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Prediction of the pollutant diffusion discharged from wind tower of the city traffic tunnel
Tunnelling and Underground Space Technology 42 (2014) 112–121
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Trenchless Technology Research
Prediction of the pollutant diffusion discharged from wind tower of the city traffic tunnel Jinghua Yu a,⇑, Liwei Tian b, Weiqian Zhuang b, Jian Yang a a b
School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China China Railway Siyuan Survey and Design Group Limited Company, Wuhan 430063, China
a r t i c l e
i n f o
Article history: Received 18 April 2013 Received in revised form 22 January 2014 Accepted 9 February 2014
Keywords: Traffic tunnel Pollutant diffusion Wind tower Numerical simulation Wind tunnel test
a b s t r a c t With the increase of the urban traffic volume, a large number of tunnels have been built in many cities; the vehicle emission from the traffic tunnel has a significant impact on the surrounding environment, it has important meaning to study the impact of pollutant diffusion discharged from wind tower on the downwind environment, and to evaluate the air pollution situation and variation tendency around the wind tower. In this study, a reduced scale model of 1:200 is used for three-dimensional numerical simulation and wind tunnel test; results show that the numerical simulation result agrees well with the result of the wind tunnel test. The diffusion of pollutant discharged from the wind tower with the actual size is simulated by the computational fluid dynamics (CFD) program; results show that, the CO concentration ratios of points on building windward side to that of the wind tower outlet (C/C0) are reduced to 1.211%–1.563% on the middle vertical line of building windward side at a distance of 200 m when the building is 39 m (10 m higher than the wind tower), 0.974%–1.215% when the heights of the building and the wind tower are both 29 m, and 0.654%–0.834% when the building is 19 m. When C/C0 of building windward is lower than the maximum allowable value of 2.67%, the required shortest distances between the building and the wind tower are 50, 150 and 200 m for the building heights of 19, 29 and 39 m, respectively. The results can be a reference for the design of wind tower of traffic tunnel and the determinations of the location and height of downwind new building. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid economic development of the city, the traffic problem is more and more prominent. The construction of urban traffic tunnels shows a sharp growth to efficiently alleviate or solve the increasing problem of urban traffic in large cities of China. The urban traffic tunnel plays a positive role in reducing urban road space, cutting running distance, and easing the urban traffic jam, but it also bring the serious air pollution problems. The ventilation system, which is the combination of wind tower and jet fans, is used to discharge the exhaust in the upper air, it shows effective to avoid high pollution concentration staying at the ground near the tunnel entrance, but the vehicle exhaust discharged from the wind tower seriously affects the downwind environment, therefore, it is particularly important to investigate the diffusion and decay rate of the pollutants on the surrounding environment. This paper focuses on the study of the dispersion distance and influence range of pollutant discharged from the wind tower of Qingchun ⇑ Correspodning author. Tel.: +86 159 7200 7990; fax: +86 27 87792101. E-mail addresses: [email protected] (J. Yu), [email protected] (L. Tian). http://dx.doi.org/10.1016/j.tust.2014.02.009 0886-7798/Ó 2014 Elsevier Ltd. All rights reserved.
Road Traffic Tunnel in Hangzhou to investigate the effect of pollutant diffusion on the building environment along the wind direction. There are some studies on pollutant diffusion discharged from the tunnel wind tower or tunnel portal using numerical simulation. The pollutant concentrations in the vicinity of the tunnel portal were conducted by Chow (1996), Oettl et al. (2002), and showed that pollution from the vehicles coming out of the tunnel portal could spread to regions far away from the tunnel portal, and the prevailing wind could disperse the harmful emissions further away from the tunnel. Wang et al. (2009) studied the pollutant diffusion discharged from the tunnel portal and tunnel shaft according to the Gauss model and an experiential model of tunnel portal prediction, results showed that the environmental effect of portal diffusion was more serious than that of shaft discharge. Ren and Xie (2010) focused on the design of the wind tower to study the CO concentration diffusion around the wind tower of an undersea tunnel; the impacts of the tower height, the exhaust air rate and the natural wind velocity on the concentration diffusion of pollutant discharged from the wind tower were analyzed, to meet the air quality standard within the scope of 15 m above the ground, the
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height of 35 m is suggested as the minimum height of this wind tower. Jiang et al. (2012) studied the dispersion of CO and NO2 discharged from the wind tower of Dapu Road underground tunnel under different characteristics of typical wind speed and directions. Two heights of the wind tower were analyzed; the results can be used as a reference for wind tower designing. Few wind tunnel tests or field experiments have been undertaken to study the environmental impact of vehicle exhaust discharged from the ventilation tower or tunnel portal. Jiang et al. (1998) carried out a wind tunnel test study with a geometric reduced scale of 1:500 to analyze the environmental impact of the exhaust discharged from the tunnel shaft and the tunnel portal. This paper focused on the CO diffusion on the ground surface, the maximum concentrations in road surface axis under different vehicle speeds and wind speeds were investigated. Brousse et al. (2005) conducted a situ measurement campaign to study the environmental impact of vehicle exhausts at the south portal of the Landy tunnel located north of Paris. Besides the numerical simulation, Jiang et al. (2012) also investigated the dispersion of vehicular exhaust pollutants discharged from the ventilation tower of Dapu Road underground tunnel by the wind tunnel test with the scale of 1:80. The features of dispersion distance and influence range were tested; the experimental results show a good agreement with the numerical simulation. In summary, few persons focus on the effect of pollutant diffusion discharged from wind tower of traffic tunnel on the downwind building environment. Three research methods for studying the pollutants emission in urban traffic tunnel are included: CFD numerical simulation, wind tunnel simulation test and field measurement method. Compared with the numerical and physical models, the field measurement on the pollutant concentration is more difficult to implement due to the complexity of airflow and the height of wind tower (about 19–49 m) for this study, so only CFD numerical simulation and scaled model tests have been used in this study, the purpose is to predict the dispersion distance and influence range of pollutant discharged from the wind tower of Qingchun Road Traffic Tunnel, and to study the effect of pollutant diffusion on the building environment along the wind direction, The present work involved the followings: (1) Study the exhaust emission discharged from the wind tower of QingChun road tunnel by using the CFD numerical simulation with a reduced scale, analyze the pollutant diffusion when the heights of downwind buildings are different. (2) Carry out the wind tunnel simulation test of QingChun road tunnel with the same scale to validate the simulation results. (3) Investigate the dispersion distance and influence range of CO discharged from the wind tower with the actual size by numerical simulation, four distances and three height differences between the wind tower and downwind building are included; study the C/C0 on four height lines of the building windward side. 2. Wind tower The research object is the wind tower of Qingchun Road Tunnel. The full-length of this tunnel is 3420 m, it is divided into left and right opening, the inner boundary dimension of each opening is: length width = 8.75 (m) 4.55 (m). Wind tower is used for discharging the pollutants to the upper air, the outlet is rectangular and the area is 48m2 (length width = 8 m 6 m), the height of the wind tower is 29 m. As the pollutants of traffic tunnel are chiefly from the vehicle exhaust, the main components are the CO, NO, NO2 and smoke, etc. Tests and operating experience indicate that, when CO level is properly diluted, other dangerous and objectionable exhaust
by-products are also diluted to acceptable levels (ASHRAE Handbook, 2007). The maximum allowable concentration of CO is stipulated in specifications of design of ventilation and lighting of highway tunnel (JTJ026.1-1999) (Ministry of Transport of the PRC, 1999) at the present stage, so this paper select CO as the primary pollutant source. The maximum allowable concentration of CO for the first road tunnel (Holland tunnel in New York) equipped with the ventilator was 400 ppm, it turned out that the design concentration of 400 ppm was too high to meet the health standard for long tunnel, and then some countries reduced the design concentration to 250 ppm. According to the Chinese standard JTJ026.11999, the design maximum allowable concentration of CO is 250 ppm for road tunnel less than 1000 m, 200 ppm for the tunnel longer than 3000 m, and 250–200 ppm for 1000–3000 m by the interpolation methods. The maximum allowable CO concentration for the worst case in traffic jam is 300 ppm, and the exposure time is less than 20 min, so 300 ppm is assumed in this paper as the initial concentration of CO at the wind tower outlet. According to Chinese standard JTJ026.1-1999, CO emissions in the whole tunnel can be calculated by the following equation:
Q CO ¼
1 6
3:6 10
qCO fa fd fh fiv L
n X
ðN m fm Þ
ð1Þ
m¼1
where QCO is CO emissions in tunnel (m3/s); qCO is base emissions of CO (m3/vehicle km), it is 0.01 m3/(vehicle km) in 1995, an annual decrease of 2% is revealed; fa is vehicle condition factor, here fa = 1.0; fd is vehicle density factor, the value is related to traffic condition, if it is in traffic jam, the vehicle speed is supposed 20 km/h , fd = 3.0; fh is height factor above the sea considering CO emissions, the altitude is 20–60 m in Hangzhou city, fh = 0.9; fiv is longitudinal slope–speed factor considering CO emissions, here fiv = 0.8; L is the length of the tunnel (m); n means the number of vehicle types; Nm means the design number of the corresponding vehicles for each type (vehicles /h); fm is the factor of vehicle types considering CO emissions; The above factors valued according to table (3.4.2-1– 3.4.2-4) in standard JTJ026.1-1999. The prediction report on the vehicle flow gave the traffic volume in Qingchun road, which is listed in Table 1; the ratio of large vehicle to small vehicle is 1:6. Saturation point was rapidly being approached in 2030 for Qingchun road tunnel, the peak hourly volume in year 2030 was taken to calculate the CO emissions. According to Eq. (1), base emissions of CO qCO is 0.00493 (=0.01 (1 0.02)35) in 2030, so the amount of CO emissions is 0.0604 m3/s. The quantity of air flow necessary in the tunnel is calculated according to the CO emissions by the following equation:
Q req ¼
Q CO P0 T 106 d P T0
ð2Þ
where Qreq is the required ventilation rate for the whole tunnel to dilute CO, m3/s; P0 is standard atmosphere pressure (kN/m2), 101.325 kN/m2; d is the CO design concentration which is the allowed maximum concentration (ppm), here is 300 ppm; P is the local atmosphere pressure (kN/m2), it is 99.98 kN/m2 in summer in Hangzhou; T0 is standard temperature (K), 273 K; T is design temperature of tunnel in summer (K), here 313 K is taken. The Qreq calculated by Eq. (2) is 234 m3/s, the exhaust velocity at wind tower outlet is 4.9 m/s according to the outlet area, so we take 5 m/s as the exhaust velocity, and 240 m3/s as the design ventilation rate.
