Air quality calculation in subway

Wen-Zhong Wu, Shi-Jun You School of Environment Science and Technology, Tianjin University.

No. 92, Weijin Road, Nankai District, Tianjin 300072, China. E-mail: wuwenzhong@tju.edu.cn (Corresponding author)

Abstract—Air quality does much to people’s health. Based on measured air flow rates, effects of different ventilation and extraction conditions, including velocity, temperature of ceiling ventilation, longitudinal ventilation and extraction on air quality at subway station platform are simulated by CFD, then strategies to optimize this air quality can be (1) decrease ventilation temperature, (2) increase ventilation velocity, (3) activate both ventilation and extraction, etc. All of these will serve as guidelines for good air quality in subway design and performance.

Keywords-air quality; health; CFD; ventilation; extraction; subway; pollutant

I. INTRODUCTION Subway provides convenient commutation mode for

people, and more and more of them travel in subway, e.g. Tianjin subway is programmed to be 9 lines with 300 stations, each of which transfers 28,000~30,000 passengers per hour in the future[1-2]. During the travel, reasonable air quality in subway environment are among passengers’ key concerns.

Air quality in subway station is affected by CO2, surplus heat and humidity released from passenger and surrounding structure, and by a variety of pollutants. Ventilation is a common method to obtain and sustain good air quality there[3-

4]. In order to assess air quality, Lu Ya-jun et al.[5] and Xian-ting Li et al.[6-7] proposed a lot of parameters or indices, such as air temperature, air flow velocity, mean age of air, etc. Mean age of air is the average time for ventilation to reach some point and evaluates ventilation effectiveness or air ‘freshness’ there. Many measurements and simulations of mean age of air[5,8] show that in ventilated space, bigger ventilation velocity results in smaller mean age of air and better ventilation effectiveness,

Ming-Tsun Ke et al.[9] found out that the under platform exhaust (UPE) has a substantial influence on the temperature. Liu Guo-fang[10] suggested that tunnel ventilation should co-activate with platform extraction to optimize ventilation effectiveness of Beijing subway stations.

Both ventilation and extraction affect air quality of subway station platform, and undoubtedly, different ventilation or extraction conditions, including volume or temperature of ceiling ventilation, longitudinal ventilation, extraction turn into a very complex issue, therefore, it is the primary objective of this paper to check these effects and establish reasonable ventilation or extraction strategy to optimize the air quality at platform. All of these will serve as guidelines for good air quality in subway design and performance.

II. MATERIALS AND METHODS

A. Air Quality Measurement In subway travel, passengers spend one-third time at

platform and this time increases in heavier and heavier traffic, therefore their most concern and complain is air quality of platform. Subway station is designed as either side- or central-platform, and more passengers commute train at central- than side- platform, therefore, more attention should be paid to air quality at central-platform. Figure 1 is a typical central-platform station, 120 m long, with 2 staircases and 2 escalators, ventilation system and an extraction level of 2 m3/(min·m2), two fans at terminals providing platform air or extract pollutant from platform[1-2].

Because of no final overall check after installment, the ventilation system does not operate in good state, and ventilation effectiveness at platform is complained to be bad, especially in summer rush hours of its early performance. Therefore, in a typical summer day, measure air temperature, humidity, air velocity, noise, particle in waiting room of platform as TABLE Ⅰ.

By ADM860C air parameter collector made in America, air velocity at each ceiling vent is also measured as TABLE Ⅱof 35 Hz and 40 Hz fan frequency, respectively, where NE1 is No.1 vent along east side of north half platform, SW10 is No.10 vent along west side of south half platform, etc.

a

Figure 1. A typical central-platform subway station.

TABLE I. MEASURED AIR QUALITY VARIABLES

Variable Measurement temperature 28 0C

humidity 76% air velocity ≤ 0.5 m/s

particle 0.01 mg/m3 noise ≤ 70 dB(A)

978-1-4244-2902-8/09/$25.00 ©2009 IEEE 1

TABLE II. MEASURED VELOCITIES (M3/H) OF CEILING VENTS

Vent No. Fan Frequency

35 Hz 40 Hz

NE1 947 1020

NE2 1382 1561

… … …

NE20 1588 1838

SE1 835 670

SE2 597 719

… … …

SE20 374 374

NW1 1445 1547

NW2 1039 1207

… … …

NW20 1258 1443

SW1 529 938

SW2 869 762

… … …

SW19 789 554

B. Air Quality Calculation Apart from measurable variables such as temperature,

humidity, velocity, another important but abstract air quality index to evaluate ventilation effectiveness at platform is mean age of air. Much difficulty exists in its measurement, and different measuring methods do not always give the same results[5]. Therefore, CFD is applied to model ventilation system and calculate its air age[6-7,11].

