Development of a holographic particle diagnostic system and its application to measurements of spray characteristics

Particle diagnostics involving three dimensional distributions are important topics in many engineering fields. The holographic system is a promising optical tool for measuring three dimensional features of particles. In this study, we developed a holographic particle diagnostic system with diffused illumination to measure the sizes and 3-D velocities of moving particles using automatic image processing. First, basic optical systems for pulse laser recording, continuous laser reconstruction, and image acquisition were constructed. One of inherent limitations of particle holography is its long depth of focus in particle images, which causes considerable difficulty in determination of particle positions in the optical axis. To solve this problem, three new auto-focusing parameters (AFPs) corresponding to particle sizes were introduced. The developed system was applied to spray droplets to validate its capabilities. Three dimensional positions of particles viewed from two sides were decided using AFPs and then three dimensional particle velocities were extracted using a particle tracking algorithm. Comparison of measured sizes and three dimensional velocities of particles with those obtained using a laser instrument, PDPA (Phase Doppler Particle Analyzer), showed that the developed holographic system produced consistent results.     

Estimation of spray flow characteristics using ensemble Kalman filter

Spray flows are widely used in several industrial applications, such as combustion engines. Accurate measurement of spray flow characteristics requires sophisticated equipment and techniques. In recent years, the discrete droplet method (DDM), which analyses droplet scattering, has become a mature technique and has been applied to various analyses. We propose an estimation system based on particle image velocimetry (PIV) measurements and an ensemble Kalman filter, together with DDM, to efficiently investigate spray flow characteristics. The proposed method performs data assimilation on the velocity distribution in a two-dimensional cross-section obtained by PIV to estimate the characteristics of the spray flow in three dimensions. In this study, the system was constructed so that droplet particle is ensembled during data assimilation to estimate the droplet diameter distribution indirectly. The proposed method can be used to estimate the spray velocity and droplet size distribution. The numerical solution obtained using DDM was used as a criterion for assimilation and validated by conducting twin experiments. The results showed that, in terms of spray velocity, the estimation error for the velocity component parallel to the main flow was 2% and that for the velocity component perpendicular to the main flow was around 10%. Finally, the velocity and particle size distributions of the spray stream and the three-dimensional droplet distribution were estimated by assimilating the velocity distributions measured by PIV. This technique predicts the spray angle and droplet size distribution from the two-dimensional velocity field of the PIV only and is expected to contribute to the development of injectors and atomizers.     

阀芯结构对双流体喷雾粒子特性的影响

为了研究阀芯结构对双流体喷雾粒子特性的影响,提升喷雾效果,运用相位多普勒粒子分析仪（PDPA）对不同阀芯结构的双流体喷雾雾滴粒径、轴向速度以及雾滴数目进行了测试,并对测量结果进行了分析和讨论。结果表明：随着轴向距离的增大,雾滴索特平均直径(SMD)、算术平均直径(AMD)呈先增大后趋于平缓的趋势,轴向速度以及湍流脉动速度均呈减小趋势,雾滴数目呈先增大后减小的趋势;随着气液压力比的增大,SMD呈先增大后减小的趋势,而AMD、轴向速度以及雾滴数目均呈减小趋势;阀芯的喉口直径、出口直径的减小均有利于喷雾效果的提升,但同时导致速度稳定性变差;当喉口直径为1.5mm、出口直径为2.5mm时,与原始阀芯结构相比,雾滴数目和雾滴轴向速度分别增大了82.43%和22.31%,SMD和AMD分别减小了52.18%和21.47%,综合喷雾效果得到了大幅提升。     

Spray jet penetration and distribution of modulated liquid jets in subsonic cross-flows

