Pascal Guiraud - Academia.edu (original) (raw)
Papers by Pascal Guiraud
The objective of this paper is to understand how and why the orifice nature (rigid or flexible) g... more The objective of this paper is to understand how and why the orifice nature (rigid or flexible) governs the bubble generation. The differences in orifice nature and properties have strong consequences on the bubbles generated. Indeed, the dynamics of the formation and the nature of the detached bubbles are fundamentally different depending on whether the bubbles are generated from the
The hydrodynamics in a raceway reactor is studied experimentally and numerically. Effects of the ... more The hydrodynamics in a raceway reactor is studied experimentally and numerically. Effects of the paddlewheel speed and geometry are investigated. Simulations successfully predict the flow rate and the impeller power consumption. Mixing rates are related to the flow low-frequency unsteadiness. Even at moderate intensities, wind has a crucial impact on the hydrodynamics. a b s t r a c t Raceways are nowadays the most used large-scale reactors for microalgae culture. This paper focuses on the hydrodynamics in such a reactor, and emphasizes on the effects of the paddlewheel geometry (two impeller configurations are tested). The global hydrodynamics behavior of the raceway (flow velocity, mixing time) is characterized experimentally by the tracer pulse injection method, and local velocity measurements are acquired by Pulsed Ultrasonic Doppler Velocimetry. Finally, the flow is modeled by using the sliding-mesh CFD technique, a method overcoming many limitations of the simulation approaches used in the literature. CFD simulations successfully estimate the flow rate in the reactor and the power consumption of the paddlewheel. It is shown that the mixing efficiency in the raceway reactor is closely related to the low-frequency flow unsteadiness that arises from the periodic motion of the blades. Concerning local velocities, CFD and experimental data are in good agreement at some positions in the reactor, but a significant disagreement is observed at some other locations. Taking into account the wind presence in the simulations reduces the discrepancy between the experimental and numerical results, showing that, even with a moderate intensity, wind has an important effect on the hydrodynamics in the reactor.
International Journal of Multiphase Flow, 2009
The collision between a contaminated spherical bubble and fine particles in suspension is conside... more The collision between a contaminated spherical bubble and fine particles in suspension is considered for r p /r b ( 1 (r p being the radius of the particles in suspension and r b the radius of the bubble). The collision probability or efficiency is defined as the number of particles colliding the bubble surface to the number of particles initially present in the volume swept out by the bubble. In this note we show that the collision probability can be expressed as P c (r p /r b ,Re) = g(r p /r b )f(Re) for both mobile and immobile interfaces. For partially contaminated bubbles a linear or quadratic dependency in r p /r b is found depending on the level of contamination and the value of r p /r b . These behaviors are given by the flux of particles near the surface which is controlled by the tangential velocity for mobile interfaces and by the velocity gradient for immobile interfaces. The threshold value (r p /r b ) th between the r p /r b and (r p /r b ) 2 evolution is shown to vary as sin n(Re) (h clean /n(Re))sin(3h clean /4), h clean being the angle describing the front clean part of the bubble and n(Re) varying from n = 2 to n = 1 from small to large Reynolds number.
Chemical Engineering Journal, 1997
An experimental approach is proposed for the measurement of the local behaviour of continuous and... more An experimental approach is proposed for the measurement of the local behaviour of continuous and dispersed phases in a stirred suspension at low particle concentrations (0.5 vol.%). The basic principle involves the simultaneous measurement of the local velocity and particle size with a phase Doppler velocimeter and the separation of the data obtained from large particles (representing the dispersed phase)
The present work investigates free damped oscillations of an oil drop in water after its release ... more The present work investigates free damped oscillations of an oil drop in water after its release from a capillary tube. Both pure heptane drops and diluted crude oil drops are considered (in the second case the interface is covered by amphiphilic species, natural components of crude oil). Shadowgraph images of the drops are taken by means of a high speed camera and the drop contour is detected by image processing. The axisymmetric drop shape is then decomposed into spherical harmonics, which constitute the eigenmodes of oscillations predicted by the Rayleigh-Lamb theory. Time evolution of each mode is then obtained. The frequency and the damping rate of the principal mode (n=2) are accurately determined and compared with theoretical values for an immobile clean drop oscillating around spherical shape. For pure heptane drops, theoretical value of the frequency agrees well with experiments whereas the damping rate is significantly underestimated by theory. The experimental results cle...
