Spray Droplet Size Distribution Research Articles

Characterization of the urea-water spray impingement in diesel selective catalytic reduction systems

Exhaust after-treatment selective catalytic reduction (SCR) systems based on urea-water solution are state-of-the-art technologies mitigating NOx emissions for diesel and lean combustion systems. Major challenges for implementing the systems are high NOx reduction performance, uniform mixture formation and solid deposits formation. This study presents a detailed analysis of the urea-water spray wall impingement and influences on reducing agent distribution and deposit formation, thus system performance. High speed images provide detailed information of the impingement process. Moreover, impinging spray mass flux distribution and droplet size distribution have been quantified under typical diesel exhaust flow conditions. The work has been performed with a commercial 3-Hole pressure-driven injector dosing into a flow channel.Under all tested conditions, the impingement is significant. At gas flow conditions of 300°C, 200kg/h, 35.6% of the injected fluid impinges on the opposed wall due to entrainment and evaporation. The entrainment level has been found to scale logarithmically with the gas flow momentum, related correlations are provided. Having an integrated analysis of a mechanical patternator and non-intrusive Phase Doppler Anemometry (PDA) results, it is concluded that droplets below 20μm are completely entrained or evaporated. The impingement rate is gradually increasing with increasing droplet diameter up to 90μm, while almost all larger droplets reach the opposed wall. High speed imaging shows in detail liquid film formation, film transport, liquid accumulation, nucleate boiling, urea crystallization and melting, as well as thin film evaporation prior to solid deposit formation. The spray impingement leads to liquid film formation. Relevant dimensions have been evaluated with digital image processing. Direct relationship of liquid accumulation to the solid deposit formation has been identified. The permanent solid deposit is consisting of cyanuric acid, biuret, ammelide and ammeline as identified by nuclear magnetic resonance spectroscopy.

Multiscale simulations and experiments on water jet atomization

Numerical simulation of primary atomization at high Reynolds number is still a challenging problem. In this work a multiscale approach for the numerical simulation of liquid jet primary atomization is applied, using an Eulerian-Lagrangian coupling. In this approach, an Eulerian volume of fluid (VOF) method, where the Reynolds stresses are closed by a Reynolds stress model is applied to model the global spreading of the liquid jet. The formation of the micro-scale droplets, which are usually smaller than the grid spacing in the computational domain, is modelled by an energy-based sub-grid model. Where the disruptive forces (turbulence and surface pressure) of turbulent eddies near the surface of the jet overcome the capillary forces, droplets are released with the local properties of the corresponding eddies. The dynamics of the generated droplets are modelled using Lagrangian particle tracking (LPT). A numerical coupling between the Eulerian and Lagrangian frames is then established via source terms in conservation equations. As a follow-up study to our investigation in Saeedipour et al. (2016a), the present paper aims at modelling drop formation from liquid jets at high Reynolds numbers in the atomization regime and validating the simulation results against in-house experiments. For this purpose, phase-Doppler anemometry (PDA) was used to measure the droplet size and velocity distributions in sprays produced by water jet breakup at different Reynolds numbers in the atomization regime. The spray properties, such as droplet size spectra, local and global Sauter-mean drop sizes and velocity distributions obtained from the simulations are compared with experiment at various locations with very good agreement.

Modulation of spray droplet number density and size distribution by an acoustic field

Multiple interactions may occur when a poly-disperse spray is exposed to an acoustic field. In the context of spray combustion instabilities, acoustic agglomeration, the formation of a droplet number density wave and the modulation of the droplet size distribution are interesting effects. A droplet number density wave, i.e. preferential concentration of droplets in space, may result from size-dependent, one-way momentum coupling between the acoustic field and the spray. The modulation of the droplet size distribution, which has been evidenced in the experimental work of Gurubaran and Sujith (AIAA 2008-1046), is thus a consequence of the droplet number density wave formation. In the present work, the mechanisms that produce these effects are simulated and analyzed in depth by means of computational fluid dynamics. The spray is modeled with both Lagrangian (particles mass-point approach) and Eulerian (continuous phase approach) descriptions. The particular Eulerian method used is a variant of the presumed density function method of moments, which allows to account for the effects of poly-dispersity, in particular the size-dependence of particle velocity. Both the Lagrangian and Eulerian models are validated against experimental data for spray dynamics and spray response to an acoustic field.

