Study on the influence of wall thermal effect coupled with tree trunk height on pollution diffusion in urban street canyons
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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.
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Key words:
- street canyon /
- numerical simulation /
- wind tunnel experiment /
- tree trunk height /
- wall thermal effect
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图 1 不同绿化高度配置城市街道峡谷物理模型
Figure 1. Physical model of city street canyon configured with different tree trunk heights
图 3 等温条件下不同树干高度街道峡谷内的流场及污染物无量纲浓度分布
Figure 3. Flow fields and dimensionless concentration distributions in street canyons with varying trunk heights under isothermal conditions
图 4 背风面加热条件下不同树干高度街道峡谷内的流场及污染物无量纲浓度分布
Figure 4. Flow fields and dimensionless concentration distributions in street canyons with varying trunk heights under leeward wall heating
图 5 迎风面加热下不同树干高度街道峡谷内的流场及污染物无量纲浓度分布
Figure 5. Flow fields and dimensionless concentration distributions in street canyons with varying trunk heights under windward wall heating
图 6 全壁面加热条件下不同树干高度街道峡谷内的流场及污染物无量纲浓度分布
Figure 6. Flow fields and dimensionless concentration distributions in street canyons with varying trunk heights under full wall heating
图 7 不同树木高度下背风面、迎风面以及人行呼吸高度面污染物平均无量纲浓度
Figure 7. Average dimensionless concentrations on leeward, windward and pedestrian breathing height under varying tree trunk heights
图 8 背风面、迎风面以及人行呼吸高度面的NEV*
Figure 8. NEV* under the leeward, windward and pedestrian breathing height planes
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 
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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
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