Drafts by Abolfazl Farasat
spray and combustion dynamics, 2009
Sprays are widely used in industry for combustion, coating, painting, and a number of other appli... more Sprays are widely used in industry for combustion, coating, painting, and a number of other applications. They have in common the principle of dividing a continuous liquid phase into a dispersed phase made of numerous droplets. Among all the techniques employed to atomize liquids, very few are fully described by a model or a convenient theory. We summarize in this work the main trends and principles of spray and atomization models. The mechanisms identified as being at the origin of atomization, such as aerodynamic drag, cavitation, turbulence, electrostatic forcing, etc., are listed and the appropriate models are described. Linear instability theory, cavitation models, electrostatic equations, along with Eulerian models, and statistical descriptions of sprays are presented. The need for intense work and improvement of knowledge is brought out in the conclusion.
The conversion of bulk liquid into a dispersion of small droplets ranging in size from submicron ... more The conversion of bulk liquid into a dispersion of small droplets ranging in size from submicron to several hundred microns (micrometers) in diameter is of importance in many industrial processes such as spray combustion, spray drying, evaporative cooling, spray coating, and drop spraying; and has many other applications in medicine, meteorology, and printing. Numerous spray devices have been developed which are generally designated as atomizers, applicators, sprayers, or nozzles.
A spray is generally considered as a system of droplets immersed in a gaseous continuous phase. Sprays may be produced in various ways. Most practical devices achieve atomization by creating a high velocity between the liquid and the surrounding gas (usually air). All forms of pressure nozzles accomplish this by discharging
the liquid at high velocity into quiescent or relatively slow-moving air. Rotary atomizers employ a similar principle, the liquid being ejected at high velocity from the rim of a rotating cup or disc. An alternative method of achieving a high relative velocity between liquid and air is to expose slow-moving liquid into a high-velocity stream of air. Devices based on this approach are usually termed air-assist, airblast or, more
generally, twin-fluid atomizers. Most practical atomizers are of the pressure, rotary, or twin-fluid type. However, many other forms of atomizers have been developed that are useful in special applications. These include “electrostatic” devices in which the driving force for atomization is intense electrical pressure, and ‘ultrasonic’ types in which the
liquid to be atomized is fed through or over a transducer which vibrates at ultrasonic frequencies to produce the short wavelengths required for the production of small droplets. Both electrical and ultrasonic atomizers are capable of achieving fine atomization, but the low liquid flow rates normally associated with these
devices have tended to curtail their range of practical application.
A device-independent framework to classify and describe atomization is developed. This framework ... more A device-independent framework to classify and describe atomization is developed. This framework divides atomizers into various classes based on the geometry of the liquid prior to breakup. These classes are general enough to encompass a wide array of existent atomizers while still describing important aspects of the atomization physics. Across these classes, a limited number of atomization regimes exist that are grouped based on the rate of the atomization processes (disturbance growth and breakdown). Existent classifications are reconsidered to show how they fit into the current construction of five classes (jet, sheet, film, prompt, and discrete parcel) and three modes (bulk fluid, mixed, and surface). The new framework also clarifies the underlying physics of the atomization process. This process consists of the initiation and growth of a disturbance, followed by its breakdown. Several categories of disturbance initiation and disturbance breakdown are described, supported by examples from the literature.
Book Reviews by Abolfazl Farasat
Papers by Abolfazl Farasat
Nuclear Science and Engineering, 1972
Journal of the Franklin Institute, 1965
Keywords: turbine a gaz ; propulsion ; mecanique ; thermodynamique ; aeronautique Reference Recor... more Keywords: turbine a gaz ; propulsion ; mecanique ; thermodynamique ; aeronautique Reference Record created on 2005-11-18, modified on 2016-08-08
Experiments in Fluids, 2009
International Journal of Spray and Combustion Dynamics, 2010
Sprays are widely used in industry for combustion, coating, painting, and a number of other appli... more Sprays are widely used in industry for combustion, coating, painting, and a number of other applications. They have in common the principle of dividing a continuous liquid phase into a dispersed phase made of numerous droplets. Among all the techniques employed to atomize liquids, very few are fully described by a model or a convenient theory. We summarize in this work the main trends and principles of spray and atomization models. The mechanisms identified as being at the origin of atomization, such as aerodynamic drag, cavitation, turbulence, electrostatic forcing, etc., are listed and the appropriate models are described. Linear instability theory, cavitation models, electrostatic equations, along with Eulerian models, and statistical descriptions of sprays are presented. The need for intense work and improvement of knowledge is brought out in the conclusion.
