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Effects of the solid curtains on natural ventilation performance in a subway tunnel
Tunnelling and Underground Space Technology 38 (2013) 526–533
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Effects of the solid curtains on natural ventilation performance in a subway tunnel Yuan-dong Huang a,b,⇑, Xiao-lu Gong a, Yue-jiao Peng a, Chang-Nyung Kim b,c,⇑ a
Department of Environmental Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China Department of Mechanical Engineering, Kyung Hee University, Yongin 449-701, Republic of Korea c Industrial Liaison Research Institute, Kyung Hee University, Yongin 449-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 27 October 2011 Received in revised form 2 August 2013 Accepted 23 August 2013 Available online 20 September 2013 Keywords: Ventilation performance Subway tunnel Natural ventilation duct Solid curtain Train-induced airflow
a b s t r a c t Solid curtains can be installed in subway tunnels for the promotion of air ventilation in ventilation ducts in association with the piston effect caused by a running train. With an aim to analyze the effects of solid curtains on duct ventilation performance in a subway tunnel, the current study adopts the tunnel and subway train geometries which are exactly the same as those in a previous model tunnel experiment, but newly incorporates two ventilation ducts connected vertically to the tunnel ceiling and two solid curtains placed at an upstream position of a duct near the tunnel inlet and at a downstream position of another duct near the tunnel outlet, respectively. A three-dimensional CFD model adopting the dynamic layering method for tracking the motion of a train, which was validated against the reported model tunnel experiment in a previous study, is employed to predict the train-induced unsteady airflows in the subway tunnel and in the ducts. The numerical results reveal that the duct ventilation performance in a subway tunnel strongly depends on the operation of the solid curtains. The suction mass flow of the air through the duct near the tunnel inlet and the exhaust mass flow of the air through the duct near the tunnel outlet are increased considerably in the case with the solid curtains in comparison with those in the case without the solid curtains. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Tunnel ventilation is crucial for environmental control in subway systems. In a subway tunnel, both the mechanical and natural ventilation modes are influenced substantially by piston wind. Recently, the Computational Fluid Dynamics (CFD) tool has been increasingly utilized to analyze problems involved in three-dimensional train-induced airflows in subway tunnels. For instance, Wang et al. (2009) performed a numerical simulation of train-induced piston effect in subway tunnels with and without ventilation ducts using CFD code FLUENT. Yuan and You (2007) evaluated numerically the time-averaged velocity and temperature fields in a side-platform station using CFD code AIRPAK. Ke et al. (2002) carried out a numerical study of the influence of train velocities on pressure distribution in a station area using CFD code PHOENICS. Juraeva et al. (2011) conducted a numerical analysis of the ventilation system in a subway tunnel to identify the best position for installation of an air-curtain using ANSYS CFX software. It should be noted that the one-dimensional tools (such as ⇑ Corresponding authors. Address: Department of Mechanical Engineering, Kyung Hee University, Yongin 449-701, Republic of Korea (C.N. Kim). E-mail addresses: [email protected] (Y.-d. Huang), [email protected] (C.-N. Kim). 0886-7798/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tust.2013.08.009
SES) are common standard and are by far more efficient than three-dimensional approaches in investigating train-induced tunnel flows. However, the one-dimensional tools cannot be utilized to simulate the airflow distributions. And the motivation of adopting the three-dimensional CFD approach is to obtain the distributions of air velocity, temperature and pressure in subway tunnels and ventilation ducts. Natural ventilation mode uses only train-induced wind to fulfill air exchange between a subway tunnel and outer atmosphere. The importance of natural ventilation has drawn many recent studies on duct ventilation achieved by train motion in subway tunnels. Using the sharp interface method (Udaykumar et al., 2001) for the moving boundaries of a train, Kim and Kim (2009) investigated numerically the effects of duct location on ventilation performance in a subway tunnel employing PSDs with the use of CFX4.4. Huang et al. (2010) conducted a numerical analysis of the train-induced unsteady airflow in a subway tunnel with natural ventilation ducts using the dynamic layering method. Their study revealed the impact of train motion on the exhaust and suction of the air through ventilation ducts and the effects of a barrier placed at the tunnel outlet on the duct ventilation performance. Huang et al. (2011) recently examined numerically the effects of the ventilation duct number and duct geometry on duct ventilation performance in a subway tunnel.
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In order to improve air quality in subway tunnels with natural ventilation ducts, searching for measures to strengthen duct ventilation effect has become a focal point in researches of subway ventilation systems. As mentioned above, much attention has been paid to the effects of duct location, duct arrangement and duct geometry on ventilation performance in a subway tunnel with natural ventilation ducts. In the study of Huang et al. (2010), it was revealed that a barrier placed at the tunnel outlet can strengthen remarkably the duct ventilation performance. In a subway tunnel, a barrier placed at the tunnel inlet may also have a great impact on the ventilation effect. But, there has been no report on how the duct ventilation is quantitatively affected by two solid barriers (curtains) placed at the inlet and outlet of a subway tunnel, respectively. Although Yu et al. (2009) conducted a numerical analysis of energy saving for the installation of two solid barriers (curtains) in a subway tunnel, they carried out a two-dimensional simulation of the air flow in a tunnel (obviously, the airflow near a train exhibits a three-dimensional characteristics) and did not account for the acceleration and deceleration processes of the train run. The overall goal of this study is to investigate the effects of solid curtains on natural ventilation performance in a subway tunnel. For this purpose, we perform numerical simulations of the train-induced unsteady airflow in a subway tunnel with natural ventilation ducts for two different cases: in Case 1 (reference case) there are no solid curtains in the subway tunnel, whereas in Case 2 two solid curtains are placed transversely near the inlet and outlet of the tunnel, respectively. In each case, the configurations of the subway tunnel and of the train are exactly the same as those used in the model tunnel experiment conducted by Kim and Kim (2007) except that the natural ventilation ducts connected vertically to the tunnel ceiling are newly installed. The simulations are carried out with FLUENT code. In the CFD model, the dynamic layering method is adopted for tracking the boundaries of a moving train. The numerical results are analyzed to clarify quantitatively the effects of solid curtains on the suction and exhaust flows through each individual duct and on the total mass flow of air sucked into and pushed out of the tunnel through all ventilation ducts.
2. Physical model and related conditions 2.1. Description of the experimental study carried out by Kim and Kim (2007) on train-induced airflow in a model tunnel Kim and Kim (2007) conducted an experimental study on the train-induced unsteady airflow in a model tunnel built to be in a 1/20th scale of a real subway tunnel. The details of this model tunnel experiment have been presented clearly in previously published literatures (Huang et al., 2010; Huang et al., 2011; Kim and Kim, 2007). However, a brief description of this experiment is given in the below since the physical model used in the present study is constructed based on the setup of the model tunnel experiment carried out by Kim and Kim (2007).
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Fig. 2. Schedule of train run (Kim and Kim, 2007).
(a) Central vertical section of the tunnel with the two identical ventilation ducts (not in scale)
(b) Top view of the tunnel with the two identical ventilation ducts (not in scale) Fig. 3. Geometry and locations of the two identical ventilation ducts installed on the tunnel ceiling in Case 1.
