Effervescent Atomizer Research Articles

A Fitting Formula for Predicting Droplet Mean Diameter for Various Liquid in Effervescent Atomization Spray

The outflow of the effervescent atomization spray is simulated by a comprehensive numerical model based on the Navier-Stokes equation and the particle tracking method. The droplet mean diameter under different operating conditions and liquid properties were calculated. Based on the extensive computations, a formula relating droplet Sauter mean diameter to the operating conditions and liquid physical parameters were obtained with curve fitting technique. The results calculated from the formulae were compared with the experimental data and a good agreement was obtained. The formulae can be used to predict the droplet mean diameter for various Newtonian liquid in axisymmetric effervescent atomization spray conveniently and effectively.

Development of an Effervescent Atomizer for Industrial Burners

The present work is conducted with the purpose of developing an effervescent atomizer for industrial burners that will generate a fine and steady spray in large turn-down ratio. The atomizer is fed with light heating oil (LHO) and uses air as an atomizing medium. First, a basic classification is made of the published design concepts of effervescent atomizers investigated by different researchers. Three distinct types of such atomizers are recognized. A single-hole, plain-orifice atomizer with an “outside-in” gas injection configuration was chosen for this study. The basic geometric parameters that may significantly influence atomizer performance are described. An experimental study of the effervescent atomizer was conducted to evaluate the influence of operational conditions and that of several geometric parameters on the drop size in the spray. The Sauter mean diameter of drops was measured using a phase/Doppler particle analyzer. The study covers the size and number of aerator holes, their location, and the diameter of the mixing chamber. The influence of these parameters on the spray quality was evaluated at atomizing pressures of 0.1, 0.3, and 0.5 MPa and gas:liquid mass flow rate ratio (GLR) values of 2%, 5%, and 10%. The main goal of this work is to develop a new procedure for the design of effervescent atomizers. This procedure is based on our experimental results and it is also supported by the findings of other authors. It allows for the determination of the key geometric parameters of the atomizer to achieve sprays of minimum mean drop size for defined values of liquid flow rate, air supply pressure, and GLR. The article also studies the optimization of the exit orifice size and of other parameters, such as the size of the mixing chamber (internal diameter and length), and the number, size, and position of aeration holes.

Emissions Reductions in Diesel and Kerosene Flames Using a Novel Fuel Injector

A SPRAY serves as the heart of almost every type of liquidfueled combustion system. Ideally, to promote combustion withmaximumefficiency andminimumharmful emissions, an injector should deliver a fuel spray that rapidly disperses and evaporates to yield a homogeneousmixture of gaseous fuel and air. Characteristics of an ideal injector include good atomization over a wide range of either steady or transient fuel flow rates, unaffected by flow instabilities, low power requirements, scalability, resistant to blockages, and delivery of a fine spray [1,2]. Reviews by McDonell and Samuelsen [3], Razdan [4],Mansour [5], andNakamura et al. [6] deal with fuel sprays optimized for lean combustion. Pressure atomization, air-assist and air-blast atomization, and effervescent atomization [7] are commonly used techniques for a variety of injector applications. In the current study, we are exploring the so-called flow-blurring injector of Ganan-Calvo [8] in a combustion system. As illustrated in Fig. 1, the flow-blurring (FB) concept consists of a liquid nozzle and an orifice plate situated downstream of the nozzle. The nozzle’s wall is tapered at the outlet, and the sharpened edge of the orifice plate is the same diameter as the inner diameter of the nozzle (d). When the axial distance between the nozzle exit and orifice plate (H) is small (i.e., H=d < 0:25), some of the gas flowing into the lateral cylindrical passageway between the nozzle exit and orifice plane is forced upstream a short distance into the nozzle carrying the liquid. The backflow of gas causes vigorous combination of turbulent mixing and effervescing, or the so-called flow-blurring, which results in a spray with very fine droplets. Simmons et al. [9] conducted cold-flow visualization experiments for a range of geometric and flow conditions to demonstrate the high efficiency of the FB injector concept. To the best of our knowledge, the FB injector concept has never been applied to combustion systems. The objective of this study is to experimentally characterize emissions performance of the FB injector in a liquid-fueled combustor operated at atmospheric pressure. Experiments using diesel and kerosene fuels are performed for fixed fuel and total airflow rates, and the atomizing airflow rate is varied. The FB injector performance (NOx and CO emissions) is compared with that of a commercial air-blast (AB) injector. II. Experimental Setup

