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Fire characteristics of rescue station inside railway tunnel with semi-transverse ventilation
Tunnelling and Underground Space Technology 98 (2020) 103303
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Fire characteristics of rescue station inside railway tunnel with semitransverse ventilation
T
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Yuanlong Zhou, Haiquan Bi , Honglin Wang, Bo Lei School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Railway tunnel Rescue station Semi-transverse ventilation Smoke exhaust velocity Fire source position Fire characteristics
Rescue stations are essential in long railway tunnels to provide a safe, smoke-free refuge for evacuated passengers in the event of a fire. The fire characteristics of rescue stations with semi-transverse ventilation are significantly different from those with longitudinal ventilation. In this work, the fire characteristics of a rescue station with semi-transverse ventilation are studied using numerical simulations. The numerical method is verified using 1:20 scale model experiment. The dependencies of the temperature and visibility on the smoke exhaust velocity, fire source position, and piston wind are discussed in detail. The results demonstrate that the smoke temperature distribution at the top of rescue station is related to the fire source location. When the fire source is under the shaft, the maximum smoke temperature at the top of the rescue station decreases with an increase in the smoke exhaust velocity; when the fire source is in the middle of two shafts, the maximum smoke temperature is independent of the smoke exhaust velocity. When the smoke exhaust velocity is 3 m/s, all of the smoke is controlled between vertical shafts no. 2 and no. 4. This conclusion is significantly different from that of longitudinal ventilation. On the evacuation platform of the rescue station, the smoke exhaust velocity has little influence on the smoke temperature distribution, and the temperature at a height of 2 m can meet the specification requirements of lower than 60 °C for a 20 MW fire source. Moreover, the piston wind has an influence on the visibility of the evacuation platform, which is another obvious fire characteristic for semi-transverse ventilation.
1. Introduction A fire in a tunnel may cause substantial casualties and property losses, particularly for extra-long railway tunnels longer than 20 km (Xu et al., 2017). A commonly used technique for ensuring the safety of passengers in an extra-long railway tunnel is to establish a rescue station inside the tunnel (Bai et al., 2016). When a train catches fire in a tunnel, it will continue to run at a speed of 80 km/h (Tanaka, 1975) if the power is not completely cut off. In such cases, the passengers can immediately be evacuated into the rescue station. A rescue station has several transverse passages through which passengers can enter the safe tunnel, therefore, rescue stations are of great significance to passenger safety. Considering the smoke spread characteristics in the tunnel, passenger safety can be strongly guaranteed by the tunnel ventilation system and its structure. Several ventilation modes, such as longitudinal (Kang, 2010; Wu and Bakar, 2000; Hwang and Edwards, 2005; Maele and Merci, 2008), transverse (Yu et al., 2018), semi-transverse ventilation, and combination of ceiling smoke extraction and longitudinal
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ventilation (Tang et al., 2017a,b), are used in railway tunnels to draw in fresh air and exhaust smoke expeditiously through the top or end of the tunnel in the case of fire (Tang et al., 2018). Owing to their advantages in terms of operability and cost, longitudinal ventilation systems are extensively used (Xu et al., 2017). However, the smoke layer may be destroyed in the longitudinal ventilation mode when the velocity is excessively high. The downstream tunnel may be filled with hightemperature smoke, which is extremely dangerous to passengers (Newman, 1984; Strang and Fernando, 2001; Ingason and Li, 2010). The semi-transverse (or transverse) ventilation mode, in which a smoke exhaust flue is added above the main tunnel (as illustrated in Fig. 2), can overcome these deficiencies. Under the semi-transverse smoke exhaust mode, the smoke will be extracted from the top of the tunnel, thereby ensuring that the smoke layer is not substantially disturbed by the airflow, and evacuations are more convenient and safer. Owing to these advantages, semi-transverse ventilation systems are becoming increasingly popular and have been applied in fire ventilation of China’s Ping'an and Gaoligong Mountain railway tunnels. The spread characteristics of rescue station fire, such as the smoke
Corresponding author. E-mail address: [email protected] (H. Bi).
https://doi.org/10.1016/j.tust.2020.103303 Received 3 July 2019; Received in revised form 10 December 2019; Accepted 17 January 2020 0886-7798/ © 2020 Elsevier Ltd. All rights reserved.
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Nomenclature a A C Fr g H HT K Km lc lm ln m p Q Qm Qn Rg
S Sv T t U Ug V Vc Vm Vn Ys Ys,m α β ρ ρm ρs φ χ Г
fire growth factor (kW/s2) surface area of a fuel pool (cm2) Constant: 8 for emitted light; 3 for reflected light Froude number gravitational acceleration (m/s2) combustion heat (MJ/kg) combustion heat of methanol (kJ/g) extinction coefficient (1/m) mass extinction coefficient (m2/kg) characteristic length length of scale model tunnel (m) length of full-scale tunnel (m) mass combustion rate (g/s cm2) pressure (Pa) total heat release rate (kW) heat release rate for scale model tunnel (kW) heat release rate for full-scale tunnel (kW) gas constant (J/kg K)
source term visibility (m) temperature (K) time (s) velocity (m/s) migration velocity (m/s) piston wind velocity (m/s) characteristic velocity velocity for scale model tunnel (m/s) velocity for full-scale tunnel (m/s) smoke yield (kg/s) smoke yield per unit mass of fuel (kg/kg) inertia resistance coefficient viscous resistance coefficient density (kg/m3) density of methanol (kg/m3) density of smoke (kg/m3) generic variable combustion efficiency generalized diffusion coefficient
the fire is stable are almost the same as in the case of static fire. Secondly, the costs of fire experiments considering trains running are excessive. Thirdly, it is very difficult to measure the temperature and observe the smoke distribution during the fire development accurately, owing to the complex flow in the tunnel environment (Takita, 1977). Therefore, the static fire experiment was used in this study. A scale model of a railway tunnel system was constructed using the rescue station in China’s Ping’an Tunnel as the protocol.
exhaust velocity and available evacuation time for passengers, are of great importance in the ventilation system design. However, most studies on the fire characteristics of tunnels with rescue stations have focused on the smoke spread characteristics, critical velocity (Li et al., 2012; Li et al., 2013), temperature distribution, and visibility during the tunnel fire process in the longitudinal ventilation mode (Li et al., 2012; Xu et al., 2017). Although these conclusions are highly valuable for the design of longitudinal ventilation systems in railway tunnel rescue stations, they cannot be directly applied in the semi-transverse ventilation mode. Limited work could be found in previously published papers on the fire characteristics in rescue stations with the semitransverse ventilation mode. It should be noted that the smoke spread characteristics in the semi-transverse ventilation mode will be significantly influenced by running trains in the tunnel, owing to the large block ratio, variable airflow, and piston wind. However, the effect of a running train on the smoke spread characteristics has not been considered in previous research. In previous paper, the authors have investigated the critical velocity in the transverse passage of rescue station with semi-transverse ventilation (Zhou et al., 2019), and explored the dependence of the critical velocity on the heat release rate, the semi-transverse smoke exhaust velocity, the height and the width of the protection door, and the tunnel blockage ratio. The paper focuses on the fire characteristics of rescue stations under the semi-transverse ventilation mode. During this study, the influence of a burning running train on the fire characteristics is considered. The remainder of this paper is organised as follows. The model experimental platform and test method are introduced in Section 2. A brief outline of the theoretical and numerical methodologies is provided in Section 3. Thereafter, in Section 4, the effects of the semi-transverse smoke exhaust velocity and fire source position on the smoke spread and temperature distribution at the top of tunnel, as well as the temperature and visibility distribution on the evacuation platform, are analysed. Finally, conclusions that are of relevance to future practical design are presented in Section 5.
2.1. Experimental platform The experimental platform is illustrated in Fig. 1. Fig. 1(a) presents a schematic of the rescue station in the railway tunnel, while Fig. 1(b) provides photographs of the fire model experiment. The scale ratio of the experimental platform was 1:20. The width and height of the railway tunnel were 0.4 and 0.45 m, respectively. The total tunnel length was 10 m, including two vertical shafts and four transverse passages. The diameter of the vertical shafts was 0.25 m. The crosssectional areas of the transverse passages were 0.25 × 0.25 m, and the protection doors located inside the transverse passages measured 0.17 × 0.1 m. The main tunnel and shaft were made of iron. The tunnel walls were wrapped in thermal insulation materials with a thickness of 0.01 m and thermal conductivity of 0.034 W/(m·K). Vortex flowmeters with an upper measurement range limit of 300 m3/h and precision of 1% were are arranged in the shaft. Moreover, K-type thermocouples were installed in the main tunnel with a spacing of 0.1 m to measure the smoke temperature, and their upper measurement range limit and precision were 800 °C and 0.5%, respectively. As illustrated in Fig. 1(a), fire source position 1 was directly under vertical shaft no. 1, while fire source position 2 was in the middle of the two vertical shafts. In this fire experiment, a methanol pool was used to generate a stable fire, owing to its stable combustion characteristics (Yi et al., 2013; Ji et al., 2012). The heat release rate of methanol is only related to the surface area of the oil pool (Yi et al., 2013; Ji et al., 2012), and can be calculated by Eq. (1):
2. Fire experimental
Q = mAχHT
Fire experiments play an important role in the study of tunnel fire characteristics. The static fire experiment, which does not consider trains running, has been used extensively in previous studies (Li et al., 2013; Yao et al., 2016; Zhong et al., 2015), for three key reasons. Firstly, when the fire is fully developed, the smoke and temperature distributions tend to be stable. The smoke spread characteristics once
(1)
where m is the mass combustion rate, x is the combustion efficiency, and HT is the combustion heat. The detailed fuel parameters are listed in Table 1.