A schematic diagram of the experimental layout is shown in Fig. 1, which illustrates the geometries of the tunnel and of the train, the initial and final positions of the train, the total distance of the train run, and the sites of the four pressure transducers (PT1, PT2, PT3 and PT4) and of the two velocity transducers (VT1 and VT2). The blockage ratio of the train to the tunnel is 0.669. Both the velocity and pressure measurements are recorded at a time interval of 0.1 s. The train connected to the cable can move back and forth on the guide rail. The speed profile of the train, which is controlled by a drive motor equipped with an electrical inverter, is shown in Fig. 2. After the train accelerates at a rate of 1.0 m/s2 for 3 s, it runs at a constant speed for 8 s. Then, the train slows down at a rate of 1.0 m/s2 until it stops. The Reynolds num-
Fig. 1. Schematic diagram of the experimental layout (Kim and Kim, 2007).
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(a) Central vertical section of the tunnel with the two ventilation ducts and the two curtains (not in scale)
(b) Top view of the tunnel with the two ventilation ducts and the two curtains (not in scale) Fig. 4. Geometries and locations of the ventilation ducts on the tunnel ceiling and of the curtains in Case 2.
Table 1 Raised/dropped states of the two curtains in Case 2 at t = 0–18 s.
Curtain 1 Curtain 2
0–2.88 s
2.88–10.62 s
10.62–18 s
Raised Dropped
Dropped Dropped
Dropped Raised
3
Experimental results Numerical results
ducts connecting the tunnel ceiling to the ground are newly installed. As shown in Fig. 3(a), the two identical ventilation ducts are connected vertically to the tunnel ceiling and the length of each duct is 0.425 m. The top view of the tunnel with the two ventilation ducts is depicted in Fig. 3(b), which illustrates that the cross sectional area of the duct is 0.15 m 0.1 m and the distance between the two ducts is 27.44 m. The initial and final positions of the train are located on the tracks in two different stations equipped with PSDs. Here, the 39 m long tunnel is divided into the two station tunnels (the length of each station tunnel is 4.6 m) and the running tunnel between the two stations. Case 2 is the same as Case 1 except that the two solid curtains (Curtains 1 and 2, see Fig. 4) with the identical geometry (each curtain is 210 mm wide, 250 mm high and 30 mm thick) are placed transversely inside the tunnel with an aim to assess the effects of solid curtains on natural ventilation performance in a subway tunnel. Here, Curtain 1 is located at an upstream position of a duct near the inlet of the tunnel while Curtain 2 is located at a downstream position of another duct near the outlet. Considering safe train passages through the positions of curtains in a real subway tunnel, in the numerical simulation the two curtains in Case 2 can be automatically raised or dropped depending upon the position of the train. As the train starts to move, Curtain 1 is raised while Curtain 2 is dropped. After the train passes through the position of Curtain 1, it will be dropped when the rear face of the train is 1 m away from Curtain 1. As the train approaches Curtain 2, it will be raised when the front surface of the train is 2.5 m away from Curtain 2. For clarity, the raised and dropped states of the two curtains during the train motion are listed in Table 1.
3. CFD model and its validation
2
3.1. CFD model with initial and boundary conditions
CP
1
0
-1
-2
-3 0
2
4
6
8
10
12
14
16
18
20
Time (s) Fig. 5. Comparison of Cp variations between experimental and numerical results at position of PT3.
ber based on the hydraulic diameter of the model tunnel and the maximum train speed (3 m/s) is 4.9 104. 2.2. Configurations of subway tunnel, train, ventilation ducts and solid curtains In this study, the numerical simulations deal with two cases. In Case 1, the geometries of the tunnel and of the subway train, as well as the initial position and running schedule of the train, are exactly the same as those used in the model tunnel experiment performed by Kim and Kim (2007), but two natural ventilation
The Reynolds-averaged Navier-Stokes (RANS) and the continuity equation, together with the RNG j e turbulence model are used to solve train-induced unsteady air flows. The governing equations are discretized using a finite volume method. The PISO algorithm is adopted for the pressure-velocity coupling. The QUICK differencing scheme is employed for the discretization of momentum, turbulent kinetic energy, and turbulent dissipation rate equations, whereas the PRESTO scheme is utilized for the pressure correction equation. For unsteady flow calculations, the temporal derivatives are discretized using the first-order implicit scheme. For modeling the moving boundaries of the train, the dynamic layering method (see ANYSY FLUENT 12.0 User’s Guide) in the dynamic mesh model supplied by FLUENT code is employed. The numerical solution is obtained using this CFD code. The grid size in the longitudinal direction (x axis) of the tunnel is set to be 0.03 m. In the cross-section of the tunnel (i.e. the y–z plane), the grid size is set to be 0.02 m in both y and z directions except in the narrow gap between the surface of moving train and the tunnel walls and in the ventilation ducts, where fine grids are applied. Initially, the entire computational domain consists of about 405,000 hexahedral cells in each of the two cases. At t = 0 s, the pressure and the air velocity in the tunnel and in the ducts are assumed to be zero. In the time domain of t > 0 s, the boundary conditions are the following: A pressure-inlet is specified at the inlet of the left station tunnel. The pressure-outlet conditions are employed at the outlet of the right station tunnel and at the ground opening of each ventilation duct. No-slip conditions are imposed on the solid surfaces of the tunnel, curtains, ventilation ducts and moving train. The events in FLUENT is used to control the raising and dropping of the solid curtains.
Y.-d. Huang et al. / Tunnelling and Underground Space Technology 38 (2013) 526–533
1.0
sults of the comparison between the measured and computed values are quite satisfactory at all measurement points including the four points of the pressure transducers and the two points of the velocity transducers (here, for brevity only the comparisons between the numerical and experimental results for pressure coefficient Cp against time at the third pressure transducer (PT3) and for u/UT_MAX variations with time at the second velocity transducer (VT2) are presented, see Figs. 5 and 6). It is thus feasible to use the current CFD model to perform numerical analysis of the problem formulated in Section 2.2.
Experimental results Numerical results
0.8
u/U T-MAX
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0.6
0.4
4. Numerical results and discussion 0.2
0.0 0
2
4
6
8
10
12
14
16
18
20
Time (s) Fig. 6. Comparison of u/UT_MAX variations between experimental and numerical results at position of VT2.
In the calculation, the size of the time step is set to be 0.005 s and thus the Courant number based on the maximum train speed and the grid size in the longitudinal direction is obtained to be 0.5 (i.e., the CFL condition is satisfied). During the process of calculation, the residual of every equation is monitored. The convergence criterion is that the residual for each variable is less than 10 5. 3.2. Model validation The current CFD model with the dynamic layering method for a moving train was validated in a previous study (Huang et al., 2010) against the data obtained from the model tunnel experiment conducted by Kim and Kim (2007). The validation showed that the re-
General characteristics of train-induced unsteady airflow through a natural ventilation duct placed on the ceiling of a subway tunnel without curtains have been investigated intensively in our previous studies (Huang et al., 2010; Huang et al., 2011), and thus in this study we focus on analyzing the flow fields in the tunnel and in the ventilation ducts for Case 2 when both the two solid curtains are operated, and on comparing the ventilation characteristics in Case 2 with those in Case 1. It can be expected that as the train completely passes Duct 1, a negative pressure is generated in the region under Duct 1, which allows the air to be sucked into the tunnel through Duct 1. Fig. 7 shows the pressure contours and velocity vectors around Duct 1 at t = 3.0 s just after Curtain 1 is dropped. Here, it can be seen obviously that there is a region with low pressure near the rear face of the train (i.e., in the near wake of the moving train) which induces a strong suction flow through Duct 1. Moreover, it is seen that just after Curtain 1 is dropped a small vortex is formed near Curtain 1 and the air within most part of the tunnel on the left side of Duct 1 still moves in the train-running direction as it did before Curtain 1 was dropped. The pressure contours and velocity vectors around Duct 1 and Duct 2 in the middle section (y = 0.105 m) of the tunnel at t = 7.0 s are shown in Figs. 8 and 9, respectively. Fig. 8 illustrates
(a) Pressure contours
(b) Velocity vectors Fig. 7. Pressure contours and velocity vectors around the duct 1 in the middle section (y = 0.105 m) of the tunnel just after the curtain 1 is closed at t = 3.0 s in Case 2.