Simulation of Droplet-gas Flow in the Effervescent Atomization Spray with an Impinging Plate

A comprehensive three-dimensional model of droplet-gas flow was presented to study the evolution of spray in the effervescent atomization spray with an impinging plate. For gas phase, the N-S equation with the k- ɛ turbulence model was solved, considering two-way coupling interaction between droplets and gas phase. Dispersed droplet phase is modeled as Lagrangian entities, accounting for the physics of droplet generation from primary andsecondary breakup, droplet collision and coalescence, droplet momentum and heat transfer. The mean size and statistical distribution of atomized droplets at various nozzle-to-plate distances were calculated. Some simulation results were compared well with experimental data. The results show that the existence of the impinging plate has a pronounced influence on the droplet mean size, size distribution and the droplet spatial distribution. The air-to-liquid ratio has obvious effects on the droplet size and distribution.

Effects of operating conditions on droplet deposition onto surface of atomization impinging spray

A comprehensive model based on the Navier–Stokes equation and particle tracking method is used to study the effervescent atomization impinging spray, and another model is used to establish the relationship between the droplet velocity near the plate and the different operating conditions. The models and numerical code are validated by comparing the numerical results with the published experimental results. The effects of air-to-liquid ratio, nozzle diameter, liquid mass flow rate, and the position of impinging plate on the Weber number and K number as well as the droplet deposition onto the plate are discussed. The results show that the droplet velocity near the plate increases with increasing air-to-liquid ratio and liquid mass flow rate, and with decreasing nozzle diameter and axial distance from the nozzle exit to the plate. The droplet diameter near the plate increases with increasing axial distance from the nozzle exit to the plate, and with decreasing air-to-liquid ratio. As a function of the nozzle diameter and liquid mass flow rate, the variation of droplet diameter is not monotonous and the effect of liquid mass flow rate on the droplet diameter is insignificant. In the studied operating conditions, it is difficult for the droplet to rebound off the plate when impinging on the plate but it is easier for the droplet to splash. In order to create a condition which can benefit the droplet deposition when impinging on the plate, the suggested ways are to reduce the air-to-liquid ratio and liquid mass flow rate, increase the nozzle diameter, and select a suitable range of axial distance from the nozzle exit to the plate.

PERFORMANCE CHARACTERISTICS OF EFFERVESCENT ATOMIZER IN DIFFERENT FLOW REGIMES

The characteristics of the two-phase flow and the spray formed in an effervescent atomizer were experimentally investigated over a range of flow rates, gas-to-liquid ratios (GLR), injection pressures, and discharge orifice diameters. The Reynolds number (Re), defined with the orifice diameter as the characteristic length, was seen to be a pertinent parameter in addition to GLR for determining the transition from the bubbly flow to slug flow. The discharge coefficient of the atomizer in the bubbly flow regime and the Sauter mean diameter of the spray in the annular flow regime were found to scale with the parameter {GLR (Re)0.5}. The structure of the spray and the measured values of cone angle of the spray showed distinctly different features in the bubbly, slug, and annular flow regimes. A two-dimensional simulation of the flow and spray structure reproduced the gross features observed in the experiments.