Powder Technology, 1996
This study deals with attrition due to the impacts of crystals against the impeller or the walls ... more This study deals with attrition due to the impacts of crystals against the impeller or the walls of a crystallizer. An attrition experiment in solution is designed in order to isolate the effect of a single impact of a suspension of crystals on a target simulating a part (fixed or mobile) of a crystallizer. The experimental devices and methods dre
Physics of Fluids, 2005
The behavior of millimetric drops rising in water and bouncing under a horizontal plate is studie... more The behavior of millimetric drops rising in water and bouncing under a horizontal plate is studied using a high-speed video camera. The position and velocity of the center of mass of the drops, as well as their deformation are analyzed. The bouncing is found to be very ...
Journal of Fluid Mechanics, 2007
... turbulence at various volume fractions SOPHIE GALINAT1, FR ´ED ´ERIC RISSO2, OLIVIER MASBERN... more ... turbulence at various volume fractions SOPHIE GALINAT1, FR ´ED ´ERIC RISSO2, OLIVIER MASBERNAT1 AND PASCAL GUIRAUD3 ... When the turbulent Weber number is large, the breakup results from the interaction of the drop with a single intense eddy. ...
International Journal of Chemical Reactor Engineering, 2000
The gas-liquid mass transfer from bubbles is estimated by Direct Numerical Simulation for fully c... more The gas-liquid mass transfer from bubbles is estimated by Direct Numerical Simulation for fully contaminated bubbles behaving as solid spheres, partially contaminated spherical bubbles and clean spherical bubbles. Partial contamination of bubble interface is accounted by the Stagnant Cap Model to show the effect of the surfactant on hydrodynamic and mass transfer at low Reynolds number. Hydrodynamics results are validated by comparison with other works of the literature. The numerical mass transfer is then analysed in term of local and averaged Sherwood numbers. The comparison of DNS results with classical relations gives the good scaling of Sherwood with Pe 1 / 3 and Pe 1 / 2 respectively for solid sphere and clean bubble in creeping flow. For partially contaminated bubble and after validation of simulated drag coefficient, the effect of the contamination on mass transfer is shown for several Peclet numbers. A correlation for Sherwood number in function of contamination angle is then proposed in creeping flow.
Chemical Engineering Science, 2005
Collision efficiency for a spherical bubble rising in a uniform concentration of small non-inerti... more Collision efficiency for a spherical bubble rising in a uniform concentration of small non-inertial particles is studied by direct numerical simulations (DNS). The Stokes number of the particles is negligibly small so that the particle trajectories follow the streamlines. The effect of the bubble interface contamination is studied for the flow surrounding the bubble using the spherical cap model. Numerical results are obtained for a wide range of bubble Reynolds number (based on bubble diameter d b ) ranging from 0.01 to 1000 and for different angles of contamination ranging from 0 • to 180 • . The collision efficiency is found to be increased with the Reynolds number and significantly decreased with the level of contamination. Correlations of the numerical results are proposed for efficiencies versus d p /d b (d p being the particle diameter), bubble Reynolds number and interface contamination degree. For clean (respectively, fully contaminated) spherical bubbles, the efficiency evolves as d p /d b (respectively (d p /d b ) 2 ) whatever the bubble Reynolds number and the particle size. For partially contaminated bubbles, efficiency can be scaled with d p /d b or (d p /d b ) 2 depending on both the level of contamination and the particle size.
Chemical Engineering Science, 2005
This work addresses the drop fragmentation process induced by a cross-sectional restriction in a ... more This work addresses the drop fragmentation process induced by a cross-sectional restriction in a pipe. An experimental device of an upward co-current oil-in-water dispersed flow (viscosity ratio λ≈0.5) in a vertical column equipped with a concentric orifice has been designed. Drop break-up downstream of the restriction has been studied using a high-speed trajectography. The first objective of this work deals
Chemical Engineering Science, 1993
-jet-stirred tubular reactor under turbulent regime provide a complete analysis of hydrodynamics ... more -jet-stirred tubular reactor under turbulent regime provide a complete analysis of hydrodynamics and mixing processes. Comparisons with the non-swirling case show how swirl promotes turbulent mixing. Local velocities are obtained by laser Doppler velocimetry. Local mixing characteristics (mean concentration and segregation) are measured by laser-induced fluorescence.