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(a) Pressure contours
(b) Velocity vectors Fig. 8. Pressure contours and velocity vectors around the duct 1 in the middle section (y = 0.105 m) of the tunnel at t = 7.0 s in Case 2.
(a) Pressure contours
(b) Velocity vectors Fig. 9. Pressure contours and velocity vectors around the duct 2 in the middle section (y = 0.105 m) of the tunnel at t = 7.0 s in Case 2.
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0.09
0.03
0.08
0.02 Case 1 Case 2
Mass flow rate (kg/s)
0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.01 -0.02
0.00 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06
-0.03
-0.07
-0.04
-0.08
-0.05 0
2
4
6
Case 1 Case 2
0.01
Mass flow rate (kg/s)
0.07
-0.09
8 10 12 14 16 18 20
0
2
4
6
8 10 12 14 16 18 20
Time (s)
Time (s)
(a) Duct 1
(b) Duct 2
Fig. 10. Comparisons of variations of mass flow rate with time at different ducts between Case 1 and Case 2.
0.10 Case 1 Case 2
0.09
0.07
Mass flow rate (kg/s)
Mass flow rate (kg/s)
0.08
0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.01 0
2
4
6
8
10 12 14 16 18 20
Time (s)
(a) Inlet of the left station tunnel
0.02 0.01 0.00 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06 -0.07 -0.08 -0.09 -0.10 -0.11 -0.12 -0.13
Case 1 Case 2
0
2
4
6
8
10 12 14 16 18 20
Time (s)
(b) Outletof the right station tunnel
Fig. 11. Comparisons of variations of mass flow rate with time at the inlet of the left station tunnel and at the outlet of the right station tunnel between Case 1 and Case 2.
apparently that the air is pulled into the tunnel through Duct 1 and the velocity vectors inside Duct 1 are found parallel to the long axis of Duct 1. Also, it is seen from Fig. 8 that the airflow near Curtain 1 is very weak and a large clockwise vortex is formed in the tunnel space on the left side of Duct 1. Fig. 9 reveals that the air is pushed out of the tunnel through Duct 2 at t = 7.0 s, and that the airflow in the tunnel on the right side of Duct 2 is very weak (especially, there is nearly no airflow in the region near Curtain 2). It is also observed from Fig. 9 that the airflow vectors vary inside Duct 2 and only a small part of velocity vectors (especially, in regions near the duct walls) are parallel to the long axis of Duct 2. Fig. 10 shows the comparisons of variations of mass flow rate with time at each duct between Case 1 and Case 2, where a positive mass flow rate indicates that the air is pulled into the tunnel through the ventilation duct while a negative mass flow rate indicates that the air is pushed out of the tunnel through the duct. From this figure it can be observed clearly that the suction mass flow rate through Duct 1 for t > 2.88 s (i.e., after Curtain1 is dropped) in Case 2 is much larger than that in Case 1 (see Fig. 10(a)) and that the exhaust mass flow rate through Duct 2 during the period of 2.88 s < t < 10.62 s (i.e., before Curtain 2 is raised)
in Case 2 is quite greater than that in Case 1 (see Fig. 10(b)), suggesting that the effect of air suction through Duct1 and the effect of air exhaust through Duct 2 are both strengthened significantly by the solid curtains placed transversally inside the tunnel. Furthermore, it can be seen that the mass flow rate through each duct in Case 2 remains almost unchanged with time as the train moves at a constant speed of 3 m/s during the period of 3 s < t < 11 s. The train moves under Duct 2 during the period of 11.13 s < t < 12.50 s and the whole train runs on the right side of Duct 2 after t > 12.50 s, which induces considerable air suction through Duct 2 for the period of 11.275 s < t < 13.25 s in Case 2. However, after this period the train speed is much reduced so that this slow-moving train has tendency to block the flow of air following the rear of the moving train, which yields the exhaust airflow through Duct 2 for the period of 13.25 s < t < 18.0 s. Fig. 11 shows the comparisons of variations of mass flow rate with time at the inlet of the left station tunnel and at the outlet of the right station tunnel between Case 1 and Case 2. Here, it can be seen clearly that there has been a remarkable reduction in air exchange between the running tunnel and the station tunnel in Case 2 as compared to that in case 1.
Y.-d. Huang et al. / Tunnelling and Underground Space Technology 38 (2013) 526–533
12
5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15
10
case 1 case 2
Case 1 Case 2
8 6 4
Pressure (Pa)
Pressure (Pa)
532
2 0 -2 -4 -6 -8 -10 -12 -14
0
2
4
6
8
10 12 14 16 18 20
0
2
4
6
8
10 12 14 16 18 20
Time (s)
Time (s)
(a) Duct 1
(b) Duct 2
Fig. 12. Comparisons of pressure variations with time at the central point of the tunnel ceiling openings of duct 1 and duct 2 between Case 1 and Case 2.
Table 2 Mass flows of the air through each of the two ventilation ducts in Case 1 at t = 0–18 s.
Duct 1 Duct 2 Inlet of the left station tunnel Outlet of the right station tunnel
Exhaust mass flow (kg)
Suction mass flow (kg)
0.0526 0.3224
0.3944 0.0040 1.1584
Net mass flow-in (kg) 0.3418 0.3184 1.1584 1.1818
1.1818
Table 3 Mass flows of the air through each of the two ventilation ducts in Case 2 at t = 0–18 s.
