多毛细管电雾化喷洒模式及稳定喷洒特征研究

通过采用传统的金属毛细管搭建了多喷管电雾化实验装置.初步获得了多管道电雾化喷洒的各种模式.并比较了乙醇在单、多管道条件下"锥-射流"模式喷洒的两个重要指标:稳定喷洒的起始电压和电流-流量关系.尽管结果显示由于加工/装配过程中的差异,各管道之间的喷洒状态会存在一定差异;但两种条件下的锥-射流喷洒模式服从相似规律:(1)稳定喷洒起始电压(V_C)与Taylor锥半锥角(θ)余弦的1/2次方成正比;(2)多管电雾化总电流(I)与流量(Q)的1/2次方成正比.它表明在常规尺度下,稳定电雾化喷洒射流的主要影响因素为锥-射流过渡区内液体界面上的极化电荷,而外界工作电场分布对稳定喷洒雾化效果影响较小.     

多毛细管稳定喷洒电雾化特征研究(一)——实验设备设计及粒径测量方法

主要对多毛细管电雾化装置开展了数值模拟和实验研究.根据线性排列单排管道电雾化现象及相应静电场的数值分析结果,初步分析了多管道条件下电场强度分布对稳定雾化的影响.进一步根据以上结果,对单排管道进行优化设计,并推广到双排电雾化毛细管.通过采用这些设计,获得了稳定的多管道电雾化射流,且雾化模式稳定可靠,可重复性较高.在此基础...     

静电雾化锥射流模式下液锥形态的研究

本文分析了静电雾化锥射流模式下液锥表面静电应力、表面张力应力分布特性,基于应力平衡建立了液锥力学模型,并对流量、荷电电压及针形喷嘴的内半径等参数对液锥结构形态的影响进行了预测。首先设计了针形喷嘴静电雾化实验装置,应用高速摄影技术观测了静电雾化的典型雾化模式和液锥形态演化特性。实验结果表明:锥射流雾化模式仅在一定的荷电电压范围内才会出现;针形喷嘴的流量增加,液锥锥角减小,液锥长度增长;随着荷电电压或针形喷嘴内半径的增加,液锥锥角增大,液锥长度缩短。实验结果与液锥力学模型的预测结果一致。     

电雾化装置及雾化模型研究

本文介绍了新研制的电雾化实验装置及其测试系统。一系列实验表明该系统运行可靠。在该装置上进行的实验表明,电雾化现象与溶液电导率、介电常数、表面张力、粘性以及电压、流量有关。电雾化现象中存在“滴”模型、“脉动”模型、“锥－射流”模型、“不稳定”模型等几种不同的流动状态。而在“锥－射流”模型时,溶液具有单分散的尺度分布。本文还介绍了酒精溶液在民雾化过程中出现“锥－射流”现象的区域。     

等离子体激励器诱导射流的湍流特性研究

为了进一步掌握等离子体流动控制机理, 完善等离子体激励器数学模型, 提升等离子体激励器扰动能力, 采用粒子图像测速技术, 在静止空气下开展了介质阻挡放电等离子体激励器诱导射流特性研究. 实验时, 将非对称布局激励器布置在平板模型上, 随后将带有激励器的模型放置在有机玻璃箱内, 从而避免环境气流对测试结果的影响. 基于激励器诱导流场, 分析了激励电压对诱导射流特性的影响, 揭示了较高电压下诱导射流近壁区的拟序结构, 获得了卷起涡、二次涡等拟序结构的演化发展过程, 计算了卷起涡脱落频率, 阐述了卷起涡与启动涡的区别, 初步探索了卷起涡的耗散机制. 结果表明: (1)层流射流不能完全概括等离子体诱导射流特性, 激励电压是影响射流特性的重要参数. 当电压较低时, 诱导射流为层流射流; 当电压较高时, 诱导射流的雷诺数提高, 射流剪切层不稳定, 层流射流逐渐发展为湍流射流. (2)等离子体诱导湍流射流包含着卷起涡、二次涡等拟序结构; 在固定电压下, 这些涡结构存在恒定的卷起频率. (3)当激励电压较高时, 流动不稳定使得卷起涡发生了拉伸、变形, 引起了流场湍动能增大, 从而加速了卷起涡的耗散. 研究结果为全面认识激励器射流特性, 进一步挖掘激励器卷吸掺混能力, 提升激励器控制能力积累基础.      