2
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Fig. 1. Schematic and actual views of 1:20 scale railway tunnel rescue station model.
Table 1 Material characteristics of methanol.
Fr =
Density, ρm (g/cm3)
Mass combustion rate, m [g/(s·cm2)]
Combustion heat, HT (kJ/g)
Combustion efficiency, χ
0.7918
0.0016
19.93
1
Vc2 glc
(2)
where g is the gravitational acceleration, Vc and lc are the characteristic values of the velocity and length, respectively. The scaling relationships between the velocity and heat release rate are as follows:
2.2. Scaling law
1/2
Vm l = ⎛ m⎞ Vn ⎝ ln ⎠ ⎜
The scaling law was applied to reflect the fire parameters of the fullscale railway tunnel system using those of the scale model. Froude modelling has been proven to be applicable for the flow and heat transfer of fire smoke driven by buoyancy, and has been used extensively in tunnel fire model experiments (McCaffrey and Quintiere, 1977; Ingason, 2007; Quintiere, 1989). The Froude number is defined as the ratio of the inertia force to the gravitational force:
⎟
(3) 5/2
Qm l = ⎛ m⎞ Qn ⎝ ln ⎠ ⎜
⎟
(4)
where m and n refer to the small scale and full scale, respectively. The values of Vm, Vn, Qm, and Qn are provided in Table 2.
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running of the burning train. The smoke flow in the tunnel was a threedimensional (3D), unsteady, and compressible turbulent flow. The lowRe number k-ε turbulent model, based on the Reynolds-averaged Navier–Stokes (RANS) equations (Mikuž and Shams, 2019) was used in the smoke flow simulation. Compared to large eddy simulation (Hwang and Edwards, 2005; Zhang et al., 2002), RANS not only meets the accuracy requirements of research on smoke spread, but also substantially reduces the computation time (Mikuž and Shams, 2019). In this study, the dynamic mesh technology used to simulate the train running and governing equations of the flow field had to take into account the influence of the grid movement on the airflow. The general governing equation is expressed as follows:
Table 2 Heat release rate and smoke exhaust velocity in the scale experiment. Pool size (cm)
18.7 × 18.7
Small-scale value
Full-scale value
Qm (kW)
Vm (m/s)
Qn (MW)
Vn (m/s)
11.2
0.447 0.559 0.671
20
2.0 2.5 3.0
3. Numerical methods 3.1. Governing equations
∂ (ρφ) + div[ρφ (U − Ug )] = div(Γgradφ) + S ∂t
In this work, the dynamic mesh technology of the STAR-CCM+ software (Harun et al., 2014; Xi et al., 2016) was used to simulate the
(5)
where φ is a generic variable, S is the source term, г is the generalized
Fig. 2. Schematic view of full-scale railway tunnel and rescue station. 4
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vertical shaft no. 2, while fire source position 2 was set in the middle of the vertical shafts no. 3 and no. 4, as illustrated in Fig. 3. The running speed of the burning train is 80 km/h. When the distance between the train and the parking position is 205 m, the brake starts, and the deceleration is 1.2 m/s2. The airflow in the tunnel is driven by the train running and is known as piston wind. Porous baffles (Wang et al., 2015; Lee and Kim, 1999) (as indicated in Fig. 3) were arranged at both ends of the tunnel to increase the airflow resistance and cause the airflow (piston wind) velocity to be consistent with the real extra-long railway tunnel. The detailed simulation method of the piston wind is introduced in Section 3.4. In this study, simulation of the train motion was realised by overset mesh technology, in which the train region overlaps with the tunnel region (as indicated in Fig. 4). When a fire occurs in the train, the passengers in the fire car are first transferred to the adjacent cars, following which the end doors of the fire car are closed. The end door has 15 mins of fire resistance. For passenger evacuation in the rescue station, all passengers must be evacuated to the safe area within 6 mins (Zhang et al., 2017). Therefore, only one fire car should be considered in the overset mesh region, as illustrated in Fig. 4. The windows and doors of the fire car are opened, allowing the high-temperature smoke to enter the tunnel. The full-scale rescue station numerical model (depicted in Fig. 5) was established according to the real structure of the rescue station (as illustrated in Fig. 2), the schematic view of the full-scale rescue station model (as depicted in Fig. 3), and the burning train model (as illustrated in Fig. 4). Fig. 5(a) illustrates the rescue station, Fig. 5(b) depicts the internal structure of the rescue station, and Fig. 5(c) presents the train model of the China Railway CRH2 model. The boundary conditions of the numerical model are displayed in Table 3. The wall condition supporting convection heat transfer was applied to all solid surfaces, and a discrete ordinates method radiation model was subsequently used to calculate the thermal radiation (Menart et al., 1993). The airflow velocity at the protection door (velocity inlet) was 2 m/s, which is the critical velocity of the transverse passage of the rescue station in China (Li and Lei, 2008). In this study, a heat release rate of 20 MW was applied to study the fire characteristics of the rescue stations (Xu et al., 2017). The volumetric heat source (VHS) model (Harun et al., 2014; Xi et al., 2016; Xue et al., 2001) was used in fire simulation. The heat release rate of the fire source was determined as follows:
diffusion coefficient, Ug is the migration velocity, and ρ and U are the airflow density and velocity, respectively. The equations are closed by regarding the air as a perfect gas satisfying the state equation:
P = ρRg T
(6)
where P is the pressure, Rg is the gas constant, and T is the temperature. In a tunnel with fire, the smoke produced by the combustion will affect the visibility significantly. The visibility can be calculated by setting the mass extinction coefficient of the smoke dust, and the definition of visibility is as follows:
Sv = C / K
(7)
K = ρs ·Km
(8)
where Sv is the visibility, and C is a constant, which is 8 for emitted light and 3 for reflected light, in this study, the value of C is 8. Moreover, K is the extinction coefficient and Km [8700 m2/kg (CDadapco, 2016)] is the mass extinction coefficient of the soot particles, ρs is the density of smoke. 3.2. Numerical model In this work, a full-scale numerical model was established based on the structure of the rescue station in China’s Ping'an railway tunnel. The total length of the Ping'an extra-long railway tunnel is 28.4 km. The rescue station is located in the middle of the tunnel, and the semitransverse ventilation mode is adopted. A schematic of the rescue station is presented in Fig. 2. Fig. 2(a) illustrates the plan view of the rescue station. The cross-section of the transverse passage, vertical shaft, and railway tunnel are presented in Fig. 2(b), (c), and (d), respectively. The actual rescue station is 550 m in length with 11 transverse passages, the length of the transverse passage is 30 m, and the spacing is 50 m. Each tunnel has five vertical shafts with a diameter of 5 m, and the height of the vertical shaft is 20 m. Moreover, the cross-sectional area of the rescue station is 62 m2, and the width of the evacuation platform is 2.3 m. Because the length of the real tunnel is excessive, to save calculation resources, limited tunnel and train lengths are usually used in numerical simulations. Fig. 3 depicts the geometry of the simulation domain. The total length of the numerical model was 2070 m, and the rescue station length was 550 m. The length of the upstream tunnel of the rescue station was approximately 1000 m, while the length of the downstream tunnel was approximately 500 m. Fig. 4 presents a schematic of the burning train model with three-car composition. The model for the train was constructed according to the specifications for the CRH2 high-speed train, which has been widely used in numerous passenger-dedicated lines in China. The train model was approximately 75 m long, 3.4 m wide, and 3.5 m high. The height and width of the protection door respectively is 2.4 m and 1.1 m. In addition, the window height is 0.84 m and its width is 1.5 m. The smoke spread in the tunnel is related to the relative position of the fire source and rescue station. Two different parking positions of the burning train were set to explore the fire characteristics of different fire source positions in this study. Fire source position 1 was set under the
Q = at 2
(9)
where a is the fire growth factor and t is the time. When the burning train enters the rescue station for evacuation, the fire has fully developed. Therefore, we set a high fire growth factor (100 kW/s2) to reduce the calculation time. To simulate the smoke particles, it is necessary to set the smoke yield in the VHS model. The smoke yield can be calculated according to the heat release rate, as follows:
Y s= Y s, m
Q H
(10)
where Ys is the smoke yield (kg/s), Ys,m is the smoke yield per unit mass of fuel [0.118 kg/kg, (CD-adapco, 2016)], H is the combustion heat
Fig. 3. Schematic view of full-scale rescue station model. 5
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Fig. 4. Schematic view of burning train model.
