Droplet Sizing by Mie Scattering Interferometry in a Spark Ignition Engine

A theoretical explanation is given of a technique based on Mie scattering interferometry (MSI), obtained by defocusing of the collecting optics, to size droplets. The originality of this study is the development of a droplet sizing method by planar laser light scattering for the case of a scattering angle range close to 90°. The feasibility of this method and its limitations are fully described. The dependence on intensity levels and refractive index variations can be neglected. After discussion of some practical details about particle size, imaging and camera constraints, the results obtained in the combustion chamber of a spark ignition (SI) engine, near the spark plug, prior to ignition and for different injection timings are described and discussed. It can be concluded that the implementation of the MSI method in this experimental set-up has been realized successfully to provide droplet distributions in an SI engine. To allow the easier use of the technique, image processing software will be developed in the Matlab environment.     

Quantitative Measurement of Planar Droplet Sauter Mean Diameter in Sprays using Planar Droplet Sizing

The purpose of this investigation is to assess and improve the accuracy of Sauter Mean Diameter measurements in dense sprays using a Planar Droplet Sizing (PDS) technique, based on the intensity ratio of scattered and fluorescence light. A novel data processing method of the PDS technique is suggested, which was derived from a theoretical light scattering investigation, and reduced possible sizing errors larger than 30% to below 10%. The novel approach for droplet sizing was applied to measure in a spray generated by a pressure swirl atomiser in a liquid‐fuelled burner operated with water at isothermal conditions, in order to avoid the effect of liquid evaporation on the accuracy of PDS technique. The Sauter Mean diameter results from the PDS technique were compared to Phase Doppler Anemometer (PDA) sizing measurements. Good agreement was obtained between the two techniques in dense regions of the spray. Discrepancies remained in dilute spray regions due to systematic statistical uncertainties of the PDS technique and the dynamic range of the intensity of the CCD cameras, which did not allow detection of large single droplets in the dilute spray region.     

Planar Droplet Sizing for the Characterization of Droplet Clusters in an Industrial Gun‐Type Burner

An important problem in spray combustion deals with the existence of dense regions of droplets, called clusters. To understand their formation mechanism, the droplet dynamics and fuel concentration profile are investigated by means of planar laser techniques in an industrial gun‐type burner. The simultaneous measurement of elastic Mie scattering and Laser Induced Fluorescence (LIF) allows the instantaneous measurement of the Sauter Mean Diameter (SMD), after proper calibration. Using two different CCDs to get the two signals requires a detailed calibration of the CCD response before getting absolute diameters. Pixels are binned 6 by 6 to obtain the final SMD map, this is a compromise between spatial accuracy and noise. Velocity field is measured on both sets of images using standard Particle Image Velocimetry (PIV) algorithms. The comparison of cross‐correlation technique with PDA results shows that the velocity measured on the LIF images are close to the velocity based on D₃₀, whereas the Mie scattering results are similar to D₂₀. On Mie scattering images, regions of high interfacial area forming clusters can be detected. A special tracking scheme is used to characterize their dynamics in terms of velocity and diameters by ensuring that the same volume of fluid is tracked. It is shown that the clusters have a velocity similar to the velocity of droplets with the same diameter as the mean SMD of the cluster. It is also shown that an increase of pressure tends to trigger the appearance of such a group of droplets, due to a smaller diameter of the droplets caused by the increase of pressure discharge. Uncertainties for the different techniques used are discussed.     

A Study of Parameters that Influence the Accuracy of the Planar Droplet Sizing (PDS) Technique

A combined theoretical and experimental study of the parameters affecting the accuracy of Planar Droplet Sizing (PDS) measurements is presented. The principle of the PDS technique relies on the assumption that the intensity emitted by a fluorescent dye added to a liquid is proportional to the volume of a resulting droplet during atomisation and that the scattered light intensity is proportional to its surface area, allowing measurement of Sauter Mean Diameter (SMD) by taking the ratio of these intensities. A geometrical optics light scattering approach was extended to calculate the fluorescence intensity emitted by a droplet, in addition to providing the scattered light intensity integrated over the collection aperture. The theoretical approach quantified the influences of scattering angle, refractive index, droplet size and dye concentration on the PDS technique. Experiments with monodisperse droplet streams confirmed the calculations in terms of dependence of the scattered and fluorescence intensities. The limitations of the technique have been established together with an appropriate calibration procedure for application in dense sprays.     