Table 1 Prediction of traffic volume in Qingchun road tunnel. Year
2010
2020
2030
The peak hourly volume (pcu/h)
4307
5459
5970
pcu/h- Passenger Car Unit per hour.
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3. Numerical simulation and wind tunnel test of pollutant diffusion discharged from tunnel wind tower CFD numerical simulation can study the pollutant diffusion by solving the three control equations of convection diffusion equation, the air motion equation and the continuity equation. In order to verify the accuracy of the CFD numerical simulation results, the wind tunnel model test is used to verify the simulation results. The geometry scale between the model (both test and the numerical simulation) and the actual wind tower is 1:200. 3.1. Numerical simulation 3.1.1. The CFD model Previous study (Huang et al., 2011) showed that when investigate the dispersion characteristics of vehicular pollutant, the prediction result is the best by using standard k–e model, the followings are realizable k–e mode and RSM mode, and however the prediction results are less good by using standard k–x model, SST k–x model and Spalart–Allmaras model. The standard k–e turbulence model (Launder and Spalding, 1974) and its variants remain the most widely used approaches to modeling wind engineering and atmospheric dispersion problems (Stathopoulos, 1997), so the standard k–e model is used for this study where k is the turbulent kinetic energy, and e is the turbulent dissipation rate. Fig. 1 shows the calculation region of model established by the numerical simulation, the lengths of AB, AC, AE are 6.0 m, 2.5 m and 1.5 m, respectively. The height of the wind tower is 0.145 m; and the distances from the center point of the wind tower to the EG, GH and FH are 2 m, 1.25 m and 4 m, respectively. The building is just opposite the wind tower along wind direction and the distance between them is 1 m. The length of the building is 0.3 m, the heights of building are assumed to be 0.145 m (case1), 0.195 m (case2) and 0.095 m (case3), respectively. In order to control the total grid number and ensure calculation accuracy, grid refinement is performed around the wind tower and building, while other regions use sparse grid. In the numerical calculation, the control equation is discrete by the finite volume method, and the SIMPLE algorithm is used for coupling effect between pressure and velocity in the airflow field calculation. The differential format is QUICK format. The quadrilateral and hexagon grids are used in the computations. The grid unit numbers of the calculation region are 1197573 for case 1, 1203079 for case 2, and 1091917 for case 3. Besides the mesh densities of the three studied cases, the finer meshes are also considered; comparison shows that mesh sizes in this study yield similar results with finer meshes. Therefore, the mesh densities of this study were adapted.
parameter in Hangzhou city; inlet 2 is the plane of wind tower outlet, the air velocity at wind tower outlet is 5 m/s, and the CO concentration at the wind tower outlet is 300 ppm; the outlet is the plane of BDHF; the air flow through outlet is set to pour freely and develop fully. The planes of ABDC, ABFE and CDHG in the Fig. 1 are symmetry planes which do not influence the flow of subjacent air. The bottom of the calculation region, the inner surface of the wind tower, and the wall surface of the building are all supposed as no-slip surface, the roughness constant is 0.5. 3.1.3. Results of CFD simulation Government health agencies differ on acceptable CO levels in the standards of workplace or ambient air quality. The American Council of Governmental and Industrial Hygienists (ACGIH, 1998) recommends a threshold limit of 29 mg/m3 (25 ppm) for 8-h exposure, the U.S. Occupational Safety and Health Administration (OSHA, 2001a) recommends 55 mg/m3 (50 ppm) for daily 8-h exposure; the U.S. Environmental Protection Agency (EPA, 2000) recommends maintaining an average of 40 mg/m3 (35 ppm) for 1-h exposure. In China, the maximum allowed 24-h average concentration is 4 mg/m3 (3 ppm), and the allowed 1-h average concentration is 10 mg/m3 (8 ppm) (Ministry of Environmental Protection of the PRC, 2012). In this study, the C/C0 which is the CO concentration at the point of building surface to that at the wind tower outlet is introduced, the maximum allowed 1-h average CO concentration of 8 ppm is used to analyze the CO concentrations on the building surface whether meet the standard or not, so the CO concentration ratio should be lower than 2.67%. The case 1 is that the heights of wind tower and building are both 0.145 m, Fig. 2a shows the C/C0 on the middle vertical line of building surface on the windward side. the concentration has
(a) The C/C0 on the middle vertical line of the building windward side
3.1.2. Boundary conditions In Fig. 1, the inlet 1 is the plane of ACGE showed, the air velocity is supposed to be 2.2 m/s according to the meteorological
Note: C represents the CO concentration of some point of building surface; C0 represents the CO concentration at the outlet of wind tower. y is the distance away from the middle point of building length
(b) The C/C0 on four horizontal lines of building windward side Fig. 1. The calculation region of model for wind tower and building.
Fig. 2. The CO concentration diffusion of building windward side for case 1 with scale of 1:200.
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dropped significantly compared with that of the wind tower outlet, the higher the point at building surface is, the larger the CO concentration is, the C/C0 are all lower than 0.982%. In addition, the C/C0 on four horizontal lines of the building windward side are analyzed (Fig. 2b). When the pollutant spread to the building windward side, the values of C/C0 vary from 0.714% to 0.982% for different height lines. As the jet uplift effect of pollutant discharged from the wind tower outlet, the CO concentration is clearly higher at the height of 0.195 m than the building height of 0.145 m. When the building surface points are lower than 0.0075 m, the pollutant concentration is very low. Case 2 is that the building is 0.05 m higher than the wind tower; Fig. 3a shows that the C/C0 is sharply decayed when the pollutant diffuses to the building windward side, and the C/C0 of different building heights are from 1.370% to 1.858%, which are a little higher than case 1. Fig. 3b shows the diffusion of CO concentration on four horizontal lines of building windward side. The higher the horizontal line is, the larger the CO concentration shows. On the same height line, CO concentration at midpoint is the highest, which is because that the building is just opposite the wind tower along the wind side. For case 3, the building is 0.05 m lower than the wind tower; Fig. 4 shows the CO attenuation when the pollutant discharges from the wind tower and diffuses to the building windward side. The values of C/C0 range from 0.605% to 0.718% on the middle vertical line of the building windward side (Fig. 4a), and 0.573% to 0.718% on four horizontal lines of the building windward side (Fig. 4b), the values on the building windward side are all very low for different height lines, but relatively higher at the height
Fig. 4. The CO concentration diffusion of building windward side for case 3 with scale of 1:200.
of 0.145 m than others, which is because the uplift effect of the wind tower outlet and jet fans. The simulation results of the three cases show that the height difference between the wind tower and the building influences the pollutant concentration on the building windward side. The C/C0 on the building windward side of the three cases are all lower than 1.858%, which indicate that the CO concentration on building windward side can meet the air quality standard for the three cases.
3.2. The wind tunnel test and the comparison with the CFD simulation
Fig. 3. The CO concentration diffusion of building windward side for case 2 with scale of 1:200.