[16.9 MJ/kg, (CD-adapco, 2016)].
Table 3 Boundary conditions of simulation.
3.3. Numerical grids Smaller grid sizes should be used near the fire source to ensure accuracy in fire simulation. However, smaller grid sizes increase the number of grids in the mesh and the subsequent calculation time. Therefore, it is important to determine a suitable grid size that balances the accuracy and calculation time. In this study, the solution domain was discretized using polyhedral grids, the grid around the burning train is illustrated in Fig. 6. In the numerical simulations, different grid sizes from 0.15 to 0.3 m near the fire source, train, and train route were compared, and the grid size of 0.35 m was adopted for the other solution domain. Fig. 7 demonstrates the temperatures measured at the top of the rescue station. According to the comparison results, as presented in Fig. 7, a grid size of 0.2 m near the fire source, train, and train route was adopted for the remaining simulations. These grid sizes
Surface
Boundary type
Tunnel wall Tunnel outlet Vertical shaft outlet Protection door inlet Train body
Wall Pressure outlet Velocity outlet Velocity inlet Wall
resulted in meshes containing seven million polyhedral grids. 3.4. Piston wind simulation method In this section, the simulation method for the piston wind is described. When a burning train is running in a tunnel, high-temperature smoke will spread through the train to the tunnel and mix with the
(a) Numerical model of rescue station
(b) Internal structure of rescue station
(c) Train model
Fig. 5. Numerical model of full-scale railway tunnel rescue station. 6
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Fig. 6. Gird around train.
of the tunnel in the 3D numerical simulations should be set to 15. 4. Results and discussion In this section, the dependence of the fire characteristics on various factors, including the smoke exhaust velocity, piston wind, and fire source position, are investigated and analysed. 4.1. Comparison with experimental results To verify the reliability of the numerical method, a numerical model that was consistent with the 1:20 scale experimental model was established, and all parameters were set as indicated in Table 2. The measured and simulated temperatures at the top of the tunnel are compared in Fig. 8. It can be observed that the smoke temperature at the top of the rescue station decreased with an increase in the smoke exhaust velocity when the fire source was under the vertical shaft (fire source position 1). When the fire source was in the middle of two vertical shafts (fire source position 2), the maximum smoke temperature at the top of the rescue station was independent of the smoke exhaust velocity, and the maximum smoke temperature was higher than that at fire source position 1. Therefore, the fire source position has an effect on the fire characteristics. Moreover, the simulated and measured values were consistent, which means that the numerical calculation results were sufficiently accurate. Thus, it is feasible to study the fire characteristics of a rescue station by means of the numerical methods described above.
Fig. 7. Top temperature distributions varying with grid size.
airflow in the tunnel. Simultaneously, the piston wind will accelerate the smoke spread in the rescue station when the train enters. Therefore, the simulation of the real tunnel environment should be as accurate as possible. By setting the inertia resistance coefficient of the porous baffle in the 3D numerical model, as indicated in Fig. 5(a), the piston wind velocity generated by the running of the burning train can be more consistent with the piston wind velocity in the real tunnel, thereby resembling a real fire scene of the rescue station more closely. To determine the inertia resistance coefficient of the porous baffle in the full-scale numerical model, the piston wind velocity in the tunnel was calculated using one-dimensional (1D) and 3D calculation methods. The 1D flow was calculated by the Subway Environment Simulation (SES) program (Ke et al., 2002), developed by the National Transportation System Center of the United States Department of Transportation (DOT), which is widely used in tunnel ventilation (DOT of USA, 1997). As the length of the real tunnel in this study was excessive, the airflow in the tunnel could be assumed as 1D and moving along the tunnel length direction. The detailed steps are as follows:
4.2. Smoke spreading and temperature distribution in rescue station 4.2.1. Smoke spread in rescue station In this study, the fire characteristics in a rescue station with semitransverse ventilation were investigated. This section describes the smoke spread characteristics of the rescue station. When passengers are evacuated to the rescue station, high-temperature smoke must be discharged to reduce the spread of smoke in the rescue station. Fig. 9 illustrates the cases of smoke spread for the semi-transverse smoke exhaust velocities of 1, 2, 3, and 4 m/s. From Fig. 9, it can be observed that, when the semi-transverse smoke exhaust velocity was 1 m/s, the smoke could spread longitudinally to the entire rescue station. With the increase in the smoke exhaust velocity, the smoke spread range decreased. When the smoke exhaust velocity increased to 3 m/s, the smoke could be controlled effectively between vertical shafts no. 2 and no. 4, which enveloped the burning car. The reason for this phenomenon is that, when the smoke exhaust velocity is low, the buoyancy force of the smoke is greater than the inertia force of the airflow, and
(1) The piston wind velocity of the Ping'an tunnel (real tunnel) was calculated using the SES program, and the piston wind velocity of a train running at 80 km/h was obtained. (2) The piston wind velocities under different inertia resistance coefficients were determined by changing the inertia resistance coefficient of the porous baffle in the 3D numerical model. (3) When the piston wind velocity obtained by the 3D numerical model was equal to that calculated by the SES program, the inertial resistance coefficient of the porous baffle was considered to be reasonable. The formula (CD-adapco, 2016) for calculating the resistance of the porous baffle in the STAR-CCM+ software is as follows:
ΔP = ρ (α |V | + β ) V
Table 4 Calculation results of piston wind velocity. SES V (m/s)
(11)
where V is the piston wind velocity, α is the inertia resistance coefficient, and β is the viscous resistance coefficient. The viscous resistance in the tunnel is very low and could be set to 0. The calculation results of the piston wind velocity are presented in Table 4. It can be observed that the inertial resistance coefficients of the porous baffles at both ends
1.23
7
STAR-CCM+ α
V (m/s)
0 8 12 15
2.28 1.70 1.55 1.25
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Fig. 8. Comparison between experiment and 1:20 scale numerical model.
source position 1, the smoke from the fire source spread to vertical shaft no. 2 at approximately 60 s, and to vertical shaft no. 4 at approximately 100 s. After approximately 250 s, the smoke distribution on both sides of the fire source gradually stabilised and was obviously stratified. At this time, all of the smoke was controlled between vertical shafts no. 2 and no. 4. For fire source position 2, the smoke spread to the vertical shafts at approximately 40 s. Similarly, approximately 250 s later, the smoke field stabilised and all of the smoke was controlled between vertical shafts no. 2 and no. 4. The soot density of the smoke on the right side of vertical shaft no. 2 vertical shaft was very low, because when the smoke crossed vertical shaft no. 3, its temperature decreased, resulting in a decrease in the buoyancy force. Therefore, when the fire source was located at position 2, the smoke was mainly discharged from the rescue station through vertical shafts no. 3 and no. 4. According to
dominates the smoke movement. With the increase in the smoke exhaust velocity, the inertia force of the airflow increases. When the inertia force of the airflow is greater than the buoyancy force of the smoke, the smoke will not spread towards both ends of the rescue station. Fig. 10 illustrates the smoke spread in different instances when the exhaust velocity was 3 m/s, and the marked times in the figures are the calculated times after the burning train stopped. It can be observed from Fig. 10 that the smoke entered the tunnel through the train while the train was entering the rescue station. When the train stopped, some smoke was present in the rescue station where the fire train passed. Owing to the disturbance of the piston wind, the smoke exhibited no obvious stratification. Thereafter, under the action of buoyancy and the ventilation system, the smoke spread to both sides of the tunnel. For fire
Fig. 9. Diagram of smoke spread in rescue station under different smoke exhaust velocities. 8
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Fig. 10. Smoke spread in rescue station under exhaust velocity of 3 m/s.
the calculated times after the burning train stopped. It can be observed from Fig. 11 that the average velocity at the entrance of rescue station decreases first and then increases with the increase of smoke exhaust velocity. At the beginning of train stop, the average velocity at the entrance of rescue station changes with time, approximately 120 s later,
the smoke spread characteristics, it can be concluded that the piston wind has an obvious effect on the smoke distribution on the left side of the fire source during the train was entering the rescue station. Fig. 11 illustrates the average velocity at the entrance of rescue station in different scenarios, and the marked times in the figures are 9
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when the smoke exhaust velocity was 1 m/s, the influence range of the high-temperature smoke was approximately 200 m; when the smoke exhaust velocity was 2, 3, and 4 m/s, the influence range was approximately 130 m, which was equal to the distance between vertical shafts no. 3 and no. 4. Moreover, for all smoke exhaust velocities, the smoke temperatures 30 m from the fire source exceeded 180 °C, and the maximum smoke temperature was approximately 330 °C. The maximum smoke temperature appeared to be independent of the smoke exhaust velocity. Therefore, when the fire source is in the middle of the vertical shafts, it can be inferred that the influence range of the hightemperature smoke can be decreased by reducing the distance between the vertical shafts. According to the temperature distribution at the top of the rescue station, it can be concluded that the fire source location has a significant influence on the spread of the high-temperature smoke. When the fire source is under the vertical shaft, the high-temperature smoke can be discharged from the top of the rescue station in a timely manner to reduce its spread. When the fire source is in the middle of two vertical shafts, the fire source is near the neutral ventilation point of the tunnel, and the smoke cannot be discharged effectively from the rescue station. The maximum smoke temperature at the top of the rescue station is independent of the smoke exhaust velocity. Therefore, if the rescue station adopts the semi-transverse ventilation mode, the fire car of the burning train should be parked close to the vertical shaft during evacuation. Moreover, the smoke temperature distribution characteristics are in strong agreement with the results of the scale fire model experiment (as illustrated in Fig. 8).
