Sheet Breakup Research Articles

Spray characterization of boron-loaded slurry fuels using high-speed imaging technique

The characterization of atomized spray in terms of its structure, break-up, and droplet morphology is crucial to analyze an engine's performance. Therefore, in this study, the spray characteristics of boron-laden slurry fuels have been evaluated experimentally through a high-speed imaging technique. The rheological properties of the boron-loaded slurry fuels have been measured and their impact on the atomization behavior has been qualitatively analyzed. The spray cone angle, penetration length, sheet break-up length, ligament length, ligament diameter, and droplet diameter distribution are obtained by processing the time-sequences images. The experiments were performed at three atomizing air-to-liquid ratios (ALR). Spray characteristics of the fuel samples with various particle loadings (10%, 20%, and 30%) have been analyzed and compared with that of the pure Jet A-1. The obtained results were qualitatively analyzed with different non-dimensional parameters, such as Momentum flux ratio, Weber number, Ohensorge number, and Reynolds number. The results show that an increase in viscosity due to particle loading significantly affects the spray characteristics. However, a better atomization behavior of boron slurry fuel at higher ALR than low ALR, even with higher particle loading, has been observed. This is possibly due to the change in momentum flux ratio at higher atomizing air velocity.

Unrelaxed tensile stress impacts on the instability of viscoelastic sheets with the gas velocity oscillation

A temporal instability analysis of a viscoelastic liquid sheet with unrelaxed tensile stresses in the oscillation gaseous flow field has been studied with the Floquet method in the present research. Effects of the oscillated amplitude and frequency of incoming gaseous flow on the instability features in the different instability regions are analyzed systematically. Results reveal that the unrelaxed tensile stress has a positive effect on stabilizing the liquid sheet, while the higher oscillation amplitude would promote the sheet breakup. Besides, it is indicated that when the stress tensor would be controlled by the viscoelastic effects, elastic impacts would suppress the breakup, while it shows an opposite impact when the stress tensor is dominated by the tensile stress. The existence of parametric instability regions is influenced by the oscillated and elastic parameters as well.

Evaluation of Liquid Atomization and Spray Drift Reduction of Hydraulic Nozzles with Four Spray Adjuvant Solutions

The droplet size distribution following pesticide application practices can significantly impact droplet drift and non-target organisms (animals and plants). However, the relationship among liquid sheet breakup, drop formation, and droplet drift is an area that has been studied over the past 65 years but is still not fully understood. The objectives of this study were severalfold: to examine the liquid sheet breakup following the use of different adjuvants (sodium dodecyl sulfate, aerosol OT, and silicone at 1%) and their effects on the drift via three commonly used commercial spray nozzles (XR, AIXR, and TXVK). The spray sheet and initial droplet size spectrum for each spray was detected by a particle image velocimetry (PIV) system, and the drift for each treatment was measured in a wind tunnel. The nozzle type and the spray solution were found to significantly affect the liquid sheet breakup characteristics. AIXR produced large droplets, and TXVK with short liquid sheet length produced more small droplets. All adjuvants used in these experiments increased the formation of large droplets upon atomization while simultaneously reducing driftable fines (e.g., drops %<150 µm). The drift potential can be reduced by up to 66.1% when switching from a fine spray quality (TXVK) to a coarse spray quality (AIXR). The SDS adjuvant provided the most effective drift reduction for XR and TXVK nozzles showing reduced drift potential by 69.2% and 66.3%, respectively, while the silicone adjuvant showed the largest drift reduction for AIXR of 78.3%. The correlation between the liquid sheet length and DV0.5 was positive for XR and TXVK nozzles, and there was a significant positive correlation between %<150 µm and drift potential for all nozzles. This work suggests that the drift can be reduced significantly by changing the nozzle type and adding pesticide adjuvant into spray solution, which provided data support for the drift reduction of plant protection drones.

