主导风向对不规则建筑群街区污染物扩散的影响

以城市道路某段典型的街道峡谷为研究对象,采用ICEM CFD数值模拟技术,分析不同风向对不规则建筑群街区污染物扩散影响。结果表明:(1)北风时,距地10.0m以下范围是污染物高浓度聚集区。与北风工况相比,西北风时街谷内污染物浓度分布变化较大,沿高度截面上升依次呈连续线状→线状和部分团状→断裂团状。(2)随着建筑高度的增加,主干道中的气流绕流作用减弱。通过控制建筑的连续界面诱导街谷中的气流横向绕流,或在临街上游设置合适的开敞空间,以增加来流通风廊道,可有效改善街谷中污染物的扩散。(3)两种风向下每条街道人员停留区内污染物停留时间排序规律相同;不同风向下每条街道人员停留区内污染物停留时间不同,说明风向对每条街道内污染物的影响存在差异,每段街谷内的污染物扩散分布不是孤立系统,而是相互关联的有机整体。     

三维街道峡谷机动车尾气时空扩散模拟研究

针对信号控制路段,采用非稳态κ-ε湍流模型、组分输运方程进行非定常三维街道峡谷数值模拟,研究了三维街道峡谷内动态交通流下机动车污染物CO的时空扩散过程,并对比了含信号、不含信号的定常模拟结果.结果表明,(1)受信号控制及峡谷内流场影响,峡谷内污染物浓度呈现显著的时空不均匀性;(2)各路段背风面浓度值要大于迎风面,且背风面和迎风面浓度峰值均位于峡谷中部的人行横道区;(3)信号周期内人行横道区污染物浓度始终远高于峡谷内其他区域.在距离背风面建筑1 m的人行横道处污染物浓度可达24.15 mg/m3,超过国家空气质量二级标准141.50％;(4)受信号控制影响,含信号控制街道峡谷污染物浓度高于不含信号控制路段,人行横道背风面污染物浓度是不含信号控制人行横道的3.5倍.     

T型街道内空气流动与污染物扩散特性研究

为研究T型街道峡谷内空气流动与污染物扩散传质的特性,利用数值模拟研究来流风向角(θ)的变化(θ为45°、90°和135°)对T型街道交叉路口内空气流动与机动车尾气污染物扩散传递的影响,并与风洞实验测量数据进行验证。3种湍流模型中,可实现k—ε模型计算的速度相对偏差小于8%,与风洞实验结果一致性最好。结果表明,来流风向角的变化,会造成从街道顶部或侧面进入街道内的气流方向及通量发生改变,从而显著影响T型街道交叉口内及其附近的流动结构和污染物浓度分布。污染物容易在建筑尾流区等流动不畅的区域产生聚集,造成污染浓度偏高。当θ=135°时,T型街道内通风条件最好,街道内行人呼吸高度和建筑临街立面附近污染物浓度水平均相对较低。由于流动结构的改善,T型街道峡谷内的污染水平低于一般街道峡谷。     

计算流体力学模拟街道峡谷特征和风向对细颗粒物污染扩散的影响

街道峡谷结构和风向会对街道峡谷内的污染物浓度和扩散特征带来一定影响。利用计算流体力学(CFD)软件,针对街道峡谷高宽比、建筑物间隔(建筑物间空隙与街道总长度的比值)和风向对街道峡谷内细颗粒物扩散的影响进行数值模拟。模拟结果表明,建筑物间隔为20%,风向为北风,风速为3m/s,街道峡谷高宽比分别为1∶2、1∶1和2∶1时,街道中心线距地面1.5m高度细颗粒物最大质量浓度分别位于-19.3、-88.0、-19.3m(以与街道中心点的距离计,正值表示在街道中心点以东,负值表示在街道中心点以西,下同)位置,为37.5、46.4、28.4μg/m3。街道峡谷高宽比为1∶1,风向为北风,风速为3m/s,建筑物间隔分别为0、20%和40%时,街道中心线距地面1.5m高度的细颗粒物最大质量浓度分别位于148.0、-92.3、-186.7m位置,为88.1、31.6、33.7μg/m3。街道峡谷高宽比为1∶1,建筑物间隔为20%,风速为3m/s,且分别处于西风、北风和西南风时,街道中心线距地面1.5m高度的细颗粒物最大质量浓度分别位于165.3、58.0、1.5m位置,为10.6、11.2、16.0μg/m3。可见,CFD模拟近地面污染物扩散时应考虑街道峡谷结构和风向的影响。     

