Breakup Of Liquid Sheet Research Articles

Experimental Study on the Spray and Self-Excitation Oscillation Characteristics of Gas-Centered Coaxial Injectors

The present article studies the spray and self-oscillation characteristics of liquid sheets generated by a series of gas-centered coaxial injectors. An experimental system has been built to investigate the impacts of gaseous core size and recess length on the breakup and self-oscillation features. The results reveal that the gaseous flow shows an apparent impact on the liquid sheet breakup length with a small gaseous core size. The atomization morphology shows the different patterns under varied pressure drops, and the spray angle is increased with larger recess ratios. Besides, there is a low oscillation frequency at which the self-oscillation phenomenon is activated. The smaller gaseous core sizes and shorter recess lengths could promote self-oscillation. The required liquid flow rate for the self-oscillation phenomena could be raised by increasing the gaseous flow rate, while the self-oscillation frequency hardly varies with it.

Experiments and visualization of sprays from beer can and turbo liquor nozzles

Industrial scale swirl-type black liquor nozzles were studied using water as the test fluid. Simple water spraying experiments were found to be very beneficial for studying and comparing nozzles for black liquor spraying. These kinds of experiments are important for finding better nozzle designs. Three nozzle designs were investigated to understand the functional differences between these nozzles. The pres-sure loss of nozzle 1 (“tangential swirl”) and nozzle 3 (“turbo”) were 97% and 38% higher compared to nozzle 2 (“tan-gential swirl”). Spray opening angles were 75°, 60°, and 35° for nozzles 1, 2, and 3, respectively. Video imaging showed that the nozzles produced sprays that were inclined a few degrees from the nozzle centerline. Spray patter-nation showed all the sprays to be asymmetric, while nozzle 2 was the most symmetric. Laser-Doppler measure-ments showed large differences in spray velocities between nozzles. The spray velocity for nozzle 1 increased from 9 m/s to 15 m/s when the flow rate was increased from 1.5 L/s to 2.5 L/s. The resulting velocity increase for nozzle 2 was from 7 m/s to 11 m/s, and for nozzle 3, it was from 8 m/s to 13 m/s. Tangential flow (swirl) directed the spray 6°–12° away from the vertical plane. Liquid sheet breakup mechanisms and lengths were estimated by analyzing high speed video images. The liquid sheet breakup mechanism for nozzle 1 was estimated to be wave formation, and the sheet length was estimated to be about 10 cm. Sheet breakup mechanisms for nozzle 2 were wave formation and sheet perforation, and the sheet length was about 20 cm. Nozzle 3 was not supposed to form a liquid sheet. Nozzle geometry was found to greatly affect spray characteristics.

Quantification of coflow effects on primary atomization of pressure swirl atomizers

Extension of high momentum jet stabilised combustors towards liquid fuel operation has shown significant injector shape and injection strategy impact on the fuel air mixing behaviour, the flame position and stability and ultimately pollutants. This can be attributed to the lack of optimised injection concepts as well as fundamental understanding of turbulence–spray interactions that dictate atomization and fuel placement in high turbulence gas flow conditions. The current work uses a canonical flow channel to investigate the bilateral interactions between a highly turbulent coaxial air flow and the primary atomization process of a liquid spray generated by pressure-swirl atomizers. A novel two-phase particle image velocimetry (PIV) method with phosphor-particles is developed to enable flow field measurements near the gas–liquid interface as well as in the dilute spray region. In addition, a shadowgraphy technique is applied to detect the disturbances on the sheet surface. Liquid fuel is replaced by deionised water due to safety concerns and the coaxial air flow velocity is varied from 0 to 80 m/s. The results show a significant influence of the aerodynamic forces on the growth rate of sheet disturbances with increasing coflow velocities. This induces an advanced breakup of comparable thick liquid sheets at large Weber numbers. The novel PIV method enabled the quantification of the turbulent kinetic energy transfer between the phases that is strongly dependent on the slip velocity at the gas–liquid interface. These findings assist the ongoing development of liquid fuel injection systems for high momentum jet based combustors and provide validation data for numerical simulations of primary atomization.

