Round Liquid Jets Research Articles

Three-dimensional large eddy simulation of round liquid jet primary breakup in coaxial gas flow using the VOF method

Large eddy simulation (LES) was performed on a liquid primary breakup process of twin-fluid atomizers with various dimensions. Volume of fluid (VOF) method was applied to track the unsteady evolution and breakup of the liquid jet. It was observed that the liquid primary breakup morphology of the thin central tube nozzle was more sensitive to the liquid inlet velocity distribution than that of the thick central tube nozzle, and the possible reasons were analyzed. The effect of liquid inlet velocity distributions on the primary breakup morphology was explored in detail. A criterion parameter was defined to assess the relative importance of liquid inlet velocity distributions on the liquid breakup shape of a selected atomizer. The combination of the present numerical scheme and the proper liquid inlet velocity distribution, which was determined by the dimensions of atomizers, was reliable and efficient for the numerical investigation of coaxial atomizer with various dimensions.

Pulsation in Bag Breakup of Round Liquid Jets in Crossflow

An experiment was conducted to investigate bag breakup process of round liquid jets in crossflow. The objective of this study is to research pulsation law. Specifically, this study measures the onset position of bag, the breakup position of bag, the breakup position of the jet. High-speed camera was used to observe the formation and breakup of bags. The diameter of the nozzle used in the experiment was 0.5mm, 0.8mm, 1mm. The test liquid was tap water. Wea number covers the range of 6~30, and liquid-to-air momentum flux ratio varied from 22 to 211. Present results indicate that in the direction perpendicular to the gas flow, the dimensionless pulsating amount of the onset point of bags (yonset/d) is linear to q, while the dimensionless pulsating amount of breakup point of bags (ybag/d) and the dimensionless pulsating amount of breakup point of the jet (yjet/d) is linear to ln (q). The dimensionless pulsating amount of these points in the direction of gas flow is irregular.

Application of Proper Orthogonal Decomposition to the morphological analysis of confined co-axial jets of immiscible liquids with comparable densities

The development of a round liquid jet under the influence of a confined coaxial flow of an immiscible liquid of comparable density (central to annular flow density ratio of 8:10) was investigated in the vicinity of the nozzle exit. Two flow regimes were considered; one where the annular flow is faster than the central jet, so the central liquid jet is accelerated and one where the annular flow is slower, so the central liquid jet is decelerated. The central jet was visualised by high speed photography. Three modes of jet development were identified and classified in terms of the Reynolds number, Re, of the central jet which was in the range of 525 < Re < 2725, a modified definition of the Weber number, We, which allows the distinction between accelerating and deceleration flows and was in the range of −22 < We < 67 and the annular to central Momentum Ratio, MR, of the two streams which was in the range of 3.6 < MR < 91. By processing the time resolved jet images using Proper Orthogonal Decomposition (POD), it was possible to reduce the description of jet morphology to a small number of spatial modes, which isolated the most significant morphologies of the jet development. In this way, the temporal and spatial characteristics of the instabilities on the interface were clearly identified which highlights the advantages of POD over direct observation of the images. Relationships between the flow parameters and the interfacial waves were established. The wavelength of the interfacial instability was found to depend on the velocity of the fastest moving stream, which is contrary to findings for fluids with large density differences.

Influence of atomizer exit area ratio on the breakup morphology of coaxial air and round water jets

The goal of this article is to study the effect of atomizer exit area ratio on atomizer performance. The experiments are performed on the round liquid jet breakup of seven coaxial air‐blast atomizers with water–air systems. The breakup morphology of liquid jet is observed first. The membrane‐type breakup can be divided into two subregimes called bag‐type breakup and membrane‐fiber breakup, and a correlation of characteristic length on bag‐type breakup regime is obtained. Then, we analyze the influence of atomizer exit area ratio on the breakup morphology of water‐air jets. To obtain reasonable atomization morphology criterions, the atomizer exit area ratio is used to modify the Weber number and momentum flux ratio per unit volume. This method is found to be able to explain different experimental results in the literature, which is also close to the results of round liquid jet in cross air flow and secondary atomization. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2335–2345, 2014

Effect of velocity profile of impinging jets on sheet characteristics formed by impingement of two round liquid jets