Droplet splashing by a slingshot mechanism

When a drop impacts onto a liquid pool, it ejects a thin horizontal sheet of liquid, which emerges from the neck region connecting the two liquid masses. The leading section of this ejecta bends down to meet the pool liquid. When the sheet touches the pool, at an "elbow," it ruptures and sends off microdroplets by a slingshot mechanism, driven by surface tension. High-speed imaging of the splashing droplets suggests the liquid sheet is of submicron thickness, as thin as 300 nm. Experiments in partial vacuum show that air resistance plays the primary role in bending the sheet. We identify a parameter regime where this slingshot occurs and also present a simple model for the sheet evolution, capable of reproducing the overall shape.     

Particle Sizing by Planar-Phase Doppler System

Conventional phase Doppler systems are useful for sizing particles in the order of microns, but sensitive to the Gaussian beam defect which can cause sizing errors. The defect can be significant when a large size is measured. In this paper, we present a new phase Doppler system using a planar optical layout which permits large particles to be measured in a forward scattering scheme without the Gaussian beam errors. The optical system design is discussed by numerical simulation based on the Mie theory.     

Droplet cluster

A new phenomenon consisting of the formation of an ordered layer of condensate microdroplets at the water free surface was observed upon the irradiation of an open horizontal water layer (and layers of some organic fluids) by a light beam that causes local heating and evaporation of the fluid.     

Droplet formation by rapid expansion of a liquid

Molecular dynamics of two- and three-dimensional liquids undergoing a homogeneous adiabatic expansion provides a direct numerical simulation of the atomization process. The Lennard-Jones potential is used with different force cutoff distances; the cluster distributions do not depend strongly on the cutoff parameter. Expansion rates, scaled by the natural molecular time unit (about a picosecond), are investigated from unity down to 0.01; over this range the mean droplet size follows the scaling behavior of an energy balance model which minimizes the sum of kinetic plus surface energy. A second model which equates the elastic stored energy to the surface energy gives better agreement with the simulation results. The simulation results indicate that both the mean and the maximum droplet size have a power-law dependence upon the expansion rate; the exponents are -2d/3 (mean) and -d/2 (maximum), where d is the dimensionality. The mean does not show a dependence upon the system size, whereas the maximum does increase with system size, and furthermore, its exponent increases with an increase in the force cutoff distance. A mean droplet size of 2.8/eta(2), where eta is the expansion rate, describes our high-density three-dimensional simulation results, and this relation is also close to experimental results from the free-jet expansion of liquid helium. Thus, one relation spans a cluster size range from one atom to over 40 million atoms. The structure and temperature of the atomic clusters are described.     

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.     

干凝胶炉法制备聚苯乙烯空心微球

研究了一步乳化法制备初始粒子的过程中相比及搅拌强度对初始粒子直径的影响 ,初步探讨了干凝胶炉内成球过程中 ,炉温、炉内气氛、压力等对聚苯乙烯 (PS)空心微球产率及同心度的影响。以分子量为 2 2万的聚苯乙烯制得了直径在 70～ 550μm的初始粒子 ,经乙醇浸泡后 ,制得了含 2 %质量分数发泡剂的干凝胶粒子 ,并在 Polymer-70 0干凝胶炉上制备出了外直径为 4 0 0～ 90 0 μm、壁厚为 2～1 0 μm、同心度≥ 95%的     

可溶性活性剂驱动的液滴铺展稳定性

对含可溶性活性剂的液滴在预置液膜上的非均匀铺展过程,基于润滑理论建立基态和扰动态下的液滴厚度、表面活性剂浓度和内部浓度的演化模型,应用非模态理论分析演化过程的稳定性,探讨活性剂溶解特性及典型参数对液滴演化特征的影响.研究表明:扰动波的引入有利于增强液滴演化的稳定性,且稳定程度与扰动波的波数呈正相关性;然而随扰动波数的持续增大,液滴演化的稳定性逐渐下降,直至失稳;相对于非溶性活性剂,可溶性活性剂减缓了液滴的铺展程度,增强了演化过程的稳定性;预置液膜厚度、Marangoni数、毛细力数及吸附系数的增大,均有利于液滴稳定演化,其中Marangoni效应和毛细力的影响较大,预置液膜厚度则主要增强液滴厚度演化的稳定性.     

