LDA/PDA measurements of instantaneous characteristics in high pressure fuel injection and swirl spray

 Transient dynamics of two injection flows, upstream and downstream a swirl injector, are investigated. Capillary n-heptane pipe flow is measured using laser Doppler anemometer to obtain instantaneous time series of centerline velocity and to reconstruct series of instantaneous and integrated flow rates and pressure gradient. A collimated laser sheet and a high-speed video camera visualize injected spray flow. Finally, the phase Doppler anemometer measurements are introduced to analyze instantaneous patterns of droplets velocity-size and number density into fuel spray. All measurements are employed at similar temporal resolution close to 30 μs. Results indicate that both flows are strongly time-dependent and well correlated in time-phases. Initial transitions are completed by 100 μs. Opening or closing of the injector valve affects both flows as strong delta oscillation causes spray penetration dynamics and a post injection effect. A combination of intrusive laser-based techniques allows indication of the basic injection and spraying characteristics need to optimize high-pressure fuel injectors and combustion late injection mode at a high speed. Received: 19 December 1998/Accepted: 13 August 1998     

Electrostatic interaction of free EHD jets

Three spray systems with multiple nozzles arranged linearly, of different geometrical configurations were tested experimentally: (1) spray nozzles without induction electrodes facing a distant plate, (2) spray nozzles with two parallel bar induction electrodes, and (3) spray nozzles with individual ring induction electrodes. The systems were set to operate at the precession mode of spraying. In all the systems, except that of individual induction electrodes, the precession mode of spraying was first set at peripheral nozzles, and next, for higher voltages, at central nozzles. In the system with ring induction electrodes, the precession mode of spraying was generated for all nozzles at the same voltage level. The size and charge distributions of the droplets were measured for each spray system. The charge distribution of the droplets was measured with a ring induction probe, and it was compared with the charge distribution determined numerically from the size distribution of the droplets, based on the Rayleigh limit formula. It was shown that both these distributions correlate quite well. Free droplets suspended and moving irregularly within the centre of the charged spray plume were observed in the system with individual rings.     

Leading-Edge Reaction Zones in Lifted-Jet Gas and Spray Flames

An investigation of the leading edge characteristics in lifted turbulent methane-air (gaseous) and ethanol-air (spray) diffusion flames is presented. Both combustion systems consist of a central nonpremixed fuel jet surrounded by low-speed air co-flow. Non-intrusive laser-based diagnostic techniques have been applied to each system to provide information regarding the behavior of the combustion structures and turbulent flow field in the regions of flame stabilization. Simultaneous sequential CH-PLIF/particle image velocimetry and CH-PLIF/Rayleigh scattering measurements are presented for the lifted gaseous flame. The CH-PLIF data for the lifted gas flame reveals the role that ``leading-edge' combustion plays as the stabilization mechanism in gaseous diffusion flames. This phenomenon, characterized by a fuel-lean premixed flame branch protruding radially outward at the flame base, permits partially premixed flame propagation against the incoming flow field. In contrast, the leading edge of the ethanol spray flame, examined using single-shot OH-PLIF imaging and smoke-based flow visualization, does not exhibit the same variety of leading-edge combustion structure, but instead develops a dual reaction zone structure as the liftoff height increases. This dual structure is a result of the partial evaporation (hence partial premixing) of the polydisperse spray and the enhanced rate of air entrainment with increased liftoff height (due to co-flow). The flame stabilizes in a region of the spray, near the edge, occupied by small fuel droplets and characterized by intense mixing due to the presence of turbulent structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental and analytical investigation of liquid sheet breakup characteristics

The main objective of this research is to study analytically and experimentally the liquid sheet breakup of a flat fan jet nozzle resulting from pressure-swirling. In this study the effects of nozzle shape and spray pressure on the liquid sheet characteristics were investigated for four nozzles with different exit widths (1.0, 1.5, 2.0 and 2.5 mm). The length of liquid sheet breakup, liquid sheet velocity and the size of formed droplets were measured by a digital high speed camera. The breakup characteristics of plane liquid sheets in atmosphere are analytically investigated by means of linear and nonlinear hydrodynamic instability analyses. The liquid sheet breakup process was studied for initial sinuous and also varicose modes of disturbance. The results presented the effect of the nozzle width and the spray pressure on the breakup length and also on the size of the formed droplets. Comparing the experimental results with the theoretical ones for all the four types of nozzles, gives a good agreement with difference ranges from 4% to 12%. Also, the comparison between the obtained results and the results due to others shows a good agreement with difference ranged from 5% to 16%. Empirical correlations have been deduced describing the relation between the liquid sheet breakup characteristics and affecting parameters; liquid sheet Reynolds number, Weber number and the nozzle width.     

