计算流体力学在膜技术及膜生物反应器研究中的应用

介绍了计算流体力学(CFD)的基本原理、内容和特点,综述了CFD在膜技术领域中的研究和发展,特别讨论了其在膜生物反应器(MBR)技术中的研究和应用,并对其应用前景进行了展望.     

膜生物反应器在污水处理与回用中的能耗分析

介绍了膜生物反应器(MBR)低能耗化研究进展,以吨水能耗(kWh/m3)和COD去除能耗(kWh/kgCOD)两种指标分析比较了MBR处理较低浓度城镇和生活污水、较高浓度工业废水及垃圾渗滤液等废水的运行能耗与成本,分析了MBR运行能耗构成和能耗较高的原因,并对MBR节能措施和对策进行了讨论.     

EPS及其在膜生物反应器中的作用和影响

胞外聚合物(EPS)普遍存在于活性污泥絮体内部及表面,具有重要的生理功能,同时也被认为是造成MBR膜污染和降低混合液可过滤性的重要因素之一.EPS的组成和含量与提取过程和分析方法有很大的关系,由于加热法的简洁性可操作性,现在的应用比较多.MBR的营养状况和反应器中环境要素对EPS有很大的影响.据已研究的结果发现,微生物产生EPS的多少取决于微生物生存环境和生长状况,即当其满足和适宜微生物生存时,微生物产生的EPS相对较少;反之,微生物产生的EPS较多.     

厌氧膜生物反应器膜污染特性研究

试验研究了厌氧膜生物反应器(MCAB)在处理酒厂高浓度有机废水时的膜污染特性.试验分析了膜阻力分布状况,结果表明外部阻力(浓差极化阻力和泥饼层阻力之和)占总阻力的98%,小于膜孔的物质进入膜孔内引起堵塞与吸附而形成的内部阻力仅占总阻力的2%.同时对泥饼阻力模型(J(t)与t的关系)进行变形推导得出新的阻力模型(阻力R与t的关系),并对数据进行线性拟合,结果表明厌氧膜生物反应器膜阻力符合新的阻力模型,并得到试验条件下的厌氧膜生物反应器的阻力数学模型方程为:R(t)=2.19×1013(1 0.14t)0.5.     

平板膜生物反应器操作运行条件对膜污染特性的影响

采用正交试验法分析评价浸没式平板膜一生物反应器操作运行条件对膜污染的影响．试验选择了影响膜污染的3个主要因素,结果表明各因素对膜污染的主次关系为：污泥浓度〉抽停时间比〉曝气强度;其优化的操作条件为：污泥浓度5500mg／L,抽停时间比10：4,曝气强度0．3m^3／h．试验同时测定了各部分膜阻力的分布情况,结果表明,外部阻力是膜阻力的主要组成部分,因而设法减轻泥饼层或凝胶层的沉积是减轻膜污染的重要手段．     

一体式厌氧膜生物反应器膜材质选择的试验研究

厌氧膜生物反应器作为一种处理高浓度有机废水行之有效的工艺,膜材质选择是保证其良好运行的关键之一.试验研究了一体式厌氧膜生物反应器中三种不同材质的膜PAN、PES和PVDF膜过滤特性.并通过数据拟合得出不同的膜通量和膜阻力随时间的数学模型表达式.结果表明,在相同的操作条件下,PAN膜通量维持在一个较高水平,膜污染速度增长缓慢,PAN膜是本试验条件下厌氧膜生物反应器的最佳膜材质.     

厌氧膜生物反应器在食品废水处理中的应用研究

分别采用小试规模(50 L/d)外置错流管式厌氧膜生物反应器和中试规模(20 t/d)浸没式厌氧旋转膜生物反应器进行了食品厂废水的处理实验研究.结果表明,加入超滤膜单元提高了厌氧系统的总有机物去除率及甲烷产量,使系统出水水质更加稳定;外置错流式超滤膜随运行时间的延长通量明显下降,而旋转式超滤膜可通过膜片旋转速度提高和保持膜通量;浸没式旋转膜组件的运行能耗较外置管式膜组件至少降低30%;但不同形式的厌氧膜生物反应器对氨氮和总磷去除能力有限,需进一步处理.     

