黏性泥沙不等速沉降絮凝的格子Boltzmann模拟

建立黏性泥沙运动的三维格子Boltzmann模型,对黏性泥沙不等速沉降絮凝过程进行全尺度直接数值模拟,从微观角度分析了泥沙颗粒不等速沉降形成的絮团特性和絮团成长过程。模拟结果与已有实验结果一致。模拟结果还表明,不等速沉降时不同泥沙浓度条件下形成的絮团若大小相近则沉降速度接近,但泥沙浓度对水体中泥沙总体平均沉速有明显影响,泥沙浓度越高,泥沙平均沉速越大,主要是因为泥沙浓度越大,颗粒之间越容易发生碰撞而形成更大、更多的絮团。     

盐度对伊利石不等速沉降絮凝的影响

盐度是影响粘性泥沙絮凝沉降的重要因素之一。将胶体化学中的XDLVO理论引入颗粒沉降三维格子玻尔兹曼模型,模拟黏土矿物质絮凝沉降过程,研究伊利石在不同盐度条件下的不等速絮凝沉降过程。模拟结果表明,伊利石黏结概率随着盐度先增加后保持不变,最佳絮凝盐度在5左右;伊利石絮凝形成的絮团粒径呈现双峰分布,盐度对粒径分布有一定影响。盐水中双电层排斥力受到抑制,起主要作用的是范德华吸引力,使得伊利石颗粒更容易产生絮凝,三维数值模型从细观尺度揭示了盐度对伊利石不等速沉降絮凝的影响机理。     

黏性细颗粒泥沙絮凝及沉降的三维模拟

鉴于试验观察黏性细颗粒泥沙悬浮体系絮凝-沉降过程存在技术困难，尝试以分形生长理论为基础，使用改进的受限反应絮团聚集(RLCCA)模型，在计算机上模拟这一复杂过程。模型考虑了絮团的分形结构与可渗透特性、泥沙浓度对悬浮液黏性的影响等因素，复演颗粒、絮团在重力沉降和布朗作用下的三维运动及相互碰撞并黏结的现象。在对模拟数据的统计分析基础上，讨论了絮凝-沉降过程的各种特性，结果表明：碰撞效率系数不同时，絮凝发育过程存在相似性。     

黏性细颗粒泥沙絮凝发育时空过程的数值模拟

以Smoluchowski方程为基础,用数值模拟方法研究黏性细颗粒泥沙絮凝-沉降的时空过程。模拟设定泥沙最小基本颗粒粒径5μm,絮团分形维数1.78,模拟高度1.75 m,模拟总时长300 min,初始条件为各级泥沙颗粒-絮团浓度各向均匀分布,入口、出口无泥沙通量。模拟给出泥沙质量浓度、絮团平均粒径、絮团粒径分布等参数的时空变化过程。模拟发现:各个高度上泥沙质量浓度随时间变化的曲线均呈平缓衰减、快速下降与趋零三个阶段,其形状相似;位置越低,曲线的显现时间越滞后。絮团质量加权平均粒径随时间变化过程存在升涨-高峰-衰落三个阶段。碰撞效率系数越大,峰值越高;位置越低,峰现时间越滞后。本文还以泥沙质量浓度随时间变化为例,比较了数值模拟与絮凝试验的测量结果,二者基本一致。     

粘性细颗粒泥沙絮团发育与絮网发育临界判据的初步研究

单颗粒沉降、絮团发育沉降、絮网压缩沉降是粘性细颗粒泥沙悬浮体系絮凝发育的三种不同相态,其中关于从絮团发育沉降转变到絮网压缩沉降这一临界状态的研究比较少见.本文从特征絮团的角度出发,考虑形成絮网的临界状态分别得出了静水、动水不同条件下区分絮团发育与絮网发育的临界判据,其分析结果与试验观察定性的一致.     