Fig. 11. Average velocity at the entrance of rescue station.
it tends to be stable.
4.2.2. Temperature distribution at the top of rescue station When the burning train stops at the rescue station, the passengers are evacuated to the safe tunnel through the evacuation platform and transverse passage. High-temperature smoke will gather at the top of the rescue station and spread to the tunnels on both sides of the fire source during the evacuation process. According to NFPA130 (2007) and PIARC (1999), the thermal radiation of the high-temperature smoke will affect the safe evacuation of passengers. The radiation heat should not exceed 2.5 kW/m2 for human bodies 30 m from the fire source, and the corresponding smoke temperature at the top of the tunnel is approximately 180–200 °C. Therefore, to ensure the safe evacuation of passengers within 6 mins, the smoke control standard used in this study was that the smoke temperature at the top of the rescue station 30 m from the fire source should not exceed 180 °C. Fig. 12 illustrates the smoke temperature distributions at the top of the rescue station at different smoke exhaust velocities. It can be observed from Fig. 12(a) that, when the fire source was at position 1, the smoke temperature at the top of the rescue station and the influence range of the high-temperature smoke (exceeding 180 °C) decreased with an increase in the smoke exhaust velocity. When the smoke exhaust velocity was 1 m/s, the influence range of the high-temperature smoke was approximately 140 m, which did not meet the thermal radiation control standard (30 m). However, when the smoke exhaust velocity was increased to 3 m/s, the influence range on both sides of the fire source was approximately 25 m, which met the thermal radiation control standard. As illustrated in Fig. 12(b), for fire source position 2,
4.3. Temperature and visibility distributions of rescue station evacuation platform 4.3.1. Temperature distribution of rescue station evacuation platform It is necessary to control the evacuation platform temperature to prevent the high-temperature smoke and airflow from burning passengers when they are evacuated to the rescue station. According to the suggestions in NFPA130 (2007) and PIARC (1999), the maximum temperature of the evacuation platform at a height of 2 m should not exceed 60 °C within 6 mins of safe evacuation in the rescue station. Fig. 13 illustrates the smoke and temperature distributions on the evacuation platform with a smoke exhaust velocity of 3 m/s. In Fig. 13(a), which indicates that the smoke spread on the evacuation platform was the same as that of the rescue station, the smoke stratification is obvious. As illustrated in Fig. 13(a) and (b), the soot density and smoke temperature of the evacuation platform were low at a height of 2 m, which is advantageous to safe passenger evacuation. Fig. 14 illustrates the smoke temperature distribution of the
Fig. 12. Temperature distribution at top of rescue station. 10
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Fig. 13. Smoke and temperature distributions of evacuation platform in rescue station.
the evacuation time. However, for the semi-transverse ventilation mode, because the evacuation platform on both sides of the fire source meets the temperature control standard, passengers can be evacuated to tunnels upstream and downstream of the fire source. This makes the evacuation more effective, safe, and convenient.
evacuation platform at a height of 2 m, which indicates that the maximum temperature was lower than 60 °C. The maximum temperature decreased slightly with the increase in the smoke exhaust velocity, and the temperature control standard of the evacuation path was met. The temperature of the downstream tunnel of the fire source is higher for the longitudinal ventilation mode (Newman, 1984; Strang and Fernando, 2001; Ingason and Li, 2010), and passengers are mainly evacuated through the upstream tunnel, which significantly increases
4.3.2. Visibility distribution of rescue station evacuation platform When a train burns in a tunnel, a large number of solid particles will
Fig. 14. Smoke temperature distribution of evacuation platform at height of 2 m. 11
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be produced, which causes the smoke to have a certain shading ability and may reduce the tunnel visibility. If the tunnel visibility becomes excessively low, passengers may panic, which is highly unfavourable for safe evacuation. Previous research (Zhang et al., 2017) has indicated that, when the tunnel visibility is less than 10 m, it is difficult to judge the passenger evacuation path. According to NFPA130 (2007) and PIARC (1999), the visibility control standard for rescue stations is that, within 6 mins of evacuation, the smoke visibility of the evacuation platform at a height of 2 m and 30 m from the fire source should exceed 10 m. Taking the smoke exhaust velocity of 3 m/s as an example, Fig. 15 illustrates the visibility distributions of the evacuation platform at a height of 2 m, with visibility of 0–50 m indicated in the figure. It can be observed that, at the initial stage of the burning train stopping, the visibility of the evacuation platform on the left side of the fire source was less than 10 m. With an increase in the smoke exhaust time, the visibility of the evacuation platform on the left side of the fire source gradually increased. After 60 s from the start of parking, the visibility of the evacuation platform 30 m from the fire source exceeded 10 m. This is because the smoke entered the tunnel through the burning train during the process of the train entering the rescue station (as illustrated in Fig. 10). Owing to the disturbance of the piston wind, the smoke spread to the entire tunnel section without stable stratification. With the increase in the smoke exhaust time, the smoke layer gradually stabilised and obvious stratification appeared, and the visibility on the evacuation platform increased accordingly. Therefore, the piston wind has an influence on the visibility distribution in the early evacuation period of the evacuation platform. The influence time is approximately 60 s, and this issue has not been considered in previous studies.
(3)
(4)
(5)
(6)
rescue station decreases with the increase in the smoke exhaust velocity. When the fire source is in the middle of two vertical shafts, the maximum smoke temperature at the top of the rescue station is independent of the smoke exhaust velocity. For fire source position 1 (under the vertical shaft), when the smoke exhaust velocity is 3 m/s, the maximum smoke temperature 30 m from the fire source is lower than 180 °C. However, for fire source position 2 (in the middle of two vertical shafts), when the smoke exhaust velocity increases to 4 m/s, the maximum smoke temperature 30 m from the fire source still exceeds 180 °C. If the rescue station adopts the semi-transverse ventilation mode, the fire car of the burning train should be parked close to the vertical shaft. For the semi-transverse ventilation mode, the maximum temperatures of the evacuation platform at a height of 2 m meet the requirements in the case of a 20 MW fire source. During the early stages after the burning train stops, the disturbance of the piston wind to the smoke causes the evacuation platform visibility to be lower than 10 m. The visibility increases gradually with an increase in the smoke exhaust time, and exceeds 10 m after 60 s. The piston wind generated by the burning train has an influence on the visibility distribution of the evacuation platform on the left side of the fire source, and the influence time is approximately 60 s.
Compared to the longitudinal ventilation mode, the smoke stratification is obvious in the semi-transverse ventilation mode, which provides an effective evacuation environment and improves the evacuation efficiency. This is of great significance for ensuring personnel safety and normal operation of the railway system. The conclusions of this study can provide guidance for the design of semi-transverse ventilation systems in rescue stations.
5. Conclusions In this study, the fire characteristics of a rescue station with semitransverse ventilation were investigated using numerical simulation. The influences of the smoke exhaust velocity, piston wind, and fire source position on the temperature and visibility distributions in the rescue station were analysed. The key findings of this study are as follows:
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(1) Under the semi-transverse ventilation mode, with an increase in the smoke exhaust velocity, the smoke spread range decreases. When the smoke exhaust velocity is 3 m/s, all of the smoke can be controlled between vertical shafts no. 2 and no. 4. (2) The fire source position has an effect on the smoke temperature distribution at the top of the rescue station. When the fire source is under the vertical shaft, the maximum temperature at the top of the
Acknowledgment This work was supported by the National Key Research and Development Program of China [Project No. 2016YFB1200403] and the Study on Fire Smoke Spread Characteristics and Safety Countermeasures for Railway Operating Tunnels (including Metro) conducted by the Institute of Science and Technology, China Railway
Fig. 15. Smoke visibility distributions at evacuation platform height of 2 m. 12
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Eryuan Engineering Group.
