Pressure-swirl Atomizer Research Articles

Numerical investigation of the impact of fuel temperature on spray characteristics in a pressure-swirl atomizer with spiral path

Numerical investigation of the impact of fuel temperature on spray characteristics in a pressure-swirl atomizer with spiral path

An Improved Comprehensive Atomization Model for Pressure Swirl Atomizers

This study presents an improved comprehensive atomization model for a pressure swirl atomizer. The model integrates internal flow predictions, linear instability analysis of a swirling annular liquid sheet, primary atomization sub-model, and droplet velocity sub-model. Measurement data combined with the inviscid theory model predict the internal flow, providing liquid sheet velocity and thickness at the atomizer outlet. The dispersion relation of surface disturbances is obtained through linear instability analysis. A primary breakup predictive model for particle size distribution is constructed based on the wavelength and growth rate within the full unstable wavenumber range of the dispersion relation. Assuming uniform circumferential distribution and a normal distribution of spray angles, the droplet velocity is assigned according to the liquid sheet velocity. The model is implemented into Eulerian–Lagrangian simulations as initial conditions for discrete phase droplets to simulate the spray field. Results show the model can accurately predict the Sauter mean diameter with an error of less than 6% and effectively predicts the spray structure and spray cone angle. The dependency of the model on its parameters is also studied, determining that the values of the ligament constant and dispersion angle have an obvious impact on the prediction of Sauter mean diameter and spray structure.

Near-field spray characteristics and steadiness of a novel twin-fluid injector with enhanced primary atomization

The present study investigates the effect of the internal swirling atomizing air on the injector near-field spray characteristics and spray stability of a novel twin-fluid injector named swirl burst (SB) injector by incorporating an internal swirl. It involves primary atomization by internal bubbling and bubble bursting, and external secondary atomization by shear layer instabilities. A previous design integrated an external swirl and successfully enhanced the secondary atomization. It generated fine droplets immediately, rather than a typical jet core/film of conventional airblast or pressure swirl atomizers. It thus resulted in compact and ultra-clean lean-premixed combustion of distinct fuels, potentially enabling small-core fuel-flexible combustors. The current work aims to further enhance the primary atomization. The near-field flow patten and droplet size distribution and dynamics are investigated using high-speed laser-driven shadowgraph imaging accompanied by the internal bubble visualization. Results reveal that the internal swirl leads to more uniform, smaller and faster-moving bubbles that concentrate at the internal liquid tube tip regardless of the increased flow rates, generating ultra-stable and finer sprays with a wider working range, compared to the injector without the internal swirl. The frequency spectrum analysis of droplet sizes consistently substantiates the significantly improved spray steadiness, enhancing clean spray combustion stability.

Influence of Nozzle Geometry and Scale-Up on Oil Droplet Breakup in the Atomization Step during Spray Drying of Emulsions

Spray drying of oil-in-water emulsions is a widespread encapsulation technique. The oil droplet size (ODS) significantly impacts encapsulation efficiency and other powder properties. The ODS is commonly set to a specific value during homogenization, assuming that it remains unchanged throughout the process, which is often inaccurate. This study investigated the impact of atomizer geometry and nozzle dimensions on oil droplet breakup during atomization using pressure-swirl atomizers. Subject of the investigation were nozzles that differ in the way the liquid is set in motion, as well as different inlet port and outlet orifice dimensions. The results indicate that nozzle inlet port area may have a significant impact on oil droplet breakup, with x90,3 values of the oil droplet size distribution decreasing from 5.29 to 2.30 µm with a decrease of the inlet area from 2.0 to 0.6 mm. Good scalability of the findings from pilot to industrial-scale was shown using larger nozzles. A simplified theoretical model, aiming to predict the ODS as a function of calculated shear rates, showed reasonable agreement to the experimental data for different atomization pressures with coefficients of determination of up to 0.99. However, it was not able to predict the impact of different nozzle dimensions, most likely due to changes in flow characteristics. These results suggest that the stress history of the oil droplets might have a larger influence than expected. Further studies will need to consider other zones of high stress in addition to the outlet orifice.

