Breakup Region Research Articles

Numerical analysis of jet breakup based on a modified compressible two-fluid-LES model

Jet breakup with a conical nozzle was studied including in-nozzle flow based on a modified compressible two-fluid model coupled with the large eddy simulation model in the OpenFOAM framework. In order to capture the interface, the phase equation and the pressure equation were modified by adding the quasi-VOF model and surface tension respectively. The effects of temperature and pressure on the air thermal properties were taken into account and the density of diesel was fitted linearly with respect to the pressure according to experimental data. In addition, the Ranz-Marshall model was applied to consider the interphase heat transfer. The modified compressible two-fluid model was verified by comparing the calculated mass flow rates, discharge coefficients, spray momentum fluxes and effective jet velocities under different injection conditions with the available experimental results. It was demonstrated that the numerical simulation results agreed well with the experimental measurements with minor discrepancies which were caused by the numerical assumptions and the experimental uncertainty. Then the mechanism of jet breakup and the effects of the ambient temperature on jet breakup were investigated. It is found that jet breakup is divided into two regions, i.e. the intact region and the jet breakup region. The intact region is dominated by the spatial instability, and the jet breakup region is dominated by the combination of the temporal instability and the spatial instability. It's worth mentioning that the surface tension plays a dual role in inhibiting atomization of droplets and promoting the pinch-off of diesel column and ligament.

Post breakup dynamical evolution of fragments produced in nuclear multifragmentation

The deexcitation of the primary hot fragments, produced in the breakup of an excited nuclear source, during their propagation under the influence of their mutual Coulomb repulsion is studied in the framework of a recently developed hybrid model. The latter is based on the Statistical Multifragmentation Model (SMM), describing the prompt breakup of the source, whereas the particle emission from the hot fragments, that decay while traveling away from each other, is treated by the Weisskopf-Ewing evaporation model. Since this treatment provides an event by event description of the process, in which the classical trajectories of the fragments are followed using molecular dynamics techniques, it allows one to study observables such as two-particle correlations and infer the extent to which the corresponding observables may provide information on the multifragment production mechanisms. Our results suggest that the framework on which these treatments are based may be considerably constrained by such analyses. They imply that information obtained from these model calculations may provide feedback to the theory of nuclear interferometry. In particular, our results do not support the hypothesis that the total momentum of the selected pair of particles used in the two-particle correlations remains constant during the post-breakup dynamics. Furthermore, the correlations may be appreciably affected by this fact. We also found that neutron deficient fragments should hold information more closely related to the breakup region than neutron rich ones, as they are produced in much earlier stages of the post breakup dynamics than the latter.

Water drop impact on a semi-cylindrical convex hot surface for a diameter ratio of unity

When a liquid drop impacts on a hot non-flat surface the curvature of the surface and its geometrical characteristics transmute the physical regimes and their boundary compared to a flat surface impact. The present experimental study is focused on water drop impingement on a mimetic solid semi-cylindrical convex hot surface, with a size equal to the drop. The thermal versus inertia map of generated regimes is obtained, while some well-known regimes associated with a flat surface are not observed for the present non-flat impacts. These include rebounding of the main drop and atomization which are common observed phenomena when the hot surface is flat. The maximum spreading of the droplet is measured at different surface temperatures and impact Weber numbers. In directrix direction it is independent of temperature at both partial contacted and breakup regions but reduces upon transitioning at high Weber numbers. The maximum spreading scaling with the impact Weber number is obtained along both directrix and generatrix directions. Finally, since the spreading is asymmetric, its aspect ratio is obtained and has shown a considerable rise at partial contacted high Weber numbers.

