燃煤细微颗粒声波团聚的机理研究

采用声波团聚疗法促使燃煤电厂产生的细微烟尘颗粒在较高强度下的声场巾团聚为较大颗粒,以此提高除尘器对细微颗粒的脱除效率.建立了以同向凝聚和流体力学作用为机理的烟尘气溶胶声波团聚动力学模型,经过数值模拟求解得到烟尘粒子团聚后的再分布曲线.模拟计算结果与声压(SPL)为130～150 dB,频率为1000～4000 Hz,烟尘浓度为2～10g/m3条件下的团聚实验相比较,得到较好的吻合.当声压SPL增加时,PM10和PM2.5质量百分比相对于初始分布均有明显下降,表明细微颗粒的团聚效果明显.     

颗粒床过滤器除尘特性数值模拟研究

运用CFD和DEM方法,建立三维随机填充固定床颗粒层除尘过滤模型,颗粒之间的接触采用"搭桥法"进行处理。数值计算得到的压降结果与实验值、关联式吻合较好,压降与床层厚度近似呈线性关系。在此基础上,数值研究了除尘效率与颗粒床床层厚度、气体流速、粉尘粒径、气流温度及流动方向的关系。粉尘粒径大于5μm时,增加床层厚度,增大气体流速,均可以有效地增加除尘效率。而粉尘粒径大于5μm时,在圆筒壁面恒热流密度冷却的条件下,气流温度越高,除尘效率越低。流动方向与重力方向相同时,可提高除尘效率。并拟合了分级除尘效率与St数的关系式,当St0.009时,分级除尘效率为一定值,当St≥0.009时,分级除尘效率随着lg(St)的增加呈线性增加的趋势。     

声波团聚技术发展现状及在火灾消烟应用前景*

声波团聚技术在细颗粒物排放控制领域具有很好的应用前景,最近衍生发展为一种全新的火灾烟雾控制手段。该文对声波团聚技术发展现状进行全面总结,介绍近年的实验和理论研究进展。随后,阐述了声波团聚消烟的基本原理,并以火灾烟雾中占比较大的电缆燃烧烟雾为实验对象,发现声波团聚技术在一分钟后就实现对电缆燃烧烟雾的完全消除,即存在突出团聚效果。该结果验证了该技术在火灾烟雾控制领域的可行性。最后,依托我国城市发展现状,探讨了声波团聚技术在火灾消烟的发展前景。     

应用傅里叶红外光谱研究强声波作用下植物壁蛋白质二级结构变化

本论文采用傅里叶红外光研究烟草花粉细胞在不同频率和不同功率的强声波作用下,细胞壁蛋白质二级结构的变化.实验表明,在酰胺Ⅰ和酰胺Ⅱ带都有变化.当频率为400Hz,功率分别为110dB、100dB时,酰胺Ⅰ带β ^⊥折叠向B‖折叠转移,振动态由ν^⊥(π,0)→ν‖(0,0);当功率为90dB,频率分别为800Hz、8000Hz时,酰胺Ⅱ带α 螺旋稍有改变.因此,本工作的结果将有助于我们了解强声波对植物细胞的作用效应.     

强声波作用下煤颗粒周围气体的振荡流动特性

根据二维非稳态层流的质量和动量守恒方程,研究强声波作用下煤颗粒周围气体的振荡流动特性.入射波的振幅远大于颗粒特征长度,声雷诺数小于20.根据通用微分方程的解,详细分析不同声雷诺数与斯特劳哈尔数下,颗粒壁面的流场分布、轴向压力梯度、切向应力及分离角的分布,发现在低频（~50 Hz）时,颗粒壁面轴向压力梯度、切向应力及流动分离角的分布主要受曲率效应影响,其变化规律与振荡速度的幅值变化相对应;在高频时（~5 000 Hz）,颗粒壁面轴向压力梯度、切向应力及流动分离角的分布同时受到曲率效应和流动加速度的影响.为进一步研究强声波强化煤颗粒燃烧提供理论基础.     