Duct 1 Duct 2 Inlet of the left station tunnel Outlet of the right station tunnel
Exhaust mass flow (kg)
Percentage of increase in exhaust mass flow relative to case 1 (%)
Suction mass flow (kg)
Percentage of increase in suction mass flow relative to case 1 (%)
0.0531 0.7217
0.95 123.9
0.8900 0.0319 0.1046
125.7 697.5 91.0
0.2517
78.7
In fact, the mass flow rate through a specific ventilation duct is determined by the pressure at the tunnel ceiling opening of the duct. Comparisons of pressure variations with time at the central point (5.705, 0.105, 0.25) of the tunnel ceiling opening of Duct 1 and at the central point (33.295, 0.105, 0.25) of the tunnel ceiling opening of Duct 2 between Case 1 and Case 2 are shown in Fig. 12. Fig. 12(a) illustrates clearly that the pressure at the point (5.705, 0.105, 0.25) for t > 2.88 s in Case 2 is much smaller than that in Case 1 and thus the suction mass flow rate through Duct 1 is greatly increased by the adoption of Curtain 1. Fig. 12(b) reveals obviously that the pressure at the point (33.295, 0.105, 0.25) during the period of 2.88 s < t < 10.62 s in Case 2 is much higher than that in Case 1 and thus the exhaust mass flow rate through Duct 2 is greatly increased by the dropping of Curtain 2. The graph of Fig. 12 can be viewed in association with the interpretation of Fig. 10. For the time period of 0 s < t < 18 s, the mass flows of the air through each duct, the inlet of the left station tunnel and the outlet of the right station tunnel in Case 1 and Case 2 are presented in Tables 2 and 3, respectively. From these two tables, it can be obtained that the suction mass flow of the air through Duct 1 and the exhaust mass flow of the air through Duct 2 are increased, respec-
Net mass flow-in (kg) 0.8369 0.6898 0.1046 0.2517
tively, by 125.7% and 123.9% in Case 2 in comparison with those in Case 1. Furthermore, the total mass flow of the air sucked into the tunnel through the two ducts and the total mass flow of the air pushed out of the tunnel through the two ducts are increased, respectively, by 131.4% and 106.6% in Case 2 as compared to those in Case 1 (the total mass flows of the air pulled into the tunnel through the two ducts are 0.3984 kg in Case 1 and 0.9219 kg in Case 2, while the total mass flows of the air pushed out of the tunnel through the two ducts are 0.3750 kg in Case 1 and 0.7748 kg in Case 2, respectively). This indicates clearly that the natural ventilation effects achieved by the ducts in the subway tunnel are greatly strengthened by the solid curtains placed transversely inside the tunnel. Also, it can be obtained from Tables 2 and 3 that the mass flows of the air across the inlet of the left station tunnel and across the outlet of the right station tunnel are decreased, respectively, by 91.0% and 78.7% in Case 2 as compared to those in Case 1. This reveals clearly that with the installation of the two solid curtains the air exchange between the station tunnels and the running tunnel is significantly lessened (especially, the air exchange between the station tunnel and the running tunnel is completely stopped when the two curtains are dropped). Moreover, it can be obtained from Tables 2 and 3 that the net mass flow into the given computational
Y.-d. Huang et al. / Tunnelling and Underground Space Technology 38 (2013) 526–533
domain is shown to be less than 0.0001 kg in Case 1 and also less than 0.0001 kg in Case 2, which demonstrates that the current numerical calculations are very accurate. It should be mentioned here that the air velocity and pressure distributions shown in Figs. 7–9, which cannot be given by the one-dimensional simulation tools, are the benefit of using the present CFD approach. 5. Conclusions In a circumstance where a train runs in a subway tunnel with many natural ventilation ducts, a solid curtain can be dropped just at an upstream (in a direction opposite to the train running) position of a ventilation duct while a train is running at a downstream (in the same direction of the train running) position of the duct, which allows a large air suction flow rate to be obtained through the duct. In a similar way, a solid curtain may be dropped just at a downstream position of another ventilation duct while a train is running at an upstream position of the duct, allowing a large exhaust airflow rate to be obtained through the duct. For the analysis of the effects of solid curtains on duct ventilation performance in a subway tunnel, the tunnel and subway train geometries which are exactly the same as those in a previous model tunnel experiment is adopted in the current study with the incorporation of two ventilation ducts connected vertically to the tunnel ceiling and two solid curtains placed at an upstream position of a duct near the tunnel inlet and at a downstream position of another duct near the tunnel outlet, respectively. In order to evaluate the effects of solid curtains on duct ventilation performance in a subway tunnel, a three-dimensional Computational Fluid Dynamics calculation is carried out. The current three-dimensional numerical model adopting the dynamic layering method for tracking the motion of a train was validated in a previous study (Huang et al., 2010) against the model tunnel experiment performed by Kim and Kim (2007). The numerical results reveal that the natural ventilation performance achieved by the ducts in the subway tunnel strongly depends on the operation of the solid curtains. The effect of air suction through the duct located near the tunnel inlet and the effect of air exhaust through the duct located near the tunnel outlet are both greatly strengthened by the solid curtains (the suction mass flow of the air through the duct near the tunnel inlet and the exhaust mass flow of the air through the duct near the tunnel outlet are increased, respectively, by 125.7% and 123.9% in the case with the solid curtains as compared to those in the case without the solid curtains). Also, with the use of the solid curtains the airflow along the tunnel across the positions of the solid curtains can be much suppressed. When the operation of solid curtains is implemented in a real subway system, the safety issues (such as the reliable droppings and raisings of solid curtains for safe running of subway trains),
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linking to signaling, acceptance procedures, the requirement of having a mechanical ventilation system, as well as the additional traction power consumption of trains induced by the solid curtains are the very practical aspects, but they are beyond the scope of this work. It should be noted that the present study is restricted to discussion in the one-tube, single-track system. And further research is expected to apply a CFD tool to analyze the effects of solid curtains on natural ventilation performance in one-tube, double-track and twin-tube, single-track systems. Also, it should be noted that the blockage ratio of the train to the tunnel is 0.669 in the present work and future research is needed to consider the smaller blockage ratio since in most modern metros the blockage ratio is below 0.5 (perhaps the smaller blockage ratio would lead to smaller airexchange enhancement by the solid curtains). Acknowledgement The work was supported by the Leading Academic Discipline Project of Shanghai Municipal Education Commission (Grant No. J50502) and the Innovation Program of Shanghai Municipal Education Commission (Grant No. 10ZZ95). References Huang, Yuan-dong, Gao, Wei, Kim, Chang-Nyung, 2010. A numerical study of the train-induced unsteady airflow in a subway tunnel with natural ventilation ducts using the dynamic layering method. Journal of Hydrodynamics 22 (2), 164–172. Huang, Yuan-dong, Gong, Xiao-lu, Peng, Yue-jiao, Lin, Xiao-yu, Kim, Chang-Nyung, 2011. Effects of the ventilation duct arrangement and duct geometry on ventilation performance in a subway tunnel. Tunnelling and Underground Space Technology 26, 725–733. Juraeva, Makhsuda, Lee, Jun-ho, Song, Dong-Joo, 2011. A computational analysis of the train-wind to identify the best position for the air-curtain installation. Journal of Wind Engineering and Industrial Aerodynamics 99 (5), 554–559. Ke, M.T., Cheng, T.C., Wang, W.P., 2002. Numerical simulation for optimizing the design of subway environmental control system. Building and Environment 37 (11), 1139–1152. Kim, J.Y., Kim, K.Y., 2007. Experimental and numerical analyses of train-induced unsteady tunnel flow in subway. Tunnelling and Underground Space Technology 22, 166–172. Kim, J.Y., Kim, K.Y., 2009. Effects of vent shaft location on the ventilation performance in a subway tunnel. Journal of Wind Engineering and Industrial Aerodynamics 97, 174–179. Udaykumar, H.S., Mittal, R., Rampunggoon, P., et al., 2001. A sharp interface Cartesian grid method for simulating flows with complex moving boundaries. Journal of Computational Physics 174 (1), 345–380. Wang, Feng, Deng, Yuan-ye, Wang, Ming-nian, 2009. Simulation research on traininduced piston effect in subway tunnels with and without shaft. Chinese Journal of Underground Space and Engineering 5 (6), 1081–1085 (in Chinese). Yu, Lian-guang, Wu, Xi-ping, Zhang, Chen, 2009. Energy saving analysis for curtains of subway. Journal of Thermal Science and Technology 8 (4), 343–349 (in Chinese). Yuan, F.D., You, S.J., 2007. CFD simulation and optimization of the ventilation for subway side-platform. Tunnelling and Underground Space Technology 22 (4), 474–482.
