扩散沉积导致的管内气溶胶穿透率简明公式

针对气溶胶粒子在长直扁平方管(通道)及圆管内受布朗扩散导致的沉积损失问题,从对流扩散方程出发,采用分离变量法,通过严格的理论求解得到了这两类问题中粒子穿透率的无穷级数解。利用Python语言中高精度计算功能确定了相应本征问题的本征值、本征函数及无穷级数展开系数。根据所得的相对严格意义上的穿透率数据,回归拟合得到了扩散沉积条件下粒子通过方管及圆管时的穿透率简明公式,并与已有研究进行了详细的比较。结果表明:本文得到的两个简明公式更加简单和准确,对方管与圆管内粒子穿透率估计的误差最大值分别仅为3.7×10~(-4)和8.3×10~(-4),可以快速有效地估计层流条件下和受扩散沉积作用时,气溶胶粒子通过两种管(通)道的穿透率。     

大气边界层内羽流扩散研究

主要研究了高架点源的污染，即``羽流扩散'. 由于大气边界层内的湍流运动，引 起污染物扩散的主要因素是湍流扩散， 研究湍流扩散有两种基本方法：统计理论和梯度理论. 采用拉格朗日粒子扩散模型 (Lagrangian particle dispersion model, LPDM)来评价羽流扩散的平均浓度，分别计算了表征扩散程度的3个参数：沿烟轴方向的 着地浓度、水平方向的羽流扩散宽度和垂直方向的羽流扩散宽度. 从计算结果和风洞实验的 数据对比来看，用该模式可以较好地模拟大气边界层内复杂气候条件下的羽流扩散.     

两种颗粒湍流扩散模型数值模拟气液两相流泄漏扩散的比较

采用相间耦合的欧拉-拉格朗日方法，模拟了装有液化气（丙烷）的容器出现小孔或裂缝时，发生泄漏后的气液两相扩散过程。分别应用随机轨道模型及颗粒群模型来考察湍流对液滴扩散的影响，并与确定性轨道进行了比较。给出了数学物理模型，计算结果与实验数据进行了对比。结果表明:采用随机轨道模型能较好地描述液滴的湍流扩散，适用于有液相蒸发的两相流扩散问题。     

三维溃坝流的光滑粒子动力学方法模拟

采用光滑粒子动力学SPH(Smoothed Particle Hydrodynamics)方法对三维溃坝流问题进行了数值模拟。为了逼真地模拟出坝内水体与壁面间相互作用而产生的水花飞溅、融合以及近壁面流动等现象,加入了混合长度形式的湍流模型。为了有效地防止粒子穿透固壁,提出了一种新型的适合三维数值模拟的固壁边界处理方法。应用SPH方法对三维溃坝流进行了数值模拟,并分别考虑了未添加障碍物和添加圆柱障碍物两种情形。计算结果表明,改进SPH方法能够精细地捕捉溃坝流在不同时刻的自由液面,并获得稳定而精确的数值结果。     

受限分散体系中粒子扩散特性的研究

当微小粒子被限制在两个距离很近的平行平面之间时，其扩散将不同于它在无限空间中的自由扩散. 利用光镊和CCD图像相关处理系统，直接测量了直径1μm的聚苯乙烯小球在不同高度，一定时间间隔内，沿着平行玻片表面方向上布朗运动的距离，并由此得到了样品池上下两个平行表面对分散体系分散相小球扩散特性的影响. 还利用小球偏离物镜焦平面不同距离时的成像状况，用相关运算法确定了小球在纵向上的位置，实现了小球扩散特性与小球在样品池中的纵向位置的关系的研究.     

飞行加速度对发动机内粒子轨迹影响研究

利用Euler-Lagrange模型方程，对发动机内的三维两相流动进行了数值模拟，分析了不同过载组合、不同粒子直径、不同壁面恢复系数对发动机燃烧室内粒子的聚集部位、浓度分布、冲刷速度、冲刷角度等的影响。计算结果表明：在纵向和横向过载相等的情形下，粒子的最大聚集密度随过载量的增加，呈先减小后增加的趋势，最大聚集点随过载量的增加向流动方向移动，过载量的大小对粒子的冲刷角度、冲刷速度影响较大；在相同工况下，随粒子直径的增加，最大聚集部位向流动方向移动，最大聚集密度增加；随粒子壁面恢复系数恢复得减小，最大聚集部位向发动机头部移动，最大聚集密度增加。     

