Simultaneous Droplet Size and Gas-Phase Turbulence Measurements in a Spray Flow using Phase-Doppler Interferometry

A recently developed method to increase the sensitivity of a phase-Doppler interferometer (PDI) has been applied to an annular air flow interacting with a spray under isothermal conditions in order to demonstrate the applicability of the method to obtaining velocity and turbulence characteristics of the gas-phase in a two-phase flow. A conventional PDI system operated in the first order refraction mode cannot be configured to efficiently detect small seed particles in the presence of large droplets due to the limited dynamic range of the photodetectors. It is therefore difficult or impossible to obtain seed particle detection rates sufficient for turbulence measurements. Doping the spray with a very small quantity of dye preferentially attenuates the light intensity scattered by large droplets, thus allowing the PDI system to detect small seed particles without damaging the photodetectors when large particles are present in the flow. Application of this method to a two-phase flow produced by a pressure-swirl atomizer coaxially located within an annular air jet has resulted in profiles of axial mean and fluctuating velocities and shear stresses as well as integral time scales and turbulent energy spectra.     

Water vapor condensation and the collapse of superheated water drops as possible causes of ball lightning

Ball lightning is modeled by the bulk of humid air heated to temperatures of 600–650 K and containing a multitude of charged drops and microbubbles of size 10⁻⁵ cm or less, as well as water vapor heated to a near-critical temperature. The condensation of the vapor in the microbubbles, followed by the cooling of resulting droplets, generates energy spent on the thermal radiation of ball lightning. The radiation of light and radio waves is explained by the motion of ions and electrons in the electric field of charged bubbles and droplets and by the thermal rotational motion of charged droplets. As a result of coagulation, the droplets overheat and tend to collapse. An external electric field, supersaturated water vapor condensation, and a number of other factors may contribute to the ball lightning explosion energy.     

Suspended water droplets as a means of noise reduction

The propagation of acoustic waves and energy absorption in a medium of air containing suspended water droplets have been investigated experimentally. It has been found that an increase in atomisation pressure or mass flow ratio reduces the size of droplets, thus yielding higher noise reduction. It should be possible to develop an appropriate system, for reducing the noise of hammers or other impact machinery, where the creation of a temporary air-water skirt around the noise source can be synchronised with the blow rate, i.e. the timing of the noise generation.     

电场对协流式微流控装置中乳液液滴生成行为的调控机理

建立了直流电场作用下协流式微流控装置中单乳液液滴乳化生成过程的非稳态理论模型,并开展了数值模拟研究,揭示了电场对液滴乳化生成动力学行为的调控机理,阐明了流场/电场参数对液滴乳化生成特性的影响规律.研究结果表明:沿流体流动方向施加静电场可在电物性参数不同的两相流体界面法线方向上产生指向内相流体的电场力,进而强化了内相流体界面的颈缩和断裂,提升了液滴生成速率和形变程度,减小了液滴生成尺寸;在同一毛细数下,随着电毛细数的增大,乳液乳化流型由每周期仅有单一液滴生成的滴式流型转变为每周期有一个主液滴并伴随有卫星液滴生成的滴式流型;随着毛细数和电毛细数的增大,黏性拖曳力以及电场力作用增强,使内相流体颈缩过程后期更容易形成细长型液线,从而有助于诱发液线上产生Rayleigh-Plateau不稳定现象,继而促进卫星液滴的形成.     

一种利用LNG冷能的空气分离装置新流程

本文介绍了一种利用LNG冷能的空分装置新流程,其特征在于采用了由LNG冷能冷却的氮气内循环和氮气外循环低温压缩节流制冷系统、LNG的余冷冷却的氟里昂为载冷剂的压缩空气冷却系统和两段式LNG与循环氮气的热交换器。对不同工况的计算结果显示,生产每公斤液氧的能耗比不用LNG法节电60％-73.5％,理论上的节能效果优于在查阅文献和专利的其它方案。     

热管喷射式空调系统中喷射器的研究

利用热管和喷射器复合所组成的空调系统是一种可利用太阳能和备类余热的无泵型新型空调系统。该系统利用喷射器代替压缩机,利用热管内的毛细芯实现工质循环。本文分析了喷射器的流动特性,采用一个简化的一维模型计算了喷射器的喷射系数。设计了一种专用的小型内置式喷射器,对一太阳能热管喷射式空调系统的制冷特性和性能系数进行了数值模拟。模拟计算表明,喷射器的结构和运行特性对热管喷射式制冷系统的性能有明显的影响。     

