外加液滴条件下固体细颗粒声凝并特性

外加声场激励下固体细颗粒的声凝并在燃烧污染物超低排放中具有应用潜力,添加大粒径液滴有望提高细颗粒声凝并效果.本文从颗粒运动、碰撞、凝并与反弹的动力学过程出发,基于直接模拟蒙特卡罗方法,建立气相中液滴与固体颗粒共存的气-液-固三相体系的声凝并模型,对外加液滴条件下固体细颗粒声凝并的过程和效果开展数值模拟.将模拟结果与实验相对比,验证模型可靠性.在此基础上,探究外加液滴条件下细颗粒声凝并动力学行为,考察外加液滴直径和数目浓度对细颗粒声凝并效果的影响规律.结果表明,外加液滴条件下,固体细颗粒迅速与大粒径液滴凝并,形成液-固混合相颗粒,细颗粒凝并效率显著提升.外加液滴直径和数目浓度是影响细颗粒声凝并的重要因素,随着直径的增大或数目浓度的提高,细颗粒凝并效率增大,而增幅趋于减小.研究结果可为复杂颗粒系统凝并模型的建立提供理论基础,并可为燃烧源细颗粒超低排放提供方法指导.     

水下环形凹槽圆柱体散射声场空间指向性调控

本文提出一种具有深度梯度的环形凹槽结构,可用于调控水中有限长刚性圆柱体散射声场空间指向性.基于声学相位阵列理论分析了环形凹槽圆柱声散射空间指向性改变的机理,研究表明:凹槽深度方向相位延迟和凹槽间Bragg散射的相互作用使得平面声波垂直于圆柱方向入射其正横方向散射声波发生偏转.采用有限元方法讨论了凹槽结构参数如占空比、梯度等对圆柱散射声场空间分布特征的影响规律.多个不同深度梯度环形凹槽单元组合圆柱体散射声场数值计算和实验结果显示:具有环形凹槽结构圆柱体正横方向散射声波均匀偏转到预定的空间范围内,使得圆柱体声散射场空间指向性均衡化,改变了圆柱整体的散射特征,这为水下目标声隐身设计和声波定向传播提供了新的方法.     

格子Boltzmann方法应用于气动声学研究

应用格子Boltzmann方法(LBM)对不同类型的气动声学问题进行数值研究.通过模拟一维平面声波和二维点源声波的传播,得到沿传播方向的声压脉动,其振荡幅值和衰减趋势与理论值相吻合.其次进行声波衍射和干涉现象的数值模拟.最后,模拟处于流场中运动声源辐射声场的多普勒效应.模拟结果说明LBM方法能较好地模拟低马赫数下的声学问题,包括声压脉动的传播,声波的波动特性以及流动与声波间的相互作用.     

微尺度稀薄效应下颗粒沉降的格子Boltzmann方法模拟

基于多松弛格子Boltzmann模型,对竖直细长微通道内颗粒自由沉降过程进行模拟,分析气体稀薄效应、初始位置以及颗粒间相互作用对微颗粒沉降特性的影响.研究表明：随Knudsen数增大,微通道内气体稀薄效应增强,颗粒表面气体滑移速度增大,气相流体有效粘度减小,颗粒相同运动状态下受到气体阻力相应减小,颗粒沉降平衡速度明显增大;不同初始位置颗粒沉降过程存在明显差异,初始位置偏离中心线颗粒将发生水平方向位移且呈振荡趋势,最终稳定于中心线平衡位置;在微尺度双颗粒沉降DKT现象过程中,气体稀薄效应影响颗粒运动特性,后颗粒跟随过程明显增长.     