Assessment of Primary Breakup of Liquid Sheet Emanating from a Pressure Swirl Atomizer for Non-Newtonian Fluids

Assessment of Primary Breakup of Liquid Sheet Emanating from a Pressure Swirl Atomizer for Non-Newtonian Fluids

Effects of elasticity number and time constant ratio on breakup and droplet formation of viscoelastic planar liquid sheet co-flowing with gases of equal velocities

Abstract

Experimental investigation and modelling of entrainment rate during bottom reflooding in a narrow rectangular channel

Entrainment rate during bottom reflooding in a narrow rectangular channel is extremely important for heat transfer and reactor security when large break loss of coolant accident (LBLOCA) happens. In order to investigate the entrainment rate under different conditions, an experimental apparatus was established to simulate the process of bottom reflooding under different initial and boundary conditions, the volume of entrainment liquid was obtained by mass conservation according to the pressure drop of entrained water tank and the experimental results revealed that the entrainment rate increased with increasing inlet velocities, coolant temperature and the heating power of the experimental section, but decreased with the increase of system pressure.Furthermore, a new entrainment rate model was developed based on the experimental results combined with the liquid sheet breakup mechanism which was verified to occur in the post-CHF region. Experimental results were used to correct the model with the dimensionless number which was relative to the vaporization of the entrained liquid in the post dryout region. Comparison of the new model with experimental data in open literature and existing model was performed to validate the accuracy of the new developed model, which suggested that the new model could accurately predict the entrainment rate of bottom reflooding in a rectangular channel with the average relative error of 13.1%.

On the nature of instabilities in externally perturbed liquid sheets

The evolution of the interfacial instabilities on a smooth and laminar liquid sheet formed by the oblique impingement of two liquid jets is investigated in the presence of external acoustic forcing. The forced liquid sheet exhibits different instability patterns, such as flapping flag, sinuous breakup and vibrating membrane structure, depending on the forcing frequency. The transition frequencies, where the instability pattern changes, are obtained using the linear theory of sheet instability and are shown to be increasing functions of the jet Weber number. These different instability patterns show distinct growth rate behaviours, which cannot be predicted using a unique theory such as the conventional linear theory of sheet breakup. The dynamics of liquid sheet breakup in the presence of forcing is further assessed with the help of established instability theories. Our measurements show that the sheet thickness distribution plays a major role in dictating the relative influence of different mechanisms on the instability characteristics. All the results are finally consolidated to arrive at the instability regime map on the Weber number versus frequency ( $We$ – $\tilde {\omega }$ ) space to serve as a guideline for adopting a suitable modelling approach.

Numerical investigation of spray self-pulsation characteristics of liquid-centered swirl coaxial injector with different recess lengths

The self-pulsation characteristics of a liquid-centered swirl coaxial (LCSC) injector are investigated numerically based on the coupled level set and volume of fluid (CLSVOF) model. Effects of recess length on self-pulsation characteristics are evaluated and discussed. The results demonstrate the crucial role of recess length in exciting self-pulsation. As the recess length increases, spray patterns vary from stable hollow cone to self-pulsated “Christmas tree” shape, and the characteristic frequency increases marginally. Compared with the stable spray, when self-pulsation occurs, the droplets are distributed more widely in the radial direction. Self-pulsation is easily excited under conditions with the critical mixing flow. Several methods to suppress self-pulsation have been proposed. The breakup of the liquid sheet is successively dominated by surface waves and spray self-pulsation as the recess length increases. The waves in both the azimuthal and the axial directions are captured and investigated. The unstable waves are composed of sinuous mode and varicose mode. The varicose waves are responsible for the primary breakup of liquid sheet. The results for the self-pulsated spray suggest that the primary breakup of the liquid sheet is coupled with pressure oscillation inside the confined region. Self-pulsation of recessed LCSC injector is suggested to be caused by the periodical blocking of the annular gap by the liquid sheet.