电场作用下流动聚焦的实验研究

通过在流动聚焦的同轴液-气射流区域施加电场, 开展了电场力和气动力共同作用下锥形以及带电射流的不稳定性特性实验研究. 实验在精密设计的流动聚焦装置上完成, 分析了外部电压、气体压力降和液体流量等主要控制参数对流动聚焦过程的影响, 获得了锥形的振动模式和稳定模式及其之间的转换, 得到了射流的滴模式、轴对称模式、共存模式和非轴对称模式及其转换并定量分析了电场对射流尺寸参数的影响. 结果表明, 相比于单一的流动聚焦, 该方法能够增强锥形的稳定性, 促进液体射流雾化, 减小颗粒的直径, 因此在科技领域和工程实际中具有重要的应用价值.      

基于正交设计方法的双锥罩结构优化设计

设计了一种双锥药型罩,借助数值模拟和实验手段研究其成形及侵彻机理,利用正交设计方法研 究不同结构的射流成形性能。结果显示:该双锥罩在爆轰波作用下形成高速射流、翻转弹丸和杵体;小锥角 2 显著影响射流头部速度;罩厚t及大锥角2 高度显著性影响弹丸速度;最佳设计参数为:t=0.14cm、2= 50、2=135、小锥角与大锥角罩口直径比N =0.4或0.5,其射流速度分别为6613、6839m/s,弹丸速度分 别为2247、2095m/s,侵彻钢靶深度分别为8.24、8.31cm,开口孔径分别为2.12、2.08cm。最终优化的双 锥罩结构合理,既保证了射流速度,又大大提高了弹丸速度,射流和弹丸先后侵彻目标,达到双重毁伤的目的; 在低密度装药和小炸高条件下侵彻效果较理想。     

等离子体激励器诱导射流的湍流特性研究

为了进一步掌握等离子体流动控制机理,完善等离子体激励器数学模型,提升等离子体激励器扰动能力,采用粒子图像测速技术,在静止空气下开展了介质阻挡放电等离子体激励器诱导射流特性研究.实验时,将非对称布局激励器布置在平板模型上,随后将带有激励器的模型放置在有机玻璃箱内,从而避免环境气流对测试结果的影响.基于激励器诱导流场,分析了激励电压对诱导射流特性的影响,揭示了较高电压下诱导射流近壁区的拟序结构,获得了卷起涡、二次涡等拟序结构的演化发展过程,计算了卷起涡脱落频率,阐述了卷起涡与启动涡的区别,初步探索了卷起涡的耗散机制.结果表明:(1)层流射流不能完全概括等离子体诱导射流特性,激励电压是影响射流特性的重要参数.当电压较低时,诱导射流为层流射流;当电压较高时,诱导射流的雷诺数提高,射流剪切层不稳定,层流射流逐渐发展为湍流射流.(2)等离子体诱导湍流射流包含着卷起涡、二次涡等拟序结构;在固定电压下,这些涡结构存在恒定的卷起频率.(3)当激励电压较高时,流动不稳定使得卷起涡发生了拉伸、变形,引起了流场湍动能增大,从而加速了卷起涡的耗散.研究结果为全面认识激励器射流特性,进一步挖掘激励器卷吸掺混能力,提升激励器控制能力积累基础.     