The instability and droplet size distribution of liquid-liquid coaxial swirling spray: An experimental investigation

An experimental study is performed to study the effect of inner and outer injection pressure on spray characteristics and the detailed structure of interacted liquid sheets of a coaxial swirl injector. A high speed camera is used to visualize the transition merging process of the coaxial swirling liquid sheet. A laser reflection system is set up to analyze the wave frequency of liquid sheet. A Malvern RTSizer is used to measure the droplet size of the coaxial spray. It is found that the structure of the interacted spray is considerably different from the individual spray, the interaction between the inner and outer liquid film promoted the breakup. At the same injection pressure, the temporal instability of a liquid sheet does not change spatially. For both individual and coaxial spray, the wave frequency increase as the injection pressure increase. The individual inner spray is more unstable and easier to breakup. Furthermore, the relationship between the surface instability and the droplet size distribution of the coaxial liquid spray is investigated quantitatively. For both individual spray and coaxial sprays, the Sauter Mean Diameter (SMD) decrease as the inner injection pressure increase. The inner spray has a higher weighing factor that influences the SMD of the coaxial spray. For coaxial spray at low inner injection pressure, the liquid film thickness would be the dominate factor to the SMD.

Characteristics of spray from a GDI fuel injector for naphtha and surrogate fuels

Characterization of the spray angle, penetration, and droplet size distribution is important to analyze the spray and atomization quality. In this paper, the spray structure development and atomization characterization of two naphtha fuels, namely light naphtha (LN) and whole naphtha (WN) and two reference fuel surrogates, i.e. toluene primary reference fuel (TPRF) and primary reference fuel (PRF) were investigated using a gasoline direct injection (GDI) fuel injector. The experimental setup included a fuel injection system, a high-speed imaging system, and a droplet size measurement system. Spray images were taken by using a high-speed camera for spray angle and penetration analysis. Sauter mean diameter, Dv(10), Dv(50), Dv(90), and particle size distribution were measured using a laser diffraction technique. Results show that the injection process is very consistent for different runs and the time averaged spray angles during the measuring period are 103.45°, 102.84°, 102.46° and 107.61° for LN, WN, TPRF and PRF, respectively. The spray front remains relatively flat during the early stage of the fuel injection process. The peak penetration velocities are 80m/s, 75m/s, 75m/s and 79m/s for LN, WN, TPRF and PRF, respectively. Then velocities decrease until the end of the injection and stay relatively stable. The transient particle size and the time-averaged particle size were also analyzed and discussed. The concentration weighted average value generally shows higher values than the arithmetic average results. The average data for WN is usually the second smallest except for Dv90, of which WN is the biggest. Generally the arithmetic average particle sizes of PRF are usually the smallest, and the sizes does not change much with the measuring locations. For droplet size distribution results, LN and WN show bimodal distributions for all the locations while TPRF and PRF shows both bimodal and single peak distribution patterns. The results imply that droplet size distribution is skewed to the larger side for locations close the axis and is skewed to the smaller side for distance away from the axis.

Droplet size and velocity distributions of wave-impact sea spray over a marine vessel

The spatial distribution of droplets in a spray cloud created by wave-impact sea spray and the distribution of their sizes and velocities over a vessel deck are investigated. Existing mono-size and mono-velocity models of sea spray are not accurate enough for modelling marine icing phenomena. Wave-impact sea spray creates numerous droplets in front of and around a vessel. Droplets are the result of sheet and droplet breakup of sea water. The velocity-size dependence of the resultant droplets causes the creation of various sizes and velocities of droplets. A droplet trajectory method employs the velocity-size dependence of the droplets to find their spatial distributions in the cloud of spray over the vessel deck. Drag and body forces overcome the initial velocities of the droplets, and consequently, they follow the wind and gravitational directions. The motion of the droplets affects the shape and extent of the spray cloud over the vessel. A numerical scheme is used to find the distribution of sizes and velocities of the droplets over a vessel. Results show that neither the smallest nor the largest droplets reach the maximum height. The medium-sized droplets can reach the maximum height of the spray cloud. As the spray cloud travels over the deck, the droplet velocities become almost the same. Comparing the numerical results with field observations shows that the predicted results are consistent and have reasonable agreement with the field measurements.