双鸭山电厂SCR脱硝反应器结构优化的数值仿真分析

选择性催化还原SCR烟气脱硝技术以其脱硝效率高、技术成熟等优点在大型燃煤电厂获得广泛的应用。采用CFD软件对双鸭山电厂200 MW燃煤机组SCR烟气脱硝反应器内烟气气流进行了数值模拟,在原有模型的基础上,通过添加导流板、微调几何模型形状等方法,达到对烟道内气流优化、提高SCR脱硝效率的目的。     

含油污水处理用旋流气浮一体化设备的CFD数值模拟

借助商业计算流体动力学(CFD)软件Fluent,对国内自主研发的含油污水处理用BIPTCFU-1型旋流气浮一体化设备进行内部流场的数值模拟研究,讨论了射流器挡板和缓流板等主要结构参数对设备分离性能的影响,并对污水处理量和回流比等运行参数对设备除油率的影响进行了评估。为下一步通过CFD数值模拟手段进行优化设计,进而成功研发含油污水旋流气浮一体化处理设备奠定了较为坚实的基础。     

周进周出辐流式沉淀池三维数值模拟与PIV实验对比研究

沉淀池是污水处理工艺流程中的重要环节,池内水流流态对沉淀池的处理效果有着较大的影响。利用计算流体力学(CFD)中常用的FLUENT软件,采用重整化RNG k-ε2方程模型对周进周出辐流式沉淀池内的湍流情况进行了数值模拟,同时利用粒子成像测速(PIV)技术对沉淀池中的流速进行了实测,研究结果表明,沉淀池顶部和底部区域流速较大,而中部区域流速相对较小,形成了一个较大的回流区。计算模拟值与实测数据吻合良好,证明了CFD数学模型能准确地模拟沉淀池内的流速分布及流场沿程变化情况,此模型可以应用于沉淀池内部流场模拟。     

袋式除尘器内部流场的数值模拟

袋式除尘器内部气流分布不均会加剧滤袋磨损,降低除尘效率。采用计算流体动力学(CFD)数值模拟方法对除尘器内部流场进行研究,分析了气流的轨迹及其在各气室中的速度分布。发现原设计中气流分布不均匀的现象比较明显,提出了在进气通道内添加气流均布板的改进措施。结果表明:改进后,除尘器箱体内部气流分布较均匀,各滤袋气流分配系数的波动幅度减小,各气室综合流量不均幅值降低。研究结果为袋式除尘器的结构优化设计提供了理论依据。     

滤筒除尘器不同进风方式对内部流场的影响

针对滤筒除尘器不同进风方式对除尘器内部流场的影响进行研究。对滤筒除尘器进行适当简化和假设,建立适合有限元计算的数值模型,对不同进风方式下滤筒除尘器的气流分布进行模拟。通过模拟分析,得出最适合滤筒除尘器的进风方式为侧中进风。     

装有气流分布板的脱硫喷淋塔流场数值模拟

以600MW机组喷淋塔为研究对象,利用Fluent软件,对装有一定开孔率气流分布板的脱硫喷淋塔进行了空塔和喷淋状态下热态流场数值模拟。计算中选用k—ε模型作为计算模型,并结合拉格朗日颗粒轨道模型,用SIMPLE算法计算。结果表明气流分布板对塔内流场、温度场和压力场都有一定的影响;引入喷淋液后,由于喷淋液滴对塔内流场强烈的整流作用,内部速度明显趋于均匀化。     

Dependence of street canyon concentrations on above-roof wind speed - implications for numerical modelling

For microscale numerical modelling of street canyon air pollution, the traffic-related component of the total ambient pollutant concentration is often assumed to be inversely dependent on the wind-speed at rooftops for idealised conditions of neutral stratification, no solar radiation, and no traffic-induced turbulence. Detailed data analyses of two comprehensive datasets from Gottinger Strase (Hannover) and Jagtvej (Copenhagen), including concentration and wind-field measurements in the street and above the rooftops, are presented to test these idealised assumptions, and to improve numerical modelling for a wider range of conditions. The experimental results show systematic deviation from the idealised inverse wind-speed law, when rooftop wind-speeds were less than 10 m/s. It was found that turbulence associated with traffic is a parameter, which is increasingly important for lower wind-speeds.     

