Pressure-swirl Atomizer Research Articles

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.

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

SWIRLING FLOW AND BREAKUP CHARACTERISTICS AT HIGH REYNOLDS NUMBER IN A PRESSURE-SWIRL ATOMIZER FOR ARTIFICIAL SNOWMAKING

The pressure-swirl atomizer is used to provide atomized droplets, which is the main atomization component of the snowmaker. The formation of snowflakes is significantly affected by the effective collision between droplets and ice nucleus produced by ice nucleus generator. Whether the collision position moves forward or lags behind, it will both cause deterioration of the snow quality. Therefore, the flow and breakup process, breakup length, and variation law of atomization are very crucial for regulating the snow quality. The fully developed swirling flow at high Reynolds number in the pressure-swirl atomizer for outdoor snowmakers were investigated based on VOF-LES approach, and its accuracy was verified by experimental results. It was found that the Kelvin-Helmholtz instability caused by the velocity difference at gas-liquid interface decisively leads to the liquid film fragmentation. The primary breakup occurs after 5.4 ~ 5.8 times orifice diameter. As the injection pressure increases from 0.6 MPa to 1.4 MPa, the breakup length reduces by 7.7%, the jet velocity increases by 66.7%. It is critical to adjust the effective collision position and enhance the convection, so as to improve the quality of snow.

MODELING INTERNAL FLOW AND PRIMARY ATOMIZATION IN A SIMPLEX PRESSURE-SWIRL ATOMIZER

Numerical simulations of simplex pressure-swirl atomizers can aid their design process toward better atomization. This work aims at studying the two-phase flow at both the internal geometry and the first millimeters of the external domain of such atomizers, where primary breakup takes place. In particular, the atomizer under study has been used in the CORIA Rouen Spray Burner (CRSB) test rig, which aims at studying lean premixed turbulent combustion. Ultimately, our goal is to complete the spray characterization in the vicinity of the injector. Such data will potentially enforce the validity of numerical simulations of nonreacting and reacting flow for this burner. Injection characteristics are analyzed through an interface capturing method within a detailed numerical simulations framework. The importance of the internal flow simulation on the final result is demonstrated in the manuscript, but it requires accurately measuring the injector internal geometry. In the present investigation, an experimental methodology combining different techniques is applied to this end, obtaining and parameterizing the actual geometry of the internal ducts within the atomizer. The numerical workflow is divided in two simulations to separately study the internal flow formation and the external spray development. This division is proposed given the difficulty to mesh the whole computational domain handling all the present length scales while still preserving the required accuracy. Several mesh refinements are studied for each simulation, also analyzing the coupling between the related internal and external simulations. The methodology is validated against experimental data for two CRSB operating conditions. The investigation then proves it is possible to couple the internal and external flow in order to describe the actual air core formation, liquid film behavior and breakup mechanism of these atomizers, extracting relevant atomization outputs in the near-field region where experimental data are scarce.

Effect of fabrication method and surface roughness on spray characteristics for small pressure-swirl atomizers

The paper investigates the effect of the fabrication method, precision and surface roughness on atomization nozzle performance. A comprehensive review of the available knowledge on the topic is an integral part and an indispensable prerequisite of the study. It focuses on the requirements for atomizing nozzles and their production from the point of view of surface quality, dimensional and shape accuracy. The published results on the influence of surface quality and roughness on internal flow and atomizer performance are discussed. Available atomizer manufacturing techniques are reviewed with a focus on Rapid prototyping methods, accuracy and geometrical imperfections.Classical Machining and two 3D printing techniques were applied to produce small pressure-swirl atomizers. They were fabricated in a number of versions. The machined nozzles featured several specific defects. Spray quality measurements of these atomizers, at selected operation regimes, were made on a cold test bench using PDA and mechanical patternation to evaluate discharge coefficient (CD), Sauter mean diameter, spray cone angle, nozzle efficiency (ηn), breakup length and liquid distribution in the spray.The results show the important and systematic effects of surface quality and manufacturing imperfections on the atomizer characteristics. The exit orifice imperfections harm the circumferential uniformity and other spray characteristics. The effect becomes significant when the exit orifice shifted out of the nozzle axis by >2 % of the chamber diameter, and the orifice edge is chamfered by >20 % of the orifice length. The swirl chamber surface modified by scratches has a minor effect.Replacement of machining with current commercial 3D printing techniques, namely Selective laser melting and PolyJet technique, is associated with increased surface roughness. It worsens spray quality, liquid distribution uniformity and ηn and increases the CD when relative surface roughness exceeds 0.007.The result presentation using dimensionless criteria allows for generalisation of the outcomes to other atomizer types and scales.