Application of maximum entropy principle for estimation of droplet-size distribution using internal flow analysis of a swirl injector

The maximum entropy principle is one of the first methods, which have been used to predict droplet size and velocity distributions of liquid sprays. Due to some drawbacks in this model, the predicted results do not match well with the experimental data. This paper presents a different approach for improving the maximum entropy principle model. It is suggested to improve the available energy source in the maximum entropy principle model equation by numerical solution of flow inside the injector based on the computational fluid dynamics technique. This will enhance the calculation accuracy of the turbulent kinetic energy of the output spray. Application of this procedure enhances the model predictions. The liquid sheet properties resulted from the analysis are also applied for calculation of the momentum source in the maximum entropy principle model. The proposed model is applied to predict the droplet size distribution of a hollow-cone spray formed by a swirl injector. The results show a better agreement with the available experimental data than the results of prior models.

Droplet trajectories of wave-impact sea spray on a marine vessel

Marine icing phenomena are strongly dependent on the rate of water impact to marine vessels. The most important source of this incoming water is wave-impact sea spray. There is limited understanding of droplet size and velocity distributions of wave-impact sea spray. Initial distributions of the size and velocity of droplets are crucial for the calculation of the droplet path and consequently for determining the water impact to every individual place on marine vessels. This paper develops a new model of wave-impact sea spray using a distribution of the size and velocity of droplets at the edge of the vessel. The concepts of water-sheet breakup and droplet breakup lead to an inverse dependence between the size and velocity of droplets after the breakup process. Droplets take different paths and form a spray cloud in front of the vessel. The liquid water content in front of the vessel can be calculated by considering the arrangement, sizes, and velocities of a set of droplets in the spray cloud. The response of the droplet trajectory model to various initial conditions with different sets of droplet sizes and velocities is examined. The numerical results are compared to real data from field observations. Droplet sizes are inversely proportional to droplet velocities, as verified by liquid water content data obtained by the field observations. This paper proposes the use of this inverse relationship based on physics of the breakup process as the initial data for calculating the wave-impact sea spray trajectory in front of a vessel.

Impacts of nozzle geometry on spray combustion of high pressure common rail injectors in a constant volume combustion chamber

Diesel engine performance and emissions are closely related to fuel atomization and spray processes, which in turn are strongly influenced by nozzle geometry. In this study, five kinds of single-hole cylindrical injectors which have different orifice diameters (0.13–0.23mm) and lengths (0.7–1.0mm) were employed to research the effects of the nozzle geometry on spray droplet size distribution and corresponding combustion characteristics. The spray droplet size spatial distribution was measured with the Phase Doppler Particle Analyzer (PDPA). The results show that the Sauter Mean Diameter (SMD) reduces with the increase of the distance from injector tip and the SMD of the central axis is bigger than that of the periphery. With the increase of the injection pressure (40–120MPa), the spray SMD decreases significantly. In addition, as the orifice diameter goes smaller, the SMD decreases and the effect of the orifice diameter on the spray SMD becomes weak. Meanwhile, as the orifice length goes longer, the SMD decreases when the orifice diameter is 0.13mm. And then, the combustion characteristics were experimentally investigated in a constant volume chamber with optical access. Time-resolved images of the natural luminosities (indicator of soot) from the combustion process were captured by high speed camera and combustion pressure was also acquired. It is found that there is a good corresponding relationship between spray SMD, combustion heat release rate and flame luminosity. That is to say, the way to decrease SMD reduces greatly the natural luminosities and improves combustion heat release rate. This article presents the effects of nozzle geometry on droplet size distribution and combustion, and provides important references for injector manufacture.