Breakup and atomization characteristics of mono-dispersed diesel droplets in a cross-flow air stream

This paper describes the microscopic and macroscopic breakup characteristics, as well as the velocity and size distributions, of mono-dispersed droplets in relation to the breakup regimes. For this experiment, a droplet generator equipped with a piezo stack produced mono-dispersed droplets. The droplet-breakup phenomenon due to the cross-flow was captured in microscopic and macroscopic views by using the following: a spark lamp, a Nd:YAG laser, a long distance microscope and a CCD camera as a function of the Weber number. Along with the analysis of the images, the droplet size and velocity distributions were measured in the near nozzle region by a phase Doppler particle analyzer system at bag, stretching and thinning, and catastrophic breakup regimes. The results of this study showed the size and velocity distributions of disintegrated droplets at the bag, stretching and thinning, and catastrophic breakup regimes. In the bag breakup regime, the droplets separated into small and large droplets during breakup. Alternatively, the droplets disintegrated at a shorter duration and formed a cloud, similar to a fuel spray injected through an injector, in the stretching and thinning and catastrophic breakup regimes.     

Droplet motion in an electrohydrodynamic fine spray

The results of a combined experimental and numerical study on droplet behavior within an electrohydrodynamic fine spray are presented. The fine spray exists in the transition region between the multiple cone-jet and rim emission spray modes. Experiments were conducted specifically to characterize the motion of droplets within the spray. Light-sheet visualizations and measurements of droplet speed and velocity using laser-based, single-particle counters were obtained. Additionally, a numerical simulation of the droplet motion within the spray was made and compared to the experimental results. The electrohydrodynamic fine spray of ethanol droplets ( 1 to 40 m diameter) was generated using a typical capillary-plate configuration, with a capillary tip electric field intensity of 10⁶ V/m and a spray charge density of 70 C/m³. Acquired images of the spray revealed a zone of rapid expansion near the capillary followed by a more gradual expansion farther from the capillary. In situ laser-diagnostic measurements confirmed these observations. Measured droplet speeds decreased rapidly with increasing axial distance from the capillary, but then increased beyond the spray's axial mid-plane as a result of a change in the sign of the axial internal electric field. Droplet axial velocity components behaved similarly. The radial velocity components exhibited a maximum value off of the spray's centerline in the near-capillary region. Farther away from the capillary, they increased monotonically with increasing radial position. These trends identified the significant role that the radial internal electric field plays in spray expansion. The numerical simulation of the normal spray verified the inferred change in sign of the axial internal field and underscored the dominant contribution of the external electric field in the near-capillary region and of the internal electric field farther away.     

Gas-phase velocity field measurements in sprays without particle seeding

A laser-based technique is presented that can be used to measure the instantaneous velocity field of the continuous phase in sprays and aerosols. In contrast to most well established laser-based velocity measurement techniques, this method is independent of particle seeding and Mie scattering. Instead of that it is based on gaseous flow tracers and laser-induced fluorescence (LIF). Inhomogeneous tracer gas distributions, which are created by an incomplete, turbulent mixing process, are exploited for flow tracing. The velocity field can be measured close to the droplets, because frequency-shifted LIF is separated from Mie scattering by optical filters. Validation tests and results from a water spray in air are given. Accuracy and spatial resolution are discussed in detail. Received: 26 April 1999/Accepted: 16 October 1999     

Nonlinear modeling of drop size distributions produced by pressure-swirl atomizers

An axisymmetric boundary element method (BEM) has been developed to simulate atomization processes in a pressure-swirl atomizer. Annular ligaments are pinched from the parent sheet and presumed to breakup via the linear stability model due to Ponstein. Corrections to Ponstein’s result are used to predict satellite droplet sizes formed during this process. The implementation provides a first-principles capability to simulate drop size distributions for low viscosity fluids. Results show reasonable agreement with measured droplet size distributions and the predicted SMD is 30–40% smaller than experiment. The model predicts a large number of very small droplets that cannot typically be resolved in an experimental observation of the spray. A quasi-3-D spray visualization is presented by tracking droplets in a Lagrangian fashion from their formation point within the ring-shaped ligaments. A complete simulation is provided for a case generating over 80,000 drops.     

