Spray Region Research Articles

Prediction and Measurement of the local extinction coefficient in sprays for 3D simulation/experiment data comparison

In the recent years, large progresses in laser imaging techniques have allowed to extract spatially resolved 2D and 3D quantitative spray information even in optically dense situations. The main breakthrough of these techniques is the possibility of suppressing unwanted effects from multiple light scattering using Structured Illumination. Thanks to this new feature, effects due to light extinction can also be corrected allowing the measurement of the local extinction coefficient. These quantitative information which is available even in challenging conditions, where Phase Doppler does not work anymore, can be used for data comparison between experiment and simulation. The local extinction coefficient is particularly valuable for the description of the droplet field, defined as the “spray region”, as it contains information related to both droplets size and concentration. In this article we detail, then, the procedure enabling the modelers to obtain numerically this local extinction coefficient over the full 3D spray system. Following this procedure, results can now be adequately compared between simulation and experiment. The proposed comparison approach can better guide model adjustments in situation where the initial droplet size distribution is unknown or approximated and presents a step towards future validations of spray simulations, especially those based on Lagrangian Particle Tracking. The approach is exemplified here for the case of a Diesel-type spray. The results reveal at which specific spray locations discrepancies occur, and highlight the sensitivity of the initial droplet size distribution on the resulting extinction coefficient.

EULERIAN MOMENT METHODS FOR AUTOMOTIVE SPRAYS

To assist industrial engine design, 3D simulations are increasingly used as they allow evaluation of a wide range of engine configurations and operating conditions and bring a comprehension of the underlying physics comple-mentary to experiments. While the gaseous flow description has reached a certain level of maturity, the multiphase flow description involving the liquid jet fuel injected into the chamber still faces some major challenges. There is a pressing need for a spray model that is time efficient and accurately describes the fuel-particle cloud dynamics downstream of the injector, which is an essential prerequisite for predictive combustion simulations. Due to the highly unsteady nature of the flow following the high-pressure injection process and the complexity of the flow regimes from separated/dense compressible phases to fully developed turbulent spray with evaporating droplets, Eulerian-Eulerian descriptions of two-phase flows are seen as very promising approaches towards realistic and predictive simulations of the mixing process. However they require some effort in terms of physical modeling and numerical analysis related to the more complex mathematical structure of the system of equations and to the unclosed terms appearing in space/time-average equations. Among the various challenges faced, one critical as-pect is to capture spray polydispersity in this framework. A review of recent developments that have permitted key advances in the spray modeling community is proposed in this paper. It is divided into four parts. First, an introduction to automotive spray modeling is provided. Then the formalisms for the description of the disperse region of an engine spray are presented with particular emphasis on the pros and cons of classical Lagrangian par-ticle methods versus Eulerian approaches. The third part presents the motivation for and the recent developments of Eulerian high-order moment methods for size polydispersion. Finally, the extension to fully two-way coupled interactions with the gas phase and the implementation of such methods for variable-geometry applications in CFD codes is described in the fourth part. Using realistic direct injection conditions computed with the IFP-C3D solver, the application and efficiency of Eulerian approaches is illustrated.

Effects of coflow temperature and composition on ethanol spray flames in hot-diluted coflow

Ethanol pressure-swirl sprays issuing in a hot-diluted oxidizer coflow with different temperature and composition were studied. The bulk coflow temperature was varied together with the oxygen volume fraction. The bulk coflow temperature was changed from 1480K to 1225K and the oxygen volume fraction from 7.1% to 10.1%. The liquid mass flow rates were chosen to yield spray flames with nearly identical Weber number. Laser Doppler anemometry, phase Doppler anemometry and coherent anti-Stokes Raman scattering were applied in the spray region and the coflow inlet. The current measurements provide a thorough description of the spray structure, droplet dispersion and gas temperature fields as well as a comprehensive database useful for validation of numerical models. Visual observations of the flame luminescence reveal that the lift-off height depends on the liquid mass flow rates as well as the coflow conditions. The lift-off height is shown to increase for lower coflow temperatures and higher liquid mass flow rates. It is found that lift-off behavior depends on the droplet convective, vaporization and chemical time scales prior to ignition. Phase Doppler anemometry results indicate that the droplet mean size and velocity distribution close to the atomizer are not influenced by the coflow conditions. A flame-front develops at the outer region of the spray where a low density of large droplets are present. A significant number of peak temperatures samples above 2000K is observed at this location. Decrease of the coflow temperature leads to a reduction of the local peak temperatures. Closer to the center axis, the local mixture composition becomes increasingly rich and the heat-release is lower than in the outer region.

