紊动特性对污染物降解的影响研究

采用潜水泵作为紊动诱发装置,利用PIV测量了流场,研究了不同流量下紊动对污染物降解的影响规律。结果表明,紊动对污染物的降解有明显的影响,流量越大,紊动强度越大,污染物的降解率越高。对紊动强度与有无紊动水箱COD、NH4^＋-N、TN和TP降解增加量的变化曲线进行指数函数拟合,拟合结果良好,可以认为紊动对污染物降解的影响可以描述为“一级反应”理论。     

连续水流和间歇水流对微囊藻生长的影响

为了解不同的水体流动方式对铜绿微囊藻生长的影响。实验在控制温度和光照条件下,室内采用野外原水培养基模拟连续水流和间歇水流微囊藻的生长。实验结果表明,间歇水流下微囊藻在各流速下的生长差异较小,连续扰动下微囊藻在各流速下的生长差异较大,在流速为35 cm/s时达到最大值;间歇性扰动条件下微囊藻密度较连续性水流条件下高。这可能是在间歇扰动下水流主要影响藻细胞对营养盐的吸收,连续扰动的低流速下主要是通过对营养盐的吸收对微囊藻产生影响,在高流速下是水流对藻细胞的机械破坏为主;同时培养基中藻细胞形成群体产生微环境有利于微囊藻的生长。水体流动的方式不同,水流对微囊藻细胞生长的作用将发生改变,这为扰动控制蓝藻生长提供科学依据。     

水流紊动特性对活性污泥沉降性能的影响

为优化活性污泥处理系统的设计和运行,在SBR反应器中通过改变搅拌浆的转速,调整水流的紊动条件,借助粒子图像测速技术分析了反应器内水流的紊动特性,研究了反应器内不同区域紊动强度对活性污泥沉降性能的影响.结果表明,搅拌浆叶周围15mm的范围内,紊动强度远高于距桨叶更远的区域,距离浆板5mm的1-1断面和距离浆板60mm的3-3断面搅拌轴心处的紊动强度值分别反映了反应器搅拌主体区和其他区的紊动特征.随搅拌浆转速的增加,反应器内各区域的紊动强度逐渐增加,活性污泥的沉降性能表现出先改善后恶化的规律.当反应器搅拌主体区紊动强度1.51%,其他区紊动强度0.31%时,本试验条件下形成的活性污泥沉降指数(SVI)达到最小值67mL/g,界面沉降速率(ZSV)达到最大值0.20mm/s,活性污泥的沉降性能最佳.搅拌主体区范围较小,紊动强度较高,增强该区的紊动强度会加剧污泥的侵蚀破坏;其他区范围较大,紊动强度较低,增强该区的紊动强度对改善污泥的沉降效果作用明显.工程中可通过降低搅拌主体区的紊动强度,选择其他区适当的紊动强度,改善活性污泥的沉降性能.     

明渠中有机质微粒沉降过程的随机游走模拟

基于明渠流动的主要特点,考虑紊动强度的垂向非均匀性,及颗粒运动的游荡效应（loitering effect）影响,对微粒运动基本方程进行改进,建立了明渠非均匀紊流中考虑游荡效应的随机游走模型,对明渠中微粒的沉降损失进行了模拟分析.模拟结果显示,不考虑游荡效应影响时,明渠中微粒的沉降损失率可用微粒沉速与水深之比计算;而在游荡效应影响下,微粒沉降损失率会发生约18%~27%的下降,下降百分比与均匀紊流中微粒沉速在游荡效应影响下的折减百分比渐近值（19~25%）基本一致.本文结论基于不考虑微粒重悬浮、游荡效应为紊流对微粒沉降的主导影响机理等前提,主要适用于对明渠中细小有机质微粒沉降至床底过程更准确的计算分析.     

The Effect of Large-Scale Turbulent Structures on a Simple 2-D Canyon-Type Flow

This paper is concerned with a preliminary experimental investigation of the interaction between large turbulent structures, generated in the wake of a circular cylinder, and the rough-wall turbulent boundary layer separated flow immediately downstream of a simple street canyon type geometry represented by backward-facing step. The motivation for the work was to provide some initial data for the validation of a 3-D k- turbulence model used for the prediction of flows and pollutant dispersion within the urban canopy. The aim has been to assess the extent of the perturbation of a simulated street canyon caused by regular large-scale eddies generated upstream. The research has involved the use of thermal anemometry to determine mean velocity and turbulence characteristics both upstream and downstream of the step, together with the mean reacttachment length for the recirculating flow. The results indicate that the presence of the cylinder in the flow reduces the reattachment length. In addition, the periodic structures generated in the cylinder wake are rapidly mixed with the turbulence in the step shear layer such that no periodicity is detected at the reattachment zone.     

