Round Liquid Jets Research Articles

Liquid jet breakup in gaseous crossflow injected through a large diameter nozzle

Near nozzle field behavior of round liquid jets injected through large injector diameter (ranging from 0.2 to 0.6 m) into subsonic uniform gaseous crossflows is numerically investigated using the Volume of Fluid (VoF) method. The liquid jet to gas momentum ratio is ranging from 4.3 to 39 with a gas Weber number ranging from 1.6×104 to 5.3×104. The large scale liquid water column main properties, such as primary breakup process, liquid column penetration height, liquid column expansion and column breakup point location are studied. Effects of several parameters on the liquid column evolution are investigated, namely the diameter injector, the liquid jet injection velocity and the gaseous crossflow velocity. It has been found that the surface breakup and column breakup mechanisms are both contributing to the liquid column fragmentation. The fragmentation at large scale is characterized by the generation of a significant number of well developed arm and leg structures that align with the airflow on the side and at the bottom of the liquid column. Correlations for the main liquid jet properties are proposed and compared with results previously obtained for smaller size liquid jet. Liquid column penetration height, width and column breakup height follow the same type of power laws that those observed for liquid jet at smaller size. The liquid jet evolution is compared to the water jet generated by the B747 airtanker.

Primary atomization of liquid jets: Identification and investigation of droplets at the instant of their formation using direct numerical simulation

A detailed investigation of droplet characteristics for the primary atomization of a round liquid jet injected into stagnant gas is performed using direct numerical simulation. The material properties correspond to a Diesel injection, and the numerical framework is based on the volume-of-fluid method. The study is carried out in the statistically steady state, which allows to collect time-averaged droplet formation statistics. A novel algorithm continuously detects droplets at the instant they detach from the liquid core jet, which are referred to as primary droplets. The main droplet characteristics, such as volume, velocity and sphericity, are captured for newly formed primary droplets as well as all other droplets in the domain. This allows a detailed comparison between droplet characteristics during and after their formation. In addition, the droplet Weber, Reynolds and Stokes numbers are computed. These dimensionless numbers govern important physical processes occurring subsequent to the droplet formation, such as potential secondary breakup and droplet motion. One central goal is to provide information relevant for large eddy simulation of sprays in the Eulerian–Lagrangian framework, where the formation of small droplets from the liquid core jet cannot be resolved, and therefore potentially needs to be modeled. Moreover, the relative importance of primary and secondary atomization is estimated for this specific setup. It is found that the distribution of the droplet sizes is very similar for primary droplets and the entire droplet collective. Furthermore, the droplet Weber numbers are significantly below the critical Weber number required for secondary droplet breakup. Together, these two observations indicate that, for the investigated configuration, primary atomization is the dominating process, while secondary atomization only plays a minor role. Moreover, the high droplet volume fraction observed shows that droplet–droplet collisions and coalescence can be expected to influence the spray behavior. To put the obtained results into perspective, empirical correlations for the Sauter Mean Diameter are used to discuss potential trends for higher Reynolds and Weber numbers.

Dynamic mode decomposition of elliptical liquid jets in crossflow

The dynamics of round and elliptical liquid jets in subsonic crossflow was studied using high-speed imaging technique. The experiments were performed at constant gaseous Weber number and liquid–gas momentum flux ratios of 6.45 and 17.87, respectively, with orifices of different aspect ratios (AR) having an equivalent diameter of 0.43 mm. All of the cases were carried out inside an open-loop subsonic wind tunnel with a test section of 100 mm × 100 mm × 750 mm. For each case, dynamic modes were generated directly from the snapshots using a variant of the Arnoldi method known as the dynamic mode decomposition (DMD). The DMD results indicate that elliptical liquid jets have more small-scaled patterns with higher frequencies compared with the round liquid jets. As the first attempt to investigate the dynamics of elliptical liquid jets in crossflow, this study captures the dominant spatiotemporal structures. We also found that the orifice AR can significantly alter the jet wavelengths. The data from this study contribute to understanding the behavior of elliptical liquid jets exposed to gas crossflow in an enhanced capillary breakup regime.