混合层中粒子扩散的实验研究

本文应用流动显示和LDA测量,对混合层中粒子的扩散问题进行了实验研究。结果表明：粒子的扩散强烈地依赖于St数,具有较小St的粒子的扩散近似地与流体示踪粒子相同,随着St的增大,扩散角度增大,也就是说粒子扩散加快,并且随着粒子惯性的增大,它对湍流脉动的响应将会减弱。     

洁净房间内亚微米粒子的沉积行为

采用二维滞止流模型，讨论了工业洁净室内不同送风速度下，气溶胶粒子分别受重力和电场力作用及二者联合作用时在工作台表面或晶片上的沉积特征。通过与相关文献提供的实验数据的对比，确认了这种预报超细粒子沉积行为的方法。     

流化床中废弃物颗粒的横向扩散特性

在试验基础上计算得到内循环流化床和鼓泡床中3种轻质废弃物颗粒和两种重质颗粒的横向扩散系数,发现废弃物颗粒的变形程度、密度、尺寸及布风板形状和布风方式均影响其扩散系数．轻质颗粒的运动符合扩散规律,重质粒子的运动则不符合该规律．文中从流化床中气体颗粒流和不同物性废弃物相互作用的角度简要分析了物料的扩散特性．     

非对称射流形成的几何理论(二)

将平板非对称正碰撞形成射流满足的定常方程组封闭条件推广到非正碰情况，提出了平面二维条件下非对称射流形成的一般理论，给出了射流形成的临界条件和出流射流的理论公式。对斜碰撞情形下非对称射流的出流方向、厚度随碰撞角和来流初始位形的变化规律进行了理论预测和数值模拟，理论解与二维欧拉程序的仿真结果符合较好。     

Formation of fibrous crystals in flowing blends of polyethylene melts

Flow induced crystallization of high density polyethylene has been studied in a two-phase flow system using low density polyethylene as the carrier phase. Extensional stresses were generated under slow flow conditions by either of two methods: one involving flow past a stationary seed, the other involving a droplet deformation and bursting mechanism. In both cases, oriented, fibrillar crystallization of the high density phase was observed optically and correlated with calculations indicating the presence of flow-induced extensional gradients. Morphological, thermal, and birefringence data indicate that the crystalline fibers produced are oriented and superheatable, and consist of a multifibrillar substructure. For fibers produced by the droplet bursting process a semi-quantitative agreement was found between fiber melting point and birefringence based on a simplified analysis for the bursting induced extensional flow. These results demonstrate that two-phase flows of crystallizable systems are a convenient means for studying the phenomenon of flow induced crystallization in polymer melts.     

Mathematical modeling of near-wall flows of two-phase mixture with evaporating droplets

ＩｎｔｒｏｄｕｃｔｉｏｎＯｐｔｉｍｉｚａｔｉｏｎｏｆｖａｒｉｏｕｓｔｈｅｒｍａｌｐｏｗｅｒｄｅｖｉｃｅｓ (ｓｔｅａｍｇｅｎｅｒａｔｏｒｓ,ｈｅａｔｅｘｃｈａｎｇｅｒｓ ,ｅｔｃ .)ａｎｄｄｅｖｅｌｏｐｍｅｎｔｏｆｔｈｉｎｆｉｌｍｔｅｃｈｎｏｌｏｇｉｅｓ(ｆｏｒｅｘａｍｐｌｅ ,ｗｉｔｈｔｈｅｕｓｅｏｆｔｗｏ＿ｐｈａｓｅｊｅｔｓ)ｐｒｏｍｐｔｓｍａｔｈｅｍａｔｉｃａｌｍｏｄｅｌｉｎｇｏｆｎｅａｒ＿ｗａｌｌｆｌｏｗｓｏｆｇａｓ＿ｄｒｏｐｌｅｔｍｉｘｔｕｒｅｓ .Ｄｕｅｔｏｔｈｅｇｒｅａｔｄｉｖｅｒｓｉｔｙｉｎ…     

Droplet heat transfer on micro-post arrays: Effect of droplet size on droplet thermal characteristics

