气泡雾化喷嘴喷雾平均直径在下游流场中的分布

文利用激光衍射粒度仪对气泡雾化喷嘴下游流场进行了实验研究,主要分析了雾化颗粒直径随径向和轴向距离变化的趋势．由于喷嘴出口处气液两相流型和颗粒自身重量的影响,液雾颗粒沿径向呈现非轴对称分布;而液雾颗粒直径随着轴向距离的增加呈现先减小、后增大的趋势,颗粒直径的减小是大量气泡爆炸的结果,而后的增加则是由于颗粒之间的相互粘结造成的。     

气体中心式离心喷嘴喷雾实验与三维仿真

在实验的基础上, 基于RNG k-ε模型对常压下气体中心式同轴离心(gas-centered swirl coaxial,GCSC)喷嘴喷雾形态和破碎模式进行了三维仿真研究。采用网格自适应加密(adaptive mesh refinement,AMR)技术、耦合水平集和流体体积(coupled level-set and volume of fluid, CLSVOF)方法对气液界面进行捕捉。结果表明, 液体质量流率($\dot{m}_{\mathrm{l}}$)不变, 随着气体质量流率($\dot{m}_{\mathrm{g}}$)的增加, 中心气流的引射作用增强, 液膜内外压差增大, 雾化锥角减小, 并对其流动特性进行了分析; 而$\dot{m}_{\mathrm{g}}$不变时, 液膜在喷嘴出口的径向速度与切向速度随$\dot{m}_{\mathrm{l}}$的增大而增大, 导致雾化锥角增大。同时根据气液质量流率比(gas-liquid mass flow rate,GLR), 将喷雾的破碎模式分为穿孔破碎、气泡破碎和气动破碎。      

自由空间内气泡破碎过程的实验研究

利用高速摄像对自由空间内悬挂气泡的自然破碎过程进行了研究,分析了气泡破碎过程中环形断面的发展规律、膜液滴的形成过程及末期射流的产生原因。气泡成孔后,在液膜表面张力和气泡内部气流喷射的综合作用下,会形成不稳定的环形断面,其铺展过程中边缘会出现诸多不稳定液线,液线末端断裂形成膜液滴,膜液滴飞行方向与断裂点切线方向一致;随着液膜的破碎,半封闭球状液膜的曲率半径逐渐减小,环状断面以一定的加速度推进;在破碎末期,环形断面将在初始破碎点的对称位置附近聚焦并形成射流。表面张力系数和液膜厚度均匀性对膜液滴分布和射流强度具有一定的影响;相对于液膜厚度,作用于液膜单位面积上的表面张力对气泡破碎时间的影响更为显著。     

超声悬浮甲醇液滴的热诱导雾化现象

采用外部高温气体加热的方式研究了超声悬浮甲醇液滴的声致破碎雾化现象,利用高速摄像手段记录并实验研究了不同直径液滴不同的破碎雾化特性.结果表明:超声悬浮液滴在高温气流吹扫后可发生边缘喷射雾化现象.破碎方式包含边缘溅射、液膜割裂和法向溅射三类.直径D_0较小的液滴直接从赤道面开始边缘溅射直至完全雾化;等效直径D_0 2.8 mm的液滴,在产生边缘溅射后,剩余液核弯曲形成液膜,其表面产生法拉第波,使得液膜割裂破碎;而D_0 3.2 mm的液滴,在变形过程中逐渐形成"碗状"空腔,并在其底部的法向方向产生溅射,同时开始液膜割裂的离散化解体过程并伴随着边缘溅射,直至完全雾化.这种声致液滴雾化现象丰富了多物理场耦合作用下的流体理论,可为相关应用研究提供新思路.     

局部沸腾液滴内部气泡动态行为研究

本文采用微加热器对液滴进行局部加热,并对其蒸发沸腾现象进行了可视化研究。液滴局部加热后产生局部沸腾现象,内部生成单气泡,气泡附着在加热基板上,持续生长,当达到某个临界点气泡破裂。在加热初期,气泡生长速度很快,随着加热过程的不断进行,气泡的生长速度逐渐放缓;随着气泡生长顺序的不断推迟,最大直径减小;加热功率的提升会增加气泡的生长速度,缩短气泡的生长时间。通过对气泡破裂过程的研究,气泡破碎过程开始于气泡上方的液膜断裂,形成不稳定的瑞利流和向上喷射的液滴,在表面张力的作用下,恢复初始状态,气泡破裂直径大小会影响液滴的波动幅度与周期。     

石灰浆液喷嘴雾化特性

石灰浆液雾化喷嘴是烟气喷雾干燥净化系统的关键设备.利用Win212-2型激光粒度分析仪,对外混及内-外混相结合双流体石灰浆液喷嘴的雾化特性进行了试验研究,分析了气液质量比、石灰浆液浓度等参数对雾化粒径分布和雾化角的影响规律.结果表明,气液质量比增大,浆液液滴平均直径减小,但当气液比增大到一定值后,粒径变化趋于平缓;浆液浓度增加,液滴平均直径增加;气液质量比增加,雾化角减小.     

