Disintegration Of Liquid Sheet Research Articles

Segment-based Eulerian-Lagrangian transition method for flat nozzle spray atomization simulation

This paper presents a novel segment-based Eulerian-Lagrangian transition method for predicting the primary and secondary breakup behavior of a flat nozzle spray. It combines the Volume of Fluid (VOF) method for simulating liquid sheet disintegration and primary breakup with the Discrete Phase Model (DPM) method for secondary breakup. The VOF simulation predicts the varying thickness and local velocity characteristics of the liquid sheet from the center region to the edge region near the nozzle exit. Meanwhile, the proposed segment-based linear stability analysis in this study is capable of predicting the initial droplet variation resulting from primary breakup, considering the varying behavior of the liquid sheet from different regions. The simulation results match reasonably well with experimental data at both macroscopic and microscopic levels, effectively predicting the overall spray structure, mass flow rate, and droplet size variations across different regions with good accuracy. Both experimental and numerical data show that the Sauter mean diameter (SMD) values differ between the center and edge regions, with the edge region displaying larger droplets than the center region. This indicates that the segment-based approach is crucial for successfully predicting the spatial distributions of droplet size characteristics.

Effect of the physical properties of liquid and ALR on the spray characteristics of a pre-filming airblast nozzle

Abstact The atomization and combustion characteristics of fuels injected in aviation gas turbine engines are very important in the research and development of gas turbine combustors and have a significant impact on engine efficiency and pollutant emissions. In this study, spray atomization characteristics of pre-filming airblast nozzles developed for use in aviation gas turbine combustors were experimentally investigated for different air to liquid ratios (ALR) and air pressure differences (ΔP (%)). Pure water and Jet A-1, which have different viscosities and surface tensions that greatly affect the atomization characteristics, were used as the working fluid. Spray atomization characteristics were investigated by simultaneous measurement of droplet sizes and velocities using PDA (phase Doppler anemometry) and spray visualization using a CCD camera coupled with a Nd:YAG laser as a light source. The atomization characteristics of water and Jet A-1 showed significant differences in spray angle, velocity distribution and mean droplet size. Jet A-1 spray with lower surface tension showed a smaller mean drop size (WMSMD) than water spray did. As the ALR increased, the mean droplet size of the two liquids decreased. From the analysis of the spray flow characteristics, the surface tension associated with aerodynamic instability was the dominant variable in the atomization process with liquid sheet disintegration. The experimental constants included in the previously proposed empirical formulas for predicting the mean droplet size of the pre-filming airblast nozzle spray were presented for the two liquids used in the experiment.

Multi-step spray modelling of a flat fan atomizer

The present study combines Eulerian CFD modeling of the liquid flow inside an atomizer, instability analysis of liquid sheet, statistical spray analysis, and Lagrangian modeling for spray transport of a flat fan atomizer widely used in agricultural spraying. High-speed spray imaging and spray size measurements carried out for validation of numerical models and the experimental results agreed reasonably with the modeling results. Although the subject of the study is a flat fan atomizer, the methodology presented could be employed to other types of spray nozzles that rely on liquid sheet disintegration mechanism. The systematic modeling of spray systems with the methodology explained in the paper not only reduces the design and prototyping time and costs but also gives engineers a better understanding of the design parameters for improved spray system performance.

Multi-scale analysis of atomizing liquid ligaments

The atomization of individual liquid ligaments appearing during the disintegration of liquid sheets issuing from a triple-disk injector is investigated. High-speed visualizations report a temporal evolution of the ligaments from their production to their disintegration into drops. The parameter of the experiments is the surface tension of the liquid. A multi-scale analysis consisting in describing the temporal evolution of the ligament shape by measuring the scale-distribution is performed. This analysis introduces the notion of scale-diameter whose temporal variation leads to the following conclusion. The ligaments are subject to elongation, capillary deformation and break up and relaxation mechanisms. These mechanisms appear concomitantly on different scales. This concomitancy depends on the surface tension, i.e., on a Weber number based on the ligament initial elongation rate and initial size: the decrease of the surface tension favor the propensity of the atomization process to cascade in the scale space towards the small scale region. A mathematical scale-distribution of the final drops is satisfactorily derived from an atomization model of the literature. Furthermore, the parameters introduced by this model are well correlated to initial ligament Weber number and size.

