Droplet Velocity Profiles Research Articles

Computational Prediction of the Effect of Microcavitation on an Atomization Mechanism in a Gasoline Injector Nozzle

The effect of microcavitation on the 3D structure of the liquid atomization process in a gasoline injector nozzle was numerically investigated and visualized by a new integrated computational fluid dynamics (CFD) technique for application in the automobile industry. The present CFD analysis focused on the primary breakup phenomenon of liquid atomization which is closely related to microcavitation, the consecutive formation of liquid film, and the generation of droplets by a lateral flow in the outlet section of the nozzle. Governing equations for a high-speed lateral atomizing injector nozzle flow taking into account the microcavitation generation based on the barotropic large eddy simulation-volume of fluid model in conjunction with the continuum surface force model were developed, and then an integrated parallel computation was performed to clarify the detailed atomization process coincident with the microcavitation of a high-speed nozzle flow. Furthermore, data on such factors as the volume fraction of microcavities, atomization length, liquid core shapes, droplet-size distribution, spray angle, and droplet velocity profiles, which are difficult to confirm by experiment, were acquired. According to the present analysis, the atomization rate and the droplets-gas atomizing flow characteristics were found to be controlled by the generation of microcavitation coincident with the primary breakup caused by the turbulence perturbation upstream of the injector nozzle, hydrodynamic instabilities at the gas-liquid interface, and shear stresses between the liquid core and periphery of the jet. Furthermore, it was found that the energy of vorticity close to the gas-liquid interface was converted to energy for microcavity generation or droplet atomization.

Evaluation of StereoPIV Measurement of Droplet Velocity in an Effervescent Spray

Particle image velocimetry (PIV) is a well known technique for measuring the instantaneous velocity field of flows. However, error may be introduced when measuring the velocity field of sprays using this technique when the spray droplets are used as the seed particles. In this study, the effect of droplet number density, droplet velocity profile, and droplet size distribution of a spray produced by an effervescent atomizer on velocity measurement using a StereoPIV has been investigated. A shadowgraph-particle tracking velocimetry (S-PTV) system provided measurement of droplet size and velocity for comparison. This investigation demonstrated that the StereoPIV under-estimates velocity at near-field dense spray region where measurement accuracy is limited by multi-scattering of the laser sheet. In the dilute far-field region of the spray, StereoPIV measurement is mostly in agreement with velocity of the droplet size-class which is close to the mean diameter based on droplet number frequency times droplet cross sectional area.

Mechanistic study on spraying of blended biodiesel using phase Doppler anemometry

Abstract Droplet size and dynamics of blended palm oil-based fatty acid methyl ester (FAME) and diesel oil spray were mechanistically investigated using a phase Doppler anemometry. A two-fluid atomizer was applied for dispersing viscous blends of blended biodiesel oil with designated flow rates. It was experimentally found that the atomizer could generate a spray with large droplets with Sauter mean diameters of ca. 30 μm at low air injection pressure. Such large droplets traveled with a low velocity along their trajectory after emerging from the nozzle tip. The viscosity of blended biodiesel could significantly affect the atomizing process, resulting in the controlled droplet size distribution. Blended biodiesel with a certain fraction of palm oil-based FAME would be consistently atomized owing to its low viscosity. However, the viscosity could exert only a small effect on the droplet velocity profile with the air injection pressure higher than 0.2 MPa.