Stagnation-point polydisperse spray flame ignition

A theory of stagnation-point flow polydisperse spray flame ignition by an isothermal hot surface is presented for the first time. The configuration investigated consists of a mixture of fuel droplets and air flowing against an isothermal hot surface (such as a hot ignition probe). The polydisperse spray of droplets is modelled using the sectional approach. A single global chemical reaction is assumed for the case when ignition occurs. The mathematical analysis makes use of a small parameter that is exploited for an asymptotic approach. An analytical criterion for ignition is derived which includes effects of the flow field, the reactants and all the fuel spray-related parameters, including the initial size distribution of the spray's droplets. Numerical calculations disclose how the actual droplet size distribution impacts on the critical stagnation point temperature necessary to promote ignition. Additionally, the analytical estimates are compared with predictions of a numerical finite difference code with very satisfactory agreement.     

Correlations to predict droplet size in ultrasonic atomisation.

In conventional two fluid nozzles, the high velocity air imparts its energy to the liquid and disrupts the liquid sheet into droplets. If the energy for liquid sheet fragmentation can be supplied by the use of ultrasonic energy, finer droplets with high sphericity and uniform size distribution can be achieved. The other advantage of ultrasound induced atomisation process is the lower momentum associated with ejected droplets compared to the momentum carried by the droplets formed using conventional nozzles. This has advantage in coating and granulation processes. An ultrasonic probe sonicator was designed with a facility for liquid feed arrangement and was used to atomise the liquid into droplets. An ingenious method of droplet measurement was attempted by capturing the droplets on a filter paper (size variation with regard to wicking was uniform in all cases) and these are subjected to image analysis to obtain the droplet sizes. This procedure was evaluated by high-speed photography of droplets ejected at one particular experimental condition and these were image analysed. The correlations proposed in the literature to predict droplet sizes using ultrasound do not take into account all the relevant parameters. In this work, a truly universal correlation is proposed which accounts for the effects of physico-chemical properties of the liquid (flow rate, viscosity, density and surface tension), and ultrasonic properties like amplitude, frequency and the area of vibrating surface. The significant contribution of this work is to define dimensionless numbers incorporating ultrasonic parameters, taking cue from the conventional numbers that define the significance of different forces involved in droplet formation. The universal correlations proposed are robust and can be used for designing ultrasonic atomisers for different applications. Among the correlations proposed here, those ones that are based on the dimensionless numbers and Davies approach predict droplet sizes within acceptable limits of deviation. Also, an empirical correlation from experimental data has been proposed in this work.     

电子膨胀阀对电动汽车空调系统的调节特性研究

电子膨胀阀是电动汽车空调系统的主要调控部件,探究其调节特性对于制定系统控制策略具有重要意义.本文搭建了以R134a为制冷剂的电动汽车空调系统实验台,研究了电子膨胀阀调节过程中,空调系统内制冷剂流量和压力的动态变化规律,分析了不同压缩机转速下,阀开度对系统制冷量、空调箱出风温度、压缩机功耗和系统COP等性能参数的影响.结果表明：阀前制冷剂相态是影响电子膨胀阀调节时系统压力变化强弱的重要因素.在阀前制冷剂为过冷液态时,调节阀开度对系统压力影响更大,并且在阀前制冷剂具有较大过冷度(大于10℃)时,下调阀开度会导致短时间的过节流造成系统压力大幅波动;系统中制冷剂循环流量与阀开度呈线性变化趋势,不受阀前制冷剂相态的影响.在实验工况下,100%阀开度对应的制冷剂循环流量为97.2～115.5 kg/h,阀开度每下调10%,系统中制冷剂循环流量下降6%～9%.     

Splash formation by a spherical body plunging into water

Splashes caused by a spherical body plunging into water were investigated experimentally using a high speed CMOS camera. We categorized types of splash according to impact velocities of the sphere. Three types of splash were found: Type-I is a thin spire-type splash, Type-II is a mushroom-type splash with many droplets, and Type-III is a crown-type splash with many droplets. The reaction to the concave water surface attached to the sinking sphere is a cause of the Type-I splash. The film flow climbing up the sphere is a dominant cause of the Type-II splash. The velocity of the film flow, which is proportional to the impact velocity of the sphere, affects the fingers of the film flow, detaching of droplets, and maximum height of the Type-II splash. The Type-III crown-type splash is characterized by water jets with many droplets. A bulky air column in water is formed behind the sinking sphere, and longitudinal ridges and ripples on the surface of the air column were observed.     