Spray impact onto flat and rigid walls: Empirical characterization and modelling

An experimental study of spray impact onto horizontal flat and rigid surfaces is presented and used as input data for a new empirical model. A phase Doppler instrument has been used to measure drop size and two components of velocity directly above the target. The average film thickness formed due to spray impact has been measured using a high-speed CCD camera. The spray–wall interaction has been characterized in terms of correlations for the velocity and trajectory of secondary droplets and the mass and number ratio of the secondary spray. The novel aspect of the model is that the correlations are based on mean statistics over many events and not on the outcome of single drop impact experiments. Furthermore a rather large range of oblique impact angles have been studied and incorporated into the empirical models as an influencing factor.     

The atomization of water–oil emulsions

The paper presents the results of experimental studies on atomization of the emulsions flowing through twin-fluid atomizers obtained by the use of the digital microphotography method. The main elements of the test installation were: nozzle, reservoir, pump and measurement units of liquid flow. The photographs were taken by a digital camera with automatic flash at exposure time of 1/8000 s and subsequently analyzed using Image Pro-Plus. The oils used were mineral oils 20–90, 20–70, 20–50 and 20–30. The studies were performed at flow rates of liquid phase changed from 0.0014 to 0.011 (dm³/s) and gas phase changed from 0.28 to 1.4 (dm³/s), respectively. The analysis of photos shows that the droplets being formed during the liquid atomization have very different sizes. The smallest droplets have diameters of the order of 10 μm. The experimental results showed that the changes in physical properties of a liquid phase lead to the significant changes in the spray characteristics. The analysis of the photos of water and emulsions atomization process showed that the droplet sizes are dependent on gas and liquid flow rates, construction of nozzle and properties of liquid. The differences between characteristics of atomization for water and emulsions have been observed. Analysis of photos on forming the droplets in air–water and air-emulsions systems showed that droplets are bigger in air-emulsion system (at the same value of gas to liquid mass ratio). The values of Sauter mean diameter (SMD) increased with increase of volume fraction of oil in emulsion. The droplet size increased with emulsion viscosity.     

Tomographic shadowgraphy for three-dimensional reconstruction of instantaneous spray distributions

Tomographic shadowgraphy is an image-based optical technique capable of reconstructing the three dimensional instantaneous spray distributions within a given volume. The method is based on a multiple view imaging setup with inline illumination provided by current-pulsed LEDs, which results in droplet shadows being projected onto multiple sensor planes. Each camera records image pairs with short inter-framing times that allow the trajectories of the individual droplets to be estimated using conventional three-dimensional particle tracking approaches. The observed volume is calibrated with a traversed micro-target. A comparison is made between several photogrammetric and polynomial least-square camera calibration techniques regarding their accuracy in deep volume calibration at magnifications close to unity. A calibration method based on volume calibration from multiple planar homographies at equally spaced z-planes was found to produce the most reliable calibration. The combination of back-projected images at each voxel plane efficiently reproduces the droplet positions in three-dimensional space by line-of-sight (LOS) intensity reconstruction. Further improvement of the reconstruction can be achieved by iterative tomographic reconstruction, namely simultaneous multiplicative algebraic reconstruction technique (SMART). The quality of spray reconstruction is investigated using experimental data from multiple view shadowgraphs of hollow cone and flat fan water sprays. The investigations are further substantiated with simulations using synthetic data.     

Phase Doppler measurements of spray impact onto rigid walls

In this work, an experimental study of spray impact onto a horizontal flat and rigid surface is presented. The phase Doppler technique has been used to characterize both the impacting and the secondary spray in terms of mass and number flux, size distribution and velocities of the droplets above the target. A high-resolution CCD camera has been used to measure the average liquid film thickness formed due to spray impact, whereas a high-speed CMOS camera has been used to characterize the splashing droplets from the wall. This visualization of the splashing phenomenon and the knowledge about the liquid film thickness are used to formulate a new physical model of the crown evolution. Furthermore, information about the incident-to-ejected mass fraction and number fraction are novel contributions of this study. Considerable data are provided comparing the impact of single drops onto a liquid film to impact of drops in a spray, and the significance of the observed differences for modelling efforts is discussed. The measurements of this study are also shown to be rather sensitive to the placement of the phase Doppler measurement volume above the surface and to the operating parameters of the instrument. These effects have been documented and discussed for this particular measurement situation.     

