EHD atomization characteristics of a point-plate system with a porous point electrode

An electrohydrodynamic (EHD) atomization from a point-to-plate system, with a wet porous point as a corona electrode, has been studied. And the atomized water droplets from the wet porous point, as well as the water droplet traces, the water droplet charge-to-mass ratios, and the water droplet number concentrations, were investigated. It was observed that the wet porous point can atomize abundant amounts of water droplet, 2.8, 2.6 and 2.2 mg/min for negative, AC and positive corona, respectively. The migrated water droplet traces were observed. The positive wet porous point atomized very fine water droplets than those obtained with the negative wet porous point. Moreover, the water droplets atomized from the AC corona showed granular-like larger traces. A weak corona discharge can atomize water droplets very effectively. On the other hand, an intensive corona discharge can eject more water droplets. As a result with the wet porous point, the maximum corona-current-based and corona-power-based water droplet atomization yields of Y_C = 3.34, 3.32 and 3.25 μg/μAs and Y_P = 0.35, 0.40 and 0.27 mg/Ws have been obtained for the negative, AC and positive corona discharges.     

Negative corona discharge from a water droplet under the pulsating DC field

Corona discharge from a fine water droplet always involves deformation of the droplet shape or Taylor-cone formation, emission of fine water jets or disruption of droplet. Therefore, corona discharge from a water droplet always manifests complicated aspects. In addition, disruption of Taylor cone simultaneously affects not only discharge current but also motion of water droplet. To confirm corona discharge phenomena from a water droplet protruded from a tip of a metal capillary tube with a diameter of 1 mm, negative corona discharge was investigated by using a water droplet located at a tip of grounded rod electrode facing a ring electrode with positive dc voltage superimposed by ac one. Since the droplet has inherent resonant vibrating frequency defined by the size or volume, the volume of water droplet was adjusted at 20 nL where the corresponding resonant frequency was 500 Hz. The period of the event of successive corona discharge is exactly consistent with resonant frequency defined by the size of the water droplet. As a result, corona pulse trains with a definite duration appeared intermittently corresponding to its resonant vibration. When dc voltage superimposed by ac voltage with resonant frequency of 500 Hz was applied to the water droplet, corona pulse trains appeared at the period corresponding to the frequency. The maximum value of corona current reasonably increased with the applied voltage. Even when the frequency of ac field superimposed on dc field was varied from the resonant frequency, corona pulse trains occur corresponding to not only the superimposed field frequency but also resonant frequency.     

Numerical study on the desorption processes of oil droplets inside oil-contaminated sand under cavitation micro-jets

The removal of the adsorbed oil droplet is critical to deoiling treatment of oil-bearing solid waste. Ultrasonic cavitation is regarded as an extremely useful method to assist the oil droplets desorption in the deoiling treatment. In this paper, the effects of cavitation micro-jets on the oil droplets desorption were studied. The adsorbed states of oil droplets in the oil-contaminated sand were investigated using a microscope. Three representative absorbed states of the oil droplets can be summarized as: (1) the individual oil droplet adsorbed on the particle surface (2) the clustered oil droplets adsorbed on the particle surface; (3) the oil droplet adsorbed in a gap between particles. The micro-jet generation during the bubble collapse near a rigid wall under different acoustic pressure amplitudes at an ultrasonic frequency of 20 kHz was investigated numerically. The desorption processes of the oil droplets at the three representative absorbed states under micro-jets were also simulated subsequently. The results showed that the acoustic pressure has a great influence on the velocity of micro-jet, and the initial diameter of cavitation bubbles is significant for the cross-sectional area of micro-jets. The wall jet caused by a micro-jet impacting on the solid wall is the most important factor for the removal of the absorbed oil droplets. The oil droplet is broken by the jet impinging, and then it breaks away from the solid wall due to the shear force generated by the wall jet. In addition to a higher sound pressure, the cavitation bubble at a larger initial diameter is more important for the desorption of the clustered oil droplets. Conversely, the micro-jet generated by the cavitation bubble at a smaller initial diameter (0.1 mm) is more appropriate for the desorption of the oil droplet in a narrow or sharp-angled gap.     