Impact of building facades and ground heating on wind flow and pollutant transport in street canyons

This paper investigates the impacts of building facades and ground heating on the wind flow and pollutant transport in street canyons using the computational fluid dynamic (CFD) technique. Street canyons of H/W (H representing the building height and W the street width) varied from 0.1 to 2, which covered the basic flow regimes of skimming flow (H/W=1 or 2), wake interference flow (H/W=0.5), and isolated roughness flow (H/W=0.1), were examined in a series of sensitivity tests. Heating that occurred on different surfaces, including ground surface and building façades, posed considerable effects on the street canyon wind flow and pollutant transport compared with those under isothermal conditions. The CFD results showed that the mechanically induced wind flow and pollutant transport were complicated by the buoyancy under temperature stratification. Individual street canyons of different H/W and surface-heating scenarios exhibited their unique wind flow structure and pollutant transport behaviors. Two counter-rotating vortices were calculated in the street canyons of H/W=1, in which the zone of higher pollutant concentration under isothermal conditions was switched from the leeward side to the windward side. In the street canyon of H/W=2, the recirculating wind pattern was perturbed by surface heating that led to the development of either one primary vortex or three closely coupled vortices. Because of the complicated wind structure, the zones of higher pollutant concentration located either on the leeward or windward ground level were subjected to the surface-heating scenarios. Only two vortices were developed inside the street canyon of H/W=0.5. The large primary vortex, centered inside the street canyon, extended above the roof level of the street canyon. Meanwhile, a small secondary vortex was found at the ground-level windward corner whose size results as a function of surface-heating configurations. Finally, in the street canyon of H/W=0.1, an isolated clockwise-rotating vortex was developed beside the leeward building while the wind in the windward side blew in the prevailing wind direction. As a result, air pollutant emitted at the street centerline was unlikely to be carried into the leeward vortex. Instead, it was dispersed rapidly on the windward side before being removed from the street canyon.     

Strategic guidelines for street canyon geometry to achieve sustainable street air quality

This paper is concerned with the motion of air within the urban street canyon and is directed towards a deeper understanding of pollutant dispersion with respect to various simple canyon geometries and source positions. Taking into account the present days typical urban configurations, three principal flow regimes “isolated roughness flow”, “skimming flow” and “wake interference flow” (Boundary Layer Climates, 2nd edition, Methuen, London) and their corresponding pollutant dispersion characteristics are studied for various canopies aspect ratios, namely relative height (h₂/h₁), canyon height to width ratio (h/w) and canyon length to height ratio (l/h). A field-size canyon has been analyzed through numerical simulations using the standard k-ε turbulence closure model. It is found that the pollutant transport and diffusion is strongly dependent upon the type of flow regime inside the canyon and exchange between canyon and the above roof air. Some rules of thumbs have been established to get urban canyon geometries for efficient dispersion of pollutants.     

Strategic guidelines for street canyon geometry to achieve sustainable street air quality

This paper is concerned with the motion of air within the urban street canyon and is directed towards a deeper understanding of pollutant dispersion with respect to various simple canyon geometries and source positions. Taking into account the present days typical urban configurations, three principal flow regimes “isolated roughness flow”, “skimming flow” and “wake interference flow” (Boundary Layer Climates, 2nd edition, Methuen, London) and their corresponding pollutant dispersion characteristics are studied for various canopies aspect ratios, namely relative height (h₂/h₁), canyon height to width ratio (h/w) and canyon length to height ratio (l/h). A field-size canyon has been analyzed through numerical simulations using the standard k-ε turbulence closure model. It is found that the pollutant transport and diffusion is strongly dependent upon the type of flow regime inside the canyon and exchange between canyon and the above roof air. Some rules of thumbs have been established to get urban canyon geometries for efficient dispersion of pollutants.     

Pollutant dispersion in urban street canopies

The effects of building configurations on pollutant dispersion around street canopies were studied numerically. The dispersion of pollutants emitted from ground sources was simulated by continuously discharging large number of particles into the computation domain. The mean wind velocities at each time-step were firstly computed by solving the time-dependent incompressible Navier–Stokes equations, while the fluctuated velocities were determined using a statistical procedure. The trajectories of the discharged particles were obtained from a Lagrangian particle model. Three categories of numerical simulation were conducted to study the effect of different canopy geometries on the pollutant dispersion. The computed wind field data were consistent with the wind field characteristics described in the previous wind tunnel studies. A counter-clockwise vortex was found resulting in high pollutant concentration at the windward side of the downstream building of the street canopy and low pollutant concentration at the leeward side of the upstream building. The increase in height of the urban roughness buildings would facilitate the pollutant dispersion in urban street canopy under certain building configurations. Two or more vortices stacked vertically in a street canopy were found when height of the upstream and downstream buildings of a street canopy was increased, preventing pollutants from escaping out of the canopy.     