飞秒激光加工微悬臂梁薄膜光纤声波传感器

设计并制作了一种光纤微悬臂梁传感器,由于悬臂梁在受迫振动过程中不会产生拉伸变形,与四周固定的圆形密闭薄膜相比,会产生更高的声波灵敏度。采用飞秒激光加工制作微悬臂梁薄膜光纤声波传感器,制备出长宽均为500μm,厚6μm的微悬臂梁结构。通过实验得到其反射光谱对比度为8.8 dB,自由光谱范围为7.72 nm,理论计算得光纤法布里-珀罗腔长为155.6μm。研究结果表明,该光纤声波传感器在2 200 Hz处出现明显的共振峰,对应的声压灵敏度为414 mV/Pa,在300 Hz时有最大的灵敏度675 mV/Pa,与普通硅橡胶薄膜声波传感器相比灵敏度显著提高。理论计算硅橡胶微悬臂梁光纤声波传感器的一阶共振频率为198 Hz,与实验测得的共振频率较为接近。同时悬臂梁传感器的声压灵敏度可达675 mV/Pa,声压响应线性度为0.994。     

换流站电力电容器塔的噪声预测

电力电容器塔是换流站的主要噪声源之一,因单元数量多、声波相干性作用复杂、单元间相互遮挡等,难以对其进行准确的噪声预测。该文通过实验方法获得电容器单元的表面振动加速度,采用边界元模型计算电容器塔辐射的噪声,并运用噪声预测评估软件对电容器塔进行建模,包括完整建模和简化为点声源、线声源及工业建筑物的建模。研究表明：边界元模型和完整建模方法能够考虑到电容器单元的声辐射指向性和单元间的遮挡作用,前者还能考虑到单元内不同壁面处相位差、单元间声波相干性的影响,比后者能较好地反映声场分布特点;简化点声源和线声源的方法预测结果相近,均无指向性且预测的总声功率偏高;简化工业建筑物的方法考虑了电容器塔的声辐射指向性和遮挡作用,方法简单且能较准确地预测总声功率。     

静电除尘器内荷电颗粒放电现象探究

电除尘器是电厂尾气处理的主要设备,粉尘在沉积边界层内的电荷输运过程,如电荷的转移、累积和反电晕现象等,对于电除尘器除尘效率有着显著的影响。本文利用Paschen定律定量分析荷电颗粒的放电条件。计算结果表明,在电除尘系统中通过场致荷电方式获得电荷的粉尘颗粒不足以引起颗粒放电,而颗粒一极板接触后发生电荷转移,导致电荷在颗粒表面局部积累会引起放电现象。     

新型气流声源发声特性及其对气溶胶的团聚 实验研究*

为提高气溶胶颗粒物的声波团聚效果,创新性地设计了一种具有气声分离特性的气流声源。新声源使乏气从侧面开口排出,避免了对团聚效果的影响。通过实验研究了侧面开口尺寸对号角出口处气流量与声压级的影响,发现侧面开口尺寸对乏气排出及声压级有较大影响,当侧面开口尺寸约为6.5 mm时排气效果较好,此时在0.3 MPa驱动压力下声压级可达159.6 dB,同时乏气已基本全部从侧面开口排出。利用该声源对超细液滴气溶胶进行团聚实验,结果显示,声源驱动压力越大,声压级越大,气溶胶受声场作用力越强,颗粒间碰撞几率增大,团聚速率越快;初始气溶胶浓度越高,前期团聚效果越好,透光率增长速度越大,而后期团聚效果则趋近一致。     

声波刺激对铁皮石斛过氧化物酶同工酶基因表达的影响

从基因表达水平探索了植物适应物理应力刺激时可能作出的反应机制。以铁皮石斛组培苗为实验材料,以特定的声波(声强100dB、频率1000Hz)刺激为应力源,检测比较了声波刺激对铁皮石斛过氧化物酶(POD)同工酶酶谱及各酶带百分含量的影响;通过设计引物扩增出铁皮石斛POD基因片段,以此为探针,利用Northern点杂交技术检测分析了声波刺激对铁皮石斛POD基因表达的影响。结果显示,声波刺激没有引起铁皮石斛产生新的酶带,但促进了其POD同工酶酶量的提高;声波刺激对铁皮石斛POD基因表达的影响与其同工酶酶谱及其总RNA量的变化基本一致。表明声波刺激对植物POD同工酶基因表达有激活作用,可能引起植物某些基因转录和表达水平的变化。     