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Effects of the solid curtains on natural ventilation performance in a subway tunnel Yuan-dong Huang a,b,⇑, Xiao-lu Gong a, Yue-jiao Peng a, Chang-Nyung Kim b,c,⇑ a
Department of Environmental Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China Department of Mechanical Engineering, Kyung Hee University, Yongin 449-701, Republic of Korea c Industrial Liaison Research Institute, Kyung Hee University, Yongin 449-701, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 27 October 2011 Received in revised form 2 August 2013 Accepted 23 August 2013 Available online 20 September 2013 Keywords: Ventilation performance Subway tunnel Natural ventilation duct Solid curtain Train-induced airflow
a b s t r a c t Solid curtains can be installed in subway tunnels for the promotion of air ventilation in ventilation ducts in association with the piston effect caused by a running train. With an aim to analyze the effects of solid curtains on duct ventilation performance in a subway tunnel, the current study adopts the tunnel and subway train geometries which are exactly the same as those in a previous model tunnel experiment, but newly incorporates two ventilation ducts connected vertically to the tunnel ceiling and two solid curtains placed at an upstream position of a duct near the tunnel inlet and at a downstream position of another duct near the tunnel outlet, respectively. A three-dimensional CFD model adopting the dynamic layering method for tracking the motion of a train, which was validated against the reported model tunnel experiment in a previous study, is employed to predict the train-induced unsteady airflows in the subway tunnel and in the ducts. The numerical results reveal that the duct ventilation performance in a subway tunnel strongly depends on the operation of the solid curtains. The suction mass flow of the air through the duct near the tunnel inlet and the exhaust mass flow of the air through the duct near the tunnel outlet are increased considerably in the case with the solid curtains in comparison with those in the case without the solid curtains. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Tunnel ventilation is crucial for environmental control in subway systems. In a subway tunnel, both the mechanical and natural ventilation modes are influenced substantially by piston wind. Recently, the Computational Fluid Dynamics (CFD) tool has been increasingly utilized to analyze problems involved in three-dimensional train-induced airflows in subway tunnels. For instance, Wang et al. (2009) performed a numerical simulation of train-induced piston effect in subway tunnels with and without ventilation ducts using CFD code FLUENT. Yuan and You (2007) evaluated numerically the time-averaged velocity and temperature fields in a side-platform station using CFD code AIRPAK. Ke et al. (2002) carried out a numerical study of the influence of train velocities on pressure distribution in a station area using CFD code PHOENICS. Juraeva et al. (2011) conducted a numerical analysis of the ventilation system in a subway tunnel to identify the best position for installation of an air-curtain using ANSYS CFX software. It should be noted that the one-dimensional tools (such as ⇑ Corresponding authors. Address: Department of Mechanical Engineering, Kyung Hee University, Yongin 449-701, Republic of Korea (C.N. Kim). E-mail addresses: [email protected] (Y.-d. Huang), [email protected] (C.-N. Kim). 0886-7798/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tust.2013.08.009
SES) are common standard and are by far more efficient than three-dimensional approaches in investigating train-induced tunnel flows. However, the one-dimensional tools cannot be utilized to simulate the airflow distributions. And the motivation of adopting the three-dimensional CFD approach is to obtain the distributions of air velocity, temperature and pressure in subway tunnels and ventilation ducts. Natural ventilation mode uses only train-induced wind to fulfill air exchange between a subway tunnel and outer atmosphere. The importance of natural ventilation has drawn many recent studies on duct ventilation achieved by train motion in subway tunnels. Using the sharp interface method (Udaykumar et al., 2001) for the moving boundaries of a train, Kim and Kim (2009) investigated numerically the effects of duct location on ventilation performance in a subway tunnel employing PSDs with the use of CFX4.4. Huang et al. (2010) conducted a numerical analysis of the train-induced unsteady airflow in a subway tunnel with natural ventilation ducts using the dynamic layering method. Their study revealed the impact of train motion on the exhaust and suction of the air through ventilation ducts and the effects of a barrier placed at the tunnel outlet on the duct ventilation performance. Huang et al. (2011) recently examined numerically the effects of the ventilation duct number and duct geometry on duct ventilation performance in a subway tunnel.
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In order to improve air quality in subway tunnels with natural ventilation ducts, searching for measures to strengthen duct ventilation effect has become a focal point in researches of subway ventilation systems. As mentioned above, much attention has been paid to the effects of duct location, duct arrangement and duct geometry on ventilation performance in a subway tunnel with natural ventilation ducts. In the study of Huang et al. (2010), it was revealed that a barrier placed at the tunnel outlet can strengthen remarkably the duct ventilation performance. In a subway tunnel, a barrier placed at the tunnel inlet may also have a great impact on the ventilation effect. But, there has been no report on how the duct ventilation is quantitatively affected by two solid barriers (curtains) placed at the inlet and outlet of a subway tunnel, respectively. Although Yu et al. (2009) conducted a numerical analysis of energy saving for the installation of two solid barriers (curtains) in a subway tunnel, they carried out a two-dimensional simulation of the air flow in a tunnel (obviously, the airflow near a train exhibits a three-dimensional characteristics) and did not account for the acceleration and deceleration processes of the train run. The overall goal of this study is to investigate the effects of solid curtains on natural ventilation performance in a subway tunnel. For this purpose, we perform numerical simulations of the train-induced unsteady airflow in a subway tunnel with natural ventilation ducts for two different cases: in Case 1 (reference case) there are no solid curtains in the subway tunnel, whereas in Case 2 two solid curtains are placed transversely near the inlet and outlet of the tunnel, respectively. In each case, the configurations of the subway tunnel and of the train are exactly the same as those used in the model tunnel experiment conducted by Kim and Kim (2007) except that the natural ventilation ducts connected vertically to the tunnel ceiling are newly installed. The simulations are carried out with FLUENT code. In the CFD model, the dynamic layering method is adopted for tracking the boundaries of a moving train. The numerical results are analyzed to clarify quantitatively the effects of solid curtains on the suction and exhaust flows through each individual duct and on the total mass flow of air sucked into and pushed out of the tunnel through all ventilation ducts.
2. Physical model and related conditions 2.1. Description of the experimental study carried out by Kim and Kim (2007) on train-induced airflow in a model tunnel Kim and Kim (2007) conducted an experimental study on the train-induced unsteady airflow in a model tunnel built to be in a 1/20th scale of a real subway tunnel. The details of this model tunnel experiment have been presented clearly in previously published literatures (Huang et al., 2010; Huang et al., 2011; Kim and Kim, 2007). However, a brief description of this experiment is given in the below since the physical model used in the present study is constructed based on the setup of the model tunnel experiment carried out by Kim and Kim (2007).
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Fig. 2. Schedule of train run (Kim and Kim, 2007).
(a) Central vertical section of the tunnel with the two identical ventilation ducts (not in scale)
(b) Top view of the tunnel with the two identical ventilation ducts (not in scale) Fig. 3. Geometry and locations of the two identical ventilation ducts installed on the tunnel ceiling in Case 1.