Breakup of swirling films issued from a pressure-swirl atomizer

Applications of pressure-swirl atomizers are found in liquid rockets, gas turbines, internal combustion engines, agriculture, food drying, pharmaceutical industry, etc. Despite extensive experimental and numerical studies, understanding of their breakup mechanism is still incomplete. Herein, we present a new theoretical breakup model of a swirling film/sheet issued from a pressure-swirl atomizer based on the quasi-one-dimensional equations of the dynamics of free swirling inviscid liquid films surrounded by viscous gas. These equations are supplemented by a novel model of the hole formation in the swirling film due to turbulent pulsations in the surrounding gas. After nucleation, the hole increase in size is driven by surface tension according to the Taylor-Culick mechanism that ultimately results in a complete breakup of an intact film and formation of a network of ligaments which eventually breakup due to the capillary instability. In particular, if the number of the holes is sufficiently high that their area locally reaches 50 % of the surface area of the liquid sheet (i.e., the ratio of holes approaches to η = 0.5), a complete breakup of the swirling sheet occurs, according to the percolation theory. Based on the above-mentioned model, the diverging shape, thickness, spreading angle, longitudinal and swirling velocities in the swirling liquid films, as well as their breakup lengths are predicted for different values of the governing dimensional parameters and the corresponding dimensionless groups. In particular, it is predicted that increasing the exit longitudinal velocity (Vτ,e), or the exit nozzle radius (Re), or thinning the exit film thickness (he) results in shorter breakup lengths. Reducing the liquid surface tension also reduced the breakup length. In addition, an increase in gas kinematic viscosity (νg) or density (ρg) reduced the breakup length.

VALIDATION OF Σ−YLIQ ATOMIZATION MODEL IN PRESSURE SWIRL ATOMIZER

In many combustion and agricultural applications atomizers are used to increase the surface area of the liquid to ensure high rates of mixing and improve evaporation. The most common, simple, and reliable atomizer is the pressure swirl atomizer. This atomizer is said to have quality and effective atomization compared to others and induces swirling motion to the liquid and gives a hollow cone spray with air core as it emerges from the exit orifice. To enhance the understanding and prediction of the atomizing characteristics various atomization models are used and need to be investigated experimentally. This paper presents a validation of the Σ−Yliq atomization model of two-phase flow in a pressure swirl atomizer using commercial CFD star-cd code and laser-diffraction based drop measurements. To obtain the best results for the droplet mean diameter between the prediction and the experiment in terms of turbulence different k-e models were evaluated. The results show that the computational predictions of Sauter Mean Diameter (SMD) for the model have a good agreement with most of the experimental measurements in the radial positions when standard k-ɛ turbulence was used. However, more divergence was observed between the predictions and the experimental measurements when the RNG and Realizable k-ɛ turbulence models were used in the predictions. It was also observed that the model has good agreement with the mean droplet measurements on the spray centreline and radial axis with a percentage error of less than five percent.

