fr critical velocity Froude number INTRODUCTION Due to the special structure of subway tunnel, long and confined, fire is always a great threaten to the passengers safety. One notable subway tunnel fire accident in Baku, Azerbaijan, led to 558 fatalities and 269 people injured. Statistics have shown that smoke is the most fatal factor in fires, and about 85% of victims in building fire were killed by hot and toxic smoke. Hence, it is significant to get smoke gas under control in the subway tunnel fire in terms of safe evacuating and casualties reduction. Like many road tunnel, longitudinal airflow play a vital role to control the movement of smoke in most fire accident [1]. However, subway tunnels have quite different features compared to the road tunnels. Firstly, The subway tunnels usually have blockage ratio over 0.5 caused by metro trains with cross-sectional area of 3.0 m 3.8 m, while the largest blockage ratio is less than 0.3 in road tunnel; Secondly, the position of fire resource is unpredictable and the tunnel is narrow, which would increase the difficulty of evacuation and smoke control because ventilation generated by station ventilation facilities is even-distributed. For subway system, there are always two parallel tunnels which are interconnected with crosspassages at interval of 600 m. These cross-passages are used for guaranteeing passengers evacuate Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8), pp Edited by Chao J., Liu N. A., Molkov V., Sunderland P., Tamanini F. and Torero J. Published by USTC Press ISBN: DOI: /c.sklfs.8thISFEH

2 Part II Fire to the adjacent metro line and rescuing operations when train stops in tunnel [2]. When passengers enter the safety tunnel from the accident tunnel, the ventilation system would supply fresh air through the cross-passage to keep the cross-passages free of smoke. The minimum ventilation velocity through an opened fireproof door that can prevent smoke from spreading into a cross-passage is called the critical velocity for smoke control in a cross-passage [3, 4]. Previous research have investigated critical velocity of longitude tunnel relative comprehensive, like Kennedy et al. propose a formula for estimating critical velocity, have been widely used in numerical simulation. However, limited work has been performed in the critical velocity for cross-passage while a few longitude tunnel velocity studies have been done. Fortunately, several cross-passage velocity model, based on critical velocity formula, proposed by Kennedy, have been present by experiment. As the aforementioned description, the main task of this paper is to investigate the critical velocity in cross-passage and the effect of train blocking in tunnel on the velocity in the cross passage. THEORETICAL ANALYSIS The most widely-used correlations for estimating the critical velocity are based upon a nondimensional Froude number analogy (Thomas, 1970). The Froude number is defined as the ratio between the buoyancy force generate by the fire and the inertial force. It is assumed that when Froude number reaches a special value, airflow would prevent the smoke from flowing into the cross-passage. Apparently, in cross-passage, the enthalpy equation should be considered in previous study [5]. Fr gh ( ρ ρ ) 0 f =, (1) 2 ρ 0v ( mc T) + ( mc T) + Q = ( mc T). (2) p t p d p f And continuity equation: ( ρmv) + ( ρmv) = ( ρmv), (3) Then t d f gh( ρ ρ ) QgH A Fr= v = vt. (4) ρ t f d t 2 d 2 tvt ρ0v0atcfr d f p Ad According to the experimental measurements of Lee et al. [6], Froude number should be 4.5 for getting a critical velocity [7]. Comparing with previous critical velocity, empirical equation of crosspassage should be taken the enthalpy balance in a control volume into account [8-10]. However, only one side tunnel was considered and the parallel accident tunnel with the train blocking are neglected. EXPERIMENTAL PROCEDURE A full-scale experiment has been carried out in two 1200 m long shield tunnels connecting with a rectangle cross-passage and each tunnel has only one track. The cross-passage is 2 m long and 1.5 m wide. The radius of two test tunnels are 5.5 m. The schematic view of the test tunnel is shown Fig. 1. In this test, there was a subway train that stayed in one tunnel to simulate the blockage effect of metro train in fire accident and 120 m away from the cross-passage. The intent of experiment is to measure the air velocities in different tunnel positions including upstream and downstream of train in the accident tunnel and the same positions of the other tunnel in order to determine the critical velocity for cross passage. Two smoke exhaust fan with volume flow rate of 110 m 3 /s at the upstream platform 93