水下低频振荡涡流场声散射调制机理与特性研究

水下涡流场对声波的散射问题是声波在复杂流场中传播的基本问题,在水下目标探测和流场声成像领域具有重要意义.针对水下低频振荡涡流场声散射调制问题建立了理论分析模型与数值计算方法,探究了其声散射调制声场的产生机理与时空频特性.首先,基于运动介质的波动方程,通过引入势函数将波动方程分解为流声耦合项和非耦合项,并对流声耦合项进行频域分析处理,揭示了水下振荡涡流场的声散射调制机理;其次,采用间断伽辽金数值方法对水下低频振荡涡流场中声传播过程进行了数值模拟,分析了低马赫数条件下,不同入射声波频率、涡流场的振荡频率和涡核尺度对涡流场声散射调制声场时空频特性的影响规律,并结合理论分析模型对其特性进行了解释.研究表明:低马赫数下,振荡涡流场对声波的散射可产生包含涡流场振荡频率双边带调制谐波的散射调制声场,且随着入射声波频率、涡核尺度的增大,散射调制声场强度增强,总散射声场空间分布具有对称性和明显主瓣,且主瓣方位角趋近于入射波传播方向;在频率比远大于1条件下,涡流场振荡频率对散射调制声场强度影响较小.     

基于多腔型超构材料的声场增强效应

构造了一种多腔型基本单元,由该基本单元构成的声学超构材料能够实现声场增强效应.此功能的实现是由基本单元的声腔和系统结构之间的相互耦合作用产生的单极子Mie共振引起.本文通过对多个基本单元进行不同形式的排列组合构造了对称型超构材料和非对称型超构材料,这两类超构材料可用于实现不同效果的声场增强.研究表明,由于对称型超构材料结构的高度对称,其声场增强效应的实现不受入射声波方向的影响;而非对称型超构材料的声场增强效应具有较强的方向依赖性,声波从不同侧入射时,超构材料对声场的增强效果也不同.本文关于这两类超构材料的研究将在隔声、声传感器、声通信、非对称性声学器件方面具有潜在的应用前景.     

基于声传播算子的声源定位实验模拟方法研究

声传播算子是一种高效的时域声场计算方法,它能够很方便地计算出给定系统参数下任意时刻任意位置的声场变化情况,本文采用这种方法计算所得的二维房间声场信息进行传声器阵列的声源定位仿真实验。计算结果表明,用该方法获取的阵列数据能有效地应用于阵列信号处理算法中,准确地估计出初始高斯脉冲声源的方向。声传播算子声场计算方法能为传声器阵列声源定位的实验提供方便,使得传声器阵列声源定位算法在不同混响时间的鲁棒性实验研究变得更加简捷。     

声驻波场中空化泡的动力学特性

以水为工作介质, 考虑了液体的可压缩性, 研究了驻波声场中空化泡的运动特性, 模拟了驻波场中各位置处空化泡的运动状态以及相关参数对各位置处空化泡在主Bjerknes力作用下运动方向的影响. 结果表明: 驻波声场中, 空化泡的运动状态分为三个区域, 即在声压波腹附近空化泡做稳态空化, 在偏离波腹处空化泡做瞬态空化, 在声压波节附近, 空化泡在主Bjerknes 力作用下, 一直向声压波节处移动, 显示不发生空化现象; 驻波场中声压幅值增加有利于空化的发生, 但声压幅值增加到一定上限时, 压力波腹区域将排斥空化泡, 并驱赶空化泡向压力波节移动, 不利于空化现象的发生; 当声频率小于初始空化泡的共振频率时, 声频率越高, 由于主Bjerknes 力的作用将有更多的空化泡向声压波节移动, 不利于空化的发生, 尤其是驻波场液面的高度不应是声波波长的1/4; 当声频率一定时, 空化泡初始半径越大越有利于空化现象的发生, 但当空化泡的初始半径超过声频率的共振半径时, 由于主Bjerknes力的作用将有更多的空化泡向声压波节移动, 不利于空化的发生.     

可吸入颗粒物在驻波声场中运动的可视化研究

采用可视化实验的方法,拍摄可吸入颗粒在驻波声场中的运动轨迹.根据拍摄结果,分别采用标尺测量、颗粒重力沉降特性和声波夹带特性计算颗粒粒径.实验表明,可吸入颗粒在声团聚室中受到声波夹带、重力作用和粘性力的影响.计算结果说明,利用声波夹带特性可以准确计算颗粒粒径,颗粒的漂移现象是由漂移力引起.     

