激光片光浓度场测量系统及其在风洞模拟中的应用

为在大型环境风洞中模拟大气污染物迁移扩散状况,开发出污染物浓度激光片光测量系统和污染物发放系统。基于激光片光浓度场测量系统,风洞模拟得到城市环境下点、线和面源污染物的扩散分布图案。基于风洞试验工况的边界条件,计算流体动力学(CFD)仿真得到污染物扩散分布,CFD仿真结果和激光片光浓度测量结果,两者定性一致,达到了相互印证的效果。将研究开发的激光片光浓度场测量系统和定量采样方法相结合,有望实现风洞试验中污染物浓度场的定量测试。     

淮南市大气颗粒物时空变化特征及成因探讨

淮南是以煤电化为支柱产业的典型能源城市,大气结构性污染十分突出.伴随煤电化"三大基地"与"生态淮南"建设,城市大气环境质量调查研究及预测对实施有效环境管理十分必要.在系统统计淮南市大气颗粒物监测数据基础上,综合剖析了城市多年降尘和TSP的年均值时空分布特征,探讨了大气颗粒物污染成因.     

塔形连续式真空干燥设备节能方向的分析

以塔形连续真空干燥设备为研究对象,从传热、设备结构、配套装置和能源利用的角度分析节能减排的方向与途经。     

Ventilation efficiency of void space surrounded by buildings with wind blowing over built-up urban area

This paper outlines methods for evaluating wind-induced ventilation efficiency in void spaces in built-up urban areas. The indices of ventilation efficiency express quantitatively the ability to deliver fresh upper-tier wind to the lower tiers of void spaces and dilute pollutants emitted therein. The evaluation of ventilation efficiency is based on detailed flow analysis. The scales for ventilation efficiency, (SVEs l, 2, 3, and 6) were originally derived by the authors for room air ventilation and are also useful when applied to the exchange of air in void spaces in built-up urban areas. The other scales, namely local purging flow rate (L-PRF), visitation frequency (VF), and residence time (RT), were also re-defined by the authors for room air ventilation and had been applied for urban street canyons. They are based on detailed flow analysis and can be evaluated more easily with computational fluid dynamics (CFD) than flow modeling experiments. CFD simulation example is used to illustrate the concept of the proposed method and it is not meant to indicate the level of CFD set up for urban flow calculations. The effectiveness of the proposed method will be demonstrated more in future study.     

CFD analysis of pollutant dispersion around buildings: Effect of cell geometry

The choice of the best mesh in terms of cost, time and accuracy of computational solutions in the CFD industry is a challenging topic and a subject of some controversy. Generating meshes based on hexahedral elements requires significant time and effort, however, these meshes are claimed to produce high quality solutions. Meshes that employ tetrahedral elements can be constructed much faster in complex geometries, but may increase the levels of numerical diffusion. The objective of this study is to better establish quantitative assessment of the influence of cell geometry in the computational mesh on the CFD results of pollutant dispersion around buildings in order to help modelers to choose the most effective mesh type for their applications. In order to achieve this objective, two widely used mesh styles, i.e., hexahedral-based and tetrahedral-based meshes, are considered in the simulation of this flow problem. Quantitative grid convergence was calculated based on a grid convergence index (GCI). The mesh style was found to have an observable effect on the calculated pollutant concentrations. For instance, the hexahedral-based mesh was observed to have GCI values that were in an order of magnitude below the tetrahedral-based mesh values for all resolutions considered, even in the very fine tetrahedral-based mesh. Furthermore, the GCI value, and hence the truncation error, remains high compared to conventional hexahedral cases. The study recommends taking special care when employing an unstructured tetrahedral-based mesh to ensure that the mesh is fine enough and any numerical errors should be documented for selected variables reported analogous to experimental uncertainty in order to assess the quality of the numerical solution.     