Measurement of the droplets sizes of a flash boiling spray using an improved extended glare point velocimetry and sizing

An improved extended glare point velocimetry and sizing (EGPVS) is proposed to investigate the droplets sizes of a flash boiling spray. When a spherical droplet with a relative refractive index from 1.16 to 1.41 is illuminated by two opposite laser sheets and a charge-coupled device camera is used to collect the s-polarization light at an observation angle of 90°, the intensities of the reflected lights are much stronger than the other order scattering lights. If the intensity of incident laser is controlled appropriately, two glare points from the reflected lights for the droplet are formed at the focused plane, while the intensities of the other order scattering lights are too weak to form any glare points. Then, the droplet diameter can be derived from the distance between the two glare points. In addition, the focused image is relative small, making it possible to measure dense spray. First, the characteristics of the improved EGPVS are discussed, and a series of standard particles are measured for validating this technique. Then, the technique is applied to investigate the droplets sizes of flash boiling spray. It is found that the minimum measurable diameter of droplets is 7.1 μm, and the relative error is less than 4.7 %. The droplet size distributions of spray are different at different stages. The Sauter mean diameter (SMD) of gasoline spray decreases gradually as the fuel temperature increases, which is different from that of a single-component fuel with a sharp decrease in SMD at the flash boiling stage.

A MARKER-BASED EULERIAN-LAGRANGIAN METHOD FOR MULTIPHASE FLOW WITH SUPERSONIC COMBUSTION APPLICATIONS

The atomization of liquid fuel is a kind of intricate dynamic process from continuous phase to discrete phase. Procedures of fuel spray in supersonic flow are modeled with an Eulerian-Lagrangian computational fluid dynamics methodology. The method combines two distinct techniques and develops an integrated numerical simulation method to simulate the atomization processes. The traditional finite volume method based on stationary (Eulerian) Cartesian grid is used to resolve the flow field, and multi-component Navier-Stokes equations are adopted in present work, with accounting for the mass exchange and heat transfer occupied by vaporization process. The marker-based moving (Lagrangian) grid is utilized to depict the behavior of atomized liquid sprays injected into a gaseous environment, and discrete droplet model 13 is adopted. To verify the current approach, the proposed method is applied to simulate processes of liquid atomization in supersonic cross flow. Three classic breakup models, TAB model, wave model and K-H/R-T hybrid model, are discussed. The numerical results are compared with multiple perspectives quantitatively, including spray penetration height and droplet size distribution. In addition, the complex flow field structures induced by the presence of liquid spray are illustrated and discussed. It is validated that the maker-based Eulerian-Lagrangian method is effective and reliable.

주거용 스프링클러 분무의 액적크기 분포에 관한 실험적 연구

주거용 스프링클러 분무의 액적크기 분포에 관한 실험적 연구

Experimental and numerical assessment of impingement and mixing of urea–water sprays for nitric oxide reduction in diesel exhaust

This study presents a joint experimental and numerical investigation of a spray used in selective catalytic NOx reduction (SCR) after-treatment systems for Diesel engine exhaust gases. The focus lies on the impingement conditions and distribution and mixing mechanisms of the spray in cross-flowing exhaust gases. These aspects are vital to system performance but still not well described.A pressure-driven SCR injector is characterized in an optically accessible flow test rig at different temperatures and mass flow rates representative of Diesel engine exhaust gas conditions by means of Mie scattering and Phase Doppler Anemometry (PDA). The effects of cross-flow velocity and temperature on spray structure, droplet size and velocity distributions are assessed resulting in a comprehensive characterization of the spray. Results show that the dense spray core, with droplets up to 200μm, only moderately reduces in density as smaller droplets are entrained at high exhaust flows. Wall impingement conditions however vary substantially as impingement angles are shallower at higher cross-flow velocities.A detailed assessment of the numerical model is presented and validation is carried out at different measurement locations of interest. The predicted droplet size distributions and velocities follow the observed trends and impingement angles as well as spray film areas on the channel floor are also in agreement with the experimental data. The validated model is subsequently used to numerically study the mixing dynamics.The findings suggest that at low cross-flow conditions two counter-rotating kidney vortices are formed which entrain reflected droplets of the spray impinging on the channel floor, which leads to improved mixing. Vapor concentrations increase close to the side walls and reflected droplets lead to film-formation on the side walls. At higher cross-flow velocities, vortex formation is not evident and spray-wall interaction is less pronounced – impairing mixing and leading to a reduction in concentration uniformity at down-stream locations. Film formation and high vapor concentrations are restricted to the bottom channel centerline.

