Three dimensional drop tracking flow chamber for coalescence studies

We have developed a novel flow chamber which imposes a controlled axisymmetric stagnation flow to enable the study of external flow effects on coalescence dynamics. This system allows for the first time the precise positioning of a drop in a three dimensional flow and additionally enforces a highly symmetric flow around the drop. We focus on the study of a single drop approaching a stationary flat plane as this is analogous to two drops approaching each other. A single drop is created and then guided along the unsteady center line of a stagnation flow. The real time computer control algorithm analyzes video images of the drop in two orthogonal planes and manipulates flow restricting valves along the four outlets of the flow. We demonstrate using particle image velocimetry that the computer control not only controls the drop position but also ensures a symmetric flow inside the flow chamber. This chamber will enable a detailed investigation of the drainage of the thin film between the drop and the lower surface in order to probe the effect of external flow on coalescence.     

A macro-microscopic investigation of high-pressure sprays injected by a common rail system

The main purpose of this research is to investigate high-pressure diesel oil sprays generated by a common-rail system through small nozzle holes. High-speed photography, phase Doppler particle analyzer (PDPA), and a combination of data obtained with high-speed photography and PDPA, based on the light extinction principle within sprays are used to deal with the objective. The sprays are characterised in an environment, which simulates in-cylinder air density of the actual diesel engine when the injection starts. However, it must be pointed out that isothermal condition at room temperature is considered and no-evaporation of drops occurs. A wide parametric study has generated evidence needed to quantify the influence of the common-rail pressure, nozzle hole diameter and environment gas density on the macroscopic evolution of sprays (spray tip penetration, and spray cone angle) as well as microscopic behaviour (spatial and temporal evolution of drop size, drop velocity and drop concentration distributions).     

Numerical simulation of the freezing process of a water drop attached to a cold plate

This paper presents a direct numerical work on the freezing process of a water drop that is either sessile on or pendant from a cold plate. The numerical technique used is an axisymmetric front-tracking method to represent interfaces that separate different phases. The sessile drop corresponds to positive Bond numbers Bo (i.e., Bo > 0), and the pendant drop represents the other values of Bo. Numerical results show that pendant drops break up into liquid drops when gravity dominates the force induced by surface tension at Bo < 0. That is, a decrease in Bo enhances the breakup of the freezing drop. The breakup also depends significantly on the initial shape of the drop in terms of the contact angle at the plate ?₀, that is, increasing ?₀ induces breakup. In addition, the drop rapidly completes freezing due to breakup. In the case of non-breakup, the increase in Bo reduces the solidified drop height and decreases the time to complete solidification. The freezing process also consumes minimal time with small ?₀. The solidified drop after solidification has a cone near the axis of symmetry due to volume expansion of water upon solidification. This shape of the solidified drop is in accordance with the experimental observation.     

光纤、电容液滴传感器的设计研究

光纤、电容液滴分析技术是利用光纤和电容测试技术制成特殊的液滴传感器，获取经过液滴的光强信号随液滴生长变化的规律以及有关液滴体积的信息。文中将具体介绍光纤、电容液滴传感器的工作原理、电容液滴传感器的数学模型和液滴滴头的结构设计，同时给出由传感器获取的与液滴生长过程相关的光纤信号和液滴体积信号。     

Theory of free fall breakup of large drops

This paper presents a theoretical investigation of the bag-mode breakup of liquid drops freely falling through the gas or another liquid of less density. A shape of flattened drop is postulated, a condition of equilibrium of drop formulated, a model of blowing up of the bag proposed and the respective definition of critical Weber number introduced. There are computed: minimal Weber number when the drop begins to flatten, degree of flattening, critical Weber number and secondary drops diameter. Appendix contains a generalization of the investigation on the drop of liquid and bubble of gas travelling upwards. The paper may be utilized in the study of precipitation, as well as in designing of cooling towers and the scrubbers.     

Study of the motion of a spheroidal drop in a linear shear flow

The motion of a spheroidal deformable drop in a simple shear flow is simulated using a finite-difference/front-tracking method. The effect of surface tension coefficient, viscosity ratio and inertia on lateral migration and deformation of the drop is investigated. It is revealed that the deformation of a spheroidal drop is directly related to the capillary and Reynolds numbers. In the limit of finite Reynolds numbers, the equilibrium position of prolate drops depends strongly on the viscosity ratio; the final position of more viscous drops is closer to the wall in contrast with the spherical ones. As the deformability of drops increases and the inertial force decreases, the rate of migration of the prolate drops increases. Although the steady-state position does not depend on the capillary and Reynolds numbers, the migration rate depends considerably on these dimensionless parameters. In addition, the rate of migration is a decreasing function of the aspect ratio due to different direction of the lift force acting on the drop.     