一种改进的聚合模型在污泥絮凝-沉降模拟中的应用

鉴于在试验中难以观测到污泥絮团的粒径分布情况,且以往类似模拟中鲜有考虑絮团分形特征及絮团黏结效率的问题,通过一种改进的聚合模型来模拟污泥颗粒间的分形聚合机理,并在群体平衡模型(PBM)和两相流Mixture模型基础上对污泥的絮凝-沉降进行三维数值模拟。模拟结果表明:建立的数学模型能较好地反映黏性污泥分形絮凝-沉降规律,具预测絮团粒径分布的能力;在同一时刻絮团的粒径分布范围随着分形维数的降低逐渐变宽,大粒径絮团数量迅速增加,小粒径絮团数量逐渐减少;初始颗粒数量的衰减速率随分形维数的增大而增大;絮团数量总体呈先增加后减少的趋势,但当分形维数不同时,其最大值随分形维数的增大而增大,出现时间随着分形维数的增大而有所缩短。     

三维分形絮团沉降的格子Boltzmann模拟

黏性泥沙絮团的形成机理和沉降特性对河口海岸细颗粒泥沙运动规律研究具有重要作用。为了从微观结构出发研究絮团运动机理，由扩散受限絮凝体聚集模型生成不同大小的分形絮团，引入格子Boltzmann方法模拟三维分形絮团的静水沉降，获得了絮团沉速的变化过程。比较格子Boltzmann算法模拟结果与采用盒子计数分维数的 Winterwerp絮团沉速公式，发现二者有较好的一致性，表明格子Boltzmann模拟方法可应用于河口海岸环境中的黏性泥沙运动研究。     

黏性泥沙絮团强度的试验研究综述

絮凝是河口地区水沙交换过程中极为重要的一环,动水絮凝过程可概括为:颗粒碰撞—有效碰撞黏结—絮团重构—絮团破裂—絮团再形成,最终絮团达到动态平衡状态,这是絮团强度在紊动剪切作用下的结果。从絮团强度的试验研究角度回顾了国内外研究进展,此方面研究多围绕絮凝的影响因素与絮团强度的特征参数之间的定性规律、经验或半经验定量关系的角度展开,采用不同的观测装置、设备和技术,以期对絮凝动力学得到更为深入的认识。     

均匀切变水流对黏性细颗粒泥沙絮凝的影响研究

黏性细颗粒泥沙的絮凝是河流动力学中一个重要且尚未完善的研究课题。本文以分形聚集生长理论为基础,考虑了絮团破碎、泥沙浓度等因素的影响,使用三维无网格方法,在MATLAB平台上通过模拟泥沙颗粒和絮团在布朗运动、重力沉降和均匀切变水流作用下的碰撞黏结过程,研究了水流剪切作用对黏性细颗粒泥沙絮凝的影响。研究结果表明：水流作用可提高颗粒之间的碰撞几率,促进絮凝,但高水流剪切力作用下絮团破碎现象明显,因此,水流速度梯度的大小对细颗粒泥沙的絮凝有一定的影响,即絮凝速度及絮团平均粒径随水流流速梯度的增大呈先增后减的规律;均匀切变水流作用下,絮团粒径分布均匀化,絮团在垂直水流方向上发育较好且分形维数较大。本文研究成果是进一步研究天然河道中黏性细颗粒泥沙絮凝机理的成功尝试。     

紊动对黏性细颗粒泥沙絮凝沉降影响的试验研究

利用室内环形水槽及高倍摄像设备,定量研究了不同悬沙浓度及紊动剪切对黏性细颗粒泥沙絮凝沉降特性的影响。试验中观测到的絮团粒径为分散颗粒的几倍到几十倍,絮团中值粒径随着水体紊动剪切的增大呈先增大后减小的变化趋势,最大的中值粒径约为150μm,出现在紊动剪切为30 1/s条件下。悬沙浓度的增大促进絮团的发育,在350 mg/L条件下形成的絮团整体粒径比150 mg/L条件下的更大。絮团中值沉降速度在0.7～3.4 mm/s之间,絮团最大的中值沉速出现在紊动强度最大时65 1/s,此时所形成的絮团结构密实,有效密度较大。絮团有效密度随着粒径的增大而减小,研究表明,采用变分形维数方法,对有效密度随粒径变化的模拟结果与试验结果吻合较好。     

Effects of floc and bubble size on the efficiency of the dissolved air flotation (DAF) process.