Numerical investigation of conjugate heat transfer on a rotating disk under round liquid jet impingement

The two-phase flow and heat transfer characteristics of a free liquid jet impinging on a heated rotating disk are investigated by the CFD method. Numerical simulations are conducted over a range of rotational Reynolds numbers, jet Reynolds numbers, jet temperatures, nozzle diameter to disk spacing ratios, and disk to nozzle diameter ratios. The results show that the local Nusselt number increases first and then decreases along the radial direction, and it reaches the peak value in the core heat transfer zone close to the rotation center. The Nusselt number of the disk surface increases with the increase of rotational Reynolds number, jet Reynolds number, jet temperature, and nozzle diameter to disk spacing ratio, and the temperature distribution of the disk surface becomes more uniform. The increase of the disk to nozzle diameter ratio is not conducive to convective heat transfer. The temperature at the disk edge rises with the larger diameter ratio. Based on the numerical simulation results, a new correlation for the average Nusselt number is presented in terms of the dimensionless variables of jet Reynolds number, rotational Reynolds number, and Prandtl number in this study.

Disintegration co-flowing gas-liquid jet coupled with forced perturbation

In this study, numerical simulation is employed to diagnose the behavior of round liquid jet under forced perturbation with the low-velocity coaxial gas flow. A sinusoidal longitudinal perturbation with high amplitude and infinite frequency is imposed on the inlet of the liquid jet. An annular gas with lower velocity flows coaxially. The volume of fluid (VOF) approach is used to capture the liquid-gas interface. Various circumstances are considered such as properties of the liquid, the ratio of gas velocity to liquid velocity, and the properties of perturbation. Different cap formations, ligament breakup, and the droplet generation processes are depicted. The results indicate that the slower gas flow can affect the liquid jet breakup behavior and droplet generation. Also, liquid properties can considerably influence the cap formation and morphology of breakup. This investigation is validated by other numerical and experimental research in the basic conditions, while co-flowing gas-liquid jet considered in the present study is completely novel.

Simulation of Elliptical Liquid Jet Primary Breakup In Supersonic Crossflow

The study of elliptical liquid jets in supersonic flow in a Mach 2.0 is performed numerically. The primary breakup process of the elliptical liquid jet is simulated for a Weber number 223, liquid/gas flux momentum 4.0. The aspect ratios of elliptical geometries are set to be 0.25, 0.5, 1, 2, and 5. The results show a remarkable difference in liquid jet disintegration morphology at different aspect ratios. Under supersonic crossflow conditions, the elliptical liquid jet shows more breakup characteristics than the round liquid jet. As the aspect ratio grows, the penetration depth decreases. The elliptical liquid jet with AR=0.25 has the largest penetration depth in all cases. Moreover, the round jet has a maximum spreading angle of 50.2°. The changing trends of the column breakup length both x direction and y direction are similar. The elliptical jet at a lower aspect ratio has a shorter breakup length due to the narrower windward area. The liquid jet has a pair of larger horseshoe vortex structure and a wider wake region at a higher aspect ratio. Two pairs of reversal vortex pairs with obvious characteristics can be observed in all the simulations.

Effect of Nanoparticles on Cavitation in a Round Liquid Jet

Nanofluid has drawngreatattentionas a new efficientenergy carrier for improving heat-transfer and combustionperformance. However, universalconsensus has not been reached on the mechanism of nanoparticlesinfluencing atomization and combustion. In this paper, the focus was thecavitation of Al2O3-H2O nanofluids inside and outside a nozzle with circular cross section. Comprehensive analyses of flow morphology were carried out based on a highspeed-macro shadow imaging system. The effects of nanoparticles on the initial development of round jet, incipient cavitation and supercavitation inside sharp-edge nozzle, and jet cavitationoutside round-edge nozzle were discussed. Results showed that: nanoparticles accelerated the formation of cavitation inside the nozzle, and reduced the critical supercavitation pressure. Meanwhile, they promoted the formation and persistence of bubbles in liquid jet. These bubbles generated near the KH-instability wave peaks below the nozzle. Compared with that of basic fluid, the wavelocationof nanofluid jet was closer to the nozzle exit. In conclusion, the addition of nanoparticlespromoted the cavitation inside and outside the nozzle.Itcan be attributed tothe following changes: nanoparticles increased the original adsorption of air, reduced the tensile strength of liquid, speeded up the disturbances of jet, and played the role of heterogeneous nucleation points.