An electrosprayed coating process for fabricating hemispherical PMMA droplets for an optical diffuser

A modified electrospraying process is proposed for fabricating uniform microsized poly(methyl methacrylate) (PMMA) hemispherical droplets on poly(ethylene terephthalate) films for use as optical diffusers. In this process, an electrode controlled by an electric field is attached to a rotating collector, and various electric field conditions are applied to the droplets ejected from a positively charged nozzle. The distribution of small hemispherical droplets and the surface roughness resulting from this modified electrospraying process are more uniform than those achieved in the normal electrospraying process. The frequency of the field applied to the electrode is not critical to the production of stable PMMA droplets. The optical diffusivity of the PMMA film fabricated using this process is greater than that of normally fabricated film, demonstrating the feasibility of using this process to create newly designed optical diffusers.     

Compact polymer multi-nozzles electrospray device with integrated microfluidic feeding system

Electrohydrodynamic (EHD) atomization consists in using an electric field for spraying a liquid flowing through a capillary. The applications are: mass spectrometry, colloid thrusters and more recently medicine nebulization processes. EHD atomization provides the ability to control the generated droplets size by adjusting electrospray parameters. It is however essential to manufacture the emitters into arrays because flow through a stable cone-jet mode electrospray can only be maintained at low flow rate and most applications require a high throughput. We propose a new design of a multiple electrospray system involving an innovative nozzle shape and flow restrictor system. Nozzles and microfluidic restrictor system are manufactured on the same polycarbonate sheet using the excimer laser technology and thus allowing a high compactness of this system.     

Nonlinear calculation of the electric field strength on the surface of a melting hailstone in a thundercloud

The contribution of aerodynamic pressure acting on the surface of a water layer to a total electric field near the free surface of the layer is considered. The layer covers a charged melting hailstone moving parallel to the external electrostatic field vector. An asymptotic analytical expression for the electric field strength near a water-covered hailstone is derived in an approximation that is quadratic in the amplitude of capillary oscillations of a charged conducting liquid layer on the surface of the hailstone. It is found that the motion of the hailstone in ambient air influences the total electric field near the hailstone only slightly but noticeably enhances energy exchange between neighboring oscillation modes. An air flow about the hailstone is shown to have an appreciable effect on the possibility of nonlinear resonance energy exchange between initially excited modes and modes due to the nonlinear interaction.     

Investigations of Droplet‐Plasma Interaction using Multi‐Dimensional Coupled Model

Recently developed multi‐dimensional coupled fluid‐droplet model is used to investigate the behavior of complex interaction between the liquid precursor droplets and atmospheric pressure plasma (APP). The significance of this droplet‐plasma interaction is not well understood under diverse realm of working conditions in two‐phase flow. In this study, we explain the implication of vaporization of liquid droplets in APP which are subsequently responsible to control major characteristics of surface coating depositions. Coalescence of water droplets is more dominant than Hexamethyldisiloxane (HMDSO) droplets because of its sluggish rate of evaporation. A disparity in the performance of evaporation is identified in two independent mediums, such as gas mixture and discharge plasma using HMDSO precursor. The length of evaporation of droplets is amplified by an increment of gas flow rate indicating with a reduction in the gas temperature and electron mean energy. In particular, the spatio‐temporal density distributions of charged particles show a clear pattern in which the typical nitrogen impurity ions are primarily effective as compared to other helium ionic species along the pulse of droplets in APP. Finally, we contrast the behavior of discharge species in the pure helium and He‐N2 gas mixtures revealing the importance of stepwise and Penning ionization processes. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New mechanism of current-induced cooling of quantum systems

A new mechanism of current-induced cooling of heterostructures is proposed and studied. As an example, a structure with two quantum wells was taken to determine the conditions under which electric current can flow in the situation where electrons and holes transfer from one well to another via indirect phonon-assisted tunneling. It is shown that this system can be used to create an inverse electron and hole population and simultaneously cool the laser active region by injection current. The universal relation is obtained according to which the cooling temperature of a quantum system is expressed only in terms of energy difference between the carrier size-quantization levels.     