Aeroponic systems design: considerations and challenges

Controlled Environment Agriculture (CEA) holds promise as a way to intensify current agricultural production systems while limiting pressures on land, water, and energy resources. However, its use has not yet been widely adopted, in part because the engineering design considerations and associated challenges are not well known. This is even more apparent for aeroponics, where the additional cost and complexities in controlling atomization have yet to establish an advantage in scale over simpler hydroponic systems To shed light on these considerations and challenges, an instrumented aeroponic system was prototyped with the goal of creating a quantitative model of growth for various species of leafy greens. As the first consideration, pressure swirl atomizers were paired with a diaphragm-type pressure tank to supply the necessary pressures needed for effective atomization. Secondly, nutrient solution was mixed on-demand from Reverse Osmosis (RO) water and concentrated nutrient stock then pumped into the pressure tank using a positive displacement pump. A bamboo-based substrate that allowed both germination and extended vegetative growth was supported on a stainless steel mesh and PVC frame acting as a grow tray. Finally, a camera microservice platform was developed to quantify plant growth using a computer vision pixel-based segmentation method.

Computational fluid dynamics characterization of the hollow-cone atomization: Newtonian and non-Newtonian spray comparison

Disintegration of liquid masses in a free-surface flow is still an open question in the field of small-scale spray applications such as dispensing of detergents or sanitizing products. In this context, the pressure-swirl atomizer is widely investigated. It allows to improve several spray characteristics through the formation and breakup of a conical liquid sheet that results in the well-known hollow-cone atomization. From this perspective, the characterization of a small-scale pressure-swirl spray under laminar flow conditions is the focus of this work. The configuration of the device and the physical properties of the discharged liquid are the key parameters that modify the attributes of such multiscale flow. In this regard, the entire picture of the fragmentation process is structured into multiple stages: internal nozzle flow, outer displacement of the liquid–gas interface, droplet spread into the atmosphere, and droplet-wall interactions on a collection surface. Through the computational fluid dynamics, we analyze the influence of the main fluid/packaging parameters on the hollow-cone spray properties, and on the whole atomization process. Reynolds and Ohnesorge numbers are coupled with the Sauter mean diameter to distinguish different breakup mechanisms and spray performances. The solution of the entire spray system is performed by implementing the volume-of-fluid-to-discrete-phase-model, which allows to capture the liquid–gas interface displacement and track the droplets produced downstream the primary atomization, simultaneously. With this Eulerian–Lagrangian hybrid model, we link key features of the hollow-cone spray process to spray pattern and droplet size distribution for both Newtonian and non-Newtonian fluid properties.

Study on inlet pressure and Reynolds number in pressure-swirl atomizer with spiral path

Studies on pressure-swirl atomizers have mainly focused on pressure-swirl atomizers with tangential input, however research on pressure-swirl atomizers with spiral path have been very limited and only the effect of some key parameters on the main properties of spray has been studied. In this research, using experimental methods and numerical details, detailed investigation has been carried out on pressure-swirl atomizers with spiral paths to better understand the effects of changes in the inlet pressure and Reynolds number on a variety of parameters of spray. In summary, the results indicated that by increasing the inlet pressure from 2.6 bar to 9.3 bar, the discharge coefficient, fluid film thickness and droplet diameters in the first and second phases of atomization decreased by 56.52, 34.48, 41.51 and 45.61%, respectively. In addition, in general, by increasing the Reynolds number from 0.21 (* 104) to 0.69 (* 104), the fluid’s film thickness and the average diameter of the droplets in the first and second phases of breakup are reduced by 22.58, 51.24 and 35.42%, respectively.It is important to note that the numerical results are in good agreement with the experimental data.

Searching for a Numerical Model for Prediction of Pressure-Swirl Atomizer Internal Flow