Chemical Engineering Science, 2003
This paper presents experimental and numerical results related to the dynamic behavior of a two-l... more This paper presents experimental and numerical results related to the dynamic behavior of a two-liquid phase mixing layer induced by a gradient of phase fraction at the ow inlet. A particle image velocimetry-based technique has been used together with a refractive index matching method in order to measure the volume phase fraction and the continuous phase velocity in the two-dimensional (2D) ow. The analysis of experimental results reveals the strong unsteady feature of the ow and the development of large-scale coherent structures. Experimental data have been compared to numerical simulations obtained using both a two-uid model and a single-uid mixture model (where only the density di erence is accounted for). The similarities and discrepancies between numerical and experimental results provide an understanding of the relative importance of variable density e ects compared to the two-phase interfacial exchanges in the momentum and the turbulent transport. ?
Chemical Engineering Science, 2007
The aim of this work is to develop a non-intrusive experimental technique to measure oxygen conce... more The aim of this work is to develop a non-intrusive experimental technique to measure oxygen concentration in a liquid phase. This technique relies on the planar laser-induced fluorescence (PLIF). In its classical version, PLIF consists in viewing the presence of a fluorescent dye in a liquid phase by means of a laser sheet. Digital image analysis gives the relationship between local grey levels and the dye concentration. The principle of the oxygen concentration measurement rests on the fact that the oxygen molecules inhibit this fluorescence, in proportion to their concentration. After calibration, analysis of the grey levels gives an image of the 2D field of oxygen concentration. ᭧
Chemical Engineering Science, 2011
A single bubble experiment has been developed for the determination of the capture efficiency of ... more A single bubble experiment has been developed for the determination of the capture efficiency of particles by bubbles in flotation under well-controlled hydrodynamics and physico-chemical conditions. In a glass column, small single bubbles (d b ¼0.22 À 1.16 mm) are produced in pure water and then rise at their terminal velocity through a suspension consisting of spherical glass particles ðd p ¼ 5256 mmÞ where bubble-particle capture takes place. The capture efficiency E capt is calculated as the ratio of the number of particles captured by one bubble to the number of particles present in the volume swept out by this bubble. Images recorded at high optical magnification show that particles slip on the interface, then adhere to air bubbles individually or as aggregates and cover the rear part of bubble surface. The bubble's effective density and interface contamination level are increased by captured particles. As a result, bubble's rising velocity U b is reduced along the experimental device. By establishing the relationship between capture efficiency E capt , bubble rise velocity U b and bubble clean angle y clean , a new approach to measure particle-bubble capture efficiency is proposed. This new experimental technique is applied to provide a new set of data for capture efficiency in the case of bubbles with a clean interface. E capt is found to grow as d b decreases and d p increases, within the range between 0.02 and 0.20, which is in the order of magnitude of experimental results of as well as of numerical results of . These data are favorably compared to numerical modeling of collision efficiency.
Chemical Engineering Science, 2006
A generalized description of the rebound of spherical drops or solid spheres over a wall is propo... more A generalized description of the rebound of spherical drops or solid spheres over a wall is proposed using two parameters: a coefficient of restitution that compares the velocity of restitution to the velocity before impact and the contact time with the wall. During the bouncing, the incident kinetic energy is transferred into deformation energy (stored on the surface for the case of liquid drops or in the bulk for the case of solid particles) and then restored into kinetic energy allowing the particle to leave or not the wall. The corresponding criteria is given by the Stokes number that compares the inertia of the particle (added mass included) and the viscous force exerted on the particle during the drainage of the film formed between the particle and the wall. The general behavior of the coefficient of restitution observed in many experiments can be modelled for solid spheres as well as spherical drops by the use of a unique simple correlation depending on this Stokes number. For solid particles, the contact time with the wall in viscous flows is found to be of the same order as that predicted by the Hertzian theory; hence, the contact with the wall can be described as a discontinuity in the particle motion. On the other hand, for liquid drops, the contact time is significant and of the same order as other characteristic time scales of the particle motion. Therefore, to properly describe the rebound process, both a restitution coefficient and a contact time must be considered. Finally, a simple model is proposed and its predictions are compared with experiments performed for millimetric toluene drops in water. ᭧
Chemical Engineering Science, 2001
An experimental study about the velocity, size and concentration of ultrasonic cavitation bubbles... more An experimental study about the velocity, size and concentration of ultrasonic cavitation bubbles is presented. Ultrasound is provided from a 20 kHz horn dipping vertically into a liquid container. Using the laser phase-Doppler technique, cavitation bubble velocity and size distributions are obtained at various locations inside the ultrasound "eld. Axial and radial pro"les of average bubble velocity, mean diameters and volumetric #ow rate are presented. The in#uence of ultrasound power input is also investigated: it increases bubble velocity quasi-linearly and has more complex e!ect on bubbles diameter. The system investigated possesses similar hydrodynamic properties with typical turbulent circular jet #ows.