室内气流模拟过程中计算流体力学(CFD)的紊流模型评价和选择

论述了采用计算流体力学技术模拟室内空气的优越性,介绍几种紊流模型的性能以及各种模型在模拟室内空气流动时的适用性.     

平板膜生物反应器膜间距与曝气气泡形态对膜污染形成的影响

以平板膜生物反应器为研究对象,研究了不同膜间距(4.0mm,8.0mm,12.0mm)和不同曝气管开孔直径(1.0mm,2.0mm,3.0mm)对膜污染的影响.同时建立了模拟膜生物反应器,采用摄像和图形分析法观测了气泡在膜间的上升运动.结果表明在相同比曝气量下,膜间距为4.0mm时,膜污染程度最轻.摄像和图形分析结果表明膜间距为4.0mm时,气泡形状为球帽形,气泡扭曲变形程度最大,单个气泡冲刷面积最大,但气泡上升速度由于"壁效应"而明显减小.在不同曝气孔径下比较了膜阻力上升速率和活性污泥悬浮液中溶解氧的差异,曝气开孔直径为1.0mm时,膜阻力上升速率最小,活性污泥悬浮液中溶解氧浓度最大,曝气开孔直径为1.0mm的管状曝气器既能有效控制膜污染,又能给微生物提供较高的溶解氧,是较为理想的曝气器.     

颗粒物质控制膜污染的增强型膜生物反应器工艺的研究

用于废水处理的膜生物反应器工艺(MBR),是指通过膜过滤实现活性污泥与产水的分离的新型生化法处理废水技术.由于MBR具有占地面积小和出水水质好的优点,膜生物反应器成功应用到废水处理中的案例正在迅速增加.膜污染是MBR废水处理工艺面临的主要问题之一.膜污染会导致膜的渗透性能降低,为此必须通过化学清洗才能恢复膜的性能.为了实现无化学清洗的MBR工艺,研究使用持续物理冲刷去除膜污染层的方法.在活性污泥中加入颗粒物质(粉料),通过这些颗粒物质的持续冲刷作用实现去除膜污染层的目的.经过8个多月的实验,膜组件的渗透性能保持不变,通量可以达到40 L/(m2.h)以上.系统安装了在线的浊度仪作为产水质量的检测,在试验过程中,浊度始终没有变化.作为对比,同时也进行了一个参照实验(MBR标准工艺,没有加入颗粒物质),实验结果表明,传统的MBR工艺的膜组件渗透性能会不断下降导致通量下降,需要进行化学清洗.新型MBR工艺高通量和无需化学清洗的优势,将极大地提高MBR工艺的成本效益.     

可反洗平板膜生物反应器处理园区生活污水研究

就可反洗平板浸没式膜生物反应器和传统平板浸没式膜生物反应器处理园区生活污水的效果进行比较研究,发现在水力停留时间同为9.6h的条件下,两者几无区别,COD和NH3-N的去除率同样分别为90%和95%,浊度均降至0.7NTU以下,也均无SS检出,出水水质都达到国家生活杂用水水质标准(GB/T18920-2002);但前者因膜具可反洗性,可以延缓膜污染,延长系统稳定运行周期,减少化学清洗频率,既降低运行成本,延长膜使用寿命,又减少化学清洗剂对环境的污染.     

计算流体力学在纳滤膜分离技术研究中应用

介绍了计算流体力学(CFD)在膜分离过程模拟中的基本原理,探讨了CFD在膜隔网优化设计、纳滤膜污染机理研究、纳滤膜对无机离子截留等领域中的应用,并对CFD在纳滤膜分离技术研究中的应用前景进行了展望.     

