Spray Characterization and CFD Simulation Workflow Analysis of Industrial Air-assist Nozzles in the Field of SCR Application

Spray and mixing characteristics of liquid jet in a tubular gas–liquid atomization mixer

Pressure swirling atomizers are wildly used in the fuel injection systems of aero-engines, marine engines, vehicle gasoline direct injection engines and gas turbines, et al. Considering about a pressure swirling atomizer liquid jet, the correlation of breakup droplet size and velocity distributions of an annular swirling viscous liquid sheet is studied. Joint probability density function of droplet size and velocity distribution of an annular swirling viscous liquid sheet is reduced based on maximum entropy method. The correlation between droplet size distribution and droplet velocity distribution are then discussed. Results show that with the right form of joint probability density function, the conservation laws of mass, momentum and energy must be included together as the constraint conditions. Droplet size and velocity distributions are closely related. And the liquid sheet swirling strength does not affect the structure of joint probability density function a lot, but the liquid swirling strength affect the distribution region to some extent.

Ammonia preparation from urea-water solutions is a key feature to ensure an effective reduction of nitrogen oxides in selective catalytic reduction (SCR) systems. Thereby, air-assisted nozzles provide fine sprays, which enhance ammonia homogenization. In the present study, a methodology was developed to model the spray formation by means of computational fluid dynamics (CFD) for this type of atomizer. Experimental validation data was generated in an optically accessible hot gas test bench using a shadowgraphy setup providing droplet velocities and size distributions at designated positions inside the duct. An adaption of the turbulence model was performed in order to correct the dispersion of the turbulent gas jet. The spray modeling in the near nozzle region is based on an experimentally determined droplet spectrum in combination with the WAVE breakup model. This methodology was applied due to the fact that the emerging two-phase flow will immediately disintegrate into a fine spray downstream the nozzle exit, which is also known from cavitating diesel nozzles. The suitability of this approach was validated against the radial velocity and droplet size distributions at the first measurement position downstream the nozzle. In addition, the simulation results serve as a basis for the investigation of turbulent dispersion phenomena and evaporation inside the spray.

A methodology based on high-speed photography has been developed to measure local droplet size and velocity distributions across the entire flow domain of a continuous, quasi-steady flat-fan water spray. The motivation is to complement phase Doppler anemometry (PDA) point measurements, which are limited to the far-field, dilute region of sprays. The nozzle was systematically shifted to capture a series of non-overlapping images of the spray, which were subsequently 'stitched' to form composite images. To reconstruct distributions along the stiches, four grids were used, each offset vertically and/or horizontally. Image analysis was employed to quantitatively evaluate the primary breakup zone and reconstruct droplet size and velocity distributions at all locations. Results showed that droplet sizes and velocities decrease downstream, with larger droplets and higher velocities observed at the periphery, attributed to rim tearing. Droplet sizes ranged from 10 to 250 μm, with velocities between 6 and 24 m/s. The local droplet size distributions were found to follow a log-normal distribution, consistent with previous observations. The local velocity distributions within the main spray region exhibited a Gaussian profile, whereas those along the spray periphery displayed a negatively skewed distribution, varying from monomodal to bimodal. At lower flow rates, the breakup zone contracted and shifted further from the nozzle. Rim tearing became dominant, leading to increased polydispersity and velocity fluctuations along the periphery.

A theoretical formulation based on maximum entropy principles is presented to predict the droplet size and velocity distributions of sprays in an isothermal environment. The joint droplet distribution function is derived subject to the constraints of mass flow rate, momentum flux, and two modes of energy fluxes (kinetic and surface). A simpler model, which reduces the number of constraints by three, is derived by choosing an adequate velocity integration range. This maximum entropy principle spray model is tested by comparing the calculated distributions with experimental measurements presented by the authors for a hollow cone, non-swirl spray nozzle and the experimental results obtained by other researchers for hollow cone, swirl spray nozzles. For a specific droplet size, the droplet velocity distribution is Gaussian. The droplet size distribution is much more complicated; three types of distributions may occur-positively skewed mono-modal, uniform size (in the limit approaching a delta function), and bi-modal. This study is concerned mainly with the bi-modal size distribution.