Breakup of thin liquid sheets through hole–hole and hole–rim merging

Abstract

Minimising atomisation drift potential by exploring the break-up of liquid sheets using multiphase methylated soybean and silicon oil emulsions

During atomisation, modified agricultural spray formulations can produce less fine spray (for drift control) or reduce the overall droplet size distribution (for efficacy). Motivated by the technical literature concerning antifoaming agents, emulsions with and without solid particles were studied to examine their effect on atomisation. For emulsions, the effects of oil type, oil droplet diameter, surfactant, and presence of solid particles in the oil phase on the sprayed droplet size distribution were examined. Test formulations were compared with a standard commercial formulation and several commercial low-drift adjuvants. Results indicated that large oil droplets and the presence of solid particles reduced the number of fine droplets produced in multi-phase emulsions. • Low-drift agricultural formulations minimise fine droplets narrowing size distribution. • Oil drops in a multiphase system facilitate liquid sheet breakup during atomisation. • Antifoam formulations (liquids, solids) help rupture of the thin films. • Tested antifoam materials as good as current industry low-drift standards.

Numerical study on the breakup mechanisms and characteristics of liquid sheets

ABSTRACT The pre-film breakup in a high-performance gas turbine combustor is simplified into a two-dimensional and three-dimensional liquid sheet breakup, and the liquid sheet breakup processes, mechanisms and characteristics are investigated under nearly representative operating conditions. First, the two-dimensional flat liquid sheet breakup processes are simulated using a Reynolds-averaged Navier–Stokes (RANS) model and a VOF (volume of fluid) method. The effect of airflow velocity on the liquid sheet breakup distance is analyzed under various high-speed airflow velocities of 80 m/s, 180 m/s and 280 m/s. Then, the three-dimensional liquid sheet breakup processes are simulated using the large eddy simulation (LES)/VOF method, and the variations of the characteristics of both the horizontal liquid sheet and the oblique liquid sheet, such as the breakup process, breakup mode, breakup distance, wave characteristics and liquid sheet thickness, are analyzed. It’s found that for the two-dimensional flat liquid sheet breakup, the liquid sheet breakup distance decreases with the increase of airflow velocity or Weber number (We). When the airflow inlet velocities are 80 m/s (We = 6.13), 180 m/s (We = 220.5) and 280 m/s (We = 741.13), the liquid sheet breakup distances are 22.2 mm, 16.0 mm and 10.5 mm, respectively. An empirical formula of the liquid sheet breakup distance was thus summarized. For the three-dimensional liquid sheet breakup, as the airflow inlet angle increases, the breakup distance is significantly shortened, which is 2.21 mm at 0° and 1.85 mm at 15°, and the oscillation angle of the liquid sheet becomes larger. For the oblique liquid sheet breakup, the liquid sheet breakup mode becomes more complicated and the surface wave fluctuation amplitude increases. The empirical formula prediction values of the liquid sheet breakup distance developed in this study are in good agreement with the numerical results.

Phase-Field-Based LBM Analysis of KHI and RTI in Wide Ranges of Density Ratio, Viscosity Ratio, and Reynolds Number

Numerous studies have elaborated the dominated roles of Kelvin-Helmholtz instability (KHI) and Rayleigh-Taylor instability (RTI) in the liquid sheet breakup and primary atomization. As for applications in aeronautics, the liquid-gas mixing generally occurs at the challenging conditions of a large density ratio and high Reynolds number. Hence, the evaluation of KHI and RTI under such challenging conditions will have great significance in better understanding the destabilizing mechanism of the liquid layer. To this end, a lattice Boltzmann multiple-relaxation-time (MRT) two-phase model, based on the conservative Allen-Cahn equation, is reconstructed for the present study. Preliminarily, the numerical stability and accuracy of this MRT model are tested by Laplace’s law under a large density ratio and high Reynolds number, including the sensitivity study to the values of mobility. Afterward, KHI and RTI are investigated in wide ranges of the Reynolds number, density ratio, and viscosity ratio. Numerical results indicate that the enhanced viscous force of light fluid with an increasing viscosity ratio notably suppresses the roll-ups of heavy fluid in KHI and RTI. As for the density ratio, it generally shows negative impacts on fluid-mixing in KHI and spike-spiraling in RTI. However, when the density ratio and the Reynolds number both arrive at high levels, the Kelvin-Helmholtz wavelets aroused by a dominated inertia force of heavy fluid trigger severe interface disintegration. The above results once more demonstrate the excellent ability of the present model in dealing with challenging conditions. Besides, the morphological characteristics of KHI and RTI at a high Reynolds number and large density ratio also greatly support the typical interface breakup mechanism observed in primary atomization.