Visualization of aggregation process of dispersed water droplets and the effect of aggregation on secondary atomization of emulsified fuel droplets

The effect of aggregation of dispersed water droplets on secondary atomization of emulsified fuel droplets in a heating process was investigated. Secondary atomization was observed using a single droplet experiment in which a water-in-oil (W/O) emulsified fuel droplet prepared using colored water was heated by a halogen heater. The initial diameter of dispersed water droplets before heating was controlled, and the change in the diameter of dispersed water droplets was measured by image analysis. As a result, the aggregation process of dispersed water droplets in the heating process was successfully visualized. The dispersed water droplet diameter increased with an increase in W/O emulsified fuel droplet temperature. The occurrence probability of micro-explosion increased with an increase in the dispersed water droplet diameter in emulsified fuel droplets. It is suggested that the occurrence probability of micro-explosion can be increased by accelerating the aggregation and coalescence of dispersed water droplets below 430 K, which is the average temperature of the starting point of puffing.     

Novel low voltage EHD spray nozzle for atomization of water in the cone jet mode

Due to the high surface tension and high conductivity, water is unsuitable for electrohydrodynamic (EHD) atomization using a DC electric field in air. The high local electric field, that is required to atomize water, is likely to generate corona discharge and consequently destabilize the atomization process. This study describes a novel low voltage EHD spray nozzle that can be used to atomize water and weak saline solutions in the stable cone jet mode. The properties of the atomization have been investigated together with the generated droplet size distribution. The nozzle operates at very low flow rates (0.5–4.0 μl/min). Due to the high dielectric constant of water and the low flow rate, the atomization takes place outside the applicability range of the scaling laws. The experimental results show that the droplet size is approximately constant when the flow rate is increased from 0.5 to 4.0 μl/min. The atomization of water was numerically simulated using computational fluid dynamics (CFD). The simulation results agree reasonably well with the experimental results with respect to the liquid cone shape and droplet size.     

Effects of geometric parameters and electric indexes on performance of a vertical wet electrostatic precipitator

A single-stage, single-wire vertical wet electrostatic precipitator was designed and operated in air–water droplets flow to investigate its performance. The efficiency was compared with a glass micro fiber filter and proposed semi-empirical efficiency model, which was in good accuracy while considering the vapor content. Effects of geometric parameters on efficiency under different charge conditions were discussed. Due to evaporation mechanism, the corona current decreases for high flow rates at the same applied voltage. Findings indicated while developing flow is created inside the ESP, there exists an optimum wire-to-flow inlet spacing that provides maximum droplet collection efficiency.     

基于机器视觉的细水雾液滴尺寸测量与分析

为了满足科研与工程中对细水雾液滴尺寸测量的高精度低成本要求,对雾滴尺寸的机器视觉测量方法进行了深入研究.在自行建立的高压喷雾系统与雾滴采集装置上对细水雾液滴进行了采样,用显微镜及其CCD相机对雾滴样本进行了图像采集,用图像处理软件对采集的雾滴图像进行了处理与分析,测量并统计了5966个雾滴,得到了雾滴尺寸的频谱分布和累积分布以及雾滴平均直径和特征直径,将测量结果与相位多普勒粒子分析仪(PDPA)的测量结果进行了比较.结果表明,机器视觉方法町测量的最小雾滴直径约4.39 μm;机器视觉测量结果与PDPA测最结果相当接近,两种方法测得的细水雾液滴平均直径和特征直径的相对误差均在5%以内,雾滴尺寸均匀度指数的相对误差为0.27%.     

非晶态合金表面的水润湿动力学

采用分子动力学模拟方法研究了改进的Simple pointcharge模型SPC/E水滴在Cu_(50)Zr_(50)非晶薄膜上的润湿行为和铺展过程.通过与CuZr(110)和(100)晶面对比研究发现,水滴在Cu_(50)Zr_(50)非晶薄膜表面上表现出较高的铺展速度.水滴在非晶合金表面的铺展过程中形成了明显的吸附层;而在晶态表面,水滴铺展前沿呈脚状形态.分析结果表明非晶表面的水分子在吸附层内呈现完全无序的单层排列方式,而在晶态表面,特别是(100)晶面,吸附层水分子呈双层有序排列.这种吸附层结构的差异导致了吸附层内水分子方向的差异:非晶表面吸附层内水分子方向倾向平行于表面,而晶态基底上吸附层内的水分子倾向于指向液滴内部.由此造成了非晶表面吸附层中的水分子与液滴内部以相对较弱的氢键相互作用,这使得上层水分子能够较容易扩散至吸附层前沿,促进液滴铺展.     