Investigation of the influence of humidity on the ultrasonic agglomeration of submicron particles in diesel exhausts

Removing very fine particles in the 0.01-1 micro m range generated in diesel combustion is important for air pollution abatement because of the impact such particles have on the environment. By forming larger particles, acoustic agglomeration of submicron particles is presented as a promising process for enhancing the efficiency of the current filtration systems for particle removal. Nevertheless, some authors have pointed out that acoustic agglomeration is much more efficient for larger particles than for smaller particles. This paper studies the effect of humidity on the acoustic agglomeration of diesel exhausts particles in the nanometer size range at 21 kHz. For the agglomeration tests, the experimental facility basically consists of a pilot scale plant with a diesel engine, an ultrasonic agglomeration chamber a dilution system, a nozzle atomizer, and an aerosol sampling and measuring station. The effect of the ultrasonic treatment, generated by a linear array of four high-power stepped-plate transducers on fumes at flow rates of 900 Nm(3)/h, was a small reduction in the number concentration of particles at the outlet of the chamber. However, the presence of humidity raised the agglomeration rate by decreasing the number particle concentration by up to 56%. A numerical study of the agglomeration process as a linear combination of the orthokinetic and hydrodynamic agglomeration coefficients resulting from mutual radiation pressure also found that acoustic agglomeration was enhanced by humidity. Both results confirm the benefit of using high-power ultrasound together with humidity to enhance the agglomeration of particles much smaller than 1 micro m.     

Effect of dissolved gases in water on acoustic cavitation and bubble growth rate in 0.83 MHz megasonic of interest to wafer cleaning

Changes in the cavitation intensity of gases dissolved in water, including H₂, N₂, and Ar, have been established in studies of acoustic bubble growth rates under ultrasonic fields. Variations in the acoustic properties of dissolved gases in water affect the cavitation intensity at a high frequency (0.83 MHz) due to changes in the rectified diffusion and bubble coalescence rate. It has been proposed that acoustic bubble growth rates rapidly increase when water contains a gas, such as hydrogen faster single bubble growth due to rectified diffusion, and a higher rate of coalescence under Bjerknes forces. The change of acoustic bubble growth rate in rectified diffusion has an effect on the damping constant and diffusivity of gas at the acoustic bubble and liquid interface. It has been suggested that the coalescence reaction of bubbles under Bjerknes forces is a reaction determined by the compressibility and density of dissolved gas in water associated with sound velocity and density in acoustic bubbles. High acoustic bubble growth rates also contribute to enhanced cavitation effects in terms of dissolved gas in water. On the other hand, when Ar gas dissolves into water under ultrasound field, cavitation behavior was reduced remarkably due to its lower acoustic bubble growth rate. It is shown that change of cavitation intensity in various dissolved gases were verified through cleaning experiments in the single type of cleaning tool such as particle removal and pattern damage based on numerically calculated acoustic bubble growth rates.     

Acoustic pressure losses in woven screen regenerators

In thermoacoustic travelling-wave engines and other Stirling cycle devices, good performance depends on the material of a regenerator being in intimate contact with the gas inside it, so that each particle of gas oscillates in temperature following the adjacent material as it is acoustically displaced. This requires that the passages are small enough for temperature waves to penetrate across the gas path with the frequencies of interest. One type of ‘regenerator’ that is commonly used for this purpose is composed of multiple layers of woven stainless steel mesh, laid on top of one another in random registration. Associated with the thermal penetration is a viscous loss of pressure and this must be quantified if efficient engines are to be designed.In the literature, reliance has been placed on the correlation of steady-flow loss data for these meshes, but for the coarser ones operating at frequencies greater than 28 Hz, the assumption of quasi steady-flow is dubious and direct acoustic measurements must be made. This paper reports acoustic pressure loss data for meshes with 34 and 75 wires per inch taken in two configurations of impedance tube, and finds that the dependence on velocity is the same as in steady-flow, but that there is indeed some enhancement of loss for frequencies above 40 Hz. (Separation of the mesh layers is probably responsible for the anomalously low loss coefficients that were recorded in one set of data.) It is shown that the acoustic pressure losses can be correlated in terms that give the acoustic impedance more directly than the friction factor correlations.     