The characteristics of a liquid sheet formed by the impingement of two round liquid jets are analyzed theoretically. Since the velocity profile of the impinging jet greatly affects the sheet characteristics, the sheet characteristics are analyzed using round impinging jets with uniform or parabolic velocity profiles. The calculated sheet shape is compared with the results of theoretical analyses reported in previous studies. The effects of the velocity profile of impinging jets on the sheet characteristics are shown theoretically. Experiments are conducted in order to verify the theoretical analysis using short and long nozzles. The sheet contour is determined by two mechanisms, i.e., the force balance at the periphery between the liquid inertia and the surface tension, and the unstable wave growth. A critical condition in which the sheet disintegrates as a result of the unstable wave growth is newly proposed. The predicted sheet shapes agree somewhat with the results of experiments using short and long nozzles. The predicted sheet breakup lengths calculated using the newly proposed critical condition agree well with the results of these experiments. The predicted sheet velocity distributions exhibit slightly smaller values compared with measurements reported in previous studies.

Bag Breakup of Turbulent Liquid Jets in Crossflows

An experiment was conducted to investigate the bag breakup process of round liquid jets in crossflows. A highspeed camera was used to observe the formation and breakup of bags in a water jet from a 1.0 mm diameter nozzle with aWeber number in the range of 11 36 and the liquid-to-air momentum ratio from 11 to 95. An analysis of the force balance on bags was conducted and a criterion for bag formation is obtained. The diameter of the bag ring tube was predicted according to a reduced-order model, and the predicted results agree qualitatively well with the experiments. Present results indicate that the normalized basal ring diameter, ring tube diameter, and the bag onset time are all constants, while the onset length of bags normalized by the initial jet diameter varieswith the liquid-to-air momentumratio and jetReynolds number.The bagbreakup lengthdecreases linearlywith the increase of theWeber number, and is independent of the liquid jet velocity. The streamwise distance of the breakup location from the nozzle exit is a constant, while the cross-stream distance increases with the liquid-to-air momentum ratio.

Mathematical modeling of the heat and mass transfer in a stationary potato sphere impinged by a single round liquid jet in a hydrofluidization system

The hydrofluidization (HF) is a method of chilling and freezing of foods which consists in a circulating system that pumps a refrigerating liquid upwards through orifices or nozzles, creating agitating jets. The objectives of this work were to develop a mathematical model to represent the combined heat and mass transfer between a food and a refrigerant liquid medium in a HF system and to validate the model using a single stationary sphere of food impinged by a single round jet of liquid. The food domain consisted of a rigid solid matrix, a liquid phase and the ice phase. Transport equations were applied to each phase and solved by a control-volume approach. The transport phenomena in the fluid domain were studied by computational fluid dynamics. The surface heat transfer coefficients obtained from fluid flow simulations were used to model the heat and mass transfer inside the food. Experimental central temperature and average solute uptake profiles were obtained when potato spheres were placed in a HF system using a NaCl–water as refrigerant and considering different temperatures (−10 and −15°C), flow rates (1 and 3Lmin−1), and orifice-sphere distances (1 and 5cm). The predicted values agreed well with the experimental ones, the maximum root mean square errors being 3.3gNaClkg−1 potato and 2.9K for the average solute concentration and temperature profiles studied, respectively. The simulations improved the understanding of the HF process and it may help to study and control different operation scenarios.

Numerical investigations in Rayleigh breakup of round liquid jets with VOF methods

Simulations of the growth of a capillary instability and of the breakup of a jet were carried out using a one-fluid model to describe the two-phase flow motion and a VOF approach to capture the interface. The model considered each phase as fictitious sub-domains and accounted implicitly for jump conditions at the interface through a unique set of equations for which a source term of surface tensions appeared in momentum equations. The predominance of capillary effects in the breakup mechanism required to accurately describe the surface tension contribution. The Brackbill surface model was chosen because of its simplicity to represent tension forces, although it was known to generate parasitic currents susceptible to limit its precision. The flow incompressibility was ensured with an augmented Lagrangian method in case of sequential calculations and by a predictor/corrector approach for 3D simulations that required parallel computations. As a first step, the numerical methods were validated by simulating the growth of a capillary instability and comparing results to those predicted by the Rayleigh theory for capillary instabilities. The consistency of the Brackbill surface tension model and the accuracy of the methods were evaluated via a convergence study. As a second step, the simulation of a jet breakup was carried out using water as injected liquid and compressed carbon dioxide as surrounding medium. It was shown that the simulation predicted accurately the breakup length and the droplet size evidenced experimentally in literature.