Chemical Engineering Science, 1991
Chemical Engineering Science, 2007
A single bubble experiment is developed for the determination of the capture efficiency by rising... more A single bubble experiment is developed for the determination of the capture efficiency by rising bubbles in a uniform concentration of small inertialess glass particles under carefully controlled hydrodynamics and physico-chemical conditions. Air bubbles (0.35-1.3 mm in diameter) rise and reach their terminal velocity in clean water before passing through a suspension of particles (15-56 m in size), where capture takes place. After passing through another zone containing pure water to remove particles trapped in their wake, bubbles release captured particles at the surface from where the particles are collected and counted. A capture efficiency is then calculated as the ratio of the number of particles captured by one rising bubble to the number of particles present in the volume swept out by this bubble. Capture efficiencies range between 10 −3 and 5 × 10 −1 and are in the order of magnitude of the experimental results presented by . The interaction between particles and bubbles. Colloids and Surfaces A: Physicochemical and Engineering Aspects 151, 3-14] as well as of numerical results for collision efficiency presented by Sarrot et al. [2005. Determination of the collision frequency between bubbles and particles in flotation. Chemical Engineering Sciene 60 (22), 6107-6117].
Chemical Engineering Science, 1997
In this paper, a correlation between axial mixing in a discs and doughnuts pulsed extraction colu... more In this paper, a correlation between axial mixing in a discs and doughnuts pulsed extraction column, the geometrical characteristics of the column, and the pulsation conditions is established by numerical experimentation. The calculation method, based on the computational fluid dynamics approach and detailed on an example, consists in evaluating the axial dispersion coefficient from simulations of tracer mixing, transported by
The objective of this paper is to understand how and why the orifice nature (rigid or flexible) g... more The objective of this paper is to understand how and why the orifice nature (rigid or flexible) governs the bubble generation. The differences in orifice nature and properties have strong consequences on the bubbles generated. Indeed, the dynamics of the formation and the nature of the detached bubbles are fundamentally different depending on whether the bubbles are generated from the
The hydrodynamics in a raceway reactor is studied experimentally and numerically. Effects of the ... more The hydrodynamics in a raceway reactor is studied experimentally and numerically. Effects of the paddlewheel speed and geometry are investigated. Simulations successfully predict the flow rate and the impeller power consumption. Mixing rates are related to the flow low-frequency unsteadiness. Even at moderate intensities, wind has a crucial impact on the hydrodynamics. a b s t r a c t Raceways are nowadays the most used large-scale reactors for microalgae culture. This paper focuses on the hydrodynamics in such a reactor, and emphasizes on the effects of the paddlewheel geometry (two impeller configurations are tested). The global hydrodynamics behavior of the raceway (flow velocity, mixing time) is characterized experimentally by the tracer pulse injection method, and local velocity measurements are acquired by Pulsed Ultrasonic Doppler Velocimetry. Finally, the flow is modeled by using the sliding-mesh CFD technique, a method overcoming many limitations of the simulation approaches used in the literature. CFD simulations successfully estimate the flow rate in the reactor and the power consumption of the paddlewheel. It is shown that the mixing efficiency in the raceway reactor is closely related to the low-frequency flow unsteadiness that arises from the periodic motion of the blades. Concerning local velocities, CFD and experimental data are in good agreement at some positions in the reactor, but a significant disagreement is observed at some other locations. Taking into account the wind presence in the simulations reduces the discrepancy between the experimental and numerical results, showing that, even with a moderate intensity, wind has an important effect on the hydrodynamics in the reactor.
