Atomized Droplet Size Research Articles

Influence of Atomizing Parameters on Droplet Properties in a Pulse Combustion Spray Dryer

A pulse combustor employed in a spray-drying system offers a new approach for liquid atomization that yields high-quality powders at low cost. Using a pulse combustion atomizer, there is no need for any form of nozzle dispersion and its atomization mechanism differs from those of conventional atomizers, such as rotary atomizers and pressure and pneumatic nozzles. In this work, based on the analysis of atomization mechanism, experiments of unsteady pulsating atomization were carried out in an experimental system of a Helmholtz-type pulse combustor. An optical analyzer was used for measuring the mean diameter of atomized droplet and droplet distribution. The effects of liquid feed rate, air flow oscillatory frequency, and liquid viscosity on atomized droplet size and size distribution were investigated and analyzed. The results indicate that the uniform droplet size distribution can be obtained under the conditions of a low feed rate, high-frequency pulsating flow, and moderate viscosity. The range of the droplets' Sauter mean diameter (SMD) is between 50 and 80 µm. The pulsating air flow from the pulse combustor can be used to atomize liquid or slurry without a nozzle and the atomizing quality can meet the requirements of spray drying.

Effect of polymer–surfactant interactions on elongational viscosity and atomization of peo solutions

The elongational viscosities of aqueous solutions of polyethylene oxide (PEO) containing either sodium dodecyl sulfate (SDS) or sodium dodecyl benzyl sulfonate (SDBS) are measured by an opposing jet elongational rheometer. The elongational viscosities of these solutions are greatly enhanced by interactions between the polymer chain and surfactant micelles, and the behavior of the elongational viscosity–strain rate curves is complex, and strongly dependent on surfactant concentration. In the case of PEO–SDS solutions, abrupt rises in elongational viscosity are observed at critical values of strain rate, which vary with the SDS concentration. Data from these measurements are also employed to estimate elongational viscosities prevailing at the nozzle atomization stress in previous atomization experiments performed on PEO–SDS and PEO–SDBS solutions. Strong similarities are found between the variation of elongational viscosity and the variation of atomization droplet size with surfactant concentration.

Feedstock atomization effects on FCC riser reactors selectivity

Feedstock atomization affects catalytic cracking reactions that take place in an FCC riser reactor, and is considered a critical parameter especially when processing heavy fuels. A finely atomized feed allows instantaneous flash vaporization and quick quenching of the catalyst, conditions that allow the feed components to be subjected to an ideal mixing temperature. Mixing conditions at the inlet of the riser reactor also affect cracking reactions selectivity. An atomization modeling scheme has been incorporated into a CFD model developed by Theologos & Markatos (1993, A.I.Ch.E. Journal, 39( 6), 1001.) and Theologos, Nikou, Lygeros & Markatos (1997, A.I.Ch.E. Journal, 43( 2), 486) and has been used to evaluate atomization effects on feedstock vaporization rates, cracking reactions initiation, reactor selectivity and overall reactor performance. Cracking reactions are simulated using a simplified 3-lump model. The atomization modeling scheme takes into account the initial feed droplet size at nozzle's exit, as well as droplet size reduction along with vaporization. Droplet vaporization parameters from the literature have been considered. To evaluate the degree of feedstock atomization effects on reactor performance, a parametric study is carried out considering three different initial feed droplet sizes, namely 30, 100, and 500 μm. The area where feedstock vaporization takes place inside the reactor is predicted and the evolution of atomized droplet size is illustrated. The effect of the initial degree of atomization on cracking reactions selectivity is discussed.

An application of the electrostatic spray technology to increase scrubbing efficiency of SO$_{2}$ emitted from thermal systems

Emission control of acid exhaust gases from coal-fired power plants and waste incinerators has become an increasing concern of both industries and regulators. Among those gaseous emissions, SO has been eliminated by a Spray Drying Absorber (SDA) system, where the exhaust gas is mixed with atomized limestone-water slurry droplets and then the chemical reaction of SO with alkaline components of the liquid feed forms sulfates. Liquid atomization is necessary because it maximizes the reaction efficiency by increasing the total surface area of the alkaline components. An experimental study was performed with a laboratory scale SDA to investigate whether the scrubbing efficiency for SO reduction increased or not with the application of a DC electric field to the limestone-water slurry. For a selected experimental condition SO concentrations exited from the reactor were measured with various applied voltages and liquid flow rates. The applied voltage varied from -10 to 10 kV by 1 kV, and the volume flow rate of slurry was set to 15, 25, 35 ml/min which were within the range of emission mode. Consequently, the SO scrubbing efficiency increased with increasing the applied voltage but was independent of the polarity of the applied voltage. For the electrical and flow conditions considered a theoretical study of estimating average size and charge of the atomized droplets was carried out based on the measured current-voltage characteristics. The droplet charge to mass ratio increased and the droplet diameter decreased as the strength of the applied voltage increased.