非线性对大气介质中阵列聚焦声场分布影响的研究

基于时域有限差分算法将大气中近似到二阶微小项的非线性声波波动方程进行离散化,得到了模拟采用的差分波动方程.在此基础上,数值模拟了初始声压强弱不同的5个点声源组成的线阵列垂直或斜向辐射的连续正弦波在大气中传播时二维声场的分布情况.将线性条件下的模拟结果与非线性条件下的模拟结果进行比较后发现:弱非线性会对声场的分布和阵列聚焦增益产生一定的影响,使声场分布波形比线性条件下的声场分布波形更加靠近阵列,聚焦效果变差;强非线性会使波形发生更严重畸变,这是由于产生了基频以外的其他频率声波引起的;非线性对斜向传播时声场分布的影响与垂直传播时的影响效果基本相同,但由于斜向辐射时的声波几何扩展造成的轴向声压衰减要大于垂直辐射时的轴向声压衰减,因此聚焦增益和强非线性的影响都将小于垂直辐射时的情况.     

Modelling of particle paths passing through an ultrasonic standing wave

Within an ultrasonic standing wave particles experience acoustic radiation forces causing agglomeration at the nodal planes of the wave. The technique can be used to agglomerate, suspend, or manipulate particles within a flow. To control agglomeration rate it is important to balance forces on the particles and, in the case where a fluid/particle mix flows across the applied acoustic field, it is also necessary to optimise fluid flow rate. To investigate the acoustic and fluid forces in such a system a particle model has been developed, extending an earlier model used to characterise the 1-dimensional field in a layered resonator. In order to simulate fluid drag forces, CFD software has been used to determine the velocity profile of the fluid/particle mix passing through the acoustic device. The profile is then incorporated into a MATLAB model. Based on particle force components, a numerical approach has been used to determine particle paths. Using particle coordinates, both particle concentration across the fluid channel and concentration through multiple outlets are calculated. Such an approach has been used to analyse the operation of a microfluidic flow-through separator, which uses a half wavelength standing wave across the main channel of the device. This causes particles to converge near the axial plane of the channel, delivering high and low particle concentrated flow through two outlets, respectively. By extending the model to analyse particle separation over a frequency range, it is possible to identify the resonant frequencies of the device and associated separation performance. This approach will also be used to improve the geometric design of the microengineered fluid channels, where the particle model can determine the limiting fluid flow rate for separation to occur, the value of which is then applied to a CFD model of the device geometry.     

细颗粒间相互作用力的研究

对微米、近亚微米尺度的细颗粒间相互作用过程的显微观察发现,细颗粒间具有"吸引-旋绕-排斥"的相互作用行为.受力分析表明,包含范德华力、静电库仑力和电像力的传统颗粒间作用力模型不能解释这种相互作用行为.根据细颗粒的荷电特性,提出细颗粒间还具有诱导偶极子作用力.通过引入偶极子作用力改进了细颗粒间作用力模型,利用新的模型对细颗粒间相互作用进行了模拟,得到了和实验相同的相互作用行为,并且对影响细颗粒相互作用的参数进行了分析.提出投入大颗粒和增加外静电场等都是促进颗粒凝并的有效措施.     

The use of ultrasonic standing waves to enhance optical particle sizing equipment

Fluid dynamics modelling augmented with routines to simulate acoustic forces on aerosol particles has been used to investigate the potential of combining ultrasonic standing wave fields with optical particle analysis equipment. Simulations of particle dynamics in airstreams incorporating acoustic forces predict that particles in the 1-10 microns diameter range may be effectively focused to the velocity nodes of the standing wave field. Particles move to the velocity nodes within tens of milliseconds for acoustic frequencies of 10-100 kHz and at an acoustic energy density of 100 Jm-3. Larger particles are predicted to move to the velocity antinodes within similar times; however, there is a crossover region at approximately 15-20 microns particle diameter where longer times are predicted due to the competing forces driving particles to the vibration node and antinode. With sufficient transverse flow velocities the models predict that disturbances due to acoustic streaming can be overcome and a useful degree of focusing achieved for the aerosol particles. Results from a model demonstrating sampling and acoustic focusing of 3-9 microns aerosol particles to a 200 microns wide analysis area are presented.     