A study on the fuel injection and atomization characteristics of soybean oil methyl ester (SME)

The spray atomization characteristics of an undiluted biodiesel fuel (soybean oil methyl ester, SME) in a diesel engine were investigated and compared with that of diesel fuel (ultra low sulfur diesel, ULSD). The experimental results were compared with numerical results predicted by the KIVA-3V code. The spray characteristics of the spray tip penetration, spray area, spray centroid and injection delay were analyzed using images obtained from a visualization system. The Sauter mean diameter (SMD) was analyzed using a droplet analyzer system to investigate the atomization characteristics.It was found that the peak injection rate increases and advances when the injection pressure increases due to the increase of the initial injection momentum. The injection rate of the SME, which has a higher density than diesel fuel, is higher than that of diesel fuel despite its low injection velocity. The high ambient pressure induces the shortening of spray tip penetration of the SME. Moreover, the predicted spray tip penetration pattern is similar to the pattern observed experimentally. The SMD of the SME decreases along the axial distance. The predicted local and overall SMD distribution patterns of diesel and SME fuels illustrate similar tendencies when compared with the experimental droplet size distribution patterns.     

Study on imaging method for measuring droplet size in large sprays

Information of droplet size and size distribution lays the basis for investigations of atomization mechanisms and performance optimization. However, the laser diffraction and phase Doppler particle analyzers have difficulty in accurately characterizing sprays with a wide range of droplet sizes and very large droplets, especially if a large number of droplets are aspherical. A method to measure size in such large-droplet sprays based on digital imaging with backward illumination was developed, including an image acquisition system and image process programs. Calibration of the measurement system was performed using a dot calibration target with different dot sizes. An experimental setup was designed and established to characterize spray nozzles under different operation loads, as well as different nozzle arrangements. Results show that the droplet size of sprays ranges from dozens of microns to several millimeters. The superiority of wide load range for such nozzles was indicated by the size-measurement results under half-load to full-load operations. The present study revealed that the image processing technique can be effectively implemented for in-line size measurements of sprays with a wide distribution of droplet size and aspherical droplets, which would be difficult to characterize by other methods.     

Experimental and numerical analysis of spray-atomization characteristics of biodiesel fuel in various fuel and ambient temperatures conditions

The purpose of this work is to reveal the effects of fuel temperatures and ambient gas conditions on the spray-atomization behavior of soybean oil methyl ester (SME) fuel. The spray-atomization behavior was analyzed through spray parameters such as the axial distance from the nozzle tip, local and overall Sauter mean diameter (SMD). These parameters were obtained from a spray visualization system and a droplet measuring system. In addition, the experimental results were compared with the numerical results calculated by the KIVA-3V code. It was revealed that the increase of the fuel temperature (from 300 K to 360 K) little affects the spray liquid tip penetration. The increase of the ambient gas temperature (from 300 K to 450 K) caused a increase in the spray liquid tip penetration. Also, biodiesel fuel evaporation actively occurred due to the increase in the fuel temperature and the ambient gas temperature. Of special significance was that the highest vapor fuel mass concentration was observed at the center region of the spray axis. In the results of the microscopic characteristics, the detected local droplet size at the axial direction and overall droplet size at the axial and radial direction in a control volume increased when the fuel temperature increased. This is believed to be due to an increase in the number of small droplets that quickly evaporated. In addition, the increased fuel temperature caused the decrease of the number of droplets and the increase of the vapor fuel mass. The mean axial velocity of droplets decreased with increasing fuel temperature.     

Spray analysis of a gasoline direct injector by means of two-phase PIV

The hollow-cone spray of a high-pressure swirl injector for a direct-injection spark-ignition (DISI) engine was investigated inside a pressure vessel by means of particle image velocimetry (PIV). As the interaction between the spray droplets and the ambient air is of particular interest for the mixture preparation process, two-phase PIV techniques were applied. To allow phase discrimination, fluorescent seeding particles were used to trace the gas phase. Because of the periodicity of piston engine injection, a statistical evaluation of ensemble-averaged fields to reduce cycle-to-cycle variations and to provide more general information about the two-phase flow was performed. Besides the general spray/air interaction process the investigation of the spray collapse at elevated ambient pressures was the main focus of the study. Future investigations of transient interaction processes require simultaneous techniques in combination with a high-speed camera to resolve the transient interaction phenomena. Therefore, optical filters that attenuate Mie-scattered light and transmit fluorescent light were used to collect both phases on the same image. Consequently, phase separation techniques were employed for data analysis. A masking and a peak separation technique are described and a comparison between the results of an instantaneous two-phase flow field in the spray cone of a DISI injector is presented in the paper.     