Dissolved air flotation (DAF) is a method for removing particles from water using micro bubbles instead of settlement. The process has proved to be successful and, since the 1960s, accepted as an alternative to the conventional sedimentation process for water and wastewater treatment. However, limited research into the process, especially the fundamental characteristics of bubbles and particles, has been carried out. The single collector collision model is not capable of determining the effects of particular characteristics, such as the size and surface charge of bubbles and particles. Han has published a set of modeling results after calculating the collision efficiency between bubbles and particles by trajectory analysis. His major conclusion was that collision efficiency is maximum when the bubbles and particles are nearly the same size but have opposite charge. However, experimental verification of this conclusion has not been carried out yet. This paper describes a new method for measuring the size of particles and bubbles developed using computational image analysis. DAF efficiency is influenced by the effect of the recycle ratio on various average floc sizes. The larger the recycle ratio, the higher the DAF efficiency at the same pressure and particle size. The treatment efficiency is also affected by the saturation pressure, because the bubble size and bubble volume concentration are controlled by the pressure. The highest efficiency is obtained when the floc size is larger than the bubble size. These results, namely that the highest collision efficiency occurs when the particles and bubbles are about the same size, are more in accordance with the trajectory model than with the white water collector model, which implies that the larger the particles, the higher is the collision efficiency.     

Effects of surface charge, micro-bubble size and particle size on removal efficiency of electro-flotation.

Flotation is a water treatment alternative to sedimentation, and uses small bubbles to remove low-density particles from potable water and wastewater. The effect of zeta potential, bubble size and particle size on removal efficiency of the electro-flotation process was investigated because previous model-simulations indicated that these attributes are critical for high collision efficiency between micro-bubbles and particles. Solutions containing Al3+ as the metal ion were subjected to various conditions. The zeta potentials of bubbles and particles were similar under identical conditions, and their charges were influenced by metal ion concentration and pH. Maximum removal efficiency was 98 and 12% in the presence and absence of flocculation, respectively. Removal efficiency was higher when particle size was similar to bubble size. These results agree with modelling simulations and indicate that collision efficiency is greater when the zeta potential of one is negative and that of the other is positive and when their sizes are similar.     

推移质颗粒平均运动特性的试验研究

采用三维粒子示踪测速技术，通过分别变化示踪粒子的粒径、比重以及水力比降等条件，观测了10种不同水沙条件下推移质颗粒的三维运动，对推移质颗粒的平均运动规律进行了统计分析。研究结果表明，推移质颗粒的纵向速度既不是Einstein假定的11.6u，也不是Bagnold假定的比清水流速小一个ω值。在分析坡降、粒径和比重对推移质平均纵向速度影响的基础上，根据Bagnold和Einstein两种表述方法以及实测资料分别得出推移质纵向平均速度的两个表达式。     

考虑不同颗粒形状的PFC~(3D)砂土直剪试验模拟研究

针对纯圆形颗粒在模拟土工直剪试验中的缺陷,以砂土直剪试验为研究对象,基于PFC~(3D)颗粒流程序,以纯圆颗粒为基本形状进行二次开发,利用FISH语言编程,引入了由pebble构成的块体(clump)单元,建立了哑铃形和椭圆形两种不同颗粒形状的簇颗粒,对纯圆颗粒及簇颗粒进行直剪试验模拟对比,并分析了颗粒初始配位数与内摩擦角的关系以及不同颗粒试样的体积变化规律,从细观上描述了不同形状颗粒的空间活跃程度。结果表明:颗粒形状直接影响着数值模拟的精度,椭圆形颗粒的吻合度最高,哑铃形颗粒次之,圆形颗粒较差,椭圆形颗粒较纯圆颗粒的精度提高了7.05%;簇颗粒的初始配位数相比纯圆颗粒较大,其试样表现出较高的强度和稳定性;颗粒形状对试样体积变化有着显著影响;纯圆颗粒的空间活跃程度较为明显,簇颗粒则相对稳定。该研究可为土工试验模拟在颗粒选择及模拟方法上提供一定的参考。     

Fundamental characteristics of bubbles and ramifications for the flotation process.

Flotation processes involve the use of very small bubbles (micro-bubbles) to separate particles from water. The process has become a good alternative to sedimentation, especially where the particles are small or of low density. Although the flotation process commences with a collision between particles and bubbles, most research has been focused only on the characteristics of the particles. In this paper, recent theoretical and experimental research on the characteristics of bubbles is summarized. The effect on the collision efficiency of the size and charge of bubbles is calculated through trajectory analysis. The size and charge of bubbles are measured under different conditions and the ramifications of the results are discussed. The results may lead to a better understanding and optimization of the existing process. In particular, we discuss an idea that a new advanced flotation process might be possible by the modification of the characteristics of the bubble alone or of both bubble and particle.     