Prediction of liquid jet trajectory in supersonic crossflow and continuous liquid column model

The trajectory of the spray is studied theoretically and experimentally when a round liquid jet is injected into a supersonic crossflow vertically. A solid model of continuous liquid column is established in three-dimensional space. The cross-section deformation equation of the continuous liquid column along the injection direction is established using a method of micro-element analysis. The stress analysis of cross section is simplified into a two-dimensional droplet. The shape of the cross section is considered to continuously change from circular to elliptical shape. And the bow shock wave in front of the jet column is simplified into an oblique shock wave with a known shock angle. Based on this, the calculation of aerodynamic force is greatly simplified. A dimensionless parameter named effective deformation time of liquid column (the logogram is <inline-formula><tex-math id="M300">\begin{document}${t_{\rm valid}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200903_M300.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20200903_M300.png"/></alternatives></inline-formula>) is defined and used to judge the end point of the liquid column quantitatively. The liquid jet trajectory and cross-section deformation can be calculated using MATLAB software. The instantaneous images of continuous liquid columns in supersonic crossflow are captured using high-spatial-resolution microscopic imaging methods. The microscopic imaging system is composed of a double pulse solid-state laser, computer, CCD camera, synchronous controller, microscope lens and laser diffuser. After passing through the laser diffuser, a plane background light with uniform distribution is formed on the scattering plate. The mean filtering method is used to filter the original image. After filtering, the range of gray distribution in the background area is obviously reduced. The distribution of gray value is more concentrated, and the background of the image is more uniform. Then the image edge detection function is used to obtain the near-field jet trajectory. The parameter variables studied include liquid injection pressure drop (1–2 MPa), liquid nozzle diameter (0.5 mm/1.0 mm), and liquid gas momentum ratio (3.32–7.27). The results show that the continuous liquid column model can better predict the jet trajectory on the center plane and the shape of the liquid column in three-dimensional space. It is indicated that the predictive result matches well with the experimental result. This study is of great significance for establishing the solid-particle coupling model of liquid jet in supersonic crossflows.

MESH RESOLUTION EFFECTS ON PRIMARY ATOMIZATION SIMULATIONS

In this work, we use direct numerical simulation through a volume of fluid solver with adaptivemesh refinement to analyze the atomization of a pulsating round liquid jet with a narrow length-scale range configuration. We propose three grid sizes based on characteristic scales we estimate from the deformation and fragmentation processes of this problem. We compute drop statistics and general spray features for each case. We found that mesh resolution affects the atomization rate and the probability density function of droplet size and position, not only for the underresolved drops but for all liquid structures. The two simulations with higher grid refinement presented volume-weighted distribution with minor differences. Therefore, we propose assessing the accuracy of atomization simulations based on the volume fraction of underresolved structures.

Experimental Breakup Characteristics of Round Liquid Jets of a Dilute Polymer Solution into Quiescent Air

Experimental Breakup Characteristics of Round Liquid Jets of a Dilute Polymer Solution into Quiescent Air