水下爆炸冲击凹陷液面诱导射流研究

利用液滴坠落冲击直管中水平液面产生半球形凹陷,并以此凹陷液面作为初始界面进行水下爆炸冲击诱导射流的实验研究。以高速摄影为主要手段,结合Fluent数值模拟,揭示了凹陷液面在水下爆炸冲击作用下的变形过程和机理。实验结果表明,随着爆炸的发生,液面凹陷中心会汇聚形成纤细光滑的射流,同时在管壁附近会产生附加环状射流,这明显区别于冲击直管中水平液面诱导射流的现象。进一步研究发现,中心射流的产生主要源于液面凹陷对爆炸能量的汇聚作用,而附加射流的产生受到液面初始形状和管壁剪切阻力的共同影响,二者经历短暂的加速过程之后均以一个近似恒定的速度向上抬升。通过考察能量对射流的影响发现,中心射流与附加射流的速度均与充电电压(爆炸能量的1/2次方)呈线性正相关;中心射流形态特征基本不变,附加射流则随能量的变化呈现不同的形态。     

气流作用下同轴带电射流的不稳定性研究

通过对气体驱动同轴电流动聚焦的实验模型进行简化,开展了电场力和惯性力共同作用下同轴带电射流的不稳定性理论研究.在流动为无黏、不可压缩、无旋的假设下,建立了三层流体带电射流物理模型并得到了扰动在时间域内发展演化的解析形式色散关系,利用正则模方法求解色散方程发现了流动的不稳定模态,进而分析了主要控制参数对不稳定模态的影响.结果表明,在参考状态下轴对称模态的最不稳定增长率最大,因此轴对称扰动控制整个流场.外层气流速度越高,气体惯性力越大,射流的界面越容易失稳.内外层液-液同轴射流之间的速度差越大,射流越不稳定.表面张力对射流不稳定性起到促进作用.轴向电场对射流不稳定性具有双重影响:当加载电场强度较小时,射流不稳定性被抑制;当施加电压大于某一临界值时,轴向电场会促进射流失稳.临界电压的大小与界面上自由电荷密度和射流表面扰动发展关系密切.这些结果与已有的实验现象吻合,能够对实验的过程控制提供理论指导.     

Droplet motion in an electrohydrodynamic fine spray

The results of a combined experimental and numerical study on droplet behavior within an electrohydrodynamic fine spray are presented. The fine spray exists in the transition region between the multiple cone-jet and rim emission spray modes. Experiments were conducted specifically to characterize the motion of droplets within the spray. Light-sheet visualizations and measurements of droplet speed and velocity using laser-based, single-particle counters were obtained. Additionally, a numerical simulation of the droplet motion within the spray was made and compared to the experimental results. The electrohydrodynamic fine spray of ethanol droplets ( 1 to 40 m diameter) was generated using a typical capillary-plate configuration, with a capillary tip electric field intensity of 10⁶ V/m and a spray charge density of 70 C/m³. Acquired images of the spray revealed a zone of rapid expansion near the capillary followed by a more gradual expansion farther from the capillary. In situ laser-diagnostic measurements confirmed these observations. Measured droplet speeds decreased rapidly with increasing axial distance from the capillary, but then increased beyond the spray's axial mid-plane as a result of a change in the sign of the axial internal electric field. Droplet axial velocity components behaved similarly. The radial velocity components exhibited a maximum value off of the spray's centerline in the near-capillary region. Farther away from the capillary, they increased monotonically with increasing radial position. These trends identified the significant role that the radial internal electric field plays in spray expansion. The numerical simulation of the normal spray verified the inferred change in sign of the axial internal field and underscored the dominant contribution of the external electric field in the near-capillary region and of the internal electric field farther away.     