Characterization of plunging liquid jets: A combined experimental and numerical investigation

This paper presents a combined experimental and numerical study of the flow characteristics of round vertical liquid jets plunging into a cylindrical liquid bath. The main objective of the experimental work consists in determining the plunging jet flow patterns, entrained air bubble sizes and the influence of the jet velocity and variations of jet falling lengths on the jet penetration depth. The instability of the jet influenced by the jet velocity and falling length is also probed. On the numerical side, two different approaches were used, namely the mixture model approach and interface-tracking approach using the level-set technique with the standard two-equation turbulence model. The numerical results are contrasted with the experimental data. Good agreements were found between experiments and the two modelling approaches on the jet penetration depth and entraining flow characteristics, with interface tracking rendering better predictions. However, visible differences are observed as to the jet instability, free surface deformation and subsequent air bubble entrainment, where interface tracking is seen to be more accurate. The CFD results support the notion that the jet with the higher flow rate thus more susceptible to surface instabilities, entrains more bubbles, reflecting in turn a smaller penetration depth as a result of momentum diffusion due to bubble concentration and generated fluctuations. The liquid average velocity field and air concentration under tank water surface were compared to existing semi-analytical correlations. Noticeable differences were revealed as to the maximum velocity at the jet centreline and associated bubble concentration. The mixture model predicts a higher velocity than the level-set and the theory at the early stage of jet penetration, due to a higher concentration of air that cannot rise to the surface and remain trapped around the jet head. The location of the maximum air content and the peak value of air holdup are also predicted differently.

ON SIMULATING PRIMARY ATOMIZATION USING THE REFINED LEVEL SET GRID METHOD

The atomization process of turbulent liquid jets is as of this day not well understood. Detailed numerical simulations can help study the fundamental mechanisms in regions where experimental access and analysis is difficult. This paper presents simulation results of the primary atomization of round turbulent liquid jets injected into stagnant high-pressure air under diesel engine conditions using the refined level set grid approach. A balanced force approach is used to accurately account for surface tension forces using an interface projected curvature method to minimize erroneous spurious currents. Broken off, small-scale nearly spherical drops are transferred into a Lagrangian point particle description allowing for full two-way coupling. The physical mechanisms resulting in the initial breakup of the jet are discussed. We analyze the impact of finite grid resolution on the phase interface geometry of the injected liquid core and discuss the impact of the automatic topology-change length scale inherent in the fixed grid interface, capturing methods like the level set method. Drop size distributions resulting from primary atomization are presented, showing that grid-independent drop sizes can be achieved for liquid structures resolved by at least six grid points.

Macroscopic analysis of a turbulent round liquid jet impinging on an air/water interface in a confined medium

In this paper, a submerged vertical water jet impinging on an air/water free surface is experimentally and numerically investigated. On the one hand, a two dimensions-two components particle image velocimetry technique is carried out to perform velocity measurements from the jet injection to the unsteady free surface. The motion of this interface is recorded by a biaxial shadowgraph imaging system. On the other hand, volume of fluid and large eddy simulation methods are coupled to compute the interaction between the jet dynamics and the interface behavior. Numerical and experimental results are confronted in both the single-phase jet area and the impinging region beneath the free surface. A statistical analysis of the interface motion is led through the maximum elevation point to highlight its organized time and spatial evolution in terms of a characteristic lifetime.

On the nonaxisymmetric instability of round liquid jets

A fully temporal linear stability analysis for round liquid jets is accomplished in this study. Three-dimensional disturbances are considered and a detailed parametric investigation is performed. The results indicate that only the nonaxisymmetric mode with azimuthal wavenumber m=1 may prevail over the axisymmetric mode and appear to dominate the maximum growth rate. The domain in which the nonaxisymmetric modes may start to grow is also clarified. A comparison with experimental observations in the literature is also presented. The results provide a complete understanding of the stability characteristics of round liquid jets.