Spray flame structure in conventional and hot-diluted combustion regime

A laboratory scale experimental setup was built to study ethanol pressure-swirl spray flames in a coflow of either air or hot-diluted oxidant. The latter case resembles conditions similar to those found in practical combustion systems of liquid fuels operating in MILD conditions.First, experiments have been performed to investigate the phenomena associated with the atomization process. High-speed visualizations show that in the presence of a hot-diluted coflow, an almost immediate disruption of the liquid jet takes place, indicating significant changes in the atomization mechanism, compared to the case with air coflow. Secondly, a comprehensive set of measurements was obtained by complementary single-point measurement techniques to reveal the gas and droplets flow fields as well as temperature fields. Measurements of droplet size and velocity components in the spray region were obtained by phase Doppler anemometry. Gas temperature was measured using coherent anti-Stokes Raman spectroscopy even in regions with droplet density as high as 105cm-3. It has been observed that in a reacting spray in air coflow, an inner and an outer flame-front are present. For a reacting spray with similar injection pressure in hot-diluted coflow, weakening of the inner flame-front is attributed to the fact that the gaseous mixture becomes increasingly rich towards the center region. Consequently, a significant reduction of occurrence of temperature samples above 2000K is observed throughout most of the spray region.

Experimental study of the air-atomized spray cooling of high-temperature metal

An experimental study of heat transfer between a hot metallic surface and water droplets sprayed by a commercial flat pattern air-atomized spray nozzle was conducted. A lattice grid was used to measure the local spray density (Q) along the horizontal (X) and vertical (Y) axes of the local spray region. By measuring the temperature evolution inside the test plate, the surface heat flux was calculated by the inverse heat conduction problem. In the case of no obvious run-off flow on the surface, the temperature TCHF and heat flux qCHF at the critical heat flux (CHF) point increased with Q. Besides, the values of TLeid and qLeid for the Leidenfrost point also showed similar variation. When V = 1.11 × 10−5 m3 s−1, the CHF points at locations around the spray center were forced to transfer to higher surface temperature, due to the run-off water flow from the spray center. Based on the results obtained, mathematical correlations were proposed, signifying the dependence of heat flux in different boiling regimes for water flow rates V ≤ 0.83 × 10−5 m3 s−1. The results obtained in this study will provide a theoretical basis for predicting the temperature of the cold-rolled strip during the spray-cooling process and for designing the cooling process.

Mass preparation of nanofibers by high pressure air‐jet split electrospinning: Effect of electric field

ABSTRACTA novel electrospinning method using airflow, namely high pressure air‐jet split electrospinning, was proposed to fabricate polymer nanofibers with ultrahigh production rate. 7 wt % polyacrylonitrile spinning solution with a 0.157 Pa s viscosity was divided into micron size droplets by the filter screen in the front of the nozzle, and then these droplets were divided and split through high pressure airflow, which were drafted into nanofibers directly in the electric field and airflow field. In this study, the electric field distributions with different positive electrodes were simulated and their effect on fiber formation was investigated. The results show that electric field distribution and its intensity depended on electrodes area, a broader electric field distribution with a stronger intensity would bring about a larger cone angle of spraying jet region, at the same time, the contrast in the spray region enhanced. When the whole nozzle was charged, thinner fibers with about 170 nm could be prepared and the fiber production was 75.6 g/h. Compared with the conventional needle electrospinning, the throughput of nanofibers could be improved by thousands of times based on this novel electrospinning method. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 993–1001