传统帘式和海藻式MBR法处理印染废水的研究

采用帘式与海藻式两种膜在一定的污泥条件下(如MLSS,pH,SV等),对印染废水各项污染物质的去除效果和膜运行过程进行研究,探索两种膜组件的各自优缺点.实验结果表明:帘式膜对污染物质的去除效果比藻式的好,而藻式膜的抗膜污染性则较好.     

A minimal element distortion strategy for computational mesh dynamics

Mesh motion strategy is one of the key points in many fluid–structure interaction (FSI) problems. Due to the increasing application of FSI to solve the current challenging engineering problems, this topic has become of great interest. There are several different strategies to solve this problem, some of them use a discrete and lumped spring–mass system to propagate the boundary motion into the volume mesh, and many others use an elastostatic problem to deform the mesh. In all these strategies there is always risk of producing an invalid mesh, i.e. a mesh with some elements inverted. Normally this condition is irreversible and once an invalid mesh is obtained it is difficult to continue. In this paper the mesh motion strategy is defined as an optimization problem. By its definition this strategy can be classified as a particular case of an elastostatic problem where the material constitutive law is defined in terms of the minimization of certain energy functional that takes into account the degree of element distortion. Some advantages of this strategy are its natural tendency to high quality meshes, its robustness and its straightforward extension to 3D problems. Several examples included in this paper show these capabilities. Even though this strategy seems to be very robust it is not able to recover a valid mesh starting from an invalid one. This improvement is left for future work. Copyright © 2006 John Wiley & Sons, Ltd.     

混凝剂对化学强化MBR深度处理印染废水的影响

在自主开发的化学强化MBR(膜生物反应器)技术的基础上,考察30mg/LFeCl3,30mg/LFeCl3和30mg/LCaCl2的组合以及30mg/LCaCl2三种混凝剂投加方式对其深度处理模拟印染废水二级出水的效果以及对膜污染的影响.结果表明,单独投加FeCl3及FeCl3和CaCl2的组合提高了化学强化MBR装置对色度、COD、氨氮和总磷的去除效果.单独投加FeCl3及FeCl3和CaCl2的组合都可以有效地缓解膜污染,其中投加FeCl3和CaCl2组合的效果更好,而单独投加CaCl2将会加重膜污染.FeCl3的絮凝作用可以降低胞外聚合物(EPS)的含量并增加污泥颗粒粒径.此外,CaCl2可以强化FeCl3的絮凝作用,提高膜污染的延缓效果.然而,单独投加CaCl2由于刺激EPS的产生并造成污泥颗粒粒径的减少,从而加重了膜污染.FeCl3和CaCl2都有利于溶解性有机物(DOM)中的小分子量片段向大分子量片段的转化,其中CaCl2的影响更大.     

Use of computational fluid dynamics to study forces exerted on prey by aquatic suction feeders

Suction feeding is the most commonly used mechanism of prey capture among aquatic vertebrates. Most previous models of the fluid flow caused by suction feeders involve making several untested assumptions. In this paper, a Chimera overset grids approach is used to solve the governing equations of fluid dynamics in order to investigate the assumptions that prey do not interact with the flow and that the flow can be modelled as a one-dimensional flow. Results show that, for small prey, both neglecting the prey and considering prey interaction give similar calculated forces exerted on the prey. However, as the prey item increases in size toward the size of the gape, its effect on the flow becomes more pronounced. This in turn affects both the magnitude of the hydrodynamic forces imparted to the prey and the time when maximum force is delivered. Maximum force is delivered most quickly to intermediate sized prey, about one-third of mouth diameter, and most slowly to prey less than 7 per cent or greater than 67 per cent of mouth diameter. This suggests that the effect of prey size on the timing of suction forces may have substantial consequences for the feeding ecology of suction feeders that are known to prefer prey between 25 and 50 per cent of mouth diameter. Moreover, for a 15 cm fish with a 15 mm gape, assuming a radial one-dimensional flow field can result in underestimating the maximum force exerted on a 5 mm diameter spherical prey 1 gape distance from the mouth by up to 28.7 per cent.     