A combined small- and wide-angle x-ray scattering detector for measurements on reactive systems

A detector with high dynamic range designed for combined small- and wide-angle x-ray scattering experiments has been developed. It allows measurements on single events and reactive systems, such as particle formation in flames and evaporation of levitating drops. The detector consists of 26 channels covering a region from 0.5° to 60° and it provides continuous monitoring of the sampled signal without readout dead time. The time resolution for fast single events is about 40 μs and for substances undergoing slower dynamics, the time resolution is set to 0.1 or 1 s with hours of continuous sampling. The detector has been used to measure soot particle formation in a flame, burning magnesium and evaporation of a toluene drop in a levitator. The results show that the detector can be used for many different applications with good outcomes and large potential.     

A numerical analysis of drop impact on liquid film by using a level set method

Splashing and spreading of a liquid by drop impact on liquid film depends on the impact velocity, drop size, drop properties and liquid film thickness. These parameters can be summarized by three main dimensionless parameters: Weber number, Ohnesorge number and non-dimensional film thickness. Upon impact of a drop on liquid film, these parameters influence the shape of the splash and the formation and propagation of the crown. In the present study, the splashing and spreading resulting from drop impact on liquid film has been numerically investigated by using a Level Set method for the interface tracking of the two-phase flow simulation. For various dimensionless parameters, characteristics of the crown formation and spreading were predicted, and the results were found to show good agreement with available experimental data in the earlier stage of crown formation and show some discrepancies in the later stage of crown spreading due to the present 2D axi-symmetric computation, which cannot predict the secondary drops.     

Contact angle of water on paper components: Sessile drops versus environmental scanning electron microscope measurements

The wetting properties of paper components and sizing agents were investigated by measuring contact angles of water drops applied by the drop test method on the sample surfaces, and by observing microscopic drops in the environmental scanning electron microscope (ESEM). The results are comparable, although measurements from the ESEM observations are approximate. The ESEM observations give insight into the wetting at the microscopic level, the formation of drops, and the spreading and absorption of water drops. Absorbent samples engulf all condensing water, resulting in a water- saturated swollen structure with no visible drop formation. On the nonabsorbent samples, water drops form and spread on the sample surfaces.     

Variation and recovery of resistivity of industrial greases — an experimental investigation

In the present work, variation in the resistivity of different greases with time in static conditions, under the influence of applied potential drops, is established. The change in resistivity and physico-chemical characteristics of the greases in electric fields, and recovery of the resistivity within the test period in the absence of applied potential drop, have been found. The properties of the greases are compared, and their correlation with resistivity is analysed. The paper also gives the results of analysis using an SRV tester, IR spectra, chemical analysis and spectrophotometry, of changes that occurred periodically in the greases, normally used in non-insulated rolling-element bearings in the presence of a potential drop across the inner and outer races. The paper highlights the behaviour and mechanism of the process of bearing failure under electric fields.     

On the breakup mechanisms of air-assisted drops in a high speed air

An experimental study was performed to investigate break-up mechanisms of liquid drops injected into a transverse high velocity air jet. The range of conditions included the three drop breakup regimes previously referred to as bag, shear or boundary layer stripping, and ‘catastrophic’ breakup regimes. The results show that the break-up mechanism consists of a series of processes in which dynamic pressure effects deform the drop into a thin liquid sheet. The flattened drop subsequently breaks up into small droplets. At high relative velocity, in the ‘catastrophic’ breakup regime, drops are flattened and fragmented by relatively large wavelength waves whose wavelengths and growth rates are consistent with estimates from Rayleigh-Taylor instability theory. The minute drops that are also produced at this high relative velocity appear to originate from short wave length of Kelvin-Helmholtz waves growing on the larger liquid fragments.     

Measurement of high-water-content oil-water two-phase flow by electromagnetic flowmeter and differential pressure based on phase-isolation

When the oil field has been exploited by long-term water-flooding, it will be in high water-content stage of production. However, it is a great challenge for high-water-content measurement due to oil droplets extremely dispersed in the water. In this paper, we developed a phase-isolation based method for high-water-content oil-water two-phase flow measurement. Phase-isolation was realized by axial-flow swirler to concentrate scattered and random oil droplets into the pipe center and change the inlet flow pattern into a particular annular flow before measuring. Owing to the axisymmetric velocity and phase distribution, the electromagnetic flow meter avoided the effect of random distribution of insulating phase, and then had a good measurement performance for total volume flow rate. Furthermore, we respectively studied using axial pressure drop, radial pressure drop and the ratio of the two pressure drops to measure water content. The results showed that the ratio of the two pressure drops not only improves the resolution of oil and water, but also effectively reduces the impact of error transfer. In the dual-parameter measurement experiment, the relative errors of total volume flow rate and water content were almost within ±5%.     