Role of axisymmetric axial velocity gradients in nonlinear disintegration of liquid sheets

The nonlinear growth of sinuous and dilatational disturbances on thin inviscid liquid sheets with a velocity profile across the thickness is determined. Variations of the velocity profile in the liquid sheet, with maximum velocity at the centre linearly decreasing to a minimum at the interfaces, show an optimum value of the velocity gradient, below which disintegration of the liquid sheet is accelerated. The ligaments are formed in local dilatational and global dilatational modes and are asymmetric, unlike the symmetric ligaments formed with a liquid sheet issuing at constant velocity. The presence of velocity gradients is seen to yield smaller ligaments when breakup occurs in the local dilatational mode.

SPRAY CHARACTERISTICS OF AN AIR-DRIVEN ROTARY ATOMIZER WITH A DOUBLE-LAYER CUP FOR USE IN AN INDUSTRIAL OIL BURNER

An experimental investigation of spray characteristics in a rotary burner with an air-driven double-layer cup atomizer is reported. The air stream generated by a centrifugal blower was used to rotate a vane propeller attached to the double-layer cup, and to assist disintegration of liquid sheets formed from the rim of the double-layer cup at the air nozzle exit. As the blower power increased, the rotational speed of the cup and the gas-to-liquid mass ratio (GLR) increased, and the atomizing crossflow velocity at an annular slot increased as well. For some given operating conditions, the effect of the crossflow velocity on the spray quality was investigated by varying the outer diameters of the air nozzle (88, 83, and 79.5 mm). The spray cone angles were measured by analyzing stroboscopic images. As the air velocity at the annular slot increased, the spray cone angle decreased. The spray radial and streamwise dispersion distributions were obtained by two liquid-collecting devices. The crossflow velocity at the annular slot, the GLR, and the diameter of air nozzles had an impact on the drop sizes and size distribution. The drop sizes of the spray were obtained using laser diffraction spectroscopy. The spray performance was described in terms of Sauter mean diameter (SMD), maximum drop diameter, relative span factor, and some representative drop diameters such as D0:9, D0:5, and D0:1. The SMD and maximum drop diameter were correlated with liquid Weber number (WeL) only. The atomization quality was improved with increasing GLR and crossflow velocity at the annular slot.

Liquid sheet disintegration at high pressure: An experimental approach

The primary atomization was studied in a 300 μm thickness water sheet, generated by a planar airblast atomizer. The research novelty consisted in increasing the airflow absolute pressure from atmospheric conditions to 6 bar. The experimental techniques employed included Oscillometry by Laser Intensity Reflexion (ORIL), Laser Doppler Velocimetry (LDV) and flow visualization by fast video camera. The atomization mechanisms, described in the literature at atmospheric environments, were observed at high pressure conditions, for a constant momentum flux ratio. Furthermore, a new atomization mechanism was observed at high values of this ratio. Finally, dimensionless relations have been proposed for the global oscillation frequency, minimum air oscillation velocity, break-up distance and transversal wavelength. To cite this article: V.G. Fernandez et al., C. R. Mecanique 337 (2009).

Liquid film disintegration regimes and proposed correlations

The process of liquid sheet disintegration and break-up into a spray is studied making use of a flat liquid film surrounded by two air streams. The work considers a 2-D liquid film with an aspect ratio of 114 and a liquid velocity between 0.7 and 6.4 m/s (543⩽ Re L⩽4963) and an air velocity up to 39 m/s ( Re G ⩽17 647 ), and allows to study the basic phenomena typical of airblast atomisation. The results include the quantification of instability amplitudes, break-up lengths and frequencies for the above-mentioned range of liquid (water) and air flow velocities. Three different experimental techniques are used: back light and laser light sheet illumination, to measure the instability amplitudes, which leads to the liquid film disintegration and the spray formation; strobe light illumination, for the quantification of either break-up lengths and frequencies, and; a laser attenuation technique for frequency measurements and comparison with the visualisation technique. The results quantify the range of application of the different techniques, and include an analysis of the liquid disintegration process as function of the liquid velocity when exposed to different air flow velocities, along with the quantification of break-up lengths, disintegration frequencies and spray angles, and show that the disintegration of the liquid sheet is associated with a periodic process, which is mainly dependent on the absolute air velocity and the air–liquid momentum ratio. Correlations are proposed for liquid sheet break-up length and frequency based on the air-to-liquid momentum ratio.