Computational study of free surface film flow and subsequent disintegration of a sheet and ligaments into droplets from a rotary disk atomizer

In the present study, a computational methodology based on computational fluid dynamics (CFD) is developed to investigate free surface film flow and its subsequent disintegration on a rotary disk atomizer. The present study provides an insight into efficiently modeling both liquid film formation and its subsequent disintegration. The presented computational methodology can easily be reproduced and can act as a benchmark for the modeling of liquid film formation as well as the disintegration phenomenon. The influence of disk speed, liquid flow and feed arrangement on the rotating disk are investigated. The film thickness profile on the disk resembles the formation of spiral waves at 500 RPM and an irregular breakup of the spiral waves for a disk speed of 1000 RPM. At higher speeds, a smooth and thin liquid film is observed on the disk. Offset feeding of the liquid on the disk alters the wave formation and breakup of the spiral waves. It is found that ligaments are formed at the lip of the disk owing to Rayleigh–Taylor instability, whereas liquid sheet breakup is due to combined rim and wave disintegration for the parameters investigated in this study.

Primary breakup of liquid fan sheet in crossflow

This paper presents the primary breakup of liquid sheet under the influence of crossflow. The atomization of liquid sheets is important for many applications, e.g., agricultural sprays, fuel atomization, and spray coatings, among others. The aerodynamic effects on the liquid breakup process is investigated experimentally to identify the breakup regime transitions. A subsonic wind tunnel was used to simulate the crosswind effects on flat fan nozzle spray. Two flat fan nozzles sizes were tested in this study. High-speed digital imaging technique was used to measure the breakup regime transitions, the location of the end of the liquid core, the breakup time, the location, and size of droplets at onset of breakup and the spray trajectory. The results were correlated using phenomenological analyses. The aerodynamic effects were responsible for initiating bag breakup and reducing the droplet sizes which are susceptible to drift. This would be problematic for spraying herbicides in windy conditions.

Experimental study on the stability and breakup of a planar liquid sheet under a standing wave acoustic field

With experiments and theory the effect of acoustic parameters on the stability and break up of a planar liquid sheet under the influence of a standing acoustic wave are studied. Atomization characteristics of slit nozzles and the effects of acoustic pressure and injector position are analyzed. Above a critical acoustic pressure the standing wave acoustic field generates a Faraday wave on the liquid sheet surface. At a specific flow rate an empirical coefficient is found which enables linear instability theory to predict the wavelength of the Faraday waves and the breakup length of the liquid sheet.

Bubble dynamics and atomization in evaporating polymeric droplets

We investigate the interaction of an aqueous polymeric droplet with a tuneable continuous laser in an acoustically levitated environment. The effect of laser irradiation intensity and polymeric concentration on various spatio-temporal parameters is unearthed using high-speed shadowgraphy and theoretical scaling analysis. We observe four temporal phases: droplet evaporation, vapour bubble growth followed by membrane inflation, bubble/membrane rupture through hole nucleation and droplet breakup. During the initial droplet evaporation phase, concentration build-up at the droplet surface beyond a critical limit leads to the formation of a skin layer. It is revealed that, at a given location inside the droplet, hot spots occur, and the maximum temperature at the hot spots scales linearly with irradiation intensity until a bubble nucleates. The low-intensity laser interaction leads to symmetric membrane inflation that eventually forms holes at droplet poles and cracks on the shell surface. On the contrary, high intensity causes early bubble nucleation followed by asymmetric membrane inflation that eventually ruptures through multiple hole formation. Furthermore, the growth and rupture of the membrane is followed by a catastrophic breakup of the droplet. Two dominant atomization modes are reported at significantly high irradiation intensities: stable sheet collapse and unstable sheet breakup. The evolution of droplets into a stable/unstable sheet follows universally observed ligament and hole dynamics. A regime map is shown to describe the influence of polymer concentration and irradiation intensity on the strength and mode of droplet atomization.

