Study on the influence of wall thermal effect coupled with tree trunk height on pollution diffusion in urban street canyons
-
摘要:
在半封闭的街道峡谷内,交通排放和二次污染物容易在通风不良的区域积聚,严重威胁人们的健康。在影响街道峡谷流场和污染物扩散特性的诸多因素中,太阳辐射引起的壁面热浮力以及不同树干高度对空气动力学的影响一直没有得到足够的重视。通过设置5种树干高度(0.18H、0.40H、0.62H、0.84H、1.06H,H为建筑高度)耦合4种壁面加热配置,研究不同树干高度(耦合树木遮阴效应)和墙体加热条件对城市街道峡谷内气流流动和污染物扩散的影响。结果表明,不同树干高度及壁面热效应对城市街道峡谷内气流流动和污染物扩散有显著影响。当树干高度低于建筑物高度时,壁面加热产生的热浮力作用会降低街谷内污染物浓度并增强通风性能;当树干高度超过建筑物高度时,迎风面加热所产生的热浮力会对污染物扩散造成阻碍。采用全壁面加热能够实现更低的污染物积累。研究结果可为城市绿色设施的优化设计,实现对局部微气候环境和空气质量精准调控提供技术指导。
Abstract:In semi-enclosed street canyons, traffic emissions and secondary pollutants tend to accumulate in poorly ventilated areas, posing a serious threat to public health. Among the various factors influencing flow dynamics and pollutant dispersion within street canyons, insufficient attention has been given to the thermal buoyancy effect caused by solar radiation on building walls and the impact of different tree trunk heights on aerodynamics. The effects of different tree trunk heights (coupled tree shading effect) and wall heating conditions on air flow and pollutant diffusion in urban street canyons were studied. Five different tree trunk heights (h=0.18H, 0.40H, 0.62H, 0.84H, 1.06H) combined with four wall heating configurations were considered for analysis purposes. The numerical results demonstrate that variations in tree trunk height and the thermal effects of walls significantly impact air flow and pollutant diffusion within urban street canyons. When the trunk height is lower than that of the building, wall heating generates thermal buoyancy which can reduce pollutant concentration in the street canyon and enhance ventilation performance. However, when the trunk height exceeds that of the building, thermal buoyancy generated by windward wall heating hinders pollutant diffusion. Using full-wall heating can achieve a lower accumulation of pollutants. The research results can provide technical guidance for the optimal design of urban green facilities and the precise control of local microclimate environment and air quality.
-
Key words:
- street canyon /
- numerical simulation /
- wind tunnel experiment /
- tree trunk height /
- wall thermal effect
-
表 1 树干高度耦合壁面热效应工况
Table 1. Trunk height coupled wall thermal effect conditions
树干高度 等温街谷(不加热) 背风面加热 迎风面加热 全加热 0.18H 0.40H 0.62H 0.84H 1.06H 表 2 实际尺度与模型尺度条件下的风速、温差及对应Ri
Table 2. Wind speed, temperature difference and corresponding Ri under actual and model scale conditions
Ri H/m Uref/(m/s) Tw / K Tref/K ΔT=(Tw−Tref)/K 模型类型 0 25 4.5 293 293 0 实际模型 0.25 1.5 293 293 0 缩尺模型 0.5 25 4.5 306 293 13 实际模型 0.25 1.5 427 293 134 缩尺模型 -
[1] 肖乾坤, 李思韬, 成雅田, 等. 行道树冠形对街谷内流场及PM10浓度场的影响[J]. 生态科学,2023,42(5):159-168.XIAO Q K, LI S T, CHENG Y T, et al. Impacts of tree crown profile on flow and PM10 concentration fields inside street canyon[J]. Ecological Science,2023,42(5):159-168. [2] XUE F, LI X F. The impact of roadside trees on traffic released PM10 in urban street canyon: aerodynamic and deposition effects[J]. Sustainable Cities and Society,2017,30:195-204. doi: 10.1016/j.scs.2017.02.001 [3] 黄远东, 王可心, 刘宇辰, 等. 壁面绿化及热效应对浅型街谷内污染物扩散与转化的影响研究[J]. 上海理工大学学报,2022,44(4):315-325.HUANG Y D, WANG K X, LIU Y C, et al. Impacts of green walls and thermal effects on pollutant dispersion and conversion within shallow street canyons[J]. Journal of University of Shanghai for Science and Technology,2022,44(4):315-325. [4] 张永林, 吴睿, 杨孝文, 等. 典型城市道路交通加密监测点大气污染特征及影响因素[J]. 环境工程技术学报,2023,13(3):929-939.ZHANG Y L, WU R, YANG X W, et al. Research on air pollution characteristics and influencing factors of typical urban road traffic densified monitoring stations[J]. Journal of Environmental Engineering Technology,2023,13(3):929-939. [5] ALLEGRINI J, DORER V, CARMELIET J. Buoyant flows in street canyons: validation of CFD simulations with wind tunnel measurements[J]. Building and Environment,2014,72:63-74. doi: 10.1016/j.buildenv.2013.10.021 [6] JIANG G Y, HU T T, YANG H K. Effects of ground heating on ventilation and pollutant transport in three-dimensional urban street canyons with unit aspect ratio[J]. Atmosphere,2019,10(5):286. doi: 10.3390/atmos10050286 [7] RICCI A, GUASCO M, CABONI F, et al. Impact of surrounding environments and vegetation on wind comfort assessment of a new tower with vertical green park[J]. Building and Environment,2022,207:108409. doi: 10.1016/j.buildenv.2021.108409 [8] ZHAO Y L, LI H W, BARDHAN R, et al. The time-evolving impact of tree size on nighttime street canyon microclimate: wind tunnel modeling of aerodynamic effects and heat removal[J]. Urban Climate,2023,49:101528. doi: 