This study addresses the effect of nanofluid synthesis on the rheological properties of the resulting fluid and their consequent effect on the characteristics (size and velocity distribution of droplets, spray cone angle, etc.) of the sprayed nanofluids. The results are discussed in the light of how the spray characteristics affect the use of the resulting nanofluid spray for cooling purposes. Nanoparticles of alumina (Al2O3) and zinc oxide (ZnO) are mixed in water-based solutions, for concentrations varying between 0.5% and 2 mass% for alumina and between 0.01% and 0.1 mass% for the zinc oxide particles. FeCl2·4H2O (0.1 mass%) was also used to infer on the effect of the nature (material) of the particles in the physicochemical properties of the resulting solutions. Among the various surfactants tested, citric acid (0.15%) was chosen for the final working mixtures, as it assured a stable behaviour of the solutions prepared during the entire study. The nanoparticles were characterized in detail, and the physicochemical properties of the fluid were measured before and after atomization, to evaluate any possible particle loss in the liquid feeding system or retention in the atomizer. The nanofluids were sprayed using a pressure-swirl atomizer at 0.5 MPa injection pressure. Droplet size and velocity in the spray were probed using phase Doppler anemometry. For the range of experimental conditions covered here, the results show that liquid viscosity is an important parameter in predetermining the spray characteristics of nanofluids, as it affects the primary liquid breakup. Despite this, only a mild increase is observed in the nanofluids viscosity, mainly for higher concentrations of alumina, which was not sufficient to significantly affect the spray characteristics, except for a small decrease in the spray cone angle and the size of the atomized droplets. Hence, for cooling purposes, the atomization mechanisms are not compromised by the addition of the nanoparticles and their using is beneficial, as they enhance the thermal properties without a significant deterioration of other fluid properties such as viscosity and spray characteristics. Present spray characteristics promote liquid adhesion to the cooling surfaces and droplet size and velocity are kept within a range that is appropriate for spray cooling, following the literature recommendations and our analysis.

Selective Catalytic Reduction (SCR) technology is extensively employed across power plants, diesel vehicles, marine engines, and industrial applications to mitigate NOx emissions, particularly in hydrogen and ammonia combustion systems. The urea injector nozzle's atomization performance critically impacts NOx conversion efficiency and urea crystallization. Pressure-swirl nozzles have gained prominence in SCR systems due to their structural simplicity, precise flow control, and superior atomization characteristics, producing hollow-cone spray patterns. This study introduces a novel Droplet Tracking Velocimetry (DTV) technique combining circular Hough transform with Voronoi tracking to analyze droplet size and velocity distributions in hollow-cone sprays. Comparative analysis with Phase Doppler Interferometry (PDI) and Malvern Laser Particle Size Analyzer revealed measurement variations due to differing accuracies and ranges. While PDI offers precise single-point diameter measurements, it fails to capture spatial spray heterogeneity. Similarly, for droplets smaller than 80 µm, the Malvern particle size analyzer faces several challenges, including light blocking, sample volume, and droplet uniformity. Additionally, the laser diffraction method tests the entire optical path, which passes through the entire hollow cone, leading to significant deviations. The DTV method demonstrated superior capability in simultaneously measuring droplet velocity and size distributions within defined ranges. Axial and radial measurements revealed distinct spray characteristics: Sauter Mean Diameter (SMD) showed progressive axial uniformity but significant radial growth, attributable to interactions between high-speed large droplets and low-speed small droplets. These findings provide crucial insights into swirl nozzle spray dynamics, supporting optimized SCR system design for enhanced NOx reduction performance.

Droplet size measurement: I. Effect of three independent variables on droplet size distribution and spray angle from a pneumatic nozzle

Novel atomizer concept for CCS applications: Impinging effervescent atomizer

Initial droplet distributions at the liquid core are examined for various Weber number and pulsing conditions. While experimental investigation in the liquid core region is nearly impossible due to difficulty in the optical access to the region, the distribution at the region is investigated computationally, and typical droplet distributions are identified. It is found that the Nukiyama–Tanasawa and log-normal distributions can best describe the droplet size and velocity distributions, respectively. By comparing computational results obtained at the liquid core (0<x<8mm) and experimental data collected at x=48mm, it is suspected that the droplet gradation occurs immediately after droplets are separated from the liquid core. Thus, the distribution shape changes rapidly in both axial and radial directions. Such droplet–gradation behavior is numerically confirmed when the Nukiyama–Tanasawa droplet size distribution is used as an initial condition for the stochastic separated flow model. When the jet velocity is increased, the width of the droplet-size distribution becomes narrower, while the droplet velocity distribution becomes broader. Possible physical mechanism for that behavior is discussed in detail. Pulsing injection prominently influences the external spray shape near the nozzle exit. However, the overall droplet size and velocity distributions of the liquid core due to the pulsing injection are relatively insignificant for a turbulent spray in the atomization regime.

