Dense Sprays Research Articles

Towards High-fidelity Multi-scale Simulation of Spray Atomization

Abstract Liquid jet atomization is an important process in internal combustion engine. This paper presents a multi-scale approach coupling a Eulerian interface-tracking method and a Lagrangian particle tracking method to simulate liquid atomization processes. This method aims to capture the complete spray atomization process, particularly primary and second breakup, paving the way for high-fidelity simulation of spray atomization in the dense spray zone and spray combustion in the dilute spray zone. The Eulerian method is based on the Coupled Level-Set and Volume-of-Fluid (CLSVOF) for interface tracking, which can accurately simulate the primary breakup process. For the coupling approach, the Eulerian method only describes large droplet and ligament structures, while small-scale droplet structures are removed from the resolved Eulerian description and transformed into Lagrangian point-source droplets. The Lagrangian method is thus used to track small droplets and their further secondary breakup and collision. In this study, two-dimensional simulation of liquid jet atomization is performed. We will analyse Lagrangian-droplet formation and motion due to the coupling and the atomization characteristics using the multi-scale approach.

Effect of the borax mass and pre-spray medium temperature on droplet size and velocity vector distributions of intermittently sprayed starchy solutions.

Spray coating technology has demonstrated great potential in the slow release fertilizers industry. The better understanding of the key spray parameters benefits both the environment and low cost coating processes. The use of starch based materials to coat the slow release fertilizers is a new development. However, the hydraulic spray jet breakup of the non-Newtonian starchy solutions is a complex phenomenon and very little known. The aim of this research was to study the axial and radial distributions of the Sauter Mean Diameter (SMD) and velocity vectors in pulsing spray patterns of native and modified tapioca starch solutions. To meet the objective, high speed imaging and Phase Doppler Anemometry (PDA) techniques were employed to characterize the four compositions of the starch-urea-borax complex namely S0, S1, S2 and S3. The unheated solutions exhibited very high viscosities ranging from 2035 to 3030 cP. No jet breakup was seen at any stage of the nozzle operation at an injection pressure of 1-5 bar. However, at 80 °C temperature and 5 bar pressure, the viscosity was reduced to 455 to 638 cP and dense spray patterns emerged from the nozzle obscuring the PDA signals. The axial size distribution revealed a significant decrease in SMD along the spray centreline. The smallest axial SMD (51 to 79 μm) was noticed in S0 spray followed by S1, S2 and S3. Unlikely, the radial SMD in S0 spray did not vary significantly at any stage of the spray injection. This trend was attributed to the continuous growth of the surface wave instabilities on the native starch sheet. However, SMD obtained with S1, S2 and S3 varied appreciably along the radial direction. The mean velocity vector profiles followed the non-Gaussian distribution. The constant vector distributions were seen in the near nozzle regions, where the spray was in the phase of development. In far regions, the velocity vectors were poly-dispersed and a series of ups and downs were seen in the respective radial distributions.

An Eulerian CFD model and X-ray radiography for coupled nozzle flow and spray in internal combustion engines

This paper implements a coupled approach to integrate the internal nozzle flow and the ensuing fuel spray using a Volume-of-Fluid (VOF) method in the finite-volume framework. A VOF method is used to model the internal nozzle two-phase flow with a cavitation description closed by the homogeneous relaxation model of Bilicki and Kestin (1990). An Eulerian single velocity field approach by Vallet et al. (2001) is implemented for near-nozzle spray modeling. This Eulerian approach considers the liquid and gas phases as a complex mixture with a highly variable density to describe near nozzle dense sprays. The liquid mass fraction is transported with a model for the turbulent liquid diffusion flux into the gas. Fully-coupled nozzle flow and spray simulations are performed in three dimensions and validated against the X-ray radiography measurements of Kastengren et al. (2014) for a diesel fuel surrogate. A standard k–∊ Reynolds Averaged Navier Stokes based turbulence model is used in this study and the influence of model constants is evaluated. First, the grid convergence study is performed. The effect of grid size is also evaluated by comparing the fuel distribution against experimental data. Finally, the fuel distribution predicted by the coupled Eulerian approach is compared against that by Lagrangian–Eulerian spray model along with experimental data. The coupled Eulerian approach provides a unique way of coupling the nozzle flow and sprays so that the effects of in-nozzle flow can be directly realized on the fuel spray. Both experiment and numerical simulations show non-cavitation occurring for this injector with convergent nozzle geometry. The study shows that the Eulerian approach has advantages over near-field dense spray distributions.