3.2.1. Introduction of wind tunnel test The large-scale environmental wind tunnel laboratory is located at the University of Shanghai for Science and Technology. It is direct-action wind tunnel with steel structure; the total length is 33 m. It includes seven parts, which are in turn the inlet section, the stable section, the contraction section, the test section, transition section, fan section and outlet diffusing section. The length of test section is 18 m, the width of the cross section is 2.5 m, and the height varies from 1.8 m to 2.1 m. The average air velocity can be adjusted within the range of 0.5 m 20 m/s, the design of uniformity in time and space for air flow parameters references to the flow field quality of internal environmental wind tunnel. Namely, speed non-uniformity 6±1%; fluctuating value of dynamic pressure 61%; pitch direction 6± 0.5°, yaw angle 6±1°, turbulence intensity 61% (Yao, 1991).
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3.2.2. The wind tunnel test model In this study, a model with the same geometry scale is adopted to study the diffusion of the pollutant discharged from the wind tower to the downwind building. The area-source pollution is selected at the wind tower outlet according with the actual situation; Many studies (Higson et al., 1996; Jiang et al., 1998; Lu and Wu, 2005) used the smoke gas, high temperature oil mist, aerosol and organic gases, which are produced by the combustion, here the smoke is produced by the smoke generator as the area-source pollution, and the intensity of smoke is maintained by changing the mass of burning material; the smoke gas is finally produced by mixing the smoke with the airflow of certain speed. The geometry of wind tunnel test model and its boundary conditions consist with the CFD numerical simulation. The three cases which the height of building are assumed to be 0.145 m (case1), 0.195 m (case2) and 0.095 m (case3), are analyzed respectively. 3.2.3. Instruments The main instruments and equipments include: SM-SEMI 2000 Laser, smoke mixing and generating devices, camera equipment, and so on. SM-SEMI 2000 Laser is a continuous wave semiconductor laser, the laser medium is Nd:YVO4, the output is laser sheet with wavelength of 532 nm, output power is 500 mW–1 W, and the beam diameter at aperture is 1.0 mm. The beam divergence is less than 4 mrad. For instantaneous concentration measurements, the AVENIR CCTV LENS is used (the Camera Lens is 16 mm, F1.2, manual-iris, C interface), the shooting speed is 15 frames per second and the maximum electronic shutter speed is 2.56 millisecond. For digital image processing, the maximum absolute error is within 8%. Camera collects scattered light to get the qualitative conclusion. Laser sheet is used to obtain some plane of concentration diffusion at the same time. The smoke gas is transported to the wind tower outlet as the pollutant source; based on digital image processing technology and instantaneous concentration measurement technology of optical particles scattering theory, the measurement of pollutant concentration from the wind tower outlet to the downwind building is carried out. 3.2.4. Results of wind tunnel test and the comparison with CFD numerical simulation Fig. 5 shows the dispersion distance and influence range of the pollutant discharged from the wind tower under the dominant wind direction, the pollutant rises steadily after leaving the wind tower outlet, the concentration is reduced to 10% when the distance is 0.2 m away from the wind tower outlet at the building height of 0.195 m (case 2). The concentration of pollutant has been greatly decayed when it spreads to the surface of the building. The concentration of pollutant is very slight at the building windward
Fig. 5. The dispersion distance and influence range of the pollutant in wind tunnel test.
side, when the wind tower is 0.145 m, only for the case that the building height is 0.195 m, the pollutant can be detected during the height of 0.17–0.195 m, and no pollutant can be detected in other cases and other positions. The C/C0 on the building windward side has relation to the height difference between the wind tower and the building, according to the test data, when the height of building is 0.095 m, 0.145 m and 0.195 m, the C/C0 at the center position of the building windward side reduce to 1.08%,1.22% and 1.75%, respectively. According to the pollutant concentration of the same position, the results of CFD simulation are compared with those of the wind tunnel test. It can be seen from dispersion distance and influence range of the pollutant in Figs. 5 and 6, the numerical simulation results in agreement with the results of the wind tunnel test. According to the concentration attenuation curve in Fig. 7, the absolute errors, namely the differences of C/C0 between the two methods, are all below 5%, the further the point away from the wind tower is, the larger the absolute error is. Since the absolute errors between the numerical simulation and the wind tunnel test are below 5%, the results obtained by numerical simulation are recognized as acceptable and reasonable.
3.2.5. The principal diffusion line of pollutant In order to analyze the pollutant diffusion easily in the actual engineering applications, the principal diffusion line of pollutant in actual size is achieved in Fig. 8, the discrete points are the CO concentrations obtained by wind tunnel test, and the continuous
Fig. 6. The dispersion distance and influence range of the pollutant in numerical simulation.
Fig. 7. The pollutant concentration decay at different distances on principal diffusion line for the case 2.
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4. The pollutant diffusion of the wind tower with actual size 4.1. Model with actual size
Fig. 8. The principal diffusion line of pollutant of the pollutant discharged from wind tower.
The diffusion of pollutant discharged from the wind tower with the actual size is simulated by CFD program. The model is the same as that showed in Fig. 1 except the dimension, the lengths of AB, AC, AE (in Fig. 1) are 1200 m, 500 m and 300 m, respectively. The distances from the center point of the wind tower to EG and GH are 400 m and 250 m, respectively, the actual height of the wind tower is 29 m, the length of the building is 60 m, the distances between the wind tower and the building are assumed to be 50 m, 100 m, 150 m and 200 m, respectively. Three cases mentioned above are investigated, namely the building height of 19 m, 29 m and 39 m. 4.2. Input conditions
line is the fitting curve. It shows that in wind days, with the increase of the vertical distance from the point on the principal diffusion line to the outlet plane of the wind tower, the horizontal distance from the point on the principal diffusion line to the wind tower outlet is increased, namely, the pollutant is keeping away from the wind tower along the wind. The fitting formula of the principal diffusion line is given, as showed in the following equation:
f ðxÞ ¼ 1:314 expð0:1739xÞ R2 ¼ 0:9569
ð3Þ
where f(x) is the horizontal distance from the point on the principal diffusion line to the vertical axes of the center of the wind tower outlet toward the flow direction (m); x is the vertical distance from the point on the principal diffusion line to the plane of the outlet of the wind tower (m). According to the formula (3), the uplift height of the pollutant on principal diffusion line can be calculated (uplift height = x + wind tower height). R2 is the correlation coefficient, the nearer it is close to 1.0, the better the fitted regression line is. Table 2 is the C/C0 of measuring points around the principal diffusion line; we can see that the CO concentration reduces sharply with the increase of the distance away from the wind tower.
Table 2 the C/C0 of measuring points around the principal diffusion line.
Air velocity Uh of some point at the height of h can be expressed as (ASHRAE Handbook, 1997):
U h ¼ U met
dmet hmet
amet a h d
ð4Þ
where dmet is the thickness of atmospheric boundary layer at the place of weather station. Umet is the average velocity of the measurement point at weather station in Hangzhou, according to the meteorological data provided by weather station, the leading wind direction is north–northwest in winter and south–southwest in summer; the average speed of dominant wind direction is 2.9 m/s in summer and 3.3 m/s in winter (Ministry of Construction of the PRC, 2012), so Umet takes the value of 3.3 m/s here; hmet is the height of the measurement point in weather station, it is 10 m; h is the height of the point where the velocity needs to be calculated; amet and a are exponents of atmospheric boundary layer thicknesses at weather station and the place where the velocity needs to be calculated. The weather station locates at the flat area of suburb, so dmet and amet are 270 m and 0.14, respectively; the Qingchun traffic tunnel lies in the central of Hangzhou where more than 50% buildings are higher than 21 m, so d and a take 460 m and 0.33, respectively (Zhu and Yan, 2005), the Uh at the wind tower of 29 m is 2.2 m/s. The air velocity at wind tower outlet is 5 m/s. The CO concentration at the wind tower outlet is 300 ppm. 4.3. Results and discussions
Scatter serial number
x (m)
f(x) (m)
C/C0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
0 4.17 6.67 8.33 9.59 9.60 11.25 14.17 17.50 18.34 19.59 20.42 20.84 21.67 22.50 23.34 24.59 25.00 25.42 26.25 26.67 27.92 28.02
0 0.42 0.83 1.25 1.67 2.08 2.50 3.75 15.00 18.75 23.34 31.25 36.26 57.51 74.59 93.76 103.76 119.60 135.02 139.60 140.85 148.77 149.19
1 0.917 0.826 0.760 0.669 0.612 0.595 0.545 0.496 0.446 0.413 0.372 0.314 0.248 0.198 0.165 0.124 0.083 0.041 0.124 0.116 0.058 0.041
The case 1 is that the heights of the building and the wind tower are both 29 m, Fig. 9a and b shows the diffusion of CO concentration ratio on the building windward side at a distance of 200 m from the wind tower outlet, the values of C/C0 are reduced to 0.974%–1.215% on the middle vertical line, and 0.825%–1.215% on the horizontal lines of 1.5 m,14.5 m and 29 m for the building windward side, while at the height of 39 m which is 10 m higher than the building top, the C/C0 are 1.015–1.745%. From Fig. 9c–e we can see that, when the horizontal line is higher than or equal to the height of building, the CO concentration of the same point is decreased with the increase of the distance between the building and the wind tower. While, when the horizontal line is near the ground (1.5 m), CO concentration is increased with the increase of the distance within 200 m, but the maximum concentration difference on the same horizontal line becomes smaller, because that the wind tower and jet fan have the uplift effect, the CO concentration at the bottom of the building is lower in the nearer distance. When the horizontal line is half of tower height, with the increase of the distance, the CO concentrations of the two edge points are increased, the concentration at the middle point is firstly increased
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Fig. 9 (continued)
Fig. 9. The concentration diffusion when the pollutant spreads to the building windward side for case 1.