Analysis on the atomization characteristics of aviation biofuel discharging from simplex swirl atomizer

The atomization characteristics of aviation biofuel discharging from a simplex swirl atomizer into quiescent atmospheric air are studied. The aviation biofuel is a mixture of 90% commercially available camelina-derived biofuel and 10% VonSol-53 (aromatics). The experiments are conducted in a spray test facility at varying fuel flow rate conditions. The measured characteristics include atomizer flow number, spray cone angle, breakup length of liquid sheet, wavelength of undulations on liquid sheet, and spray droplet size. The characteristics of biofuel sheet breakup are deduced from the captured images of biofuel spray. The measurements of spray droplet size distribution are obtained using Spraytec. The experimentally measured characteristics of the biofuel sheet breakup are compared with the predictions obtained from the liquid film breakup model proposed by Senecal et al. (1999). The measurements of wavelength and breakup length of the biofuel sheet discharging from the simplex swirl atomizer agree well with the model predictions. The model-predicted droplet size for the biofuel spray is significantly higher than the experimentally measured Sauter mean diameter (SMD). The spray droplets formed from the liquid sheet breakup undergo secondary atomization until 35–45mm from the atomizer exit and thereafter the SMD increases downstream due to the combined effect of fuel evaporation and droplet coalescence. A good comparison is observed between the experimentally measured SMD of the biofuel spray and the predictions obtained using the empirical correlation reported in literature for sprays discharging from simplex swirl atomizers.

On the role of physiochemical properties on evaporation behavior of DISI biofuel sprays

Biofuels and alternative fuels are increasingly being blended to conventional gasoline fuel to reduce the overall CO2 emissions. The effect on NOx and soot formation is still unclear as the atomization and evaporation of gasoline with biocomponents differ depending on fuel specific physiochemical properties. This work focuses on describing the biofuel evaporation behavior of gasoline sprays at homogeneous charge (early injection timing) and stratified-charge conditions (late injection timing mode) used in modern direct injection spark ignition engines (DISI). A spray plume of a 6-hole solenoid injector is analyzed in terms of liquid spray propagation, and local droplet sizes studied in an injection chamber. Depending on the operating conditions, different physiochemical properties are found to dominate the atomization and evaporation processes: For low and moderate ambient temperature and pressure, high-boiling point components show a strong influence on the spray droplet size distribution. However, at elevated temperature and pressure, the evaporation behavior changes completely. Due to a high degree of evaporation, the evaporation cooling effect dominates the local droplet sizes. Fuel mixtures owing a larger heat of vaporization show larger droplet sizes—even if these fuels have a lower boiling point. Depending on the local evaporation behavior, the different remaining droplet momentum in the spray controls the air entrainment and the subsequent progress of evaporation and mixing. Overall, it can be stated that the heat of vaporization is a dominating physiochemical property for the droplet evaporation rate at high-level supercharged conditions.

Spray drying behaviour and functionality of emulsions with β-lactoglobulin/pectin interfacial complexes

Aim of the present study was to investigate the impact of atomization and drying on the functionality of emulsions with a bilayered oil–water interface consisting of a globular protein (β-lactoglobulin, bLG) and anionic polysaccharides (pectins with varying degree of methoxylation). With regard to the atomization process, the emulsion spray droplet size generally decreased with increasing atomization energy. The spray droplet size distribution was narrower with rotary atomization compared to two-fluid nozzle atomization. The single droplet drying behaviour of the differently stabilized emulsions was similar as examined by acoustic levitation. With more than 95%, microencapsulation efficiency was high in all spray-dried particles. However, a shift in oil droplet size upon reconstitution indicated that oil droplet coalescence occurred within the process which was less pronounced in bilayer emulsions compared to the bLG-stabilised single layer emulsion. Data from interfacial viscoelasticity measurements showed distinct differences, which may explain oil droplet coalescence. The oxidative stability of encapsulated oil was influenced by both the physical state of the emulsions and the different constituents at the o/w-interface with bLG and low methoxylated pectin giving the best protection of the oil.