Droplet size and velocity characteristics of water-air impinging jet atomizer

A water-air impinging jets atomizer is investigated in this study, which consists of flow visualization using high speed photography and mean droplet size and velocity distribution measurements of the spray using Phase Doppler Anemometry (PDA). Topological structures and break up details of the generated spray in the far and near fields are presented with and without air jet and for an impinging angle of 90°. Spray angle increases with the water jet velocity, air flow rate and impinging angle. PDA results indicate that droplet size is smallest in the spray center, with minimum value of Sauter mean diameter (SMD) of 50 µm at the air flow rate of Q_m = 13.50 g/min. SMD of droplets increases towards the spray outer region gradually to about 120 µm. The mean droplet velocity component W along the air-jet axis is highest in the spray center and decreases gradually with increasing distance from the spray center. SMD normalized by the air nozzle diameter is found firstly to decrease with gas-to-liquid mass ratio (GLR) and air-to-liquid momentum ratio (ALMR) and then remain almost constant. Its increasing with aerodynamic Weber number indicates an exponential variation. The study sheds light on the performance of water-air impinging jets atomizers providing useful information for future CFD simulation works.     

On simulations investigating droplet diameter–charge distributions in electrostatically atomized dielectric liquid sprays

A general procedure has been developed for the simulation of charged liquid and electrostatically atomized sprays. The procedure follows a Lagrangian approach for simulation of spray droplets and a Eulerian approach for gas‐phase variables, including the electric field generated by the charge presence on droplets. Validation of the procedure was examined through simulations of previously published charged spray experiments. Results showed that for the specification of initial droplet charge, modelling the droplet charge–diameter relationship through a scaling law is as reliable a method as using a directly obtained charge–diameter relationship from experimental measurements. The normalized root‐mean‐square errors for sprays using the two methods were shown to be within 12% of one another, for the prediction of spatially averaged profiles of mean droplet diameters, mean axial velocities and mean radial droplet velocities. Results showed that the general spatial characteristics and dynamics of a charged liquid spray can successfully be reproduced, including the axial and radial dispersal pattern of droplets and the distribution of mean droplet diameters throughout the spray plume. For all sprays with droplet charges defined through a scaling law relationship, the normalized root‐mean‐square errors range from 9.0% to 31.6% for mean droplet diameters, 10.4% to 67.9% for mean axial droplet velocities and 16.8% to 38.6% for mean radial droplet velocities. Lastly, we present a brief set of general recommendations for simulating electrostatically atomized dielectric liquid sprays.Copyright © 2013 John Wiley & Sons, Ltd.     

A STUDY ON DROPLET VELOCITY OF ETHANOL DURING ELECTROSPRAYING PROCESS AT CONE-JET MODE

研究液滴在静电喷雾下的速度特性是理解喷雾形态的形成及演化的关键.结合锥射流模式下乙醇静电喷雾实验数据,建立了静电喷雾二维轴对称模型.基于离散相液滴运动方程、连续相空气运动方程、电场方程以及用户自定义函数,进行了数值求解,获得了锥射流模式下的乙醇静电喷雾形态、空间电场分布以及液滴速度场分布.考虑了不同空气入口流速的影响,得到了乙醇/空气同轴射流静电喷雾形态的变化规律.结果表明,喷雾外围液滴与空气流场有较强的相互作用,导致喷雾中轴线附近的液滴速度分布变化较小,而在喷雾外围处的液滴速度分布沿径向剧烈变化;随着空气入口速度的增大,乙醇/空气同轴射流静电喷雾形态先趋于发散,当空气入口速度大于喷雾外围液滴轴向速度时,喷雾形态则趋于聚拢.因此,除改变施加电压、液体流量和电极结构外,通过控制空气入口速度来影响喷雾液滴速度场,也可获得不同的静电喷雾效果.     

Spray Penetration in a Turbulent Flow

Analytical expressions for mass concentration of liquid fuel in a spray are derived taking into account the effects of gas turbulence, and assuming that the influence of droplets on gas is small (intitial stage of spray development). Beyond a certain distance the spray is expected to be fully dispersed. This distance is identified with the maximum spray penetration. Then the influence of turbulence on the spray stopping distance is discussed and the rms spray penetration is computed from a trajectory (Lagrangian) approach. Finally, the problem of spray penetration is investigated in a homogeneous two-phase flow regime taking into account the dispersion of spray away from its axis. It is predicted that for realistic values of spray parameters the spray penetration at large distances from the nozzle is expected to be proportional to t ^2/3 (in the case when this dispersion is not taken into account this distance is proportional to t ^1/2). The t ^2/3 law is supported by experimental observations for a high pressure injector. This revised version was published online in July 2006 with corrections to the Cover Date.     

