粒度分布函数在柴油机喷雾测量中的应用和比较

本文根据激光衍射粒子分析仪在各种不同实验条件下测得大量柴油机喷雾数据，对测量中应用的几种粒度分布模型（函数）做了比较研究，发现对于柴油机喷雾测量，独立分布效果最好。但若欲用经验分布预测未知喷雾特性，则Ｒ－Ｒ分布较好。文中还对实验数据做了统计，给出了Ｒ－Ｒ分布的分布参数的数值范围及其在数据组中的出现频数。     

柴油机SCR系统尿素溶液喷雾特性的试验研究

应用多普勒粒子动态分析仪(PDA)研究了SCR系统尿素溶液喷雾的粒径及其分布特征。试验对比了有、无空气辅助喷射系统的喷雾粒径分布特征;测量了空气辅助喷射系统尿素溶液喷雾在垂直喷雾轴线不同截面上的粒径分布;研究了空气压力及喷嘴的喷孔结构对喷雾粒径的影响。通过高速摄影得到空气辅助喷射系统尿素溶液喷雾的发展过程。试验结果表明:空气辅助喷射系统沿喷雾轴向喷雾粒径的大小及粒径分布变化不大,大直径液滴所占比例随着辅助空气压力的减小而增大,有倒角喷孔较无倒角喷孔更有利于喷雾雾化;无空气辅助喷射系统喷雾雾化效果较差,大直径液滴所占的比例明显增大。     

PIVS方法测雾滴粒径的影响因素及其实验验证

在分析讨论PIVS(particle imageve locimetry and sizing)方法测量喷雾场粒子粒径的影响因素的基础上，通过建立基于标准粒子的粒子场实验模拟装置，对PIVS系统粒径测量结果进行了验证。结果表明：现有系统只适用于对大粒子(如平均粒径大干115μm)粒径的精确测量，要实现对小粒子(如粒径小于50μm)粒径的测量，仍需改善其成像系统的性能。     

基于激光衍射原理的柴油机瞬变喷雾场粒径分布测量系统研究

摘要柴油机喷雾场是高速运动的持续时间只有数毫秒的瞬变喷雾场,实际测量柴油机喷雾场粒径分布随时间的变化对柴油机燃烧过程的研究具有重大意义.本文介绍的测量系统是我们针对柴油机的雾场特点研制的激光衍射瞬变雾场测量系统.该系统可以在一个喷雾周期内连续多次对柴油机雾场进行取样,可以测量在一个喷雾周期内喷油雾场粒径分布随时间的变化特性.本文还介绍了使用该系统进行的柴油机瞬变喷雾场粒径分布实测研究.     

基于时间分割法的柴油机交叉孔喷油嘴喷雾场粒子特性

采用相位多普勒粒子分析仪(PDPA)对柴油机V形交叉孔油嘴和单孔油嘴的喷雾场粒子特性进行测量,通过时间分割法对测量结果进行深入分析.结果表明,在喷雾轴向Z=40,mm的位置上,靠近喷雾轴线的区域内两种交叉孔油嘴与单孔油嘴的粒径大小差别不大,但随着测量点的径向坐标增大,交叉孔油嘴的粒子直径逐渐小于单孔油嘴.在Z=60,mm位置上,与单孔油嘴的结果相比,两种交叉孔喷油嘴的粒子算术平均直径(AMD)和索特平均直径(SMD)更小.同时,交叉孔的交叉角增大会对液滴的进一步细化起促进作用,相比于单孔油嘴,交叉孔2喷油嘴的粒子AMD和SMD减小了近30%.对时间分割法划分的4个阶段粒子特性进行分析发现,F阶段的粒子速度高达70,m/s,喷射过程中的系统数据率较低,PDPA的大量粒子信息均来自于喷油结束的T阶段,此时粒子速度为零.交叉孔喷油嘴的扇状喷雾结构对油气混合的促进作用使粒子AMD随时间变化而不断减小.同时,与喷雾内侧相比,在喷雾外侧两交叉孔喷油嘴之间的粒径差异变小,相对于单孔喷油嘴,在粒径方面的优势变得明显.     