Aqueous Droplet Sizing by Inertial Classification

The particle size distribution of an aerosol generated from an aqueous system is difficult to analyse because of the shrinkage of the droplets due to solvent evaporation. These problems are very important for the characterization of medical nebulizers, since most of the drugs delivered via inhalation are water soluble. In situ methods for droplet size analysis, such as laser diffraction, phase Doppler anemometry and light scattering, do not determine either the initial or the equilibrium size distribution. With the residual technique, which means evaporating the droplets and measuring the size and concentration of the residuals, the instability of the aqueous droplets plays no role and the necessary radioactive labelling of the sprayed material allows a direct determination of the mass flow rate at the mouthpiece of the nebulizer. In this way it is possible to discriminate between the delivered drug solution and the water necessary to humidify the incoming air. The output of nebulizers of different designs is given for various operating conditions, filling volumes and solution concentrations. The measured droplet size distribution of a nebulizer is found to be fixed mainly by its internal impaction system.     

Characteristics of electrohydrodynamically prepared titanium dioxide films

A titanium dioxide precursor sol flowing through a needle at a flow rate of 10^-10 m³ s^-1 was subjected to an electric field of 4.5 kV to generate droplets in the size range 0.3–6 μm. The droplets were collected on a silicon substrate to form uniformly thick, dense films. Raman spectroscopy, X-ray diffraction, field emission scanning electron microscopy and UV/Vis spectroscopy were used to characterize as-deposited and annealed films. Raman spectra show the annealed films were anatase phase with annealing converting it to the rutile phase. The energy bandgap of the titanium dioxide film annealed to 500 °C shows an indirect bandgap energy of 3.50 eV and a direct bandgap energy of 3.95 eV. PACS 81.15.Rs; 81.07.-b; 78.20.-e; 78.30.-j; 78.67.-n; 78.70.ck     

Vortex formation in a shock-accelerated gas induced by particle seeding

An instability forms in gas of constant density (air) with an initial nonuniform seeding of small particles or droplets as a planar shock wave passes through the two-phase medium. The seeding nonuniformity is produced by vertical injection of a slow-moving jet of air premixed with glycol droplets or smoke particles into the test section of a shock tube, with the plane of the shock parallel to the axis of the jet. After the shock passage, two counterrotating vortices form in the plane normal to that axis. The physical mechanism of the instability we observe is peculiar to multiphase flow, where the shock acceleration causes the second (embedded) phase to move with respect to the embedding medium. With sufficient seeding concentration, this leads to entrainment of the embedding phase that acquires a relative velocity dependent on the initial seeding, resulting in vortex formation in the flow.     

Fabrication of nano-structured gold films by electrohydrodynamic atomisation

A monodisperse alcosol containing 0.1 wt. % 15 nm gold particles was subjected to controlled flow through a metal capillary exposed to an electric field at the ambient temperature to generate an electrohydrodynamic jet, which subsequently disintegrated into droplets. A silicon substrate was used to collect the droplets and prepare gold films. By varying the deposition time, we have prepared gold films in the thickness range of ∼500–∼ 2000 nm in a maximum spray time of 450 s. This is a significant achievement considering the initial concentration of gold and the spray time involved. The characteristics of the jet, droplets and films prepared were evaluated using advanced analytical techniques. PACS 81.15.Rs; 81.15.-z; 81.07.-b; 81.20.-n; 68.55.jd     

Simulation of the two-phase flow in a thermosyphon using an inclined transparent tube with the lower-end closed and the upper-end open

The initial liquid charge of a vertically orientated two-phase closed thermosyphon for adequate thermal performance as determined theoretically by assuming that the condensate is in the form of a relatively thin film underestimates the amount determined by using experimental correlations. Knowing the physical details of the two-phase flow within the thermosyphon could explain this discrepancy. Because, however, of the difficulty of directly observing two-phase flow in an actual metal thermosyphon it was decided to investigate the two-phase flow by using air and water in a transparent tube. The tube that was used is closed at the lower end and open at the top end, was partially charged with water, and air was introduced into the closed end at increasing air flow rates until water droplets were just about to be expelled from the open end. The flow patterns occurring as a function of air flow rate were identified. The average liquid fraction in four sections of the tube was determined for different initial charge fractions and inclinations. It was observed that even at low air flow rates significant quantities of liquid were propelled up into the tube and that the flow is oscillatory. It was concluded that care would have to be taken in assuming a relatively thin and uniform liquid film in theoretically modelling a thermosyphon.