Modulated liquid jets injected into subsonic cross-flows are empirically studied by using a mechanical liquid jet modulation apparatus. Experimental investigations were conducted using water over a range of cross-flow velocities from 5 m/s to 143 m/s and with modulated liquid jet frequencies from 35.7 Hz to 166.2 Hz and so on. PDPA(phase Doppler particle anemometry) was employed to measure droplet diameter and velocity with various spray cross-sections from Z/d=20 to Z/d=60. The spray structure, penetration depth, SMD(Sauter mean diameter), volume flux and velocity characteristics of modulated liquid jets injected into cross-flows were examined. As oscillation of the periodic pressure that could make liquid jet moved up and down in cross-flow field, the mixing process was facilitated. This phenomenon has the advantage of mixing the spray concentration from the center area to the outer area. Also, a bulk liquid jet puff was detected in the upper field of the liquid jet surface. The modulation effect appears significant in the extent of the spray oscillation. The correlation equations for the liquid jet boundary of the upper and lower regions which related to the Strouhal number have been presented to predict the spray structure under modulation conditions. Because of the modulation frequency, an inclination of averaged SMD for the structured layer was evanescent which contributed to the promotion of the macroscopic spray mixing process. Cross-sectional characteristics of SMD had the same tendency over a range of various modulation frequencies. As the modulation frequency increased, the region of volume flux distribution also increased.     

Spray characteristics of the rotating fuel injection system of a micro-jet engine

In micro-turbojet engines with less than 350 kW power, it is not easy to find a suitable fuel injector with good spray quality. However, the rotating fuel injection system can potentially provide high atomization quality without the high-pressure fuel pump through the centrifugal forces of the engine shaft. With this motivation, a very small rotating fuel injector with 40 mm diameter is designed for the micro-turbo jet engine. It is directly linked to a high-speed rotational spindle capable of a speed up to 100,000 rpm. The droplet size, velocity, and spray distribution from the PDPA (Phase Doppler Particle Analyzer) system are measured. The spray is also visualized by a high-speed camera. The test results show that the length of liquid column from injection orifice is controlled by the rotational speeds and that SMD (Sauter Mean Diameter) is decreased with increasing rotational speeds. At a rotational speed of 73.3 m/s (35,000 rpm), SMD is lower than 60 μm at the entirety of the measuring space in the case of Type 2 (injection orifice diameter of 1.5 mm) and Type 3 (injection orifice diameter of 2.2 mm). Therefore, conceptually, it is possible to apply this small rotating fuel injection system to the micro-turbojet engine combustor.     

Note: Accuracy of velocity correction for impact of a laser-accelerated miniature flyer with lithium fluoride shock-compressed along the [100] axis

We performed miniature flyer impact experiments to investigate the relationship between the apparent (u(a)) and actual (u(A)) particle velocities measured by a velocity interferometer in single-crystal lithium fluoride (LiF) that was shock-compressed along the [100] axis. The miniature flyer was accelerated to velocities in the range 652.5-1937.6 m/s by a tabletop pulsed laser. An empirical relationship of u(a) = (1.2749 ± 0.0102)u(A) was obtained. The obtained relationship agreed well with the results of a previous study within the experimental errors and its uncertainty was less than ±1%. This result indicates that the present experimental technique is effective for measuring the relationship between u(a) and u(A) of shocked transparent materials with a comparable accuracy to conventional methods.     

A study on the spray characteristics of a piezo pintle-type injector for DI gasoline engines

We studied the spray flow initiated from a piezo pintle-type injector for DI gasoline engines in an environment supplied by a constant volume vessel by means of laser diagnostics. To fully grasp the effects of the characteristic parameters, including designed spray angle, needle lift, injection pressure (P _inj ) and ambient pressure (P _b ), on the spray atomization and mixture preparation, particle image velocimetry (PIV) and phase Doppler anemometry (PDA) are used in the experiment, respectively. The gas perpendicularly enters into the outer periphery of the conical spray injected through the pintle-type injector activated by piezo, which creates two large-scale vortices: the vortex A and vortex B. The velocity standard deviation of the spray field is introduced to analyze the gas flow motion in the vicinity of nozzle. The droplet information of spray field is also recorded by PDA in variable boundary conditions. The time dividing method is used to study the droplet characteristics in four parts of spray. The injector with 98° designed spray angle has smaller droplet mean diameter (D10 and D32), due to a larger spray distribution. When the droplet velocity of the spray field is close to 0 m/s, the D10 and D32 hold at around 10 μm and 20 μm, respectively, in atmospheric pressure condition, which are about 20 μm and 40 μm, respectively, at ambient pressure of 1.1 MPa.     