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Fire characteristics of rescue station inside railway tunnel with semitransverse ventilation
T
⁎
Yuanlong Zhou, Haiquan Bi , Honglin Wang, Bo Lei School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Railway tunnel Rescue station Semi-transverse ventilation Smoke exhaust velocity Fire source position Fire characteristics
Rescue stations are essential in long railway tunnels to provide a safe, smoke-free refuge for evacuated passengers in the event of a fire. The fire characteristics of rescue stations with semi-transverse ventilation are significantly different from those with longitudinal ventilation. In this work, the fire characteristics of a rescue station with semi-transverse ventilation are studied using numerical simulations. The numerical method is verified using 1:20 scale model experiment. The dependencies of the temperature and visibility on the smoke exhaust velocity, fire source position, and piston wind are discussed in detail. The results demonstrate that the smoke temperature distribution at the top of rescue station is related to the fire source location. When the fire source is under the shaft, the maximum smoke temperature at the top of the rescue station decreases with an increase in the smoke exhaust velocity; when the fire source is in the middle of two shafts, the maximum smoke temperature is independent of the smoke exhaust velocity. When the smoke exhaust velocity is 3 m/s, all of the smoke is controlled between vertical shafts no. 2 and no. 4. This conclusion is significantly different from that of longitudinal ventilation. On the evacuation platform of the rescue station, the smoke exhaust velocity has little influence on the smoke temperature distribution, and the temperature at a height of 2 m can meet the specification requirements of lower than 60 °C for a 20 MW fire source. Moreover, the piston wind has an influence on the visibility of the evacuation platform, which is another obvious fire characteristic for semi-transverse ventilation.
1. Introduction A fire in a tunnel may cause substantial casualties and property losses, particularly for extra-long railway tunnels longer than 20 km (Xu et al., 2017). A commonly used technique for ensuring the safety of passengers in an extra-long railway tunnel is to establish a rescue station inside the tunnel (Bai et al., 2016). When a train catches fire in a tunnel, it will continue to run at a speed of 80 km/h (Tanaka, 1975) if the power is not completely cut off. In such cases, the passengers can immediately be evacuated into the rescue station. A rescue station has several transverse passages through which passengers can enter the safe tunnel, therefore, rescue stations are of great significance to passenger safety. Considering the smoke spread characteristics in the tunnel, passenger safety can be strongly guaranteed by the tunnel ventilation system and its structure. Several ventilation modes, such as longitudinal (Kang, 2010; Wu and Bakar, 2000; Hwang and Edwards, 2005; Maele and Merci, 2008), transverse (Yu et al., 2018), semi-transverse ventilation, and combination of ceiling smoke extraction and longitudinal
⁎
ventilation (Tang et al., 2017a,b), are used in railway tunnels to draw in fresh air and exhaust smoke expeditiously through the top or end of the tunnel in the case of fire (Tang et al., 2018). Owing to their advantages in terms of operability and cost, longitudinal ventilation systems are extensively used (Xu et al., 2017). However, the smoke layer may be destroyed in the longitudinal ventilation mode when the velocity is excessively high. The downstream tunnel may be filled with hightemperature smoke, which is extremely dangerous to passengers (Newman, 1984; Strang and Fernando, 2001; Ingason and Li, 2010). The semi-transverse (or transverse) ventilation mode, in which a smoke exhaust flue is added above the main tunnel (as illustrated in Fig. 2), can overcome these deficiencies. Under the semi-transverse smoke exhaust mode, the smoke will be extracted from the top of the tunnel, thereby ensuring that the smoke layer is not substantially disturbed by the airflow, and evacuations are more convenient and safer. Owing to these advantages, semi-transverse ventilation systems are becoming increasingly popular and have been applied in fire ventilation of China’s Ping'an and Gaoligong Mountain railway tunnels. The spread characteristics of rescue station fire, such as the smoke
Corresponding author. E-mail address: [email protected] (H. Bi).
https://doi.org/10.1016/j.tust.2020.103303 Received 3 July 2019; Received in revised form 10 December 2019; Accepted 17 January 2020 0886-7798/ © 2020 Elsevier Ltd. All rights reserved.
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Nomenclature a A C Fr g H HT K Km lc lm ln m p Q Qm Qn Rg
S Sv T t U Ug V Vc Vm Vn Ys Ys,m α β ρ ρm ρs φ χ Г
fire growth factor (kW/s2) surface area of a fuel pool (cm2) Constant: 8 for emitted light; 3 for reflected light Froude number gravitational acceleration (m/s2) combustion heat (MJ/kg) combustion heat of methanol (kJ/g) extinction coefficient (1/m) mass extinction coefficient (m2/kg) characteristic length length of scale model tunnel (m) length of full-scale tunnel (m) mass combustion rate (g/s cm2) pressure (Pa) total heat release rate (kW) heat release rate for scale model tunnel (kW) heat release rate for full-scale tunnel (kW) gas constant (J/kg K)
source term visibility (m) temperature (K) time (s) velocity (m/s) migration velocity (m/s) piston wind velocity (m/s) characteristic velocity velocity for scale model tunnel (m/s) velocity for full-scale tunnel (m/s) smoke yield (kg/s) smoke yield per unit mass of fuel (kg/kg) inertia resistance coefficient viscous resistance coefficient density (kg/m3) density of methanol (kg/m3) density of smoke (kg/m3) generic variable combustion efficiency generalized diffusion coefficient
the fire is stable are almost the same as in the case of static fire. Secondly, the costs of fire experiments considering trains running are excessive. Thirdly, it is very difficult to measure the temperature and observe the smoke distribution during the fire development accurately, owing to the complex flow in the tunnel environment (Takita, 1977). Therefore, the static fire experiment was used in this study. A scale model of a railway tunnel system was constructed using the rescue station in China’s Ping’an Tunnel as the protocol.
exhaust velocity and available evacuation time for passengers, are of great importance in the ventilation system design. However, most studies on the fire characteristics of tunnels with rescue stations have focused on the smoke spread characteristics, critical velocity (Li et al., 2012; Li et al., 2013), temperature distribution, and visibility during the tunnel fire process in the longitudinal ventilation mode (Li et al., 2012; Xu et al., 2017). Although these conclusions are highly valuable for the design of longitudinal ventilation systems in railway tunnel rescue stations, they cannot be directly applied in the semi-transverse ventilation mode. Limited work could be found in previously published papers on the fire characteristics in rescue stations with the semitransverse ventilation mode. It should be noted that the smoke spread characteristics in the semi-transverse ventilation mode will be significantly influenced by running trains in the tunnel, owing to the large block ratio, variable airflow, and piston wind. However, the effect of a running train on the smoke spread characteristics has not been considered in previous research. In previous paper, the authors have investigated the critical velocity in the transverse passage of rescue station with semi-transverse ventilation (Zhou et al., 2019), and explored the dependence of the critical velocity on the heat release rate, the semi-transverse smoke exhaust velocity, the height and the width of the protection door, and the tunnel blockage ratio. The paper focuses on the fire characteristics of rescue stations under the semi-transverse ventilation mode. During this study, the influence of a burning running train on the fire characteristics is considered. The remainder of this paper is organised as follows. The model experimental platform and test method are introduced in Section 2. A brief outline of the theoretical and numerical methodologies is provided in Section 3. Thereafter, in Section 4, the effects of the semi-transverse smoke exhaust velocity and fire source position on the smoke spread and temperature distribution at the top of tunnel, as well as the temperature and visibility distribution on the evacuation platform, are analysed. Finally, conclusions that are of relevance to future practical design are presented in Section 5.
2.1. Experimental platform The experimental platform is illustrated in Fig. 1. Fig. 1(a) presents a schematic of the rescue station in the railway tunnel, while Fig. 1(b) provides photographs of the fire model experiment. The scale ratio of the experimental platform was 1:20. The width and height of the railway tunnel were 0.4 and 0.45 m, respectively. The total tunnel length was 10 m, including two vertical shafts and four transverse passages. The diameter of the vertical shafts was 0.25 m. The crosssectional areas of the transverse passages were 0.25 × 0.25 m, and the protection doors located inside the transverse passages measured 0.17 × 0.1 m. The main tunnel and shaft were made of iron. The tunnel walls were wrapped in thermal insulation materials with a thickness of 0.01 m and thermal conductivity of 0.034 W/(m·K). Vortex flowmeters with an upper measurement range limit of 300 m3/h and precision of 1% were are arranged in the shaft. Moreover, K-type thermocouples were installed in the main tunnel with a spacing of 0.1 m to measure the smoke temperature, and their upper measurement range limit and precision were 800 °C and 0.5%, respectively. As illustrated in Fig. 1(a), fire source position 1 was directly under vertical shaft no. 1, while fire source position 2 was in the middle of the two vertical shafts. In this fire experiment, a methanol pool was used to generate a stable fire, owing to its stable combustion characteristics (Yi et al., 2013; Ji et al., 2012). The heat release rate of methanol is only related to the surface area of the oil pool (Yi et al., 2013; Ji et al., 2012), and can be calculated by Eq. (1):
2. Fire experimental
Q = mAχHT
Fire experiments play an important role in the study of tunnel fire characteristics. The static fire experiment, which does not consider trains running, has been used extensively in previous studies (Li et al., 2013; Yao et al., 2016; Zhong et al., 2015), for three key reasons. Firstly, when the fire is fully developed, the smoke and temperature distributions tend to be stable. The smoke spread characteristics once
(1)
where m is the mass combustion rate, x is the combustion efficiency, and HT is the combustion heat. The detailed fuel parameters are listed in Table 1.