Vorticity dynamics in a spatially developing liquid jet inside a co-flowing gas

A three-dimensional transient round liquid jet within a low-speed coaxial outer gas flow is numerically simulated and analysed via vortex dynamics ($\unicode[STIX]{x1D706}_{2}$ analysis). Two types of surface deformations are distinguished, which are separated by a large indentation on the jet stem. First, there are those inside the recirculation zone behind the leading cap – directly affecting the cap dynamics and well explained by the local vortices. Second, deformations upstream of the cap are mainly driven by the Kelvin–Helmholtz (KH) instability, unaffected by the vortices in the behind-the-cap region (BCR), and are important in the eventual atomization process. Different atomization mechanisms are identified and are delineated on a gas Weber number ($We_{g}$) versus liquid Reynolds number ($Re_{l}$) map based on the relative gas–liquid velocity. In a frame moving with the liquid velocity, this result is consistent with prior temporal studies. A simpler and clearer portrait of similarity of the atomization domains is shown by using the relative gas–liquid axial velocity, i.e. $We_{r}$ and $Re_{r}$, and avoiding the widely used velocity ratio as a third key parameter. A detailed comparison of vorticity along the axis in an Eulerian frame versus a frame fixed to a surface wave reveals that the vortex development and surface deformations are periodic in the upstream region, but this periodicity is lost closer to the BCR. In the practical range of the density ratio and for early times in the process, axial vorticity is mainly generated by baroclinicity while streamwise vortex stretching becomes more important at later times and only at lower relative velocities when pressure gradients are reduced. The inertia, vortex, pressure, viscous and surface tension forces are analysed to delineate the dominant causes of the three-dimensional instability of the axisymmetric KH structure due to surface acceleration in the axial, radial and azimuthal directions. The inertia force related to the axial gradient of kinetic energy is the main cause of the axial acceleration of the waves, while the azimuthal acceleration is mainly caused by the pressure and viscous forces. The viscous forces are negligible in the radial direction and away from the nozzle exit in the axial direction. It is interesting to note that azimuthal viscous forces are important even at high $Re_{l}$, indicating that inertia is not totally dominant in this instability occurring early in the atomization cascade.

Shear instability of an axisymmetric air–water coaxial jet

We study the destabilization of a round liquid jet by a fast annular gas stream. We measure the frequency of the shear instability waves for several geometries and air/water velocities. We then carry out a linear stability analysis, and show that there are three competing mechanisms for the destabilization: a convective instability, an absolute instability driven by surface tension and an absolute instability driven by confinement. We compare the predictions of this analysis with experimental results, and propose scaling laws for wave frequency in each regime. We finally introduce criteria to predict the boundaries between these three regimes.

Planar liquid jet: Early deformation and atomization cascades

The temporal evolution of three-dimensional instabilities on a planar liquid sheet segment is studied using direct numerical simulation, and the level-set and volume-of-fluid methods for the liquid-gas interface tracking. Three atomization cascades are distinguished at early breakup, which are well categorized on a gas Weber number (Weg) versus liquid Reynolds number (Rel) map. These atomization processes include lobe stretching that occurs at low Rel and low Weg, hole and bridge formation that occurs at moderate Rel and high Weg, and lobe corrugation occurring at high Rel and low Weg. Qualitative comparison between the sizes of the ligaments and droplets that result from each process is presented. A transitional region between the prescribed atomization domains is found. At high Rel, the transitional boundary is a constant Ohnesorge line defined based on gas We and liquid Re (Ohm≡Weg∕Rel). At low Rel, the transitional region follows a hyperbolic line on the Weg–Rel plot. These atomization processes are qualitatively independent of the jet geometry—seen in both planar and round liquid jets. At a constant density ratio, characteristic times for the hole formation and for the lobe and ligament stretching are different—the former depending on surface tension and the latter on liquid viscosity. In the transitional region, both characteristic times are of the same order.

Experimental and numerical investigation of liquid jet impingement on superhydrophobic and hydrophobic convex surfaces

Experiments and numerical simulations were carried out to examine the vertical impingement a round liquid jet on the edges of horizontal convex surfaces that were either superhydrophobic or hydrophobic. The experiments examine the effects on the flow behaviour of curvature, wettability, inertia of the jet, and the impingement rate. Three copper pipes with outer diameters of 15, 22, and 35 mm were investigated. The pipes were wrapped with a piece of a Brassica oleracea leaf or a smooth Teflon sheet, which have apparent contact angles of 160° and 113°. The Reynolds number ranged from 1000 to 4500, and the impingement rates of the liquid jets were varied. Numerical results show good agreement with the experimental results for explaining flow and provide detailed information about the impingement on the surfaces. The liquid jet reflected off the superhydrophobic surfaces for all conditions. However, the jet reflected or deflected off the hydrophobic surface, depending on the inertia of the jet, the curvature of the surface, and the impingement rate. The results suggest that pressure is not the main reason for the bending of the jet around the curved hydrophobic surface.