Integrated numerical prediction of atomization process of liquid hydrogen jet

The 3-D structure of the liquid atomization behavior of an LH 2 jet flow through a pinhole nozzle is numerically investigated and visualized by a new type of integrated simulation technique. The present computational fluid dynamics (CFD) analysis focuses on the thermodynamic effect on the consecutive breakup of a cryogenic liquid column, the formation of a liquid film, and the generation of droplets in the outlet section of the pinhole nozzle. Utilizing the governing equations for a high-speed turbulent cryogenic jet flow through a pinhole nozzle based on the thermal nonequilibrium LES-VOF model in conjunction with the CSF model, an integrated parallel computation is performed to clarify the detailed atomization process of a high-speed LH 2 jet flow through a pinhole nozzle and to acquire data, which is difficult to confirm by experiment, such as atomization length, liquid core shape, droplet-size distribution, spray angle, droplet velocity profiles, and thermal field surrounding the atomizing jet flow. According to the present computation, the cryogenic atomization rate and the LH 2 droplets–gas two-phase flow characteristics are found to be controlled by the turbulence perturbation upstream of the pinhole nozzle, hydrodynamic instabilities at the gas–liquid interface and shear stress between the liquid core and the periphery of the LH 2 jet. Furthermore, calculation of the effect of cryogenic atomization on the jet thermal field shows that such atomization extensively enhances the thermal diffusion surrounding the LH 2 jet flow.

Integrated Simulation of the Atomization Process of a Liquid Jet Through a Cylindrical Nozzle

The 3-D structure of liquid atomization behavior through a cylindrical nozzle is numerically investigated and visualized by a new type of integrated simulation technique. CFD (Computational Fluid Dynamics) analysis focused on the consecutive breakup of a liquid column, formation of liquid film, and generation of droplets of a cylindrical flow in the outlet section of the nozzle. Utilizing the governing equations for high-speed atomizing nozzle flow based on the LES-VOF model in conjunction with the CSF model, an integrated parallel computation is performed to clarify the detailed atomization process of a cylindrical nozzle flow and to acquire data, which is difficult to confirm by experiment, such as atomization length, liquid core shape, distribution droplet sizes, spray angle and droplet velocity profiles. According to the present analysis, the atomization rate and the droplets-gas two-phase flow characteristics are found to be controlled by the turbulence perturbation upstream of the injector nozzle, hydrodynamic instabilities at the gas-liquid interface, and shear stresses between the liquid core and periphery of the jet.

Dynamics of sessile droplet evaporation: A comparison of the spine and the elliptic mesh generation methods

The focus of this paper is the numerical study of the fluid dynamics inside an evaporating liquid sessile droplet. The governing equations are solved numerically using the Galerkin/finite element method (G/FEM) for discretization of the spatial domain and an adaptive finite difference method for discretization in the time domain. A mesh generation method needs to be carefully chosen for this problem, preferably the method which continuously gives sufficient mesh resolution, in the region where the numerical solution needs to be resolved, throughout the drop evaporation time. After comparing two mesh generation methods, the elliptic mesh generation method is preferred over the spine mesh method. Successfully, the droplet shape, and velocity, and pressure profiles inside the droplet are solved simultaneously for the case where inertial forces are important (Reynolds number ∼1) and where the gravitational force is either absent or present.

Spray Characteristics of a Multi-hole Injector for Direct-Injection Gasoline Engines

The sprays from a high-pressure multi-hole nozzle injected into a constant-volume chamber have been visualized and quantified in terms of droplet velocity and diameter with a two-component phase Doppler anemometry (PDA) system at injection pressures up to 200 bar and chamber pressures varying from atmospheric to 12 bar. The flow characteristics within the injection system were quantified by means of a fuel injection equipment (FIE) one-dimensional model, providing the injection rate and the injection velocity in the presence of hole cavitation, by an in-house three-dimensional computational fluid dynamics (CFD) model providing the detailed flow distribution for various combinations of nozzle hole configurations, and by a fuel atomization model giving estimates of the droplet size very near to the nozzle exit. The overall spray angle relative to the axis of the injector was found to be almost independent of injection and chamber pressure, a significant advantage relative to swirl pressure atomizers. Temporal droplet velocities were found to increase sharply at the start of injection and then to remain unchanged during the main part of injection, before decreasing rapidly towards the end of injection. The spatial droplet velocity profiles were jet-like at all axial locations, with the local velocity maximum found at the centre of the jet. Within the measured range, the effect of injection pressure on droplet size was rather small while the increase in chamber pressure from atmospheric to 12 bar resulted in much smaller droplet velocities, by up to four-fold, and larger droplet sizes by up to 40 per cent.