Visualization of two phase flow inside an effervescent atomizer

Effervescent atomization is one of the twin-fluid atomization methods while it has better performance in terms of smaller drop sizes and/or lower injection pressures. In order to investigate the effects of the internal flow patterns on droplet characteristics, a new kind of effervescent atomizer was designed and manufactured. The bubble forming process was visualized with a high-speed camera, while the droplet size was characterized with a LDV/PDA system. The experimental results show that there are three regimes of the two-phase flows inside the discharge orifice, one is bubbly flow, another is annular flow while the other is the intermittent flow. The flow patterns transfered from bubbly flow to intermittent flow and then to annular flow with decreasing of the water flow rate. In addition, with increasing of the working pressure or decreasing of the water flow rate, the SMD (Sauter mean diameter) of the droplets decreased and the axial mean velocity increased.

Relationship between deposition properties and operating parameters for droplet onto surface in the atomization impinging spray

Spray coating is one of the most important applications in the industry. The deposition properties of droplet onto surface can be determined by Weber number and K number. While the relationship between the Weber number, the K number and the operating conditions is not understood. In this paper, a comprehensive numerical model based on the Navier–Stokes equation and the particle tracking method was used to simulate numerically the effervescent atomization spray with an impinging surface, and an analytical model was used to predict the droplet velocity. After performing extensive computations at different operating conditions, the formulae of droplet diameter and droplet velocity were obtained with curve fitting technique. The results calculated from the formulae agree with the experimental data. Finally the new formulae relating the Weber number and the K number to the operating parameters were deduced out. The new formulae can be conveniently used to judge the deposition property of droplet onto surface.

A 3D simulation of two-phase flow in an effervescent atomizer for suspension plasma spray

In the suspension spray of nanoparticles, where the attempt is to reach nano-scaled uniform coatings, there is a vital demand to produce a controllable and non-pulsating spray. Effervescent atomizers, in which a gas is bubbled into the bulk of liquid through an aerator, have shown to be a technological alternative to the conventional atomizers when liquid atomization with various concentrations of nano-particles is required. Thus, understanding the behavior of gas and liquid flow through the nozzle is crucial to predict the condition of resultant spray. The two-phase flow inside an effervescent atomizer is numerically investigated. Using an incompressible Eulerian/Eulerian approach, the three-dimensional structure of two-phase flow inside an aerated-liquid injector is modeled. The behaviour of liquid film carrying nano-particles in the discharge passage is studied using different Gas to Liquid mass flow Ratios (GLR), ranging from 0.08% to 1.25%. These numerical results are compared with the experimental data available in literature. The effect of nano-sized solid particles concentration on the liquid film thickness at the exit of the atomizer is studied through the change in liquid bulk density and viscosity. The results show that the atomizing gas-to-liquid mass flow ratio (GLR) does play a key role on the flow behaviour inside the atomizer. At low GLRs of 0.15%, the thickness of the liquid film decreases rapidly and as GLR increases to 1.25% the liquid film thickness dependency on GLR reduces. The results also show that there is no significant effect of particle concentrations, varying within the range of Newtonian fluid, on the liquid film thickness. This feature makes effervescent atomizers a technology choice for controllable suspension thermal spray processes.

Miniature conductivity wire-mesh sensor for gas-liquid two-phase flow measurement

A miniature conductivity wire-mesh sensor for gas-liquid two-phase flow measurement in small channels is presented. The sensor design is similar to the conventional wire-mesh sensor for larger flow cross sections with wire electrodes stretched across the flow channel in two adjacent planes and with perpendicular wire orientation between planes. Conductivity measurement is performed by special electronics which consecutively applies bipolar voltage pulse excitation to the sender wires and measures electrical current flow in the wire crossings at the receiver wires. The new design is based on printed circuit board technology. A prototypical sensor made of 2×16 stainless steel wires each of 50 μm diameter was manufactured and applied to two-phase flow measurement inside the mixing chamber of an effervescent atomizer. Accuracy of the sensor was studied for static liquid distributions using microphotography and for dynamic two-phase flow by comparison of wire-mesh sensor data with radial gas fraction profiles obtained from X-ray microtomography measurements.