Effect of Viscosity and Heating Temperature on an Intermittent Spray Jet of Starch Dispersion

In this paper, an attempt was made to integrate the effect of jet injection time, load pressure and solution physical properties on jet breakup parameters. A starch-urea-borax complex solution was prepared and tested with an axi-symmetric full cone nozzle. The jet injection time was set to 100, 200, 300 and 400, the solution heating temperature was set to 20°C and 80°C ms, the load pressure was set to 1, 2, 3, 4, and 5 bar, and corresponding spray patterns were imaged by using a high speed camera. The imaging study of the developing spray patterns revealed that the unheated solution forms only spinning jets for all used load pressures. No jet breakup was seen in the near and far-nozzle imaged regions except at 5 bar load pressure, where minor spreading in the jet was seen after 80 mm downstream of the nozzle exit. At 80°C temperature and 5 bar load pressure, very dense spray patterns with an increased spray cone angle were emerging from the nozzle. After 300 ms of injection time, these developing spray jets were changed into fine spray patterns.

Characterization of atomization and combustion in moderately dense turbulent spray flames

This paper presents a study of spray jets and flames where the liquid fuel loading in the carrier air is such that there are substantial liquid–liquid interactions close to the exit plane. The burner consists of an air-blast atomizer located within a wide hot co-flow used to stabilize turbulent auto-igniting spray flames. Laser/phase Doppler anemometry, microscopic backlit imaging coupled with advanced image processing, broadband chemiluminescence and OH-PLIF imaging are utilized. A key focus here is on a novel characterization of the spray boundary condition in terms of non-spherical shapes of fluid fragments. Three different classes of shapes, namely: ligaments, droplets, and large, generally irregular objects are examined. Statistics for each of these fragments are computed in a range of sprays and it is found that their size and probability of occurence depends on the initial Weber number and fuel/air mass ratio (F/A). The change in chemiluminescence emission that is measured as a function of the F/A ratio trends in the same manner as the change in the size of the largest non-spherical objects in the spray. In addition, it is found that changes in the growth of reaction zone width occur as estimated from OH images of laser induced fluorescence, and this is also controlled by the F/A ratio. Therefore, the spray structure at the exit plane may partly dictate the downstream flame characteristics; and this could be largely due to the presence of non-spherical fragments and hence the different rates of atomization and vapourization amongst the different sprays. The burner, as well as the classification of the spray introduced here, while complex, can form a platform for the improvement of models for moderately dense reacting sprays.

Droplet/ligament modulation of local small-scale turbulence and scalar mixing in a dense fuel spray

In this study, the modulation of turbulence and scalar mixing by finite-size droplets/ligaments in a dense fuel spray is investigated using a DNS (Direct Numerical Simulation) dataset. Ejected from a spray nozzle with a high speed, a liquid-fuel jet deforms and the fuel spray is atomized into many ligaments and droplets. During these processes, the gas flow becomes turbulent due to droplet/ligament dynamics. At the same time, droplet evaporation and mixing with ambient air are affected by the small-scale gas turbulence. An understanding of the mixing characteristics in the dense spray zone is important for modeling spray combustion. In a region where the droplet number density is relatively low, a universal feature of isotropic turbulence was found, although the alignments of strain eigenvectors with vorticity and the mixture fraction gradient are slightly modulated by the presence of droplets, which is a characteristic of particle-laden flows. In gas-phase regions close to droplet surfaces, where the dissipation rate of turbulent kinetic energy is strongly increased, the alignments are more modulated, especially those of the scalar gradient with strain eigenvectors. This can also be seen in the topology similarity among energy dissipation, enstrophy and scalar dissipation in the near field of droplet/ligament surfaces. For the first time, it is found that droplets whose size is comparable to turbulence scales do affect the mixing characteristics in a realistic turbulent spray. This finding has shed new light upon the modeling of flow turbulence and scalar mixing in an evaporating and atomizing fuel spray.

