Prefilming Atomizer Research Articles

Experimental investigation on a novel external-mixing prefilming atomizer for advanced aircraft engines

As a crucial component of aircraft engine combustors, the external-mixing prefilming atomizer exhibits significant advantages in enhancing combustion efficiency and reducing pollutant emissions. However, the intricate relationship between spray characteristics, droplet breakup mechanisms, flow fields, and geometric parameters of the external-mixing prefilming atomizer remains poorly understood. This paper presents a novel external-mixing prefilming atomizer, aiming to address this knowledge gap. Additionally, particle image velocimetry (PIV), high-speed Schlieren photography, and particle image analysis (PIA) measuring systems were employed to gain insights into the spray characteristics and droplet breakup mechanism of the external-mixing prefilming atomizer under various swirl flow fields. The obtained results reveal the formation of two distinct regions, namely a low-density area and a high-density area. Notably, the number of droplets in the low-density area, located within 0–2 cm from the central axis, is significantly lower compared to the high-density area situated further away from the central axis. The breakup of droplets in the low-density region is primarily attributed to the inner swirl airflow, while the outer swirl airflow induces droplet breakup in the high-density region. As the Reynolds number of the incoming flow increases, the air-liquid ratio (ALR) and Weber number also increase, thereby enhancing the fuel breakup effect. The Schlieren results reveal a two-stage spray development process. The initial stage is characterized by the dominant momentum of the dense spray, with the spray tip penetration (STP) exhibiting a relatively slow increase. The aerodynamic force of airflow plays a crucial role in governing the breakup and transport of droplets, leading to a linear variation of STP over time. As the swirl number increases, there is an enhanced momentum exchange between droplets and airflow, resulting in accelerated spray penetration and droplet breakup.

Spray, flowfield, and combustion characteristics of an external mixing atomizer in a novel cavity-swirler-based combustor

This paper presents an investigation into the performance of two external mixing atomizers designed for a novel cavity-swirler-based combustor, with the aim of achieving high combustion efficiency and uniform outlet temperature distribution. Experimental and numerical analyses were conducted to study the spray characteristics of the atomizers, including the Sauter mean diameter (SMD) and spray angle. The flow field, combustion efficiency, outlet temperature distribution, and pattern factor of the two atomizers were examined under atmospheric pressure and a temperature of 473 K. The results show that the external mixing prefilming atomizer (case-1) exhibits larger SMD and spray angle compared to the external mixing pressure-swirl atomizer (case-2). Furthermore, case-1 demonstrates higher combustion efficiency than case-2 under pilot-only fueling conditions, which is attributed to the larger wake regions achieved by case-1. Interestingly, under pilot-main fueling conditions, case-2 achieves higher combustion efficiency than case-1. This is due to the smaller SMD and longer residence time achieved by case-2. Moreover, case-1 demonstrates more uniform outlet temperature distribution and smaller pattern factor than case-2. This is mainly attributed to the large-scale axial vortex generated by case-2, which significantly improves the mixing intensity between cavity and mainstream.

Experimental investigations on dynamic wave structures of a prefilming atomizer

An experimental study was performed on a planar prefilming airblast atomizer to investigate the dynamic wave behavior on a prefilmer. Using a high-speed camera, backlighting visualization and reflection imaging were applied to record the breakup process of the liquid film and the temporary evolution of wave structures. Image post-processing methods were used to obtain the film thickness, surface wave frequency, breakup length, and ligament breakup frequency. Depending on non-dimensional aerodynamic numbers, surface waves on the prefilmer were classified into three mechanisms. In addition, correlations were derived from experimental data and used to predict atomization characteristics. The deviations of surface wave frequency and breakup length were limited to ±10%. These predictive correlations can assist in resolving engineering issues regarding prefilming atomizers.