低温光滑壁面上水滴撞击结冰行为

采用高速摄像技术测试低温光滑壁面上水滴撞击结冰过程, 分析了撞击速度、壁面温度和材料热导率对水滴撞击铺展、振荡及结冰行为的影响规律. 结果表明, 低温壁面造成水滴最大铺展直径缩小, 且结冰时间随温度降低而缩短; 当撞击We数提高时, 水滴最大铺展直径增大, 而振荡和结冰时间减小; 同时材料热导率越高, 最大铺展直径越小, 结冰越迅速. 另外, 从热力学角度推导出水滴撞击结冰时间的理论公式, 预测误差<5.3%.     

过冷水滴碰撞结冰的实验与模拟研究

以宏观结冰/霜过程中过冷水滴的碰撞结冰现象为背景,实验对比了亲水和超疏水表面上常温水滴碰撞、常温水滴碰撞结冰和过冷水滴碰撞结冰的过程,建立了过冷水滴碰撞结冰过程的数值模型,研究了We数和接触角对碰撞结冰的影响。结果表明:相比于常温水滴的碰撞及其碰撞结冰过程,过冷水滴碰撞结冰过程的稳态铺展系数更大;随着过冷度和We数的增大以及接触角的减小,过冷水滴的碰撞结冰与常温水滴的碰撞在水滴形态和铺展系数上的差异逐渐增大。     

Multi-nozzle electrospray system for gas cleaning processes

A multi-nozzle electrospray system was developed as a charged droplet source for cleaning a gas contaminated with fine particles. The efficiency of removal of fine particles from the gas can be significantly increased, as compared to uncharged sprays, when the droplets are electrically charged. In the presented experiments, the spray of the droplets of size lower than 100 μm was charged either positively or negatively. Cigarette smoke was used as a source of submicrometer particles. The suppression of the particle concentration was determined after different time intervals of spraying of water. Further improvement in gas cleaning was obtained after charging the smoke particles using a specially designed corona charger. The efficiency of the cleaning process was similar to that obtained for droplets generated by mechanical atomisers with induction charging, but the electrospraying allowed decreasing the water consumption up to about three times.     

液滴撞击加热壁面传热实验研究

本文采用高速摄像仪对水滴和乙醇液滴撞击加热壁面后的蒸发过程进行了实验观测, 分析了液滴撞击加热壁面后的蒸发特性参数. 实验中, 两种液体初始温度均为20 ℃, 不锈钢壁面初始温度范围为68-126℃. 水滴初始直径为2.07 mm, 撞击壁面时Weber 数为2-44; 乙醇液滴初始直径为1.64 mm, Weber数为3-88. 结果表明, 液滴受到重力、表面张力及流动性的影响, 在蒸发过程的大部分时间内, 水滴高度持续降低而接触直径几乎不变; 蒸发后期, 液滴发生回缩, 水滴的接触直径、高度和接触角出现振荡现象. 乙醇液滴的接触角随时间的增加呈现先减小随后保持不变的趋势, 而接触直径和高度则持续减小, 直到液滴完全蒸发. 液滴蒸发总时长与液体物性和壁面温度有关, 随壁面温度的升高而减小, 与液滴撞击壁面时的Weber 数无关. 同时, 随着壁面温度的升高, 液滴显热部分占总换热量的比重增大, 显热部分能量不可忽略, 本文实验条件下得到水滴的平均热流密度为0.014-0.110 W·mm^-2.     

Numerical assessment of ultrasound supported coalescence of water droplets in crude oil

In this study, a numerical assessment of the coalescence of binary water droplets in water-in-oil emulsion was conducted. The investigation addressed the effect of various parameters on the acoustic pressure and coalescence time of water droplets in oil phase. These include transducer material, initial droplet diameter (0.05–0.2 in), interfacial tension (0.012–0.082 N/m), dynamic viscosity (10.6–530 mPas), temperature (20–100 °C), US (ultra sound) frequency (26.04–43.53 kHz) and transducer power (2.5–40 W). The materials assessed are lead zirconate titanate (PZT), lithium niobate (LiNbO₃), zinc oxide (ZnO), aluminum nitride (AlN), polyvinylidene fluoride (PVDF), and barium titanate (BaTiO₃). The numerical simulation of the binary droplet coalescence showed good agreement with experimental data in the literature. The US implementation at a fixed frequency produced enhanced coalescence (t = 5.9–8.5 ms) as compared to gravitational settling (t = 9.8 ms). At different ultrasound (US) frequencies and transducer materials, variation in the acoustic pressure distribution was observed. Possible attenuation of the US waves, and the subsequent inhibitive coalescence effect under various US frequencies and viscosities, were discussed. Moreover, the results showed that the coalescence time reduced across the range of interfacial tensions which was considered. This reduction can be attributed to the fact that lower interfacial tension produces emulsions which are relatively more stable. Hence, at lower interface tension between the water and crude oil, there was more resistance to the coalescence of the water droplets due to their improved emulsion stability. The increment of the Weber number at higher droplet sizes leads to a delay in the recovery of the droplet to spherical forms after their starting deformation. These findings provide significant insights that could aid further developments in demulsification of crude oil emulsions under varying US and emulsion properties.     