Numerical prediction of discharge parameters allows design of a pressure-swirl atomizer in a fast and cheap manner, yet it must provide reliable results for a wide range of geometries and operating regimes. Many authors have used different numerical setups for similar cases and often concluded opposite suggestions on numerical setup. This paper compares 2D (two-dimensional) axisymmetric, 3D (three-dimensional) periodic and full 3D numerical models used for estimation of the internal flow characteristics of a pressure-swirl atomizer. The computed results are compared with experimental data in terms of spray cone angle, discharge coefficient (CD), internal air-core dimensions, and velocity profiles. The three-component velocity was experimentally measured using a Laser Doppler Anemometry in a scaled transparent model of the atomizer. The internal air-core was visualized by a high-speed camera with backlit illumination. Tested conditions covered a wide range of the Reynolds numbers within the inlet ports, Re = 1000, 2000, 4000. The flow was treated as both steady and transient flow. The numerical solver used laminar and several turbulence models, represented by k-ε and k-ω models, Reynolds Stress model (RSM) and Large Eddy Simulation (LES). The laminar solver was capable of closely predicting the CD, air-core dimensions and velocity profiles compared with the experimental results in both 2D and 3D simulations. The LES performed similarly to the laminar solver for low Re and was slightly superior for Re = 4000. The two-equation models were sensitive to proper solving of the near wall flow and were not accurate for low Re. Surprisingly, the RSM produced the worst results.

Experimental investigation of flameless combustion of biodiesel

A laboratory-scale combustor was investigated under flameless biodiesel combustion. The biofuel was used due to its importance as a green fuel substitute for conventional fossil diesel, in order to reduce the emission of greenhouse gases. The combustor design was based on the phenomenon of internal recirculation, whose intensity is determined by the airflow jet momentum rate through its air intake nozzle. This investigation is important to identify the physicochemical phenomena that govern flameless combustion of liquid fuels, in addition to determining the operating parameters of the burner. A pressure swirl atomizer was used to atomize the biodiesel. The influence of biodiesel temperature and pressure on the droplet size was investigated. Results show that after a certain liquid pressure and preheating temperature, the droplet size does not vary. The combustor aerodynamics promoted adequate mixing of fuel vapor in the vicinity of the droplet interface with diluted oxidant, as a result of high airstream jet momentum rate, leading to distributed combustion reactions. The experimental results showed that combustion at high rates of excess air and preheated air fulfilled the typical characteristics of flameless combustion due to the indistinguishable flame limits, reduced combustion noise levels, absence of soot emissions and low emissions of NOx and CO, simultaneously. On the other hand, combustion in low excess air resulted in the stabilization of a bright yellowish flame with high soot emissions together with a sharp increase in CO emissions. The droplet size has a significant influence on the establishment of stable combustion. For droplet sizes above 35 μm it was impossible to keep the burning of the biodiesel. NOx and CO emissions were affected by the excess air and temperature variations.

An improved theoretical formulation for Sauter mean diameter of pressure-swirl atomizers using geometrical parameters of atomization

This study discusses the development of a mathematical model that is capable of predicting the drop size mean diameter of the spray generated by a pressure swirl atomizer, considering the effects of the liquid's viscosity and the geometrical parameters of this type of injector, as well as the angle of incidence of the inlet channels (ψ and β) and atomization parameters (k, ϰ), obtained from hyperbolic relations. Additionally, this model investigates the phenomena of rupture and stability that are observed in the conical liquid film, in which the importance of a new geometrical parameter of atomization, “ϰ”, which immediately influences the drop size diameter of the spray, should be highlighted. The results that are obtained using this model are compared with analytical results of Couto, Wang and Lefebvre, Jasuja, Radcliffe and Lefebvre, experimental results and numerics (Hollow cone atomization model), using the Ansys Fluent software for the validation and consistency of the model proposed in Rivas (2015). This model yields good approximations as compared to that yielded using other alternative mathematical models, demonstrating that the new atomization geometric parameter “ϰ” is an “adjustment” factor that exhibits considerable significance while designing pressure swirl atomizers according to the required SMD. Furthermore, this model is easy to use, with reliable results, and has the advantage of saving computational time.