In the graphics window of Airpak, create a 3D rectangular room with the full dimensions of platform, the physical properties of ceiling, floor and walls are specified as Fig. 2. Then, add 79 air supply vents, staircase and tunnel portal into the room. The platform is meshed coarsely to save the computational effort due to the limited computing devices available.

With an eddy-viscosity model, the indoor airflow is described by the time-averaged Navier-Stokes equations, combined with the equation for mean age of air[6-7], and their free boundary conditions. A second-order upwind scheme discretizes these equations into algebraic equations, then Fluent solved them by integrating over all the cells. The iterations used the SIMPLE algorithm to couple pressure and velocity with the segregated steady-state solver. The continuity and momentum equations were thought to reach convergent when the ratio of the sum of the mass gain and loss on all boundaries to the overall mass gain in the domain was less than 1.0e-3. In a similar method the convergent ratio limit for energy was 1.0e-6 and for the CO2 mass was 1.0e-3.

7200

120000

ceiling 33 0C

floor 28 0C

wal

l 25

0C

wal

l 25

0C

0.5 m/s, 30 0C, outward staircase opening

y

x z

Figure 2. Sketch and physical property of the platform.

7 different cases in TABLE Ⅲ are simulated to find out affecters to mean age of air. In each case, CO2 concentration is input by default. The CFD solution provided their mean age of air data, and those on z = 9 m longitudinal section of cases 1~7 are as Fig. 3.

TABLE III. BOUNDARY CONDITIONS FOR AIR SUPPLY AND EXTRACTION

Case Velocity (m/s) / Temperature (0C)

Ceiling supply air Longitudinal ventilation Extraction level

1 Measured1 / 22 0.5, 0.4 / 30 0 / -

2 Measured2 / 22 0.5, 0.4 / 30 0 / -

3 Measured2 / 25 0.5, 0.4 / 30 0 / -

4 Measured2 / 22 0.5, 0.4 / 22 0 / -

5 Measured3 / 22 0.5, 0.4 / 30 0 / -

6 Measured2 / 22 2.5~3.5 / 30 0 / -

7 Measured2 / 22 2.5 / 30 0~1.0 / - 1measured data before checking through the ventilation system, 2data in TABLE Ⅱ, 3increased ventilation velocity at middle platform by 10~30% based on TABLE Ⅱ.

2

300 263 225 188 150 113 75 38 0 s

Figure 3. Air ages on longitudinal section in cases 1~7 (downwards).

III. RESULTS AND DISCUSSIONS Shown by the measured data in TABLE Ⅰ, air

temperature, humidity, velocity, particle and noise at platform meet ‘Criteria of Air Quality in waiting room of public traffic’ (GB9672-1996) quite well, therefore, reason for air quality complaint of passenger may lie in 7 cases of Fig. 3.

A. Air Quality Affectors 1) Ceiling air velocity: The 1st case shows mean age of air

extremely high at middle part platform, and most part platform has 300 s mean age of air, as the 1st contour in Fig.3.

After checking through the ventilation system, repaired and turn on all valves along ventilation path, most ceiling vents have bigger velocity than before. Both average value and maximum of mean age of air decrease, and area of high air age shrinks to central platform with the highest 227 s, 100 s less than that of the 1st case, shown in the 2nd contour in Fig.3.

2) Air temperature: When ventilation temperature of the 2nd case increases to 25 0C, its mean age of air contour is the 3rd contour in Fig.3. Air spends longer time to drive away pollutant at many points than the 2nd case, therefore the 3rd contour in Fig.3 shows many more points of high mean age of air than the 2nd case.

3) Ambient condition: Having modified ambient environment of the 2nd case model into winter, that is, ambient temperature -5 0C, pressure 1atm, mean age of air at platform becomes lower than those in all the foregoing cases, as the 4th contour of Fig.3.