Liquid Sheet Instability and Breakup in Primary Atomization for a Stirling Engine

Liquid Sheet Instability and Breakup in Primary Atomization for a Stirling Engine

Modal analysis of the liquid sheet breakup with and without acoustic forcing

• The coupling between the acoustic field and the liquid sheet atomization process is studied. • Modal decomposition techniques like Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are employed. • Frequency response of the liquid sheet is found to be dependent on the laminar or turbulent nature of the impacting jets. • Liquid sheet breakup gets coupled with the external forcing when the acoustic energy goes into low frequency modes. • Decoupling occurs when the acoustic energy goes into the dominant modes which are higher ones having a higher frequency. The objective of the present study is to understand the coupling between the acoustics and the sheet atomization process. The phenomenon is studied through modal decompositions of time-resolved images of acoustically perturbed liquid sheets by employing the proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) techniques. The injector operating conditions are selected in order to allow the transition of a smooth liquid sheet to a highly turbulent sheet or fully developed spray. The sheet is destabilized by the traveling acoustic waves of known frequency and sound pressure level. POD modes of the smooth sheets and DMD modes of the turbulent sheets clearly highlight distinct effects of the acoustic perturbations on the liquid sheets. The frequency response of the liquid sheet is found to be dependent on the laminar or turbulent nature of the impacting jets. When the impinging jets are laminar, interestingly the dominant POD mode frequencies are either equal to or harmonics of the external acoustic excitation frequency which confirms the coupling of the atomization process with the acoustic perturbations. Below the lower cutoff frequency, the POD mode having frequency equal to the forcing frequency always dominates other modes. Also, for laminar jets, the DMD growth rate is maximum for the mode having frequency equal to the external forcing. However, for the turbulent impinging jets, the first DMD mode frequency is not always equal to the external forcing frequency but one of the mode in the DMD frequency spectrum show the frequency which is either close to the forcing frequency or close to its integer multiple.

Identification and Mapping of Three Distinct Breakup Morphologies in the Turbulent Inertial Regime of Emulsification—Effect of Weber Number and Viscosity Ratio

Turbulent emulsification is an important unit operation in chemical engineering. Due to its high energy cost, there is substantial interest in increasing the fundamental understanding of drop breakup in these devices, e.g., for optimization. In this study, numerical breakup experiments are used to study turbulent fragmentation of viscous drops, under conditions similar to emulsification devices such as high-pressure homogenizers and rotor-stator mixers. The drop diameter was kept larger than the Kolmogorov length scale (i.e., turbulent inertial breakup). When varying the Weber number (We) and the disperse-to-continuous phase viscosity ratio in a range applicable to emulsification, three distinct breakup morphologies are identified: sheet breakup (large We and/or low viscosity ratio), thread breakup (intermediary We and viscosity ratio > 5), and bulb breakup (low We). The number and size of resulting fragments differ between these three morphologies. Moreover, results also confirm previous findings showing drops with different We differing in how they attenuate the surrounding turbulent flow. This can create ‘exclaves’ in the phase space, i.e., narrow We-intervals, where drops with lower We break and drops with higher We do not (due to the latter attenuating the surrounding turbulence stresses more).

Effects of bulk elasticity on sheet formation and expansion

The destabilization, fragmentation, and atomization of thin fluid sheets governs processes such as the aerosolization of sneeze ejecta, agrochemical spraying, and fuel injection in liquid rocket engines. Although the evolution, stability, and breakup of fluid sheets composed of a Newtonian liquid has been extensively studied, the morphology and dynamics of viscoelastic fluid sheets remains poorly understood. This manuscript provides a theoretical and numerical framework that integrates the effects of fluid elasticity, surface tension, inertia, and viscosity to predict the morphology, velocity, and stress within stable fluid sheets composed of viscoelastic fluids as a function of the dimensionless Weber, Reynolds, and Weissenberg numbers. We find a non-monotonic behavior in the sheet’s size, velocity, and stress distribution as a function of the ratio between the Weissenberg and the Weber numbers. In particular, a minimum in the sheet’s size and a maximum in the stress occur when such a ratio is of the order of unity. We interpret these results as the consequence of the competing effects of the growth-favoring inertia and the restoring elastic forces acting within the sheet. • We study the dynamics of viscoelastic liquid sheets formed by two impinging jets. • Three dimensionless groups: Weber ( W e ), Reynolds ( R e ), and Weissenberg ( W i ) numbers. • Viscosity, elasticity, inertia, and capillarity are quantified by W e , R e , and W i . • A set of governing equations is developed and solved in terms of W e , R e , and W i . • A non-monotonicity in sheet size and stress is revealed for W i / W e ∼ 1. • The non-monotonicity represents a departure from Newtonian behavior. • At W i / W e ∼ 1, sheets also encounter a maximum in the extensional stress. • W i / W e is the ratio between the fluid relaxation time and the process timescale. • Large W e and low R e induce larger non-monotonicities.