Heat transfer towards a water droplet from hydrophobic micro-post array surface is considered while mimicking the environmental temperatures. Micro-post arrays are created on a silicon wafer surface via lithography technique. The textured surfaces are replicated by polydimethylsiloxane (PDMS) to achieve an optical transmittance. The droplet adhesion on micro-post array surface is presented and the influence of droplet size on the heat transfer and droplet internal flow characteristics is examined. The flow predictions are validated via the particle image velocimetry data. It is found that adhesion force between the water droplet and the micro-post arrays surface depends on the geometric size and the orientation of the micro-post arrays on the surface. Temperature and flow fields are influenced by the droplet size. The Nusselt and the Bond numbers increase with the droplet volume; however, the Bond number remains less than unity indicating that the Marangoni current dominates over the buoyancy current in the droplet. The Nusselt number attains larger values for micro-post array surface than that of the plain surface. This is because of temperature and velocity oscillations along the contact lines at the droplet bottom due to the pitches of the micro-post arrays.     

Electrokinetic Mixing and Displacement of Charged Droplets in Hydrogels

Mixing in droplets is an essential task in a variety of microfluidic systems. Inspired by electrokinetic mixing, electric field-induced hydrodynamic flow inside a charged droplet embedded in an unbounded polyelectrolyte hydrogel is investigated theoretically. In this study, the polyelectrolyte hydrogel is modeled as a soft, and electrically charged porous solid saturated with a salted Newtonian fluid, and the droplet is considered an incompressible Newtonian fluid. The droplet-hydrogel interface is modeled as a surface, which is located at the plane of shear, with the electrostatic potential \(\zeta \) . The fluid inside the droplet attains a finite velocity owing to hydrodynamic coupling with the electroosmotic flow arising from the droplet and polymer charge. The fluid velocity inside the droplet is linearly proportional to the electroosmotic flow velocity in the charged gel and the electroosmotic flow velocity beyond the electrical double layer of a charged interface. It is found that the polymer boundary condition at the droplet surface and the viscosities of the fluids inside and outside the droplet significantly modulate the interior fluid flow. The ionic strength and the permeability of the polymer network impact the flow differently depending on whether the flow arises from the droplet or polymer charge. Finally, the displacement of a charged droplet embedded in a gel under the influence of an external electric field is undertaken. This work is motivated by experimental attempts, which can register sub-nanometer-scale inclusion displacements in hydrogels, to advance electrical microrheology as a diagnostic tool for probing inclusion-hydrogel interfaces. In the absence of polymer charge, a close connection is found between the electrical response of a charged droplet when it is immobilized in an uncharged incompressible gel and when it is dispersed in a Newtonian electrolyte.     

Heat and mass transfer of a fuel droplet evaporating in oscillatory flow

A numerical study of the heat and mass transfer from an evaporating fuel droplet in oscillatory flow was performed. The flow was assumed to be laminar and axisymmetric, and the droplet was assumed to maintain its spherical shape during its lifetime. Based on these assumptions, the conservation equations in a general curvilinear coordinate were solved numerically. The behaviors of droplet evaporation in the oscillatory flow were investigated by analyzing the effects of flow oscillation on the evaporation process of a n-heptane fuel droplet at high pressure.The response of the time history of the square of droplet diameter and space-averaged Nusselt numbers to the main flow oscillation were investigated in frequency band of 1–75 Hz with various oscillation amplitudes. Results showed that, depending on the frequency and amplitude of the oscillation, there are different modes of response of the evaporation process to the flow oscillation. One response mode is synchronous with the main flow oscillation, and thus the quasi-steady condition is attained. Another mode is asynchronous with the flow oscillation and is highly unsteady. As for the evaporation rate, however, in all conditions is more greatly enhanced in oscillatory flow than in quiescent air.To quantify the conditions of the transition from quasi-steady to unsteady, the response of the boundary layer around the droplet surface to the flow oscillation was investigated. The results led to including the oscillation Strouhal number as a criteria for the transition. The numerical results showed that at a low Strouhal number, a quasi-steady boundary layer is formed in response to the flow oscillation, whereas by increasing the oscillation Strouhal number, the phenomena become unsteady.     