A one-way coupling approach for simulating in-nozzle flow and spray characteristics of a pressure-swirl atomizer

This article proposes a model framework coupling in-nozzle flow and external spray and presents its application to the simulation of a commercial pressure-swirl atomizer, focusing on the transient characteristics of the internal flow and subsequently the impact on the spray characteristics. High-fidelity in-nozzle simulation of the liquid–gas interactions is performed using the volume-of-fluid (VOF) method. Then, a corresponding Lagrangian simulation of sprays is performed where the parcels are injected using the information from the VOF predictions instead of phenomenological models. Both the internal flow and the spray are compared to the experimental data that are available in the literature, and satisfactory agreement is obtained in terms of the in-nozzle velocity, film thickness, and Sauter mean diameter. The effect of the different liquid properties and geometric features on the air–core formation, and consequently, on the spray characteristics have been obtained directly through spray simulation coupled with nozzle flow. As indicated by the Eulerian simulation results, the viscosity plays a key role in the formation of the air core, as the hollow-cone shape can degenerate into a solid cylindrical liquid jet under high viscosity conditions. Additionally, significantly distinct spray characteristics in terms of droplet velocity, mean diameter, and penetration were predicted depending on the formation of air core. Even if there is no stable air core in the nozzle, the spray is still discharged in a swirling motion. As opposed to the converging angle and orifice length, the nozzle diameter has a direct correlation with the formation of air core and spray atomization. This study implies that the in-nozzle flow field, which is usually ignored in fuel spray simulation, has a substantial impact on the spray characteristics and should be taken into account for design optimization by applying the developed one-way coupling approach.

Experimental investigation on the impact of air flow rates and back pressures on kerosene microscopic spray characteristics discharged from an air-assisted pressure-swirl atomizer

The air-assisted pressure swirl atomizer has attracted considerable interest owing to its exceptional capacity to generate fine droplets and precisely regulate spray characteristics. The primary objective of the study was to investigate the impact of auxiliary air flow rate, orifice diameter, and ambient back pressure on the kerosene spray cone angle, droplet size and distribution. The results indicate that employing a nozzle with a smaller orifice size leads to a larger spray cone angle. The spray cone angles of the other two orifice sizes exhibit an increasing trend as the auxiliary air flow rate increases. Also, increasing the auxiliary air flow rate effectively reduces the Sauter Mean Diameter (SMD). Additionally, as the auxiliary air flow rate gradually increases, the peak of the particle size distribution shifts towards smaller diameters. Furthermore, as the environmental back pressure increases, there is a noticeable decrease in the SMD. With an increase in ambient backpressure, there is a consistent shift of the peak particle size towards smaller diameters. At an elevated ambient pressure of 1.4 MPa, an increase in the axial distance from the nozzle results in a decrease in the volume percentage of the peak diameter, while simultaneously causing the particle size distribution range to broaden.

THE EFFECTS OF SHAPE AND LIQUID PROPERTIES ON PRESSURE SWIRL ATOMIZER IN-NOZZLE FLOW

The internal flow in pressure swirl atomizers is numerically predicted by performing large eddy simulations and using a volume of fluid approach. The output of the numerical model is validated by comparing it with three databases of experimental measurements obtained on large-scale pressure swirl atomizers available in the open literature. A simplified analytical model previously developed by the authors, which relates the swirl intensity to the thickness of the fluid exiting the nozzle, is used to analyze the flow behavior in three pressure swirl atomizers, with large differences in the injector geometry, the operating conditions, and the fluid thermophysical properties. This simple relationship is found to hold for the three pressure swirl atomizers, with small changes of the parameter that accounts for energy losses, while data obtained with relatively small variations of the injector geometry are found to collapse on the same curve. The effects of operating conditions and fluid thermophysical properties on this relation are found to be irrelevant.

EXPERIMENTAL STUDY OF THE SPRAY PATTERN AND MERGER PROCESS OF DUAL-LAYER, ROTATING CONICAL LIQUID SHEETS

To obtain a comprehensive understanding of the primary breakup mechanism of dual-layer, rotating conical liquid sheets, the spray field of a dual-orifice, pressure-swirl atomizer is observed using high-speed shadowgraphy. The influences of the pressure drop in the primary and pilot flow channels and of the merger of the dual-layer liquid sheet on spray morphology, spray cone angle, and liquid sheet surface fluctuations are investigated on the basis of an analysis of spray field images. Attention is focused on the mechanisms underlying the behavior of the disturbance waves generated during the merger of the dual-layer liquid sheet. The results of this study reveal that the pressure drops in the primary and pilot flow channels affect the spray pattern, spray cone angle, and liquid sheet surface fluctuations. It is found that the influence of the pressure drop in the primary channel is dominant, and the changes in the liquid sheet surface fluctuations are related to the spray pattern. During liquid sheet merger (after the inner sheet has reached the expected spray cone angle), the spray cone angle of the outer sheet decreases, and only after the sheets are in contact with each other does the amplitude of the surface fluctuations become significantly larger and generate more medium- and high-frequency disturbance waves.