Revisiting and improving models for the breakup of compact dry powder agglomerates in turbulent flows within Eulerian–Lagrangian simulations

The present study is concerned with breakup models for agglomerates in turbulent flows. First, a brief literature review is provided describing the state of the art concerning the description and modeling of breakup of agglomerates with special emphasis on the role of fluid forces. That comprises turbulent and drag (inertia) stresses. Furthermore, the breakup by rotary stresses is taken into account. Based on the idea to describe these processes using first principles within an Eulerian–Lagrangian approach relying on the large-eddy simulation methodology, modeling ideas from the literature are revisited and extended in order to be applied in a four-way coupled simulation. Building on a deterministic collision and agglomeration model, the breakup process is tackled in a similar manner leading to a highly efficient procedure, which allows to track a huge number of agglomerates and particles but with the restriction to compact dry powder agglomerates. For this kind of agglomerates an enhanced evaluation of the strength is derived relying on an improved determination of the average porosities and coordination number. Furthermore, special emphasis is put on the post-breakup treatment, i.e., models for the sizes and velocities of the generated fragments. Finally, the proposed models are applied to a test case inspired by the experimental study of Weiler [1] for the deagglomeration in a dry powder disperser. The effect of the different physical mechanisms (turbulence, drag, rotation) is analyzed concerning breakup rates and resulting size distributions. Furthermore, the dominant breakup regions are identified separately. Since agglomeration processes are simultaneously taken into account, re-agglomeration of the fragments is found to play a non negligible role. Finally, the effect of the properties of the powder on the breakup and re-agglomeration processes is investigated.

Multi-pulse shadowgraphic RGB illumination and detection for flow tracking

This work demonstrates the application of a multi-color LED and a consumer color camera for visualizing phase boundaries in two-phase flows, in particular for particle tracking velocimetry. The LED emits a sequence of short light pulses, red, green, then blue (RGB), and through its color-filter array, the camera captures all three pulses on a single RGB frame. In a backlit configuration, liquid droplets appear as shadows in each color channel. Color reversal and color cross-talk correction yield a series of three frozen-flow images that can be used for further analysis, e.g., determining the droplet velocity by particle tracking. Three example flows are presented, solid particles suspended in water, the penetrating front of a gasoline direct-injection spray, and the liquid break-up region of an “air-assisted” nozzle. Because of the shadowgraphic arrangement, long path lengths through scattering media lower image contrast, while visualization of phase boundaries with high resolution is a strength of this method. Apart from a pulse-and-delay generator, the overall system cost is very low.

Characterization of impinging jet sprays of gelled propellants loaded with nanoparticles in the impact wave regime

This study presents impact wave and spray properties for sheet breakup of rheologically complex gelled propellants using high-speed visualization and image processing. Two converging high-aspect ratio nozzles formed sheets for pressure drops in the range 414–828 kPa. Metalized gelled propellants are simulated using 0–20 wt% alumina nanoparticles suspended in a gelled rocket propellant. Spatio-temporal analysis of the impact waves formed on the sheet surface is presented for the first time and wave properties are compared against other liquids undergoing sheet breakup. Impact waves propagate in primary breakup region with constant velocity much less than jets. Unlike in Newtonian liquids, wave propagation velocity smaller than jet velocity delays the breakup of gelled propellants. Breakup length of the gelled propellant sheet varied with Weber number in a similar manner to impinging water jets. However, linear instability theories describing sheet breakup of Newtonian fluids could not describe the relationship between breakup length and Weber number. Bulk jets of 0.001 m diameter broke up into droplets of Sauter mean diameters 200–400 µm. Smaller size in this range is observed in the sheet periphery and near to the impingement region. Larger size occurs in the middle regions of sheet towards downstream. Externally impinging jets must overcome the rheological resistance of gelled propellants by imparting higher kinetic energy to the sheet. This invariably results in higher velocities of the propellant jets for a given orifice diameter. Thus, a smaller injector parameter is obtained and results of this study suggest that this operational limitation could induce combustion instability.

Effects of gas liquid ratio on the atomization characteristics of gas-liquid swirl coaxial injectors

To understand the atomization characteristics and atomization mechanism of the gas-liquid swirl coaxial (GLSC) injector, a back-lighting photography technique has been employed to capture the instantaneous spray images with a high speed camera. The diameter and velocity of the droplets in the spray have been characterized with a Dantec Phase Doppler Anemometry (PDA) system. The effects of gas liquid ratio (GLR) on the spray pattern, Sauter mean diameter (SMD), diameter-velocity distribution and mass flow rate distribution were analyzed and discussed. The results show that the atomization of the GLSC injector is dominated by the film breakup when the GLR is small, and violent gas-liquid interaction when the GLR is large enough. The film breakup dominated spray can be divided into gas acceleration region and film breakup region while the violent gas-liquid interaction dominated spray can be divided into the gas acceleration region, violent gas-liquid interaction region and big droplets breakup region. The atomization characteristics of the GLSC injector is significantly influenced by the GLR. From the point of atomization performance, the increase of GLR has positive effects. It decreases the global Sauter mean diameter (GSMD) and varies the SMD distribution from a hollow cone shape (GLR = 0) to an inverted V shape, and finally slanted N shape. However, from the point of spatial distribution, the increase of GLR has negative effects, because the mass flow rate distribution becomes more nonuniform.