Effect of breakup model on large-eddy simulation of diesel spray evolution under high back pressures

A breakup model has been developed and optimized for diesel spray simulation with a large-eddy simulation. Previously, diesel spray analysis has been carried out using a breakup model such as the Kelvin–Helmholtz and Rayleigh–Taylor model. The Kelvin–Helmholtz and Rayleigh–Taylor models are used for high Weber number conditions. However, even when fuel is injected at high pressures, the downstream region of spray corresponds to relatively low Weber number conditions. Hence, a hybrid breakup model that combines the Kelvin–Helmholtz and modified Taylor analogy breakup models (Kelvin–Helmholtz–modified Taylor analogy breakup) has been developed. The Kelvin–Helmholtz and modified Taylor analogy breakup models are used to model the primary and secondary breakup, respectively. The modified Taylor analogy breakup model is more suitable than the Rayleigh–Taylor model to describe secondary breakup. Spray simulations under non-evaporative and evaporative conditions were performed to validate the Kelvin–Helmholtz–modified Taylor analogy breakup model. It is found that the simulation results of Kelvin–Helmholtz–modified Taylor analogy breakup are in good agreement with experimental measurements of non-evaporative and evaporative spray.

Light intensity and image visualization of GDI injector sprays according to nozzle hole arrangements

The light intensity measurement and image visualization of multi-hole injection spray due to different hole arrangements and hole numbers were investigated. The light intensities and behavior characteristics of the GDI spray were analyzed through the axial and diagonal spray penetration, cone angle, and spray area from the spray images by using the image visualization system and image analysis system. The atomization performance of GDI injectors was analyzed by the local and overall Sauter mean diameter (SMD) measurement.It is revealed that the higher injection pressure shows higher light intensity levels due to the strong vortices and collision by a high injection pressure. The light intensity level at the outer and end region of the spray shows lower value compared to that at the center spray region. In all of test injectors, the increased injection pressure leads to the decrease of the droplet size distribution in the initial spray. But, there is little difference of the droplet size at low and high injection pressure in middle and latter period after the injection. The overall SMD of GDI injectors showed from 14 to 23.14μm.

Three-Dimensional Mass Fraction Distribution of a Spray Measured by X-Ray Computed Tomography

In order to design a spraying system with the desired characteristics, the atomization process has to be understood in detail, including the primary breakup of the liquid core. Accurate prediction of primary breakup is a major barrier to computer-based analysis of spray combustion. The development of models is hindered by the lack of validation data in a region where the fluid is dense, and optical access is therefore limited. The present experimental study is aimed at probing the spray structure by means of X-ray computed tomography (CT). A full-cone atomizer (0.79 mm orifice diameter) spraying in air at ambient pressure is investigated as a proof of concept. A mixture of water and iodine is used as the working fluid, providing elevated X-ray absorption and therefore, improved signal-to-noise ratio. Several hundreds of X-ray projections are acquired as the spraying atomizer is rotated in front of the detector. Standard software for medical imaging is used to reconstruct the three-dimensional time-averaged distribution of liquid mass fraction in the full field of view, from the intact liquid core to the dilute spray region. A spatial resolution of 0.6 mm is obtained along the spraying direction, while the resolution is 0.3 mm in the other two directions. Significant asymmetries in the structure of the spray are revealed.