International Journal of Multiphase Flow, 2009
The collision between a contaminated spherical bubble and fine particles in suspension is conside... more The collision between a contaminated spherical bubble and fine particles in suspension is considered for r p /r b ( 1 (r p being the radius of the particles in suspension and r b the radius of the bubble). The collision probability or efficiency is defined as the number of particles colliding the bubble surface to the number of particles initially present in the volume swept out by the bubble. In this note we show that the collision probability can be expressed as P c (r p /r b ,Re) = g(r p /r b )f(Re) for both mobile and immobile interfaces. For partially contaminated bubbles a linear or quadratic dependency in r p /r b is found depending on the level of contamination and the value of r p /r b . These behaviors are given by the flux of particles near the surface which is controlled by the tangential velocity for mobile interfaces and by the velocity gradient for immobile interfaces. The threshold value (r p /r b ) th between the r p /r b and (r p /r b ) 2 evolution is shown to vary as sin n(Re) (h clean /n(Re))sin(3h clean /4), h clean being the angle describing the front clean part of the bubble and n(Re) varying from n = 2 to n = 1 from small to large Reynolds number.
Chemical Engineering Journal, 1997
An experimental approach is proposed for the measurement of the local behaviour of continuous and... more An experimental approach is proposed for the measurement of the local behaviour of continuous and dispersed phases in a stirred suspension at low particle concentrations (0.5 vol.%). The basic principle involves the simultaneous measurement of the local velocity and particle size with a phase Doppler velocimeter and the separation of the data obtained from large particles (representing the dispersed phase)
The present work investigates free damped oscillations of an oil drop in water after its release ... more The present work investigates free damped oscillations of an oil drop in water after its release from a capillary tube. Both pure heptane drops and diluted crude oil drops are considered (in the second case the interface is covered by amphiphilic species, natural components of crude oil). Shadowgraph images of the drops are taken by means of a high speed camera and the drop contour is detected by image processing. The axisymmetric drop shape is then decomposed into spherical harmonics, which constitute the eigenmodes of oscillations predicted by the Rayleigh-Lamb theory. Time evolution of each mode is then obtained. The frequency and the damping rate of the principal mode (n=2) are accurately determined and compared with theoretical values for an immobile clean drop oscillating around spherical shape. For pure heptane drops, theoretical value of the frequency agrees well with experiments whereas the damping rate is significantly underestimated by theory. The experimental results cle...
Powder Technology, 1996
This study deals with attrition due to the impacts of crystals against the impeller or the walls ... more This study deals with attrition due to the impacts of crystals against the impeller or the walls of a crystallizer. An attrition experiment in solution is designed in order to isolate the effect of a single impact of a suspension of crystals on a target simulating a part (fixed or mobile) of a crystallizer. The experimental devices and methods dre
Physics of Fluids, 2005
The behavior of millimetric drops rising in water and bouncing under a horizontal plate is studie... more The behavior of millimetric drops rising in water and bouncing under a horizontal plate is studied using a high-speed video camera. The position and velocity of the center of mass of the drops, as well as their deformation are analyzed. The bouncing is found to be very ...
Journal of Fluid Mechanics, 2007
... turbulence at various volume fractions SOPHIE GALINAT1, FR ´ED ´ERIC RISSO2, OLIVIER MASBERN... more ... turbulence at various volume fractions SOPHIE GALINAT1, FR ´ED ´ERIC RISSO2, OLIVIER MASBERNAT1 AND PASCAL GUIRAUD3 ... When the turbulent Weber number is large, the breakup results from the interaction of the drop with a single intense eddy. ...
International Journal of Chemical Reactor Engineering, 2000
The gas-liquid mass transfer from bubbles is estimated by Direct Numerical Simulation for fully c... more The gas-liquid mass transfer from bubbles is estimated by Direct Numerical Simulation for fully contaminated bubbles behaving as solid spheres, partially contaminated spherical bubbles and clean spherical bubbles. Partial contamination of bubble interface is accounted by the Stagnant Cap Model to show the effect of the surfactant on hydrodynamic and mass transfer at low Reynolds number. Hydrodynamics results are validated by comparison with other works of the literature. The numerical mass transfer is then analysed in term of local and averaged Sherwood numbers. The comparison of DNS results with classical relations gives the good scaling of Sherwood with Pe 1 / 3 and Pe 1 / 2 respectively for solid sphere and clean bubble in creeping flow. For partially contaminated bubble and after validation of simulated drag coefficient, the effect of the contamination on mass transfer is shown for several Peclet numbers. A correlation for Sherwood number in function of contamination angle is then proposed in creeping flow.