Macroscopic and microscopic characteristics of a fuel spray impinged on the wall

An experimental study was performed to investigate the macroscopic behavior and atomization characteristics of a high-speed diesel spray impinged on the wall at various injection and impinging conditions. The development processes of sprays impinged on the wall were visualized using the spray visualization system composed of a Nd:YAG laser and an intensified charge-coupled device (ICCD) camera. The atomization characteristics of the impinged spray on the wall were also explored in terms of mean droplet diameter and velocity distributions by using a phase Doppler particle analyzer (PDPA) system. The results provide the effects of injection parameters, wall conditions, and the other various experimental conditions on the macroscopic behavior and atomization characteristics of the impinged sprays on the wall.     

Modelling spray impingement using linear stability theories for droplet shattering

This paper compares several linear‐theory‐based models for droplet shattering employed for simulations of spray impingement on flat wall surface or a circular cylinder. Numerical simulations are conducted using a stochastic separated flow (SSF) technique that includes sub‐models for droplet dynamics and impact. Results for spray impingement over a flat wall indicate that the linear theory applicable for a single droplet impact over‐predicts the number of satellite (or secondary) droplets upon shattering when compared to experimental data. The causes for the observed discrepancies are discussed. Numerical simulation results for spray impingement over a circular cylinder in cross flow are obtained and discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

温度和浓度对甲醇喷雾爆炸特性参数的影响

采用20 L球形喷雾爆炸实验系统，探究甲醇在不同环境温度、物料温度及喷雾浓度下的爆炸特性规律。结果表明:20 L爆炸球内甲醇喷雾液滴爆炸极限范围为118.8～594.0 g/cm3，与纯气相爆炸极限范围（78.6～628.6 g/cm3）相比，甲醇喷雾液滴爆炸极限范围较窄，喷雾液滴的爆炸敏感性比纯气相甲醇蒸汽低。随着爆炸球内环境温度的升高，甲醇喷雾爆炸极限范围变宽，受限空间内甲醇气液喷雾点火成功概率增大。当甲醇物料自身温度或爆炸容器内环境温度保持不变时，相应爆炸特性参数在Φ=1.8拐点处均呈现先增大后减小的趋势。当Φ=1.8时，甲醇喷雾爆炸存在最大超压峰值。环境温度、物料温度的升高可以提高雾化质量，促进扩散燃烧。但是环境温度的变化较之物料温度的改变对于甲醇液滴喷雾爆炸特性参数的影响更为显著。环境温度和化学当量比二元变量共同影响着甲醇喷雾爆炸强度值，当Φ=1.8，环境温度为303.15 K时，甲醇的喷雾爆炸强度大于甲烷气体爆炸的爆炸强度。     

Similarity solutions for the evolution of polydisperse droplets in vortex flows

A new mathematical analysis of the dynamics of evaporating sprays in the vicinity of a vortex flow field is presented. The governing equations for a polydisperse spray evaporating in an unsteady viscous vortex flow are formulated using the sectional approach. First, new similarity solutions are found for the dynamics of the spray in a mono-sectional framework. It is shown that similarity for the droplets’ drag term exists, and an explicit model for the drag is found using perturbation theory. Numerical simulations are conducted to validate the main assumptions of the analytic approach adopted in this study. An extension of the mono-sectional solution of the spray equations to a polydisperse spray solution is then derived and the dynamics of polydisperse spray in an Oseen type vortex are presented. It is shown that for a given radial location, the droplets in each section reach a maximal radial velocity due to the effect of vorticity. A simple model is derived for the prediction of this maximal radial velocity of the droplets using perturbation theory, which agrees very well with the full similarity solution. The present study shows that spray dynamics is highly affected by the droplets’ size, but also by the spray initial size distribution, even when the same Sauter mean diameter is considered. This may have far reaching implications, especially in spray combustion applications.