Effervescent Atomization of Viscoelastic Liquids: Experiment and Modeling

The effervescent atomization of viscoelastic liquids is reported. A total of 23 fluids, formulated from a 60wt% glycerine/40wt% water solvent to which were added varying concentrations (0.001–0.5wt%) of poly(ethylene oxide) polymers whose molecular weights ranged from 12,000 to 900,000, were sprayed through a conventional effervescent atomizer. Mean drop sizes were measured using a forward light scattering instrument. The drop size (D32) data show the expected decrease with an increase in air-liquid ratio by mass (ALR), the expected increase with an increase in polymer concentration, plus an increase with an increase in polymer molecular weight for most cases. However, no significant change in D32 was observed for polymer solutions whose molecular weights ranged from 12,000 to 35,000, suggesting the presence of a critical molecular weight below which spray performance is unaltered. This argues for two different factors controlling drop size: Polymer molecular weight is most influential at the highest polymer concentrations while polymer concentration is most influential at the lowest polymer concentrations. Analysis of the spray formation process was carried out using a ligament formation model previously developed for the effervescent atomization of Newtonian liquids coupled with a linear stability model for the breakup of viscoelastic liquid jets. The jet breakup model assumes that an unrelaxed axial tension exists within the fluid. A comparison of model predictions and experimental data indicates that the model predicts the observed dependencies of mean drop size on ALR, polymer concentration, and polymer molecular weight. Quantitative agreement is within 10–50% of experimental values in all cases. Finally, a shortcoming of the model is noted and a means of avoiding this limitation reported.

UNSTEADINESS IN EFFERVESCENT SPRAYS: A NEW EVALUATION METHOD AND THE INFLUENCE OF OPERATIONAL CONDITIONS

A new evaluation method of unsteadiness of the spray generated by an effervescent atomizer is presented. The method is based on measurements of pressure fluctuations in the atomizer mixing chamber. Measurements, made under different atomizer operational conditions, show the spray unsteadiness depends mainly on Gas-to-Liquid-Ratio (GLR). Decrease in GLR causes the spray to become more unsteady. The relation between atomizer internal two-phase flow pattern and the spray unsteadiness is elucidated by visualization of the internal twophase flow using a digital camera and the use of published two-phase flow maps. Findings of the new method are complemented and confronted with the results obtained by the use of spray unsteadiness evaluation method of Edwars & Marx.

Atomization performance of effervescent atomizers with gas-liquid internal mixing

Atomization performance of effervescent atomizers with gas-liquid internal mixing The paper contains the results of experimental studies on water atomization process in effervescent nozzle in one-phase and two-phase systems. The gas phase flow rate causes the changes in characteristic parameters of atomization. The gas flow rate has certain effect on the Sauter Mean Diameter SMD values. SMD for airliquid systems investigated is decreasing with increase of gas volume rate. It is observed that SMD values are increasing with increase of liquid volume rate. The larger SMD values for one-phase flow in comparison with two-phase flow have also been observed. The increase of SMD value has been observed with a decrease of an outlet diameter of an effervescent nozzle.

スクラムジェットエンジン燃焼器内におけるケロシンの燃焼特性

An experimental research on supersonic combustion of kerosene in a model scramjet combustor has been conducted. Kerosene was injected normally into a Mach 2 by three types of methods. First, liquid kerosene was directly injected. In comparison with hydrogen, combustion did not take place at low total temperature or in the fuel lean condition. Secondly, “effervescent atomization” method was used. Effervescent atomization method could control penetration height and mass flow rate independently, and improve ignition limits of liquid kerosene. Finally, gaseous kerosene was used. While only intensive combustion mode and choke mode were observed when liquid kerosene was used, existence of transition mode was observed when gaseous kerosene was used.