Mechanistic Assessment of Swirl Coaxial Injection by Quantitative X-Ray Radiography

Detailed x-ray radiographic experiments were conducted to evaluate the time-averaged spray characteristics of a liquid rocket swirl injector. Sprays issued from a single liquid-centered swirl coaxial element—with and without coannular gas flow—were exposed to focused, monochromatic x rays produced by a synchrotron light source. Two-dimensional attenuation data are presented at various axial stations and reveal both projected mass distribution of liquid in the injector near field and mass-weighted axial velocity integrated over time. Measurements describing spray morphology are also inferred from the x-ray data and compared with those measured through objective image processing of visible light imagery. The gas flow is observed to reduce spray cone angle up to 50% and increase liquid film thickness up to 20% in the near field. Measurements compare well with those extracted from imagery of the pressure-swirl spray; however, the x-ray technique is more robust in resolving liquid film thickness for the optically dense swirl coaxial spray.

Studies on Microscopic Structure of Diesel Sprays under Atmospheric and High Gas Pressures

In the present work, the spray structure of diesel from a 200-μm, single-hole solenoid injector is studied using microscopic imaging at injection pressures of 700, 1000 and 1400 bar for various gas pressures. A long-distance microscope with a high resolution camera is used for spray visualization with a direct imaging technique. This study shows that even at very high injection pressures, the spray structure in an ambient environment of atmospheric pressure reveals presence of entangled ligaments and non-spherical droplets during the injection period. With increase in the injection pressure, the ligaments tend to get smaller and spread radially. The spray structure studies are also conducted at high gas pressures in a specially designed high pressure chamber with optical access. The near nozzle spray structure at the end of the injection shows that the liquid jet breakup is improved with increase in gas density. The droplet size measurement is possible only late in the injection duration when the breakup appears to be complete and mostly spherical droplets are observed. Hence, droplet size measurements are performed after 1.3 ms from start of the injection pulse. Spatial and temporal variation in Sauter Mean Diameter (SMD) is observed and reported for the case corresponding to an injection pressure of 700 bar. Overall, this study has highlighted the importance of verifying the extentof atomization and droplet shape even in dense sprays before using conventional dropsizing methods such as PDPA.

Performance testing of iron based thermally sprayed HVOF coatings in a biomass-fired fluidised bed boiler

Managing high temperature corrosion problems in biomass firing boilers has been challenging especially due to high amounts of chemically active compounds, in particular alkali chlorides. Thermally sprayed coatings with high chromium content can offer a solution for protecting low alloyed substrate materials in locations prone to high temperature corrosion. Two thermally sprayed (HVOF — high velocity oxy-fuel) iron based coatings (Fe–27Cr–11Ni–4Mo and Fe–19Cr–9W–7Nb–4Mo) were exposed to biomass boiler conditions for two years. The fluidised bed boiler for district heating used mainly wood-based fuels mixed with small amounts of peat. The coated tubes were located at the hot economiser of the boiler, where the estimated material temperature was about 200°C maximum. After the exposure the coatings and the carbon steel St35.8 substrate material were analysed with SEM–EDX. It was detected that corrosion due to elements such as chlorine, potassium, zinc, lead and copper had caused severe material wastage in the biomass boiler with relatively low heat exchanger surface temperatures. The low alloyed boiler tubes had suffered severely with a corrosion rate as high as 2mm/year, whereas dense thermal spray coatings offered excellent protection during the exposure.

Liquid phase temperature determination in dense water sprays using linear Raman scattering

Linear Raman scattering has been applied for the determination of the temperature of the liquid phase in water sprays under normal and superheated conditions. The envelope of the Raman OH-stretching vibration band of water is deconvoluted into five Gaussian peaks which can be assigned to five different intermolecular interactions (hydrogen bonding). The intensity of each of the peaks is a function of the temperature and the phase of the water under investigation. The interference of the Raman signals originating from the water vapor is eliminated from the Raman signals originating from the liquid water. Consequently the temperature of the liquid water droplets surrounded by water vapor is accessible which is favorable for the investigation of non-equilibrium sprays where the droplet temperature is different to the vapor temperature.

Development of an efficient statistical volumes of fluid–Lagrangian particle tracking coupling method

SUMMARYThe breakup of a liquid jet into irregular liquid structures and droplets leading to the formation of a dilute spray has been simulated numerically. To overcome the shortcomings of certain numerical methods in specific flow regimes, a combined approach has been chosen. The intact liquid core, its primary breakup and the dense spray regime are simulated using the volumes of fluid (VOF) method in combination with LES, whereas the Lagrangian particle tracking (LPT) approach in the LES context is applied to the dilute spray regime and the secondary breakup of droplets. A method has been developed to couple both simulation types on a statistical basis. This statistical coupling approach (SCA) reflects the dominating physical mechanisms of the two‐phase flow in each regime to a high degree. The main benefit of the SCA is computational efficiency as compared with the more straightforward approach where one follows each structure, denoted here as the direct coupling approach. The computational benefits stem from the reduction of computational time since the VOF simulation is run only until statistical convergence and not during the whole spray development. A second benefit using the SCA is the possibility to use the stochastic parcel method in the LPT simulation whereby a large number of droplets may be handled. The coupling approach is applied to the atomization of a fuel jet in a high pressure chamber, demonstrating the gain of efficiency of the SCA as compared with direct coupling approach. Copyright © 2014 The Authors. International Journal for Numerical Methods in Fluids published by John Wiley & Sons Ltd.