Thermomechanical finite element analysis of slider-disk impact in magnetic storage thin film disks

Oblique impact of a slider with a rotating disk in a hard disk drive was analyzed using the finite element method. A three dimensional, thermomechanical, impact model was developed to study the mechanical and thermal response during the impact of a spherical slider corner with a rotating disk. The model was validated by comparing the finite element results with analytical solutions for a homogeneous glass substrate disk. Impact penetration, stress and incurred flash temperature were obtained for various normal impact velocities. The effects of material layers on the disk were also investigated by introducing layers with different material properties and thicknesses. It was found that for a rounded slider corner and a critical normal impact velocity of 0.03 m/s studied in this work, the layers have insignificant effects on the mechanical response and small but predictable effects on the flash temperature.     

Spray characteristics on the electrostatic rotating bell applicator

The current trend in automotive finishing industry is to use more electrostatic rotating bell (ESRB) need space to their higher transfer efficiency. The flow physics related with the transfer efficiency is strongly influenced by operating parameters. In order to improve their high transfer efficiency without compromising the coating quality, a better understanding is necessary to the ESRB application of metallic basecoat painting for the automobile exterior. This paper presents the results from experimental investigation of the ESRB spray to apply water-borne painting. The visualization, the droplet size, and velocity measurements of the spray flow were conducted under the operating conditions such as liquid flow rate, shaping airflow rate, bell rotational speed, and electrostatic voltage setting. The optical techniques used in here were a microscopic and light sheet visualization by a copper vapor laser, and a phase Doppler particle analyzer (PDPA) system. Water was used as paint surrogate for simplicity. The results show that the bell rotating speed is the most important influencing parameter for atomization processes. Liquid flow rate and shaping airflow rate significantly influence the spray structure. Based on the microscopic visualization, the atomization process occurs in ligament breakup mode, which is one of three atomization modes in rotating atomizer. In the spray transport zone, droplets tend to distribute according to size with the larger drops on the outer periphery of spray. In addition, the results of present study provide detailed information on the paint spray structure and transfer processes.     

Measurements of the characteristics of spray droplets using in-line digital particle holography

In-line digital particle holography is applied to measure the characteristics of spray droplets. Common reconstruction methods were considered and the best one was selected. Several important parameters at the time of hologram recording, such as the object distance and the region of laser beam used, are discussed. The feasibility of the correlation coefficient (CC) method for focal plane determination of 3-D droplets was verified. A double exposure hologram recording system with synchronization system for time control was established, and two digital spray holograms were obtained in a short time interval. For post-processing of reconstruction images, the two-threshold and the image segmentation methods were used in binary image transformation. Using the CC method and some image processing techniques applied to droplets in each double exposure image, the spatial positions of droplets used to evaluate the three dimensional droplet velocities were easily located, which proved the feasibility of in-line digital particle holographic technology as a good measurement tool for spray droplets. This paper was recommended for publication in revised form by Associate Editor Gihun Son Boseon Kang received his B.S. and M.S. degrees in Mechanical Engineering from Seoul National University in 1986 and 1988, respectively. He then went on to receive his Ph.D. degree from University of Illinois, Chicago in 1995. He is currently Professor at School of Mechanical Systems Engineering, Chonnam National University in Gwangju, Korea. His research interests are in the area of sprays, holographic techniques in thermofluid measurements. Yan Yang received his B.S. degree in Mechanical Engineering from Chongqing Insitute of Technology in 1997, and received his M.S. degree in Mechanics from Chongqing University in 2005. He is doctoral student of Department of Mechanical Systems Engineering, Chonnam National University in Gwangju, Korea. He is also currently Associate Professor at Automobile College, Chongqing University of Technology in Chongqing, China. His research direction is digital holographic techniques     