Thermofluid characterization of nanofluids in spray cooling

The current study addresses a parametric characterization of nanofluid sprays as a way to improve the thermal performance of spray cooling systems. The nanofluids are prepared using as base solution of distilled water with 0.05% (m/m) cetyltrimethylammonium bromide – CTAB in which nanoparticles of alumina and silver are mixed at different concentrations, namely 0.5%, 1% and 2% (m/m). The effect of the shape of the nanoparticles is also considered. These nanofluids are sprayed using a pressure-swirl atomizer and impact, vertically at two different heights, onto an AISI 304 stainless steel foil. This foil is heated by Joule effect, with two imposed currents, delivering 915 and 2100 W/m2. Infrared thermography is used to measure the thermal footprint from which the dissipated heat flux is calculated. Results show that, in steady state operation, the lowest surface temperature is obtained with the 0.5% (m/m) alumina nanofluid, while increasing alumina nanoparticles, concentration increases the radial temperature distributions on the surface. Results also show a mild effect of the shape of the nanoparticles. Hence, lowest surface foil temperatures are achieved with the silver nanofluid with spherical particles, when compared with the same nanofluid with triangular particles. Heat transfer coefficients decreased with increasing thermal conductivity and dynamic viscosity of the nanofluids. On the other hand, a positive correlation is found between the specific heat capacity of the nanofluids and the spray cooling capacity. For the range of experimental conditions covered in this work, nanofluids have proven to increase the thermal performance of the spray.

Numerical study of the secondary atomization characteristics and droplet distribution of pressure swirl atomizers

The droplet distribution in a spray field directly represents the secondary atomization of the spray field, an issue that is currently receiving significant attention in engineering research. In this study, based on a simple pressure swirl atomizer, the atomization process is numerically simulated by using coupled calculations of internal and external flow at different pressure drops, and the corresponding experimental measurements were also made. For different pressure drops, the numerically calculated macroscopic morphology of the spray field is consistent with the experimental results, and the maximum differences in the numerically calculated average diameter, spray angle, and flow rate are 8.2%, 6.7%, and 9.5%, respectively. The droplet-size distributions in simulation and experiment are acquired in the spatial and temporal domains, respectively, and are quantitatively analyzed in the former by fitting with a probability density function. The advantage of simulations is that they are not affected by the random fluctuations in measurement position and can thus accurately and comprehensively characterize the spatial distribution and atomization characteristics of droplets in the entire spray field, which is conducive to the overall analysis of the spray field and to the prediction of a high-temperature zone in the combustion chamber. This study should thus advance the macroscopic understanding of the overall spatial distribution of the spray field and provide a reference method for the design and optimization of pressure swirl atomizers in engineering applications.

Oil droplet breakup during pressure swirl atomization of emulsions: Influence of emulsion viscosity and viscosity ratio

The atomization of oil-in-water emulsions for spray drying is a common task in food engineering. During atomization with pressure swirl atomizers, the emulsions are subjected to high stresses that can lead to deformation and breakup not only of the liquid itself but also of the disperse oil droplets therein. In this study, the effect of the viscosity ratio λ of the emulsion (0.2–33) and of the emulsion viscosity (5–50 mPa⋅s) on oil droplet breakup during atomization was investigated at atomization pressures between 50 and 250 bar. A systematic influence of λ on the oil droplet size was observed with maximum oil droplet breakup for λ between 0.2 and 1.2. At a constant viscosity ratio of ∼ 1, the resulting oil droplet size was almost independent of the emulsion viscosity, even when volume flow rates and spray droplet sizes during atomization showed a clear dependence on this parameter. A model based on existing knowledge on droplet breakup in laminar shear flow is developed, by which oil droplet sizes after atomization can be predicted. This model may serve as a tool to control oil droplet size after atomization by defining appropriate atomization conditions and emulsion formulation. This is especially relevant for spray drying applications in which the oil droplet size in the powder is an important quality parameter. • Viscosity ratio λ influences oil droplet breakup during pressure swirl atomization. • At λ ∼ 1 the emulsion viscosity per se has little influence on oil droplet breakup. • Oil droplet breakup follows the theory of droplet breakup in laminar shear flow. • A model is proposed that serves as a tool to predict oil droplet breakup.