1110 ジェットエンジン用エアーブラスト式噴射弁の微粒化用空気供給路の形状が噴霧特性に及ぼす影響

Emission of nitrogen oxides (NOx) caused by exhaust gas from airplane is computed approximately 60% at high-altitude. Lean premixed-prevaporized combustion is given for one of methods of NOx reduction in jet engines. Improvement of spray characteristics before combustion is important for fuel injection of jet engine, which is leaded to redaction of exhaust gas emission at high-load conditions and high-dispersion of fuel. However, air-blast atomizer has the following characteristics that velocity of atomizing air decreases with an increase in rapidly large droplets of spray. Therefore, it is required that uniform and small droplets are obtained independent of atomizing air flow rate. The purpose of this study is that uniform and small droplets are obtained under low atomizing air velocity, to improve spray characteristics and homogeneous droplets is generated at wide region of spray. It was investigated that effects of shapes of supplying passage for atomizing air of air-blast atomizer on spray characteristics. As a results, it was cleared that excellent spray characteristics, that is, large spray angle of about 140 deg. and relatively small Sauter mean diameter of about 40 μm, was obtained by using the air-blast atomizer which was designed in this study.

Development of an analytical model to quantify downward smoke displacement caused by a water spray for zone model simulations

A stand-alone analytical model for downward smoke layer displacement caused by a water spray is developed. The model can be implemented into two-zone model calculations. Smoke flow into and out of the water spray envelope is incorporated, as well as a model for the heat exchange between the smoke and the water droplets. The model input quantities are water flow rate, orifice diameter, droplet diameter, spray angle, initial smoke layer thickness and temperature, and ambient air temperature. The quality of the model predictions is illustrated for a range of experimental conditions. Results are shown to be sensitive to the mean droplet diameter and the spray angle. Then, the combinations of values of these parameters are suggested for the sprinkler considered. A practically important phenomenon of abrupt strong downward smoke layer displacement is explained in that the upward buoyancy flow diminishes due to the entrainment of cool air into the water spray envelope. This only occurs for high enough water flow rates and as long as the smoke layer thickness is below a critical value. Therefore, this is particularly dangerous during the smoke layer build-up phase. Finally, the accuracy of the model for the heat transfer rate from the hot smoke layer to the droplets and the smoke temperature inside the spray region are evaluated by comparison to experimental data.

Self-extinguishing behavior of kerosene spray fire in a completely enclosed compartment

The self-extinguishing behavior of kerosene spray fire was investigated in a completely enclosed compartment with the size of 3 m × 3 m × 3.4 m. The spray was generated by locating one BETE nozzle at the center of the bottom wall. A series of spray fire videos were obtained by changing BETE nozzle type and injecting pressure. The results show that spray fire undergoes four stages: the growth stage, the quasi-steady stage, the stretch stage and the self-extinguishing stage. Consumption of large quantities of oxygen causes spray fire to first be stretched and then quench. In this process, fire base migrates away from spray region and leads to the emergence of ghosting fire. Ghosting fire promotes the instability of spray fire and large fluctuation of its height, which provides help to its self-extinguishing. With increasing the injecting pressure or the nozzle diameter, the self-extinguishing time decreases. It is found that the self-extinguishing time is approximately in inverse relation with injecting flow rate. Additionally, we also observed the occurrence of two-phase deflagration just after ignition, and it accelerates the spray fire growth and induces a larger fire height than the following quasi-steady spray fire. The deflagration turns stronger with increasing the injecting pressure.

Multi-scale modeling of a spray coating process in a paddle mixer/coater: the effect of particle size distribution on particle segregation and coating uniformity

The purpose of this paper is to investigate the effect of particle size distribution on the flow dynamics, segregation, and coating uniformity in an industrial paddle mixer/coater. First principles modeling approaches with the discrete element method (DEM) and spray post-processing analysis were applied to model particle motion and coating growth. A monosize seed distribution with a particle size of 10.52mm was used as a baseline DEM simulation. Important spray region variables such as the residence time in the spray region, number of spray region visits, and area exposure percentage were analyzed to characterize the mixing dynamics in the spray. The seed particles were then changed to a log normal size distribution with a relative standard deviation varying from 0.134 to 0.389. Small particles appear preferentially in the spray zone in all polydisperse particle systems, and further analysis on the size dependence of the spray variables shows that small particles have a relatively high frequency of spray zone visits and low shielding. The binary particle DEM simulations with 11.69mm large particles and 9.01mm small particles demonstrate segregation patterns in which large particles collect in the end wall region and small particles collect in the spray region, which explains the observed spray preference for small particles. The results of this study highlight the need to understand the effect of mixer geometry on segregation in the particle flow field in order to predict the coating preference with particle size.