高速电弧喷涂粒子速度和雾化特性研究

对比研究了高速电弧喷涂和普通电弧喷涂ＡＩ和３Ｃｒ１３的粒子速度和雾化特性,用Ｐｉｔｏｔ管总压法。双转盘法和扫描电镜、图象分析仪分别测定了喷枪出口雾化气流速度和粒子速度的轴向分布,以及不同工艺条件下雾化粒子的粒度分布。用ＰｅａｒｓｏｎＸ２分布拟合检验法对粒子的粒度分布进行统计分布检验。结果表明,高速电弧喷涂ＡＩ和３Ｃｒ１３粒子的平均飞行速度为３４２ｍ／ｓ和３８８ｍ／ｓ;雾化粒子平均粒度分别为普通电弧喷涂的１／３和１／８;雾化气流轴向速度在主要雾化区间（ｄ＜１００ｍｍ）为７００－５５０ｍ／ｓ;高速电弧喷涂和普通电弧喷涂川粒子的粒度具有相同的分布规律,而高速电弧喷涂３Ｃｒ１３粒子的粒度却具有不同的分布规律。探讨了高速电弧喷涂雾化机理和影响粒子速度的因素。     

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.     

The μPIVOT: an integrated particle image velocimeter and optical tweezers instrument for microenvironment investigations

A novel instrument to manipulate and characterize the mechanical environment in and around microscale objects in a fluidic environment has been developed by integrating two laser-based techniques: micron-resolution particle image velocimetry (μPIV) and optical tweezers (OT). This instrument, the μPIVOT, enables a new realm of microscale studies, yet still maintains the individual capabilities of each optical technique. This was demonstrated with individual measurements of optical trap stiffness (～70 pN μm(-1) for a 20 μm polystyrene sphere and a linear relationship between trap stiffness and laser power) and fluid velocities within 436 nm of a microchannel wall. The integrated device was validated by comparing computational flow predictions to the measured velocity profile around a trapped particle in either a uniform flow or an imposed, gravity-driven microchannel flow (R(2) = 0.988, RMS error = 13.04 μm s(-1)). Interaction between both techniques is shown to be negligible for 15 μm to 35 μm diameter trapped particles subjected to fluid velocities from 50 μm s(-1) to 500 μm s(-1) even at the highest laser power (1.45 W). The integrated techniques will provide a unique perspective toward understanding microscale phenomena including single-cell biomechanics, non-Newtonian fluid mechanics and single particle or particle-particle hydrodynamics.     

光学互相关测速系统设计与验证

针对两相流颗粒运动速度测量及基于互相关原理设计了双光路激光测速系统。采用搭建的变频电机带动绕丝产生已知旋转线速度的装置开展光学互相关测速验证实验。通过测量绕丝经双光路激光的光强衰减信号,再对两路信号进行互相关分析,从而得到测点绕丝旋转线速度。以电机转速计算的测点绕丝线速度作为参考值进行测量精度验证,得到光学互相关测速方法的测量相对误差在6%以内,验证了光学互相关测速方法的准确性。     

Characteristics of a liquefied butane spray

The characteristics of a butane spray from pintle-type injector were studied by droplet velocity and diameter measurements and high speed photography. The accumulator type injector operated off a common rail fuel supply system operated at 13 MPa, and was controlled by a high-speed solenoid valve. Injection was carried out in a chamber at ambient temperature and at the pressure above (0.37 MPa) and below (0.15 MPa) the fuel vapor pressure. Two component phase/Doppler particle analyzer and traverser were used to obtain the droplet diameter and the velocity at numerous locations in the spray. The entire injection event was analyzed as a time-average and also subdivided into three temporal intervals. A, B, and C. The high-speed photographs showed a narrower cone angle during the quasi-steady spray period at the 0.37 MPa chamber pressure compared to the 0.15 MPa case.     