Strategies for single particle manipulation using acoustic and flow fields

Acoustic radiation forces have often been used for the manipulation of large amounts of micrometer sized suspended particles. The nature of acoustic standing wave fields is such that they are present throughout the whole fluidic volume; this means they are well suited to such operations, with all suspended particles reacting at the same time upon exposure. Here, this simultaneous positioning capability is exploited to pre-align particles along the centerline of channels, so that they can successively be removed by means of an external tool for further analysis. This permits a certain degree of automation in single particle manipulation processes to be achieved as initial identification of particles’ location is no longer necessary, rather predetermined. Two research fields in which applications are found have been identified. First, the manipulation of copolymer beads and cells using a microgripper is presented. Then, sample preparation for crystallographic analysis by positioning crystals into a loop using acoustic manipulation and a laminar flow will be presented.     

Single fiber model of particle retention in an acoustically driven porous mesh

A method for the capture of small particles (tens of microns in diameter) from a continuously flowing suspension has recently been reported. This technique relies on a standing acoustic wave resonating in a rectangular chamber filled with a high-porosity mesh. Particles are retained in this chamber via a complex interaction between the acoustic field and the porous mesh. Although the mesh has a pore size two orders of magnitude larger than the particle diameter, collection efficiencies of 90% have been measured. A mathematical model has been developed to understand the experimentally observed phenomena and to be able to predict filtration performance. By examining a small region (a single fiber) of the porous mesh, the model has duplicated several experimental events such as the focusing of particles near an element of the mesh and the levitation of particles within pores. The single-fiber analysis forms the basis of modeling the overall performance of the particle filtration system.     

Ultrasonic particle size fractionation in a moving air stream

Identification of bio-aerosol particles may be enhanced by size sorting before applying analytical techniques. In this paper, the use of ultrasonic acoustic radiation pressure to continuously size fractionate particles in a moving air stream is described. Separate particle-laden and clean air streams are introduced into a channel and merged under laminar flow conditions. An ultrasonic transducer, mounted flush to one wall of the channel, excites a standing ultrasonic wave perpendicular to the flow of the combined air stream. Acoustic radiation forces on the particles cause them to move transverse to the flow direction. Since the radiation force is dependent upon the particle size, larger particles move a greater transverse distance as they pass through the standing wave. The outlet flow is then separated into streams, each containing a range of particle sizes. Experiments were performed with air streams containing glass microspheres with a size distribution from 2-22 μm, using a centerline air stream velocity of approximately 20 cm/s. An electrostatic transducer operating at a nominal frequency of 50 kHz was used to drive an ultrasonic standing wave of 150 dB in pressure amplitude. The microsphere size distributions measured at the outlet were compared with the predictions of a theoretical model. Experiments and theory show reasonable correspondence. The theoretical model also indicates an optimal partitioning of the particle-laden and clean air inlet streams.     

Multiphysics modelling of the separation of suspended particles via frequency ramping of ultrasonic standing waves