某导弹惯性敏感装置跌落仿真分析

为了保证勤务处理惯性敏感装置跌落时结构完好和安全,运用ADAMS软件仿真分析了某导弹惯性点火器中作为惯性敏感装置的弹簧-质量系统的跌落响应特性,得到了针对不同跌落目标时惯性弹簧受力曲线。惯性敏感装置跌向不同介质目标时,惯性体与其下方的座体平面仅有一次碰撞,且由该碰撞产生的释放钟表机构的窗口时间远小于钟表机构的解除保险时间,之后惯性体又在惯性簧的作用下复位,因此该惯性敏感装置设计是能够保证跌落安全和结构正确性的。     

Pressure drop of wet gas flow with ultra-low liquid loading through DP meters

The most common method to predict the gas and liquid flow rates in a wet gas flow simultaneously is to use dual pressure drops (dual-DPs) from two or even one single DP meter. In this paper, the metering mechanism of applying dual-DPs were overviewed. To fully understand the response of DP meters to wet gas flows, the pressure drops of wet gas flow with ultra-low liquid loading through three typical DP meters were experimentally investigated, including an orifice plate meter, a cone meter and a Venturi meter. The equivalent diameter ratio is 0.45. The experimental fluids are air and tap water. The pressure is in the range of 0.1–0.3 MPa and the Lockhart-Martinelli parameter (X_LM) is less than approximately 0.02. The results show that the upstream-throat pressure drop, the downstream-throat pressure drop and the permanent pressure loss of individual DP meters have unique response to liquid loading. The upstream-throat pressure drop of the orifice plate meter decreases at first and then increases as the liquid loading increases, while that of the cone meter and the Venturi meter increase monotonically. The non-monotonicity of the pressure drop for the orifice plate meter can be attributed to the flow modulation of trace liquid. The downstream-throat pressure drops of all the three test sections decrease at first and then increase. The reason is that the liquid presence in a gas flow increases the downstream friction and vortex dissipation. The permanent pressure loss of the orifice plate meter also shows non-monotonicity. To avoid non-monotonicity, the pressure loss ratio is introduced, which is defined as the ratio of the permanent pressure loss to the upstream-throat pressure drop. Results show that the pressure loss ratio of the Venturi meter has the highest sensitivity to the liquid loading.     

Recent progress of spray-wall interaction research

In the present article, recent progress of spray-wall interaction research has been reviewed. Studies on the spray-wall interaction phenomena can be categorized mainly into three groups: experiments on single drop impact and spray (multiple-drop) impingement, and development of comprehensive models. The criteria of wall-impingement regimes (i.e., stick, rebound, spread, splash, boiling induced breakup, breakup, and rebound with breakup) and the post-impingement characteristics (mostly for splash and rebound) are the main subjects of the single-drop impingement studies. Experimental studies on spray-wall impingement phenomena cover examination of the outline shape and internal structure of a spray after the wall impact. Various prediction models for the spray-wall impingement phenomena have been developed based on the experiments on the single drop impact and the spray impingement. In the present article, details on the wall-impingement criteria and post-impingement characteristics of single drops, external and internal structures of the spray after the wall impact, and their prediction models are reviewed.     

Characterization of drop-on-demand microdroplet printing

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been used extensively in the fabrication of polymer light-emitting diode displays by drop-on-demand inkjet printing technique. In order to make a good thin film with high uniformity, it is crucial to control the drying behavior of inkjet-printed PEDOT:PSS droplets. In this work, the morphology of printed drops of ～10 pL after drying was examined and explained in a qualitative manner, which would extend the comparison with previous work described in published literature. Results show that drying behavior is sensitive to substrate temperature due to the high surface-to-volume ratio of a microfluid. There exists an appropriate range of temperatures to obtain a good drop with sharp edge and flat surface. On the other hand, when the temperature exceeds a limit, a ring-like structure was observed. Results from this experiment would be considered as a reference to achieve uniform film after drying and has potential to be applied in the modeling of drop impact.     

A Cell Design for Minimizing iR Error in Controlled-Potential Polarography of High Specific Resistance Solutions

SUMMARY

A design is presented for a polarographic cell that minimizes the iR error in controlled-potential polarography of solutions possessing a high specific resistance. A sharpened Smoler 90° vertical orifice DME is used. With this electrode, the mercury drop grows principally in the downward direction. This and the fact that the end of the capillary is sharpened to a point allows the positioning of a micro reference electrode probe directly above the mercury drop and to within 0.1 of the maximum radius of the mercury drop from the mercury surface during the entire drop life time. With this apparatus it is also possible to observe the iR error as a function of the distance between the reference electrode tip and the mercury drop surface. The effect of reference electrode position on iR error introduced is illustrated by a series of uranium and cadmium polarograms in high specific resistance media. Controlled-potential polarograms with little distortion attributable to iR error can be obtained with this cell design in solutions with specific resistances up to at least 22,000 Ω-cm.     

