CFD simulation and optimization of the ventilation for subway side-platform

CFD simulation and optimization of the ventilation for subway

side-platform

Feng-Dong Yuan *,Shi-Jun You

School of Environment Science and Technology,Tianjin University,No.72,Weijin Road,Nankai District,Tianjin 300072,China

Received 27May 2006;received in revised form 23September 2006;accepted 19October 2006

Available online 5December 2006

Abstract

To obtain the velocity and temperature ?eld of subway station and the optimized ventilation mode of subway side-platform station,this paper takes the evaluation and optimization of the ventilation for subway side-platform station as main line,builds three dimen-sional models of original and optimization design of the existed and rebuilt station.And using the two-equation turbulence model as its physics model,the thesis makes computational ?uid dynamics (CFD)simulation to subway side-platform station with the boundary conditions collected for simulation computation through ?eld measurement.It is found that the two-equation turbulence model can be used to predict velocity ?eld and temperature ?eld at the station under some reasonable presumptions in the investigation and study.At last,an optimization ventilation mode of subway side-platform station was put forward.ó2006Elsevier Ltd.All rights reserved.

Keywords:Ventilation;CFD simulation;k -e model;Subway;Side-platform

1.Introduction

Computational ?uid dynamics (CFD)software is com-monly used to simulate ?uid ?ows,particularly in complex environments (Chow and Li,1999;Zhang et al.,2006;Moureh and Flick,2003).CFD is capable of simulating a wide variety of ?uid problems (Gan and Ri?at,2004;Somarathne et al.,2005;Papakonstantinou et al.,2000;Karimipanah and Awbi,2002).CFD models can be realis-tically modeled without investing in more costly experi-mental method (Betta et al.,2004;Allocca et al.,2003;Moureh and Flick,2003).So CFD is now a popular design tool for engineers from di?erent disciplines for pursuing an optimum design due to the high cost,complexity,and lim-ited information obtained from experimental methods (Li and Chow,2003;Vardy et al.,2003;Katolidoy and Jicha,2003).Tunnel ventilation system design can be developed in depth from the predictions of various parameters,such

as vehicle emission dispersion,visibility,air velocity,etc.(Li and Chow,2003;Yau et al.,2003;Gehrke et al.,2003).Earlier CFD simulations of tunnel ventilation system mainly focus on emergency situation as ?re condition (Modic,2003;Carvel et al.,2001;Casale,2003).Many sci-entists and research workers (Waterson and Lavedrine,2003;Sigl and Rieker,2000;Gao et al.,2004;Tajadura et al.,2006)have done much work on this.This paper stud-ied the performance of CFD simulation on subway envi-ronment control system which has not been studied by other paper or research report.It is essential to calculate and simulate the di?erent designs before the construction begins,since the investment in subway’s construction is huge and the subway should run up for a few decade years.The ventilation of subway is crucial that the passengers should have fresh and high quality air (Lowndes et al.,2004;Luo and Roux,2004).Then if emergency occurred that the well-designed ventilation system can save many people’s life and belongings (Chow and Li,1999;Modic,2003;Carvel et al.,2001).The characteristics of emergency situation have been well investigated,but there have been

0886-7798/$-see front matter ó2006Elsevier Ltd.All rights reserved.doi:10.1016/j.tust.2006.10.004

*

Corresponding author.Tel.:+862287401917;fax:+862227892626.E-mail address:yuanfd@https://www.360docs.net/doc/fd5670286.html, (F.-D.Yuan).

https://www.360docs.net/doc/fd5670286.html,/locate/tust

Tunnelling and Underground Space Technology 22(2007)

474–482

Tunnelling and

Underground Space Technology

incorporating Trenchless Technology Research

few studies in air distribution of side-platform in normal conditions.

The development of large capacity and high speed com-puter and computational?uid dynamics technology makes it possible to use CFD technology to predict the air distri-bution and optimize the design project of subway ventila-tion system.Based on the human-oriented design intention in subway ventilation system,this study simu-lated and analyzed the ventilation system of existent station and original design of rebuilt stations of Tianjin subway in China with the professional software AIRPAK,and then found the optimum ventilation project for the ventilation and structure of rebuilt stations.