Spray atomisation in multiphase flows with reference to tank mixes of agricultural products

First-principle models describing the breakup of liquid sheets into droplets are used to screen spray adjuvants and agricultural tank mix partners to eliminate combinations that might negatively impact droplet size, reducing the need for extensive experimental testing. An agent-based model is developed to predict droplet spectra following spray atomisation from a flat-fan nozzle for a continuous oil-in-water liquid sheet that comprises the mechanisms of hole nucleation, hole growth, hole coalescence, ligament formation and rupture into atomised drops. Hole nucleation is driven by Marangoni spreading of oil globules at the interface that initiates sheet breakup, followed ultimately by fluid ligament rupture into droplets of sizes conforming to a gamma distribution. This mechanistic model is used to predict the effect of various relevant parameters on the volume fraction of driftable fines (DFV, fine spray that can easily become entrained in wind, typically <150 μm diameter) resulting from spray atomisation. An increase in the concentration of the emulsion immiscible phase increases the rate of hole nucleation in the spray sheet and decreases the DFV. In addition, increasing the surface tension of the emulsion aqueous phase is also found to substantially decrease the DFV. A small decrease in DFV is observed with increase in the mean oil globule diameter, and negligible change is observed when varying the viscosity by 10× over water. Experimental droplet spectra from commercially available formulations mixed together are used to validate this numerical approach for mixture performance with excellent agreement.

Investigation of Water Films Shed From an Airfoil in a High-Speed Flow

Abstract The breakup of liquid films in high-speed flows is found in many applications. These include prefilming air blast atomization found in fuel injectors and shedding of droplet off of airfoils. In this work, a test rig has been developed and applied to study the breakup of water films from the trailing edge of an NACA 0012 airfoil placed in a high-speed air flow. The results provided reflect a new detailed dataset focused on air velocities above 75 m/s. Computational fluid dynamic (CFD) simulations were used to guide the development of the rig and the associated boundary and inlet conditions. In this work, air velocities up to 175 m/s were used with water films between 0.4 cm2/s and 3.6 cm2/s. The water film was introduced onto one surface through a series of 0.5 mm holes separated by 1 mm at a location 35 mm downstream of the leading edge of the vane. High speed video was used to document the sheet breakup behavior from the edge of the airfoil. Image analysis was used to identify the breakup length of ligaments formed. Laser diffraction was used 50 mm downstream of the trailing edge of the vane to determine the droplet size distribution and associated representative average diameters generated. A series of test conditions were run between 50 m/s and 175 m/s using multilevel factorial designed experiments. The results obtained were subjected to analysis of variance to generate correlations for breakup length and droplet representative diameters as a function of the conditions studied. The analysis was conducted on dimensionless versions of the variables studied to help connect the results to the physics of the situation. For example, rather than using air velocity to correlate the behavior, the Weber number (based on gas phase density and velocity) was used owing to its historical ability to describe liquid breakup processes. The results obtained are compared with other studies in the literature, and good agreement was found for the conditions at which a comparison could be made. It is observed that, for the conditions studied, liquid film thickness has little effect on the resulting droplet sizes—especially for the higher air velocity cases studied. The results also illustrate the sensitivity to choice of the characteristic length, which is generally consisted of trailing edge geometry or boundary layer thickness. In contrast, liquid film does influence the ligament breakup length, although the time to break up is less affected. Overall, the correlations developed provide engineering tools to help estimate ligament and drop size behavior for water films shed from airfoils in a high-speed flow.