Evaluation of hot-film,dual optical and Pitot tube probes for liquid–liquid two-phase flow measurements

An experimental study of kerosene–water upward two-phase flow in a vertical pipe was carried out using hot-film, dual optical and Pitot tube probes to measure the water, kerosene drops and mixture velocities. Experiments were conducted in a vertical pipe of 77.8 mm inner diameter at 4.2 m from the inlet (L/D=54). The tests were carried out for constant superficial water velocities of 0.29, 0.59 and 0.77 m/s (flow rates = 83, 167 and 220 l/min) and volume fractions of 4.2%, 9.2%, 18.6% and 28.2%. The Fluent 6.3.26 was used to model the single and two-phase flow and to reproduce the results for the experimental study. Two methods were used to evaluate the accuracy of the probes for the measurement of the velocities of water, drops and mixture for two-phase flow: (i) comparison of measured local velocities with predictions from the CFD simulation; (ii) comparison between the area-averaged velocities calculated from the integration of the local measurements of water, drops and mixture velocities and velocities calculated from flow meters’ measurements.The results for single phase flow measured using Pitot tube and hot-film probe agree well with CFD predictions. In the case of two-phase flow, the water and drops velocities were measured by hot-film and dual optical probes respectively. The latter was also used to measure the volume fraction. These three measured parameters were used to calculate the mixture velocity. The Pitot tube was also used to measure the mixture velocity by applying the same principle used for single phase flow velocity. Overall the mixture local velocity measured by Pitot tube and that calculated from hot-film and dual optical probe measurements agreed well with Fluent predictions. The discrepancy between the mixture area-averaged velocity and velocity calculated from flow meters was less than 10% except for one test case. It is concluded that the combined hot-film and optical approach can be used for water and drop velocity measurements with good accuracy for the flow conditions considered in this study. The Pitot tube can also be used for the measurement of mixture velocities for conditions of mixture velocities greater than 0.4 m/s. The small discrepancy between the predictions and experimental data from the present study and literature demonstrated that both instrumentation and CFD simulations have the potential for two-phase flow investigation and industrial applications.     

Numerical investigation of dynamic behavior of a compound drop in shear flow

We present a numerical investigation of the deformation and breakup of a compound drop in shear flow. The numerical method used in this study is a two-dimensional front-tracking/finite difference technique for representing the interface separating two fluids by connected elements. The compound drop with the initially circular and concentric inner and outer fronts is placed at the center of a domain whose top and bottom boundaries move in the opposite direction. Because of the shear rate, the compound drop deforms and can break up into drops, depending on the flow conditions based on the Reynolds number Re, the Capillary number Ca and the interfacial tension ratio σ₂₁ of the outer to inner interfaces. We vary Re in the range of 0.1-3.16, Ca in the range of 0.05-0.6 and σ₂₁ in the range of 0.8-3.2 to reveal the transition from the non-breakup to breakup regimes. Numerical results indicate that the compound drop breaks up into drops when there's an increase in Re or Ca or a decrease in σ₂₁ beyond the corresponding critical values. We also propose a phase diagram of Ca versus Re that shows the region in which the compound drop changes from the deformation mode to the breakup mode.     

Volume ratio and pressure drop on hydraulic dynamic pressure calibration system

The calibration of a pressure transducer that works in dynamic conditions is an unavoidable challenge. To address this challenge, an experimental setup has been designed and developed to simultaneously generate and calibrate the dynamic pressure. The system mainly comprises a pressure chamber that accommodates a step pressure generator and test chamber with a quick open valve and pressure sensor under calibration. Accordingly, an aperiodic type of step pressure generator with a quick open valve is particularly designed. This instrument can generate a positive step pressure with a rise time of up to 11 ms and a negative step pressure with a fall time of up to 12 ms within a working pressure range from the atmospheric pressure to 200 MPa. The volume ratio of the test chamber to the pressure chamber and its effect on pressure drop is critical in such systems. The effect of volume ratio on the dynamic performance parameters, namely, rise/fall time, is explored and inferences for optimizing these parameters are derived. A mathematical model for pressure drop in the test chamber has been developed and validated to precisely determine the magnitude of the step pressure input. Lastly, the experimental study of the effect of volume ratio on pressure drop has been conducted to diminish the pressure drop, thereby minimizing the uncertainty.