2.Ventilation system

Tianjin Metro,the secondly-built subway in China,will be rebuilt to meet the demand of urban development and expected to be available for Beijing2008Olympic Games. The existent subway has eight stations,with a total length of7.335km and a0.972km average interval.For sake of saving the cost of engineering,the existent subway will con-tinue to run and the stations will be rebuilt in the rebuilding Line1of Tianjin subway.Although di?erent existent sta-tions of Tianjin Metro have di?erent structures and geom-etries,the Southwest Station is the most typical one.So the Southwest Station model was used to simulate and analyze in the study.Its geometry model is shown in Fig.1.

2.1.The structure and original ventilation mode of existent station

The subway has two run-lines.The structure of South-west Station is,length·width·height=74.4m(L)·18.7m(W)·4.4m(H),which is a typical side-platform station.Each side has only one passageway(length·height=6.4m(L)·2.9m(H)).The middle of station is the space for passengers to wait for the vehicle.The plat-form mechanical ventilation is realized with two jet open-ings located at each end of station and the supply air jets towards train and track.There is no mechanical exhaust system at the station and air is removed mechanically by tunnel fans and naturally by the exits of the station.

2.2.The design structure and ventilation of rebuilt station

The predicted passenger?ow volume increase greatly and the dimension of the original station is too small,so in the rebuilding design,the structure of subway station is changed to,(length·width·height=132m(L)·17.438m(W)·4.65m(H)),and each side has two passage-ways.The design volume?ow of Southwest Station is 400000m3/h.For most existent stations,the platform height is only2.9m,which is too low to set ceiling ducts. So in the original design,there are two grille vents at each end of the platform to supply fresh air along the platform length direction and two grille vents to jet air breadthways towards trains.The design velocity of each lengthways grille vent is5.54m/s.For each breadthways vent,it is 5.28m/s.Under the platform,80grille vents of the same velocity(4.62m/s,40for each platform of the station) are responsible for exhaust.

3.CFD simulation and optimization

The application of CFD simulation in the indoor envi-ronment is based on conversation equations of energy, mass and momentum of incompressible air.The

study Fig.1.The location of test section and the layout of measuring points.

F.-D.Yuan,S.-J.You/Tunnelling and Underground Space Technology22(2007)474–482475

adopted a turbulence energy model that is the two-equa-tion turbulence model advanced by Launder and Spalding. And it integrated the governing equation on the capital control volumes and discretized in the de?nite grids,at last simulated and computed with the AIRPAK software.

3.1.Preceding simpli?cations and presumptions

Because of mechanical ventilation and the existence of train-driven piston wind,the turbulence on platform is transient and complex.Unless some simpli?cations and presumptions are made,the mathematics model of three-dimensional?ow is not expressed and the result is diver-gent.While ensuring the reliability of the computation results,some preceding simpli?cations and presumptions have to be taken.

(1)The period of maximum air velocity is paid attention

to in the transient process.Apparently the maximum air velocity is reached at the period when train stops at or starts away from the station(Yau et al.,2003;

Gehrke et al.,2003),so the period the simulation con-cerns about the best period of time for simulation is from the point when at the section of‘x=0.0m’(Fig.1)and the air velocity begin to change under piston-e?ect to the point when train totally stops at the station(de?ned as a‘pulling-in cycle’).

(2)Though the pulling-in cycle is a transient process,it is

simpli?ed to a steady process.

(3)Because the process is presumed to a steady process,

the transient velocity of test sections,which was tested in Southwest Station in pulling-in cycle,is pre-sumed to the time-averaged velocity of test sections.

(4)The volume?ow driven into the station by pulling-in

train is determined by such factors as BR(blocking ratio,the ratio of train cross-section area to tunnel cross-section area),the length of the train and the resistance of station etc.For existent and new sta-tions,BRs are almost the same.Although the length of the latter train doubles that of the former which may increase the piston?ow volume,the resistance of latter is greater than that of the former which may counteract this increase.So it is presumed that the piston?ow volume is same for both existent and new station and that the volume?ow through the passenger exits is also same.Based on this pre-

sumption,the results of the?eld measurements at the existent station can be used as velocity boundary conditions to predict velocity?led of new station. 3.2.Original conditions

To obtain the boundary conditions for computation and simulation,such as the air velocity and temperature of enclosure,measures were done by times at Southwest Sta-tion.All data are recorded during a complete pulling-in cycle.The air velocities were measured by the multichannel anemonmaster hotwire anemoscope and infrared ther-mometer is used to measure the temperature of the walls of the station which are taken as the constant temperature thermal conditions in the simulation.