基于高速摄像技术的生物柴油粒径的研究

基于高速摄像技术及先进的相位多普勒粒子分析仪,在不同的喷油泵转速、启喷压力以及喷孔 直径下,对生物柴油的喷雾粒径进行了研究,并拍摄了几幅喷雾图片.结果表明:随着启喷压力的增加 和喷孔直径的减小,生物柴油索特平均直径不断减小;在不同的喷油泵转速下,生物柴油索特平均直径 变化不大.     

燃油喷嘴气液两相流雾化特性研究

以空气,水为工质,利用马尔文粒度分析仪对气液两相流雾化器喷嘴的雾化特性进行了详细的实验研究。测量了气,液两相流不同入口压力比条件下通过喷嘴后形成的液体雾化粒子的粒径分布,详细讨论分析了气,液两相压力及进气,进液方式对喷雾效果的影响,得出了喷嘴雾化过程中气液两相流量与气液两相压力之间的规律和 化原则,并对喷嘴的雾化机理进行了探讨。     

空燃比对柴油机颗粒物排放特性的影响

在一台单缸柴油机上利用TSI 3090 EEPS(发动机排气粒径分析仪)测量不同空燃比下柴油机尾气颗粒物粒径分布。结果表明:随着空燃比的降低,颗粒物数量浓度、质量浓度、表面积浓度和体积浓度随之增大,粒径分布向大直径粒子方向偏移,核态粒子减少,积聚态粒子增加。     

节流轴针式喷油嘴的喷雾特性研究

利用激光马尔文粒子测量系统，实测了新型节流轴针式喷油嘴的喷雾情况，对喷注的发展、油粒直径的分布、每循环喷油量、针阀开启压力及转速等参数的影响进行了分析。     

新型喷雾光学测量仪的研制

饱和器是HAT循环中的关键部件,如何测量其内部水滴粒径、浓度及其变化是现在研究工作中的一个难题。文中描述了一种新型光学喷雾粒径、浓度测量系统,利用该系统对双流体喷嘴在空气中喷雾时水滴粒径及在不同位置的变化进行了测量,同时在相应位置和相同工况下用PDA对喷雾特性进行了比较测量。对得到的实验数据进行定量分析,说明测量结果的准确性。实验测量结果表明,用该系统测量喷嘴喷雾粒径是可行的,可以进一步用于饱和器内部水滴的测量。     

Experimental investigation of decomposition and evaporation characteristics of HAN-based monopropellants

Hydroxylammonium nitrate (HAN)-based monopropellants are among the most promising candidates for eco-friendly rocket engine propellants. They are not carcinogenic or mutagenic, and their thermal decomposition reactions are sufficiently exothermic for military and aerospace applications. Here, an experimental analysis was performed to investigate the characteristics of HAN-based mixtures for monopropellant applications. Three kinds of propellants were prepared for this study: a HAN–water solution, a HAN–water solution with methanol added at a stoichiometric ratio, and a third solution where water was added to further dilute the solution. Two different experimental techniques were used under atmospheric pressure of nitrogen gas environment: thermal analysis and droplet evaporation analysis. Thermal and catalytic decomposition were analyzed using thermo gravimetric analysis. Droplet evaporation was analyzed using a cylindrical vessel equipped with a heating system and a droplet feed, and a high-speed charge-coupled device (CCD) camera. These data were post-processed to calculate the temporal variation of the droplet diameters.     

Mechanistic study on spraying of blended biodiesel using phase Doppler anemometry

Droplet size and dynamics of blended palm oil-based fatty acid methyl ester (FAME) and diesel oil spray were mechanistically investigated using a phase Doppler anemometry. A two-fluid atomizer was applied for dispersing viscous blends of blended biodiesel oil with designated flow rates. It was experimentally found that the atomizer could generate a spray with large droplets with Sauter mean diameters of ca. 30 μm at low air injection pressure. Such large droplets traveled with a low velocity along their trajectory after emerging from the nozzle tip. The viscosity of blended biodiesel could significantly affect the atomizing process, resulting in the controlled droplet size distribution. Blended biodiesel with a certain fraction of palm oil-based FAME would be consistently atomized owing to its low viscosity. However, the viscosity could exert only a small effect on the droplet velocity profile with the air injection pressure higher than 0.2 MPa.     