Initial instability of round liquid jet at subcritical and supercritical environments

In the present experimental work, the behavior of laminar liquid jet in its own vapor as well as supercritical fluid environment is conducted. Also the study of liquid jet injection into nitrogen (N2) environment is carried out at supercritical conditions. It is expected that the injected liquid jet would undergo thermodynamic transition to the chamber condition and this would alter the behavior of the injected jet. Moreover at such conditions there is a strong dependence between thermodynamic and fluid dynamic processes. Thus the thermodynamic transition has its effect on the initial instability as well as the breakup nature of the injected liquid jet. In the present study, the interfacial disturbance wavelength, breakup characteristics, and mixing behavior are analysed for the fluoroketone liquid jet that is injected into N2 environment as well as into its own vapor at subcritical to supercritical conditions. It is observed that at subcritical chamber conditions, the injected liquid jet exhibits classical liquid jet characteristics with Rayleigh breakup at lower Weber number and Taylor breakup at higher Weber number for both N2 and its own environment. At supercritical chamber conditions with its own environment, the injected liquid jet undergoes sudden thermodynamic transition to chamber conditions and single phase mixing characteristics is observed. However, the supercritical chamber conditions with N2 as ambient fluid does not have significant effect on the thermodynamic transition of the injected liquid jet.

Early spray development at high gas density: hole, ligament and bridge formations

Three-dimensional temporal instabilities, leading to spray formation of a round liquid jet segment with an outer, coaxial high-density gas flow, are studied with Navier–Stokes and level-set computations. These computations predict the liquid surface shape showing the smaller structures on the conical wave crests, i.e. lobes, holes, bridges and ligaments, which are the precursors to droplet and spray formations. These structures and their time scales affect droplet size and velocity distributions as well as spray cone angles. The gas-to-liquid density ratio, liquid Reynolds number ($Re$) and liquid Weber number ($We$) range between 0.02–0.9, 320–16 000 and 2000–230 000, respectively, which cover three distinct physical domains. (1) At higher $Re$ and $We$, ligaments and then drops develop following hole and liquid bridge formations. (2) At higher gas densities throughout the $Re$ range, several holes merge forming two bridges per lobe before breaking to form ligaments; this hole merging is explained by slower development of hairpin vortices and lobe shape. (3) In cases where both gas density and $Re$ or $We$ are lower, the well-ordered lobes are replaced by more irregular, smaller-scale corrugations along the conical wave crest edge; ligaments form differently by stretching from the lobes before holes form. Thicker ligaments and larger droplets form in the low $Re$, low gas density range. The surface wave dynamics, vortex dynamics and their interactions are explained. Understandings of liquid stream break up and concurrent smaller structure formation are built upon an examination of both translation and rotation of the fluid. In all cases, hole formation is correlated with hairpin and helical vortices; fluid motion through a perforation in the thin sheet near the wave crest corresponds to these vortices. The hole formation process is dominated by inertial forces rather than capillary action, which differs from mechanisms suggested previously for other configurations. Circulation due to streamwise vorticity increases while the lobes thin and holes form. For larger surface tension, cavities in the jet core rather than perforations in a sheet occur. The more rapid radial extension of the two-phase mixture with increasing gas density is explained by greater circulation in the ring (i.e. wave crest) region. Experimental descriptions of the smaller structures are available only at lower $Re$ and lower density, agreeing with the computations. Computed scales of bridges, ligaments, early droplets and emerging spray radii agree qualitatively with experimental evidence through the high $Re$ and $We$ domains.