3.2.1.Temperatures of enclosure

Divide the platform into?ve segments and select some typical test positions.The distributing temperature of enclosure is shown in Table1.It can be seen from Table 1that all temperatures of enclosure are between23°C and25°C,there is little di?erence in all test positions, and the average temperature is24°C.So all temperatures of subway station’s walls is24°C in CFD computation and simulation.

3.2.2.Time-averaged air velocity above the platform

Fig.1is the location of test section and the layout of measuring points.The data measured include12transient velocities in each section(A–H in Fig.1),which were deal with section’s time-averaged velocities in the period,12 point’s velocities of passageway,which is used to acquire the average?ow,and the velocities of each end of station, which is used to acquire the average piston?ow volume.

Fig.2is the lengthways velocities measured of platform sections,V max is the maximum air velocity,V min is the minimum air velocity and V ave is the average air velocity. Fig.2shows that the maximum air velocity is at the pas-sageway.At the passageway the change of air velocity is about2.25m/s,which is the maximum and indicates that the passageway is the position e?ected most by the piston wind e?ect,and the air velocity of section D and E after the passageway is almost the same,which indicates that the piston wind can hardly e?ect the air velocity after the passageway.

Table1

Distributing temperature of enclosure

Test positions Station walls Platform

Top Railway ground Lateral wall Ground

123123123123 Temperature(°C)when x=12.8m24.824.624.823.823.823.824.223.823.623.823.823.8 Temperature(°C)when x=25.6m24.824.624.624242424.42423.623.423.824 Temperature(°C)when x=38.4m24.82524.824242424.423.823.623.623.223.6 Temperature(°C)when x=51.2m24.424.424.424.224.224.224.224.223.42323.423.4 476 F.-D.Yuan,S.-J.You/Tunnelling and Underground Space Technology22(2007)474–482

In addition,because of the movement of train,the veloc-ity cannot be measured in section U of Fig.1,the air veloc-ity of section U was computed in CFD simulation.

3.2.3.Boundary conditions of simulation

Through analyzing the boundary data measured at the Southwest Station,the boundary conditions of the existent station in CFD simulation is shown in Table2.Addition-ally,the Tianjin Metro is out of run during the measuring time,so the heat load is not typical and is di?cult to be determined so that the distributing temperature was not simulated in this study.

Referred to the feasibility research report of No.3 design institute of China’s ministry of railway and the mea-sured data in Southwest Station,the boundary conditions of new station in CFD simulation of the existent station is shown in Table3.

3.3.Solution process

3.3.1.Method of solution

The governing equations to calculate original variables, such as velocity and temperature etc.,can be shown (Moureh and Flick,2003).

o

o t

eq/Ttdiveq~u/T?diveC grad/TtSe1Twhere/is common variable and denotes(u,v,w,T),~u is velocity vector,q is density,C is dissipation function,S is the source item.u is air velocity of direction X,v and w are air velocities of direction Y and Z,respectively,and T is air temperature.

Eq.(1)is integrated on the spatial control volumes,and through discretizing in the de?nite

grids Fig.2.Measured lengthways velocities at platform sections.

Table2

Boundary condition of existent stations in CFD simulation

Sections Section G Section U

(estimated)Section H Passageway of the train-

stopping side:EXIT1

Passageway of the non-train

stopping side:EXIT2

The exit of

train F

Air velocity(m/s)0.670.670.670.670.670.67 Air?ow(m3/s)0.670.670.670.670.670.67 Table3

Boundary condition of rebuilt stations in CFD simulation

(a)Heat load

Persons Lamps(on the top

of platform)Advertisement lamps(lateral

wall of platform)

Heat emitted from train

Heat

load 44kW/

person

13W/m215kW/station320kW(acceleration)and200kW

(heat from applying brake)

(b)Air velocity

Supply air(lengthways)Supply air(breadthways)Under the platform vents Air velocity(m/s) 5.54 5.28 4.62

(c)Boundary conditions of air velocity

The entrance of train The exit of

train

Passageway of the train-stopping

side

Passageway of the non-train-stopping

side

Boundary conditions

of air velocity(m/s)

0.2–0.590.41 1.440.97

(d)Temperature

Train walls(expert for excluding top and bottom of train)Outdoor air

(summer)

Supply air

(summer)

Tunnel

air

Enclosure

walls

Temperature (°C)2727272524

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a p/?