Chemical Engineering Science, 2005
Collision efficiency for a spherical bubble rising in a uniform concentration of small non-inerti... more Collision efficiency for a spherical bubble rising in a uniform concentration of small non-inertial particles is studied by direct numerical simulations (DNS). The Stokes number of the particles is negligibly small so that the particle trajectories follow the streamlines. The effect of the bubble interface contamination is studied for the flow surrounding the bubble using the spherical cap model. Numerical results are obtained for a wide range of bubble Reynolds number (based on bubble diameter d b ) ranging from 0.01 to 1000 and for different angles of contamination ranging from 0 • to 180 • . The collision efficiency is found to be increased with the Reynolds number and significantly decreased with the level of contamination. Correlations of the numerical results are proposed for efficiencies versus d p /d b (d p being the particle diameter), bubble Reynolds number and interface contamination degree. For clean (respectively, fully contaminated) spherical bubbles, the efficiency evolves as d p /d b (respectively (d p /d b ) 2 ) whatever the bubble Reynolds number and the particle size. For partially contaminated bubbles, efficiency can be scaled with d p /d b or (d p /d b ) 2 depending on both the level of contamination and the particle size.
Chemical Engineering Science, 2005
This work addresses the drop fragmentation process induced by a cross-sectional restriction in a ... more This work addresses the drop fragmentation process induced by a cross-sectional restriction in a pipe. An experimental device of an upward co-current oil-in-water dispersed flow (viscosity ratio λ≈0.5) in a vertical column equipped with a concentric orifice has been designed. Drop break-up downstream of the restriction has been studied using a high-speed trajectography. The first objective of this work deals
Chemical Engineering Science, 1993
-jet-stirred tubular reactor under turbulent regime provide a complete analysis of hydrodynamics ... more -jet-stirred tubular reactor under turbulent regime provide a complete analysis of hydrodynamics and mixing processes. Comparisons with the non-swirling case show how swirl promotes turbulent mixing. Local velocities are obtained by laser Doppler velocimetry. Local mixing characteristics (mean concentration and segregation) are measured by laser-induced fluorescence.
Chemical Engineering Science, 2003
This paper presents experimental and numerical results related to the dynamic behavior of a two-l... more This paper presents experimental and numerical results related to the dynamic behavior of a two-liquid phase mixing layer induced by a gradient of phase fraction at the ow inlet. A particle image velocimetry-based technique has been used together with a refractive index matching method in order to measure the volume phase fraction and the continuous phase velocity in the two-dimensional (2D) ow. The analysis of experimental results reveals the strong unsteady feature of the ow and the development of large-scale coherent structures. Experimental data have been compared to numerical simulations obtained using both a two-uid model and a single-uid mixture model (where only the density di erence is accounted for). The similarities and discrepancies between numerical and experimental results provide an understanding of the relative importance of variable density e ects compared to the two-phase interfacial exchanges in the momentum and the turbulent transport. ?
Chemical Engineering Science, 2007
The aim of this work is to develop a non-intrusive experimental technique to measure oxygen conce... more The aim of this work is to develop a non-intrusive experimental technique to measure oxygen concentration in a liquid phase. This technique relies on the planar laser-induced fluorescence (PLIF). In its classical version, PLIF consists in viewing the presence of a fluorescent dye in a liquid phase by means of a laser sheet. Digital image analysis gives the relationship between local grey levels and the dye concentration. The principle of the oxygen concentration measurement rests on the fact that the oxygen molecules inhibit this fluorescence, in proportion to their concentration. After calibration, analysis of the grey levels gives an image of the 2D field of oxygen concentration. ᭧
Chemical Engineering Science, 2011
A single bubble experiment has been developed for the determination of the capture efficiency of ... more A single bubble experiment has been developed for the determination of the capture efficiency of particles by bubbles in flotation under well-controlled hydrodynamics and physico-chemical conditions. In a glass column, small single bubbles (d b ¼0.22 À 1.16 mm) are produced in pure water and then rise at their terminal velocity through a suspension consisting of spherical glass particles ðd p ¼ 5256 mmÞ where bubble-particle capture takes place. The capture efficiency E capt is calculated as the ratio of the number of particles captured by one bubble to the number of particles present in the volume swept out by this bubble. Images recorded at high optical magnification show that particles slip on the interface, then adhere to air bubbles individually or as aggregates and cover the rear part of bubble surface. The bubble's effective density and interface contamination level are increased by captured particles. As a result, bubble's rising velocity U b is reduced along the experimental device. By establishing the relationship between capture efficiency E capt , bubble rise velocity U b and bubble clean angle y clean , a new approach to measure particle-bubble capture efficiency is proposed. This new experimental technique is applied to provide a new set of data for capture efficiency in the case of bubbles with a clean interface. E capt is found to grow as d b decreases and d p increases, within the range between 0.02 and 0.20, which is in the order of magnitude of experimental results of as well as of numerical results of . These data are favorably compared to numerical modeling of collision efficiency.