Dense Spray Patternation using Optical Tomography

Optical tomography was used to measure the pattern of spray cross-section. The maximum-likelihood estimation (MLE) algorithm was used to reconstruct the spray cross-section from the measured transmission rate of the spray. A swirl-type injector was used to form an optically dense spray, and the test was carried out in a high-pressure chamber, to control the pressure condition of the test site. Before the experiment, the reliability of the MLE-based reconstruction algorithm was verified, by comparing it with a conventional filtered back projection reconstruction (FBP) method. The MLE algorithm showed superior reconstruction of the image. In the spray patternation experiment, the results of the optical tomography and optical line patternator, which uses Mie scattering signal information, were compared. While measuring the cross-section of optically dense spray, the intensity of the scattering signal had attenuated to an uncorrectable level, which led to incorrect spray pattern measurement by the optical line patternator. However, reliable results were obtained by optical tomography, under the same condition. Finally, the pattern of the optically dense spray was measured at various chamber pressures, of up to 3 MPa. As the chamber pressure increased, the hollow cone-shaped swirl spray shrank, and the attenuation coefficient value of the inner region increased.

Transmitted light microscopy for visualizing the turbulent primary breakup of a microscale liquid jet

Aiming at a maximum spatial resolution and a minimum motion blur, a new simple double-imaging transmitted light microscopy technique is developed in this work enabling a fundamental investigation of primary breakup of a microscale liquid jet. Contrary to conventional far-field visualization techniques, the working distance is minimized to increase the numerical aperture. The resulting images provide information about shapes, length scales and velocities of primary liquid structures. The method is applied to an optically dense spray leaving a 109-μm diesel nozzle at various injection pressures under atmospheric conditions. A phenomenological study on the temporal spray evolution is done with focus on droplet and ligament formation. Different breakup processes are identified and described. It is found that the jet is characterized by long ligaments parallel or angular to the inner jet region. These ligaments result from collapsing films developing at the spray edge. A significant influence of outlet velocity variation on shape and velocity of these ligaments is observed. The experimental results prove that a transmitted light microscopy technique with reduced working distance is an appropriate tool for a better understanding of primary breakup for small-scaled diesel nozzles and a valuable complement to highly complex measurement techniques.

Time-gated holography to provide a glimpse into dense sprays

Time-gated holography to provide a glimpse into dense sprays

Transient two-phase boundary layer modeling for hollow cone sprays

This paper presents a spray model suited for dense sprays. It captures the transient evolution of the two-phase jet characteristics resulting from hollow cone injection. The model is designed for fast model response as needed in engine system simulation. It is based on the description of the gas phase boundary layer surrounding the dense spray. Mass and momentum equations are solved for both the dispersed liquid and the continuous gas phase. Spatial gradients are resolved along one dimension, namely the main injection direction. The conservation equations are expressed in conical coordinates. The model’s response is studied qualitatively and global characteristics such as the penetration behavior are compared to both experimental and CFD data.

Experimental investigation of explosive vaporization of C6F14

Depressurization experiments were conducted with C6F14 for superheats between 5 °C and 90 °C in order to determine how the drop characteristics evolve with the thermodynamic disequilibrium. High-speed imaging indicates that flashing is predominant and leads to very dense sprays. Phase Doppler Interferometer (PDI) was adapted to these optically thick conditions and its ability to provide unbiased velocity and size distributions was carefully checked (even though the flux was underestimated). Experiments show that, for initial superheats above 40 °C, the mean drop velocity linearly increases with the initial superheat while the mean drop diameter remains nearly constant. Such behaviors, not reported in the literature, are tentatively related with a front boiling process in which the drop velocity is driven by the vapor velocity while the drop size is controlled by bubbles exploding at the front. In addition, phase detection optical probes used in combination with PDI data provided number density flux measurements. Global vaporization rates deduced from these local data happen to be fairly consistent with alternate techniques, and the vapor flux exhibits a somewhat weak increase with the superheat.