X

nb a nb/

nb

tbe2T

In Eq.(2),nb denotes neighbor grids;a p and a nb are the coe?cients of/and/nb,respectively,b is the source item.

(2)Tables4and5shows solution,cycle type and convergence criterion and relaxation coe?cient in CFD simulation of the existent station ventilation and the new station.

3.4.Simulation and discussion

Used the boundary conditions measured in Southwest Station,the air velocity of subway side-platform station was simulated in AIRPAK.Fig.3is the air velocity com-parison of simulation and measurement in section A–E, which shows that the two-equation model and the preced-ing simpli?cations are acceptable in predicting?ow at the station.Although the computed velocity at section C and E is not equal to the values of the?eld measurement,whose error is about0.1–0.2m/s,the di?erences are acceptable and the overall variance trend of the velocity along the platform tallies with the measurement.The simulation result of air velocity of1.7m height above platform of exis-tent station can be seen in Fig.4.Fig.4shows that the majority of piston winds pour out from the passageway and the air velocity of the zone after the passageway is lower,which is below0.4m/s.The maximum air velocity at1.7m height above platform is the passageway of the train-stopping side,which is about1.89m/s.

The simulation results indicate that it is applicable to use two-equation turbulence model to predict time-averaged ?ow and temperature distribution at subway station so the veracity is reliable.

Fig.5is the simulation result,which is the original design of air velocity at1.7m height above platform of new station.It is shown from Fig.5that at the train-ing-stopping side of platform the supply air does not reach the zone between the two exits,which is for the passenger to wait for the train,and pour out from the passageway because of the leakage of passageway.The air velocity between two passageways,which is below 0.4m/s,is a little low so that the‘Dead Zone’is formed. The simulation also shows that the maximum velocity exists at the exits,which agrees with the references and experience.

Fig.6is the temperature distribution at1.7m height above platform of the new station,which is used to simu-late for original design ventilation mode.It shows that the temperature distribution is imbalanced for the leakage of exits at the training-stop side of platform,and at each end of platform,the temperature should be approximately between24°C and26°C while the temperature is between 32°C and34°C because the heat away from the train can not be cooled and removed by the cold supply air in time, even at the x=61m point,the temperature reaches above 40°C,which makes people feel uncomfortable.

The simulation result of temperature and air velocity of original design at the lengthways center section is shown in

Table4

Settlement of existent station ventilation CFD simulation

Variable Pressure Kinetic k e

Discretization Body force weighted Second order upwind Second order upwind Second order upwind Relaxation coe?cient0.3–1.00.3–1.00.3–1.00.3–1.0

Solution Pressure AMG a Kinetic AMG k AMG e AMG

Cycle type V-type Flexible Flexible Flexible

a AMG is algebra multi-grid.

Table5

Settlement of new station ventilation CFD simulation

Variable Pressure Temperature Kinetic k e

Discretization Body force weighted Second order upwind First order upwind First order upwind First order upwind Relaxation coe?cient0.3–1.00.3–1.00.3–1.00.3–1.00.3–1.0

Solution Pressure AMG Temperature AMG Kinetic AMG k AMG e AMG

Cycle type V-type Flexible Flexible Flexible

Flexible Fig.3.Air velocity of section A–E simulation and measurement.

478 F.-D.Yuan,S.-J.You/Tunnelling and Underground Space Technology22(2007)474–482

Fig.7,which shows that the distribution of temperature and air velocity is not balanceable and the zone between two exits is ‘Dead Zone’.

From the simulation of original design,the following can be seen:(1)The ventilation mode that the feasibility research

report design for the side-platform station of subway makes the distribution of temperature and air veloc-ity unbalanced,so the passengers will feel uncomfort-able at the middle of

platform.

Fig.4.Simulation result of air velocity at 1.7m height above platform of existent

station.