Time–frequency analysis of submerged synthetic jet

The coherent structures transport the finite body of fluid mass through rolling which plays an important role in heat transfer, boundary layer control, mixing, cooling, propulsion and other engineering applications. A synthetic jet in the form of a train of vortex rings having coherent structures of different length scales is expected to be useful in these applications. The propagation and sustainability of these coherent structures (vortex rings) in downstream direction characterize the performance of synthetic jet. In the present study, the velocity signal acquired using the S-type hot-film probe along the synthetic jet centerline has been taken for the spectral analysis. One circular and three rectangular orifices of aspect ratio 1, 2 and 4 actuating at 1, 6 and 18 Hz frequency have been used for creating different synthetic jets. The laser induced fluorescence images are used to study the flow structures qualitatively and help in explaining the velocity signal for detection of coherent structures. The study depicts four regions as vortex rollup and suction region (X/Dh ≤ 3), steadily translating region (X/Dh ≤ 3–8), vortex breakup region (X/Dh ≤ 4–8) and dissipation of small-scale vortices (X/Dh ≤ 8–15). The presence of coherent structures localized in physical and temporal domain is analyzed for the characterization of synthetic jet. Due to pulsatile nature of synthetic jet, analysis of velocity time trace or signal in time, frequency and combined time–frequency domain assist in characterizing the signatures of coherent structures. It has been observed that the maximum energy is in the first harmonic of actuation frequency, which decreases slowly in downstream direction at 6 Hz compared to 1 and 18 Hz of actuation.

A method for quantifying recurrent patterns of local wavefront direction during atrial fibrillation

IntroductionSpiral wave reentry is a potential mechanism of atrial fibrillation (AF), but is difficult to differentiate clinically from multiple wavelet breakup using standard bipolar recordings. We developed a new methodology using bipolar recordings to estimate the direction of local activation wavefronts during AF by calculating the electrogram conformation (Egm-C). We subsequently used recurrence quantification analysis (RQA) of Egm-C to differentiate regions of spiral wave reentry from wavelet breakup. MethodsA 2D computer simulation was created with regions containing a stable spiral wave and also regions of wavebreak. A grid of 40 × 40 unipolar electrodes was superimposed. At each site, the actual wavefront direction (WD) was determined by comparing relative activation timings of the local intracellular recordings, and the estimated wavefront direction (Egm-C) was determined from the morphology of the local bipolar electrogram. RQA of Egm-C was compared to RQA of actual WD in order to differentiate AF mechanisms. ResultsRQA of actual WD and Egm-C both distinguished regions of spiral wave reentry from wavelet breakup with high correlation between the two methods (recurrence rate, r = 0.96; determinism, r = 0.61; line max, r = 0.95; entropy, r = 0.84; p < 0.001 for all). In areas of stable spiral wave reentry, the recurrence plots of both Egm-C and actual WD demonstrated stable, periodic dynamics, while regions of wavelet breakup demonstrated chaotic behavior largely devoid of repetitive activation patterns. ConclusionCalculation of Egm-C allows RQA to be performed on bipolar electrograms during AF and differentiates regions of spiral wave reentry from multiple wavelet breakup.