Eulerian versus Lagrangian simulation of unsteady two-way coupled coalescing two-phase flows in solid propellant combustion

Solid rocket motor flows depend strongly on alumina granulometry, which spreads up with coalescence. Solving droplet sizes in unsteady flows is a modeling and numerical challenge, specially when high loadings generate retrocoupling. As an alternative to Lagrangian approaches, the Eulerian Multi-Fluid model describes polydispersity with size-sorted droplet fluids called sections. A two size moment method, that spares sections, is implemented in the CEDRE code. A new transport and coupling method allows to capture moderately dense spray structures with low diffusion. The aim is threefold: compare CEDREʼs Lagrangian and new Eulerian two-phase approaches, prove the feasibility of unsteady and complex motor flow simulations, and show the effect of coalescence on instabilities.

A method for measurement of planar liquid volume fraction in dense sprays

A methodology for measurement of planar liquid volume fraction in dense sprays using a combination of Planar Laser-Induced Fluorescence (PLIF) and Particle/Droplet Imaging Analysis (PDIA) is presented in this work. The PLIF images are corrected for loss of signal intensity due to laser sheet scattering, absorption and auto-absorption. The key aspect of this work pertains to simultaneously solving the equations involving the corrected PLIF signal and liquid volume fraction. From this, a quantitative estimate of the planar liquid volume fraction is obtained. The corrected PLIF signal and the corrected planar Mie scattering can be also used together to obtain the Sauter Mean Diameter (SMD) distribution by using data from the PDIA technique at a particular location for calibration. This methodology is applied to non-evaporating sprays of diesel and a more viscous pure plant oil at an injection pressure of 1000bar and a gas pressure of 30bar in a high pressure chamber. These two fuels are selected since their viscosity values are very different with a consequently very different spray structure. The spatial distribution of liquid volume fraction and SMD is obtained for two fuels. The proposed method is validated by comparing liquid volume fraction obtained by the current method with data from PDIA technique.

Quantitative imaging of a non-combusting diesel spray using structured laser illumination planar imaging

Due to its transient nature, high atomization process, and rapid generation of fine evaporating droplets, diesel sprays have been, and still remain, one of the most challenging sprays to be fully analyzed and understood by means of non-intrusive diagnostics. The main limitation of laser techniques for quantitative measurements of diesel sprays concerns the detection of the multiple light scattering resulting from the high optical density of such a scattering medium. A second limitation is the extinction of the incident laser radiation as it crosses the spray, as well as the attenuation of the signal which is to be detected. All these issues have strongly motivated, during the past decade, the use of X-ray instead of visible light for dense spray diagnostics. However, we demonstrate in this paper that based on an affordable Nd:YAG laser system, structured laser illumination planar imaging (SLIPI) can provide accurate quantitative description of a non-reacting diesel spray injected at 1,100 bar within a room temperature vessel pressurized at 18.6 bar. The technique is used at λ = 355 nm excitation wavelength with 1.0 mol% TMPD dye concentration, for simultaneous LIF/Mie imaging. Furthermore, a novel dual-SLIPI configuration is tested with Mie scattering detection only. The results confirm that a mapping of both the droplet Sauter mean diameter and extinction coefficient can be obtained by such complementary approaches. These new insights are provided in this article at late times after injection start. It is demonstrated that the application of SLIPI to diesel sprays provides valuable quantitative information which was not previously accessible.

Observation of the Binary Coalescence and Equilibration of Micrometer-Sized Droplets of Aqueous Aerosol in a Single-Beam Gradient-Force Optical Trap

The binary coalescence of aqueous droplets has been observed in a single-beam gradient-force optical trap. By measuring the time-dependent intensity for elastic scattering of light from the trapping laser, the dynamics of binary coalescence have been examined and the time scale for equilibration of a composite droplet to ambient conditions has been determined. These data are required for modeling the agglomeration of aqueous droplets in dense sprays and atmospheric aerosol. Elastic-light scattering from optically trapped particles has not been used previously to study the time-resolved dynamics of mixing. It is shown to offer a unique opportunity to characterize the binary coalescence of aqueous droplets with radii from 1 to 6 μm. The study of this size regime, which cannot be achieved by conventional imaging methods, is critical for understanding the interactions of droplets in the environment of dense sprays.