An experimental investigation on seawater SO2 scrubbing for marine application

A seawater spray scrubber was set up and tested with focus to desulfurization of marine engine exhaust gas. Experimental tests were carried out simulating typical exhaust conditions of a marine diesel engine burning heavy fuel oil and using real seawater. Different flue gas flow rates, seawater flow rates, and SO2 concentrations were tested. Liquid‐to‐gas ratios were in the range 1–10 × 10−3. A comparison between seawater and distilled water was also carried out to elucidate the effect of seawater alkalinity. In all conditions, the liquid phase was analyzed for alkalinity, pH, and sulfate content before and after the tests. Finally, the spray droplet size distribution was measured as a function of liquid flow rate.Experimental results indicated that seawater performed better than distilled water, by exploiting its inherent alkalinity. The desulfurization performance was improved by increasing the liquid flow rate and the gas residence time and by decreasing the SO2 concentration. SO2 capture efficiencies up to 93% were measured under the present operating conditions. The experimental data were further compared with calculations performed with an available model for SO2 absorption in fall‐down droplets within a simple plug‐flow reactor. Model calculations were carried out by using all parameters' values as measured in the present experimental campaign. Model results well fit the experimental data, even if a slight underestimation of the desulfurization efficiency is reported. The likely reasons are the assumed plug flow of gas as well as the neglected contribution to SO2 capture by water film falling at the wall. © 2012 American Institute of Chemical Engineers Environ Prog, 32: 1179–1186, 2013

Effects of fuel droplet size distribution on soot formation in spray flames formed in a laminar counterflow

The effects of fuel droplet size distribution of the fuel spray on the soot formation of spray flames, which is stabilized in a laminar counterflow field, were investigated numerically and experimentally. To investigate the effects of droplet size distribution of fuel spray on soot formation, fuel sprays with quasi-mono-dispersed droplet size distribution (QM-DSD) spray and poly-dispersed droplet size distribution (P-DSD) spray were generated using a frequency-tunable vibratory orifice atomizer. The differences in the spray flame structures and soot formation characteristics between QM-DSD and P-DSD were examined in detail using an experiment with two-dimensional laser induced incandescence of soot and using three-dimensional direct numerical simulations (3D-DNS) employing a CIP method. Results showed that the soot formation area in the cases of P-DSD decreased compared to the cases of the QM-DSD under the same Sauter mean diameter (SMD) condition. This tendency was caused by differences of the flame structures. The results of the fuel and oxygen concentration and temperature obtained from the 3D-DNS indicated that the portion of the premixed-like flame structure increased in the case of P-DSD. As a result, the combustion reaction region of the spray flames is enlarged in axis direction in the case of P-DSD in the counterflow field.

Spray characterization of gasoline-ethanol blends from a multi-hole port fuel injector

This work reports the measured spray structure and droplet size distributions of ethanol-gasoline blends for a low-pressure, multi-hole, port fuel injector (PFI). This study presents previously unavailable data for this class of injectors which are widely used in automotive applications. Specifically, gasoline, ethanol, and gasoline-ethanol blends containing 10%, 20% and 50% ethanol were studied using laser backlight imaging, and particle/droplet image analysis (PDIA) techniques. The fuel mass injected, spray structure and tip penetrations, droplet size distributions, and Sauter mean diameter were determined for the blends, at two different injection pressures. Results indicate that the gasoline and ethanol sprays have similar characteristics in terms of spray progression and droplet sizes in spite of the large difference in viscosity. It appears that the complex mode of atomization utilized in these injectors involving interaction of multiple fuel jets is fairly insensitive to the fuel viscosity over a range of values. This result has interesting ramifications for existing gasoline fuel systems which need to handle blends and even pure ethanol, which is one of the renewable fuels of the future.

Modeling and Simulation of Evaporating Spray Flows with Coalescence in an Eulerian Framework

AbstractThe study concerns the modeling and simulation of gas‐droplet and droplet‐droplet interactions on spray dynamics and droplet size distribution in turbulent spray flows where the spray is described in an Eulerian framework. The primary focus is to develop a suitable mathematical model that can describe the underlying physics including the polydispersion of the spray, evaporation, drag, coalescence, and interaction with the convective gas flow. This is achieved by applying the direct quadrature method of moments, which is extended to account for evaporation, drag force and coalescence by implementing well‐established standard models available in literature. The model is developed and formulated in three‐dimensional physical space, whereas the computations are carried out in a one‐dimensional geometric configuration since the reference experimental setup is only one‐dimensional. A water spray carried by nitrogen was injected through a hollow cone nozzle into a cylindrical spray chamber. The results reveal that evaporation and coalescence have a significant effect on the droplet size distribution whereas the droplet velocity is influenced by drag force as anticipated. Computational and experimental results agree very well when coalescence is considered.