Begell House Inc.
Atomization and Sprays
AAS
1044-5110
16
7
2006
CHALLENGES FOR FUTURE RESEARCH IN ATOMIZATION AND SPRAY TECHNOLOGY: ARTHUR LEFEBVRE MEMORIAL LECTURE
727-736
10.1615/AtomizSpr.v16.i7.10
Norman
Chigier
Department of Mechanical Engineering,
Carnegie-Mellon University, Pittsburgh, PA 15213-3890, USA
The current state-of-the-art of atomizer design, aircraft engines, diesel and gasoline engines, medical sprays, spray painting of automobiles, and laser optical instruments is reviewed. Future research for several decades will be driven by increasingly urgent demands to improve fuel and energy efficiency, and to drastically reduce the emission of pollutants. A much greater degree of control is required, not only in spray systems, but for pressure and flow rates which are introduced into atomizers. An improved understanding and control of liquid and air flows inside atomizers will be necessary. The breakup of liquid jets and sheets in many current spray systems results in chaotic generation of drop sizes and velocities. Electrosprays and ultrasonic sprays provide the means to generate more steady and controlled sprays. Major breakthroughs in the physics of diesel injector sprays have been achieved by using high intensity x-rays from the Synchroton at Argonne Labs. Moving away from single injectors to multiple injectors and replacing drilling of holes with etching and lamination allows multiple nozzles, with air and liquid swirl passages, to generate micro-sized droplets, fully dispersed in combustion chambers. The Federal Drug Administration has authorized the inhalation of insulin for treatment of diabetes. Aerosol particle sizers must be in the range of one to four microns. Stents, for the support of collapsed arteries, must be spray-coated with medication. Cryogenic sprays are used for cooling the skin during dermatological skin surgery. These are examples of important developments in the field of medicine. Future collaboration between physicists, mechanical, chemical and materials engineers, mathematicians, computationalists, and experimentalists, with industrial designers and engineers, will offer many opportunities for young engineers to establish careers in atomization and spray technology.
INEXPENSIVE AIR-ASSIST ATOMIZATION FROM 80,000 ORIFICES
737-748
10.1615/AtomizSpr.v16.i7.20
Thomas J.
Hoverman
School of Aeronautics and Astronautics, Purdue University, 1375 Aviation Drive, West Lafayette, IN 47907-2015, USA
Steven
Collicott
Purdue University
A new method to produce small drop sizes with large liquid mass flow rates is demonstrated. In the present example, droplets with Sauter mean diameter of 16−80 μ;m are produced with liquid mass flow rates of 0.4−1.5 L/min (0.1−0.4 gallons/min). The application of inexpensive existing technology has produced a device operating up to 80,000 orifices in parallel in an air-assist mode. Capillary techniques from decades of successful on-orbit control of liquid rocket propellants in satellites are used to deliver the liquid and the air to the multitude of orifices in a controlled manner. The air and liquid are maintained as separate flows until the atomization process. Pressure drops and flow rates are presented for several configurations. Droplet size distributions are measured by laser diffraction droplet sizing.
FURTHER DEVELOPMENTS OF A NOVEL SELF-DRIVEN SPRAY NOZZLE
749-762
10.1615/AtomizSpr.v16.i7.30
Edward H.
Owens
Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, United Kingdom
Weiping
Liu
Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, United Kingdom
George H.
Smith
Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, United Kingdom
Mark T.
Leonard
Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, United Kingdom
An experimental study of a further development of a novel atomizing nozzle is presented. The novelty of the nozzle lies in the incorporation of a self-driven, hydraulic turbine, which provides improved atomization performance. The inclusion of this rotating component promotes smaller particle sizes within the plume for a given pressure and flow rate. A new version of the nozzle is described in terms of both design and performance. Previous versions of the nozzle produced spray plumes by a variety of atomization processes. This new version incorporates a series of enclosed spray channels that provide a spinning disk-type atomization effect. The effect on the spray plume is to produce a Sauter mean diameter that varies with fluid feed pressure and a plume spray density that is similar to a solid cone-type plume. The Sauter mean diameter was found to vary within the range of 80−190 μ;m. The observed droplet size distributions suggest that the nozzle produces a spray similar to that produced by spinning disk atomizers in the outer region of the spray plume. The volumetric concentration in the inner spray is reduced to 40% of that in the outer plume, and the droplets present in this region are smaller than in the outer plume.