Bag breakup of nonturbulent liquid jets in crossflow

An experimental investigation of the bag breakup of round nonturbulent liquid jets in gaseous crossflow at room temperature and pressure is described. Pulsed photography, pulsed shadowgraphy, and high-speed imaging were used to observe the column and surface waves along the liquid jet and the formation and breakup of bags. Measurements included: wavelengths of column and surface waves, jet velocities, the number of bags along the liquid jet, the number of nodes per bag, droplets sizes and velocities, and trajectories of droplets. Present results show that the column waves of a nonturbulent liquid jet in crossflow within bag breakup regime can be explained based on Rayleigh–Taylor instability. The number of nodes per bag affected the breakup mechanism of the bags. Three distinctive sizes of droplets were produced due the breakup of the bag membrane, the ring strings and the ring nodes. The size of the droplets resulting from the breakup of the bag membrane was constant independent of the crossflow Weber number. Finally different trajectories were observed for the three groups of droplets.

Primary Breakup of Turbulent Round Liquid Jets in Uniform Crossflows

An experimental investigation of the deformation and breakup properties of turbulent round liquid jets in uniform gaseous crossflows is described. Pulsed shadowgraph and holograph observations were obtained for turbulent round liquid jets injected normal to air crossflow in a shock tube. Crossflow velocities of the air behind the shock wave relative to the liquid jet were subsonic (36-90 m/s) and the air in this region was at normal temperature and pressure. Liquid injection was done by a pressure feed system through round tubes having inside diameters of 1 and 2 mm and length-to-diameter ratios greater than 100 to provide fully developed turbulent pipe flow at the jet exit. Test conditions were as follows: water and ethyl alcohol as test liquids, crossflow Weber numbers based on gas properties of 0-282, streamwise Weber numbers based on liquid properties of 1400-32,200, liquid/gas density ratios of 683 and 845, and jet exit Reynolds numbers based on liquid properties of 7100-48,200, all at conditions in which direct effects of liquid viscosity were small (Ohnesorge numbers were less than 0.12). Measurements were carried out to determine conditions required for the onset of breakup, ligament and drop sizes along the liquid surface, drop velocities after breakup, liquid column breakup as whole, rates of turbulent primary breakup, and liquid column trajectories. Phenomenological theories proved to be quite successful in interpreting and correlating the measurements.

A numerical study on the breakup process of laminar liquid jets into a gas

Surface phenomena of liquid jets into still air are numerically investigated. The liquid and air are treated as a single continuum with dynamically evolving interfaces captured by the level-set method. The jets considered are laminar: the jet exit Reynolds number ranges from 480 to 2300, with the Weber number of 3.1–28 000. The liquid/air density ratios are about 103. In comparison with an empirical correlation, the present simulations reasonably predict the breakup lengths of round liquid jets at low Weber numbers. The development of the surface wave is captured and the breakup mechanisms are discussed referring to a conventional classification chart. The breakup phenomenon at high Weber and Ohnesorge numbers is also analyzed. It is then found that large-scale longitudinal-vortex motions, which are initially generated inside the liquid core by relaxation of the axial liquid velocity profile and surface shear, are amplified by surface motions due to instability. They grow up and eventually overcome inertial and surface-tension forces leading to a sudden breakup of the liquid column.

Breakup of Round Nonturbulent Liquid Jets in Gaseous Crossflow

An experimental investigation of the primary breakup of nonturbulent round liquid jets in gas crossflow is described. Pulsed shadowgraph and holograph observations of jet primary breakup regimes, conditions for the onset of breakup, properties of waves observed along the liquid surface, drop sue and velocity properties resulting from breakup and conditions required for the breakup of the liquid column as a whole, were obtained for air crossflows at normal temperature and pressure. When combined with the earlier studies of Mazallon et al. (1999), the test range included crossflow Weber numbers of 02000, liquidgas momentum ratios of 100-8000, liquidgas density ratios of 683-1021, and Ohnesorge numbers of 0.003-0.12. The results suggest qualitative similarities between the primary breakup of nonturbulent round liquid jets in crossflows and the secondary breakup of drops subjected to shock wave disturbances (e.g., bag, multimode and shear breakup regimes are observed in both instances) with relatively little effect of the liquidgas momentum ratio on breakup properties over the present test range. Effects of liquid viscosity were also small for present observations where Ohnesorge numbers were less than 0.4. Phenomenological analyses were successful for helping to interpret and correlate the properties of primary breakup of round liquid jets in gas crossflows that were measured during the present investigation.