Effect of nozzle geometry for swirl type twin-fluid water mist nozzle on the spray characteristic

Experimental investigations on the atomization characteristics of twin-fluid water mist nozzle were conducted using particle image velocimetry (PIV) system and particle motion analysis system (PMAS). The twin-fluid water mist nozzles with swirlers designed two types of swirl angles such as 0°, 90° and three different size nozzle hole diameters such as 0.5mm, 1mm, 1.5mm were employed. The experiments were carried out by the injection pressure of water and air divided into 1bar, 2bar respectively. The droplet size of the spray was measured using PMAS. The velocity and turbulence intensity were measured using PIV. The velocity, turbulence intensity and SMD distributions of the sprays were measured along the centerline and radial direction. As the experimental results, swirl angle controlled to droplet sizes. It was found that SMD distribution decreases with the increase of swirl angle. The developed twin-fluid water mist nozzle was satisfied to the criteria of NFPA 750, Class 1. It was proven that the developed nozzle under low pressures could be applied to fire protection system.     

Experimental analysis and numerical modeling using lisa-ddb hybrid breakup model of direct injected gasoline spray

This paper presents the effect of injection pressure on the atomization characteristics of high-pressure injector in a direct injection gasoline engine both experimentally and numerically. The atomization characteristics such as mean droplet size, mean velocity, and velocity distribution were measured by phase Doppler particle analyzer. The spray development, spray penetration, and global spray structure were visualized using a laser sheet method. In order to investigate the atomization process in more detail, the calculations with the LISA-DDB hybrid model were performed. The results provide the effect of injection pressure on the macroscopic and microscopic behaviors such as spray development, spray penetration, mean droplet size, and mean velocity distribution. It is revealed that the accuracy of prediction is promoted by using the LISA-DDB hybrid breakup model, comparing to the original LISA model or TAB model alone. And the characteristics of the primary and secondary breakups have been investigated by numerical approach.     

A Superheterodyne Three-Component Laser Doppler Anemometer

The design, method of tuning, and operating principle of a three-component laser Doppler anemometer with a symmetrical heterodyning optical system are described. The use of an optical frequency modulator with a 100% modulation factor and its possibility of continuously changing the light frequency allows the anemometer to operate in a superheterodyne mode. The range of measured velocities is from 1 mm/s to 50 m/s. The relative error of the velocity measurements in a multiparticle regime is at most 5%. The methodological capabilities of the instrument are demonstrated for an example of measuring the gas velocity field in a prototype of a vortex tube. It is pointed out that the anemometer can operate under forward and backward laser-light scattering conditions.     

均匀液滴喷射过程仿真与试验研究

针对液滴喷射增材制造试验参数调整困难、实施难度较大的现存问题,基于流体体积(Volume of fluid,VOF)两相流模型,建立均匀液滴喷射过程流场的计算模型。采用数值模拟的方法,对液滴喷射过程中的液滴流形态、压力场和速度场及其影响因素进行了研究,揭示了形成均匀液滴流的内在变化规律,得到了均匀液滴喷射过程的最优频率。在模拟结果的基础上,建立了液滴喷射装置并配置了相应的高速拍照系统,对射流断裂形态、喷射过程、喷射速度进行了试验研究。结果表明,射流速度主要取决于喷射压强,液滴流均匀性主要取决于扰动频率和扰动振幅,射流的压力场则呈周期性变化。模拟结果与试验结果吻合较好,说明所提出的建模方法是可行的,为不同情况下射流内部流场的计算提供了实用的方法,也为液滴喷射增材制造技术的应用奠定了理论基础。     

A study on the behavior and heat transfer characteristics of impinging sprays

The spray/wall interaction is considered as an important phenomenon influencing air-fuel mixing in the internal combustion engines. In order to adequately represent the spray/wall interaction process, impingement regimes and post-impingement behavior have been modeled using experimental data and conservation constraints. The modeled regimes were stick, rebound, spread and splash. The tangential velocities of splashing droplets were obtained using a theoretical relationship. The continuous phase was modeled using the Eulerian conservation equations, and the dispersed phase was calculated using a discrete droplet model. The numerical simulations were compared to experimental results for spray impingement normal to the wall. The predictions for the secondary droplet velocities and droplet sizes were in good agreement with the experimental data.