3 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) B are operated. On the contrary, there are two fans supplying fresh air at downstream station C. Their volume flow rate also achieve 110 m 3 /s. 5.5 m (a) Plan view of the tunnel Figure 1. The schematic view of the test tunnel. (b) Section view of the tunnel In order to measure the air velocity of tunnel portals in accident side, two multi-point anemometers is used. Similarly, two handle anemometers are used to measure the air velocity of two portals in the other side of tunnel without train, as shown in Fig. 2. Each measurement position is 100 m away from the portal. The whole value of air velocity is determined by the average value of in each measurement section. In the test, train head is 120 m away from the cross-passage and stoke in the accident tunnel. (a) Arrangement of anemometer on cross section (b) Arrangement of anemometer in cross-passage Figure 2. Survey point arrangement in the tunnel. The result of air velocity is shown in Table 1. The average air velocity of cross-passage reach 1.36 m/s, and the flow direction is from safety tunnel to accident side. Table 1. Result of air velocity in the text. Velocity of the ventilation (m/s) Section A 2.64 Section B 2.81 Section C 3.01 Section D

4 Part II Fire NUMERICAL SIMULATION An introduction to FDS The software package, Fire Dynamics Simulator (FDS) [18], a LES code coupling with a post processing visualization tool, SMOKEVIEW, developed by National Institute of Standards and Technology (NIST), USA, could now be regarded as a practical tool for simulating fire-induced environment. Numerous former researches have been carried out using FDS to model the existing experiments to justify the feasibility of FDS modelling tunnel fires. Physical model The schematic view of FDS model is shown in Fig. 3. The model two shield tunnels in current study are specified as 1200 m long and 5.5 m in diameter, The tunnel cross-passage is rectangle with size of 1.5 m 2 m (width high), Fire resource has a HRR of 7.5 MW and area of 2 m 2 m (length width). As the practical engineering, the boundary conditions are assign in four portal of the tunnel as shown in Fig. 3. Grid system setup (a) Figure 3. Schematic of tunnel modeling (a) plan form (b) side view. In FDS modeling, the grid size is a key parameter to be considered. A criterion related fire characteristics diameter (D*) has been widely used for assessing the grid resolution. Many researches suggest that the mesh size must be no larger than 0.1 D* to obtain viable simulation results. The grid sensitivity analysis is conducted for the fire scenario without smoke control system in order to test the grid size under larger gradient. To get reliable numerical grid for simulation, finer mesh sizes are used near the fire location and cross-passage as shown in Table 2. The total number of cells in the whole region is (b) Table 2. Summary of grid systems. Section 1 Section 2 Section 3 Distance form fire source (m) Size of the mush (m) Boundary conditions When fire happens in carriage, the train is assumed to be stopped in tunnel immediately and passengers can escape from the train to enter the safety tunnel through the cross-passage. Then metro 95

5 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) platform, on two portal of the tunnel would operate exhausting system to control the smoke in the accident. 2 m Figure 4. Schematic picture of a 7.5 MW fire set near the cross-passage gate. The portals of two tunnels A~D are set as air SUPPLY with 2.64 m/s, 2.81 m/s, 3.01 m/s, 3.5 m/s, and have different air supply velocities. Fire is set on the top of the train with an area of 4 m 2. In whole cases, the values of ambient temperature, pressure and relative humidity are the same as the full-scale experiment with the pressure Pa, temperature of 14.5 and relative humidity of 30.18%. Figure 5. Boundary conditions. RESULTS AND DISCUSSION Comparison of critical velocity between theoretical and numerical simulation By using the cross-passage critical velocity equation (Eq. (4)), we can determine the cross-passage critical velocity. 6 3 c p = 1040 J/(kg K) ; Q c = W ; ρ 0 = 1.1kg/m ; c = ; T 0 = 300 K; 2m 2 A t = m ; p 1040 J/(Kg K) H = ; Fr = 4.5. d Figure 6. Relationship between longitude flow and critical velocity of cross-passage. 96

6 Part II Fire In order to determine the critical velocity of cross-passage in different longitudinal ventilation velocity, several cases are considered in Table 3 (shown below). Table 3. Results of the simulated tunnel ventilation velocities. Project number HRR (MW) Cross sectional area of tunnel (m 2 ) Shape of the tunnel Distance from fire source to cross-passage (m) Air velocity of main tunnel (m/s) Critical velocity (m/s) Rectangle Shield The critical velocity of cross passage is shown in Fig. 7 as a function of longitudinal ventilation velocity in the main tunnel. It can be indicated that the critical velocity of shield tunnel is less than rectangle tunnels, and the critical velocity in cross-passage is similar with the result from empirical formula. However the cases in shield tunnel is deviate from the rectangle shape tunnel. Figure 7. Critical velocity of cross-passage. 97