Physical Modelling of Urban Roughness using Arrays of Regular Roughness Elements

The aerodynamic behaviour of large urbanagglomerations must be represented in increasingly greaterdetail, as large-scale numerical weather prediction and airpollution dispersion models are refined. The present studyprovides detailed measurements of the flow field in regulararrays of obstacles to obtain representative data on meanflow and turbulence statistics in urban-type areas. Obstacle arrays consisting of simple cubes and flat plateroughness commonly used in boundary layer simulations wereplaced in a simulated atmospheric boundary layer flow in ahydraulic flume. The scale factor was about 1:200 based onthe obstacle height (50 mm). The results show noappreciable `constant stress' region in the internalboundary layer above the buildings, since in any finite-length test array the boundary layer is always developing.However, if the RMS turbulence components are scaled by thelocal values of the shear stress, then there seems to be auniversal scaling, with _u/u_* = 2.1,_v/u_* = 1.65 and _w/u_* = 1.2. This greatly simplifies the parameterization of the first orderturbulence statistics in obstacle arrays. It was alsoobserved during the experiments that, compared to results inthe cube arrays, the turbulence kinetic energy and theReynolds stresses were almost doubled in the flat plateroughness arrays.     

双气体射流作用下燃煤可吸入颗粒团聚研究

大气中的可吸入颗粒物对有机体危害很大,已经成为世界范围内重要的大气污染源。在可吸入颗粒团聚室内引入双射流,通过双气体射流在团聚室内形成大小不同尺度涡旋,促进流场可吸入颗粒团聚。实验结果表明:10μm以下的可吸入颗粒在单射流作用下减少了21.9%,在双射流作用下减少了58.0%。异侧双射流的团聚清除率高于同侧双射流的清除效率,异侧双射流管间距增加更有利于可吸入颗粒的团聚。双射流管出口Re增加,颗粒质量清除率增大;但不同粒径颗粒的单级团聚效率则与粒径密切相关。颗粒的质量清除率随着主气流与射流流量比增大呈现逐渐减小的趋势。     

潮汐河口环流、湍流、混合与层化的物理学

简要回顾了潮汐河口环流、湍流、混合与层化的基本物理概念、内涵、研究方法、研究成果,指出了主要的研究进展,最后,展望了今后的研究方向。本文不考虑悬沙和风浪的影响。经典的河口环流也因潮汐应变的出现而受到挑战,河口环流由重力环流和潮汐应变环流构成。“涡黏度-剪切协方差(ESCO)”概念的提出,又区分出重力ESCO环流与潮汐ESCO环流。横向环流,尤其具有曲率的弯道中的横向环流,也得到进一步的理解。涡度方法的应用,揭示横向环流不仅由各种不同物理机制造成,而且对纵向河口重力环流有重要的影响。分层流中剪切湍流的理论加深了人们对潮汐河口湍流、混合的物理学的认识,势能差异方程更是使得定量理解潮汐河口混合与层化的三维时间、空间变化成为可能。     

Assessing the influence of real releases on explosions: Selected results from large-scale experiments

The research activities in the project Assessing the Influence of Real Releases on Explosions (AIRRE) included a unique series of large-scale explosion experiments with high-momentum jet releases directed into congested geometries with subsequent ignition. The primary objective for the AIRRE project was to gain improved understanding of the effect that realistic releases and turbulent flow conditions have on the consequences of accidental gas explosions in the petroleum industry. A secondary objective was to develop a methodology that can facilitate safe and optimal design of process facilities. This paper presents selected results from experiments involving ignition of a highly turbulent gas cloud, generated by a large-scale, pressurised release of natural gas. The paper gives an overview of the effect on maximum explosion overpressures of varying the ignition position relative to the release point of the jet and a congested region placed inside the flammable cloud, with either a high or a medium level of congestion. For two of the tests, involving a jet release and the medium congestion rig, the maximum overpressures significantly exceeded those obtained in a quiescent reference test. The paper presents detailed results for selected tests and discusses the effect of the initial flow field generated by realistic releases – including turbulence, net flow and concentration gradients – on relevant explosion phenomena.     

Upstream Turbulence Effect on Pollution Dispersion

Experiments have been carried out to investigate turbulence at and above roof-level in an urban environment, and to predict the behaviour of street pollution from experiments using dye dispersion, for different roughness conditions and bed geometries. The flow in the boundary layer above an idealised urban environment has been simulated in a laboratory water flume. Comparisons have been made for the same model street canyon with and without the presence of upstream roughness. In the tests reported here, model street canyons were aligned perpendicular to the flow direction, and velocity measurements made within and above the model street canyons using a laser Doppler velocimeter (LDV). Flow visualisation techniques have also been used to confirm the gross flow features from streak images. Turbulence generated from the upstream roughness has a significant effect on the turbulence production and dispersion behaviour of the dye simulating pollution in street canyons.