PLANAR TWO-PHOTON FLUORESCENCE IMAGING OF DENSE SPRAY TO ESTIMATE SPRAY CHARACTERISTICS: APPLICATION IN PRESSURE-SWIRL ATOMIZERS

The dense spray produced at the primary stage of atomization in a pressure-swirl atomizer is characterized in this work. The optically dense regime, from continuous liquid stream to first-step breakup into liquid structures, is acquired using a two-photon planar laser-induced fluorescence (2p-PLIF) technique. A notable advantage of 2p-PLIF over conventional PLIF is the attenuation of multiple scattering by simultaneous absorption of two photons in an ultra-short pulse duration. This approach is able to capture the complex interface morphology of spray structures. A curvature-based analysis of the near field is carried out to predict far-field spray characteristics. This methodology was recently introduced by Palanti et al. (2022) to investigate numerical simulation of atomizing liquid flows. The present work extends its application to experimental images. The atomization process is described through the curvature distribution in different regimes. The spray characteristics are predicted from the early stage of atomization and are reasonably comparable with those of direct measurement by phase Doppler anemometry (PDA) in the later stage of atomization. The present analysis shows how it is possible to obtain information about the dispersed phase of the spray in advance based on the dense spray curvature distribution.

Usability of a low-end 3D printer for prototyping of pressure-swirl atomizers

Usability of a low-end 3D printer for prototyping of pressure-swirl atomizers

Adaptation of conical liquid sheet and spray morphologies to cross-flowing gas

A variety of industrial devices use liquid injection into the traverse stream of air. Ambient flow changes the spray characteristics, which can alter a system's function. Two different spill-return pressure-swirl atomizers (SRA) were investigated under cross-flow. One of the tested atomizers generates a conical liquid sheet with shorter breakup lengths due to reduced internal air-core stability. Phase Doppler anemometer (PDA) was used for 2D velocity and droplet size measurements, along with high-speed visualization to record instantaneous spray behaviour. A wide range of operating parameters was tested, e.g., inlet pressure (pl) ranging from 0.25 MPa to 1 MPa, spill-to-feed ratio (SFR) from 0 to 0.9, and cross-flow velocity from 0 to 32 m/s. As the cross-flow velocity increases, the spray starts to tilt and the breakup length (lb) shortens as a result of increased aerodynamic forces. This reduces the differences between atomizers. Moreover, the droplet diameter depends on the lb, yet no semi-empirical model was found to accurately predict the droplet sizes. A map of breakup modes was established, and a transition point from long- to short-wave mode was identified. The Linear Stability Analysis of the liquid sheet breakup was used and compared with a semi-empirical approach for the first time for the SRA and cross-flow conditions. The results imply that considering only quiescent ambient conditions during atomizer development might not lead to the optimal design.

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.

Effects of secondary breakup, collision dynamics, gravity and evaporation on droplet size distribution in a pressure-swirl JET A-1 spray

Size and velocity of droplets in pressure-swirl sprays vary with distance downstream the nozzle. This study aims to explain the phenomena contributing to size-resolved spatial variation of the drop size distribution for Jet A-1 spray from a small pressure swirl atomizer injected into quiescent ambient air at atmospheric conditions. The effects of secondary breakup, droplet collisions, gravity, drag-driven spray dispersion, and evaporation are evaluated. Phase Doppler anemometer (PDA) was used to resolve the velocity and size of the droplets in radial profiles at several axial distances (Z) from the nozzle exit at injection pressures between 0.5 and 1.5 MPa. The droplet motion and collision dynamics were qualitatively characterised by high-speed imaging. The analysis focused on areas along 1) the spray axis and 2) the liquid sheet direction. The first area covers small droplets with marginal evolution, while the mean drop size in the second area significantly increases downstream. The local drop size distribution and its axial evolution results from a combined effect of ballistic filtering (drifting of small droplets from periphery to spray centre and large droplets vice versa), centrifugal and turbulent droplet dispersion, and droplet collisions (increasing drop size with distance). The relative droplet-gas velocity was found at investigated Z positions too small for drag-driven secondary droplet breakup to occur. Evaporation of Jet A-1 sprayed at room temperature and pressure into still atmosphere reduces the drop size marginally. Trajectory of all drop sizes is insignificantly altered by gravity. The smallest droplets are strongly drag-driven to the spray axis, while large ones disperse centrifugally and have sufficient momentum to neglect the gravity. Combining results from imaging and laser diagnostics, various collision outcomes were identified, with a conclusion that these depend on the size and velocity of colliding droplets. Coalescing collisions dominate and strongly increase the mean drop size downstream the spray at off-axis positions while insignificantly contribute along the spray axis. The drop size span factor reduces with Z as separative collisions between large droplets and mixed-type collisions between small and large droplets narrow the drop size distribution. The above effects can be most easily amplified for drop size reduction via modification of physical properties of sprayed liquid by its heating.