Experiments and modeling of the breakup mechanisms of an attenuating liquid sheet

We present a combined experimental-numerical study of the primary breakup of a single-phase attenuating liquid sheet emerging from a flat fan nozzle. An experimental setup has been built to produce flat fan liquid sheets/sprays at near industrial conditions, i.e. high pressure and turbulent. The process of sheet disintegration, for a transitional and a turbulent sheet, is visualized using high-speed shadowgraphy. The shadowgrams resolve features of the primary breakup at very fine spatial and temporal resolutions, revealing new information about attenuating liquid sheets. Three breakup mechanisms are identified in the central region of the sheet; two of these, aerodynamic instability and disruption by perforations, are well known. The third mechanism, bag breakup, appears not to have been previously reported in the context of flat fan sprays. Highly resolved Computational Fluid Dynamics (CFD) is employed to model the phenomena of sheet breakup. Features of the attenuating liquid sheet are computed and compared with experimental measurements where strong agreement is found. Breakup mechanisms similar to those observed experimentally are resolved in the CFD simulations, confirming the ability of the simulation methodology to extend studies to scales that are not achievable via laboratory experiments. The mechanisms of holes nucleation and growth are studied, we find that hole nucleation in a turbulent single-phase liquid sheet is due to the growth of local surface disturbance which is initiated by nozzle turbulence.

Liquid Sheet Breakup Mode and Droplet Size of Free Opposed Impinging Jets by Particle Image Velocimetry

A comprehensive comparison between water and different mass fraction of glycerol-water (40 wt%, 50 wt%, and 60 wt%) was performed to study the breakup characteristics in free opposed impinging jets...

Retracted: “Symmetric and Asymmetric Disturbances in the Rayleigh Zone of an Air-Assisted Liquid Sheet: Theoretical and Experimental Analysis,” [ASME Journal of Fluids Engineering, 2020, 142(7), p. 071302; DOI: 10.1115/1.4045998

AbstractThe temporal analysis of symmetric (dilatational) and asymmetric (sinusoidal) perturbations at the interface of a water sheet in a coflowing air stream focuses on low gas Weber number region (Weg < 0.4), namely, Rayleigh breakup zone. The motive for this investigation is to acquire a better insight of breakup phenomena involved, rather than technical relevance, by utilizing Kelvin–Helmholtz instability. Accordingly, perturbations are introduced on the basic flow whose stability is to be examined by the method of normal (Fourier) modes. The temporal growth-rate of perturbations is traced to extract the wavenumber associated with maximum growth-rate. Thus, the critical wavelength, in conjunction with the phase velocity of the disturbance will facilitate to obtain the corresponding breakup frequency of the liquid sheet. The analytical findings on liquid sheet breakup frequency with increasing Weber number ratio exhibit the dominance of symmetric wave over asymmetric wave. It also shows independent evolution of breakup frequency with respect to Weber number ratio for the respective perturbation modes, which appears to be a pointed profile. That is, the frequency contour for dilatational mode dips, whereas it rises for the sinusoidal mode and at the Weber number ratio of 0.518 the crossover occur. The theoretical results were substantiated by high-speed flow visualization studies that discern the coexistence of low-frequency (primary) and high-frequency (intermediate) breakup events. Furthermore, the empirical average frequency data track reasonably well with the dilatational instability.

Efficient preservation and breakup of liquid sheets using screen-projected particles.