Measurement of Size and Velocity of Atomized Droplets by Dual-Beam Light-Scatter Interferometry.

The size and velocity of atomized droplets are simultaneously measured by dual-beam light-scatter interferometry. The relationship of scattered beam intensity against a broad range of particle size parameters (101103) is theoretically analyzed, and the effect of the aperture of collimating lens is numerically studied to clarify the range to show the linear relation. It is implied that this measurement is available for particles of diameter 10200 μm when the numerical aperture of focusing lens is 5.2×10-2 in the optical system. This method is applied to the cold model of the atomized droplets. The behavior of the velocity and the droplet particle size measured by using the open optical system which has a comparatively long focus length corresponded to the conventional one.

Simulation of removal of HCI gas in a spray dry tower

AbstractA mathematical model of the absorption of HCI gas in a spray dry tower has been proposed. The proposed model describes the mechanism of HCI removal talking into account atomized droplet size distribution and the change in the droplet state from “slurry droplet” to “dry particle”. It is found that the removal efficiency can be simulated by using the model with the constant mean diameter of droplet from the simulation results. Simulation results of the removal of HCI gas are in good agreement with the experimental data in a spray dry tower.

Electrospray Atomization of Electrolytic Solutions

Results of experiments on droplet production by electrospray atomization of electrolytic solutions are described. The spray results from the formation of liquid droplets at the tip of a Taylor cone. These experiments using sodium iodide dissolved in n-propyl alcohol show that as the solution concentration increases, the volumetric flow rate decreases, the electrical current increases, and the aerosol size distribution of the solid residue particles shifts to smaller sizes, a counter-intuitive result which occurs because the atomized droplet size decreases with increasing specific electrical conductivity of the solution. There is no complete analytical description of the electrospray but some analytical insights will be discussed regarding the behavior of its three distinct yet interacting parts: the Taylor cone, the jet, and the charged droplet spray.

Atomized Droplet Size Distribution in Wake of Fixed Blade.

The presence of water droplets in the flow of a steam turbine leads to erosion of moving blades because of the large blade tip speed and higher local wetness. Since erosion resistance is strongly influenced by the impact droplet velocity and size as well as the hardness of the blade material, estimating the distributions of droplet size and frequency is important for establishing erosion criteria. In this paper we describe an estimation method for droplet size of dispersed water flow in the wake of a fixed blade, which can be extended to a moving blade. The LDV system was applied to measure velocity and size of droplets in the wake near the trailing edge of a fixed blade, under conditions simulating those found in the last stage of a low-pressure steam turbine. Test results show that droplet sizes in the wake of a fixed blade can be approximately evaluated by a normal distribution function depending on the mean droplet diameter. It is concluded that the size and frequency distributions of the droplets which impact on a moving blade can be calculated using these characteristics.

Process of dispersing a molten jet by an annular stream of high-pressure water. I. Hydrodynamic model of molten jet transformation into droplets

A theoretical model is constructed for dispersion based on analyzing processes of hydrodynamics, instability, and heat exchange of a molten jet under the action of a high pressure stream of water. It includes description of successively proceeding transformations of the original jet into microjets and droplets. A dependence is obtained which established a correlation of atomized droplet size with the physical characteristics of water and melt (density, viscosity, and specific surface energy) as well as with such atomization parameters as melt flow rate, water stream velocity, and jet diameter. It is shown that it is necessary to supplement the model with empirical factors which consider relative melt and water flow rate, the attack angle, and the slot width of an annular atomizer nozzle.

Low-rank coal-water fuel combustion in a laboratory-scale furnace

A detailed study of hot-water dried lignite slurry combustion and the formation of nitrogen-containing pollutants was performed in a vertical, laboratory-scale combustor. Space-resolved local measurements of solid and gaseous combustion products were obtained from throughout the combustion zone using a stainless steel, water-quenched sample probe. Coal burnout (daf) of greater than 99% was achieved without supplementary fuel support, in an estimated residence time of 1.4 s. Flame stability was strongly affected by the atomized droplet size, which is controlled by the atomizing air to slurry mass ratio ( A S ). For A S greater than 0.7, coal burnout was relatively insensitive to further increases in A S , yet burnout decreased rapidly as A S was decreased. Nitric oxide (NO) emissions were not affected greatly by changes in A S . Decreasing stoichiometric ratio (SR) to about 0.8 caused coal burnout to decrease from about 98% to 94% and NO emissions to decrease from around 600 ppm to less than 100 ppm. Changes in secondary air swirl number from 0 to 4.25 had little or no effect on coal burnout or NO emissions for a SR of 1.1 and an A S of 0.75. At low A S (0.24), high secondary air swirl was required to stabilize the slurry flame. Reactor mapping tests showed rapid mixing between the slurry and the combustion air. CO was found only near the slurry inlet at a maximum concentration of 0.3%. No other fuel-rich species were detected in measurable quantities.