Characterization of Spray Field for Water-Emulsified Diesel Using a Pressure Swirl Atomizer Under a Nonreacting Environment

Abstract Employing a mixture or an emulsion of water and diesel fuel is considered a way to reduce gas emissions such as NOx and soot in a gas turbine. This study presents detailed experimental results on the spray characteristics of a water–diesel emulsion injected by a pressure swirl atomizer with a 90-degree spray angle and a flow number of 0.58 under a nonreacting environment at high pressure and temperature conditions. Acquiring these data is a key step when configuring a combustor that will employ emulsified fuels. In addition, this study seeks to confirm that the emulsion stays intact when it gets sprayed into the combustor. Furthermore, this study attempts to understand if a water–diesel emulsion prepared by a sonicator improves fuel atomization as compared to a water–diesel mixture prepared by a static mixer, i.e., not a proper emulsion. Tests are conducted in a high pressure and temperature testing facility at two ambient pressures and three ambient temperatures and the water to diesel ratio (W/D) is varied from 11% to 100% by mass. Phase Doppler Particle Anemometry (PDPA) is employed to measure the spray characteristics. Through a backlit high-speed photography, overall spray patterns over different test conditions are visualized. Mie-scattering and planar laser-induced fluorescence imaging are utilized to visualize the mixture field. In general, the results indicate that emulsion stays intact as it gets sprayed into the combustor; and emulsion is a better solution to reduce emissions than a statically mixed mixture.

Droplet characterization of a pressure-swirl atomizer by means of high-fidelity modelling based on DNS simulations

Liquid injection optimization is key to enhance the efficiency of many processes in a wide variety of fields. Combustion processes are one of the most challenging ones, due to the direct emissions and greenhouse gases implications. This work aims at studying the external two-phase flow produced when injecting fuel with a pressure-swirl atomizer in an operating condition typical of an academic burner. The main objective is to have a better comprehension of the atomization process in this type of nozzles, getting information on the droplets’ characteristic size and how they are produced and arranged. For that regard, a high-fidelity Direct Numerical Simulation (DNS) is used to analyse the very near field of the spray produced, where primary atomization takes place. Pre-processing tasks are explained in terms of geometry comprehension, computational domain and mesh selection, boundary conditions and main simulation setup parameters. Then, through post-processing tasks, both quantitative and qualitative results are extracted, which will serve to validate the modelling against previous works and to provide novel data about the atomization process in pressure-swirl atomizers. Results show that smaller droplets predominate over bigger ones as expected, since just the first millimetres of the spray are modelled. However, there is a clear trend of droplet’s size growing when increasing both axial and radial distances, indicating coalescence in regions relatively far from the nozzle. The achieved results, together with results from simulating the injection event with other fuels or at other operating conditions, can be used to develop a phenomenological model able to predict how atomization is going to be as a function of the non-dimensional Reynolds and Weber numbers that could be implemented in lower-resolution RANS and LES codes for modelling atomization. This investigation proves that it is possible to faithfully predict the near field of these sprays through DNS simulation, getting similar trends to those of the experimental data, and that the study through numerical models is necessary in the investigation process. The information they can bring, together with the experimental knowledge, can make a good synergy that will eventually lead to a better understanding of this type of atomizers.