VISCOUS POTENTIAL FLOW ANALYSIS OF STRESS-INDUCED CAVITATION IN AN APERTURE FLOW
763-776
10.1615/AtomizSpr.v16.i7.40
T.
Funada
Department of Digital Engineering, Numazu College of Technology, 3600 Ooka, Numazu, Shizuoka, 410-8501, Japan
J.
Wang
Department of Aerospace Engineering and Mechanics, University of Minnesota, 110 Union St. SE, Minneapolis, MN 55455
Daniel D.
Joseph
University of Minnesota, AEM, 107 Akerman Hall, 110 Union Street, Minneapolis, MN 55455, USA
Cavitation in an aperture flow in a flat plate is studied using viscous potential flow. The maximum tension criterion for cavitation used here was proposed by Joseph [Phy. Rev. E, vol. 51, pp. 1649−1650, 1995; J. Fluid Mech., vol. 366, 367−378, 1998]: “Liquids at atmospheric pressure which cannot withstand tension will cavitate when and where tensile stresses due to motion exceed one atmosphere. A cavity will open in the direction of the maximum tensile stress which is 45° from the plane of shearing in pure shear of a Newtonian fluid.” The aperture flow is expressed using a complex potential and the stress is calculated using viscous potential flow. We find that the viscous stress is huge near the tips of the aperture, thus cavitation could be induced.
QUANTIFYING AIR ATOMIZATION OF VISCOELASTIC FLUIDS THROUGH FLUID RELAXATION TIMES
777-790
10.1615/AtomizSpr.v16.i7.50
Lynn M.
Walker
Center for Complex Fluids Engineering, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213
Yenny
Christanti
Center for Complex Fluids Engineering, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213
The industrial need to control the atomization of non-Newtonian fluids necessitates an understanding of the influence of fluid rheology and different viscoelastic properties on this complex process. Previous studies of atomization of a pertinent class of non-Newtonian fluids—polymer solutions—provide a wealth of qualitative observations of the effects of viscoelasticity but are often hindered by the inability to decouple different types of non-Newtonian behavior. Here we use a series of well-characterized polymer solutions whose non-Newtonian behavior are dominated by extensional hardening and utilize a fluid relaxation time τ—the key viscoelastic parameter characterizing the onset of extensional hardening—to quantify the atomization process. The model fluids are sprayed via an air atomizer, and the drop size distributions are measured using a diffraction-based size analyzer. The atomization study shows that viscoelasticity increases the mean drop diameter and broadens the size distribution. We incorporate fluid relaxation time into a drop size correlation that predicts the Sauter mean diameter.
IMPACT WAVE-BASED MODEL OF IMPINGING JET ATOMIZATION
791-806
10.1615/AtomizSpr.v16.i7.60
Robert J.
Santoro
Propulsion Engineering Research Center and Department of Mechanical Engineering, The Pennsylvania State University, University Park, Pennsylvania
Harry M.
Ryan, III
NASA, Stennis Space Center, MS 39529
William E.
Anderson
315 N. Grant Street, Purdue University, West Lafayette, IN 47907
Previous studies of impinging jet atomizers indicated that “impact waves” may dominate the atomization of high-speed turbulent impinging jets. An experiment was conducted to characterize the formation and effects of impact waves on the atomization process. The model flow consisted of opposed turbulent water jets at atmospheric conditions. The impact waves are formed with a characteristic wavelength of about one jet diameter, and the distance between the waves was found to increase with distance from the impingement point due to wave merging, which helps explain discrepancies reported in the previous studies. A computational study of the flow structure around the stagnation point showed that the effects of impingement extend about one jet diameter upstream and that maximum gradients and incipient disruption of the surface occur at a normalized radius of 1.2, where an inflection in the jet flow from predominantly axial to predominantly radial occurs. Using these observations and measurements, and existing correlations for breakup length and drop size, a three-step phenomenological model of atomization (impact wave formation and propagation, sheet breakup into ligaments, and ligament disintegration into drops) was developed.