Effect of confinement on weakly nonlinear temporal instability analysis of a viscous planar liquid sheet sandwiched between two co-flowing gas streams at equal velocities

Extensive number of applications like rocket jet engine and gas turbines carry out atomization within a confined environment as it provides faster atomization and good mixing efficiency. Literature suggests that the effect of confinement on breakup of droplets, liquid threads, and annular jets is not elementary, but it varies with liquid viscosity, surrounding medium viscosity, and the confinement strength. Through the present weakly nonlinear temporal analysis, we unveil if a similar dynamic effect of confinement persists on the breakup of planar two-dimensional liquid sheets of varying viscosities. Moreover, to mimic air assisted atomization, the surrounding gases are considered to be flowing with non-zero velocities. Our study reveals that presence of confinement at higher gas-to-liquid velocity ratios of 2.5 and 3 produces no apparent improvement in atomization at all liquid Reynolds numbers. However, at relatively low gas-to-liquid velocity ratios of 2 and 2.25, confinement produces faster breakup as compared to an unconfined one, more so when Reynolds number is relatively low (Re &amp;lt; 100). The overall influence of confinement on non-dimensional breakup time is, however, weak as compared to how it affects the linear growth rate. A counteracting effect of confinement on the linear maximum growth rate and normalized second order disturbance amplitude is identified as the primary reason. Our study also identifies the minimum confinement height CH-1 which is just enough for the liquid sheet to achieve breakup without touching the solid walls. Interestingly, the maximum linear growth rate and minimum breakup time for a particular flow condition are always registered when confinement height equals to CH-1. The values of CH-1 display a strong dependence upon Reynolds number and gas-to-liquid velocity ratios.

Investigation of electromagnetic plasma waves in the process of formation and growth of plasmoid instability in solar corona in two regimes ωce&gt;&lt; ωpe

Using two-dimensional particle-in-cell simulation of collisionless plasmoid instability, the waves generated during formation and growth of plasmoids are investigated. Magnetic reconnection is recognized as a fundamental phenomenon in the conversion of magnetic energy into heating energy and particles acceleration in space and astrophysical plasmas. In this phenomenon stored magnetic energy abruptly released in form of violent plasma jets and non-thermal particles acceleration and relativistic particle acceleration. In large systems such as solar systems that the length of the current layer is much greater than its thickness ,current sheets are affected by complex photospheric motion or external disturbances such as the eruption motion of magnetic structures. This violent plasma instability results in the breakup, fragmentation and formation of multiple current sheets. Therefore reconnection happens at several X points and magnetic islands or plasmids appear. Plasmoid chains are highly dynamic configurations and can fastly grow by coalescence bounce and repel smaller plasmoids. Understanding of the effects of charged particles on the earth's atmosphere depends on a better understanding of the plasmoid instability that we have addressed an aspect of it in this article. The acceleration of charged particles toward the earth's atmosphere is one of the most important consequences of magnetic reconnection and the formation of plasmids in the solar corona as well as in magnetopause and in the magnetotail. The study of electric field components, which are the most important factor of particle acceleration, is very important in recognizing these charged particles. Fourier analysis of electric field components shows different types of electromagnetic and electrostatic waves during the formation and growth of plasmoid instability in both states ωce> ωpe and ωce< ωpe. This analysis reveals that two electromagnetic right and left-hand polarized modes and whistler wave parallel to the initial magnetic field and also the extraordinary mode, ordinary mode and magnetosonic wave perpendicular to the current sheet are excited.