A Comprehensive Model to Predict Simplex Atomizer Performance

The pressure swirl atomizer, or simplex atomizer, is widely used in liquid fuel combustion devices in the aerospace and power generation industries. A computational, experimental, and theoretical study was conducted to predict its performance. The Arbitrary-Lagrangian-Eulerian method with a finite-volume scheme is employed in the CFD model. Internal flow characteristics of the simplex atomizer, as well as its performance parameters such as discharge coefficient, spray angle and film thickness, are predicted. A temporal linear stability analysis is performed for cylindrical liquid sheets under three-dimensional disturbances. The model incorporates the swirling velocity component, finite film thickness and radius that are essential features of conical liquid sheets emanating from simplex atomizers. It is observed that the relative velocity between the liquid and gas phases, density ratio and surface curvature enhance the interfacial aerodynamic instability. The combination of axial and swirling velocity components is more effective than only the axial component for disintegration of liquid sheet. For both large and small-scale fuel nozzles, mean droplet sizes are predicted based on the linear stability analysis and the proposed breakup model. The predictions agree well with experimental data at both large and small scale.

Direct droplet production from a liquid film: a new gas-assisted atomization mechanism

X-ray lithography and micro-machining have been used to study gas-assisted liquid atomization in which a liquid film was impinged by a large number of sonic micro-gas jets. Three distinct breakup regimes were demonstrated. Two of these regimes share characteristics with previously observed atomization processes: a bubble bursting at a free surface (Newitt et al. 1954; Boulton-Stone & Blake 1993) and liquid sheet disintegration in a high gas/liquid relative velocity environment (Dombrowski & Johns 1963). The present work shows that suitable control of the gas/liquid interface creates a third regime, a new primary atomization mechanism, in which single liquid droplets are ejected directly from the liquid film without experiencing an intermediate ligament formation stage. The interaction produces a stretched liquid sheet directly above each gas orifice. This effectively pre-films the liquid prior to its breakup. Following this, surface tension contracts the stretched film of liquid into a sphere which subsequently detaches from the liquid sheet and is entrained by the gas jet that momentarily pierces the film. After droplet ejection, the stretched liquid film collapses, covering the gas orifice, and the process repeats. This new mechanism is capable of the efficient creation of finely atomized sprays at low droplet ejection velocities (e.g. 20 μm Sauter mean diameter methanol sprays using air at 239 kPa, with air-to-liquid mass ratios below 1.0, and droplet velocities lower than 2.0 m s−1). Independent control of the gas and the liquid flows allows the droplet creation process to be effectively de-coupled from the initial droplet momentum, a characteristic not observed with standard gas-assisted atomization mechanisms.

Disintegration of liquid sheets

The development, stability, and disintegration of liquid sheets issuing from a two-dimensional air-assisted nozzle have been studied. Detailed measurements of mean drop size (SMD) and velocity have been made using a phase Doppler particle analyzer. Without air flow the liquid sheet converges toward the axis as a result of surface tension forces. There is a linear increase in convergence length with increases in liquid flow rate. With air flow a quasi-two-dimensional expanding spray is formed. It is shown that the air flow is responsible for the formation of large, ordered, and small, chaotic ‘‘cell’’ structures. These structures are bounded by large diameter ligaments containing thin membranes inside. The ligaments are the origin of the large droplets in the spray and the membranes contribute to the formation of the smaller droplets. The air flow causes small variations in sheet thickness to develop into major disturbances with the result that disruption starts before the formation of the main breakup region. The phase Doppler measurements show that the spray two-dimensionality breaks down at a short distance from the nozzle in the downstream region. Boundary layer growth and turbulent mixing result in the redistribution of droplets according to their size class. The droplets detached from the central part of the sheet are subject to further breakup because of high local relative velocities. Droplets detached from the rims are outside the main air flow field and have diameters close to that of the rims. The spray acquires a Gaussian velocity profile in both the Y and Z directions.

The mechanisms of disintegration of liquid sheets in cross-current air streams

A photographic study has been made of the disintegration of thin circular liquid sheets in air streams flowing normal to the liquid sheet. The results may be summarised as follows: (1) Two principal mechanisms of disintegration have been observed: (a) Continuous atomisation proceeds through the formation of circum-ferential waves, and fragments of the sheet are subsequently torn off and atomised. (b) A vibratory system is set up between the two fluids, and the sheet breaks up by resonance between the fluids into periodic clusters of drops. (2) The occurrence of each mechanism is markedly affected by the operating conditions. The vibratory mechanism is pronounced at air/liquid momentum ratios above 18, particularly with values of air energy above 2000 ft.lbs/min. At all values of air energy wave disintegration occurs with air/liquid momentum ratios below 18. (3) A sheet undergoing wave disintegration produces smaller drops than a vibrating sheet. Since for given air flow conditions the momentum ratio can be reduced by increasing the liquid flow rate these phenomena demon-strate the surprising result that smaller drop sizes can be achieved by an increase in liquid flow rate; tha.t is to say, a decrease in air/liquid mass ratio. (4) At any level of air energy atomisation is improved when the air is distributed from a narrower annular gap, the air stream being utilised more efficiently. (5) When the sheet is atomised by a wide air stream close to the orifice, an increase of 50% in sheet thickness produces a coarser spray. When the sheet is atomised by a narrow air stream at a greater distance from the orifice, the effect of sheet thickness is less marked. (6) Spatial drop dispersion in the atmosphere is affected by the mechanism of disintegration. Wave disintegration results in a relatively well dispersed spray, whilst the vibratory mechanism produces a dense core of drops along the nozzle axis. (7) Imparting a rotary motion tothe atomising air stream improves the quality of atomisation and spatial dispersion, particularly with vibratory disintegration.