Mechanical bioeffects of acoustic droplet vaporization in vessel-mimicking phantoms

This study investigated the mechanical bioeffects exerted by acoustic droplet vaporization (ADV) under different experimental conditions using vessel phantoms with a 200-μm inner diameter but different stiffness for imitating the microvasculature in various tumors. High-speed microscopy, passive cavitation detection, and ultrasound attenuation measurement were conducted to determine the morphological characteristics of vascular damage and clarify the mechanisms by which the damage was initiated and developed. The results show that phantom erosion was initiated under successive ultrasound exposure (2 MHz, 3 cycles) at above 8-MPa peak negative pressures (PNPs) when ADV occurred with inertial cavitation (IC), producing lesions whose morphological characteristics were dependent on the amount of vaporized droplets. Slight injury occurred at droplet concentrations below (2.6 ± 0.2) × 10⁶ droplets/mL, forming shallow and rugged surfaces on both sides of the vessel walls. Increasing the droplet concentration to up to (2.6 ± 0.2) × 10⁷ droplets/mL gradually suppressed the damage on the distal wall, and turned the rugged surface on the proximal wall into tunnels rapidly elongating in the direction opposite to ultrasound propagation. Increasing the PNP did not increase the maximum tunnel depth after the ADV efficiency reached a plateau (about 71.6 ± 2.7% at 10 MPa). Increasing the pulse duration effectively increased the maximum tunnel depth to more than 10 times the diameter of the vessel even though there was no marked enhancement in IC dose. It can be inferred that substantial bubble generation in single ADV events may simultaneously distort the acoustic pressure distribution. The backward ultrasound reinforcement and forward ultrasound shielding relative to the direction of wave propagation augment the propensity of backward erosion. The results of the present work provide information that is valuable for the prevention or utilization of ADV-mediated mechanical bioeffects in clinical applications.     

An experimental study on the motion of water droplets in oil under ultrasonic irradiation

The motion of a single water droplet in oil under ultrasonic irradiation is investigated with high-speed photography in this paper. First, we described the trajectory of water droplet in oil under ultrasonic irradiation. Results indicate that in acoustic field the motion of water droplet subjected to intermittent positive and negative ultrasonic pressure shows obvious quasi-sinusoidal oscillation. Afterwards, the influence of major parameters on the motion characteristics of water droplet was studied, such as acoustic intensity, ultrasonic frequency, continuous phase viscosity, interfacial tension, and droplet diameter, etc. It is found that when the acoustic intensity and frequency are 4.89 W cm⁻² and 20 kHz respectively, which are the critical conditions, the droplet varying from 250 to 300 μm in lower viscous oil has the largest oscillation amplitude and highest oscillation frequency.     

Transient convective burning of interactive fuel droplets in single-layer arrays

The transient convective burning of n-octane droplets interacting within single-layer arrays in a hot gas flow perpendicular to the layer is studied numerically, with considerations of droplet surface regression, deceleration due to the drag of the droplets, internal liquid motion, variable properties, non-uniform liquid temperature and surface tension. Infinite periodic arrays, semi-infinite periodic arrays with one row of droplets (linear array) or two rows of droplets, and finite arrays with nine droplets with centers in a plane are investigated. All arrays are aligned orthogonal to the free stream direction. This paper compares the behavior of semi-infinite periodic arrays and finite arrays with the behavior of previously studied infinite periodic arrays. Furthermore, it identifies the critical values of the initial Damköhler number for bifurcations in flame behavior at various initial droplet spacing for all these arrays. The initial flame shape is either an envelope flame or a wake flame as determined by the initial Damköhler number, the array configuration and the initial droplet spacing. The critical initial Damköhler number separating initial wake flames from initial envelope flames decreases with increasing interaction amongst droplets at intermediate droplet spacing (when the number of rows in the array increases or the initial droplet spacing decreases for a specific number of rows in the array). In the transient process, an initial wake flame has a tendency to develop from a wake flame to an envelope flame, with the moment of wake-to-envelope transition advanced for the increasing interaction amongst droplets at intermediate droplet spacing. For the array with nine droplets with centers in a plane, the droplets at different types of positions have different critical initial Damköhler number and different wake-to-envelope transition time for initial wake flame.     