<p>Emulsions are the basis for many commercial products such as foodstuffs and paint due in part to their highly tunable flow properties. It is qualitatively understood that factors such as the dispersed phase droplet size and size distribution should affect how an emulsion flows because they influence how droplets can deform or pack. Since standard emulsification techniques such as blending and homogenization cannot produce emulsions with well-defined size distributions, little work has been done to, in particular, quantitatively determine the influence of droplet size distribution on emulsion flow properties. Consequently, in this investigation we have probed how the droplet size distribution affects emulsion flow properties by using model monodisperse emulsion systems with narrow, controllable droplet size distributions. Using a microfluidic flow focusing device, dodecane-in-water emulsions with diameters between 50 to 100 m with polydispersities less than 5% were produced, as characterized by pulsed field gradient nuclear magnetic resonance and optical microscopy. Due to the relatively large size of the droplets, it was only possible to examine the creamed phase of the emulsion. Samples of known polydispersity were made by mixing known quantities of two monodisperse emulsions. The monodisperse and bimodal emulsions were then subjected to rotational and oscillatory shear flow using a controlled stress rheometer to determine the effects of droplet size and size distribution on emulsion flow properties. Rotational and oscillatory rheological experiments showed that the monodisperse emulsions had two distinct behaviours: foam-like with appreciable thixotropy and yield stresses as well as emulsion-like with no evident thixotropy. The transition between these two behaviours appears to happen at a critical droplet radius between 33 and 37 micrometres. The rheological properties of the bimodal emulsions was split into three distinct behaviours. In samples that could be considered a matrix of large droplets perturbed by smaller droplets, the flow properties were similar to those of the constituent emulsion with the larger droplets. Increasing the number fraction of smaller droplets to a 1:1 ratio creates an entirely new phase with significantly reduced elastic properties. Surprisingly, when the emulsion primarily consists of small droplets, the flow properties are most similar to that of the large droplets. Additionally, despite the microstructural differences, all emulsions showed flow characteristics typical of soft glassy materials above the glass transition temperature. These results demonstrate the significant influence of microstructure on emulsion rheology, where altering the droplet size or polydispersity essentially creates a new phase with its own unique flow properties that is not simply a combination of the properties of the individual monodisperse components that make up the sample</p>

<p>Emulsions are the basis for many commercial products such as foodstuffs and paint due in part to their highly tunable flow properties. It is qualitatively understood that factors such as the dispersed phase droplet size and size distribution should affect how an emulsion flows because they influence how droplets can deform or pack. Since standard emulsification techniques such as blending and homogenization cannot produce emulsions with well-defined size distributions, little work has been done to, in particular, quantitatively determine the influence of droplet size distribution on emulsion flow properties. Consequently, in this investigation we have probed how the droplet size distribution affects emulsion flow properties by using model monodisperse emulsion systems with narrow, controllable droplet size distributions. Using a microfluidic flow focusing device, dodecane-in-water emulsions with diameters between 50 to 100 m with polydispersities less than 5% were produced, as characterized by pulsed field gradient nuclear magnetic resonance and optical microscopy. Due to the relatively large size of the droplets, it was only possible to examine the creamed phase of the emulsion. Samples of known polydispersity were made by mixing known quantities of two monodisperse emulsions. The monodisperse and bimodal emulsions were then subjected to rotational and oscillatory shear flow using a controlled stress rheometer to determine the effects of droplet size and size distribution on emulsion flow properties. Rotational and oscillatory rheological experiments showed that the monodisperse emulsions had two distinct behaviours: foam-like with appreciable thixotropy and yield stresses as well as emulsion-like with no evident thixotropy. The transition between these two behaviours appears to happen at a critical droplet radius between 33 and 37 micrometres. The rheological properties of the bimodal emulsions was split into three distinct behaviours. In samples that could be considered a matrix of large droplets perturbed by smaller droplets, the flow properties were similar to those of the constituent emulsion with the larger droplets. Increasing the number fraction of smaller droplets to a 1:1 ratio creates an entirely new phase with significantly reduced elastic properties. Surprisingly, when the emulsion primarily consists of small droplets, the flow properties are most similar to that of the large droplets. Additionally, despite the microstructural differences, all emulsions showed flow characteristics typical of soft glassy materials above the glass transition temperature. These results demonstrate the significant influence of microstructure on emulsion rheology, where altering the droplet size or polydispersity essentially creates a new phase with its own unique flow properties that is not simply a combination of the properties of the individual monodisperse components that make up the sample</p>