Effects of the structured lips on spray characteristics in prefilming airblast atomization

Nowadays, prefilming atomizers have been widely applied in aero-engine, industrial fields, and agricultural irrigation. In this study, atomization characteristics of the prefilming atomizers with structured lips have been investigated. The influence of eight structured lips on the spray breakup process has been measured using high-speed planner laser introduced fluorescence and particle/droplet image analysis in the planner prefilming air-blast atomizer. Synchronously, multi-directional visualization was used to measure hydrodynamic behavior. Rec-3 with a depth of 1 mm, a width of 1 mm, and a pitch of 0.5 mm has the smallest droplet size although it has medium breakup length and ligament velocity. For all studied structured prefilmers, rectangle and triangle structured lips performed good atomization effect than no structured lips except tri-1.

Lattice Boltzmann modeling of primary liquid film breakup at the prefilmer edge: Effects of surface wettability

In this paper, numerical predictions are performed for a planar prefilming atomizer using the lattice Boltzmann (LB) method. The main target is to inspect the effects of surface wettability on the primary liquid film breakup at the prefilmer edge. To this end, a phase-field-based multiple-relaxation-time LB model for two-phase flow is derived and verified for the present study, as well as the wetting boundary scheme. After that, the effects of surface wettability are assessed by specifying different contact angles during the simulations. Numerical results indicate that the whole primary breakup process of the liquid film can be roughly divided into three interactive stages: (i) liquid accumulation at the trailing edge; (ii) ligament formation and its flapping movements; (iii) ligament breakup and droplet formation. It is visible that the liquid accumulation and the following ligament breakup show similar periodic patterns with quite consistent frequencies. Besides, when the contact angle is increased, the volume of liquid accumulated at the prefilmer edge declines, the maximum flapping region of ligaments shrinks, the ligament breakup length reduces, but the ligament breakup frequency increases. Based on the above-mentioned results, it can be concluded that the prefilmer with hydrophobic surfaces benefits the primary liquid film breakup in the close vicinity of the prefilmer edge.

Air-Core–Liquid-Ring (ACLR) Atomization Part II: Influence of Process Parameters on the Stability of Internal Liquid Film Thickness and Resulting Spray Droplet Sizes

Air-core–liquid-ring (ACLR) atomization presents a specific type of internal mixing pneumatic atomization. It can be used for disintegration of high viscous feed liquids into small droplets at relatively low gas consumptions. However, the specific principle of ACLR atomization is still under research and no guidelines for process and atomizer design are available. Regarding literature on pre-filming atomizers, it can be hypothesized for ACLR atomization that the liquid film thickness inside the exit orifice of the atomizer, as well as the resulting spray droplet sizes decrease with increasing air-to-liquid ratio (ALR) and decreasing feed viscosity. In this study, the time dependent liquid film thickness inside the exit orifice of the atomizer was predicted by means of computational fluid dynamics (CFD) analysis. Results were compared to high speed video images and correlated to measured spray droplet sizes. In conclusion, the hypothesis could be validated by simulation and experimental data, however, at high viscosity and low ALR, periodic gas core breakups were detected in optical measurements. These breakups could not be predicted in CFD simulations, as the simplification of an incompressible gas phase was applied in order to reduce computational costs and time. Nevertheless, the presented methods show good potential for improvement of atomizer geometry and process design as well as for further investigation of the ACLR atomization principle.

Nonmonotonic Effects of Aerodynamic Force on Droplet Size of Prefilming Air-Blast Atomization

The goal of this investigation is to improve the understanding of the mechanism on prefilming atomization. The coaxial two-fluid prefilming air-blast atomization has two airflows: inner air jet and outer air jet. It is generally believed that droplet size decreases monotonically with increasing gas velocity in air-blast atomization. However, this work shows that in prefilming atomization the droplet size increases with the increase of gas velocity and then decreases with the increase of gas velocity when the velocity of the other airflow is high. The relationship between two air jets in prefilming air-blast atomization is mutual restriction rather than assistance. To investigate the nonmonotonic phenomenon, the modified Kelvin-Helmhotz and Rayleigh-Taylor instability atomization model for the prefilming air-blast nozzle is induced. This study presents the model developed to predict the performance of coaxial atomizers, and the experimental results are consistent with the model estimates.