2
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Fig. 1. Schematic and actual views of 1:20 scale railway tunnel rescue station model.
Table 1 Material characteristics of methanol.
Fr =
Density, ρm (g/cm3)
Mass combustion rate, m [g/(s·cm2)]
Combustion heat, HT (kJ/g)
Combustion efficiency, χ
0.7918
0.0016
19.93
1
Vc2 glc
(2)
where g is the gravitational acceleration, Vc and lc are the characteristic values of the velocity and length, respectively. The scaling relationships between the velocity and heat release rate are as follows:
2.2. Scaling law
1/2
Vm l = ⎛ m⎞ Vn ⎝ ln ⎠ ⎜
The scaling law was applied to reflect the fire parameters of the fullscale railway tunnel system using those of the scale model. Froude modelling has been proven to be applicable for the flow and heat transfer of fire smoke driven by buoyancy, and has been used extensively in tunnel fire model experiments (McCaffrey and Quintiere, 1977; Ingason, 2007; Quintiere, 1989). The Froude number is defined as the ratio of the inertia force to the gravitational force:
⎟
(3) 5/2
Qm l = ⎛ m⎞ Qn ⎝ ln ⎠ ⎜
⎟
(4)
where m and n refer to the small scale and full scale, respectively. The values of Vm, Vn, Qm, and Qn are provided in Table 2.
3
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running of the burning train. The smoke flow in the tunnel was a threedimensional (3D), unsteady, and compressible turbulent flow. The lowRe number k-ε turbulent model, based on the Reynolds-averaged Navier–Stokes (RANS) equations (Mikuž and Shams, 2019) was used in the smoke flow simulation. Compared to large eddy simulation (Hwang and Edwards, 2005; Zhang et al., 2002), RANS not only meets the accuracy requirements of research on smoke spread, but also substantially reduces the computation time (Mikuž and Shams, 2019). In this study, the dynamic mesh technology used to simulate the train running and governing equations of the flow field had to take into account the influence of the grid movement on the airflow. The general governing equation is expressed as follows:
Table 2 Heat release rate and smoke exhaust velocity in the scale experiment. Pool size (cm)
18.7 × 18.7
Small-scale value
Full-scale value
Qm (kW)
Vm (m/s)
Qn (MW)
Vn (m/s)
11.2
0.447 0.559 0.671
20
2.0 2.5 3.0
3. Numerical methods 3.1. Governing equations
∂ (ρφ) + div[ρφ (U − Ug )] = div(Γgradφ) + S ∂t
In this work, the dynamic mesh technology of the STAR-CCM+ software (Harun et al., 2014; Xi et al., 2016) was used to simulate the
(5)
where φ is a generic variable, S is the source term, г is the generalized
Fig. 2. Schematic view of full-scale railway tunnel and rescue station. 4
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vertical shaft no. 2, while fire source position 2 was set in the middle of the vertical shafts no. 3 and no. 4, as illustrated in Fig. 3. The running speed of the burning train is 80 km/h. When the distance between the train and the parking position is 205 m, the brake starts, and the deceleration is 1.2 m/s2. The airflow in the tunnel is driven by the train running and is known as piston wind. Porous baffles (Wang et al., 2015; Lee and Kim, 1999) (as indicated in Fig. 3) were arranged at both ends of the tunnel to increase the airflow resistance and cause the airflow (piston wind) velocity to be consistent with the real extra-long railway tunnel. The detailed simulation method of the piston wind is introduced in Section 3.4. In this study, simulation of the train motion was realised by overset mesh technology, in which the train region overlaps with the tunnel region (as indicated in Fig. 4). When a fire occurs in the train, the passengers in the fire car are first transferred to the adjacent cars, following which the end doors of the fire car are closed. The end door has 15 mins of fire resistance. For passenger evacuation in the rescue station, all passengers must be evacuated to the safe area within 6 mins (Zhang et al., 2017). Therefore, only one fire car should be considered in the overset mesh region, as illustrated in Fig. 4. The windows and doors of the fire car are opened, allowing the high-temperature smoke to enter the tunnel. The full-scale rescue station numerical model (depicted in Fig. 5) was established according to the real structure of the rescue station (as illustrated in Fig. 2), the schematic view of the full-scale rescue station model (as depicted in Fig. 3), and the burning train model (as illustrated in Fig. 4). Fig. 5(a) illustrates the rescue station, Fig. 5(b) depicts the internal structure of the rescue station, and Fig. 5(c) presents the train model of the China Railway CRH2 model. The boundary conditions of the numerical model are displayed in Table 3. The wall condition supporting convection heat transfer was applied to all solid surfaces, and a discrete ordinates method radiation model was subsequently used to calculate the thermal radiation (Menart et al., 1993). The airflow velocity at the protection door (velocity inlet) was 2 m/s, which is the critical velocity of the transverse passage of the rescue station in China (Li and Lei, 2008). In this study, a heat release rate of 20 MW was applied to study the fire characteristics of the rescue stations (Xu et al., 2017). The volumetric heat source (VHS) model (Harun et al., 2014; Xi et al., 2016; Xue et al., 2001) was used in fire simulation. The heat release rate of the fire source was determined as follows:
diffusion coefficient, Ug is the migration velocity, and ρ and U are the airflow density and velocity, respectively. The equations are closed by regarding the air as a perfect gas satisfying the state equation:
P = ρRg T
(6)
where P is the pressure, Rg is the gas constant, and T is the temperature. In a tunnel with fire, the smoke produced by the combustion will affect the visibility significantly. The visibility can be calculated by setting the mass extinction coefficient of the smoke dust, and the definition of visibility is as follows:
Sv = C / K
(7)
K = ρs ·Km
(8)
where Sv is the visibility, and C is a constant, which is 8 for emitted light and 3 for reflected light, in this study, the value of C is 8. Moreover, K is the extinction coefficient and Km [8700 m2/kg (CDadapco, 2016)] is the mass extinction coefficient of the soot particles, ρs is the density of smoke. 3.2. Numerical model In this work, a full-scale numerical model was established based on the structure of the rescue station in China’s Ping'an railway tunnel. The total length of the Ping'an extra-long railway tunnel is 28.4 km. The rescue station is located in the middle of the tunnel, and the semitransverse ventilation mode is adopted. A schematic of the rescue station is presented in Fig. 2. Fig. 2(a) illustrates the plan view of the rescue station. The cross-section of the transverse passage, vertical shaft, and railway tunnel are presented in Fig. 2(b), (c), and (d), respectively. The actual rescue station is 550 m in length with 11 transverse passages, the length of the transverse passage is 30 m, and the spacing is 50 m. Each tunnel has five vertical shafts with a diameter of 5 m, and the height of the vertical shaft is 20 m. Moreover, the cross-sectional area of the rescue station is 62 m2, and the width of the evacuation platform is 2.3 m. Because the length of the real tunnel is excessive, to save calculation resources, limited tunnel and train lengths are usually used in numerical simulations. Fig. 3 depicts the geometry of the simulation domain. The total length of the numerical model was 2070 m, and the rescue station length was 550 m. The length of the upstream tunnel of the rescue station was approximately 1000 m, while the length of the downstream tunnel was approximately 500 m. Fig. 4 presents a schematic of the burning train model with three-car composition. The model for the train was constructed according to the specifications for the CRH2 high-speed train, which has been widely used in numerous passenger-dedicated lines in China. The train model was approximately 75 m long, 3.4 m wide, and 3.5 m high. The height and width of the protection door respectively is 2.4 m and 1.1 m. In addition, the window height is 0.84 m and its width is 1.5 m. The smoke spread in the tunnel is related to the relative position of the fire source and rescue station. Two different parking positions of the burning train were set to explore the fire characteristics of different fire source positions in this study. Fire source position 1 was set under the
Q = at 2
(9)
where a is the fire growth factor and t is the time. When the burning train enters the rescue station for evacuation, the fire has fully developed. Therefore, we set a high fire growth factor (100 kW/s2) to reduce the calculation time. To simulate the smoke particles, it is necessary to set the smoke yield in the VHS model. The smoke yield can be calculated according to the heat release rate, as follows:
Y s= Y s, m
Q H
(10)
where Ys is the smoke yield (kg/s), Ys,m is the smoke yield per unit mass of fuel [0.118 kg/kg, (CD-adapco, 2016)], H is the combustion heat
Fig. 3. Schematic view of full-scale rescue station model. 5
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Fig. 4. Schematic view of burning train model.