Atomization of a liquid jet

Generation of ripples by wind blowing over a viscous fluid was investigated by G. I. Taylor [The Scientific Papers of G. I. Taylor (Cambridge U. P., Cambridge, 1963), Vol. 3, No. 25] with linear stability analysis. Taylor considered the case of temporally growing disturbances in a low density gas and applied his results to explain the process of atomization of a liquid jet injected into a low density gas. Taylor’s analysis is extended here to investigate the case of a spatially growing disturbance in a dense gas. Taylor showed that temporal disturbances of wavelength shorter than the capillary length are stable. The same is found for the spatial disturbances. Each type of disturbance possesses a maximum growth rate with a specific wavelength and frequency. The atomized droplet size corresponding to the maximum growth rate is shown in both theories to decrease inversely as the square of the jet velocity. While the maximum growth rate increases as the square root of the gas-to-liquid density ratio when A2 exceeds 1 for the temporal disturbances, the same dependence on the density ratio does not hold for spatial disturbances until A2 exceeds 100, where A2 is a flow parameter representing the ratio of surface force to the viscous force. When A2 exceeds 100 the growth rates predicted by two theories deviate significantly only at air pressure higher than 10 atm for most liquids at room temperature. However, for all parameters, the spray angle changes along the jet axis according to the spatial theory, but remains constant according to the temporal theory. It is shown that the viscous force in the liquid may be increased relative to the surface tension force to the point that no discernable spray angle may be observed in practice. Then an intact jet without atomization may result. It is shown that the onset of atomization is primarily caused by the pressure fluctuation which resonates the capillary waves. The results on the interfacial amplification rate suggest that a sufficiently large initial amplitude at the nozzle exit is essential for the onset of atomization.

Atomization of fuel mixtures

The combustion process of a coal/oil slurry begins by atomization of the slurry. This project is a presentation of experimental system and results on atomization of coal/oil(COM) and coal/methanol mixtures with a twin-fluid and a wheel atomizer. The slurries are consisted of coal/methanol and coal/oil mixtures with two different concentrations and two coal particile sizes. The volume median diameters of the coal particles were 45 and 130 microns. The oil was No. 2 diesel oil. The droplet size was determined by photographing the spray and the photographs were analyzed to determine droplet size and distribution. The results show that the inclusion of particles in the liquid(both methanol and diesel oil) does not appreciably affect the atomized droplet size. The tendency is for the slurry droplets to be somewhat smaller than the droplets atomized with the pure liquid.

Ignition and combustion of coal-water slurry in a confined turbulent diffusion flame

Characteristics times were estimated for the heating, vaporization, ignition, and combustion of 10 to 100 μm coal-water slurry droplets under conditions measured in 1 MW turbulent diffusion flames with high swirl. Agglomeration of the coal particles in a droplet was assumed to result in a coal particle size identical to the atomized droplet size. The locations of the ignition and combustion processes along the flame axis were determined from droplet trajectories based on estimates of the initial droplet velocity and measured gas velocities. Gas temperature and oxygen profiles recalculated using the predicted spatial distribution of the combustion processes, an estimate of the droplet size distribution, and the properties of the externally recirculated combustion products, were consistent with the measurements. The droplet velocity leaving the atomizer, and the gas velocity in the region of the fuel spray have important effects on the spatial distribution of ignition and combustion through their determination of the droplet trajectories. The presence of 32 to 36 wt% water in a slurry droplet results in an increase of about 50% over the distance required for ignition of an equivalent coal particle under the conditions which existed in the flames.

Some aspects of splat-quenching in an inert atmosphere and of the formation of non-crystalline phases in Al-17.3 at. % Cu, germanium and tellurium

An apparatus for splat-quenching by the gun technique in a sealed, inert atmosphere is described. The importance of a low-oxygen quenching atmosphere in promoting efficient spreading of liquid particles and good thermal contact with the quenching surface is shown. A cooling rate of ∼1010 K sec−1 was estimated from the interlamellar spacing in a quenched Al-17.3 at. % Cu alloy. The process mechanisms of the gun technique are discussed with particular reference to the atomized droplet size and the effective specimen thickness for heat transfer.

Flame characteristics and emission

Several combustion characteristics which play a role in the radiation of flames are reviewed. The limits of inflammability of various combustible mixtures used in analytical flame spectroscopy, together with their ignition temperature, burning and detonation velocity and detonation pressure are examined briefly. The structure of premixed and diffusion flames at various pressures is described in some details and the radiation of several elements excited in the various zones of premixed flames is examined. A procedure for eliminating the background radiations from the blue cone and outer cone of various flames is suggested. The flame temperature and its distribution is analyzed in relation to flame structure, and the meaning of this combustion characteristic in relation to the radiation phenomenon is discussed. Brief considerations of some reactions and ionization in flames follow, the article being concluded with a discussion of the size of atomized droplets and its influence in the calciumphosphorus interference.