电极感应熔化气雾化制粉技术中非限制式喷嘴雾化过程模拟

电极感应熔化气雾化(electrode induction melting gas atomization, EIGA)是一种制备超洁净无夹杂物的先进制粉技术,本文以粉末高温合金的氩气雾化过程为研究示例,对现有用于实际生产的国内某厂家提供的EIGA用非限制式喷嘴进行建模,采用商用计算流体力学软件FLUENT,分布采用欧拉-欧拉VOF(volume of fluid)多相流方法与欧拉-拉格朗日DPM (discrete phase model)离散相方法,对非限制式环缝喷嘴主雾化与二次雾化过程进行了数值模拟.通过对主雾化过程中多相流大涡模拟速度流场,主雾化过程中不同阶段高温熔体云图模拟以及二次雾化过程中TAB (Taylor analogy breakup)模型速度流场及TAB模型粒度分布的模拟研究,实现了对EIGA制粉技术中非限制式喷嘴雾化过程的全过程模拟,并预测了雾化后的粉末粒度分布.在此基础上,采用本文模拟使用的非限制式环缝喷嘴,设定与模拟条件一致(进气压力4 MPa,液流直径约4 mm)的实验条件,制备的粉末大部分颗粒的直径大小在100μm左右,该实验结果与模拟得到的粉末直径D50=100μm大小一致,进一步验证了模拟数据的合理性.该方法也适用于非限制式喷嘴里,其他金属或合金的雾化过的模拟研究.     

气体-颗粒掺混的数值模拟研究

通过数值模拟方法研究了圆管内湍流气体和压力旋流雾化喷嘴产生的液滴群的掺混规律.定义了掺混度、不均匀浓度因子和无量纲颗粒浓度定量评价掺混水平,根据这些特征参数,获得了掺混度曲线和截面等浓度线.分析了不同水气比、喷嘴雾化角度和颗粒直径情况下颗粒群对横向气流的影响以及气相对颗粒群掺混过程的作用.结果表明:颗粒群的加入使气相产生了尺度不同的涡,促进了气相湍流发展;近喷雾区,大尺度的涡造成了颗粒局部浓度较高,掺混度较低,但是经过较短的距离就能充分掺混,此时掺混度值较高,同时初始的喷雾形态和壁面处的较大速度梯度使得颗粒存在趋壁现象;水气比1:1、中等雾化角度90°和较大的颗粒更有利于掺混.     

湍动雾化射流液雾粒径分布的数值模拟

对水在空气中湍动雾化射流的气液两相流场进行了数值模拟.其中,气体流场采用k-ε湍流模型进行模拟,给出了载气轴向速度的分布情况.对喷雾粒子的运动采用颗粒轨道法,建立粒子破碎和碰撞的数值模型,研究了液雾粒径在不同工况下沿轴向的变化趋势.数值模拟结果与实验结果在多种气液比下进行了比较,两者吻合较好.同时,分析了初始粒径和粒子总数及喷嘴尺寸对液雾粒径变化趋势的影响,讨论了有助于粒径均匀分布的条件.     

紫外光诱导纳米颗粒胶体微射流碰撞特性（英文）北大核心CSCD

采用流体力学模拟方法,建立了垂直非淹没射流的计算流体动力学模型,研究了在紫外光诱导纳米颗粒胶体射流中用直径D为500μm的微孔光-液耦合喷嘴进行抛光加工的冲击动力学,分析了非淹没射流条件下光-液耦合喷嘴内、外的流场分布情况及其对工件表面的喷射冲击特征,对紫外光诱导纳米颗粒胶体射流冲击动力学过程进行了理论描述。计算结果表明,在1MPa入射压力时,微孔光-液耦合喷嘴口TiO2胶体的喷射速度约为30m/s,其集束匀速喷射距离约为5mm。在此喷射距离时进行垂直喷射,在胶束与工件表面的冲击射流作用区域,其射流静压最大值分布在射流冲击作用中心,但射流动压及射流合成速度在此区域的截面分布呈"W"形状,射流动压及速度最大值出现在胶体射流束的外环直径约2mm处。     

A Predictive Model for the Initial Droplet Size and Velocity Distributions in Sprays and Comparison with Experiments