缝隙抽吸对二次水滴直径及直径分布影响的试验研究

在空气-水膜两相流动试验装置上，利用Malvem粒度分析仪对空心静叶缝隙抽吸和无抽吸条件下的静叶尾迹区二次水滴直径及直径分布进行了测量和分析。结果表明：缝隙抽吸可以有效地去除静叶表面上的流动液膜，大大减小了静叶尾迹区由于撕裂和二次雾化所形成的二次水滴直径；且水滴直径分布的范围较窄，有利于减轻或防止动叶的水蚀。     

Advances in droplet array combustion theory and modeling

A review of research on the subject of the vaporization and burning of fuel droplets configured in a prescribed array is presented, including both classical works and research over the past decade or two. Droplet arrays and groups and the relation to sprays are discussed. The classical works are reviewed. Recent research on transient burning and vaporization of finite arrays with Stefan convection but without forced convection is presented, including extensions to non-unitary Lewis number and multi-component, liquid fuels. Recent results on transient, convective burning of droplets in arrays are also examined. In particular, transient convective burning of infinite (single-layer periodic and double-layer periodic) and finite droplet arrays are discussed; attention is given to the effects of droplet deceleration due to aerodynamic drag, diameter decrease due to vaporization, internal liquid circulation, and arrays with moving droplets in tandem and staggered configurations. Flame structure is examined as a function of spacing between neighboring droplets and Damköhler number: individual droplet flames versus group flames and wake flames versus envelope flames. Based on existing knowledge of laminar droplet array and spray combustion theory, experimental evidence, and turbulent studies for non-vaporizing and non-reacting two-phase flows, comments are made on the needs and implications for the study of turbulent spray and array combustion.     

High-pressure combustion of submillimeter-sized nonane droplets in a low convection environment

An experimental study of droplet combustion of nonane (C₉H₂₀) at elevated pressures burning in air is reported using low gravity and small droplets to promote spherical gas-phase symmetry at pressures up to 30 atm (absolute). The initial droplet diameters range from 0.57 to 0.63 mm and they were ignited by two electrically heated hot wires positioned horizontally on opposite sides of the droplet. The droplet and flame characteristics were recorded by a 16-mm high-speed movie and a high-resolution video camera, respectively. A photodiode is used to measure broadband gray-body emission from the droplet flames and to track its dependence on pressure. Increasing the pressure significantly influences the ability to make quantitative measurements of droplet, soot cloud, and luminous zone diameters. At pressures as low as 2 atm, soot aggregates surrounding the droplet show significant coagulation and agglomeration and at higher pressures the soot cloud completely obscures the droplet, with the result being that the droplet could not be measured. Above 10 atm radiant emissions from hot soot particles are extensive and the resulting flame luminosity further obscures the droplet. Photographs of the luminous zone in subcritical pressures show qualitatively that increasing pressure produces more soot, and the mean photodiode voltage output increases monotonically with pressure. The maximum flame and soot shell diameters shift to later times as pressure increases and the soot shell is located closer to the flame at higher pressure. The soot shell and flame diameter data are correlated by a functional relationship of reduced pressure derived from scaling the drag and thermophoretic forces on aggregates that consolidates all of the data onto a single curve.     

Surface tension effects during low-Reynolds-number methanol droplet combustion

A numerical investigation of methanol droplet combustion in a zero-gravity, low-pressure, and low-temperature environment is presented. Simulations have been carried out using a predictive, transient, and axisymmetric model, which includes droplet heating, liquid-phase circulation, and water absorption. A low initial Reynolds number (Re₀=0.01) is used to impose a weak gas-phase convective flow, introducing a deviation from spherical symmetry. The resulting weak liquid-phase circulation is greatly enhanced due to surface tension effects, which create a complex, time-varying, multicellular flow pattern within the liquid droplet. The complex flow pattern, which results in nearly perfect mixing, causes increased water absorption within the droplet, leading to larger extinction diameters. It is shown that, for combustion of a 0.43-mm droplet in a nearly quiescent environment (Re₀=0.01) composed of dry air, the extinction diameter is 0.11 mm when surface tension effects are included, and 0.054 mm when surface tension effects are neglected. Experimental work available in the literature for a 0.43-mm droplet reported extinction diameters in the range of 0.16 to 0.19 mm. Results for combustion in a nearly quiescent environment (Re₀=0.01) with varying initial droplet diameters (0.16 to 1.72 mm) show that including the effect of surface tension results in approximately linear variation of the extinction diameter with the initial droplet diameter, which is in agreement with theoretical predictions and experimental measurements. In addition, surface tension effects are shown to be important even at initial Reynolds numbers as high as 5.     

