High-speed Spray Research Articles

High-speed jetting and spray formation from bubble collapse

A method to create impacting jets at the micrometer length scale by means of a collapsing cavitation bubble is presented. A focused shock wave from a lithotripter leads to the nucleation of a cavitation bubble below a hole of 25 μm diameter etched in a silicon plate. The plate is placed at an air-water interface. The expansion and collapse of the bubble leads to two separate jets--an initial slow jet of velocity ∼10 m/s and a later faster jet of velocity ∼50 m/s. The jets subsequently impact coaxially, resulting in a circular sheet of liquid in the plane perpendicular to their axis. The sheet is characterized by a ring of droplets at its rim and breaks up into a spray as the shock pressure is increased. The results demonstrate an approach to create a high-speed jet and fine spray on demand at the micrometer scale.

603 高速噴霧流実験による液滴衝撃エロージョンの評価と減肉モデルの提案(応用熱工学I,オーガナイズドセッション,学術講演)

603 高速噴霧流実験による液滴衝撃エロージョンの評価と減肉モデルの提案(応用熱工学I,オーガナイズドセッション,学術講演)

304 洗浄用ノズル内の高速液滴噴霧流の数値解析(流体工学I)

304 洗浄用ノズル内の高速液滴噴霧流の数値解析(流体工学I)

Interaction Mechanism between <i>In Situ</i> Nb<sub>2</sub>C-V<sub>2</sub>C Nanoparticles and S/L Interface during the Rapid Solidification of Fe-Nb-V Alloy

This paper deals with the interaction mechanism between in situ Nb2C-V2C nanoparticles and solid/liquid interface during the rapid solidification of Fe-Nb-V alloy ribbons. In situ Nb2C-V2C nanoparticles reinforced Fe alloy matrix composite were carried out by in situ reaction and melt spun with a high-speed centrifugal spray. According to the setting of a force balance for the in situ nanoparticles in front of the S/L interface during the high-speed centrifugal spray of Fe-Nb-V alloy melt as following: F = Fbuoyant force + Frepulsive + Fviscous force + Fcentrifugal force, authors get the critical velocity that S/L interface engulfs particles is proportional to the radius of particles. TEM observation indicates that the less the size of particle is, the more easily S/L interface engulfs particles.

The Research of Spraying SiC/Cu Electronic Packaging Composites

The SiC/Cu electronic packaging composites with excellent performance were successfully prepared by the chemical plating copper on the surface of SiC powders and high-speed flame spraying technology. The results showed that the homogeneous dense coated layers can be obtained on the surface of SiC powder by optimizing process parameters. The volume fraction of SiC powders in the composites could significantly increase and figure was beyond 55vol% after spraying Copper. The SiC and Cu were the main phases in the spraying SiC/Cu electronic packaging composite, at the same time Cu2O can be tested as the trace phase. The interface combination properties of SiC/Cu in the hot-pressed samples can obviously improve. The thermal expansion coefficient and thermal conductivity of SiC/Cu electronic packaging composite basic can satisfy the requirements for electronic packaging materials.

Low sooting combustion of narrow-angle wall-guided sprays in an HSDI diesel engine with retarded injection timings

An optically accessible single-cylinder high speed direct-injection (HSDI) diesel engine was used to investigate the spray and combustion processes with narrow-angle wall-guided sprays. Influences of injection timings and injection pressure on combustion characteristics and emissions were studied. In-cylinder pressure was measured and used for heat release analysis. High-speed spray and combustion videos were captured. NOx emissions were measured in the exhaust pipe. With significantly retarded post-top dead center (TDC) injections, smokeless combustion was achieved for wall-guided diesel spray. Premixed-combustion was observed from the heat release rates and the combustion images. Natural luminosity was found significantly lower for smokeless combustion case. However, NOx emissions were higher for the low sooting combustion cases. In addition, retarding injection timing lead to more complete combustion with more heat released from the same amount of fuel. Spray images revealed significant fuel impingement for all the conditions and the spray development was controlled and guided by the piston bowl curvature. NOx and natural luminosity trade-off trend was observed for these conditions. However, quite different from conventional diesel combustion, retarding post-TDC injection timing leads to lower natural luminosity and higher NOx emissions for narrow-angle wall-guided spray combustion. For the smokeless combustion case under moderate operating load, both homogeneous combustion and low-luminosity pool fires were observed during combustion process and the latter was due to fuel-piston impingement. The findings in this study could be used to solve the smoke issues associated with narrow-angle injection technique under high load conditions. With narrow-angle injectors, ignition could occur for significantly retarded post-TDC injections, which provides a unique mixing approach for diesel engines.