Complex chemistry DNS of n-heptane spray autoignition at high pressure and intermediate temperature conditions

Direct Numerical Simulations (DNS) of turbulent n-heptane sprays autoigniting at high pressure (P=24bar) and intermediate air temperature (Tair=1000K) have been performed to investigate the physical mechanisms present under conditions where low-temperature chemistry is expected to be important. The initial turbulence in the carrier gas, the global equivalence ratio in the spray region, and the initial droplet size distribution of the spray were varied. Results show that spray ignition exhibits a spotty nature, with several kernels developing independently in those regions where the mixture fraction is close to its most reactive value ξMR (as determined from homogeneous reactor calculations) and the scalar dissipation rate is low. Turbulence reduces the ignition delay time as it promotes mixing between air and the fuel vapor, eventually resulting in lower values of scalar dissipation. High values of the global equivalence ratio are responsible for a larger number of ignition kernels, due to the higher probability of finding regions where ξ=ξMR. Spray polydispersity results in the occurrence of ignition over a wider range of mixture fraction values. This is a consequence of the inhomogeneities in the mixing field that characterize these sprays, where poorly mixed rich spots are seen to alternate with leaner ones which are well-mixed. The DNS simulations presented in this work have also been used to assess the applicability of the Conditional Moment Closure (CMC) method to the simulation of spray combustion. CMC is found to be a valid method for capturing spray autoignition, although care should be taken in the modelling of the unclosed terms appearing in the CMC equations.

Direct numerical simulation and CMC (conditional moment closure) sub-model validation of spray combustion

Direct numerical simulation (DNS) is used to study spray combustion in a slab layer configuration. The gas phase is simulated in an Eulerian way, while the droplets are tracked with a Lagrangian method. The coupling between the two phases is considered. A fourth-order explicit Runge–Kutta method for time integration and an eighth-order central differencing scheme for spatial discretization are used to solve the fully compressible Navier–Stokes (N–S) equation system. One-step reaction mechanism for heptane is adopted. Several cases with different droplet diameters, global equivalence ratios and turbulence intensities are investigated. The general characteristics of spray combustion are described. It is shown that ignition is always found in low scalar dissipation rate regions. Equivalence ratio has the most significant effect on the evolution of spray combustion, while turbulence intensity only has a limited effect. The presumed beta PDF (probability density function) for the mixture fraction is studied, and excellent agreement is found for the predictions and the DNS results. It is demonstrated that the beta PDF is more accurate without evaporation. Conditional moment closure (CMC) sub-models are validated for the scalar dissipation rate, evaporation rate and reaction rate. The scalar dissipation rate is modeled using the amplitude mapping closure (AMC) model. The AMC model has the ability to predict the dissipation rate well except in the rich side. A linear model for evaporate rate captures the correlations of the conditional averaged evaporation rate and the mixture fraction very well for the stoichiometric and fuel-rich cases. Finally, the first-order closure for the reaction rate is examined. It is found that the predictions qualitatively agree with the DNS results at all the timings. The discrepancy is mainly attributed to ignition and extinction in the spray region.

Droplet/turbulence interaction and early flame kernel development in an autoigniting realistic dense spray

A very detailed evaporating/reacting spray simulation has been conducted extending our previous liquid atomization data in order to understand the physical processes leading to group combustion. The head region of a realistic dense Diesel jet spray is considered, and the droplets and diffusive layers are resolved. At this stage, the droplets have the relative velocity to the high-temperature air flow and this generates comparable-size turbulent eddies around the droplets. By these eddies, mixing is enhanced more strongly than molecular diffusion and merged vapor clouds are formed especially in the area of large droplet number density. This suggests that external group combustion is likely to occur and this is also confirmed by the estimation of group combustion number. This finding requires a modification to the point-source model so that the droplet-generated mixing enhancement effect should be more accurately included. A simple time scale analysis shows that final ignition occurs after the liquid jet head disappears and the resident time is sufficiently large enough.