10.1016/j.uclim.2023.101528 [9] GROMKE C, RUCK B. Influence of trees on the dispersion of pollutants in an urban street canyon: experimental investigation of the flow and concentration field[J]. Atmospheric Environment,2007,41(16):3287-3302. doi: 10.1016/j.atmosenv.2006.12.043 [10] GROMKE C, BUCCOLIERI R, di SABATINO S, et al. Dispersion study in a street canyon with tree planting by means of wind tunnel and numerical investigations: evaluation of CFD data with experimental data[J]. Atmospheric Environment,2008,42(37):8640-8650. doi: 10.1016/j.atmosenv.2008.08.019 [11] WANG Y F, BAKKER F, de GROOT R, et al. Effects of urban green infrastructure (UGI) on local outdoor microclimate during the growing season[J]. Environmental Monitoring and Assessment,2015,187(12):732. doi: 10.1007/s10661-015-4943-2 [12] HUANG Y D, LI M Z, REN S Q, et al. Impacts of tree-planting pattern and trunk height on the airflow and pollutant dispersion inside a street canyon[J]. Building and Environment,2019,165:106385. doi: 10.1016/j.buildenv.2019.106385 [13] ALLEGRINI J, DORER V, CARMELIET J. Analysis of convective heat transfer at building façades in street canyons and its influence on the predictions of space cooling demand in buildings[J]. Journal of Wind Engineering and Industrial Aerodynamics,2012,104/105/106:464-473. [14] HANG J, LIN M, WONG D C, et al. On the influence of viaduct and ground heating on pollutant dispersion in 2D street canyons and toward single-sided ventilated buildings[J]. Atmospheric Pollution Research,2016,7(5):817-832. doi: 10.1016/j.apr.2016.04.009 [15] CHEN G W, WANG D Y, WANG Q, et al. Scaled outdoor experimental studies of urban thermal environment in street canyon models with various aspect ratios and thermal storage[J]. Science of the Total Environment,2020,726:138147. doi: 10.1016/j.scitotenv.2020.138147 [16] MU D, GAO N P, ZHU T. CFD investigation on the effects of wind and thermal wall-flow on pollutant transmission in a high-rise building[J]. Building and Environment,2018,137:185-197. doi: 10.1016/j.buildenv.2018.03.051 [17] ZHAO Y L, CHEW L W, KUBILAY A, et al. Isothermal and non-isothermal flow in street canyons: a review from theoretical, experimental and numerical perspectives[J]. Building and Environment,2020,184:107163. doi: 10.1016/j.buildenv.2020.107163 [18] CUI P Y, ZHANG Y, ZHANG J H, et al. Application and numerical error analysis of multiscale method for air flow, heat and pollutant transfer through different scale urban areas[J]. Building and Environment,2019,149:349-365. doi: 10.1016/j.buildenv.2018.12.029 [19] CHEN T H, YANG H Y, CHEN G W, et al. Integrated impacts of tree planting and aspect ratios on thermal environment in street canyons by scaled outdoor experiments[J]. Science of the Total Environment,2021,764:142920. doi: 10.1016/j.scitotenv.2020.142920 [20] TOMINAGA Y, MOCHIDA A, YOSHIE R, et al. AIJ guidelines for practical applications of CFD to pedestrian wind environment around buildings[J]. Journal of Wind Engineering and Industrial Aerodynamics,2008,96(10/11):1749-1761. [21] HUANG X T, HUANG Y D, XU N, et al. Thermal effects on the dispersion of rooftop stack emission in the wake of a tall building within suburban areas by wind-tunnel experiments[J]. Journal of Wind Engineering and Industrial Aerodynamics,2020,205:104295. doi: 10.1016/j.jweia.2020.104295 [22] CHANG J C, HANNA S R. Air quality model performance evaluation[J]. Meteorology and Atmospheric Physics,2004,87(1):167-196. [23] MOONEN P, GROMKE C, DORER V. Performance assessment of Large Eddy Simulation (LES) for modeling dispersion in an urban street canyon with tree planting[J]. Atmospheric Environment,2013,75:66-76. doi: 10.1016/j.atmosenv.2013.04.016 [24] ALLEGRINI J, DORER V, CARMELIET J. Wind tunnel measurements of buoyant flows in street canyons[J]. Building and Environment,2013,59:315-326. doi: 10.1016/j.buildenv.2012.08.029 [25] BADY M, KATO S, HUANG H. Towards the application of indoor ventilation efficiency indices to evaluate the air quality of urban areas[J]. Building and Environment,2008,43(12):1991-2004. doi: 10.1016/j.buildenv.2007.11.013 [26] LIM E, ITO K, SANDBERG M. New ventilation index for evaluating imperfect mixing conditions: analysis of Net Escape Velocity based on RANS approach[J]. Building and Environment,2013,61:45-56. doi: 10.1016/j.buildenv.2012.11.022 [27] HANG J, WANG Q, CHEN X Y, et al. City breathability in medium density urban-like geometries evaluated through the pollutant transport rate and the net escape velocity[J]. Building and Environment,2015,94:166-182. □ doi: 10.1016/j.buildenv.2015.08.002