Chemical Engineering Science, 2006
A generalized description of the rebound of spherical drops or solid spheres over a wall is propo... more A generalized description of the rebound of spherical drops or solid spheres over a wall is proposed using two parameters: a coefficient of restitution that compares the velocity of restitution to the velocity before impact and the contact time with the wall. During the bouncing, the incident kinetic energy is transferred into deformation energy (stored on the surface for the case of liquid drops or in the bulk for the case of solid particles) and then restored into kinetic energy allowing the particle to leave or not the wall. The corresponding criteria is given by the Stokes number that compares the inertia of the particle (added mass included) and the viscous force exerted on the particle during the drainage of the film formed between the particle and the wall. The general behavior of the coefficient of restitution observed in many experiments can be modelled for solid spheres as well as spherical drops by the use of a unique simple correlation depending on this Stokes number. For solid particles, the contact time with the wall in viscous flows is found to be of the same order as that predicted by the Hertzian theory; hence, the contact with the wall can be described as a discontinuity in the particle motion. On the other hand, for liquid drops, the contact time is significant and of the same order as other characteristic time scales of the particle motion. Therefore, to properly describe the rebound process, both a restitution coefficient and a contact time must be considered. Finally, a simple model is proposed and its predictions are compared with experiments performed for millimetric toluene drops in water. ᭧
Chemical Engineering Science, 2001
An experimental study about the velocity, size and concentration of ultrasonic cavitation bubbles... more An experimental study about the velocity, size and concentration of ultrasonic cavitation bubbles is presented. Ultrasound is provided from a 20 kHz horn dipping vertically into a liquid container. Using the laser phase-Doppler technique, cavitation bubble velocity and size distributions are obtained at various locations inside the ultrasound "eld. Axial and radial pro"les of average bubble velocity, mean diameters and volumetric #ow rate are presented. The in#uence of ultrasound power input is also investigated: it increases bubble velocity quasi-linearly and has more complex e!ect on bubbles diameter. The system investigated possesses similar hydrodynamic properties with typical turbulent circular jet #ows.
Chemical Engineering Science, 1991
Chemical Engineering Science, 2007
A single bubble experiment is developed for the determination of the capture efficiency by rising... more A single bubble experiment is developed for the determination of the capture efficiency by rising bubbles in a uniform concentration of small inertialess glass particles under carefully controlled hydrodynamics and physico-chemical conditions. Air bubbles (0.35-1.3 mm in diameter) rise and reach their terminal velocity in clean water before passing through a suspension of particles (15-56 m in size), where capture takes place. After passing through another zone containing pure water to remove particles trapped in their wake, bubbles release captured particles at the surface from where the particles are collected and counted. A capture efficiency is then calculated as the ratio of the number of particles captured by one rising bubble to the number of particles present in the volume swept out by this bubble. Capture efficiencies range between 10 −3 and 5 × 10 −1 and are in the order of magnitude of the experimental results presented by . The interaction between particles and bubbles. Colloids and Surfaces A: Physicochemical and Engineering Aspects 151, 3-14] as well as of numerical results for collision efficiency presented by Sarrot et al. [2005. Determination of the collision frequency between bubbles and particles in flotation. Chemical Engineering Sciene 60 (22), 6107-6117].
Chemical Engineering Science, 1997
In this paper, a correlation between axial mixing in a discs and doughnuts pulsed extraction colu... more In this paper, a correlation between axial mixing in a discs and doughnuts pulsed extraction column, the geometrical characteristics of the column, and the pulsation conditions is established by numerical experimentation. The calculation method, based on the computational fluid dynamics approach and detailed on an example, consists in evaluating the axial dispersion coefficient from simulations of tracer mixing, transported by