Experiment on Liquid Droplet Impinging Erosion by High-speed Spray and Measurement of Droplet Parameters

原子力発電プラントの配管減肉の一要因である液滴衝撃エロージョン現象の解明を目的として，高速噴霧流を用いた液滴衝撃エロージョン実験を行った．本論文では，液滴パラメータである速度，液滴径，液滴の衝突数を，それぞれ，PIV, シャドーグラフ法，レーザー光減衰法によって計測する方法を示し，さらに，高速噴霧流の液滴パラメータの計測結果，ステンレス材料のエロージョン実験結果を示した．本実験結果によると，高速噴霧流装置で発生した実機の推定液滴径（数十μm）によって液滴衝撃エロージョンが発生し，減肉したステンレス材料表面には梨地状の凹凸パターンが観測された．ただし、減肉痕の大きさは数百μmオーダーであり、衝突する液滴径よりかなり大きい．

Original use of a direct injection high efficiency nebulizer for the standardization of liquid fuels spray flames

It is of practical importance to lead laboratory-scale experiments allowing a better understanding of the impact of commercial fuels composition on the formation of combustion residues such as soot particles. To this end, a hybrid burner has been designed recently to burn high-speed sprays of small liquid fuel droplets. It consists of a Holthuis (previously McKenna) burner originally equipped with a direct injection high efficiency nebulizer for the atomization of liquid hydrocarbons. A detailed description of this original setup is given in this paper. A priori estimations of atomization and evaporation times and length scales are then proposed and compared with experimental data. Droplet-size distribution measurements obtained in nonreacting conditions using a Malvern Spraytec particle sizer are presented and compared with values estimated by calculation. Cold sprays contours and liquid jet lengths in flames determined by Mie scattering at 532 and 1064 nm, respectively, are also presented. The results discussed in this work indicate that the hydrodynamic characteristics of the sprays generated with our system are relatively independent of the physical properties of fuels leading to comparable flames with identical liquid jet lengths, dimensions, and global structure. This feature facilitates an accurate comparison of flames burning various liquid hydrocarbons, which is of interest to emphasize differences in pollutants emissions and to highlight chemical effects for soot formation analysis.

Study on Multiphase Flow and Mixing in Semidry Flue Gas Desulfurization with a Multifluid Alkaline Spray Generator Using Particle Image Velocimetry

Particle image velocimetry (PIV) technique was used to measure the velocity fields of gas-droplet-solid multiphase flow in the experimental setup of a novel semidry flue gas desulfurization process with a multifluid alkaline spray generator. The flow structure, mixing characteristic, and interphase interaction of gas-droplet-solid multiphase flow were investigated both in the confined alkaline spray generator and in the duct bent pipe section. The results show that sorbent particles in the confined alkaline spray generator are entrained into the spray core zone by a high-speed spray jet and most of the sorbent particles can be effectively humidified by spray water fine droplets to form aqueous lime slurry droplets. Moreover, a minimum amount of air stream in the generator is necessary to achieve higher collision humidification efficiency between sorbent particles and spray water droplets and to prevent the possible deposition of fine droplets on the wall. The appropriate penetration length of the slurry droplets from the generator can make uniform mixing between the formed slurry droplets and main air stream in the duct bent pipe section, which is beneficial to improving sulfur dioxide removal efficiency and to preventing the deposition of droplets on the wall.

Impact of high-speed steam-droplet spray on solid surface

A novel technique for the generation of impact by a high-speed steam-droplet spray is proposed. Relatively low-pressure super-purified steam (0.1–0.2 MPa) is mixed with super-purified water in a nozzle, and then sprayed on a solid surface, which is located at approximately 10 mm from the nozzle. This spray is found to cause harsh erosion. The most striking result of this experiment is that the degree of erosion is strongly dependent on temperature; this dependence is hardly explained by the classical droplet impact theory. We recognize the occurrence of a strong focused rarefaction wave in the middle of the droplet; this rarefaction wave may cause cavitation. The existence of cavitation may be supported by the temperature susceptibility of erosion. We experimentally measure both the droplet velocity and diameter distributions by a Phase-Doppler Anemometer. We also numerically study the dynamics of a high-speed liquid droplet impact on a solid surface by solving the Euler equation using the conditions obtained by the experiments. We discuss the possibility of the formation of cavitation bubbles as the primary cause of the experimentally observed harsh erosion on a solid surface.