Experimental and numerical investigation of the primary breakup of an airblasted liquid sheet

The primary breakup of airblast atomization is governed by complex mechanisms and is still not well understood. In recent years high speed shadowgraphy experiments and Direct Numerical Simulations of prefilming airblast atomization have been performed independently. In this paper detailed results of a combined experimental and numerical study are presented. A single operating point of a planar prefilming airblast atomizer is investigated, based on a spatial resolution of 10 µm and a consistent analysis of the liquid film in both the experimental and the numerical studies. For the analysis the three-dimensional DNS data is projected on a plane, corresponding to the data obtained by shadowgraphy. The experiment is characterized by back light illumination in conjunction with particle and ligament tracking velocimetry. A Depth of Field correction is applied to further improve the measurement accuracy. For the numerical investigation the embedded DNS approach is utilized: The primary breakup region is simulated with a highly resolved DNS, embedded in a coarser Large Eddy Simulation. The comparison comprises a phenomenological discussion of the disintegration process and quantitative results. Distributions for the breakup length, the liquid film deformation velocity, the droplet sizes and velocities are presented. The results are in good agreement and confirm the applicability of the embedded DNS and the particle and ligament tracking velocimetry for the analysis of the primary breakup of airblast atomization. This work also shows the intrinsic limitation of a diffusive interface technique as the results depend on the filtering parameter of the diffuse interface.

Features of Capillary Breakup of a Liquid Jet at Ohnesorge Numbers Larger Than Unity

A theoretical study has been made of the forced capillary breakup of a jet of viscous liquid at a value of the Ohnesorge number larger than unity. The regions of breakup of the jet without the formation of satellites have been determined. The dependences of the dimensions of main and satellite droplets on the wave number have been obtained for different Ohnesorge numbers.

Breakup of free liquid jets influenced by external mechanical vibrations

The breakup of liquid jets has been studied with various test liquids using externally imposed mechanical vibrations. Images of the jets were captured by a high speed camera up to the speed of 1000 frames per second, and analyzed to obtain the profile of the jet and breakup length. The dynamics of the jets have also been studied to understand the effects of additives—a surfactant and polymer—incorporating externally imposed mechanical vibrations. Different types of breakup modes have been explored with respect to the Weber number and Ohnesorge number. The introduction of mechanical vibrations have caused jet breakup with separated droplets at a comparatively lower Weber number. The region of jet breakup by neck formation at constant jet velocities also contracted due to mechanical vibrations.

Numerical description of a pressure-swirl nozzle spray

Abstract Pressure-swirl nozzles are widely used in both industrial and experimental processes. However, the spray flow fields are difficult to predict due to the complexities of the dense break-up region. Herein, an inflow boundary condition approach is employed whereby droplets are computationally injected at 9 mm downstream of the orifice based on experimentally derived descriptions of the mean droplet and gas velocities, combined with empirical relations for turbulent kinetic energy and dissipation. The gas flow is then predicted with a Reynolds-Averaged Navier-Stokes solution while the particle trajectories are computed with a discrete random walk model. This approach was found to reasonably describe downstream spatial distribution of gas and droplet velocity, with moderate mesh resolution and CPU time. In addition, the evolution of the droplet velocity fluctuations was found to be consistent with turbulent diffusion theory based on the local Stokes numbers and turbulent kinetic energy.

Coherent Structures in the Transition Process of a Laminar Separation Bubble

Coherent structures formed within the laminar separation bubble on the suction side of a NACA 0018 airfoil at a chord Reynolds number of 100,000 and an angle of attack of 5 deg are investigated experimentally using a combination of surface pressure measurements; time-resolved flow visualization; and time-resolved planar two-component particle image velocimetry. The results show that strongly periodic shear-layer vortices form in the separation bubble due to the amplification of disturbances in the fore portion of the bubble. These structures feature strong spanwise coherence at rollup; however, they deform rapidly upstream of the mean reattachment location. Spanwise undulations in the vortex filaments develop in a nonperiodic fashion, with the spanwise wavelength shown to be approximately two times the streamwise spacing of the shed structures. It is demonstrated that these spanwise deformations lead to regions of local vortex breakup, which expand rapidly as the vortices approach the mean reattachment point. This is associated with a rapid decay of the spanwise coherence length, which reaches a constant minimum value typical of turbulent boundary layers just downstream of mean reattachment. The results demonstrate that the development of shear-layer vortices plays the fundamental role in the formation of the separation bubble, and the later stages of transition are directly related to the breakup of these structures in the aft portion of the separation bubble.