In this paper, we propose a technique that can efficiently express the preservation and breakup of liquid sheets by eliminating over-preserved liquid sheets using the motion of water particles projected onto the screen. First, we project three-dimensional water particles onto a two-dimensional screen. When multiple particles are projected onto the same pixel, we select one of the front most particles as a screened particle by comparing their depth values. Based on the anisotropic kernel and density, the motion of the screened water particles is tracked to determine whether preservation and breakup should be performed. As a result, new water particles are added or existing ones are deleted, which makes it possible to express two characteristics of particle-based fluids: preservation and breakup of liquid sheets. The proposed technique is based on particle-based fluids, which can be used to remove the over-preserved liquid sheets and thus improve the quality of liquid sheets without surface noise.

Experimental study on flashing spray characteristics of pressure swirl nozzle with ethanol solution

Flashing spray has the potential to enhance the performance of aero-engine combustor, compared with conventional spray. In this study, the kerosene was replaced by ethanol solution, the influence of flashing degree of ethanol on spray profile, velocity distribution and droplet size distribution was investigated based on the ratio of the ambient pressure to saturation pressure using the particle image velocity and laser particle size analyzer. Results indicated that when the ratio of ambient pressure to saturation pressure was reduced, the micro-explosion effect was intensified, spray cone angle was first increased and then decreased, and the radial velocity peak and velocity gradient were improved. The downtrend of sauter mean diameter became faster and it was dominated by the ratio of ambient pressure to saturation pressure, the cumulative distribution ratio and frequency distribution were changed, small droplets produced by the micro-explosion effect mainly existed in 16% of the total droplet volume. Moreover, the formation mechanism of recirculation zone upstream of the spray field was proposed, and the difference of formation and breakup of liquid sheet between conventional spray and flashing spray was analyzed. These results provide insightful information for understanding flashing spray characteristics with pressure swirl nozzle.

On the correlation of the primary breakup length with fuel temperature in pressure swirl nozzle

This work is focused on the effect of viscosity on the estimation of liquid sheet thickness and breakup length of a pressure swirl nozzle that is undergoing fuel temperature variation. The pressure swirl nozzle tested in this study was developed for aviation gas turbine engines, where the ignition characteristics included an importantly low-temperature environment. Jet A-1 aviation fuel was used for various injection pressures at different fuel temperatures. With the aid of a laser sheet planar imaging, instantaneous spatial spray images were acquired. In lower fuel temperature cases, the fuel viscosity increased, resulting in a slowing of the liquid sheet breakup process. An increased mass flowrate and a decreased spray cone angle were observed at lower fuel temperatures and fixed injection pressures. With a decrease in fuel temperature, the primary breakup length and secondary breakup length increased dramatically. A newly proposed expression for predicting the primary breakup length of the liquid sheet and accounting for viscosity effects with the change of fuel temperature is presented.

Drop impact close to the edge of an inclined substrate: Liquid sheet formation and breakup

Experiments on the impact of a drop near the edge of an inclined substrate are reported. A liquid sheet forms beyond the edge, then fragments into droplets. This configuration is a minimal model of the crucial raindrop impacts on plant leaves that are responsible for the dispersal of crop diseases.

입자 기반 유체 시물레이션에서 디테일한 액체 시트를 표현하기 위한 보존과 분해 기법

In this paper, we propose a new method to improve the details of the fluid surface by removing liquid sheets that are overpreserved in particle-based water simulation. A variety of anisotropic approaches have been proposed to address the surface noise problem, one of the chronic problems in particle-based fluid simulation. However, a method of stably expressing the preservation and breakup of the liquid sheet has not been proposed. We propose a new framework that can dynamically add and remove the water particles based on anisotropic kernel and density to simultaneously represent two features of liquid sheet preservation and breakup in particle-based fluid simulations. The proposed technique well represented the characteristics of a fluid sheet that was breakup by removing the excessively preserved liquid sheet in a particle-based fluid simulation approach. As a result, the quality of the liquid sheet was improved without noise.