Observation of droplet motion during flame spread on three-fuel-droplet array with a pendulum suspender

Flame spread on a fuel droplet array has been studied as a simple model of spray combustion. A three-fuel-droplet array with a pendulum suspender was employed to investigate interactions between flame spread and droplet motion in the axial direction. Initial droplet diameter was 0.8 mm, and fuel was n-heptane. A silicon carbide pendulum suspender of 15 μm in diameter and 30 mm in length was used for the third droplet. The first fixed droplet was ignited by electric spark. Behavior of the flame and the third droplet was observed using a high-speed video camera with an image intensifier. Particle tracking velocimetry (PTV) measurements were performed to explain the behavior of the third movable droplet. The dimensionless droplet span, which is the average of droplet-to-droplet distances divided by the average initial diameter of the three droplets, was varied from 2.5 to 8 for observing flame spread, and fixed at 5.5 for PTV measurements. It was observed that the third droplet moved away from the second droplet before the flame spread to the third droplet. The displacement of the third droplet is remarkable when the dimensionless droplet span is close to the limit of flame spread. This implies that the movement of the droplet decreases the dimensionless span of the flame spread limit and the flame spread speed near the flame spread limit. Results of PTV measurements suggest that the heat expansion wave, caused by ignition of the premixture which was accumulated around the second droplet, and the burned gas flow from the second droplet pushed away the third droplet; then natural convection, induced by the flames of the first and second droplets, drew the third droplet to the second droplet. The heat expansion wave and the burned gas flow of the second droplet reached nearly 12 in dimensionless span.     

液滴室内的氢液滴超冷条件下的结晶(英文)

液滴室内超冷现象的存在使得氢微球在自身温度低于其三相点温度的情况下依然处于液态。这可能导致液滴在真空注入过程中发生碎裂。结合液氢的超冷知识, 对液滴室内的氢液滴进行了热力学模拟, 理论上提出了利于氢液滴尽快冷却成固体小丸的最佳实验条件, 即保持液滴室内氦背景气体和氢蒸汽的压强尽可能低, 并将液滴室长度延长到6 cm以上。Because of the existence of supercooling in a droplet formation chamber, the hydrogen micro spheres are still in liquid phase even though their temperature is lower than the triple point temperature. This may cause the droplets to shatter in the vacuum injection capillary. Based on the knowledge about supercooling of liquid hydrogen, we have done a thermodynamic simulation of the droplets in the droplet formation chamber, and theoretically suggested the optimal working conditions under which the droplets will most properly nucleate to solid pellets. The suggested working conditions are that the helium gas pressure and the hydrogen vapor pressure in the droplet formation chamber are kept as low as possible, and the droplet formation chamber should be no less than 6 cm in the length.     

纳米结构表面上部分润湿液滴合并诱导弹跳的理论研究

部分润湿液滴是适宜纳米结构表面上滴状冷凝传热的主要载体,研究纳米结构参数与部分润湿液滴合并弹跳之间的关系有重要意义,本文依据冷凝液滴生长过程中能量增加最小的原理来判断其是否为部分润湿状态,并根据液滴合并前后的体积和界面自由能守恒,确定了合并液滴的初始形状,进而对合并液滴变形过程的动力学方程进行了求解,结果表明:部分润湿冷凝液滴仅在纳米柱具有一定高度、直径间距比较大的表面上形成,而当纳米柱高度过低、直径间距比小于0.1时则形成完全润湿的冷凝液滴;液滴合并后能否弹跳与纳米结构参数紧密相关,仅在纳米柱较高、直径间距比适宜的表面上,部分润湿液滴合并后才能诱发弹跳;液滴尺度及待合并液滴间的尺度比对合并弹跳也有重要影响;多个部分润湿液滴合并后由于具有更多的过剩界面自由能而比两个液滴合并更容易诱发弹跳,本模型对纳米结构表面上冷凝液滴是否合并诱发弹跳的计算结果与绝大部分实测结果相一致,准确率达到95%。     

Monitoring the presence of water and water–sand droplets in a horizontal pipe with Acoustic Emission technology

Monitoring of multiphase flow is a process that has been established over several decades. This paper demonstrates the use of Acoustic Emission (AE) technology to detect and monitor moving water and water–sand droplets in a horizontal pipe. The experimental investigation considered two types of droplets, water and water–sand with average droplet volumes ranging from 1 ml to 5 ml. The experimental findings show good correlation between AE energy, droplet volume and the superficial gas velocity (V_SG).