[16.9 MJ/kg, (CD-adapco, 2016)].
Table 3 Boundary conditions of simulation.
3.3. Numerical grids Smaller grid sizes should be used near the fire source to ensure accuracy in fire simulation. However, smaller grid sizes increase the number of grids in the mesh and the subsequent calculation time. Therefore, it is important to determine a suitable grid size that balances the accuracy and calculation time. In this study, the solution domain was discretized using polyhedral grids, the grid around the burning train is illustrated in Fig. 6. In the numerical simulations, different grid sizes from 0.15 to 0.3 m near the fire source, train, and train route were compared, and the grid size of 0.35 m was adopted for the other solution domain. Fig. 7 demonstrates the temperatures measured at the top of the rescue station. According to the comparison results, as presented in Fig. 7, a grid size of 0.2 m near the fire source, train, and train route was adopted for the remaining simulations. These grid sizes
Surface
Boundary type
Tunnel wall Tunnel outlet Vertical shaft outlet Protection door inlet Train body
Wall Pressure outlet Velocity outlet Velocity inlet Wall
resulted in meshes containing seven million polyhedral grids. 3.4. Piston wind simulation method In this section, the simulation method for the piston wind is described. When a burning train is running in a tunnel, high-temperature smoke will spread through the train to the tunnel and mix with the
(a) Numerical model of rescue station
(b) Internal structure of rescue station
(c) Train model
Fig. 5. Numerical model of full-scale railway tunnel rescue station. 6
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Fig. 6. Gird around train.
of the tunnel in the 3D numerical simulations should be set to 15. 4. Results and discussion In this section, the dependence of the fire characteristics on various factors, including the smoke exhaust velocity, piston wind, and fire source position, are investigated and analysed. 4.1. Comparison with experimental results To verify the reliability of the numerical method, a numerical model that was consistent with the 1:20 scale experimental model was established, and all parameters were set as indicated in Table 2. The measured and simulated temperatures at the top of the tunnel are compared in Fig. 8. It can be observed that the smoke temperature at the top of the rescue station decreased with an increase in the smoke exhaust velocity when the fire source was under the vertical shaft (fire source position 1). When the fire source was in the middle of two vertical shafts (fire source position 2), the maximum smoke temperature at the top of the rescue station was independent of the smoke exhaust velocity, and the maximum smoke temperature was higher than that at fire source position 1. Therefore, the fire source position has an effect on the fire characteristics. Moreover, the simulated and measured values were consistent, which means that the numerical calculation results were sufficiently accurate. Thus, it is feasible to study the fire characteristics of a rescue station by means of the numerical methods described above.
Fig. 7. Top temperature distributions varying with grid size.
airflow in the tunnel. Simultaneously, the piston wind will accelerate the smoke spread in the rescue station when the train enters. Therefore, the simulation of the real tunnel environment should be as accurate as possible. By setting the inertia resistance coefficient of the porous baffle in the 3D numerical model, as indicated in Fig. 5(a), the piston wind velocity generated by the running of the burning train can be more consistent with the piston wind velocity in the real tunnel, thereby resembling a real fire scene of the rescue station more closely. To determine the inertia resistance coefficient of the porous baffle in the full-scale numerical model, the piston wind velocity in the tunnel was calculated using one-dimensional (1D) and 3D calculation methods. The 1D flow was calculated by the Subway Environment Simulation (SES) program (Ke et al., 2002), developed by the National Transportation System Center of the United States Department of Transportation (DOT), which is widely used in tunnel ventilation (DOT of USA, 1997). As the length of the real tunnel in this study was excessive, the airflow in the tunnel could be assumed as 1D and moving along the tunnel length direction. The detailed steps are as follows:
4.2. Smoke spreading and temperature distribution in rescue station 4.2.1. Smoke spread in rescue station In this study, the fire characteristics in a rescue station with semitransverse ventilation were investigated. This section describes the smoke spread characteristics of the rescue station. When passengers are evacuated to the rescue station, high-temperature smoke must be discharged to reduce the spread of smoke in the rescue station. Fig. 9 illustrates the cases of smoke spread for the semi-transverse smoke exhaust velocities of 1, 2, 3, and 4 m/s. From Fig. 9, it can be observed that, when the semi-transverse smoke exhaust velocity was 1 m/s, the smoke could spread longitudinally to the entire rescue station. With the increase in the smoke exhaust velocity, the smoke spread range decreased. When the smoke exhaust velocity increased to 3 m/s, the smoke could be controlled effectively between vertical shafts no. 2 and no. 4, which enveloped the burning car. The reason for this phenomenon is that, when the smoke exhaust velocity is low, the buoyancy force of the smoke is greater than the inertia force of the airflow, and
(1) The piston wind velocity of the Ping'an tunnel (real tunnel) was calculated using the SES program, and the piston wind velocity of a train running at 80 km/h was obtained. (2) The piston wind velocities under different inertia resistance coefficients were determined by changing the inertia resistance coefficient of the porous baffle in the 3D numerical model. (3) When the piston wind velocity obtained by the 3D numerical model was equal to that calculated by the SES program, the inertial resistance coefficient of the porous baffle was considered to be reasonable. The formula (CD-adapco, 2016) for calculating the resistance of the porous baffle in the STAR-CCM+ software is as follows:
ΔP = ρ (α |V | + β ) V
Table 4 Calculation results of piston wind velocity. SES V (m/s)
(11)
where V is the piston wind velocity, α is the inertia resistance coefficient, and β is the viscous resistance coefficient. The viscous resistance in the tunnel is very low and could be set to 0. The calculation results of the piston wind velocity are presented in Table 4. It can be observed that the inertial resistance coefficients of the porous baffles at both ends
1.23
7
STAR-CCM+ α
V (m/s)
0 8 12 15
2.28 1.70 1.55 1.25
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Fig. 8. Comparison between experiment and 1:20 scale numerical model.
source position 1, the smoke from the fire source spread to vertical shaft no. 2 at approximately 60 s, and to vertical shaft no. 4 at approximately 100 s. After approximately 250 s, the smoke distribution on both sides of the fire source gradually stabilised and was obviously stratified. At this time, all of the smoke was controlled between vertical shafts no. 2 and no. 4. For fire source position 2, the smoke spread to the vertical shafts at approximately 40 s. Similarly, approximately 250 s later, the smoke field stabilised and all of the smoke was controlled between vertical shafts no. 2 and no. 4. The soot density of the smoke on the right side of vertical shaft no. 2 vertical shaft was very low, because when the smoke crossed vertical shaft no. 3, its temperature decreased, resulting in a decrease in the buoyancy force. Therefore, when the fire source was located at position 2, the smoke was mainly discharged from the rescue station through vertical shafts no. 3 and no. 4. According to
dominates the smoke movement. With the increase in the smoke exhaust velocity, the inertia force of the airflow increases. When the inertia force of the airflow is greater than the buoyancy force of the smoke, the smoke will not spread towards both ends of the rescue station. Fig. 10 illustrates the smoke spread in different instances when the exhaust velocity was 3 m/s, and the marked times in the figures are the calculated times after the burning train stopped. It can be observed from Fig. 10 that the smoke entered the tunnel through the train while the train was entering the rescue station. When the train stopped, some smoke was present in the rescue station where the fire train passed. Owing to the disturbance of the piston wind, the smoke exhibited no obvious stratification. Thereafter, under the action of buoyancy and the ventilation system, the smoke spread to both sides of the tunnel. For fire
Fig. 9. Diagram of smoke spread in rescue station under different smoke exhaust velocities. 8
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Fig. 10. Smoke spread in rescue station under exhaust velocity of 3 m/s.
the calculated times after the burning train stopped. It can be observed from Fig. 11 that the average velocity at the entrance of rescue station decreases first and then increases with the increase of smoke exhaust velocity. At the beginning of train stop, the average velocity at the entrance of rescue station changes with time, approximately 120 s later,
the smoke spread characteristics, it can be concluded that the piston wind has an obvious effect on the smoke distribution on the left side of the fire source during the train was entering the rescue station. Fig. 11 illustrates the average velocity at the entrance of rescue station in different scenarios, and the marked times in the figures are 9
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when the smoke exhaust velocity was 1 m/s, the influence range of the high-temperature smoke was approximately 200 m; when the smoke exhaust velocity was 2, 3, and 4 m/s, the influence range was approximately 130 m, which was equal to the distance between vertical shafts no. 3 and no. 4. Moreover, for all smoke exhaust velocities, the smoke temperatures 30 m from the fire source exceeded 180 °C, and the maximum smoke temperature was approximately 330 °C. The maximum smoke temperature appeared to be independent of the smoke exhaust velocity. Therefore, when the fire source is in the middle of the vertical shafts, it can be inferred that the influence range of the hightemperature smoke can be decreased by reducing the distance between the vertical shafts. According to the temperature distribution at the top of the rescue station, it can be concluded that the fire source location has a significant influence on the spread of the high-temperature smoke. When the fire source is under the vertical shaft, the high-temperature smoke can be discharged from the top of the rescue station in a timely manner to reduce its spread. When the fire source is in the middle of two vertical shafts, the fire source is near the neutral ventilation point of the tunnel, and the smoke cannot be discharged effectively from the rescue station. The maximum smoke temperature at the top of the rescue station is independent of the smoke exhaust velocity. Therefore, if the rescue station adopts the semi-transverse ventilation mode, the fire car of the burning train should be parked close to the vertical shaft during evacuation. Moreover, the smoke temperature distribution characteristics are in strong agreement with the results of the scale fire model experiment (as illustrated in Fig. 8).