4) Light intensity: The try to increase light intensity turns into the 3rd case with air temperature increased, and worsens air quality as the 3rd contour in Fig.3 shows. However, seen from the intuitive viewpoint of health, bigger light intensity

can provide some better feel of air quality with better eyesight and less pollutant density.

B. Optimize Air Quality Seen from the results of 1~4 cases, high air temperature

causes poor air quality, therefore, in the case of no enough air supply, decreasing the temperature will do much help to improve air quality at platform, at least do little harm to it.

As the 1st and 2nd contours in Fig.3 show, long air age appears at two terminals of platform, and very high mean age of air, 50% higher than other platform part, exists from x = -20 to x = 20 m nearby staircase.

1) Increase ceiling air velocity: Increase the ventilation velocity at middle platform by 10~30% can decrease mean age of air from x = -10 to x = 15 m, as the 5th contour in Fig.3 shows, but causes higher air age at all other platform parts.

2) Activate tunnel ventilation: Activate tunnel ventilation with velocity 2.5~3.5 m/s can decrease mean age of air at two-third platform part, including its central part x= -20~20 m, as the 6th contour in Fig.3 shows, only increases that of its terminal part with x bigger than 25 m.

3) Co-activate ventilation and extraction: Following the second policy, as a try to decrease high air age of part with x bigger than 25 m, activate both left tunnel ventilation 2.5 m/s and right tunnel extraction 0~1.0 m/s, mean age of air along platform is calculated as the 7th contour in Fig.3.

Just as expected, right tunnel extraction decreases high air age within x = 25~60 m in great efficiency. When right tunnel extraction velocity surpasses 0.5 m/s, mean age of air at every platform point becomes not more than 160 s.

C. More Discussions of Air Quality On air quality of polluted subway station platform, more

characteristics of pollutant, such as movement of smoke[12-13], critical ventilation velocity[14-17], etc. have effects in different ways. CFD and Data-based mechanistic modeling (DBM) approach[18], etc. can be used to simulate air quality in this case, and propose more discussions of the air quality.

1) Air quality of different pollutant sites: It seems that pollutant diffuses itself in mass wave[8,12], and out of different source sites, it pollutes platform in different density with different velocities of wave, so ventilation spends different time to clean pollutant here and there. For example, when a smoke source is near lift, much higher air age exists both upstream and downstream source, and maximum difference of air age exceeds 300 s on longitudinal section, but when smoke source is near staircase, few pulsations of air age appear, and air age keeps relatively stable along longitudinal direction.

2) Level-off of air quality: Those simulations of pollutant case show that the staircase and some air flow turbulence delay longitudinal ventilation for a long time, so that mean age of air becomes high, but it decreases when velocity of longitudinal ventilation increases, and this effect always keeps quite well at any velocity bigger than a certain value. For a fire of 30 MW and smoke volume flow rate 80 m3/s[19], as ventilation velocity is not less than 2.0 m/s, stratifications of

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temperature, CO2 mass rate and mean age of air upstream pollutant source appear; as ventilation velocity increases gradually, these stratifications are kept with an obvious dropping tendency of averaged temperature, mean age of air

and CO2 mass rate, constant values of avg

floorceiling

TTT −

,

avg,CO

floor,COceiling,CO

2

22

ρρρ −

, avg

floorceiling

τττ −

and their level-offs

on cross-sections after ventilation velocity surpasses 2.5 m/s.

IV. CONCLUSIONS This paper checks effects of different ventilation and

extraction conditions on air quality at subway station platform in series: (1) Bigger air velocity decreases both average value and maximum of mean age of air, and high mean age of air area shrinks to platform central part; (2) Increasing air temperature worsens air quality without change of air supply; (3) Change of ambient environment affects air quality. For example, in winter, lower mean age of air than other cases appears.

Based on these analysis, reasonable strategies for optimization of air quality are established: with suitable air temperature, (1) increase the ceiling ventilation velocity can decrease mean age of air but with some imbalance along platform; (2) activate tunnel ventilation can decrease mean age of air at two-third of platform; (3) following the second policy, activate both ventilation and extraction can decrease high air age in great efficiency at every platform point.

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