Numerical study of the effects of Planetary Boundary Layer structure on the pollutant dispersion within built-up areas

The effects of different Planetary Boundary Layer (PBL) structures on pollutant dispersion processes within two idealized street canyon configurations and a realistic urban area were numerically examined by a Computational Fluid Dynamics (CFD) model. The boundary conditions of different PBL structures/conditions were provided by simulations of the Weather Researching and Forecasting model. The simulated results of the idealized 2D and 3D street canyon experiments showed that the increment of PBL instability favored the downward transport of momentum from the upper flow above the roof to the pedestrian level within the street canyon. As a result, the flow and turbulent fields within the street canyon under the more unstable PBL condition are stronger. Therefore, more pollutants within the street canyon would be removed by the stronger advection and turbulent diffusion processes under the unstable PBL condition. On the contrary, more pollutants would be concentrated in the street canyon under the stable PBL condition. In addition, the simulations of the realistic building cluster experiments showed that the density of buildings was a crucial factor determining the dynamic effects of the PBL structure on the flow patterns. The momentum field within a denser building configuration was mostly transported from the upper flow, and was more sensitive to the PBL structures than that of the sparser building configuration. Finally, it was recommended to use the Mellor–Yamada–Nakanishi–Niino (MYNN) PBL scheme, which can explicitly output the needed turbulent variables, to provide the boundary conditions to the CFD simulation.     

基于结构参数响应的内循环流化床流体特性优化数值模拟

流化床是一种结构复杂而能量转化高效的反应器型式,为了实现系统化的内循环流化床优化设计,利用欧拉-欧拉双流体模型构建了不同结构的CFD流化床模型,在分别改变高径比、导流筒与反应器的直径比和底隙高度3个结构因素的情况下,考察流化床内全流场、局部流场、液相运动速度及气含率等气液两相流的响应特性,分析结构因素及操作因素对流体运动的内在影响,阐明工程优化设计的方向.数值模拟结果表明:高径比主要影响气液流动型态,低高径比时容易出现漩涡和返混,工程中应采用导流内构件并重视气体分布装置的合理设计;导流筒与反应器的直径比主要影响液体循环速度,存在一个合理的区间,考虑流动速度与气含率,流化床直径比可取0.6～0.8,最佳取值为0.7;底隙高度影响流化床底泥区的流体运动,应约等于流化床下降区的缝隙长度.CFD模拟可作为污染控制技术工业放大和优化设计的辅助工具.     

Numerical study of the effects of local atmospheric circulations on a pollution event over Beijing-Tianjin-Hebei, China

Currently, the Chinese central government is considering plans to build a trilateral economic sphere in the Bohai Bay area, including Beijing, Tianjin and Hebei(BTH), where haze pollution frequently occurs. To achieve sustainable development, it is necessary to understand the physical mechanism of the haze pollution there. Therefore, the pollutant transport mechanisms of a haze event over the BTH region from 23 to 24 September 2011 were studied using the Weather Research and Forecasting model and the FLEXible-PARTicle dispersion model to understand the effects of the local atmospheric circulations and atmospheric boundary layer structure. Results suggested that the penetration by sea-breeze could strengthen the vertical dispersion by lifting up the planetary boundary layer height(PBLH) and carry the local pollutants to the downstream areas; in the early night, two elevated pollution layers(EPLs) may be generated over the mountain areas: the pollutants in the upper EPL at the altitude of 2–2.5 km were favored to disperse by long-range transport, while the lower EPL at the altitude of 1 km may serve as a reservoir, and the pollutants there could be transported downward and contribute to the surface air pollution.The intensity of the sea–land and mountain–valley breeze circulations played an important role in the vertical transport and distribution of pollutants. It was also found that the diurnal evolution of the PBLH is important for the vertical dispersion of the pollutants,which is strongly affected by the local atmospheric circulations and the distribution of urban areas.