and then decreased beyond the distance of 100 m, namely, the CO concentration will be the same if the building is far enough from the tower. That is because the length of building is much larger than the wind tower; the middle of the building will be polluted
firstly when it is near the wind tower, with the increase of the distance, the CO concentration on the same horizontal line finally tend to be the same, and then will become smaller and smaller. As a whole, the CO concentrations on the horizontal line of 1.5 m height and half of tower height are all very low for three cases. The pollutant concentration on the building windward side is decided by the distance away from the wind tower and the height of the building, when the distance is less than 150 m, the C/C0 on the horizontal line of h = 29 are beyond the allowed maximum value of 2.67%, if the building height is reduced to 14.5 m or less, the CO concentration of building windward side can be acceptable to meet the health standard. For case 2 (the building is 39 m which is 10 m higher than the wind tower), The C/C0 on middle vertical line of building windward side at a distance of 200 m from the wind tower is showed in Fig. 10a, the higher the point is, the larger the pollutant concentration is, values of C/C0 are reduced to 1.211%–1.563%. Fig. 10b–e shows C/C0 on four horizontal lines of building wall surface at a distance of 200, 50, 100, 150 m from the wind tower outlet. As the building is just located at the downwind direction, the concentrations on the middle vertical line of the building are higher than the two sides of the same height. The maximum Values C/C0 appear at height of 39 m at the distances of 50, 100, 150 m and at height of 49 m at the distance of 200 m, the result indicate that with the increase of the distance, the uplift height of the pollutant rises, it is about 10 m when the distance is less than 150 m, and is about
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Fig. 10 (continued)
Fig. 10. The concentration diffusion when the pollutant spreads to the building windward side for case 2.
20 m when the distance is further than 150 m. The CO concentration can be negligible when the points are lower than half of tower
height. However, when the distance is less than 200 m, the C/C0 on the top of the building (39 m) are beyond the allowed maximum value of 2.67%, so the safe distance between the wind tower and the building is equal to or further than 200 m. For the case 3, the building height is 19 m; Fig. 11a is the CO concentration diffusion on the middle vertical line of building windward side at a distance of 200 m away from the wind tower. The C/C0 are reduced to 0.654%–0.834% from the bottom to the top. Fig. 11b–e show the diffusion of CO concentration ratio on four horizontal lines of building windward side at distances of 200, 50, 100, 150 m away from the wind tower. Because the building is 10 m lower than the wind tower, the maximal C/C0 at the height of 29 m are 5.7%, 3.7%, 2.5% and 1.4% at the distances of 50, 100, 150 and 200 m, respectively. In Fig. 11b, there is a minimum at y = 0 for h = 14, 19 and 29 m, the reason is that, when the air flows towards the building, some air will flow to the two sides, while the air in the central of the building will climb up, low concentration on lower point will replace the higher concentration on higher point, so the pollutant concentration shows a little decrease at y = 0. The phenomenon is more obvious in low pollutant concentrations, namely for this case when the building height is lowest (19 m) and the distance away from the wind tower is the furthest
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Fig. 11 (continued)
Fig. 11. The concentration diffusion when the pollutant spreads to the building windward side for case 3.
izontal line at h 6 19 m are below the allowed maximum value of 2.67%. From the simulation results of the three cases we can conclude that, for a specific wind tower and a constant distance between wind tower and the building, the higher the downstream building is, the greater the concentration of pollutant on the windward side goes. But for the three cases in this study, the concentration of the pollutant has significantly attenuated when they diffuse to the round of the downstream building because of the long distance between the downstream buildings and the wind tower. When C/C0 of building windward is lower than the maximum allowable value of 2.67%, the required shortest distances between the building and the wind tower are 50, 150 and 200 m for the building heights of 19 m, 29 m and 39 m, respectively.
5. Conclusions (200 m), but the concentration difference between this point and the maximum value of the same line is only about 0.05%. When the distance is further than or equal to 50 m, the C/C0 on the hor-
The purpose of this study is to predict the pollutant diffusion discharged from the tunnel shaft of Qingchun road. Firstly, the pol-
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lutant concentration diffusion from the wind tower to downstream building is calculated for three different cases using CFD numerical simulation and wind tunnel test with the same reduced scale to verify the simulation results. Then, the diffusion of pollutant discharged from the wind tower with the actual size is simulated by CFD program for three height differences and four distances between the wind tower and the building. The major findings and conclusions derived from this study are summarized as follows: (1) The principal diffusion line of pollutant discharged from wind tower and the fitting formula are obtained by the wind tunnel test (Eq. (3)). The comparative analysis on the CO concentration shows that the numerical simulation results agree well with the test results. (2) The height difference between the wind tower and the building influences the pollutant concentration of building windward side. For a specific wind tower and a constant distance between wind tower and the building, the higher the downstream building is, the greater the concentration of pollutant on the windward side goes. When the wind tower is 29 m and the distance is 200 m, C/C0 on the middle vertical line of the building windward are 0.653–0834%, 0.974– 1.215% and 1.211–1.563% for the building heights are 19 m, 29 m and 39 m, respectively. (3) With the increase of distance between the wind tower and the downstream building, the values of C/C0 on building windward decrease, the decrement for the higher point of building windward is much more obvious. When C/C0 of building windward is lower than the maximum allowable value of 2.67%, the required shortest distances between the building and the wind tower are 50, 150 and 200 m for the building heights of 19, 29 and 39 m, respectively. The pollutant concentration is negligible when the point is lower than the half of tower height, where the values of C/C0 are all below 1.5% for all the cases.
Acknowledgments The work of this paper is financially supported by the Doctoral Funds for Higher Education of China (Grant No. 20110142120084) and Project supported by the National Natural Science Foundation of China (Grant No. 51208221).