The Structure of the Auto-Ignition Region of Turbulent Dilute Methanol Sprays Issuing in a Vitiated Co-flow

This paper presents planar imaging of laser induced fluorescence (LIF) from key reactive species in the auto-ignition region of dilute turbulent spray flames of methanol. High-speed (5 kHz) LIF-OH imaging as well as low speed (10 Hz) imaging of joint LIF-OH-CH2O is performed. The product of the OH and CH2O signals is used as a qualitative indicator of local heat release. The burner is kept intentionally simple to facilitate computations and the spray is formed upstream of the jet exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The studied flames are all lifted but differ in the shape of their leading edge and heat release zones. Similarities with auto-ignition of gaseous fuels, as well as differences, are noted here. Formaldehyde is detected earlier than OH implying that the former is a key precursor in the initiation of auto-ignition. Growing kernels of OH that are advected from upstream, close in on the jet centreline and ignite the main flame. The existence of double reaction zones in some flames may be due to ignitable mixtures formed subsequent to local evaporation of droplets and subsequent mixing. When air is used as spray carrier, reaction zones broaden with distance, possibly due to increased partial premixing and regions of intense heat release occur near the flame centreline further downstream. With nitrogen as carrier, the flame maintains a nominal diffusion-like structure with reaction zones of uniform width and substantially less concentration of heat release on the flame centreline.

External and Near-Nozzle Spray Characteristics of a Continuous Spray Atomized from a Nasal Spray Device

Computational fluid dynamics (CFD) simulations of nasal drug delivery often neglect the initial spray particle conditions (e.g., particle velocity) and instead introduce particles entrained by the inhaled airflow into the nasal cavity. The results presented in this study aim to provide some insight into spray droplet formation after atomization of a drug formulation from a nasal spray device. A commercial nasal spray device was tested under a constant flow in order to better understand its spray formation and characteristics. External characteristics such as the spray cone angle define the shape of the atomized spray that exits from the device, while the internal characteristics such as the droplet size distribution help to determine the likelihood of inertial impaction within the nasal cavity. The experimental method makes use of particle image velocimetry (PIV) and particle/droplet image analysis (PDIA) to obtain droplet diameters and spray velocities in different spray regions. Image processing techniq...

G709 L2Fによるディーゼル噴霧の液滴サイズと数密度の評価(GS7-3 流体一般,一般セッション)

A laser 2-focus velocimeter (L2F) was used for the measurements of the velocity and size of droplets in the dense region of diesel spray which was intermittently injected into the atmosphere. The diameter of the nozzle orifice was 0.113 mm. The injection pressure was set at 40 MPa by using a common rail system. The L2F had a micro-scale measuring volume which consisted of two foci. The distance between two foci was 17 p.m, and the focal diameter was about 3|am. The maximum data sampling rate of the L2F system wajs markedly high as 15 MHz. Measurements were conducted at 15mm and 20mm downstream from the nozzle. The number density was calculated at several specified times from the measured velocity, size and arrival time of droplets. The sizes of droplets were larger at the spray center and the spray periphery than at the spray off-center. The result shows a strong inverse correlation between the droplet size and droplet number density.

Modeling, Asymptotic Analysis, and Simulation of an Energy Tower

A new model for the description of an energy tower is presented. For the modeling of the transient flow in an energy tower and the related power production a one-dimensional approach is taken. Although many simplifying assumptions are imposed, this approach is thought to incorporate most of the main physical effects. The model is derived from the general one-dimensional Euler equations of gas dynamics for a compressible humid air gas mixture. Low Mach number asymptotics allow us to handle the related problems. Numerical simulations are performed. Optimal values for the spray rate and the height of the spraying region are identified. The model gives results which are in accordance with our physical understanding. For lack of an energy tower prototype, a quantitative comparison is not possible at the moment.