MODELING INTERPHASE TURBULENT KINETIC ENERGY TRANSFER IN LAGRANGIAN-EULERIAN SPRAY COMPUTATIONS
807-826
10.1615/AtomizSpr.v16.i7.70
M. G.
Pai
Department of Mechanical Engineering, Iowa State University, Ames, IA 50011
Shankar
Subramaniam
Iowa State University
Modeling turbulent multiphase flows, such as sprays, is a major challenge owing to droplet (or solid-particle) interactions with a wide range of turbulence length and time scales. In a broad class of Lagrangian-Eulerian models, the instantaneous Lagrangian dispersed-phase velocity evolves on a time scale that is proportional to the particle response time τp = (ρddp2)/ (ρf18νf). Numerical simulations of a model from this class reveal a nonmonotonic and unphysical increase of the turbulent kinetic energy (TKE) in the dispersed phase kd that is not seen in direct numerical simulations (DNS) of decaying, homogeneous turbulence laden with solid particles. Analysis of this class of models shows that for a linear drag law corresponding to the Stokes regime, the entire class of models will predict an anomalous increase in kd for decaying turbulent flow laden with solid particles or droplets. Even though the particle response time is the appropriate time scale to characterize momentum transfer between sub-Kolmogorov-size dispersed-phase particles and the smallest turbulent eddies (for droplet/particle Reynolds number of < 1), it is incapable of capturing the range of time- and length-scale interactions that are reflected in the evolution of kd. A new model that employs a time scale based on a multiscale analysis is proposed. This model succeeds in capturing the dispersed-phase TKE and fluid-phase TKE evolution observed in DNS. The model also correctly predicts the trends of TKE evolution in both phases for different Stokes numbers.
INFLUENCE OF WATER SPRAYS AND HEAT LOSS ON NEGATIVELY AND POSITIVELY STRETCHED CURVED PREMIXED FLAMES
827-842
10.1615/AtomizSpr.v16.i7.80
Jiann-Chang
Lin
Department of General Education, Transworld Institute of Technology, Touliu City, Yunlin County 640, Taiwan, R.O.C.
Shuhn-Shyurng
Hou
Department of Mechanical Engineering, Kung Shan Institute of Technology, Tainan, Taiwan, Republic of China
In the present study, the structure of normal (or inverted) Bunsen flame tips under the influence of a monodisperse dilute water spray and external heat loss is investigated using large activation energy asymptotics. We consider two flame structures: normal and inverted Bunsen flames, and two spray modes: completely and partially prevaporized burnings. In this way, a complete parametric study of flame tip intensification or extinction (opening) can be conducted. Five parameters are used in the analysis. Three are the droplet size, amount of liquid-water loading (which indicates internal heat loss for inert spray), and the external heat loss. The other two are the stretch and Lewis number (Le). Stretch is negative for a normal Bunsen flame but positive for an inverted Bunsen flame. Stretch strengthens (or weakens) the burning intensity of the Le > 1 (or Le < 1) normal Bunsen flame but decreases (or increases) the burning intensity of the Le > 1 (or Le < 1) inverted Bunsen flame. Burning intensity of the flame tip weakens when the water spray has a smaller droplet size or a larger amount of liquid loading, or when the curved flame experiences a larger amount external heat loss. For a rich methane-air normal Bunsen flame with Le > 1 or a lean methane-air inverted Bunsen flame with Le < 1, closed-tip solutions are obtained. Conversely, stretch weakens the burning intensity of a rich methane-air inverted Bunsen flame with Le > 1, or a lean methane-air normal Bunsen flame with Le < 1, eventually leading to tip opening. Moreover, the opening becomes wider when the droplet size decreases, liquid loading increases, or external heat loss enlarges.
TWO-DIMENSIONAL DROPLET SIZE AND VOLUME FRACTION DISTRIBUTIONS FROM THE NEAR-INJECTOR REGION OF HIGH-PRESSURE DIESEL SPRAYS
843-855
10.1615/AtomizSpr.v16.i7.90
Jennifer
Labs
Division of Engineering, Colorado School of Mines, Golden, Colorado, USA
Terry
Parker
Florida Polytechnic University, Lakeland, FL 33805, USA
Droplet diameter and volume fraction measurements are reported as a function of radial and axial position near the injector orifice within a high-pressure spray typical of diesel systems. Injection system parameters were peak pressures of ∼ 80 MPa and a single orifice injector with a 0.16 mm diameter and an L/D ratio of ∼ 4. Two cases are presented and discussed in detail; injection into room ambient conditions and injection with combustion (initial conditions: 873 K, 12.5 atm). Scattered light at two infrared wavelengths was collected from a spatially resolved probe volume and, through scattering theory, both Sauter mean diameter and liquid volume fraction were produced. Spray properties were determined as a function of time at a number of points, and these points form a grid based on multiple axial and radial positions within the spray. Results from multiple, yet identical, events were used to construct two-dimensional contour plots of the Sauter mean diameter and volume fraction within the spray.