Fig. 11. Average velocity at the entrance of rescue station.
it tends to be stable.
4.2.2. Temperature distribution at the top of rescue station When the burning train stops at the rescue station, the passengers are evacuated to the safe tunnel through the evacuation platform and transverse passage. High-temperature smoke will gather at the top of the rescue station and spread to the tunnels on both sides of the fire source during the evacuation process. According to NFPA130 (2007) and PIARC (1999), the thermal radiation of the high-temperature smoke will affect the safe evacuation of passengers. The radiation heat should not exceed 2.5 kW/m2 for human bodies 30 m from the fire source, and the corresponding smoke temperature at the top of the tunnel is approximately 180–200 °C. Therefore, to ensure the safe evacuation of passengers within 6 mins, the smoke control standard used in this study was that the smoke temperature at the top of the rescue station 30 m from the fire source should not exceed 180 °C. Fig. 12 illustrates the smoke temperature distributions at the top of the rescue station at different smoke exhaust velocities. It can be observed from Fig. 12(a) that, when the fire source was at position 1, the smoke temperature at the top of the rescue station and the influence range of the high-temperature smoke (exceeding 180 °C) decreased with an increase in the smoke exhaust velocity. When the smoke exhaust velocity was 1 m/s, the influence range of the high-temperature smoke was approximately 140 m, which did not meet the thermal radiation control standard (30 m). However, when the smoke exhaust velocity was increased to 3 m/s, the influence range on both sides of the fire source was approximately 25 m, which met the thermal radiation control standard. As illustrated in Fig. 12(b), for fire source position 2,
4.3. Temperature and visibility distributions of rescue station evacuation platform 4.3.1. Temperature distribution of rescue station evacuation platform It is necessary to control the evacuation platform temperature to prevent the high-temperature smoke and airflow from burning passengers when they are evacuated to the rescue station. According to the suggestions in NFPA130 (2007) and PIARC (1999), the maximum temperature of the evacuation platform at a height of 2 m should not exceed 60 °C within 6 mins of safe evacuation in the rescue station. Fig. 13 illustrates the smoke and temperature distributions on the evacuation platform with a smoke exhaust velocity of 3 m/s. In Fig. 13(a), which indicates that the smoke spread on the evacuation platform was the same as that of the rescue station, the smoke stratification is obvious. As illustrated in Fig. 13(a) and (b), the soot density and smoke temperature of the evacuation platform were low at a height of 2 m, which is advantageous to safe passenger evacuation. Fig. 14 illustrates the smoke temperature distribution of the
Fig. 12. Temperature distribution at top of rescue station. 10
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Fig. 13. Smoke and temperature distributions of evacuation platform in rescue station.
the evacuation time. However, for the semi-transverse ventilation mode, because the evacuation platform on both sides of the fire source meets the temperature control standard, passengers can be evacuated to tunnels upstream and downstream of the fire source. This makes the evacuation more effective, safe, and convenient.
evacuation platform at a height of 2 m, which indicates that the maximum temperature was lower than 60 °C. The maximum temperature decreased slightly with the increase in the smoke exhaust velocity, and the temperature control standard of the evacuation path was met. The temperature of the downstream tunnel of the fire source is higher for the longitudinal ventilation mode (Newman, 1984; Strang and Fernando, 2001; Ingason and Li, 2010), and passengers are mainly evacuated through the upstream tunnel, which significantly increases
4.3.2. Visibility distribution of rescue station evacuation platform When a train burns in a tunnel, a large number of solid particles will
Fig. 14. Smoke temperature distribution of evacuation platform at height of 2 m. 11
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be produced, which causes the smoke to have a certain shading ability and may reduce the tunnel visibility. If the tunnel visibility becomes excessively low, passengers may panic, which is highly unfavourable for safe evacuation. Previous research (Zhang et al., 2017) has indicated that, when the tunnel visibility is less than 10 m, it is difficult to judge the passenger evacuation path. According to NFPA130 (2007) and PIARC (1999), the visibility control standard for rescue stations is that, within 6 mins of evacuation, the smoke visibility of the evacuation platform at a height of 2 m and 30 m from the fire source should exceed 10 m. Taking the smoke exhaust velocity of 3 m/s as an example, Fig. 15 illustrates the visibility distributions of the evacuation platform at a height of 2 m, with visibility of 0–50 m indicated in the figure. It can be observed that, at the initial stage of the burning train stopping, the visibility of the evacuation platform on the left side of the fire source was less than 10 m. With an increase in the smoke exhaust time, the visibility of the evacuation platform on the left side of the fire source gradually increased. After 60 s from the start of parking, the visibility of the evacuation platform 30 m from the fire source exceeded 10 m. This is because the smoke entered the tunnel through the burning train during the process of the train entering the rescue station (as illustrated in Fig. 10). Owing to the disturbance of the piston wind, the smoke spread to the entire tunnel section without stable stratification. With the increase in the smoke exhaust time, the smoke layer gradually stabilised and obvious stratification appeared, and the visibility on the evacuation platform increased accordingly. Therefore, the piston wind has an influence on the visibility distribution in the early evacuation period of the evacuation platform. The influence time is approximately 60 s, and this issue has not been considered in previous studies.
(3)
(4)
(5)
(6)
rescue station decreases with the increase in the smoke exhaust velocity. When the fire source is in the middle of two vertical shafts, the maximum smoke temperature at the top of the rescue station is independent of the smoke exhaust velocity. For fire source position 1 (under the vertical shaft), when the smoke exhaust velocity is 3 m/s, the maximum smoke temperature 30 m from the fire source is lower than 180 °C. However, for fire source position 2 (in the middle of two vertical shafts), when the smoke exhaust velocity increases to 4 m/s, the maximum smoke temperature 30 m from the fire source still exceeds 180 °C. If the rescue station adopts the semi-transverse ventilation mode, the fire car of the burning train should be parked close to the vertical shaft. For the semi-transverse ventilation mode, the maximum temperatures of the evacuation platform at a height of 2 m meet the requirements in the case of a 20 MW fire source. During the early stages after the burning train stops, the disturbance of the piston wind to the smoke causes the evacuation platform visibility to be lower than 10 m. The visibility increases gradually with an increase in the smoke exhaust time, and exceeds 10 m after 60 s. The piston wind generated by the burning train has an influence on the visibility distribution of the evacuation platform on the left side of the fire source, and the influence time is approximately 60 s.
Compared to the longitudinal ventilation mode, the smoke stratification is obvious in the semi-transverse ventilation mode, which provides an effective evacuation environment and improves the evacuation efficiency. This is of great significance for ensuring personnel safety and normal operation of the railway system. The conclusions of this study can provide guidance for the design of semi-transverse ventilation systems in rescue stations.
5. Conclusions In this study, the fire characteristics of a rescue station with semitransverse ventilation were investigated using numerical simulation. The influences of the smoke exhaust velocity, piston wind, and fire source position on the temperature and visibility distributions in the rescue station were analysed. The key findings of this study are as follows:
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(1) Under the semi-transverse ventilation mode, with an increase in the smoke exhaust velocity, the smoke spread range decreases. When the smoke exhaust velocity is 3 m/s, all of the smoke can be controlled between vertical shafts no. 2 and no. 4. (2) The fire source position has an effect on the smoke temperature distribution at the top of the rescue station. When the fire source is under the vertical shaft, the maximum temperature at the top of the
Acknowledgment This work was supported by the National Key Research and Development Program of China [Project No. 2016YFB1200403] and the Study on Fire Smoke Spread Characteristics and Safety Countermeasures for Railway Operating Tunnels (including Metro) conducted by the Institute of Science and Technology, China Railway
Fig. 15. Smoke visibility distributions at evacuation platform height of 2 m. 12
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Eryuan Engineering Group.
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