Atomization characteristics on the surface of a round liquid jet

Fundamental mechanisms of liquid jet breakup are identified and quantified. The quality of the atomization of liquids is an important parameter of many technological processes and is, e.g. for fuels and propellants critical in defining engine performance. This investigation takes a look at the jet behavior for a single injector element to determine the influence of the injection conditions on a round liquid jet. The study focuses on the atomization of a liquid forming a classical spray. To adjust the relative velocity between the liquid jet and the gaseous ambient a wind tunnel-like coaxial flow configuration was used. This made it possible to distinguish between effects of aerodynamic forces, chamber pressure and jet velocity, which determine the liquid Reynolds number and thereby the internal jet turbulence. Shadowgraphy and a novel image-processing approach was used to determine the jet surface characteristics: wavelength and amplitude. The absolute injection velocity of the jet seems to affect the structures the most with an increasing velocity causing the wavelengths to be smaller. An increase in chamber pressure seemed to have little influence on the jet with no relative velocity between the gas and liquid jet, but increased the amplitude and drop formation frequency at other testing conditions with relative motion. The wave amplitude trends provide information about the likelihood of drop formation but are limited in maximum size due to this breakup phenomenon of the jet. The study of the direction of the relative velocity demonstrated that injector performance cannot simply be described by scalar geometrical and operational injection parameters (e.g., We , Re or Oh), but has to include the injection direction of the atomizing fluids in relation to each other and to the ambient (e.g., combustion chamber). The undisturbed jet length and the spread angle were investigated, and a correlation for the droplet separation position was proposed. The data led to an extended classification of liquid jet breakup regimes. Large wave instabilities were experimentally analyzed and compared with linear stability theory.

Growth and structures of surface disturbances of a round liquid jet in a coaxial airflow

An experimental study is carried out to investigate the growth and the structures of surface disturbances in a round turbulent water jet with a coaxial airflow. The experimental conditions include a liquid jet Reynolds number of 2940, an airflow Reynolds number of 0-3680 and an aerodynamic Weber number of 0.02–52.7. The growth rate of surface disturbances and the time–space spectra between two different disturbances on the jet surface are obtained from the surface displacements measured using a high-speed video camera and an image analysis system. When the momentum of the airflow at the nozzle exit is superior to that of the water jet, asymmetric disturbances caused by the shear stress on the water jet surface develop significantly. As a result, the effect of the asymmetric disturbances on the jet breakup is comparable to that of the axi-symmetric disturbances.

Surface Properties During Primary Breakup of Turbulent Liquid Jets in Still Air

The formation of ligaments and drops at the liquid surface during primary breakup of turbulent liquid jets in still air, that is, during turbulent primary breakup, was studied experimentally using pulsed shadowgraphy and holography. Experimental conditions included round and plane (the latter actually being annular with a large aspect ratio) turbulent liquid jets in still air at normal temperature and pressure for noncavitating water and ethanol flows, long length/hydraulic-diameter (greater than 40:1) injector passages to provide fully developed turbulent pipe flow at the jet exit, jet exit Reynolds numbers of 6 x 10 3 -4.24 × 10 5 , jet exit Weber numbers of 200-300,000, and liquid/gas density ratios of 690 and 860 at conditions where direct effects of liquid viscosity were small (for example, jet exit Ohnesorge numbers were smaller than 0.0053). Measured properties included liquid surface velocities, conditions at the onset of ligament and drop formation, ligament and drop sizes along the surface, and ligament and drop velocities along the surface and rates of drop formation along the surface. Simplified phenomenological theories were used to help interpret and correlate the measurements

Life of a flapping liquid sheet

A round liquid jet with density ρ, surface tension σ and diameter D0 impacting a solid circular surface at normal incidence with velocity U0 takes the form of a radially expanding sheet whose thickness decreases with distance from the impact point. When the sheet develops in a still environment with density ρa = αρ, it destabilizes, provided the impacting Weber number We = ρU20D0/σ is larger than about 40α−1/2, as a result of a shear instability with the surrounding medium, in a sinuous, flag-like motion. We show how the instability properties set both the radial extent of the liquid sheet and the drop formation process at its rim. The shear instability gives the liquid a flag-like motion, ultimately triggering a Rayleigh–Taylor instability at the rim of the sheet which disintegrates, at the radial location R, into disjointed droplets of size d such thatR/D0 ∼ α−2/3We−1/3 and d/D0 ∼ α−2/3We−1.The features of the sheet instability, its radius and the droplet sizes are determined experimentally for a broad range of control parameters, using different liquids and ambient-medium densities.