Agglomeration of sub-micron particles by a non-thermal plasma electrostatic precipitator

Non-thermal Plasma agglomeration is presented as a promising process to reduce the number concentration of sub-micron particles in an acrylic duct, which included a saw-tooth electrode and a wire-plate electrostatic precipitator (ESP). The generated plasma by pulse-energized ESP, the particle agglomerations were controlled under operating conditions such as pulse voltages, pulse frequencies, dust loadings, and gas velocities. When gas velocity increased from 0.5 to 1 m s⁻¹ at 45 kV_p, 20 kHz, it was found that efficiency was increased. At gas velocity of 1 m s⁻¹, the sub-micron particle number reduction efficiency for all particle sizes was over 90% in ESP.     

A new approach to the acoustic conditioning of industrial aerosol emissions

Although the ability of finite-amplitude sound to agglomerate aerosols has been known for some time, the process has enjoyed limited commercial application. It is argued that this has resulted primarily from limitations imposed by high capital cost and high specific power requirements. With these twin criteria as a starting point, a new approach to acoustic dust conditioning is examined, with particular emphasis on economic feasibility. Related air quality criteria and the historical development of acoustic agglomeration are reviewed in moderate depth. Possible advantages of using progressive saw-tooth waves for acoustic agglomeration, when compared with the standing-wave fields conventionally used, are identified. The possible advantages of pulse-jet sound generation also are noted. These two new concepts have been tested in a near full-scale experimental facility, and the results indicate economic feasibility when compared with other gas cleaning systems.     

Broadband high-efficiency controllable asymmetric propagation by pentamode acoustic metasurface

We utilise the pentamode metasurface to realise broadband high-efficiency and controllable asymmetric transmission. The designed metasurface can manipulate the acoustic waves, as expected from the generalised Snell's law, and exhibits unique characteristics such as extraordinary broadband acoustic control, apparent negative refraction, and conversion from the propagating wave to surface mode. The asymmetric transmission features of positive refraction for the forward incidence (FI) and negative refraction for the reverse incidence (RI) can be realised within the range of 2600 Hz to 5600 Hz by controlling the incident angle from 0° to 35° with the transmission efficiency higher than 85.4% for the FI and RI. In addition, by further adjusting the angle of incidence in the range of 25° to 90°, asymmetric transmission characteristics can be expressed as surface wave transmission for the FI and transmitted wave transmission for the RI within the same frequency ranges.     

Analytical Study of Nonlinear Dust Acoustic Waves in Two-Dimensional Dust Plasma with Dust Charge Variation

The nonlinear dust acoustic waves in two-dimensional dust plasma with dust charge variation is analytically investigated by using the formally variable separation approach. New analytical solutions for the governing equation of this system have been obtained for dust acoustic waves in a dust plasma for the first time. We derive exact analytical expressions for the general case of the nonlinear dust acoustic waves in two-dimensional dust plasma with dust charge variation.     

Analytical Study of Nonlinear Dust Acoustic Waves in Two-Dimensional Dust Plasma with Dust Charge Variation

The nonlinear dust acoustic waves in two-dimensional dust plasma with dust charge variation is analytically investigated by using the formally variable separation approach. New analytical solutions for the governing equation of this system have been obtained for dust acoustic waves in a dust plasma for the first time. We derive exact analytical expressions for the general case of the nonlinear dust acoustic waves in two-dimensional dust plasma with dust charge variation.     

Pattern phenomena in an rf discharge dusty plasma system

Various dust patterns are observed in an rf discharge dusty plasma system. According to the dust growth process from small to large in size, the formation of different dust patterns can be divided into two stages: the small-particle stage (or dust cloud stage), and the large-particle stage (or dust crystal stage). The evolution relations between different dust patterns with gas pressure changing are investigated. Dust voids, dust acoustic waves and strong turbulence modes are presented at the small-particle stage. The self-organized dust lattices and dust clusters are investigated at the large-particle stage. The static structure of a dust lattice is characterized by means of the pair correlation function. Dust clusters formed by particles with different numbers and the regular evolution of the clusters with gas pressure are also investigated. The packing sequences of dust clusters are verified through two-dimensional confined molecular dynamics simulations.