动态光散射图像法测量纳米颗粒粒度研究

提出了一种基于动态光散射原理的图像法测量纳米颗粒粒径的新方法,采用面阵CCD数以万计的像素同时并行测量处于布朗运动的纳米颗粒空间分布的动态散射光信号,对测得的信号进行数据处理,得到了纳米颗粒粒径。对27、79、482、948nm 4种不同粒径的纳米标准颗粒进行了实验研究,针对面阵CCD拍摄帧率远低于光电倍增管测量频率的特点,采用质量分数为55%高粘度甘油水溶液作为分散介质,在CCD拍摄帧率为8290frame/s时,27nm颗粒的测量误差从以水为分散介质时的15.1%降至1.9%。与目前动态光散射纳米颗粒测量方法相比,该方法大幅度减少了测量时间,仅为现有方法的1%以下,并可大幅度简化测量装置。     

Inertial Reciprocating Photomotor

JETP Letters - We present a theory for an inertial reciprocating Brownian motor driven by deterministic dichotomous fluctuations in the parameters of a Brownian particle and confinement potentials...     

Droplet mass formula fit

A mass fit of the droplet model is presented ¹). To gain more confidence in the values of the thirteen coefficients used, we fitted in addition the best-known isobar parabolae. By this procedure the position and the width of the β-valley are fitted and the symmetry J and the surface stiffness Q proved to be the only relevant parameters. Finally the binding energy at the bottom of the β-valley is refitted by altering the other parameters. The thus obtained nuclear masses are compared with experimental data as well as with the TCH model presented earlier.     

基于单模光纤的DLS粒径测量研究

动态光散射技术是一种测量纳米颗粒粒径分布的最有效方法。为了克服高浓度样品溶液中存在的多重散射效应, 本文基于单模光纤搭建了一套DLS实验系统, 传输光纤的输出端与接收光纤的接收端都作了抛光磨平处理。在模拟复杂工业环境下, 分别用低浓度单分散、不同浓度单分散以及高浓度多分散性标准聚苯乙烯乳胶球悬浮液检测了该系统的适用性。实验结果表明, 利用该系统可快速准确的测量体积分数达40%的聚苯乙烯乳胶球悬浮液中的纳米颗粒粒径及其分布。     

Inertial cavitation initiated by polytetrafluoroethylene nanoparticles under pulsed ultrasound stimulation

Nanoscale gas bubbles residing on a macroscale hydrophobic surface have a surprising long lifetime (on the order of days) and can serve as cavitation nuclei for initiating inertial cavitation (IC). Whether interfacial nanobubbles (NBs) reside on the infinite surface of a hydrophobic nanoparticle (NP) and could serve as cavitation nuclei is unknown, but this would be very meaningful for the development of sonosensitive NPs. To address this problem, we investigated the IC activity of polytetrafluoroethylene (PTFE) NPs, which are regarded as benchmark superhydrophobic NPs due to their low surface energy caused by the presence of fluorocarbon. Both a passive cavitation detection system and terephthalic dosimetry was applied to quantify the intensity of IC. The IC intensities of the suspension with PTFE NPs were 10.30 and 48.41 times stronger than those of deionized water for peak negative pressures of 2 and 5 MPa, respectively. However, the IC activities were nearly completely inhibited when the suspension was degassed or ethanol was used to suspend PTFE NPs, and they were recovered when suspended in saturated water, which may indicates the presence of interfacial NBs on PTFE NPs surfaces. Importantly, these PTFE NPs could sustainably initiate IC for excitation by a sequence of at least 6000 pulses, whereas lipid microbubbles were completely depleted after the application of no more than 50 pulses under the same conditions. The terephthalic dosimetry has shown that much higher hydroxyl yields were achieved when PTFE NPs were present as cavitation nuclei when using ultrasound parameters that otherwise did not produce significant amounts of free radicals. These results show that superhydrophobic NPs may be an outstanding candidate for use in IC-related applications.