Experimental Investigation of Spray and Combustion Performances of a Fuel-Staged Low Emission Combustor: Effects of Main Swirl Angle

In order to reduce NOx emissions, modern gas turbines are often equipped with lean-burn combustion systems, where the high-velocity fuel-lean conditions that limit NOx formation in combustors also inhibit the ability of ignition, high altitude relight, and lean combustion stability. To face these issues, internally staged scheme of fuel injection is proposed. Primary and main fuel staging enable fuel distribution control, and multi-injections of main fuel lead to a fast and efficient mixing. A fuel-staged low emission combustor in the framework of lean-burn combustion is developed in the present study, i.e., the central pilot stage for low power conditions is swirl-cup prefilming atomization, the main stage is jet-in-crossflow multi-injection, and a combination of primary and main stage injection is provided for higher power output conditions. In lean-burn combustors, the swirling main air naturally tends to entrain the pilot flame and quench it at low power conditions, which is adverse to the operability specifications, such as ignition, lean blow-out (LBO), and high-altitude relight. In order to investigate the effects of the main swirl angle on combustion performances, the ignition and LBO performances were evaluated in a single dome rectangular combustor. Furthermore, the spray patterns and flow field are characterized by kerosene-planar laser induced fluorescence and particle image velocimetry (PIV) to provide insight into spray and combustion performances. Flow–flow interactions between pilot and main air streams, spray–flow interactions between pilot spray and main air streams, and flame–flow interactions between pilot flame and main air streams are comprehensively analyzed. The entrainment of recirculating main air streams on pilot air streams enhances with the increase of main swirl angle, because of the upward motion and increasing width of main recirculation zone. A small part of droplets are entrained by the recirculating main air streams at periphery of combustor and a majority of droplets concentrate near the centerline of combustor, making that entrainment of recirculating main air streams on pilot spray and quenching effects of recirculating main air streams on pilot flame are slight, and the extinguishing effects can be ignored. The contributions of main swirl strength to improvement of ignition and LBO performances are due to enhancement of air/fuel mixing by strengthening turbulence level in pilot zone.

Experimental study on the influence of liquid and air boundary conditions on a planar air-blasted liquid sheet, Part II: prefilming zone length

In this work, experiments were performed on a prefilming atomizer. For this purpose, an injector used for liquid sheet pulverization was adapted to highlight the influence of the prefilming zone. Breakup length, oscillation frequency and drop size were studied for various prefilming zone lengths and airflow configurations (convergent or divergent and different air thicknesses). Various influences were observed depending on the air configuration, with an improvement of the atomization process for convergent configuration when an optimum prefilming zone length is chosen. At the same time, visualizations of liquid dynamics and liquid thickness measurements on the prefilming zone have enabled the classification of liquid evolution in three regimes: “smooth”, “waves” and “accumulation”. The use of primary atomization parameters enables a cartography gathering all of the experimental conditions studied during this work to be proposed. Finally, a comparison of these results with previous correlations highlights the necessity to take into account the geometrical aspect of injection systems to predict the atomization characteristics.

INFLUENCE OF SOME GEOMETRICAL PARAMETERS ON THE CHARACTERISTICS OF PREFILMING TWIN-FLUID ATOMIZATION

Geometries are considered to have a great influence on the spray characteristics of atomizers. In the present study, we studied a prefilming twin-fluid atomizer patented by Sadatomi and Kawahara (2012), in which liquid atomization is implemented by supplying compressed air alone into an internal mixing chamber, and water is automatically sucked by the negative pressure induced by an orifice. In the experiments, we studied spray characteristics influenced by the geometrical parameters, such as orifices in different opening area ratios and different shapes, porous rings with different porous diameters, and different atomizer sizes. Higher spray performance can be obtained by a small sized atomizer with a circular orifice in opening area ratio of 0.429 and a porous fiber ring with porosity of 25 μm. The present results provide a significant guidance for practical applications with different requirements of spray characteristics.