Study of Internal Flow in Open-End and Closed Pressure-Swirl Atomizers with Variation of Geometrical Parameters

This study delves into the examination of internal flow characteristics within closed (with nozzles) and open-end pressure-swirl atomizers (lacking nozzles). The number of inlet channels “n” and the opening parameter “C” were manipulated in this study, as they play a pivotal role in understanding various atomizer attributes, such as uniformity of the air-core diameter, the discharge coefficient, spray angle, and more, all of which hold significance in the design of bipropellant atomizers for liquid rocket engines (LREs). To validate our findings, six distinct hexahedral meshes were generated using Ansys ICEM software 2023. Subsequently, we employed Ansys Fluent, considering the RNG k-ε turbulence model and the VOF (volume-of-fluid) multiphase model to identify the liquid–gas interface, to aid in analyzing the uniformity of the air core, which is directly linked to the even distribution of mass, the mixing ratio of propellants, combustion efficiency, and stability. The results indicate that the uniformity of the air core is not solely contingent on an increase in parameter “n” but is also influenced by an increase in the parameter “C”. It is worth noting that the key dimensions of these six atomizers were determined using a mathematical model based on Abramovich and Kliachko theories.

Propagation behaviors of kerosene-fueled rotating detonation wave with varied atomizer locations

In this study, we tested a rotating detonation engine (RDE) fueled by liquid kerosene. A scheme of circumferential array atomization with an injection throat upstream of the annular combustor was used to realize the kerosene-fueled rotating detonation. The propagation characteristics of rotating detonation waves (RDWs) were analyzed by varying the distance between the pressure-swirl atomizers and the injection throat. The dynamic pressure signals and the visualized high-speed images were captured in the combustor. When the fuel atomizers were close to the injection throat, an unstable detonation mode appeared during the operation of the combustor. There was a periodical extinction and reinitiation of detonation waves in the combustor because the wave formed in the combustor could not propagate self-sustainably. In the fuel-lean tests, the rotating detonation degenerated to the combustion mode intermittently. Subsequently, unstable detonation was detected during the recovery stage from combustion to detonation mode. As the equivalence ratio increased, the rotating detonation process exhibited better continuity, but the duration of unstable detonation also increased. The continuity and stability of the rotating detonation process could be significantly improved by lengthening a certain distance between the atomizers and the injection throat. The experimental results indicated an optimal location of the fuel atomizers for stable operation, which could serve as a reference for the design of the injection structure of liquid-fueled RDEs.

Design of a Cryogenic Duplex Pressure-Swirl Atomizer through CFDs for the Cold Conservation of Marine Products

Design of a Cryogenic Duplex Pressure-Swirl Atomizer through CFDs for the Cold Conservation of Marine Products

Simulation of Carbon Monoxide Emission during Combustion of a Liquid Fuel Injected by a Pressure Swirl Atomizer into the Combustion Chamber

Simulation of Carbon Monoxide Emission during Combustion of a Liquid Fuel Injected by a Pressure Swirl Atomizer into the Combustion Chamber

Perforation induced ligament evolution in low-temperature swirl spray via off-axis holography

Ligaments generated from a perforated sheet are commonly observed in the primary breakup process and have a fundamental influence on spray characteristics. Ignition and combustion instabilities of aeroengines at high altitudes raise the demand for dynamic measurement of ligaments in a low-temperature spray. Picosecond pulsed digital off-axis holography (DOH) system with 65 kHz high-speed and 4.5μm equivalent pixel size high-resolution setups is established. Three-dimensional (3D) evolution of ligaments in a pressure swirl spray under fuel temperature of 245–250 K and fuel pressure of 0.2–0.8 MPa is characterized. The stringy object extraction method optimized by FMCs cross correlation algorithm successfully extracts a 3D arc-shaped ligament in the spray at 0.2 MPa. The transition of ligaments into droplets during two modes of sheet perforation as hole–edge and two-hole interactions at 0.5 MPa is quantified with time resolution up to 15.4μs. Both the nucleation and growth of perforations are found involved with surface waves, and their early diameter growth velocities remain stable in the short term, consistent with Taylor–Culick velocity characteristics. The 3D velocity of a pair of ligaments in the spray at 0.5 MPa is obtained based on the spatial displacement of their daughter droplets within double frames, indicating a downward helical movement trend after separation from the conical swirling sheet. This work demonstrates DOH a powerful tool for 3D morphology extraction and dynamic analysis of ligaments and perforations in the pressure swirl atomization scenario.