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References ACGIH. 1998. Industrial ventilation: a manual of recommended practice. In: Appendix A. American Conference of Governmental Industrial Hygienists, 23rd ed., Cincinnati, OH. ASHRAE Handbook, Fundamental, 1997. American Society of Heating, Refrigerating and Air-Conditioning. Engineers, Inc., 1791 Tullie circle, N.E., Atalanta. ASHRAE Handbook, Fundamental, 2007. HVAC Applications (SI Edition). American Society of Heating, Refrigerating and Air-Conditioning. Engineers, Inc. Brousse, B., Vidal, B., Ponticq, X., Goupil, G., Alary, R., 2005. Pollution dispersion at an urban motor way tunnel portal: comparison of the small-scale predictive study with the actual conditions measured on the site. Atmos. Environ. 39, 2459–2473. Chow, W.K., 1996. Numerical Studies of Carbon Monoxide level Contour from a Vehicular Tunnel. Tunnel Engineering Handbook. Chapman and Hall, New York, pp. 232–233. EPA. 2000. Air Quality Criteria for Carbon Monoxide. EPA/600/P-99/001F.U.S. Environmental Protection Agency, Research Triangle Park, NC. Higson, H.L., Griffiths, R.F., Jones, C.D., Hall, D.J., 1996. Flow and dispersion around an isolated building. Atmos. Environ. 30 (16), 2859–2870. Huang, Y.D., Gong, X.L., Peng, Y.J., Lin, X.Y., Kim, C.N., 2011. Effects of the ventilation duct arrangement and duct geometry on ventilation performance in a subway tunnel. Tunn. Undergr. Space Technol. 26 (6), 725–733. Jiang, W.M., Yu, H.B., Xie, G.L., Wu, J., 1998. Experimental study of urban traffic tunnel environmental impact of vehicle emissions. Environ. Sci. 18, 188–193. Jiang, L., Wang, J.S., Xiao, J., Yu, B., Ye, G.Q., 2012. Numerical simulation of the dispersion of vehicular exhausted gaseous pollutants discharged from the tunnel ventilation tower. Chin. J. Hydrodynam. 27, 87–92. Launder, B.E., Spalding, D.B., 1974. The numerical computation of turbulent flows. Int. J. Numer. Methods Fluids. 15, 127–146. Lu, X., Wu, W.Q., 2005. Experimental investigation of the instantaneous Concentration distribution with Laser sheet technique. J. Eng. Thermophys. 26 (4), 587–589. Ministry of Construction of the PRC, 2012. Design Code for Heating Ventilation and Air Conditioning of Civil Buildings (GB 50736-2012), first ed. China Architecture and Building Press, Beijing. Ministry of Environmental Protection of the PRC, 2012. Ambient Air Quality Standard. (GB 3095-2012), third ed. China Architecture and Building Press, Beijing. Ministry of Transport of the PRC, 1999. Specifications of Design of Ventilation and Lighting of Highway Tunnel (JTJ026.1-1999), first ed. China Architecture and Building Press, Beijing. Oettl, D., Sturm, P.J., Bacher, M., Pretterhofer, G., Almbauer, R.A., 2002. A simple model for the dispersion of pollutants from a road tunnel portal. Atmos. Environ. 36, 2943–2953. OSHA. 2001a. Occupational Safety and Health Standards. 29CFR1910.1000.Code of Federal Regulations, U.S. Department of Labor, Occupational Safety and Health Administration, Washington, D.C. Ren, R., Xie, Y.L., 2010. Numerical analysis of pollutant exhaust of immersed tunnel and engineering application. J. Zhengzhou Univ. 31, 90–94. Stathopoulos, T., 1997. Computational wind engineering: past achievements and future challenges. J. Wind Eng. Ind. Aerodyn. 67–68, 509–532. Wang, J., Zhang, X., Zhang, R.P., 2009. Analysis of environmental effect of centralized discharge from the long tunnel in city. Chin. J. Underground Space Eng. 5, 197–200. Yao, X.S., 1991. Flow analysis and design of three-dimensional wind tunnel contractions. AIAA J. 29, 1912–1920. Zhu, Y.X., Yan, Q.S., 2005. Building Environment, third ed. China Architecture and Building Press, Beijing.
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Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust
Trenchless Technology Research
Prediction of the pollutant diffusion discharged from wind tower of the city traffic tunnel Jinghua Yu a,⇑, Liwei Tian b, Weiqian Zhuang b, Jian Yang a a b
School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China China Railway Siyuan Survey and Design Group Limited Company, Wuhan 430063, China
a r t i c l e
i n f o
Article history: Received 18 April 2013 Received in revised form 22 January 2014 Accepted 9 February 2014
Keywords: Traffic tunnel Pollutant diffusion Wind tower Numerical simulation Wind tunnel test
a b s t r a c t With the increase of the urban traffic volume, a large number of tunnels have been built in many cities; the vehicle emission from the traffic tunnel has a significant impact on the surrounding environment, it has important meaning to study the impact of pollutant diffusion discharged from wind tower on the downwind environment, and to evaluate the air pollution situation and variation tendency around the wind tower. In this study, a reduced scale model of 1:200 is used for three-dimensional numerical simulation and wind tunnel test; results show that the numerical simulation result agrees well with the result of the wind tunnel test. The diffusion of pollutant discharged from the wind tower with the actual size is simulated by the computational fluid dynamics (CFD) program; results show that, the CO concentration ratios of points on building windward side to that of the wind tower outlet (C/C0) are reduced to 1.211%–1.563% on the middle vertical line of building windward side at a distance of 200 m when the building is 39 m (10 m higher than the wind tower), 0.974%–1.215% when the heights of the building and the wind tower are both 29 m, and 0.654%–0.834% when the building is 19 m. When C/C0 of building windward is lower than the maximum allowable value of 2.67%, the required shortest distances between the building and the wind tower are 50, 150 and 200 m for the building heights of 19, 29 and 39 m, respectively. The results can be a reference for the design of wind tower of traffic tunnel and the determinations of the location and height of downwind new building. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid economic development of the city, the traffic problem is more and more prominent. The construction of urban traffic tunnels shows a sharp growth to efficiently alleviate or solve the increasing problem of urban traffic in large cities of China. The urban traffic tunnel plays a positive role in reducing urban road space, cutting running distance, and easing the urban traffic jam, but it also bring the serious air pollution problems. The ventilation system, which is the combination of wind tower and jet fans, is used to discharge the exhaust in the upper air, it shows effective to avoid high pollution concentration staying at the ground near the tunnel entrance, but the vehicle exhaust discharged from the wind tower seriously affects the downwind environment, therefore, it is particularly important to investigate the diffusion and decay rate of the pollutants on the surrounding environment. This paper focuses on the study of the dispersion distance and influence range of pollutant discharged from the wind tower of Qingchun ⇑ Correspodning author. Tel.: +86 159 7200 7990; fax: +86 27 87792101. E-mail addresses: [email protected] (J. Yu), [email protected] (L. Tian). http://dx.doi.org/10.1016/j.tust.2014.02.009 0886-7798/Ó 2014 Elsevier Ltd. All rights reserved.
Road Traffic Tunnel in Hangzhou to investigate the effect of pollutant diffusion on the building environment along the wind direction. There are some studies on pollutant diffusion discharged from the tunnel wind tower or tunnel portal using numerical simulation. The pollutant concentrations in the vicinity of the tunnel portal were conducted by Chow (1996), Oettl et al. (2002), and showed that pollution from the vehicles coming out of the tunnel portal could spread to regions far away from the tunnel portal, and the prevailing wind could disperse the harmful emissions further away from the tunnel. Wang et al. (2009) studied the pollutant diffusion discharged from the tunnel portal and tunnel shaft according to the Gauss model and an experiential model of tunnel portal prediction, results showed that the environmental effect of portal diffusion was more serious than that of shaft discharge. Ren and Xie (2010) focused on the design of the wind tower to study the CO concentration diffusion around the wind tower of an undersea tunnel; the impacts of the tower height, the exhaust air rate and the natural wind velocity on the concentration diffusion of pollutant discharged from the wind tower were analyzed, to meet the air quality standard within the scope of 15 m above the ground, the
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height of 35 m is suggested as the minimum height of this wind tower. Jiang et al. (2012) studied the dispersion of CO and NO2 discharged from the wind tower of Dapu Road underground tunnel under different characteristics of typical wind speed and directions. Two heights of the wind tower were analyzed; the results can be used as a reference for wind tower designing. Few wind tunnel tests or field experiments have been undertaken to study the environmental impact of vehicle exhaust discharged from the ventilation tower or tunnel portal. Jiang et al. (1998) carried out a wind tunnel test study with a geometric reduced scale of 1:500 to analyze the environmental impact of the exhaust discharged from the tunnel shaft and the tunnel portal. This paper focused on the CO diffusion on the ground surface, the maximum concentrations in road surface axis under different vehicle speeds and wind speeds were investigated. Brousse et al. (2005) conducted a situ measurement campaign to study the environmental impact of vehicle exhausts at the south portal of the Landy tunnel located north of Paris. Besides the numerical simulation, Jiang et al. (2012) also investigated the dispersion of vehicular exhaust pollutants discharged from the ventilation tower of Dapu Road underground tunnel by the wind tunnel test with the scale of 1:80. The features of dispersion distance and influence range were tested; the experimental results show a good agreement with the numerical simulation. In summary, few persons focus on the effect of pollutant diffusion discharged from wind tower of traffic tunnel on the downwind building environment. Three research methods for studying the pollutants emission in urban traffic tunnel are included: CFD numerical simulation, wind tunnel simulation test and field measurement method. Compared with the numerical and physical models, the field measurement on the pollutant concentration is more difficult to implement due to the complexity of