Combustion and Pollutant Emission Characteristics of Lignite Dried by Low Temperature Air

Lignite is a kind of coal that has high moisture content and needs to be dried before being utilized. In this article, a Chinese lignite was dried in air at 120–180°C and the changes in its physical and chemical structures after drying were investigated. The results showed that the pore volume and specific surface area of the lignite decreased after drying. Some of the methylene and methyl groups were oxidized by the oxygen in the drying air, resulting in an increase in oxygen functional groups. The combustion characteristics of the dried coals and parent coal (dry basis) were studied via thermogravimetric analysis. The total volatile yields of the dried coals increased compared to the parent coal. The burnout temperatures of the dried coals were higher than the parent coal, whereas the ignition temperatures stayed almost unchanged. An entrained flow system was set up to study the release of nitrogenous gas products during rapid pyrolysis and combustion. The HCN yields of the dried coals during pyrolysis were higher than that of the parent coal, and a similar trend was found for the NO yield during combustion. The mechanism changes of combustion and pollutant emission characteristics were discussed according to the results of the physical and chemical structure analyses.     

Cotton Stalk Combustion in a Circulating Fluidized Bed

The basic properties of cotton stalk (CS) of length 10–100 mm and its ash were investigated. Studies concerning the combustion characteristics of CS were performed in a circulating fluidized bed (CFB) test facility with a heat input of 0.5 MW. According to previous cold tests, there is very little segregation during the mixing of CS with this size profile and bed material at a fluidization number of N > 7, but the hot experimental results indicate that slight segregation has a small effect on the steady combustion of the dense region. Experiments were carried out with the aim of investigating the effects of operating conditions on the axial temperature and gas concentration profiles along the combustor height, as well as the emission performance and combustion efficiency of the CFB. The experimental results indicate it is difficult to react alumina bed material with alkali metals from CS ash following 26 h of combustion. The overall conclusions appear to indicate that the application of circulating fluidized bed boilers to fire pure CS of length 10–100 mm, is feasible.     

Contributions of wastewater, runoff and sewer deposit erosion to wet weather pollutant loads in combined sewer systems

An observatory of urban pollutants was created in Paris for the purpose of assessing the dynamics of wastewater and wet weather flow (WW and WWF) pollutant loads within combined sewers. This observatory is composed of six urban catchments, covering land areas ranging in size from 42 ha to 2581 ha. For a wide array of parameters including total suspended solids (TSS), chemical and biochemical oxygen demand (COD and BOD₅), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), heavy metals (Cu and Zn) and polycyclic aromatic hydrocarbons (PAHs), this article is intended to evaluate the contributions of wastewater, runoff and in-sewer processes to WWF pollutant loads through the use of an entry–exit mass balance approach. To achieve this objective, a total of 16 rain events were sampled on these sites between May 2003 and February 2006. This study has confirmed that at the considered catchment scale (i.e. from 42 ha to 2581 ha) the production and transfer processes associated with WWF pollutant loads do not vary with basin scale. Entry–exit chemical mass balances over all catchments and for a large number of rain events indicate that wastewater constitutes the main source of organic and nitrogenous pollution, while runoff is the predominant source of Zn. For Cu, PAHs and TSS, the calculation underscores the major role played by in-sewer processes, specifically by sediment erosion, as a source of WWF pollution. A significant loss of dissolved metals was also observed during their transfer within the sewer network, likely as a consequence of the adsorption of dissolved metals on TSS and/or on sewer deposits. Moreover, the nature of eroded particles was examined and compared to the various sewer deposits. This comparison has highlighted that such particles exhibit similar organic and PAH contents to those measured in the organic layer, thus suggesting that the deposit eroded during a wet weather period is organic and of a nature comparable to the organic layer. Despite the extent of initial field investigations, no organic deposit was observed to be present on sewer lines within the catchments, which implies that this organic deposit is probably present in another form or to be found elsewhere in the main trunks.     

Dispersion in a street canyon for a wind direction parallel to the street axis

We investigate pollutant dispersion in a street canyon for an external wind direction parallel to the street axis, a case which has been poorly documented in the literature. The study is performed numerically and analytically by means of a model based on a series of simplifying assumptions. The range of validity of these assumptions is discussed by comparing analytical and numerical results for two different street aspect ratios. Our results show that, for a critical length of the street, ground level concentration can be higher than those observed in a street canyon whose axis is perpendicular to the external wind direction. We show that this critical length depends on the street aspect ratio.