A schematic diagram of the experimental layout is shown in Fig. 1, which illustrates the geometries of the tunnel and of the train, the initial and final positions of the train, the total distance of the train run, and the sites of the four pressure transducers (PT1, PT2, PT3 and PT4) and of the two velocity transducers (VT1 and VT2). The blockage ratio of the train to the tunnel is 0.669. Both the velocity and pressure measurements are recorded at a time interval of 0.1 s. The train connected to the cable can move back and forth on the guide rail. The speed profile of the train, which is controlled by a drive motor equipped with an electrical inverter, is shown in Fig. 2. After the train accelerates at a rate of 1.0 m/s2 for 3 s, it runs at a constant speed for 8 s. Then, the train slows down at a rate of 1.0 m/s2 until it stops. The Reynolds num-
Fig. 1. Schematic diagram of the experimental layout (Kim and Kim, 2007).
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(a) Central vertical section of the tunnel with the two ventilation ducts and the two curtains (not in scale)
(b) Top view of the tunnel with the two ventilation ducts and the two curtains (not in scale) Fig. 4. Geometries and locations of the ventilation ducts on the tunnel ceiling and of the curtains in Case 2.
Table 1 Raised/dropped states of the two curtains in Case 2 at t = 0–18 s.
Curtain 1 Curtain 2
0–2.88 s
2.88–10.62 s
10.62–18 s
Raised Dropped
Dropped Dropped
Dropped Raised
3
Experimental results Numerical results
ducts connecting the tunnel ceiling to the ground are newly installed. As shown in Fig. 3(a), the two identical ventilation ducts are connected vertically to the tunnel ceiling and the length of each duct is 0.425 m. The top view of the tunnel with the two ventilation ducts is depicted in Fig. 3(b), which illustrates that the cross sectional area of the duct is 0.15 m 0.1 m and the distance between the two ducts is 27.44 m. The initial and final positions of the train are located on the tracks in two different stations equipped with PSDs. Here, the 39 m long tunnel is divided into the two station tunnels (the length of each station tunnel is 4.6 m) and the running tunnel between the two stations. Case 2 is the same as Case 1 except that the two solid curtains (Curtains 1 and 2, see Fig. 4) with the identical geometry (each curtain is 210 mm wide, 250 mm high and 30 mm thick) are placed transversely inside the tunnel with an aim to assess the effects of solid curtains on natural ventilation performance in a subway tunnel. Here, Curtain 1 is located at an upstream position of a duct near the inlet of the tunnel while Curtain 2 is located at a downstream position of another duct near the outlet. Considering safe train passages through the positions of curtains in a real subway tunnel, in the numerical simulation the two curtains in Case 2 can be automatically raised or dropped depending upon the position of the train. As the train starts to move, Curtain 1 is raised while Curtain 2 is dropped. After the train passes through the position of Curtain 1, it will be dropped when the rear face of the train is 1 m away from Curtain 1. As the train approaches Curtain 2, it will be raised when the front surface of the train is 2.5 m away from Curtain 2. For clarity, the raised and dropped states of the two curtains during the train motion are listed in Table 1.
3. CFD model and its validation
2
3.1. CFD model with initial and boundary conditions
CP
1
0
-1
-2
-3 0
2
4
6
8
10
12
14
16
18
20
Time (s) Fig. 5. Comparison of Cp variations between experimental and numerical results at position of PT3.
ber based on the hydraulic diameter of the model tunnel and the maximum train speed (3 m/s) is 4.9 104. 2.2. Configurations of subway tunnel, train, ventilation ducts and solid curtains In this study, the numerical simulations deal with two cases. In Case 1, the geometries of the tunnel and of the subway train, as well as the initial position and running schedule of the train, are exactly the same as those used in the model tunnel experiment performed by Kim and Kim (2007), but two natural ventilation
The Reynolds-averaged Navier-Stokes (RANS) and the continuity equation, together with the RNG j e turbulence model are used to solve train-induced unsteady air flows. The governing equations are discretized using a finite volume method. The PISO algorithm is adopted for the pressure-velocity coupling. The QUICK differencing scheme is employed for the discretization of momentum, turbulent kinetic energy, and turbulent dissipation rate equations, whereas the PRESTO scheme is utilized for the pressure correction equation. For unsteady flow calculations, the temporal derivatives are discretized using the first-order implicit scheme. For modeling the moving boundaries of the train, the dynamic layering method (see ANYSY FLUENT 12.0 User’s Guide) in the dynamic mesh model supplied by FLUENT code is employed. The numerical solution is obtained using this CFD code. The grid size in the longitudinal direction (x axis) of the tunnel is set to be 0.03 m. In the cross-section of the tunnel (i.e. the y–z plane), the grid size is set to be 0.02 m in both y and z directions except in the narrow gap between the surface of moving train and the tunnel walls and in the ventilation ducts, where fine grids are applied. Initially, the entire computational domain consists of about 405,000 hexahedral cells in each of the two cases. At t = 0 s, the pressure and the air velocity in the tunnel and in the ducts are assumed to be zero. In the time domain of t > 0 s, the boundary conditions are the following: A pressure-inlet is specified at the inlet of the left station tunnel. The pressure-outlet conditions are employed at the outlet of the right station tunnel and at the ground opening of each ventilation duct. No-slip conditions are imposed on the solid surfaces of the tunnel, curtains, ventilation ducts and moving train. The events in FLUENT is used to control the raising and dropping of the solid curtains.
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1.0
sults of the comparison between the measured and computed values are quite satisfactory at all measurement points including the four points of the pressure transducers and the two points of the velocity transducers (here, for brevity only the comparisons between the numerical and experimental results for pressure coefficient Cp against time at the third pressure transducer (PT3) and for u/UT_MAX variations with time at the second velocity transducer (VT2) are presented, see Figs. 5 and 6). It is thus feasible to use the current CFD model to perform numerical analysis of the problem formulated in Section 2.2.
Experimental results Numerical results
0.8
u/U T-MAX
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0.6
0.4
4. Numerical results and discussion 0.2
0.0 0
2
4
6
8
10
12
14
16
18
20
Time (s) Fig. 6. Comparison of u/UT_MAX variations between experimental and numerical results at position of VT2.
In the calculation, the size of the time step is set to be 0.005 s and thus the Courant number based on the maximum train speed and the grid size in the longitudinal direction is obtained to be 0.5 (i.e., the CFL condition is satisfied). During the process of calculation, the residual of every equation is monitored. The convergence criterion is that the residual for each variable is less than 10 5. 3.2. Model validation The current CFD model with the dynamic layering method for a moving train was validated in a previous study (Huang et al., 2010) against the data obtained from the model tunnel experiment conducted by Kim and Kim (2007). The validation showed that the re-
General characteristics of train-induced unsteady airflow through a natural ventilation duct placed on the ceiling of a subway tunnel without curtains have been investigated intensively in our previous studies (Huang et al., 2010; Huang et al., 2011), and thus in this study we focus on analyzing the flow fields in the tunnel and in the ventilation ducts for Case 2 when both the two solid curtains are operated, and on comparing the ventilation characteristics in Case 2 with those in Case 1. It can be expected that as the train completely passes Duct 1, a negative pressure is generated in the region under Duct 1, which allows the air to be sucked into the tunnel through Duct 1. Fig. 7 shows the pressure contours and velocity vectors around Duct 1 at t = 3.0 s just after Curtain 1 is dropped. Here, it can be seen obviously that there is a region with low pressure near the rear face of the train (i.e., in the near wake of the moving train) which induces a strong suction flow through Duct 1. Moreover, it is seen that just after Curtain 1 is dropped a small vortex is formed near Curtain 1 and the air within most part of the tunnel on the left side of Duct 1 still moves in the train-running direction as it did before Curtain 1 was dropped. The pressure contours and velocity vectors around Duct 1 and Duct 2 in the middle section (y = 0.105 m) of the tunnel at t = 7.0 s are shown in Figs. 8 and 9, respectively. Fig. 8 illustrates
(a) Pressure contours
(b) Velocity vectors Fig. 7. Pressure contours and velocity vectors around the duct 1 in the middle section (y = 0.105 m) of the tunnel just after the curtain 1 is closed at t = 3.0 s in Case 2.