A model was developed to determine the local changes of concentration of particles and the formations of bands induced by a standing acoustic wave field subjected to a sawtooth frequency ramping pattern. The mass transport equation was modified to incorporate the effect of acoustic forces on the concentration of particles. This was achieved by balancing the forces acting on particles. The frequency ramping was implemented as a parametric sweep for the time harmonic frequency response in time steps of 0.1 s. The physics phenomena of piezoelectricity, acoustic fields and diffusion of particles were coupled and solved in COMSOL Multiphysics? (COMSOL AB, Stockholm, Sweden) following a three step approach. The first step solves the governing partial differential equations describing the acoustic field by assuming that the pressure field achieves a pseudo steady state. In the second step, the acoustic radiation force is calculated from the pressure field. The final step allows calculating the locally changing concentration of particles as a function of time by solving the modified equation of particle transport. The diffusivity was calculated as function of concentration following the Garg and Ruthven [1] equation which describes the steep increase of diffusivity when the concentration approaches saturation. However, it was found that this steep increase creates numerical instabilities at high voltages (in the piezoelectricity equations) and high initial particle concentration. The model was simplified to a pseudo one-dimensional case due to computation power limitations. The predicted particle distribution calculated with the model is in good agreement with the experimental data as it follows accurately the movement of the bands in the centre of the chamber.     

基于分形理论的驻波声场中颗粒团运动特性数值预测*

基于分形理论,建立驻波声场中颗粒团动力学模型,对颗粒团的夹带系数、相位滞后和漂移系数进行数值预测。将预测结果和实验进行对比,二者吻合良好。在此基础上,研究了组成颗粒团的原生颗粒半径、数目以及排列情况对于颗粒团运动特性参数的影响。结果表明,对于由两个原生颗粒组成的颗粒团,原生颗粒半径越接近,颗粒团与等体积球形颗粒运动特性的差异越大;在分形维数一定时,随着原生颗粒数目的增多,颗粒团的夹带系数减小,相位滞后增加,漂移系数先增大后减小,颗粒团与等体积球形颗粒的动力学行为存在显著差异;原生颗粒排列趋于致密时,颗粒团的夹带系数增大,相位滞后减小,漂移系数发生单调变化,与等体积球形颗粒运动特性的差异缩小。     

The selection of layer thicknesses to control acoustic radiation force profiles in layered resonators

Ultrasonic standing waves can be used to generate radiation forces on particles within a fluid. A number of authors have derived detailed representations of these forces but these are most commonly applied using an approximation to the energy distribution based upon an idealized standing wave within a mode based upon rigid boundaries. An electro-acoustic model of the acoustic energy distribution within a standing wave with arbitrary thickness boundaries has been expanded to model the radiation force on an example particle within the acoustic field. This is used to examine the force profile on a particle at resonances other than those predicted with rigid boundaries, and with pressure nodes at different positions. A simple analytical method for predicting modal conditions for combinations of frequencies and layer thickness characteristics is presented, which predicts that resonances can exist that will produce a pressure node at arbitrary positions in the fluid layer of such a system. This can be used to design resonators that will drive particles to positions other than the center of the fluid layer, including the fluid/solid boundary of the layer, with significant potential applications in sensing systems. Further, the model also predicts conditions for multiple subwavelength resonances within the fluid layer of a single resonator, each resonance having different nodal planes for particle concentration.     

Mode-switching: A new technique for electronically varying the agglomeration position in an acoustic particle manipulator

Acoustic radiation forces offer a means of manipulating particles within a fluid. Much interest in recent years has focussed on the use of radiation forces in microfluidic (or “lab on a chip”) devices. Such devices are well matched to the use of ultrasonic standing waves in which the resonant dimensions of the chamber are smaller than the ultrasonic wavelength in use. However, such devices have typically been limited to moving particles to one or two predetermined planes, whose positions are determined by acoustic pressure nodes/anti-nodes set up in the ultrasonic standing wave. In most cases devices have been designed to move particles to either the centre or (more recently) the side of a flow channel using ultrasonic frequencies that produce a half or quarter wavelength over the channel, respectively.It is demonstrated here that by rapidly switching back and forth between half and quarter wavelength frequencies - mode-switching - a new agglomeration position is established that permits beads to be brought to any arbitrary point between the half and quarter-wave nodes. This new agglomeration position is effectively a position of stable equilibrium. This has many potential applications, particularly in cell sorting and manipulation. It should also enable precise control of agglomeration position to be maintained regardless of manufacturing tolerances, temperature variations, fluid medium characteristics and particle concentration.