Effects of fuel injection on the combustion and emission performance of a trapped vortex combustor

An experimental study was conducted at atmospheric pressure at 473 K to investigate the effects of fuel injection strategy on the combustion and emission characteristics of a trapped vortex combustor. In the test, the combustor was fueled by a simple pressure-swirl atomizer and novel atomizer to realize a normal circular spray pattern and fan spray pattern, respectively. The combustion and emission characteristics of the combustor were fully examined under a variety of Mach numbers and fuel-to-air ratios. The effects were directly explored in terms of combustion efficiency, pattern factor, and pollutant emissions. The results indicated that the combustion and emission characteristics of the combustor are highly dependent on the fuel injection strategy. When compared to the simple pressure-swirl atomizer, the novel atomizer realized evident advantages in terms of combustion efficiency, pattern factor, and pollutant emissions of carbon monoxide and unburned hydrocarbons under a wide range of operating conditions. This is mainly attributed to the suitable spray pattern and smaller droplet size realized by the novel atomizer.

Structure of the ethanol spray flame under conventional and MILD conditions

A numerical examination is carried out on the experimental MILD turbulent spray burner of Delft Spray in Hot Co-flow, DSHC. The RANS approach is combined with the Lagrangian tracking of droplets for modeling the pressure-swirl atomizer. The EDC combustion model is employed, to take into account detailed chemical mechanisms. Predictions show that the structure of the triple-flame of liquid fuel in the atmospheric condition is different from the conventional gaseous triple-flames. Furthermore, the structure of the MILD spray flame is compared to the non-MILD spray flame. The flame index shows that the triple-flame structure in the conventional spray flame that consists of two outer and middle diffusion branches, and a premixed inner branch converts to a double-flame structure under MILD condition, including an outer diffusion and a premixed inner branch. In another step, sensitivity analysis relative to the temperature, and oxygen concentration of the co-flow stream is performed independently, which was not possible in the experiment. The parameter of ignition delay time is used for the selection of parametric study cases. Moderate preheating could lead to deviation from the MILD combustion regime, while high preheating is beneficial. NO pollutant generation shows more sensitivity to dilution rather than preheating in MILD spray flame. This study showed that increasing or decreasing each of the temperature and oxygen concentration doesn’t improve all the effective aspects toward achieving the MILD condition. So, the effective parameters should be changed according to a specific and more precise goal.

Quantification of coflow effects on primary atomization of pressure swirl atomizers

Extension of high momentum jet stabilised combustors towards liquid fuel operation has shown significant injector shape and injection strategy impact on the fuel air mixing behaviour, the flame position and stability and ultimately pollutants. This can be attributed to the lack of optimised injection concepts as well as fundamental understanding of turbulence–spray interactions that dictate atomization and fuel placement in high turbulence gas flow conditions. The current work uses a canonical flow channel to investigate the bilateral interactions between a highly turbulent coaxial air flow and the primary atomization process of a liquid spray generated by pressure-swirl atomizers. A novel two-phase particle image velocimetry (PIV) method with phosphor-particles is developed to enable flow field measurements near the gas–liquid interface as well as in the dilute spray region. In addition, a shadowgraphy technique is applied to detect the disturbances on the sheet surface. Liquid fuel is replaced by deionised water due to safety concerns and the coaxial air flow velocity is varied from 0 to 80 m/s. The results show a significant influence of the aerodynamic forces on the growth rate of sheet disturbances with increasing coflow velocities. This induces an advanced breakup of comparable thick liquid sheets at large Weber numbers. The novel PIV method enabled the quantification of the turbulent kinetic energy transfer between the phases that is strongly dependent on the slip velocity at the gas–liquid interface. These findings assist the ongoing development of liquid fuel injection systems for high momentum jet based combustors and provide validation data for numerical simulations of primary atomization.

Near-Nozzle Characteristics of Jet A-1 Fuel Sprays from a Pressure Swirl Atomizer

Near-Nozzle Characteristics of Jet A-1 Fuel Sprays from a Pressure Swirl Atomizer