Underwater oil jet: Hydrodynamics and droplet size distribution

We conducted a large scale experiment of underwater oil release of 6.3L/s through a 25.4mm (one inch) horizontal pipe. Detailed measurements of plume trajectory, velocity, oil droplet size distribution, and oil holdup were obtained. The obtained experimental data were used for the validation of the models JETLAG and VDROP-J. Key findings include: (1) formation of two plumes, one due to momentum and subsequently plume buoyancy, and another due mostly to the buoyancy of individual oil droplets that separate upward from the first plume; (2) modeling results indicated that the traditional miscible plume models matched the momentum and buoyancy plume, but were not able to simulate the upward motion plume induced by individual oil droplets; (3) high resolution images in the jet primary breakup region showed the formation of ligaments and drops in a process known as “primary breakup”. These threads re-entered the plume to re-break in a process known as “secondary breakup”; (4) the plume velocity was highly heterogeneous with regions of high velocity surrounded by stagnant regions for various durations. The results from this study revealed that the primary breakup is a key factor for quantifying the droplet size distribution which plays a crucial role in determining the ultimate fate and transport of the released oil in the marine environment. The observed spatial heterogeneity in the oil plume implies that the effectiveness of applied dispersants may vary greatly when applying directly in the discharged oil flow.

Gas–Liquid Flow Dispersion in Micro-Orifices and Bubble Coalescence With High Flow Rates

The flow of microbubbles in millichannels with typical dimensions in the range of few millimeters offers a reduced pressure loss with simultaneous large specific contact surface. The transformation of pressure into kinetic energy creates secondary flow in micro-orifices, which results in continuous bubble dispersion. In this work, bubble flow through different orifices and channel modules with widths up to 7 mm are experimentally and numerically studied. The effect of the orifice dimensions on bubble sizes is evaluated for hydraulic diameters of 0.25–0.5 mm with different aspect ratios. To provide larger residence times of the generated dispersions in the reactor, several channel structures are analyzed to offer less coalescence. Volume flow rates of 10–250 mL/min are studied with various phase ratios. Bubble diameters are generated in the range of less than 0.1–0.7 mm with narrow size distributions depending on the entire flow rate. Opening angles of the orifices above 6 deg resulted in flow detachments and recirculation zones around the effluent jet. The first break-up point is shifted closer to the orifice outlet with increasing velocity and hydraulic diameter. The entire break-up region stays nearly constant for each orifice indicating stronger velocity oscillations acting on the bubble surface. Linear relation of smaller bubble diameters with larger energy input was identified independent from Reynolds number. Flow detachment and coalescence in bends were avoided by an additional bend within the curve based on systematically varied geometrical dimensions.

Effects of droplet ratio and void fraction on the attenuation of radiative heat flux in water curtain

The attenuation effects of radiative heat flux in water curtain are theoretically and experimentally investigated along the vertical direction from the nozzle. Based on the boundary conditions of water curtain, the general form of the attenuation efficiency of radiative heat flux is introduced as a function of droplet ratio and void fraction using the vertical 180 nozzle. The droplet ratio is applied to a 50% cumulative distributions (d50) for droplet mean diameters and a 99% that (d99) for the maximum diameters. In addition, the ratio of d50/D (hydraulic diameter of the injection nozzle) is correlated with the −0.39 index of Webber number to estimate the decrease tendency of the measurement values. The void fractions are well curve-fitted by flow rate and distance within±3% error along the vertical direction for the vertical 180 nozzle. The radiative heat fluxes are obtained by considering the convective term in energy equation. To verify the reliability of the present equation, the attenuation efficiencies of radiative heat flux are obtained from the vertical 60 nozzle. The results show that the predictions and experiments are in good agreement within a±10% error as flow rate increases in the fully break-up region.

CAPILLARY FORCES BETWEEN TWO PARALLEL PLATES CONNECTED BY A LIQUID BRIDGE

Liquid flow between porous and nonporous materials plays an important role in many science and engineering applications such as oil recovery from fractured porous media. The capillary continuity between porous matrix blocks via formation of liquid bridges is a key contributor to the gas−oil gravity drainage mechanism in a gas invaded zone of naturally fractured reservoirs, which increases the height of the continuous liquid column in a fractured formation, thereby enhancing the recovery of oil. However, the role of capillary forces information or break-up of liquid bridges between porous matrix blocks remains a controversial topic. In an attempt to improve an understanding of this problem, a force balance is presented for the concave liquid bridges formed between two horizontal parallel plates. The force balance allows development of a simple model that can be used to find a relationship between the net capillary force, contact angle, and liquid bridge volume. Three different regions including: (I) repulsive net capillary force, (II) attractive net capillary force, and (III) nonexistence regions have been identified. Region I is considered as a region of liquid bridge break-up while Region II is considered as a region of liquid bridge formation. The findings improve an understanding of the formation and break-up of the liquid bridges, which is important in oil recovery from naturally fractured reservoirs during a gravity drainage process.