A photographic investigation into the disintegration of liquid sheets

The types of apparatus used to produce liquid sheets are classified according to the manner in which the energy is imparted to the liquid. The factors influencing the development, stability and manner of disintegration of a liquid sheet are examined more particularly with flat sheets produced from the single-hole fan-spray nozzle and the spinning disk. The development of the liquid sheet is influenced by the liquid properties. As the working pressure is raised the width of the sheet increases, but this development is hindered by high surface tension. It is shown that the effect of a surface-active agent on the development is only influential where the surface is not expanding or changing rapidly. Consequently its effect is more pronounced as the liquid moves farther away from the orifice. Increase of viscosity at the same pressure causes the region of disintegration to move away from the orifice, and high viscosity maintains the sheet undisturbed by air friction. Density has little effect on the area of the sheet. The effect of turbulence in the orifice is shown to be responsible for at least two types of disturbance in the sheet which results in holes being formed near the orifice. The depth of the disturbance in the sheet has to be equal to the thickness before disruption occurs. Similar disruption through the formation of holes can be caused by suspensions of unwettable particles. Wettable particles in low concentration, irrespective of their size, have no effect on the manner of disintegration. The most placid, stable and resistant sheet is obtained with a liquid of high surface tension, high viscosity, low density, giving low turbulence in the nozzle. Such a sheet will disintegrate when the velocity is raised and disintegration can occur through air friction. The easiest sheet to disintegrate is obtained with a liquid of low surface tension, low viscosity, low density and with low turbulence in the nozzle. Disintegration will occur near the nozzle at low velocities through waves caused by air friction. Disintegration through the formation of holes in the sheet can occur at low velocity with liquids of high surface-tension, low viscosity and high density where turbulence obtains in the nozzle. The formation of ligaments or threads is a necessary stage before the production of drops. Threads can be formed directly from any free edge or in the boundary. A free edge is formed when equilibrium exists between surface tension and inertia forces. In the spinning disk, at low flow rates, where the sheet is in contact with the surface of the disk, drops are formed at the ends of threads which break down into a limited number of sizes. At high flow rates a free edge of liquid exists outside the periphery of the disk with the formation of more irregular threads and a wider spectrum of drop sizes results. Where perforations occur in the sheet, expansion of the hole by surface tension occurs very regularly so that the holes remain nearly circular until they coalesce forming long threads. These long threads quickly become unstable and break down into drops. Threads being approximately uniform in diameter produce uniform drops, but the irregular areas of liquid which occur when a number of holes expand towards each other produce a wide variety of drop sizes. When the velocity of the sheet in the atmosphere is high, air friction causes slight variations in the sheet to develop rapidly into major wave disturbances, and these can result in holes being blown through the sheet so that disruption starts before the formation of a leading edge. With liquids having visco-elastic properties the sheet disintegrates through the formation of waves, but the rapid increase of viscosity, as the rate of shear is reduced, prevents further break-up of the threads into drops and a web of fine threads only is produced.

The Mechanism of Disintegration of Liquid Sheets

Abstract The disintegration of a plane sheet of liquid of finite thickness, moving tangentially relative to a surrounding fluid, is subjected to mathematical and experimental analysis. Instability and wave formation at the interface are established as the major factors in the breakup of the sheet of liquid into drops. An equation is derived relating four significant dimensionless groups, and a graphical presentation of the quantitative relationship among these groups is included. The force balance between the interfacial tension and aerodynamic forces is the basic concept, and the criteria for stability and instability are determined from it. It is possible to predict the conditions for maximum instability, and therefore for the most rapid disintegration of the sheet. Short-exposure photographs of the actual disintegration of the sheet are included to support the analysis, and a prediction of drop size in the spray compares favorably with measurements by other methods.