airflow and the height of wind tower (about 19–49 m) for this study, so only CFD numerical simulation and scaled model tests have been used in this study, the purpose is to predict the dispersion distance and influence range of pollutant discharged from the wind tower of Qingchun Road Traffic Tunnel, and to study the effect of pollutant diffusion on the building environment along the wind direction, The present work involved the followings: (1) Study the exhaust emission discharged from the wind tower of QingChun road tunnel by using the CFD numerical simulation with a reduced scale, analyze the pollutant diffusion when the heights of downwind buildings are different. (2) Carry out the wind tunnel simulation test of QingChun road tunnel with the same scale to validate the simulation results. (3) Investigate the dispersion distance and influence range of CO discharged from the wind tower with the actual size by numerical simulation, four distances and three height differences between the wind tower and downwind building are included; study the C/C0 on four height lines of the building windward side. 2. Wind tower The research object is the wind tower of Qingchun Road Tunnel. The full-length of this tunnel is 3420 m, it is divided into left and right opening, the inner boundary dimension of each opening is: length width = 8.75 (m) 4.55 (m). Wind tower is used for discharging the pollutants to the upper air, the outlet is rectangular and the area is 48m2 (length width = 8 m 6 m), the height of the wind tower is 29 m. As the pollutants of traffic tunnel are chiefly from the vehicle exhaust, the main components are the CO, NO, NO2 and smoke, etc. Tests and operating experience indicate that, when CO level is properly diluted, other dangerous and objectionable exhaust
by-products are also diluted to acceptable levels (ASHRAE Handbook, 2007). The maximum allowable concentration of CO is stipulated in specifications of design of ventilation and lighting of highway tunnel (JTJ026.1-1999) (Ministry of Transport of the PRC, 1999) at the present stage, so this paper select CO as the primary pollutant source. The maximum allowable concentration of CO for the first road tunnel (Holland tunnel in New York) equipped with the ventilator was 400 ppm, it turned out that the design concentration of 400 ppm was too high to meet the health standard for long tunnel, and then some countries reduced the design concentration to 250 ppm. According to the Chinese standard JTJ026.11999, the design maximum allowable concentration of CO is 250 ppm for road tunnel less than 1000 m, 200 ppm for the tunnel longer than 3000 m, and 250–200 ppm for 1000–3000 m by the interpolation methods. The maximum allowable CO concentration for the worst case in traffic jam is 300 ppm, and the exposure time is less than 20 min, so 300 ppm is assumed in this paper as the initial concentration of CO at the wind tower outlet. According to Chinese standard JTJ026.1-1999, CO emissions in the whole tunnel can be calculated by the following equation:
Q CO ¼
1 6
3:6 10
qCO fa fd fh fiv L
n X
ðN m fm Þ
ð1Þ
m¼1
where QCO is CO emissions in tunnel (m3/s); qCO is base emissions of CO (m3/vehicle km), it is 0.01 m3/(vehicle km) in 1995, an annual decrease of 2% is revealed; fa is vehicle condition factor, here fa = 1.0; fd is vehicle density factor, the value is related to traffic condition, if it is in traffic jam, the vehicle speed is supposed 20 km/h , fd = 3.0; fh is height factor above the sea considering CO emissions, the altitude is 20–60 m in Hangzhou city, fh = 0.9; fiv is longitudinal slope–speed factor considering CO emissions, here fiv = 0.8; L is the length of the tunnel (m); n means the number of vehicle types; Nm means the design number of the corresponding vehicles for each type (vehicles /h); fm is the factor of vehicle types considering CO emissions; The above factors valued according to table (3.4.2-1– 3.4.2-4) in standard JTJ026.1-1999. The prediction report on the vehicle flow gave the traffic volume in Qingchun road, which is listed in Table 1; the ratio of large vehicle to small vehicle is 1:6. Saturation point was rapidly being approached in 2030 for Qingchun road tunnel, the peak hourly volume in year 2030 was taken to calculate the CO emissions. According to Eq. (1), base emissions of CO qCO is 0.00493 (=0.01 (1 0.02)35) in 2030, so the amount of CO emissions is 0.0604 m3/s. The quantity of air flow necessary in the tunnel is calculated according to the CO emissions by the following equation:
Q req ¼
Q CO P0 T 106 d P T0
ð2Þ
where Qreq is the required ventilation rate for the whole tunnel to dilute CO, m3/s; P0 is standard atmosphere pressure (kN/m2), 101.325 kN/m2; d is the CO design concentration which is the allowed maximum concentration (ppm), here is 300 ppm; P is the local atmosphere pressure (kN/m2), it is 99.98 kN/m2 in summer in Hangzhou; T0 is standard temperature (K), 273 K; T is design temperature of tunnel in summer (K), here 313 K is taken. The Qreq calculated by Eq. (2) is 234 m3/s, the exhaust velocity at wind tower outlet is 4.9 m/s according to the outlet area, so we take 5 m/s as the exhaust velocity, and 240 m3/s as the design ventilation rate.
Table 1 Prediction of traffic volume in Qingchun road tunnel. Year
2010
2020
2030
The peak hourly volume (pcu/h)
4307
5459
5970
pcu/h- Passenger Car Unit per hour.
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3. Numerical simulation and wind tunnel test of pollutant diffusion discharged from tunnel wind tower CFD numerical simulation can study the pollutant diffusion by solving the three control equations of convection diffusion equation, the air motion equation and the continuity equation. In order to verify the accuracy of the CFD numerical simulation results, the wind tunnel model test is used to verify the simulation results. The geometry scale between the model (both test and the numerical simulation) and the actual wind tower is 1:200. 3.1. Numerical simulation 3.1.1. The CFD model Previous study (Huang et al., 2011) showed that when investigate the dispersion characteristics of vehicular pollutant, the prediction result is the best by using standard k–e model, the followings are realizable k–e mode and RSM mode, and however the prediction results are less good by using standard k–x model, SST k–x model and Spalart–Allmaras model. The standard k–e turbulence model (Launder and Spalding, 1974) and its variants remain the most widely used approaches to modeling wind engineering and atmospheric dispersion problems (Stathopoulos, 1997), so the standard k–e model is used for this study where k is the turbulent kinetic energy, and e is the turbulent dissipation rate. Fig. 1 shows the calculation region of model established by the numerical simulation, the lengths of AB, AC, AE are 6.0 m, 2.5 m and 1.5 m, respectively. The height of the wind tower is 0.145 m; and the distances from the center point of the wind tower to the EG, GH and FH are 2 m, 1.25 m and 4 m, respectively. The building is just opposite the wind tower along wind direction and the distance between them is 1 m. The length of the building is 0.3 m, the heights of building are assumed to be 0.145 m (case1), 0.195 m (case2) and 0.095 m (case3), respectively. In order to control the total grid number and ensure calculation accuracy, grid refinement is performed around the wind tower and building, while other regions use sparse grid. In the numerical calculation, the control equation is discrete by the finite volume method, and the SIMPLE algorithm is used for coupling effect between pressure and velocity in the airflow field calculation. The differential format is QUICK format. The quadrilateral and hexagon grids are used in the computations. The grid unit numbers of the calculation region are 1197573 for case 1, 1203079 for case 2, and 1091917 for case 3. Besides the mesh densities of the three studied cases, the finer meshes are also considered; comparison shows that mesh sizes in this study yield similar results with finer meshes. Therefore, the mesh densities of this study were adapted.
parameter in Hangzhou city; inlet 2 is the plane of wind tower outlet, the air velocity at wind tower outlet is 5 m/s, and the CO concentration at the wind tower outlet is 300 ppm; the outlet is the plane of BDHF; the air flow through outlet is set to pour freely and develop fully. The planes of ABDC, ABFE and CDHG in the Fig. 1 are symmetry planes which do not influence the flow of subjacent air. The bottom of the calculation region, the inner surface of the wind tower, and the wall surface of the building are all supposed as no-slip surface, the roughness constant is 0.5. 3.1.3. Results of CFD simulation Government health agencies differ on acceptable CO levels in the standards of workplace or ambient air quality. The American Council of Governmental and Industrial Hygienists (ACGIH, 1998) recommends a threshold limit of 29 mg/m3 (25 ppm) for 8-h exposure, the U.S. Occupational Safety and Health Administration (OSHA, 2001a) recommends 55 mg/m3 (50 ppm) for daily 8-h exposure; the U.S. Environmental Protection Agency (EPA, 2000) recommends maintaining an average of 40 mg/m3 (35 ppm) for 1-h exposure. In China, the maximum allowed 24-h average concentration is 4 mg/m3 (3 ppm), and the allowed 1-h average concentration is 10 mg/m3 (8 ppm) (Ministry of Environmental Protection of the PRC, 2012). In this study, the C/C0 which is the CO concentration at the point of building surface to that at the wind tower outlet is introduced, the maximum allowed 1-h average CO concentration of 8 ppm is used to analyze the CO concentrations on the building surface whether meet the standard or not, so the CO concentration ratio should be lower than 2.67%. The case 1 is that the heights of wind tower and building are both 0.145 m, Fig. 2a shows the C/C0 on the middle vertical line of building surface on the windward side. the concentration has
(a) The C/C0 on the middle vertical line of the building windward side
3.1.2. Boundary conditions In Fig. 1, the inlet 1 is the plane of ACGE showed, the air velocity is supposed to be 2.2 m/s according to the meteorological
Note: C represents the CO concentration of some point of building surface; C0 represents the CO concentration at the outlet of wind tower. y is the distance away from the middle point of building length
(b) The C/C0 on four horizontal lines of building windward side Fig. 1. The calculation region of model for wind tower and building.
Fig. 2. The CO concentration diffusion of building windward side for case 1 with scale of 1:200.