Preparation of Emodin‐Polyethylene Glycol Composite Microparticles Using a Supercritical Antisolvent Process

AbstractEmodin‐polyethylene glycol (PEG) composite microparticles were obtained from a dichloromethane‐methanol mixture via the solution‐enhanced dispersion by supercritical fluids through prefilming atomization (SEDS‐PA) process. Morphologies, particle sizes (PSs), and emodin contents of the composite microparticles were analyzed by scanning electron microscopy and UV‐visible spectrophotometry. The crystallinity change of emodin before and after the SEDS‐PA process was demonstrated by X‐ray powder diffraction (XRD). The composite microparticles present nubbly, rod‐like emodin dispersed in PEG or a nubbly, sheet emodin inlay on PEG, with PSs ranging between 3 and 12 μm. The PSs of the composite microparticles increase with the increase of temperature, decrease with the increase of pressure, and do not seem to depend on the emodin content of the initial solute and on the solution flow rate. The emodin contents of the composite microparticles increase with the increasing emodin content in the initial solute and temperature and decrease with increasing solution flow rate.

Mechanism of dispersing an active component into a polymeric carrier by the SEDS-PA process

Supercritical fluid anti-solvent precipitation has been attracting widespread attention due to its distinctive advantages, and has exhibited a great perspective of application in the production of polymer-based composite micro- and nanoparticles. In this study, based on the experiment results of production of carotene-loaded polymer PEG or l-PLA composite microparticles using solution enhanced dispersion by supercritical fluids through prefilming atomization (SEDS-PA) process, the possible mechanism of dispersing an active component in a polymeric carrier by the SEDS-PA co-precipitation was deduced. The mechanism is mainly the formation and growth of the active component (carotene) nuclei in the polymer-rich phase induced by mass transfer and phase transition, and the polymer capture/encapsulation of active component particles generated in an expanded solution droplet caused by the collision among these particles and polymer-rich phase. There are four factors that could influence the sizes and morphologies of the SEDS-PA precipitates. They are, respectively, atomization of solution, prompt and persistent super-saturation of the expanded droplets, breakup of the expanded droplets with interstices and the particle agglomeration caused by collision in the SEDS-PA process. The integrated effect of these factors dominates the sizes and morphologies of the SEDS precipitates.

Study on Precipitation of Astaxanthin in Supercritical Fluid

As health hazards of synthetic pigments are continuously found, more and more natural food pigments have been applied to the food industry. Astaxanthin is one of carotenoids that have been widely used in the food, cosmetic and pharmaceutical industries as natural colorants. In this paper, the micronization of astaxanthin in dichloromethane via the solution enhanced dispersion by supercritical fluids through prefilming atomization (SEDS-PA) process has been successfully performed. Morphologies and particle sizes (PSs) of the astaxanthin microparticles were analyzed by scanning electron microscopy (SEM). The results of micronization show that astaxanthin crystals with PSs about 20–30 µm have successfully been micronized to the microparticles with PSs about 0.5–6.0 µm by the SEDS-PA process. The effect of experimental variables on micronization of the astaxanthin has been studied. With the increase of initial solution concentration, the PSs of astaxanthin microparticles decrease initially, then increase. With the increase of solution flow rate and temperature, the PSs of astaxanthin microparticles increase. With the increase of pressure, the PSs of astaxanthin microparticles decrease. The nubbly particles were obtained in SEDS-PA process, and the crystallinity change of astaxanthin was showed by X-ray powder diffraction patterns (XRD).