Fig.5.Simulation result of air velocity at 1.7m height above platform of new station (original

design).

Fig.6.Simulation result of temperature at 1.7m height above platform of new station (original design).

F.-D.Yuan,S.-J.You /Tunnelling and Underground Space Technology 22(2007)474–482479

(2)The air velocity is too high at some position of plat-

form and the passengers feel easily the change of air velocity,whose scope is between0.4m/s and2.5m/s, and feel uncomfortable when they go to exits from the middle of platform.

(3)The e?ciency of ventilation is not as good as expec-

tation and the supply air from jet fans cannot reach the middle of platform adequately and part of them will?ow away from the passageways.

Because of the unbalanced distribution of temperature and air velocity,original design’s ventilation mode was optimized.The exhaust vents are as same as that of original ones.The?ow of supplied air remains the same and under the platform there are tubes of supplied fresh air.Also there are some vertical tubes fetched out by platform.As a result,the air is supplied widthways and reasonably the ?ow of supplied air is delivered in balance,which elimi-nates the unbalanced distribution of temperature and air velocity on the platform.On the other hand the breadth-ways supplied air prevents the heat brought by train from di?using onto the platform,and the heat can be vented in time from the exhaust vents under the platform.At the same time the fresh air from the breadthways vents passes through the passengers,takes away heat and humidity,and the e?ciency of ventilation is quite satisfactory.

To validate the optimization ventilation mode,the sim-ulation adopted the same boundary conditions and set up with original design’s CFD simulation.The simulation results are showed in Figs.8and9.

Fig.8is the air velocity distribution at1.7m height above platform of new station,which is simulated for optimization design ventilation mode.It shows that the distribution of air velocity is good enough and balanced. The breadthways supplied air makes the‘Dead Zone’disappeared,and the maximum air velocity is only about 1.67m/s at 1.7m height above platform of new station,which makes it di?cult for passengers to feel windy.

Fig.9is the simulation result of temperature and air velocity of original design at the lengthways center section, which shows that the‘Dead Zone’disappears at the length-ways center section.And the breadthways supplied air pre-vents the heat,which is from the air conditioning condenser and the friction produced by train-stopping, from di?using to the platform,so the temperature is about below28°C.

Compared with the ventilation mode in original design, the optimization ventilation mode has several advantages as follows:

(1)The supply air is delivered to every zone in a balanced

way and the distribution of temperature and air velocity of side-platform is also balanced.

(2)The air velocity is balanced and the maximum is only

about1.67

m/s.

Fig.7.Simulation result of temperature and air velocity at the lengthways center section(original

design).

Fig.8.Simulation result of air velocity at1.7m height above platform of new station(optimization design). 480 F.-D.Yuan,S.-J.You/Tunnelling and Underground Space Technology22(2007)474–482

(3)The supplied air removes the heat of platform better

and the whole platform’s temperature is lower than that of the original design,which can lower tempera-ture standard and can decrease the ?ow of supply air to save energy.

(4)The supplied air from breadthways vents also well

prevents the hot contaminated train-driven air from spreading onto the platform,and on the other hand the fresh air from the breadthways vents passes through the passengers,takes away heat and humid-ity,collects and is exhausted by the under-platform vents.

(5)The ventilation e?ciency is quite satisfactory.

4.Conclusions

The following can be concluded from this study:

The air ?ow in platform of subway is a complex three-dimensional transient turbulence.Simpli?cation of the air ?ow to steady process and presumption of the transient velocity to the time-averaged velocity are applicable to sim-ulate the distribution of temperature and air velocity of subway platform in the pulling-in cycle.

It is applicable to use two-equation turbulence model to predict time-averaged velocity and temperature distribu-tion at subway station.The simulation results are in?u-enced greatly by the boundary conditions such as boundary velocity and temperature.

The optimization ventilation mode has more advantages than the original design’s.The breadthways ventilation is suggested when the height of platform is too low to have ceiling ducts.It makes the distribution of temperature and air velocity balanced.

Before a design is accepted it is very necessary to evalu-ate and optimize it by such techniques as CFD,etc.It will help a lot to build a reliable,e?ective and economical system.

Acknowledgement

This project was supported by the funding generated by the Tianjin Municipal Science and Technology Commis-sion of China (Project No:033112911).

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