Investigation and Comparison of Performance of Effervescent and Standard Pneumatic Atomizer Intended for Soluble Aqueous Coating

Effervescent atomizers belong to the group of internal mixing atomizers. The effervescent approach might be a potential alternative to traditional atomization techniques, e.g., for applications where low atomization air consumption is advantageous In this paper, performance of one proposed design of the effervescent atomizer is investigated and compared to that of a standard pneumatic atomizer. The purpose of the comparison is to evaluate the actual potential of the specific effervescent atomizer in pharmaceutical relevant aqueous coating applications. Aqueous solutions of Hypromellose 5 as well as Povidone K-90F were characterized in terms of rheological properties and surface tension. Solutions were atomized by means of a standard Schlick pneumatic atomizer as well as a customized inside-out type effervescent atomizer. Spray droplet size distributions were recorded by a Spraytec instrument. Increased shear viscosity in the range 24–836 mPa.s had a modest effect on spray mean diameters for pneumatic sprays of the Newtonian solutions of Hypromellose 5. In contrast, mean droplet diameters increased by a factor of 3–5 in pneumatic sprays of Povidone K-90F solutions 11–175 mPa.s in viscosity, where non-Newtonian behavior was observed. Further, sprays of all solutions of Povidone K-90F have considerably larger mean droplet size. The effervescent atomizer atomized low viscosity solutions of Povidone K-90F more efficiently than Hypromellose 5 solutions of corresponding shear viscosity. However, atomization of high viscosity Povidone K-90F results in a coarser spray than that of the corresponding Hypromellose 5 solution. Viscosity, visco-elasticity, and surface tension of solutions all seem to affect atomization efficiency. The pneumatic atomizer was not sensitive to changes in airflow above 8.4 kg/h and liquid flow only had a considerable effect at suboptimal air flows. In its current design the effervescent atomizer improved efficiency throughout the investigated range of air flow of 0.18–0.84 kg/h and consistently produced smaller drops at liquid flow of 10 g/min compared to 35 g/min. In spite of the very low level of air consumption, the effervescent atomizer can produce fine sprays. Within its working range, the standard pneumatic atomizer, however, is capable of producing sprays of even smaller mean droplet size. All together this suggests the described effervescent atomizer as an alternative for applications where advantages of reduced atomization air flow outweigh the disadvantages of a less fine spray, e.g., in coating of attrition- prone substrate.

Investigation of Vaporized Kerosene Injection and Combustion in a Supersonic Model Combustor

Injection and combustion of vaporized kerosene was experimentally investigated in a Mach 2.5 model combustor at various fuel temperatures and injection pressures. A unique kerosene heating and delivery system, which can prepare heated kerosene up to 820 K at a pressure of 5.5 MPa with negligible fuel coking, was developed. A three-species surrogate was employed to simulate the thermophysical properties of kerosene. The calculated thermophysical properties of surrogate provided insight into the fuel flow control in experiments. Kerosene jet structures at various preheat temperatures injecting into both quiescent environment and a Mach 2.5 crossflow were characterized. It was shown that the use ofvaporized kerosene injection holds the potential of enhancing fuel-air mixing and promoting overall burning. Supersonic combustion tests further confirmed the preceding conjecture by comparing the combustor performances of supercritical kerosene with those of liquid kerosene and effervescent atomization with hydrogen barbotage. Under the similar flow conditions and overall kerosene equivalence ratios, experimental results illustrated that the combustion efficiency of supercritical kerosene increased approximately 10-15% over that of liquid kerosene, which was comparable to that of effervescent atomization.

An experimental study of kerosene combustion in a supersonic model combustor using effervescent atomization

Investigation of kerosene combustion in a Mach 2.5 flow was carried out using a model supersonic combustor with cross-section area of 51 mm × 70 mm and different integrated fuel injector/flameholder cavity modules. Experiments with pure liquid atomization and with effervescent atomization were characterized and compared. Direct photography, Schlieren imaging, and planar laser induced fluorescence (PLIF) imaging of OH radical were utilized to examine the cavity characteristics and spray structure. Schlieren images illustrate the effectiveness of gas barbotage in facilitating atomization and the importance of secondary atomization when kerosene sprays interacting with a supersonic crossflow. OH PLIF images further substantiate our previous finding that there exists a local high-temperature radical pool within the cavity flameholder, and this radical pool plays a crucial role in promoting kerosene combustion in a supersonic combustor. Under the same operation conditions, comparison of the measured static pressure distributions along the combustor also shows that effervescent atomization generally leads to better combustion performance than the use of pure liquid atomization. Furthermore, the present results demonstrate that the cavity characteristics can be different in non-reacting and reacting supersonic flows. As such, the conventional definition of cavity characteristics based on non-reacting flows needs to be revised.