Droplet deformation of a strongly shear thinning dense suspension of polymeric micelles

We investigated the deformation of a strong shear thinning droplet undergoing simple shear flow in a Newtonian liquid. The droplet was an aqueous solution of poly(ethylene oxide) end capped with an alkyl group that forms spherical micelles in aqueous solution. At high concentrations and below a critical temperature, the jammed micelles showed strong shear thinning behaviour, and neither a yield stress nor a Newtonian viscosity was observed. At small shear rates, the droplet rotated and aligned in the flow, but did not deform or only very weakly. At high shear rates, the droplet deformation increased with increasing shear rate. The deformed droplet did not relax after the shear was stopped except for a modest rounding of the edges. For each shear rate, an apparent viscosity, η _ad, of the equivalent Newtonian droplet was calculated assuming affine deformation. η _ad showed a power law dependence on the capillary number Ca with an exponent of − 1.8 and was larger than the shear viscosity of the micelle suspension at the same shear rates. The results were explained by the existence of a strong gradient of the viscosity inside the droplet leading to a very low viscosity fluid layer near the droplet/matrix interface.     

Excess thermal noise and low-frequency oscillations during capillary flow of aqueous solutions of poly(ethylene oxide)

The excess thermal noise generated in polymer solutions through narrow capillaries is studied in detail for aqueous solutions of poly(ethylene oxide), , of varying concentration. With increasing flow rate, the excess noise level increases, the noise spectrum assuming a 1/f -form with 1.5. Within a critical flow range, distinct peaks appear in the spectrum, their frequencies being multiples of a fundamental frequency. The latter frequency (f ₀) is found to increase with the flow rate; this variation, as well as that brought about by varying concentration and capillary dimensions, can be accommodated in a single curve correlatingf ₀ with the shear rate at the capillary wall. No such correlation was found for the total noise level. The value off ₀ appeared to be determined by transversal oscillations of the liquid stream entering the capillary. Addition of small amounts of silica particles (Aerosil) led to the disappearance of the peaks in the spectrum.     

Unstable capillary flow of reinforced polymer melts

The injection capillary flow of various unfilled and glass fibre or calcium carbonate filled polypropylene and nylon 6.6 melts is studied using either a single capillary of five capillaries in series, separated by small reservoirs. Only unfilled nylon 6.6 yields instability during flow through a single capillary due to mechanochemical degradation in the capillary at extremely high shear rates above 5 × 10⁵ s^?1. It is found that only short glass fibre reinforced polypropylene yields high frequency oscillations in the reservoir pressure and extrudate diameter and has discontinuity in the flow curve when the apparent shear rate is above 4 × 10⁵ s^?1 and the flow is through multiple capillaries. Further increase in the shear rate restores the stable flow. The intensity of the oscillations and the range of shear rate during which unstable flow occurs are increased with increasing melt temperature. The mechanism of this unstable flow is investigated by studying fibre orientation at the capillary entrance and exit using mouldings simulating capillary entry-exit flows.     

Dynamics of turbulent air-flow in droplet driven sprays

This paper presents a study of the fundamental mechanics of droplet and gas motion in sprays. Only vertical sprays are considered and our theoretical analysis identifies two main flow zones, corresponding to where the droplet velocity is much greater than or of the same order as the induced air velocity. Analytical asymptotic results for the induced air velocity for small and large downstream distances confirmed a full numerical calculation and also agreed with experimental results. The second half of the paper deals with some of the most important aspects of spray jets in a cross flow. We find that the ratio of the cross-wind speed to the induced air jet speedU ₀/V _j is a crucial factor for specifying the dynamical behaviour. We present results for an axi-symmetric spray in uniform cross flow for bothweak andstrong cross-winds.