The distribution of sizes and velocities of droplets initially formed in sprays is an important piece of information needed in the spray modelling, because it defines the initial condition of the spray droplets in the predictive calculations of the downstream two‐phase flow fields. A predictive model for the initial droplet size and velocity distributions in sprays is formulated in this study. The present model incorporates both the deterministic and the stochastic aspect of spray formation process. The deterministic aspect takes into account of the unstable wave motion before the liquid bulk breakup through the linear and nonlinear instability analysis, which provides information for the liquid bulk breakup length, the mass‐mean diameter and a prior distribution for the droplet sizes corresponding to the unstable wave growth of various wavelengths. The stochastic aspect deals with the final stage of droplet formation after the liquid bulk breakup by statistical means through the maximum entropy principle based on Bayesian entropy. The two sub‐models are coupled together by the various source terms signifying the liquid‐gas interaction, the mass mean diameter and the prior distribution based on the instability analysis. The initial droplet size and velocity distributions are measured experimentally by phase‐Doppler interferometry for sprays generated by a planar research nozzle and a practical gas turbine airblast nozzle. For the two nozzles, the liquid bulk sheet is formed before its breakup in a coflowing air stream. It is found that the model predictions are in satisfactory agreement with the experimental data for all the cases measured. Hence the present model may be applied to a variety of practical sprays to specify the initial conditions for the spray droplets formed in practical spray systems.     

超声速气流中液体横向射流雾化过程数值模拟

为了更加深入了解超燃冲压发动机燃烧室中的燃料雾化机理,对来流Mach数为1.94的超声速气流中液体横向射流的雾化过程进行了数值模拟研究.计算采用Euler-Lagrange方法,液滴二次破碎模型采用K-H/R-T模型.计算结果表明:考虑液滴二次破碎时,采用雾化锥模型获得的射流穿透深度以及液滴速度分布与实验结果符合得很好;初始液滴直径对射流穿透深度和液滴分布的影响很小;随着初始雾化锥角的增加,相同横截面上的射流穿透深度逐渐减小.当不考虑液滴二次破碎时,液滴穿透深度及分布与所选的初始液滴直径有很大关系.      

Drop-Size Distributions Produced by Flat-Spray Nozzles

This study deals with measurements of the drop-size distributions of different types of fan spray atomisers using a scattered-light particle size counting analyser. The drop-size distributions were determined at various locations in the spray cone. These local distributions change systematically from the fan's axis to its border. Superimposing these local distributions, adequately weighted, one acquires the entire distribution of all the drops. A comparison of the experimental results is made with those yielded by mathematical equations.     

Ligament-mediated spray formation

The spray formed when a fast gas stream blows over a liquid volume presents a wide distribution of fragment sizes. The process involves a succession of changes of the liquid topology, the last being the elongation and capillary breakup of ligaments torn off from the liquid surface. The coalescence of the liquid volumes constitutive of a ligament at the very moment it detaches from the liquid bulk produces larger drops. This aggregation process has its counterpart on the shape of the size distribution associated with the ligament breakup, found to be very well represented by gamma distributions. The exponential shape of the overall distribution in the spray coincides with the large excursion wing of these elementary distributions, underlying the crucial role played by the ligament dynamics in building up the broad statistics of sprays.     

Bubble splitting in oscillatory flows on ground and in reduced gravity

The stability of centimeter scale air bubbles is studied in quiescent suspending liquid under an imposed oscillatory acceleration field. Experiments were performed in reduced- and normal-gravity environments. A strong acceleration resulted in an instability leading to the breakups of the bubbles in both gravity environments. The breakup onset was investigated and found to be characterized by a critical acceleration a _cr . The influence of the liquid viscosity and the gravitational environment was studied. Empirical correlations for the onset are presented and discussed with the intention to reveal splitting mechanism. The inertial mechanism often deemed to cause the breakup of drops subjected to a rapid gas stream is shown to give explanations consistent with the experiments. A breakup criterion for both gravitational environments is proposed through discussions from an energetic point of view.     

Detonation waves in bubble-drop media

Wave processes in chemically active multicomponent media: liquid — gas bubbles — liquid drops have been studied experimentally. Existence of detonation waves in multicomponent (bubble-drop) media has been proved. Structure of detonation waves in bubble-drop and bubble media is qualitatively identical: detonation waves are solitary waves with pulsation profile the pressure behind which is close in value to the one in unperturbed medium. Propagation velocity of detonation waves in bubble and bubble-drop media drops with the increase in medium gas phase concentration and with the decrease in carrier liquid viscosity. Presence of liquid drops decreases detonation wave velocity compared with bubble medium that does not contain liquid drops. Detonation wave propagation in multicomponent media causes gas bubbles fragmentation as well as fragmentation of individual liquid drops. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 04-03-33106).     

Droplet growth by coalescence in binary fluid mixtures

The evolution of the drop-size distribution in immiscible fluid mixtures following well-specified shear histories is investigated by in situ microscopy, allowing determination of the shear-induced coalescence efficiency epsilon. At small capillary number Ca, epsilon is constant, whereas at larger values of Ca, epsilon decreases, in agreement with theory accounting for slight deformation of the drops in close approach. Coalescence causes the drop-size distribution to broaden in general, but greater deformation of the larger drops at high shear rates causes the drop-size distribution to remain narrow.