Effervescent atomization under sub-sonic and choked conditions—a theoretical approach

Spray formation of a bi-component mixture under sub- and choked-flow conditions has been studied. Special attention has been drawn to the processes inside the atomizer, i.e., the expansion chamber and two orifices. The relevant processes, which include the pressure drop at the inlet orifice, nuclei formation, bubble growth inside the expansion chamber, pressure drop at the discharge orifice, the velocity slip between the bubbles and the liquid bulk, and the flow regime (sub- and choked-flow) at the discharge orifice, have been analyzed by using a one-dimensional model approach. Three different operating regions have been identified. In the 1st, when the inlet to discharge orifices' diameter ratio is small, subsonic flow is anticipated, and no noticeable slip between the bubbles and the bulk liquid is expected. As the orifices' diameter ratio increases, the slip becomes more and more significant (2nd region). When the pressure at the expansion chamber exceeds the critical pressure, the flow chokes, and the slip maximizes. Further increase results in maximum slip (3rd region). The two main roles of the expansion chamber were described: (a) to provide the required time for the bubbles to grow till one bubble touches the other, and (b) to provide the discharge orifice with a well-mixed mixture to allow dominant flashing enhanced by shear stress disintegration. Optimized operation conditions for best atomization, optimized expansion chamber volume, and optimized orifices' diameter ratio are proposed in terms of the thermodynamic properties of the mixture.

Microscopic spray characteristics in the effervescent atomizer with two aerator tubes

An experimental study is performed on atomization characteristics and stable operating conditions for the injection of high viscous waste vegetable oil using an effervescent atomizer with 2 aerator tubes. Consideration is given to the effects of ALR and liquid viscosity on the velocity and mean diameter of the injected droplet. It is found that (i) as ALR increases, the axial velocity of the droplet is increased, while half-velocity width and SMD are decreased regardless of the change in liquid viscosities, (ii) the rate of fine drop distribution occupied in the total spray field is increased with an increase in ALR, and (iii) the effect of viscosity on the atomization characteristics is minor. Consequently, it is expected that the effervescent atomizer will exhibit an excellent atomization performance at the high ALR condition, regardless of liquid viscosities.

Design and Atomization Properties for an Inside‐Out Type Effervescent Atomizer

Atomization of aqueous polymer solutions is a key step in the formulation of several pharmaceutical products. Droplet size control is essential in order to produce pharmaceutical products with the desired properties. The purpose of this paper is to investigate design issues for an inside‐out type of effervescent atomizer used to spray water and aqueous solutions of polyvinylpyrrolidone (Kollidon® K‐30) and hydroxypropyl methylcellulose (Pharmacoat® 603). The atomizer was operated at air‐to‐liquid mass ratios of 0.1, 0.3, and 0.5 and a feed pressure of 1172 kPa. Fluid viscosities ranged from 1 to 47 mPa.s. The influence of several atomizer design features was considered, including exit orifice length‐to‐diameter ratio, exit orifice diameter, the total area of the air injection holes, the distance between the air injection point and the exit orifice, the diameter of the mixing chamber, and the orientation of both air and liquid flows. Droplet size distributions were shown to vary significantly with the atomizer's exit orifice diameter, air injector design, and air injector distance to the exit orifice. In all cases air‐to‐liquid mass ratio played a key role in the mean droplet size. The design of the atomizer was shown to have the most pronounced effect on the mean droplet size at the lowest air‐to‐liquid mass ratios. Optimization of the atomizer design is very important in order to obtain small droplet sizes in pharmaceutical processes where the amount of air/gas should be minimized, e.g., closed‐cycle spray drying and agglomeration processes.