Atomization and Sprays, 2009
Springer, 2009
When a drop is subjected to a surrounding dispersed phase that is moving at an initial relative v... more When a drop is subjected to a surrounding dispersed phase that is moving at an initial relative velocity, aerodynamic forces will cause it to deform and fragment. This is referred to as secondary atomization. In this paper, the abundant literature on secondary atomiza-tion experimental methods, breakup morphology, breakup times, fragment size and velocity distributions, and mod-eling efforts is reviewed and discussed. Focus is placed on experimental and numerical results which clarify the physical processes that lead to breakup. From this, a consistent theory is presented which explains the observed behavior. It is concluded that viscous shear plays little role in the breakup of liquid drops in a gaseous environment. Correlations are given which will be useful to the designer, and a number of areas are highlighted where more work is needed. List of symbols Dimensional a drop acceleration (m/s 2) c velocity of sound (m/s) D 10 drop or fragment arithmetic mean diameter (m) D 30 drop or fragment volume mean diameter (m) D 32 drop or fragment Sauter mean diameter (m) D 43 drop or fragment de Brouckere mean diameter (m) d 0 drop initial spherical diameter (m) d core diameter of drop core at end of sheet-thinning breakup (m) d cro drop cross-stream diameter (m) d str drop stream-wise diameter (m) F D aerodynamic drag force (kg m/s 2) F surf net surface force (kg m/s 2) F l shear force (kg m/s 2) f 0 (D) fragment number PDF (1/m) f 3 (D) fragment volume PDF (1/m) K power-law fluid consistency index (kg/m s (2-n)) k wave number; 2p/k (1/m) MMD drop or fragment mass median diameter (m) q net electrostatic charge (C) q Ra Rayleigh charge limit (C) t time (s) U 0 initial relative velocity between drop and ambient fluid in main flow direction (m/s) U core velocity of drop core relative to ambient fluid (m/s) " U f mean relative velocity of fragments in main flow direction (m/s) V 0 initial relative velocity between drop and ambient fluid perpendicular to main flow direction (m/s) " V f mean relative velocity of fragments in cross-stream direction (m/s) d boundary layer thickness (m) e a electrical permittivity of ambient (C 2 /N m 2) k wavelength (m) k (1) elastic fluid relaxation time (s) l a ambient viscosity (kg/m s) l d drop viscosity (kg/m s) l eff power-law effective viscosity (kg/m s) l sol solvent shear viscosity (kg/m s) q a ambient density (kg/m 3)
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Drafts by Abolfazl Farasat
A spray is generally considered as a system of droplets immersed in a gaseous continuous phase. Sprays may be produced in various ways. Most practical devices achieve atomization by creating a high velocity between the liquid and the surrounding gas (usually air). All forms of pressure nozzles accomplish this by discharging
the liquid at high velocity into quiescent or relatively slow-moving air. Rotary atomizers employ a similar principle, the liquid being ejected at high velocity from the rim of a rotating cup or disc. An alternative method of achieving a high relative velocity between liquid and air is to expose slow-moving liquid into a high-velocity stream of air. Devices based on this approach are usually termed air-assist, airblast or, more
generally, twin-fluid atomizers. Most practical atomizers are of the pressure, rotary, or twin-fluid type. However, many other forms of atomizers have been developed that are useful in special applications. These include “electrostatic” devices in which the driving force for atomization is intense electrical pressure, and ‘ultrasonic’ types in which the
liquid to be atomized is fed through or over a transducer which vibrates at ultrasonic frequencies to produce the short wavelengths required for the production of small droplets. Both electrical and ultrasonic atomizers are capable of achieving fine atomization, but the low liquid flow rates normally associated with these
devices have tended to curtail their range of practical application.
Book Reviews by Abolfazl Farasat
Papers by Abolfazl Farasat
A spray is generally considered as a system of droplets immersed in a gaseous continuous phase. Sprays may be produced in various ways. Most practical devices achieve atomization by creating a high velocity between the liquid and the surrounding gas (usually air). All forms of pressure nozzles accomplish this by discharging
the liquid at high velocity into quiescent or relatively slow-moving air. Rotary atomizers employ a similar principle, the liquid being ejected at high velocity from the rim of a rotating cup or disc. An alternative method of achieving a high relative velocity between liquid and air is to expose slow-moving liquid into a high-velocity stream of air. Devices based on this approach are usually termed air-assist, airblast or, more
generally, twin-fluid atomizers. Most practical atomizers are of the pressure, rotary, or twin-fluid type. However, many other forms of atomizers have been developed that are useful in special applications. These include “electrostatic” devices in which the driving force for atomization is intense electrical pressure, and ‘ultrasonic’ types in which the
liquid to be atomized is fed through or over a transducer which vibrates at ultrasonic frequencies to produce the short wavelengths required for the production of small droplets. Both electrical and ultrasonic atomizers are capable of achieving fine atomization, but the low liquid flow rates normally associated with these
devices have tended to curtail their range of practical application.