A model for droplet entrainment in heated annular flow

The ability to accurately predict droplet entrainment in annular two-phase flow is required to effectively calculate the interfacial mass, momentum, and energy transfer, which characterizes nuclear reactor safety, system design, analysis, and performance. Most annular flow entrainment models in the open literature are formulated in terms of dimensionless groups, which do not directly account for interfacial instabilities. However, many researchers agree that there is a clear presence of interfacial instability phenomena having a direct impact on droplet entrainment. The present study proposes a model for droplet entrainment, based on the underlying physics of droplet entrainment from upward co-current annular film flow that is characteristic to light water reactor safety analysis. The model is developed based on a force balance and stability analysis that can be implemented into a transient three-field (continuous liquid, droplet, and vapor) two-phase heat transfer and fluid flow systems analysis computer code.     

PDA measurements of droplet size and mass flux in the three-dimensional atomisation region of water jet in air cross-flow

The main objective of this paper is to develop a technique to measure the global droplet properties in the atomisation region of a water jet issuing in a high-speed air cross-flow. Knowledge of these global properties allows comparison of the break-up outcome of geometrically different water nozzles. This is achieved by extending a PDA system to enable measurements in three-dimensional droplet flows. First, the droplet size and the spatial droplet distribution are measured by the PDA method. The global droplet properties are then obtained by using the measured local mass flux as a weighting factor in integrating the local droplet size. To facilitate the measurement of mass flux in three-dimensional flows, the PDA method is extended so that the reference area for the mass flux is derived as a function of both the geometry of the measurement volume and the flow direction. In the present application of three-dimensional droplet flow (a water jet in air cross-flow), a simple method is developed to measure the three velocity components of droplets by means of a two-component PDA system. The paper outlines the measurement technique and the procedure of estimating the global droplet size and the global droplet size spectra from local droplet properties and local mass flux. Received: 26 July 1998/Accepted: 23 February 1999     

流动聚焦装置中液滴的形成:喉部尺寸的影响

在本文中,研究了液滴在一个流动聚焦微流体设备中的形成过程,分析了喉部长度和宽度以及连续相的流速和分散相的粘度对液滴尺寸的影响。在固定的分散相流速(Qd)下,连续相流速(Q_c)对于液滴尺寸有重要的影响。当Q_c0.7mL/h,液滴尺寸在喉部长度到达一个临界值之前先趋于增加,之后随着喉部尺寸的继续增加逐渐下降;当Q_c0.7mL/h,液滴尺寸随着喉部长度的增加而降低。而越大的喉部宽度会产生越大尺寸的液滴。在Q_c继续增长的过程中通常会出现从挤压模式到滴模式的转变,最终液滴尺寸呈现出随着Q_c的增加指数降低的特征。归因于流速控制破碎机制,低粘度分散相下,液滴尺寸随粘度的增加而增加。     

Abnormal rotation of a deformed liquid crystal droplet immersed in an isotropic fluid after transient flow

The morphology evolution of liquid crystal droplets immersed in an isotropic fluid in flow field is found to be different from flexible polymer droplets. In this paper, we investigated the retraction of a liquid crystal droplet after transient flow. It is found that the liquid crystal droplet will rotate during the shape recovery, which has never been observed for an isotropic droplet. The factors that influence the rotational angle of a single liquid crystal droplet during retraction progress were studied, including the temperature, the dimension of the droplets, the time of shear flow, the shear rate, the flow type, and the properties of liquid crystal molecules. The rotation of liquid crystal droplet during shape recovery is ascribed to both the bulk elasticity of liquid crystal droplets and the anisotropic properties of the interface between liquid crystal and isotropic fluid.     

The influence of thermal expansion flow on droplet evaporation

It has been demonstrated recently that it follows from conservation of mass that unsteady temperature fields create flow in an incompressible fluid with a temperature-dependent density even in the absence of gravity. The paper studies the influence of thermal expansion flow on spherically symmetric evaporation of an isolated droplet. A model problem of a droplet evaporating at a constant rate is first considered. In this idealized situation one can use the assumption of a thin thermal boundary layer to solve analytically the unsteady moving-boundary heat conduction problem to find the temperature field inside the droplet both with and without the thermal expansion flow. Next evaporation of a fuel droplet in a diesel engine is studied numerically. The heat diffusion equation is solved in the liquid phase while the standard quasi-steady model is used for the gas phase. The results of the calculation show that for high ambient temperatures the influence of the thermal expansion flow on the droplet lifetime can be considerable.