叶栅含沙水流中沙粒运动的数值模拟

利用Lagrangian颗粒运动方程，数值求解了叶栅含沙水流中沙粒的运动。定义了撞击效率，给出了来流雷诺数(或来流速度)，来流冲角和沙粒尺寸对撞击效率的影响。数值模拟了不同来流雷诺数，来流冲角和尺寸的沙粒在栅中的运动轨迹，考虑了壁面、边界层、尾迹及少粒与壁面碰撞对颗粒运动的影响。数值分析了沙粒对叶栅固壁的冲击情况，数值模拟结果与水涡轮机械中所表现的磨损情况较为一致。     

Multiphase modeling study for storm water solids treatment in experimental storm water settling chamber

This study tests four different types of multiphase models to determine the most appropriate model for predicting the behaviors of various types of storm water solids in a settling chamber. The Lagrangian reference frame discrete phase models of uncoupled and coupled models based on the interaction between the discrete phase and the continuous phase were tested. The rigid moving objects model providing six degrees of freedom particle motion was also tested to model non-spherical particle motion. The fourth model was a sediment transport model using the Eulerian reference frame model. This study tested five different storm water solids consisting of bulk, gross, coarse, sediment and fine which are classified by particle size and settling characteristics. Particle settling efficiency and computational time were considered in determining the most appropriate multiphase model. The coupled model provided better solid settling than the uncoupled model, but required 8.2% more computational time in this study. The Eulerian model matched settling efficiency for the high density finer solids. Although the Eulerian model showed reliable settling prediction, the Lagrangian coupled model can be an effective alternative requiring significantly reduced computational time.     

沙漠沙的筛分粒径与粒径计粒径的比较实验

筛分法与粒径计法是两种重要的颗粒分析方法。本文通过较为系统的实验发现采用这两种方法所得到的实验结果之间存在较大的偏差。采用粒径计法测得的样品粒径较之用筛分法所得的结果明显偏大 ,说明用这两种原理不同的颗分方法所测出的粒径数据不能不加换算地直接采用。由于目前有关筛分粒径与粒径计粒径转换关系的研究结果并不一致 ,所以文中比较了几家代表性研究成果之间的差异 ,重点讨论了采用不同沉降速度公式进行计算时所得粒径计粒径之间的差异 ,探讨了各家结果不同的原因 ,对与颗粒粒径相关的研究具有参考价值     

温度对黏性泥沙絮凝影响的机理研究

用格子玻耳兹曼方法,引入胶体作用理论(e Xtended Derjaguin-Landau-Verwey-Overbeek theory,XDLVO理论)模拟不同温度下两个黏性泥沙颗粒静水不等速沉降絮凝过程。结果表明,温度升高缩短泥沙絮凝时间,促进絮凝。同时分析了泥沙颗粒的运动轨迹、相对运动速度和颗粒间近距离作用力,探讨温度对泥沙絮凝的影响机理。分析结果认为温度升高一方面减小水体黏性系数,加大泥沙沉速,加快泥沙颗粒相互靠近;另一方面,增大泥沙颗粒表面吸附水膜厚度的同时,也增大泥沙颗粒之间的近距离排斥力,对吸附水膜厚度和排斥力影响的综合结果也促进了絮凝。     

水库疏浚底泥絮凝沉降室内试验研究

泥浆絮凝沉降效率是影响湖库疏浚工程进度的重要因素。为探索不同絮凝剂(有机、无机)对疏浚泥浆絮凝沉降效率的影响,以通济桥水库疏浚底泥为研究对象,通过室内试验先分析1种有机絮凝剂(PAM)和5种无机絮凝剂(PAC,FeCl₃,Na₂CO₃,Na₂SiO₃·9H₂O,KAl(SO₄)₂·12H₂O)单独作用下疏浚泥浆的絮凝沉降效果及上清液水质的优劣情况,然后对有机、无机絮凝剂进行优化组合试验,确定絮凝沉降效果和上清液水质最优的设计配比方案。试验结果表明:①PAM可以有效促进疏浚泥浆土颗粒的絮凝沉降,但对上清液的去浊率较低;CO₃^2-可以促进细颗粒絮凝结合成大颗粒,提高颗粒的沉降速率;Fe³⁺,Al³⁺具有改善上清液浊度的作用,但对促进泥浆絮凝沉降效果有限。②每1 000 mL疏浚泥浆中采用10 mL PAM+2 g PAC+2 g Na₂CO₃的有机-无机絮凝剂组合为最佳方案,既能快速促进疏浚泥浆的絮凝沉降,又能有效降低上清液的浊度。试验成果可为类似疏浚工程中选择絮凝剂最佳配比方案提供参考。