Study on transient structure characteristics of round liquid jet in supersonic crossflows

Study on transient structure characteristics of round liquid jet in supersonic crossflows

Unsteady oscillation distribution model of liquid jet in supersonic crossflows

Unsteady distribution of spray is experimentally studied when a round liquid jet is injected into a supersonic crossflow vertically. An oscillation distribution model for the liquid column and spray is established. Tyndall scattering caused by the sol medium is put forward to eliminate the interference effect of monochromatic laser passing through the supersonic gas flow field. The scattering causes the disordering of laser propagation direction and phase, thus makes the planar light source uniform and eliminate the interference effect of laser at the same time. Then a uniform light source is formed and can be set as the uniform background with a pulse width of 7 ns. The camera, with dimension of CCD pixel space of 40002672 pixel, is located directly in front of planar light source, and the shooting area is between both. The frozen liquid jet/spray images with high spatiotemporal resolution are captured using the pulsed laser background imaging (PLBI) method in supersonic crossflows. And the drag phenomenon caused by the too-long exposure time in the ordinary and traditional high-speed imaging process is avoided. Based on the maximizing inter-class variance method (Otsu) and Canny method, the out boundary of liquid jet/spray are extracted from an instantaneous image. A dimensionless parameter named intermittency factor (the logogram is r) is defined and used to quantitatively analyze the oscillation distribution characteristics of jet/spray. The intermittency factor of the whole spray field could be calculated by sample probability statistic method. An empirical jet/spray oscillation distribution model, in supersonic crossflows, is summarized based on parameter studies. Various conditions are studied, including stagnation pressure range of gas (642 kPa to 1010 kPa), practical pressure range (0.36 MPa to 4.61 MPa), nozzle diameters (0.48 mm/1.0 mm/1.25 mm/1.52 mm), distances down from nozzle (10 mm to 125 mm), and jet-gas momentum flux ratio range (0.11 to 7.49). The empirical model is used to predict the oscillation distribution of water jet penetrated in a Ma2.1 supersonic crossflow. It is indicated that the predictive result matches well with the experimental result. It could be concluded that the PLBI method presented in this paper reasonably utilizes the high energy and short pulse characteristics of the laser to successfully complete the frozen image of liquid jet/spray under the condition of supersonic crossflow. The dimensionless parameter r defined in the study can be used to quantitatively analyze the oscillation distribution characteristics of jet/spray well. This study has important significance for understanding the diffusion characteristics of liquid jet in supersonic crossflows.

Circular hydraulic jump on finite surfaces with capillary limit

A round liquid jet impinging on a circular disc with free edge generates a thin liquid film in the center surrounded by a thick film. For low flow rates, the thick film is bounded by a stable rim around the disc edge and flows off only from one spot at the edge. The outer Froude number remains constant for varied flow rates and disc sizes but changes with the surface tension of the fluid. The jump radius increases linearly with the flow rate, and the linear slope varies with the surface tension. Due to the stable-rim edge condition, the constant outer Froude numbers observed in our study are different from the constant value reported by Duchesne et al. [“Constant Froude number in a circular hydraulic jump and its implication on the jump radius selection,” Europhys. Lett. 107, 54002 (2014)]. Despite the constant outer Froude numbers being independent of the flow rate, the inner Froude number changes with the flow rate, which is due to the surface tension force at the jump location. Force analysis is conducted by taking into account the stable rim, and the derived equations provide the relationship of jump radius with the contact angle of the stable rim, disc size, and jet flow. The maximum outer Froude number and the minimum inner Froude number are theoretically analyzed. Depending on the pre-jump velocity profile and the surface tension force at the jump, the maximum outer Froude number could be larger than unity. The shape of free surface at the jump is analyzed to evaluate the theoretical assumption of steep jump.

On the rupture of a round liquid jet: the primary atomization process

The present study analyzes the process of fragmentation of a liquid volume into a cloud of droplets, a well-known phenomenon usually denoted as primary atomization. The experimental work was performed in a single-orifice pressure-swirl atomizer. For the first time, a transition was observed in the droplet size distribution function (DSDF) that transforms from a Nukiyama–Tanasawa type to a power law dependence described by the function d −x . A non-dimensional group including atomizing pressure was identified as the controlling parameter in the transition, as in other similar fracture processes, for example in solid materials. The value obtained for the critical exponent of a power law function that best fitted the DSDF for the maximum atomizing pressure is a quantitative evidence of the three-dimensional characteristics of this process.