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dropped significantly compared with that of the wind tower outlet, the higher the point at building surface is, the larger the CO concentration is, the C/C0 are all lower than 0.982%. In addition, the C/C0 on four horizontal lines of the building windward side are analyzed (Fig. 2b). When the pollutant spread to the building windward side, the values of C/C0 vary from 0.714% to 0.982% for different height lines. As the jet uplift effect of pollutant discharged from the wind tower outlet, the CO concentration is clearly higher at the height of 0.195 m than the building height of 0.145 m. When the building surface points are lower than 0.0075 m, the pollutant concentration is very low. Case 2 is that the building is 0.05 m higher than the wind tower; Fig. 3a shows that the C/C0 is sharply decayed when the pollutant diffuses to the building windward side, and the C/C0 of different building heights are from 1.370% to 1.858%, which are a little higher than case 1. Fig. 3b shows the diffusion of CO concentration on four horizontal lines of building windward side. The higher the horizontal line is, the larger the CO concentration shows. On the same height line, CO concentration at midpoint is the highest, which is because that the building is just opposite the wind tower along the wind side. For case 3, the building is 0.05 m lower than the wind tower; Fig. 4 shows the CO attenuation when the pollutant discharges from the wind tower and diffuses to the building windward side. The values of C/C0 range from 0.605% to 0.718% on the middle vertical line of the building windward side (Fig. 4a), and 0.573% to 0.718% on four horizontal lines of the building windward side (Fig. 4b), the values on the building windward side are all very low for different height lines, but relatively higher at the height
Fig. 4. The CO concentration diffusion of building windward side for case 3 with scale of 1:200.
of 0.145 m than others, which is because the uplift effect of the wind tower outlet and jet fans. The simulation results of the three cases show that the height difference between the wind tower and the building influences the pollutant concentration on the building windward side. The C/C0 on the building windward side of the three cases are all lower than 1.858%, which indicate that the CO concentration on building windward side can meet the air quality standard for the three cases.
3.2. The wind tunnel test and the comparison with the CFD simulation
Fig. 3. The CO concentration diffusion of building windward side for case 2 with scale of 1:200.
3.2.1. Introduction of wind tunnel test The large-scale environmental wind tunnel laboratory is located at the University of Shanghai for Science and Technology. It is direct-action wind tunnel with steel structure; the total length is 33 m. It includes seven parts, which are in turn the inlet section, the stable section, the contraction section, the test section, transition section, fan section and outlet diffusing section. The length of test section is 18 m, the width of the cross section is 2.5 m, and the height varies from 1.8 m to 2.1 m. The average air velocity can be adjusted within the range of 0.5 m 20 m/s, the design of uniformity in time and space for air flow parameters references to the flow field quality of internal environmental wind tunnel. Namely, speed non-uniformity 6±1%; fluctuating value of dynamic pressure 61%; pitch direction 6± 0.5°, yaw angle 6±1°, turbulence intensity 61% (Yao, 1991).
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3.2.2. The wind tunnel test model In this study, a model with the same geometry scale is adopted to study the diffusion of the pollutant discharged from the wind tower to the downwind building. The area-source pollution is selected at the wind tower outlet according with the actual situation; Many studies (Higson et al., 1996; Jiang et al., 1998; Lu and Wu, 2005) used the smoke gas, high temperature oil mist, aerosol and organic gases, which are produced by the combustion, here the smoke is produced by the smoke generator as the area-source pollution, and the intensity of smoke is maintained by changing the mass of burning material; the smoke gas is finally produced by mixing the smoke with the airflow of certain speed. The geometry of wind tunnel test model and its boundary conditions consist with the CFD numerical simulation. The three cases which the height of building are assumed to be 0.145 m (case1), 0.195 m (case2) and 0.095 m (case3), are analyzed respectively. 3.2.3. Instruments The main instruments and equipments include: SM-SEMI 2000 Laser, smoke mixing and generating devices, camera equipment, and so on. SM-SEMI 2000 Laser is a continuous wave semiconductor laser, the laser medium is Nd:YVO4, the output is laser sheet with wavelength of 532 nm, output power is 500 mW–1 W, and the beam diameter at aperture is 1.0 mm. The beam divergence is less than 4 mrad. For instantaneous concentration measurements, the AVENIR CCTV LENS is used (the Camera Lens is 16 mm, F1.2, manual-iris, C interface), the shooting speed is 15 frames per second and the maximum electronic shutter speed is 2.56 millisecond. For digital image processing, the maximum absolute error is within 8%. Camera collects scattered light to get the qualitative conclusion. Laser sheet is used to obtain some plane of concentration diffusion at the same time. The smoke gas is transported to the wind tower outlet as the pollutant source; based on digital image processing technology and instantaneous concentration measurement technology of optical particles scattering theory, the measurement of pollutant concentration from the wind tower outlet to the downwind building is carried out. 3.2.4. Results of wind tunnel test and the comparison with CFD numerical simulation Fig. 5 shows the dispersion distance and influence range of the pollutant discharged from the wind tower under the dominant wind direction, the pollutant rises steadily after leaving the wind tower outlet, the concentration is reduced to 10% when the distance is 0.2 m away from the wind tower outlet at the building height of 0.195 m (case 2). The concentration of pollutant has been greatly decayed when it spreads to the surface of the building. The concentration of pollutant is very slight at the building windward
Fig. 5. The dispersion distance and influence range of the pollutant in wind tunnel test.
side, when the wind tower is 0.145 m, only for the case that the building height is 0.195 m, the pollutant can be detected during the height of 0.17–0.195 m, and no pollutant can be detected in other cases and other positions. The C/C0 on the building windward side has relation to the height difference between the wind tower and the building, according to the test data, when the height of building is 0.095 m, 0.145 m and 0.195 m, the C/C0 at the center position of the building windward side reduce to 1.08%,1.22% and 1.75%, respectively. According to the pollutant concentration of the same position, the results of CFD simulation are compared with those of the wind tunnel test. It can be seen from dispersion distance and influence range of the pollutant in Figs. 5 and 6, the numerical simulation results in agreement with the results of the wind tunnel test. According to the concentration attenuation curve in Fig. 7, the absolute errors, namely the differences of C/C0 between the two methods, are all below 5%, the further the point away from the wind tower is, the larger the absolute error is. Since the absolute errors between the numerical simulation and the wind tunnel test are below 5%, the results obtained by numerical simulation are recognized as acceptable and reasonable.
3.2.5. The principal diffusion line of pollutant In order to analyze the pollutant diffusion easily in the actual engineering applications, the principal diffusion line of pollutant in actual size is achieved in Fig. 8, the discrete points are the CO concentrations obtained by wind tunnel test, and the continuous
Fig. 6. The dispersion distance and influence range of the pollutant in numerical simulation.
Fig. 7. The pollutant concentration decay at different distances on principal diffusion line for the case 2.
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4. The pollutant diffusion of the wind tower with actual size 4.1. Model with actual size
Fig. 8. The principal diffusion line of pollutant of the pollutant discharged from wind tower.
The diffusion of pollutant discharged from the wind tower with the actual size is simulated by CFD program. The model is the same as that showed in Fig. 1 except the dimension, the lengths of AB, AC, AE (in Fig. 1) are 1200 m, 500 m and 300 m, respectively. The distances from the center point of the wind tower to EG and GH are 400 m and 250 m, respectively, the actual height of the wind tower is 29 m, the length of the building is 60 m, the distances between the wind tower and the building are assumed to be 50 m, 100 m, 150 m and 200 m, respectively. Three cases mentioned above are investigated, namely the building height of 19 m, 29 m and 39 m. 4.2. Input conditions
line is the fitting curve. It shows that in wind days, with the increase of the vertical distance from the point on the principal diffusion line to the outlet plane of the wind tower, the horizontal distance from the point on the principal diffusion line to the wind tower outlet is increased, namely, the pollutant is keeping away from the wind tower along the wind. The fitting formula of the principal diffusion line is given, as showed in the following equation:
f ðxÞ ¼ 1:314 expð0:1739xÞ R2 ¼ 0:9569
ð3Þ
where f(x) is the horizontal distance from the point on the principal diffusion line to the vertical axes of the center of the wind tower outlet toward the flow direction (m); x is the vertical distance from the point on the principal diffusion line to the plane of the outlet of the wind tower (m). According to the formula (3), the uplift height of the pollutant on principal diffusion line can be calculated (uplift height = x + wind tower height). R2 is the correlation coefficient, the nearer it is close to 1.0, the better the fitted regression line is. Table 2 is the C/C0 of measuring points around the principal diffusion line; we can see that the CO concentration reduces sharply with the increase of the distance away from the wind tower.
Table 2 the C/C0 of measuring points around the principal diffusion line.
