Large-eddy simulation of turbulent transports in urban street canyons in different thermal stabilities

Five sets of large-eddy simulations (LES) were performed to examine the characteristics of flows and pollutant dispersion in two-dimensional (2D) urban street canyons of unity building-height-to-street-width ratio in neutral, unstable, and stable thermal stratifications. The characteristic flows fall into the skimming flow regime for all the cases tested. The mean wind speed is increased and decreased, respectively, in unstable and stable conditions. Turbulence is enhanced in unstable conditions. Whereas, in stable conditions, the low-level temperature inversion weakens the recirculating flows forming another layer of stagnant air in the vicinity of the ground level. Unexpectedly, an increase in turbulence is found in the street canyon core in the slightly stable condition (Richardson number Rb=0.18). The turbulence promotion could be caused by the unique geometry of 2D street canyon in which the stable stratification slows down the primary recirculation. The rather stagnant flows in turn sharpen the roof-level vertical velocity gradient and deter the entrainment penetrating down to the ground level, leading to a substantial pollutant accumulation. While the pollutant tends to be well mixed in the street canyons in neutral and unstable conditions, a mildly improved pollutant removal in unstable conditions is observed because of the enhanced roof-level buoyancy-driven turbulence.     

Numerical simulation of dispersion in urban street canyons with avenue-like tree plantings: Comparison between RANS and LES

Previous CFD studies on pollution dispersion problems have largely centred on employing Reynolds-averaged Navier–Stokes (RANS) turbulence closure schemes, which have often been reported to overpredict pollutant concentration levels in comparison to wind tunnel measurement data. In addition, the majority of experimental and numerical investigations have failed to account for the aerodynamic effects of trees, which can occupy a significant proportion of typical urban street canyons. In the present work, the prediction accuracy of pollutant dispersion within urban street canyons of width to height ratio, W/H = 1 lined with avenue-like tree plantings are examined using two steady-state RANS models (the standard k-ε and RSM), and Large Eddy Simulation (LES) to compare their performance against wind tunnel experiments available on the online database CODASC [1]. Two cases of tree crown porosities are investigated, one for a loosely (P_vol = 97.5%) and another for a densely (P_vol = 96%) packed tree crown, corresponding to pressure loss coefficients of λ = 80 m⁻¹ and λ = 200 m⁻¹, respectively. Results of the tree-lined cases are then compared to a tree-free street canyon in order to demonstrate the impact of trees on the flow field and pollutant dispersion, and it is observed that the presence of trees reduces the in-canyon circulation and air exchange, and increases the overall concentration levels. Between the two numerical methods employed, LES performs better than RANS, because it captures the unsteady and intermittent fluctuations of the flow field, and hence, successfully resolves the transient mixing process within the canyons.     

Phosphorene-Based Heterostructured Photocatalysts

Semiconductor photocatalysis is a potential pathway to solve the problems of global energy shortage and environmental pollution. Black phosphorus (BP) has been widely used in the field of photocatalysis owing to its features of high hole mobility, adjustable bandgap, and wide optical absorption range. Nevertheless, pristine BP still exhibits unsatisfactory photocatalytic activity due to the low separation efficiency of photoinduced charge carriers. In recent years, the construction of heterostructured photocatalysts based on BP has become a research hotspot in photocatalysis with the remarkable improvement of photoexcited charge-separation efficiency. Herein, progress on the design, synthesis, properties, and applications of BP and its corresponding heterostructured photocatalysts is summarized. Furthermore, the photocatalytic applications of BP-based heterostructured photocatalysts in water splitting, pollutant degradation, carbon dioxide reduction, nitrogen fixation, bacterial disinfection, and organic synthesis are reviewed. Opportunities and challenges for the exploration of advanced BP-based heterostructured photocatalysts are presented. This review will promote the development and applications of BP-based heterostructured photocatalysts in energy conversion and environmental remediation.     

Numerical prediction of gas concentrations and fluctuations above a triangular hill within a turbulent boundary layer

The objective of the present work is to propose a numerical and statistical approach, using computational fluid dynamics, for the study of the atmospheric pollutant dispersion. Modifications in the standard k-ε turbulence model and additional equations for the calculation of the variance of concentration are introduced to enhance the prediction of the flow field and scalar quantities. The flow field, the mean concentration and the variance of a flow over a two-dimensional triangular hill, with a finite-size point pollutant source, are calculated by a finite volume code and compared with published experimental results. A modified low Reynolds k-ε turbulence model was employed in this work, using the constant of the k-ε model C_μ=0.03 to take into account the inactive atmospheric turbulence. The numerical results for the velocity profiles and the position of the reattachment point are in good agreement with the experimental results. The results for the mean and the variance of the concentration are also in good agreement with experimental results from the literature.     