Measurement of droplet size distribution in core region of high-speed spray by micro-probe L2F

In order to investigate the distribution of droplet sizes in the core region of diesel fuel spray, instantaneous measurement of droplet sizes was conducted by an advanced laser 2-focus velocimeter (L2F). The micro-scale probe of the L2F is made up of two foci and the distance between them is 36 µm. The tested nozzle had a 0.2 mm diameter single-hole. The measurements of injection pressure, needle lift, and crank angle were synchronized with the measurement by the L2F at the position 10 mm downstream from the nozzle exit. It is clearly shown that the droplet near the spray axis is larger than that in the off-axis region under the needle full lift condition and that the spatial distribution of droplet sizes varies temporally. It is found that the probability density distribution of droplet sizes in the spray core region can be fitted to the Nukiyama-Tanasawa distribution in most injection periods.

Simulation and Analysis of High-Speed Droplet Spray Dynamics

Abstract An experimentally validated computer simulation model has been developed for the analysis of gas-phase and droplet characteristics of isothermal sprays generated by pressure jet atomizers. Employing a coupled Euler-Lagrange approach for the gas-droplet flow, secondary droplet breakup (based on the ETAB model), was assumed to be dominant and the k-ε model was selected for simulating the gas flow. Specifically, transient spray formation in terms of turbulent gas flow as well as droplet velocities and size distributions are provided for different back pressures. Clearly, two-way coupling of the phases is important because of the impact of significant gas entrainment, droplet momentum transfer, and turbulent dispersion. Several spray phenomena are discussed in light of low back-pressure (1atm) and high back-pressure (30atm) environments. At low back-pressure, sprays have long thin geometric features and penetrate faster and deeper than at high back-pressures because of the measurable change in air density and hence drag force. Away from the nozzle exit under relatively high back pressures, there is no distinct droplet size difference between peripheral and core regions because of the high droplet Weber numbers, leading to very small droplets which move randomly. In contrast to transient spray developments, under steady-state conditions droplets are subject to smaller drag forces due to the fully-developed gas entrainment velocities which reduce gas-liquid slip. Turbulent dispersion influences droplet trajectories significantly because of the impact of random gas-phase fluctuations.

A digital image analysis technique for quantitative characterisation of high-speed sprays

A digital image analysis technique developed as a particle or droplet sizing tool and capable of measuring non-spherical objects has been examined in terms of its suitability for quantitative measurements in moderately dense sprays and in particular the potential capability for the characterisation of small diameter, high-speed two-phase flows by employing high-intensity pulsed lasers for illumination. In order to evaluate robustness of the image analysis technique (PDIA), measurement certainty and also to assess whether measurement performance is sensitive to the optical set-up, the technique was applied to data obtained from a hollow cone spray via two independent optical configurations which employed firstly a diode laser and secondly an Nd:YAG laser. The calibration response of the two optical set-ups revealed significant differences in terms of the depth-of-field characteristics and thus effective measurement volume dimensions. Despite these differences, a comparison of PDIA spray data revealed excellent agreement between the two datasets for measured diameters in the range 10–90 μm in the number distributions which not only confirmed robustness of the technique but also the potential of PDIA for the measurement of fast, small diameter objects. Subsequent comparisons of the PDIA data were made with PDA data obtained within the same spray in space and time and showed excellent agreement between the two techniques for droplets larger than approximately 25 μm in diameter. Discrepancies between PDIA and PDA were observed in the volume size distributions for the larger droplets measured whose diameters were greater than approximately 40 μm. This discrepancy is due to the ability of PDIA to measure the diameter of non-spherical droplets which were shown to exist in significant numbers at this measurement location within the spray. In contrast, the well-established technique PDA, which relies on the assumption of droplet sphericity clearly does not detect the presence of these larger deformed droplets.