Computational fluid dynamics characterization of the hollow-cone atomization: Newtonian and non-Newtonian spray comparison

Disintegration of liquid masses in a free-surface flow is still an open question in the field of small-scale spray applications such as dispensing of detergents or sanitizing products. In this context, the pressure-swirl atomizer is widely investigated. It allows to improve several spray characteristics through the formation and breakup of a conical liquid sheet that results in the well-known hollow-cone atomization. From this perspective, the characterization of a small-scale pressure-swirl spray under laminar flow conditions is the focus of this work. The configuration of the device and the physical properties of the discharged liquid are the key parameters that modify the attributes of such multiscale flow. In this regard, the entire picture of the fragmentation process is structured into multiple stages: internal nozzle flow, outer displacement of the liquid–gas interface, droplet spread into the atmosphere, and droplet-wall interactions on a collection surface. Through the computational fluid dynamics, we analyze the influence of the main fluid/packaging parameters on the hollow-cone spray properties, and on the whole atomization process. Reynolds and Ohnesorge numbers are coupled with the Sauter mean diameter to distinguish different breakup mechanisms and spray performances. The solution of the entire spray system is performed by implementing the volume-of-fluid-to-discrete-phase-model, which allows to capture the liquid–gas interface displacement and track the droplets produced downstream the primary atomization, simultaneously. With this Eulerian–Lagrangian hybrid model, we link key features of the hollow-cone spray process to spray pattern and droplet size distribution for both Newtonian and non-Newtonian fluid properties.

Impingement Atomization of Carbopol Gels

Experiments were performed on the atomization of Carbopol–water gels of compositions in the range 0.10–0.30 gellant wt.% on a like-on-like jet impingement configuration. Analysis indicates two primary modes of sheet breakup: perforation driven and wave driven. While perforation-driven breakup is not seen for the lowest gellant concentration case, 0.10 wt.%, the wave-driven breakup is present in all cases within the range of investigated jet velocities. Based on the observations, a mechanism involving feedback between the impingement point and the sheet vertex, and a separation front initiated in between the impingement point and the sheet vertex are proposed for the wave-driven breakup.

Aerosol generation from tear film during non-contact tonometer measurement

Aerosols, generated and expelled during common human physiological activities or medical procedures, become a vital carrier for the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). During non-contact intraocular pressure measurements, aerosols can be produced from the tear film on eyes and potentially convey the SARS-CoV-2 in tears, creating a high risk for eye care practitioners and patients. Herein, we numerically investigate deformation and fragmentation of the tear films with various thicknesses and surface tensions that are impinged by an air jet. Evolution of the tear films manifests several types of breakup mechanisms, including both the bag breakup and ligament breakup of tear film on the eyeball, the ligament breakup of tear film on the eyelid margin, and the sheet breakup near the eyelid margin. The sheet near the eyelid margin is critical for generating large droplets and can be formed only if the jet velocity is high enough and the film is sufficiently thick. A criterion based on Weber number and capillary number is proposed for the breakup of tear film into droplets in which three regions are used to classify the film evolution. Our results indicate that eyes with excessive tears have a greater probability of generating aerosols than eyes under normal conditions. We recommend that enhanced protections should be adopted upon measurement for the patients with watery eyes, and the time interval between two adjacent measurements for the same individual should be also prolonged during the COVID-19 pandemic.

What determines the drop size in sprays of polymer solutions?

The effect of viscoelasticity on sprays produced from agricultural flat fan nozzles is investigated experimentally using dilute aqueous solutions of polyethylene oxide (PEO). Measurements of the droplet size distribution reveal that polymer addition to water results in the formation of overall bigger droplets with a broader size distribution. The median droplet size is found to increase linearly with the extensional relaxation time of the liquid and to be well captured by a simple empirical expression in terms of non-dimensional Weber and Deborah numbers. We show that the increase in droplet size with increasing viscoelasticity can be partly attributed to an increase of the wavelength of the flapping motion responsible for the sheet breakup, resulting in the formation of larger ligaments from which droplets are formed. We also show that droplet size distributions, rescaled by the average drop size, are well described by a compound gamma distribution with parameters n and m encoding for the ligament corrugation and the width of the ligament size distribution, respectively. These parameters are found to saturate to values n = 4 and m = 4 at sufficiently high polymer concentrations. • Viscoelasticity leads to larger droplets in sprays for agricultural flat fan nozzles. • Polymer additives can therefore be used to reduce airborne spray drift. • The median droplet size increases linearly with the extensional relaxation time. • Droplet sizes are Gamma distributed with broad drop and ligament sizes with polymers.

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.