Macroscopic spray characteristics of a fuel injection system in an internal combustion (IC) engine have a direct impact on engine performance, emissions, and combustion characteristics. Nonintrusive in-cylinder measurements provide insights into the spray formation process for greater understanding of fuel-air mixing and combustion processes in an IC engine. In this paper, there are two parts: (a) procedure and methodology to configure the Artium phase Doppler interferometer (PDI) for in situ measurements through a cylindrical window and (b) comparative macroscopic spray characteristics in a firing Gasoline Direct Injection (GDI) optical engine to a constant volume spray chamber (CVSC) for spray droplet size-velocity distributions. Binned average velocity and average Sauter mean diameter of spray droplets in a firing engine were compared with that of a CVSC. Probability density function of droplet diameters in the CVSC under ambient conditions and in the engine combustion chamber provides an insight into the comparative droplet size distributions and droplet dynamics. Discussion on challenges encountered during PDI measurements in the firing engine environment, safety protocols, and tools required is also included. In addition, shadowgraphy images have been used to discuss the details on spray boundaries and spray evolution. The droplet size distribution inside the engine combustion chamber was found to be significantly different from the one observed in the CVSC. An engine simulation model can be developed/validated by using the data reported in this manuscript for attaining superior accuracy in the model. This paper describes the comparisons of the spray droplet size and velocity distributions in a CVSC and in situ for a working GDI engine. Maximum spray droplet velocity components (Vx, Vy) under engine combustion chamber conditions were 29.8 m/s, 14.2 m/s whereas the corresponding velocities in the CVSC under ambient conditions they were 78.41 m/s, 23.92 m/s, respectively, showing a large difference between the traditional measurements in the CVSC simulating engine conditions, and actual firing engine conditions. This study also reports the very first attempt in the open literature to measure spray droplet size and velocity distribution measurements in a firing IC engine.

A predictive model for droplet size and velocity distributions of a pressure swirl atomizer has been proposed based on the maximum entropy formalism (MEF). The constraint conditions of the MEF model include the conservation laws of mass, momentum, and energy. The effects of liquid swirling strength, Weber number, gas-to-liquid axial velocity ratio and gas-to-liquid density ratio on the droplet size and velocity distributions of a pressure swirl atomizer are investigated. Results show that model based on maximum entropy formalism works well to predict droplet size and velocity distributions under different spray conditions. Liquid swirling strength, Weber number, gas-to-liquid axial velocity ratio and gas-to-liquid density ratio have different effects on droplet size and velocity distributions of a pressure swirl atomizer.

A flashing phenomenon is often met in liquid propulsion of safety fields in industrial environments. This violent evaporation occurs when a liquid finds itself suddenly in a thermodynamic non-equilibrium and becomes superheated. To investigate theoretically the source processes and validate models for design and safety assessments, knowledge of accurate and reliable data such as distribution of droplet size, velocity and temperature in the closest field of flashing occurrence is mandatory. In this present work, an experimental study is undertaken in order to characterize the two-phase jet after a sudden accidental release and aims to quantify the effects of initial conditions such as initial storage pressure, temperature, geometrical effects of the release points etc on the spray characteristics. To fulfil this goal, a laser-based optical technique like Phase Doppler Anemometry (PDA) is used to obtain information for particle diameter and velocity evolution in this harsh environment. Cases for different initial pressures, temperatures and orifice diameters are studied and the droplet size and velocity evolution are presented in function of initial parameters.