宁波市区汽车行驶工况和污染物排放调查研究

利用GPS技术调查了宁波市不同道路汽车行驶工况,记录了实际道路汽车行驶瞬时速度的大量数据。根据数据进行了不同道路汽车行驶工况的解析,分别建立起适合宁波市汽车排放污染物测量的一般道路汽车行驶工况、快速道路汽车行驶工况、城区综合道路汽车行驶工况。分析了城市机动车排放污染的状况及影响因素,讨论了机动车车型和汽车城市道路运行工况对机动车排放污染物的影响。根据行驶工况和流量调查计算了宁波市部分道路机动车污染物排放量。     

Recent advances in polydopamine and its derivatives assisted electrocatalysis and photocatalysis

In recent years, polydopamine (PDA) has attracted extensive attention of researchers due to its unique chemical structure and functional groups. However, a comprehensive review of PDA-based materials in electrocatalysis and photocatalysis is still lacking. The aim of this review is to summarize recent progress and development of PDA-assisted electrocatalytic and photocatalytic reactions. Owing to its unique advantages in surface modification, it has been a good platform for the construction of electrocatalytic and photocatalytic materials. With the development of biological sources, PDA has been increasingly applied in the field of materials science. However, it is still lacking of a comprehensive review which focuses on PDA surface modification strategies and its derivative function for electrocatalysis and photocatalysis applications. Here in this article the preparation and polymerization mechanism of PDA and its derivatives has been firstly reviewed. Then their physical and chemical properties have been introduced, including chemical reactivity, optical property, conductivity, and cohesiveness. Thirdly, the electrocatalytic and photocatalytic activities of PDA and its derivatives have been demonstrated. Finally, the advances of their electrocatalytic and photocatalytic applications have been summarized including hydrogen production, pollutant degradation, and CO₂ reduction. We believe this synthetic study can provide critical information for mussel-inspired chemistry, which will be of great interest for researchers from chemistry, materials, biology, medicine, and interdisciplinary. This paper can provide readers with a general framework of PDA and its derivatives in the field of photocatalysis and electrocatalysis including hydrogen production, pollutant degradation, and CO₂ reduction.     

小缸径多缸柴油机贯彻国家排放标准的探索

随着国家新的排放标准的实施，控制柴油机的排放越来越受到人们的重视．本文详细介绍了小缸径多缸柴油机在控制排放污染物过程中所做的试验分析及技术探索，提出了值得商讨的排放标准问题。     

A review on alloyed quantum dots and their applications as photocatalysts

Semiconductor driven artificial photocatalysis is the most sustainable technology towards addressing the growing energy and environmental pollution issues. In this context, alloyed quantum dots (QDs) are an emerging class of promising nanomaterials gathering tremendous attention in this area due to several beneficial features. Compared to other bulk semiconductors, alloyed QDs are cost-effective, stable, less-toxic with superior optoelectronic features, which significantly enhances their solar energy conversion efficiency. Herein, the present review summarizes the fundamentals of alloyed QDs, various synthesis techniques, and discusses optical as well as structural properties from data interpretation point of view taking suitably reported literature. Moreover, we have provided a comprehensive summary of recent state of art metal chalcogenides based alloyed QD systems towards H₂ evolution, CO₂ reduction, and pollutant degradation. Finally, the review discusses the associated challenges and future prospects of alloyed QDs with a special focus on preparation, property engineering, theoretical aspect, stability and other field application. Additionally, the overarching aim is to provide researchers an in-depth understanding in the field of alloyed QDs relating to synthesis, characterisation, and promotes their photocatalytic applications, and can foster as a manual to future researchers.     

水煤浆热解气与氨复合还原超低NOx排放控制技术

为达到火电厂大气污染物排放标准,应控制燃煤锅炉烟气中污染物排放,研发污染物控制技术和超低排放技术。针对烟气中NOx的治理,提出了水煤浆热解气与氨复合还原超低NOx排放技术,并针对水煤浆气化炉控制,提出一个有效气流量和气化炉温度协同控制策略。以煤浆流量和空气流量为输入量、有效气流量和气化炉温度为输出量,建立水煤浆气化炉双输入、双输出耦合模型。在双回路完全解耦的基础上,设计单回路控制系统并进行适当优化,实现热解气的稳定制备和气化炉的安全运行。     

Research progress in the application of metal organic frameworks and its complex in microbial fuel cells: Present status,opportunities, future directions and prospects

Metal organic frameworks (MOFs) could greatly improve the power generation and degradation performance of microbial fuel cells (MFCs). MOFs and their compound derivatives played key role in cathode, anode and proton exchange membrane of MFCs, which greatly promoted the power generation of MFC and the degradation efficiency of various pollutants. However, MOFs were still possessed some defects, such as complex synthesis process, difficult regulation, instability, etc. Moreover, the application of MFC was limited in low power density, system internal resistance, microbial consumption, etc. Which further limited the degradation of pollutants by MFC. The existing problems and various improvement schemes of MOFs for MFCs were further summarized, which would provide references for promoting the application of MOFs materials in MFC system. It was expected to enhance the application of MOFs materials and promote the performance of MFC.