분사각 변화에 따른 횡단류에 분사되는 액체제트의 분무특성에 대한 수치적 연구

The spray characteristics of liquid jet in cross flow with variation of injection angle are numerically studied. Numerical analysis was carried out using KIVA code, which was modified to be suitable for simulating liquid jet ejected into cross flow. Wave model and Kelvin- Helmholtz(KH)/Rayleigh-Taylor(RT) hybrid model were used for the purpose of analyzing liquid column, ligament, and the breakup of droplet. Numerical results were compared with experimental data in order to verify the reliability of the physical model. Liquid jet penetration length, volume flux, droplet velocity profile and SMD were obtained. Penetration length increases as flow velocity decreases and injection velocity increases. From the bottom wall, the SMD increases as vertical distance increases. Also the SMD decreases as injection angle increases.

Droplet size and velocity profiles in liquid–liquid horizontal flows

The size and vertical distribution of drops were studied experimentally in dispersed liquid–liquid pipeline flows. Under most conditions the pattern was dual continuous where both phases retain their continuity and there is entrainment in the form of drops of one phase into the other. The investigations were carried out in a stainless steel test section with 38 mm ID with water and oil (density 828 kg/m 3 and viscosity 6 mPa s ) as test fluids. Mixture velocities from 1.5 to 2.5 m/s and input oil volume fractions from 20% to 80% were used. A dual sensor impedance probe allowed drop chord length and drop velocity measurements at different locations in a pipe cross section. It was found that in dual continuous flows drop concentration and size decreased with increasing distance from the interface. There were only small differences in size between oil drops in the lower water continuous layer of the flow and water drops in the upper oil continuous layer. Mixture velocity did not affect significantly the drop size of either phase since higher velocities that would result in smaller drops were accompanied by increased entrainment of one phase dispersed into the other that favoured larger drops. The Rosin–Rammler function was found to fit satisfactorily the experimental drop size distributions, while literature correlations on entrained and maximum drop sizes in a turbulent field underpredicted the values found experimentally.

Spatial droplet velocity and size profiles in effervescent atomizer-produced sprays

Effervescent atomizer produced sprays were studied for application to rotary kiln incinerators. The sprays were characterized by their number averaged droplet velocity profiles, Sauter mean diameter (SMD) profiles, and size-velocity correlations. Spray mass flow rates ranged from 30 to 120 g/s. Air-liquid-ratios by mass (ALRs) were between 2% and 10%. Data were obtained at seven to ten radial positions across the diameter of the spray at three separate axial locations, all in the far-field (or dilute region) of the spray. The number averaged droplet velocity profiles were bell-shaped across any diameter, irrespective of ALR, liquid mass flow rate, or axial location. Velocity magnitudes were found to increase with an increase in either liquid mass flow rate or ALR, and to decrease with an increase in axial distance from the atomizer exit. SMD was found to be nearly constant across any spray diameter. On the basis of velocity-size-number flux contour plots, it was determined that the droplet size-velocity correlation is minimal. This observation, coupled with the minimal variation in SMD across a diameter, indicates that the spray can be modeled as a variable-density single-phase jet. A model was developed, based on previous gas-phase studies, to explain the development of the drop velocity profiles in the dilute spray region. Agreement between the measured drop velocities and model predictions was within 30% in all cases.

Liquid Jet Disintegration and Spray Characteristics in Subsonic Air Streams.

The present study focuses on the problem of a liquid jet injected transversely into a subsonic air stream. Droplet sizes and droplet velocities were measured, and empirical equations of mean droplet size and mean droplet velocity profile were deduced. On the basis of the observation, the spray formation mechanism is classified into two mechanisms : one due to the instability of the liquid jet surface and the other due to the instability of the liquid jet itself. The maximum size of the mean droplet size profile in the liquid injection direction decreases with increasing air velocity and decreasing momentum ratio of the liquid and the air. The location in the liquid injection direction where the mean droplet velocity becomes minimum is coincident with the location where the mean droplet size becomes maximum. There is a strong correlation between the droplet size and droplet velocity, and larger particles have smaller velocities.