Precipitation and Characterization of Chelerythrine Microparticles by the Supercritical Antisolvent Process

AbstractChelerythrine was successfully micronized from methanol solution using Supercritical Carbon Dioxide (SC‐CO2) as an antisolvent via the Solution Enhanced Dispersion by Supercritical Fluids through the Prefilming Atomization (SEDS‐PA) process. The morphology and particle size of the chelerythrine microparticles were visually analyzed by scanning electron microscopy (SEM). For the purpose of the optimizing operating conditions of the SEDS‐PA process, the influences of the experimental variables, i.e., temperature, pressure, solution flow rate and initial solution concentration, on the particle size and morphology of chelerythrine microparticles are discussed in detail. The results show that the best process conditions for the micronization of chelerythrine are: T = 313 K, P = 20 MPa, C = 2.0 g/L and F = 2.0 mL/min. The precipitates obtained under the optimized experimental conditions are short rod‐like chelerythrine microparticles with a mean particle size of 0.1–1 μm in width.

Micronization of the officinal component emodin via the SEDS process through prefilming atomization

AbstractThe micronization of emodin in a mixed solution of dichloromethane and methanol by the Solution Enhanced Dispersion by Supercritical Fluids through Prefilming Atomization (SEDS‐PA) process has been successfully performed. Morphologies and particle sizes (PSs) of the emodin microparticles were analyzed by scanning electron microscopy. The results of micronization show that the acicular or rod‐like emodin crystals with PSs of about 3‐10 µm wide and 100‐300 µm long have successfully been micronized to needle‐like, rod‐like, twisted fiber‐like and coalesced net‐like microparticles with PSs of about 0.1‐1.0μm wide and 2‐40μm long by the SEDS‐PA process. The effect of technological conditions on micronization has been studied. With the increase of initial solution concentration and temperature, the PSs of emodin microparticles increase. With the increase of solution flow rate and pressure, the PSs of emodin microparticles decrease. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of Carotene/Proanthocyanidin Composite Microparticles via the Solution-Enhanced Dispersion by Supercritical Fluids (SEDS) Process through Prefilming Atomization

The Solution-Enhanced Dispersion by Supercritical Fluids (SEDS) process is a promising technology for the production of ultrafine powders, composite microparticles, and other powders. Carotene/proanthocyanidin composite microparticles were obtained using supercritical CO2 (SC-CO2) as an antisolvent in a mixed solution of dichloromethane and ethanol. For the purpose of evaluating the efficiency of the supercritical fluid precipitation technologies in processing the composite microparticles and studying the effects of operating variables on the composite microparticle, the SEDS process through Prefilming Atomization (SEDS-PA) was used in this paper. Morphologies and particle sizes of the composite microparticles were analyzed by scanning electron microscopy and optical microscopy. The carotene concentrations in the composite microparticles were determined by spectrophotometry. The results show that the precipitates obtained under the experimental conditions are platelet-shaped carotene and proanthocyanidin ...

Production of natural carotene-dispersed polymer microparticles by SEDS-PA co-precipitation

In this work, composite microparticles of poly (ethylene glycol) (PEG) loaded with natural carotene were successfully produced using solution enhanced dispersion by supercritical fluids through prefilming atomization (SEDS-PA) process. The characteristics of dispersion of carotene in the polymeric drug carrier were analyzed by color difference between carotene and PEG using optical micrographs. The morphologies and particle sizes (PSs) of the composite microparticles were studied by SEM micrographs. The precipitates were amorphous particles of PEG embedded with flake-like natural carotene crystals. The carotene degradation of the carotene/PEG composite microparticles was substantially lessened under the protection of PEG carrier in comparison with that of carotene microparticles. With the increase of the carotene content added in solution, the PSs of carotene crystals dispersed into the PEG carrier increase, the carotene loading yields in the composite microparticles (Wc2) obviously increased, whereas PSs of these composite particles slightly decreased. Higher operating temperature resulted in formation of bigger coalesced composite particles and the substantial increase of Wc2. With increase of operating pressure PSs decreased, whereas no clear dependence of Wc2 was obtained on it.