Using design of experiments methods for applied computational fluid dynamics: A case study

This article presents an application of design of experiments (DOE) in a computational fluid dynamics (CFD) environment to study forces and moments acting on a missile through various speeds and angles of attack. Researchers employed a four-factor Latin hypercube space-filling design and the Gaussian Process to build a surrogate model of the CFD environment. The surrogate model was used to characterize missile aerodynamic coefficients across the transonic flight regime. The DOE process completed the task with less computational resources than a traditional one-factor-at-a-time (OFAT) approach. To validate the surrogate model, specific OFAT angle of attack sweeps were performed. This provided a direct comparison between the Gaussian Process model and OFAT analysis. In most cases, the surrogate computer model was able to accurately capture the nonlinear response variables. Moreover, the surrogate model enabled a dynamic prediction tool that could investigate untested scenarios, a capability not available with OFAT. The DOE process consequently received support from engineers who do not typically use DOE.     

The flow field near a venous needle in hemodialysis: A computational study

The vascular access used in hemodialysis can suffer from numerous complications, which may lead to failure of the access, patient morbidity, and significant costs. The flow field in the region of the venous needle may be a source of damaging hemodynamics and hence adverse effects on the fistula. In this study, the venous needle flow has been considered, using three‐dimensional computational methods. Four scenarios where the venous needle flow could potentially influence dialysis treatment outcome were identified and examined: Variation of the needle placement angle (10°, 20°, 30°), variation of the blood flow rate settings (200, 300, 400 mL/min), variation of the needle depth (top, middle, bottom), and the inclusion of a back eye in the needle design. The presence of the needle has significant effect on the flow field, with different scenarios having varying influence. In general, wall shear stresses were elevated above normal physiological values, and increased presence of areas of low velocity and recirculation—indicating increased likelihood of intimal hyperplasia development—were found. Computational results showed that the presence of the venous needle in a hemodialysis fistula leads to abnormal and potentially damaging flow conditions and that optimization of needle parameters could aid in the reduction of vascular access complications. Results indicate shallow needle angles and lower blood flow rates may minimize vessel damage.     

Shape optimization of bi-directional flow passage components based on a genetic algorithm and computational fluid dynamics

The inlet/outlet is an important part of a water conveyance system in a pumped storage power station (PSPS). Its hydraulic characteristics are directly related to the operation and economic benefit of the PSPS. Frequent changes between inflow and outflow operations pose significant challenges in the design of the inlet/outlet diffusion segment shape. In this study, an effective optimization method, including three-dimensional parametric modelling, computational fluid dynamics and a genetic algorithm, is introduced and coupled to the design of the diffusion segment shape. The hydraulic characteristics of bi-directional flow, including the head loss, velocity uneven distribution and uneven discharge distribution, are selected as the objective function in the optimization method. Using this method, the recommended shape of the inlet/outlet is studied and its hydraulic characteristics are discussed. The results indicate that the optimized inlet/outlet has better performance.     

Rapid pivot feeding in pipefish: flow effects on prey and evaluation of simple dynamic modelling via computational fluid dynamics

Most theoretical models of unsteady aquatic movement in organisms assume that including steady-state drag force and added mass approximates the hydrodynamic force exerted on an organism''s body. However, animals often perform explosively quick movements where high accelerations are realized in a few milliseconds and are followed closely by rapid decelerations. For such highly unsteady movements, the accuracy of this modelling approach may be limited. This type of movement can be found during pivot feeding in pipefish that abruptly rotate their head and snout towards prey. We used computational fluid dynamics (CFD) to validate a simple analytical model of cranial rotation in pipefish. CFD simulations also allowed us to assess prey displacement by head rotation. CFD showed that the analytical model accurately calculates the forces exerted on the pipefish. Although the initial phase of acceleration changes the flow patterns during the subsequent deceleration phase, the accuracy of the analytical model was not reduced during this deceleration phase. Our analysis also showed that prey are left approximately stationary despite the quickly approaching pipefish snout. This suggests that pivot-feeding fish need little or no suction to compensate for the effects of the flow induced by cranial rotation.