Air velocity Uh of some point at the height of h can be expressed as (ASHRAE Handbook, 1997):
U h ¼ U met
dmet hmet
amet a h d
ð4Þ
where dmet is the thickness of atmospheric boundary layer at the place of weather station. Umet is the average velocity of the measurement point at weather station in Hangzhou, according to the meteorological data provided by weather station, the leading wind direction is north–northwest in winter and south–southwest in summer; the average speed of dominant wind direction is 2.9 m/s in summer and 3.3 m/s in winter (Ministry of Construction of the PRC, 2012), so Umet takes the value of 3.3 m/s here; hmet is the height of the measurement point in weather station, it is 10 m; h is the height of the point where the velocity needs to be calculated; amet and a are exponents of atmospheric boundary layer thicknesses at weather station and the place where the velocity needs to be calculated. The weather station locates at the flat area of suburb, so dmet and amet are 270 m and 0.14, respectively; the Qingchun traffic tunnel lies in the central of Hangzhou where more than 50% buildings are higher than 21 m, so d and a take 460 m and 0.33, respectively (Zhu and Yan, 2005), the Uh at the wind tower of 29 m is 2.2 m/s. The air velocity at wind tower outlet is 5 m/s. The CO concentration at the wind tower outlet is 300 ppm. 4.3. Results and discussions
Scatter serial number
x (m)
f(x) (m)
C/C0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
0 4.17 6.67 8.33 9.59 9.60 11.25 14.17 17.50 18.34 19.59 20.42 20.84 21.67 22.50 23.34 24.59 25.00 25.42 26.25 26.67 27.92 28.02
0 0.42 0.83 1.25 1.67 2.08 2.50 3.75 15.00 18.75 23.34 31.25 36.26 57.51 74.59 93.76 103.76 119.60 135.02 139.60 140.85 148.77 149.19
1 0.917 0.826 0.760 0.669 0.612 0.595 0.545 0.496 0.446 0.413 0.372 0.314 0.248 0.198 0.165 0.124 0.083 0.041 0.124 0.116 0.058 0.041
The case 1 is that the heights of the building and the wind tower are both 29 m, Fig. 9a and b shows the diffusion of CO concentration ratio on the building windward side at a distance of 200 m from the wind tower outlet, the values of C/C0 are reduced to 0.974%–1.215% on the middle vertical line, and 0.825%–1.215% on the horizontal lines of 1.5 m,14.5 m and 29 m for the building windward side, while at the height of 39 m which is 10 m higher than the building top, the C/C0 are 1.015–1.745%. From Fig. 9c–e we can see that, when the horizontal line is higher than or equal to the height of building, the CO concentration of the same point is decreased with the increase of the distance between the building and the wind tower. While, when the horizontal line is near the ground (1.5 m), CO concentration is increased with the increase of the distance within 200 m, but the maximum concentration difference on the same horizontal line becomes smaller, because that the wind tower and jet fan have the uplift effect, the CO concentration at the bottom of the building is lower in the nearer distance. When the horizontal line is half of tower height, with the increase of the distance, the CO concentrations of the two edge points are increased, the concentration at the middle point is firstly increased
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Fig. 9 (continued)
Fig. 9. The concentration diffusion when the pollutant spreads to the building windward side for case 1.
and then decreased beyond the distance of 100 m, namely, the CO concentration will be the same if the building is far enough from the tower. That is because the length of building is much larger than the wind tower; the middle of the building will be polluted
firstly when it is near the wind tower, with the increase of the distance, the CO concentration on the same horizontal line finally tend to be the same, and then will become smaller and smaller. As a whole, the CO concentrations on the horizontal line of 1.5 m height and half of tower height are all very low for three cases. The pollutant concentration on the building windward side is decided by the distance away from the wind tower and the height of the building, when the distance is less than 150 m, the C/C0 on the horizontal line of h = 29 are beyond the allowed maximum value of 2.67%, if the building height is reduced to 14.5 m or less, the CO concentration of building windward side can be acceptable to meet the health standard. For case 2 (the building is 39 m which is 10 m higher than the wind tower), The C/C0 on middle vertical line of building windward side at a distance of 200 m from the wind tower is showed in Fig. 10a, the higher the point is, the larger the pollutant concentration is, values of C/C0 are reduced to 1.211%–1.563%. Fig. 10b–e shows C/C0 on four horizontal lines of building wall surface at a distance of 200, 50, 100, 150 m from the wind tower outlet. As the building is just located at the downwind direction, the concentrations on the middle vertical line of the building are higher than the two sides of the same height. The maximum Values C/C0 appear at height of 39 m at the distances of 50, 100, 150 m and at height of 49 m at the distance of 200 m, the result indicate that with the increase of the distance, the uplift height of the pollutant rises, it is about 10 m when the distance is less than 150 m, and is about
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Fig. 10 (continued)
Fig. 10. The concentration diffusion when the pollutant spreads to the building windward side for case 2.
20 m when the distance is further than 150 m. The CO concentration can be negligible when the points are lower than half of tower
height. However, when the distance is less than 200 m, the C/C0 on the top of the building (39 m) are beyond the allowed maximum value of 2.67%, so the safe distance between the wind tower and the building is equal to or further than 200 m. For the case 3, the building height is 19 m; Fig. 11a is the CO concentration diffusion on the middle vertical line of building windward side at a distance of 200 m away from the wind tower. The C/C0 are reduced to 0.654%–0.834% from the bottom to the top. Fig. 11b–e show the diffusion of CO concentration ratio on four horizontal lines of building windward side at distances of 200, 50, 100, 150 m away from the wind tower. Because the building is 10 m lower than the wind tower, the maximal C/C0 at the height of 29 m are 5.7%, 3.7%, 2.5% and 1.4% at the distances of 50, 100, 150 and 200 m, respectively. In Fig. 11b, there is a minimum at y = 0 for h = 14, 19 and 29 m, the reason is that, when the air flows towards the building, some air will flow to the two sides, while the air in the central of the building will climb up, low concentration on lower point will replace the higher concentration on higher point, so the pollutant concentration shows a little decrease at y = 0. The phenomenon is more obvious in low pollutant concentrations, namely for this case when the building height is lowest (19 m) and the distance away from the wind tower is the furthest
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Fig. 11 (continued)
Fig. 11. The concentration diffusion when the pollutant spreads to the building windward side for case 3.
izontal line at h 6 19 m are below the allowed maximum value of 2.67%. From the simulation results of the three cases we can conclude that, for a specific wind tower and a constant distance between wind tower and the building, the higher the downstream building is, the greater the concentration of pollutant on the windward side goes. But for the three cases in this study, the concentration of the pollutant has significantly attenuated when they diffuse to the round of the downstream building because of the long distance between the downstream buildings and the wind tower. When C/C0 of building windward is lower than the maximum allowable value of 2.67%, the required shortest distances between the building and the wind tower are 50, 150 and 200 m for the building heights of 19 m, 29 m and 39 m, respectively.
5. Conclusions (200 m), but the concentration difference between this point and the maximum value of the same line is only about 0.05%. When the distance is further than or equal to 50 m, the C/C0 on the hor-
The purpose of this study is to predict the pollutant diffusion discharged from the tunnel shaft of Qingchun road. Firstly, the pol-
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lutant concentration diffusion from the wind tower to downstream building is calculated for three different cases using CFD numerical simulation and wind tunnel test with the same reduced scale to verify the simulation results. Then, the diffusion of pollutant discharged from the wind tower with the actual size is simulated by CFD program for three height differences and four distances between the wind tower and the building. The major findings and conclusions derived from this study are summarized as follows: (1) The principal diffusion line of pollutant discharged from wind tower and the fitting formula are obtained by the wind tunnel test (Eq. (3)). The comparative analysis on the CO concentration shows that the numerical simulation results agree well with the test results. (2) The height difference between the wind tower and the building influences the pollutant concentration of building windward side. For a specific wind tower and a constant distance between wind tower and the building, the higher the downstream building is, the greater the concentration of pollutant on the windward side goes. When the wind tower is 29 m and the distance is 200 m, C/C0 on the middle vertical line of the building windward are 0.653–0834%, 0.974– 1.215% and 1.211–1.563% for the building heights are 19 m, 29 m and 39 m, respectively. (3) With the increase of distance between the wind tower and the downstream building, the values of C/C0 on building windward decrease, the decrement for the higher point of building windward is much more obvious. When C/C0 of building windward is lower than the maximum allowable value of 2.67%, the required shortest distances between the building and the wind tower are 50, 150 and 200 m for the building heights of 19, 29 and 39 m, respectively. The pollutant concentration is negligible when the point is lower than the half of tower height, where the values of C/C0 are all below 1.5% for all the cases.
Acknowledgments The work of this paper is financially supported by the Doctoral Funds for Higher Education of China (Grant No. 20110142120084) and Project supported by the National Natural Science Foundation of China (Grant No. 51208221).
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