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(a) Pressure contours
(b) Velocity vectors Fig. 8. Pressure contours and velocity vectors around the duct 1 in the middle section (y = 0.105 m) of the tunnel at t = 7.0 s in Case 2.
(a) Pressure contours
(b) Velocity vectors Fig. 9. Pressure contours and velocity vectors around the duct 2 in the middle section (y = 0.105 m) of the tunnel at t = 7.0 s in Case 2.
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0.09
0.03
0.08
0.02 Case 1 Case 2
Mass flow rate (kg/s)
0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.01 -0.02
0.00 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06
-0.03
-0.07
-0.04
-0.08
-0.05 0
2
4
6
Case 1 Case 2
0.01
Mass flow rate (kg/s)
0.07
-0.09
8 10 12 14 16 18 20
0
2
4
6
8 10 12 14 16 18 20
Time (s)
Time (s)
(a) Duct 1
(b) Duct 2
Fig. 10. Comparisons of variations of mass flow rate with time at different ducts between Case 1 and Case 2.
0.10 Case 1 Case 2
0.09
0.07
Mass flow rate (kg/s)
Mass flow rate (kg/s)
0.08
0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.01 0
2
4
6
8
10 12 14 16 18 20
Time (s)
(a) Inlet of the left station tunnel
0.02 0.01 0.00 -0.01 -0.02 -0.03 -0.04 -0.05 -0.06 -0.07 -0.08 -0.09 -0.10 -0.11 -0.12 -0.13
Case 1 Case 2
0
2
4
6
8
10 12 14 16 18 20
Time (s)
(b) Outletof the right station tunnel
Fig. 11. Comparisons of variations of mass flow rate with time at the inlet of the left station tunnel and at the outlet of the right station tunnel between Case 1 and Case 2.
apparently that the air is pulled into the tunnel through Duct 1 and the velocity vectors inside Duct 1 are found parallel to the long axis of Duct 1. Also, it is seen from Fig. 8 that the airflow near Curtain 1 is very weak and a large clockwise vortex is formed in the tunnel space on the left side of Duct 1. Fig. 9 reveals that the air is pushed out of the tunnel through Duct 2 at t = 7.0 s, and that the airflow in the tunnel on the right side of Duct 2 is very weak (especially, there is nearly no airflow in the region near Curtain 2). It is also observed from Fig. 9 that the airflow vectors vary inside Duct 2 and only a small part of velocity vectors (especially, in regions near the duct walls) are parallel to the long axis of Duct 2. Fig. 10 shows the comparisons of variations of mass flow rate with time at each duct between Case 1 and Case 2, where a positive mass flow rate indicates that the air is pulled into the tunnel through the ventilation duct while a negative mass flow rate indicates that the air is pushed out of the tunnel through the duct. From this figure it can be observed clearly that the suction mass flow rate through Duct 1 for t > 2.88 s (i.e., after Curtain1 is dropped) in Case 2 is much larger than that in Case 1 (see Fig. 10(a)) and that the exhaust mass flow rate through Duct 2 during the period of 2.88 s < t < 10.62 s (i.e., before Curtain 2 is raised)
in Case 2 is quite greater than that in Case 1 (see Fig. 10(b)), suggesting that the effect of air suction through Duct1 and the effect of air exhaust through Duct 2 are both strengthened significantly by the solid curtains placed transversally inside the tunnel. Furthermore, it can be seen that the mass flow rate through each duct in Case 2 remains almost unchanged with time as the train moves at a constant speed of 3 m/s during the period of 3 s < t < 11 s. The train moves under Duct 2 during the period of 11.13 s < t < 12.50 s and the whole train runs on the right side of Duct 2 after t > 12.50 s, which induces considerable air suction through Duct 2 for the period of 11.275 s < t < 13.25 s in Case 2. However, after this period the train speed is much reduced so that this slow-moving train has tendency to block the flow of air following the rear of the moving train, which yields the exhaust airflow through Duct 2 for the period of 13.25 s < t < 18.0 s. Fig. 11 shows the comparisons of variations of mass flow rate with time at the inlet of the left station tunnel and at the outlet of the right station tunnel between Case 1 and Case 2. Here, it can be seen clearly that there has been a remarkable reduction in air exchange between the running tunnel and the station tunnel in Case 2 as compared to that in case 1.
Y.-d. Huang et al. / Tunnelling and Underground Space Technology 38 (2013) 526–533
12
5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15
10
case 1 case 2
Case 1 Case 2
8 6 4
Pressure (Pa)
Pressure (Pa)
532
2 0 -2 -4 -6 -8 -10 -12 -14
0
2
4
6
8
10 12 14 16 18 20
0
2
4
6
8
10 12 14 16 18 20
Time (s)
Time (s)
(a) Duct 1
(b) Duct 2
Fig. 12. Comparisons of pressure variations with time at the central point of the tunnel ceiling openings of duct 1 and duct 2 between Case 1 and Case 2.
Table 2 Mass flows of the air through each of the two ventilation ducts in Case 1 at t = 0–18 s.
Duct 1 Duct 2 Inlet of the left station tunnel Outlet of the right station tunnel
Exhaust mass flow (kg)
Suction mass flow (kg)
0.0526 0.3224
0.3944 0.0040 1.1584
Net mass flow-in (kg) 0.3418 0.3184 1.1584 1.1818
1.1818
Table 3 Mass flows of the air through each of the two ventilation ducts in Case 2 at t = 0–18 s.