Fringe probing of an evaporating microdroplet on a hot surface

The phenomenon of droplets impacting and evaporating on a hot surface is of interest in many areas of engineering. Quantitative measurement of these processes provides great help to reveal the physics behind. A novel technique was developed to quantitatively measure the volume evolution and contact diameter of an evaporating microdroplet on a hot surface utilizing interference fringe scattering method. In this method, fine fringes produced by the interference of two coherent laser beams was scattered by the droplet and projected onto a screen. The profile and volume of the droplet can be derived from the spatial fringe spacing on the screen. The number of total fringes measurable on the screen was used to determine the instantaneous contact diameter of the microdroplet. Validation experiments demonstrated that the measurement errors are less than ±5% and ±1% for microdroplet volume and contact diameter, respectively. By using this method, the dynamic of droplet impingement, evaporation and boiling using ethanol, pure water and water solution of a surfactant (sodium dodecyl sulfate) with impact velocity of 7.5 m/s and diameters ranged from 0.19 to 0.46 mm were investigated.     

Ex-situ probing of PEFC liquid droplet dynamics in the presence of vibration

PEFC performance degradation when exposed to vibration has been attributed to liquid water redistribution; however, fundamental liquid droplet behavior under such conditions is not well understood. As this technology continues to penetrate automotive, aerospace, and marine markets, where vibration and mechanical shock may be common, it is necessary to characterize droplet dynamics under such conditions. This work uses ex-situ experimental techniques to study the effects of sinusoidal vibration on water and gas-diffusion layer interactions for open cathode and flow channel based designs. Parameters such as contact angle, key droplet dimensions, adhesion force, and detachment velocity were measured for a range of frequencies and amplitudes. Vertical and horizontal oriented cells were investigated. The results show that vibration can significantly impact wetting; in some cases, increasing the barrier for liquid removal. Sessile droplet contact angle and dimensions such as height and wetting diameter, shifted toward more hydrophilic wetting under elevated frequency. Droplet detachment velocities were measured to be higher at 100 Hz then at 0 Hz for air speeds up to 10 m s^?1. These results may be of interest to PEFC modelers and flow-field designers.     

Interacting effects of uniform flow, plane shear, and near-wall proximity on the heat and mass transfer of respiratory aerosols

Individual and interacting effects of uniform flow, plane shear, and near-wall proximity on spherical droplet heat and mass transfer have been assessed for low Reynolds number conditions beyond the creeping flow regime. Validated resolved volume simulations were used to compute heat and mass transfer surface gradients of two-dimensional axisymmetric droplets and three-dimensional spherical droplets near planar wall boundaries for conditions consistent with inhalable aerosols (5 ? d ? 300 μm) in the upper respiratory tract. Results indicate that planar shear significantly impacts droplet heat and mass transfer for shear-based Reynolds numbers greater than 1, which occur for near-wall respiratory aerosols with diameters in excess of 50 μm. Wall proximity is shown to significantly enhance heat and mass transfer due to conduction and diffusion at separation distances less than five particle diameters and for small Reynolds numbers. For the Reynolds number conditions of interest, significant non-linear effects arise due to the concurrent interaction of uniform flow and shear such that linear superposition of Sherwood or Nusselt number terms is not allowable. Based on the validated numeric simulations, multivariable Sherwood and Nusselt number correlations are provided to account for individual flow characteristics and concurrent non-linear interactions of uniform flow, planar shear, and near-wall proximity. These heat and mass transfer correlations can be applied to effectively compute condensation and evaporation rates of potentially toxic or therapeutic aerosols in the upper respiratory tract, where non-uniform flow and wall proximity are expected to significantly affect droplet transport, deposition, and vapor formation.