Stemflow chemistry of urban street trees

pH and ion concentrations (Na(+), K(+), Mg(2+), Ca(2+), NO(-)(3)) in the stemflow of the evergreen broad-leaved tree, Ilex rotunda, planted in the median strip of a highway and nitrogen oxides concentration in the air in an urban site were compared with a suburban site in Fukuoka city, Japan. The annual average of the nitrogen oxides concentration in the air was higher and NO(-)(3) concentration in the stemflow at the urban site was higher or similar compared with the suburban site. However, the annual average of pH in the stemflow at the urban site was higher than at the suburban. The annual average cation concentrations in the stemflow at the urban site were higher than at the suburban except Na(+). In particular, K(+) and Ca(2+) were higher throughout the measurement period. Therefore, higher pH in the urban stemflow was probably due to neutralization by higher concentrations of K(+), Mg(2+) and Ca(2+).     

Flow and dispersion in street intersections

Street intersections play an important role in determining pollutant concentrations in the urban canopy – vehicle emissions often increase in the vicinity of road intersections, and the complex flow patterns that occur within the intersection determine the pollutant fluxes into adjoining streets and into the atmosphere. Operational models for urban air quality therefore need to take account of the particular characteristics of street intersections. We have performed an experimental and numerical investigation of flow and dispersion mechanisms within an urban intersection, and on the basis of our observations and results, we have developed a new operational model for pollutant exchanges in the intersection, which takes account of the non-uniformity of the pollutant fluxes entering and leaving the intersection. The intersection is created by two streets of square cross-section, crossing orthogonally; concentrations were measured by releasing a neutrally buoyant tracer gas from a line source located in one of the streets. As a general result, the numerical simulations agree well with the measurements made in the wind tunnel experiments, except for the case of ground-level concentrations, where the computed concentrations far from the axis of the line source are significantly lower than the measured values. In the first part of the study we investigate the influence of an intersection on the velocity and concentration fields in the adjoining streets; we show that the immediate influence of the intersection extends within the adjoining streets, to a distance of the order of the characteristic size of the streets. A large recirculating vortex is formed at the entrance to the cross-wind streets, and this determines the exchange of pollutants between the streets and the intersection. For some wind directions the average velocity in the street segment between intersections is the same as that which occurs in an infinitely long street with the same wind, but for other angles the average velocity in the finite-length street is significantly lower. The average concentration along a finite-length street is significantly different from that observed in an infinitely long street. In the second part of the study we investigate how the pollutant fluxes in the incoming streets are redistributed amongst the outgoing streets. An analysis of the mean streamlines shows that the flows remain relatively planar, with little variation over the vertical, and we have exploited this result to develop a simple operational model for the redistribution of pollutant fluxes within the intersection. This model has been further adapted to take account of the influence of fluctuations in wind direction over typical averaging periods. The resulting model is used in the street network model SIRANE.     

Nitrogen dioxide distribution in street canyons

Weekly average NO₂ concentrations have been measured at over 40 locations within two canyonlike streets in central London, using passive diffusion tube samplers. A steady decline in concentrations with height is found, levels being close to local background at about 20 m. There is a rapid decline over the first 10–15 m horizontally from the centre of the road and concentrations are close to local background at a distance of 30 m. Concentrations are 10–15% higher close to traffic lights than they are 60 m upstream. Possible reasons for this are discussed. The results provide valuable information to assist in the selection of sites for chemiluminescence monitors, which are required to check compliance with the new European Council air quality standard for NO₂.     

Nomograms for calculating pollution within street canyons

The Environment Act 1995 has introduced the notion of local air quality management which requires that air quality in towns be reviewed and assessed. There is a need to identify those streets that are worst affected by vehicular pollutants. Such worst cases are likely to be narrow congested streets with tall buildings on each side. A nomogram presented here allows rapid screening of pollution in congested street canyons. The strong dependence on wind direction is reduced to the two extremes, namely wind along and wind across the canyon. Then canyon concentrations are estimated according to street geometry and traffic flow. The nomogram is designed for use by local authorities, is quick and easy to use, and paper or computer versions are available. It is suggested that detailed monitoring or modelling may only be required when simple screening methods predict high air pollution.