Duct 1 Duct 2 Inlet of the left station tunnel Outlet of the right station tunnel
Exhaust mass flow (kg)
Percentage of increase in exhaust mass flow relative to case 1 (%)
Suction mass flow (kg)
Percentage of increase in suction mass flow relative to case 1 (%)
0.0531 0.7217
0.95 123.9
0.8900 0.0319 0.1046
125.7 697.5 91.0
0.2517
78.7
In fact, the mass flow rate through a specific ventilation duct is determined by the pressure at the tunnel ceiling opening of the duct. Comparisons of pressure variations with time at the central point (5.705, 0.105, 0.25) of the tunnel ceiling opening of Duct 1 and at the central point (33.295, 0.105, 0.25) of the tunnel ceiling opening of Duct 2 between Case 1 and Case 2 are shown in Fig. 12. Fig. 12(a) illustrates clearly that the pressure at the point (5.705, 0.105, 0.25) for t > 2.88 s in Case 2 is much smaller than that in Case 1 and thus the suction mass flow rate through Duct 1 is greatly increased by the adoption of Curtain 1. Fig. 12(b) reveals obviously that the pressure at the point (33.295, 0.105, 0.25) during the period of 2.88 s < t < 10.62 s in Case 2 is much higher than that in Case 1 and thus the exhaust mass flow rate through Duct 2 is greatly increased by the dropping of Curtain 2. The graph of Fig. 12 can be viewed in association with the interpretation of Fig. 10. For the time period of 0 s < t < 18 s, the mass flows of the air through each duct, the inlet of the left station tunnel and the outlet of the right station tunnel in Case 1 and Case 2 are presented in Tables 2 and 3, respectively. From these two tables, it can be obtained that the suction mass flow of the air through Duct 1 and the exhaust mass flow of the air through Duct 2 are increased, respec-
Net mass flow-in (kg) 0.8369 0.6898 0.1046 0.2517
tively, by 125.7% and 123.9% in Case 2 in comparison with those in Case 1. Furthermore, the total mass flow of the air sucked into the tunnel through the two ducts and the total mass flow of the air pushed out of the tunnel through the two ducts are increased, respectively, by 131.4% and 106.6% in Case 2 as compared to those in Case 1 (the total mass flows of the air pulled into the tunnel through the two ducts are 0.3984 kg in Case 1 and 0.9219 kg in Case 2, while the total mass flows of the air pushed out of the tunnel through the two ducts are 0.3750 kg in Case 1 and 0.7748 kg in Case 2, respectively). This indicates clearly that the natural ventilation effects achieved by the ducts in the subway tunnel are greatly strengthened by the solid curtains placed transversely inside the tunnel. Also, it can be obtained from Tables 2 and 3 that the mass flows of the air across the inlet of the left station tunnel and across the outlet of the right station tunnel are decreased, respectively, by 91.0% and 78.7% in Case 2 as compared to those in Case 1. This reveals clearly that with the installation of the two solid curtains the air exchange between the station tunnels and the running tunnel is significantly lessened (especially, the air exchange between the station tunnel and the running tunnel is completely stopped when the two curtains are dropped). Moreover, it can be obtained from Tables 2 and 3 that the net mass flow into the given computational
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domain is shown to be less than 0.0001 kg in Case 1 and also less than 0.0001 kg in Case 2, which demonstrates that the current numerical calculations are very accurate. It should be mentioned here that the air velocity and pressure distributions shown in Figs. 7–9, which cannot be given by the one-dimensional simulation tools, are the benefit of using the present CFD approach. 5. Conclusions In a circumstance where a train runs in a subway tunnel with many natural ventilation ducts, a solid curtain can be dropped just at an upstream (in a direction opposite to the train running) position of a ventilation duct while a train is running at a downstream (in the same direction of the train running) position of the duct, which allows a large air suction flow rate to be obtained through the duct. In a similar way, a solid curtain may be dropped just at a downstream position of another ventilation duct while a train is running at an upstream position of the duct, allowing a large exhaust airflow rate to be obtained through the duct. For the analysis of the effects of solid curtains on duct ventilation performance in a subway tunnel, the tunnel and subway train geometries which are exactly the same as those in a previous model tunnel experiment is adopted in the current study with the incorporation of two ventilation ducts connected vertically to the tunnel ceiling and two solid curtains placed at an upstream position of a duct near the tunnel inlet and at a downstream position of another duct near the tunnel outlet, respectively. In order to evaluate the effects of solid curtains on duct ventilation performance in a subway tunnel, a three-dimensional Computational Fluid Dynamics calculation is carried out. The current three-dimensional numerical model adopting the dynamic layering method for tracking the motion of a train was validated in a previous study (Huang et al., 2010) against the model tunnel experiment performed by Kim and Kim (2007). The numerical results reveal that the natural ventilation performance achieved by the ducts in the subway tunnel strongly depends on the operation of the solid curtains. The effect of air suction through the duct located near the tunnel inlet and the effect of air exhaust through the duct located near the tunnel outlet are both greatly strengthened by the solid curtains (the suction mass flow of the air through the duct near the tunnel inlet and the exhaust mass flow of the air through the duct near the tunnel outlet are increased, respectively, by 125.7% and 123.9% in the case with the solid curtains as compared to those in the case without the solid curtains). Also, with the use of the solid curtains the airflow along the tunnel across the positions of the solid curtains can be much suppressed. When the operation of solid curtains is implemented in a real subway system, the safety issues (such as the reliable droppings and raisings of solid curtains for safe running of subway trains),
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linking to signaling, acceptance procedures, the requirement of having a mechanical ventilation system, as well as the additional traction power consumption of trains induced by the solid curtains are the very practical aspects, but they are beyond the scope of this work. It should be noted that the present study is restricted to discussion in the one-tube, single-track system. And further research is expected to apply a CFD tool to analyze the effects of solid curtains on natural ventilation performance in one-tube, double-track and twin-tube, single-track systems. Also, it should be noted that the blockage ratio of the train to the tunnel is 0.669 in the present work and future research is needed to consider the smaller blockage ratio since in most modern metros the blockage ratio is below 0.5 (perhaps the smaller blockage ratio would lead to smaller airexchange enhancement by the solid curtains). Acknowledgement The work was supported by the Leading Academic Discipline Project of Shanghai Municipal Education Commission (Grant No. J50502) and the Innovation Program of Shanghai Municipal Education Commission (Grant No. 10ZZ95). References Huang, Yuan-dong, Gao, Wei, Kim, Chang-Nyung, 2010. A numerical study of the train-induced unsteady airflow in a subway tunnel with natural ventilation ducts using the dynamic layering method. Journal of Hydrodynamics 22 (2), 164–172. Huang, Yuan-dong, Gong, Xiao-lu, Peng, Yue-jiao, Lin, Xiao-yu, Kim, Chang-Nyung, 2011. Effects of the ventilation duct arrangement and duct geometry on ventilation performance in a subway tunnel. Tunnelling and Underground Space Technology 26, 725–733. Juraeva, Makhsuda, Lee, Jun-ho, Song, Dong-Joo, 2011. A computational analysis of the train-wind to identify the best position for the air-curtain installation. Journal of Wind Engineering and Industrial Aerodynamics 99 (5), 554–559. Ke, M.T., Cheng, T.C., Wang, W.P., 2002. Numerical simulation for optimizing the design of subway environmental control system. Building and Environment 37 (11), 1139–1152. Kim, J.Y., Kim, K.Y., 2007. Experimental and numerical analyses of train-induced unsteady tunnel flow in subway. Tunnelling and Underground Space Technology 22, 166–172. Kim, J.Y., Kim, K.Y., 2009. Effects of vent shaft location on the ventilation performance in a subway tunnel. Journal of Wind Engineering and Industrial Aerodynamics 97, 174–179. Udaykumar, H.S., Mittal, R., Rampunggoon, P., et al., 2001. A sharp interface Cartesian grid method for simulating flows with complex moving boundaries. Journal of Computational Physics 174 (1), 345–380. Wang, Feng, Deng, Yuan-ye, Wang, Ming-nian, 2009. Simulation research on traininduced piston effect in subway tunnels with and without shaft. Chinese Journal of Underground Space and Engineering 5 (6), 1081–1085 (in Chinese). Yu, Lian-guang, Wu, Xi-ping, Zhang, Chen, 2009. Energy saving analysis for curtains of subway. Journal of Thermal Science and Technology 8 (4), 343–349 (in Chinese). Yuan, F.D., You, S.J., 2007. CFD simulation and optimization of the ventilation for subway side-platform. Tunnelling and Underground Space Technology 22 (4), 474–482.