Numerical Analysis of the Primary Breakup Applying the Embedded DNS Approach to a Generic Prefilming Airblast Atomizer

An improved understanding of the breakup processes of two-phase flows is essential to effectively control the fuel atomization for future aircraft engines. A detailed insight into the phenomena of primary breakup is a major limitation in gaining this knowledge. Aircraft engines use airblast atomizers to provide the fuel atomization. The geometries of airblast atomizers are complex, the operating conditions are characterized by high Reynolds- and Weber numbers. Direct Numerical Simulations (DNS) of liquid breakup under realistic conditions and geometries are hardly possible. The embedded DNS (eDNS) concept aims to fill this gap. The concept consists of three steps: a geometry simplification, the generation of realistic boundary conditions for the DNS and the DNS of the breakup region. The realistic annular airblast atomizer geometry is simplified to a planar geometry. Inside this domain the eDNS is located. The eDNS domain requires the generation of boundary conditions. A zonal Large Eddy Simulation (LES) of the turbulent channel flow is performed prior to the DNS. The parameters are stored transiently on the “virtual” DNS inlet planes. These variables are then mapped to the DNS. The Volume of fluid (VOF) method is used to solve for the two-phase flow. DNS are performed for a shear-driven liquid wall film and for a generic planar prefilming airblast atomizer. As the Reynolds and Weber number for the first operating point (OP) are low (Reair = 5,333/Wefilm = 1.9), the liquid wall film as well as the liquid sheet show no surface waves. For the second case with Reair = 13,333 and We film = 11.9, the surface appears more wrinkled and streamwise waves are transported along the wall for the shear-driven wall film. Instantaneous snapshots in 2–D and 3–D illustrate the qualitative behavior of the liquid sheet in time. Leaving the prefilmer trailing edge, the liquid sheet starts to oscillate in a sinusoidal fashion. This oscillation appears crucial for the onset of primary breakup. The qualitative characterization of the breakup for OP 1 and OP 2, yield to the distinction of the stretched ligament breakup for the former, and the torn-sheet breakup for the latter O P. The breakup time for OP 1 is longer than for OP 2. This study proves the applicability of the eDNS concept for investigating breakup processes as the transient nature of the phase interface behavior can be captured. The approach offers the potential of simulating realistic annular highly-swirled airblast atomizer geometries under realistic conditions.

Experimental Study on Diesel Engine and Analysis the Spray Characteristics of Diesel and Biodiesel by Varying Injection Pressure

— The spray characteristic of the injected fuel is mainly depends upon fuel injection pressure, temperature, ambient pressure, fuel viscosity and fuel density. An experimental study was conducted to examine the effect of injection pressure on the spray was injected into direct injection (DI) diesel engine in the atmospheric condition. In Diesel engine, the window of 20 mm diameter hole and the transparent quartz glass materials were used for visualizing spray characteristics of combustion chamber at right angle triangle position. The varying Injection pressure of 180 - 240 bar and the engine was hand cranked for conducting the experiments. Spray characteristics for Jatropha oil methyl ester (JOME) and diesel were studied experimentally. Spray tip penetration and spray cone angle were measured in a combustion chamber of Direct Injection diesel engine by employing high speed Digital camera using Mie Scattering Technique and ImageJ software. The study shows the JOME gives longer spray tip penetration and smaller spray cone angle than those of diesel fuels. The Spray breakup region (Reynolds number, Weber number), Injection velocity and Sauter Mean Diameter (SMD) were determined for diesel and JOME. SMD decreases for JOME than diesel and the Injection velocity, Reynolds Number, Weber Number Increases for JOME than diesel.