Supercritical Antisolvent Micronization of Natural Carotene by the SEDS Process through Prefilming Atomization

Natural carotene is micronized by the SEDS process through prefilming atomization (SEDS−PA) with the aim of evaluating the efficiency of prefilming atomization, examining the variation of carotene purity induced by the SEDS−PA process and studying the influence of operating variables on the particle size (PS) of the carotene precipitates. The carotene/dichloromethane solution to be atomized is driven through a liquid distributor with spiral slots in the prefilming atomizer as a thin film swirling at 45°. At the exit of the atomizer, the atomizing supercritical CO2 (SC-CO2) stream impinges on the film at 45°. Through the impingement and the use of swirling, the annular solution film is disintegrated into fine drops, and the mixing of the SC-CO2 and solution is intensified. Compared to the process in which solution is driven through the inner capillary of the atomizer while SC-CO2 is driven through its annular passage, the SEDS−PA process can obtain smaller droplets and finer microparticles with a narrower particle size distribution (PSD). After micronization by the SEDS−PA process, the purity of carotene microparticles is increased for unprocessed carotene with purity ranging from 30% to 87%, but it decreased for carotene with purity higher than 87%. The CO2 flow rate, solution flow rate, solution concentration, and pressure have marked influences on the particle size (PS), and the PSD broadens with increasing PS. Mechanisms that control PS are explained in terms of liquid atomization, agglomeration of particles, volumetric expansion of solution in SC-CO2, and nucleation and growth processes of particles. The dependence of the PS on temperature is not clear from all of the experiments performed.

Micronization of the natural pigment-bixin by the SEDS process through prefilming atomization

Bixin was micronized from dichloromethane solution using Supercritical CO 2 (SC-CO 2) as antisolvent by the SEDS process through Prefilming Atomization (SEDS–PA) with the aim of evaluating the efficiency of the supercritical fluid precipitation technologies in processing natural pigments and studying the effects of operating variables in the SEDS–PA process. Morphologies and sizes of microparticles were analyzed using scanning electron microscopy. This study has shown that the sizes of the bixin microparticles could be substantially reduced by the SEDS–PA process. The precipitates obtained at all experimental conditions were needle-like or coalesced net-like particles. With the increase of SC-CO 2 flow rate the impingement of SC-CO 2 on the liquid film and the consequent mixing of SC-CO 2 and droplets are reinforced, producing smaller particles. For constant SC-CO 2 flow rate, with the increase of solution flow rate the impingement of SC-CO 2 on the liquid film becomes weak leading to formation of bigger particles. Higher solution concentration results in formation of bigger particles and increase of agglomeration. Sizes of bixin microparticles decrease with pressure. Serious agglomeration and coalescence phenomena at the high operating temperature result in precipitation of net-like products.

Precipitation of ephedrine by SEDS process using a specially designed prefilming atomizer

One of the key processes of SEDS precipitation is droplet formation due to jet break-up at the exit of an injection device. In this work, a prefilming atomizer was designed on the basis of the mechanisms of atomization and applied to the SEDS process to precipitate ephedrine from ethanol solution using supercritical CO 2 as antisolvent with the aim of evaluating the efficiency of the atomizer and studying the influence of operating variables (concentration, pressure, temperature, solution flow rate, and CO 2 flow rate) on sizes of the particles micronized. The solution to be atomized was driven through a coaxial annular passage with spiral slots in the atomizer as a thin film swirling with 45°. The atomizing CO 2 was driven through another passage (the inner capillary) to form a jet stream to impinge on the film at 45°. Upon violent interaction with jet streams, the solution sheet was effectively disintegrated into fine drops and the mixing of supercritical fluid (SF) and solution was intensified for increased transfer rates. Long needle-like or short rod-like uncoagulated particles were obtained by the SEDS process in a wide range of experimental conditions. The mechanisms that control particle sizes were explained in terms of liquid atomizing mechanism, nucleation, and growth processes of particles. Particle size did not seem to depend on pressure and temperature in all the experiments performed.