7 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) Effect of train location on smoke control As shown in Fig. 8, in fire scenarios, location of the train will appear in tree area of the accident tunnel, upstream, middle and downstream. We can see apparently in the Fig. 9. The fire set in the bottle stop near the cross-passage and the train head is 2 m apart from connect path. The train head distance to cross-passage door 78.3 m and fire set in bottle of the train. The train head travel through the connected 110 m and the fire set in bottle of the train. Compare to the other scenario the train staying in middle of the accident tunnel owe higher flow velocity through the connected path as showed in Fig. 10. Figure 8. Train stay the upstream of the tunnel. Figure 9. Train stay in middle of tunnel. Figure 10. Train stops in downstream of the tunnel. Fig. 11 shows the relationship between the location of the train and the velocity in cross-passage. However none of the point, in Fig. 8, can attain critical velocity of the cross-passage derived from experimental model equation in practical smoke exhausting system operation. The distance of the train to the connected aisle in three cases with various velocity values are 0.41, 0.7 and 0.3 m/s. 98

8 Part II Fire Figure 11. Relationship between blocking train. CONCLUSION In this study, full-scale experiment and CFD simulation will use to investigate the critical velocity of cross-passage when consider the effect of train blockage and varied longitude ventilation. In the fullscale experiment, the result of air velocity in four portals of the tunnel intent to provide boundary velocity for CFD model. (1) The result of critical velocity indicate that the critical velocity of shield tunnel is less than rectangle tunnels, and the critical velocity in cross-passage is similar with the critical velocity calculate by empirical formula. (2) In full-scale experiment, with considering the train blockage, the velocity of ventilation in crosspassage in exceed the critical velocity. In practical, the longitudinal ventilation can prevent the smoke spreading in to the tunnel without accident. (3) In practical engineering, the smoke evacuation system will prevent smoke spread from accident side to safety tunnel in cross-passage when consider the effect of train blockage. A full-scale experiment has been carried out in two 1200 m long shield tunnels connecting with a rectangle cross-passage and each tunnel has only one track. The cross-passage is 2 m long and 1.5 m wide. The radius of two test tunnels are 5.5 m. The schematic view of the test tunnel is shown Fig. 1. In this test, there was a subway train that stayed in one tunnel to simulate the blockage effect of metro train in fire accident and 120 m away from the cross-passage. The intent of experiment is to measure the air velocities in different tunnel positions. ACKNOWLEDGEMENT This work was supported by Beijing Natural Science Foundation (Grant No: ) and Natural Science Foundation of China (NSFC) (Grant No: ). REFERENCES 1. Kayili, S., Yozgatligil, A., and Eralp, O. C. Effect of Ventilation and Geometrical Parameters of the Burning Object on the Heat Release Rate in Tunnel Fires, Combustion Science and Technology, 184(2): 165, Li, Y. Z., and Lei, B. Model of Critical Velocity in a Tunnel Cross-Passage, Journal of the China Railway Society, 30(3): 87-90,

9 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) 3. Tarada, F., Bopp, R., Nyfeler, S., Jegal, K. S., and Kim, D. S. Ventilation and Risk Control of the Young Dong Rail Tunnel in Korea, First International Conference on Major Tunnel and Infrastructure Projects, Taiwan, Hall, R. C. Ventilation During Road Tunnel Emergencies, Published Project Report, PPR140. TRL Limited, Berkshir. 5. Xue, H., Chew, T. C., Tay, K. L., and Cheng, Y. M. Control of Ventilation Airflow for Tunnel Fire Safety, Combustion Science and Technology, 152(1): , Lee, C. K., Hwang, C. C., Singer, J. M., and Chauken, R. F. Influence of Passageway Fires on Ventilation Flows, Second International Mine Ventilation Congress, Reno, Nevada, Kennedy, W. D. Critical Velocity: Past, Present and Future, One Day Seminar on Smoke and Critical Velocity in Tunnels, ITC, Li, Y. Z., Lei, B., and Ingason, H. Theoretical and Experimental Study of Critical Velocity for Smoke Control in a Tunnel Cross-Passage, Fire Technology, 49(2): , Tarada, F. Critical Velocities for Smoke Control in Tunnel Cross-Passages, First International Conference on Major Tunnel and Infrastructure Projects, Taiwan, Thomas, P. H. Movement of Smoke in Horizontal Corridors against an Air Flow, Institution of Fire Engineers, 30(2): 45,