Effect of Injector Type on Fuel-Air Mixture Formation of High-Speed Diesel Sprays

This study was performed to analyse the characteristics of fuel sprays issued from a piezo-driven injector and to compare both experimental results and numerical predictions with those from a solenoid-driven injector. In order to study the effect of the injector type, two injectors with the same specifications - except for their drive type - were used in the experiments. In this work, the profiles of injection rates, processes of spray development, mean droplet size distributions, and mean velocities were obtained using an injection rate meter, a spray visualization system, and a phase Doppler particle analyser (PDPA) system. In order to calculate the atomization process of the injected spray, the Kelvin-Helmholtz instability/droplet deformation and break-up ((KH-DDB) model was used, and the prediction accuracies were verified by comparing them with experimental results. For the numerical approach using the KIVA code, the nozzle flow and droplet break-up models were modified to consider the conditions of a common-rail injection system. The input parameters for the KIVA calculations were determined based on the nozzle flow model considering the effect of cavitation. In addition, the effects of injector type on the evaporation characteristics were calculated using the modified Spalding evaporation model. This study reveals that the injection delay of the piezo-driven injector is shorter than that of the solenoid-driven injector by 0.06 ms, and that the atomization and evaporation performances of the fuel spray can be enhanced by using the piezo injector instead of the solenoid injector.

Discrete Bubble Modeling of Unsteady Cavitating Flow

A discrete vapor bubble model is developed to simulate unsteady cavitating flows. In this model, the mixed vapor-liquid mixture is modeled as a system of pure phase domains (vapor and liquid) separated by free interfaces. On the phase boundary, a numerical solution for the phase transition is developed for compressible flows. This model is used to study the effect of cavitation bubbles on atomization, i.e., the breakup of a high-speed jet and spray formation. The major conclusion is that a multiscale (three-scale) model is sufficient to achieve agreement with quantitative macroscale flow parameters, such as spray opening angle and spray volume fraction or density, or as a qualitative measure, the occurrence of spray formation. The authors believe this to be the first numerical study of the atomization process at such a level of detail in modeling of the related physics.

C20 Ultrafast X-Radiography and X-Tomography of High-Pressure and High-Speed Fuel Sprays—Invited

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Dynamic Properties of Surfactants: Influence on Processes and Technologies

Abstract Industrial processes become faster and faster. The corresponding formulations have to keep pace with new technical solutions like spray coating in paper production or the substitution of isopropanol in fountain solutions for offset printing. Fast diffusion of amphiphiles to interfaces and the formation of interfacial films to lower the interfacial energy are the fundamental molecular mechanisms which have to be optimised for all high speed spraying and wetting processes. Our results show that the interfacial tension at water/air and at water/solid interfaces primarily depends on the concentration of free surfactants. Micelles act as depots for free surfactants which adjust the concentration in solution as soon as surfactants disappear by interface adsorption. By using synergistic mixtures it is possible to extend the application range of dynamic surfactants towards detergency by combining fast adsorption at interfaces with micellar properties.

Flash x-ray radiography using imaging plates for the observation of hypervelocity objects

Flash x-ray radiography was conducted using imaging plates (IP) to observe high-speed thermal spray jets and debris clouds produced from hypervelocity impact. The radiographs of the spray jets or debris cloud shadows on the IPs were analyzed to estimate the distribution of mass per unit area, i.e., Areal mass density, due to the distribution of the intensities of stimulated emissions from the IPs. The wide dynamic range of the IPs led to the detection of an Areal mass density one hundred times as large as the minimum Areal mass density and the very detailed densities. The availability of the IPs for the flash x-ray radiography of a high-speed thermal spray jet and a hypervelocity-impact-produced debris cloud was demonstrated.

501 マイクロプローブL2Fによる高速高濃度噴霧コア部における粒径分布計測(OS5-1 混相流の微細流動構造(相間作用),OS5 混相流の微細流動構造,オーガナイズドセッション)

501 マイクロプローブL2Fによる高速高濃度噴霧コア部における粒径分布計測(OS5-1 混相流の微細流動構造(相間作用),OS5 混相流の微細流動構造,オーガナイズドセッション)

325 液体微粒化メカニズムに関する一体型シミュレーションと可視化(J11-2 流体情報学と可視化(2),J11 流体情報学と可視化,2005年度年次大会)

The 2-D and 3-D structure of liquid atomization behavior is numerically investigated and vizualized by the new type of integrated simulation technique. The present CFD analysis is focusing on the consecutive breakup of liquid column, formation of liquid film and droplets at the exit section of nozzle outlet. The governing equations for high-speed spray nozzle flow based on the LES-VOF model are presented, and then an integrateed parallel simulation are performed to clarify the detailed atomization process and to clarify the atomized droplets-gas two-phase flow characteristics.