Jyeshtharaj Joshi - Academia.edu (original) (raw)
Papers by Jyeshtharaj Joshi
Expansion behaviour of a binary solid-liquid fluidised bed (SLFB) system with different initial m... more Expansion behaviour of a binary solid-liquid fluidised bed (SLFB) system with different initial mass of solids was studied both experimentally and numerically. Three different sizes (3, 5 & 8 mm diameter) of borosilicate glass beads of equal density (2230 kgm-3) were used as fluidised particles. Three different combinations of particle size pairs of both equal and unequal mass ratios were used using a constant liquid (water) superficial velocity of 0.17 ms-1 in all the cases. Numerically, a two dimensional Eulerian-Eulerian (E-E) CFD model incorporating kinetic theory of granular flow (KTGF) was developed to predict the bed expansion behaviour. It was observed that complete bed segregation occurred when the difference between the solid particle diameters was higher while lower difference in particle diameters led to partial bed segregation. The CFD model also predicted these behaviours which were in good agreement with the experimental data.
AIChE Journal, 2015
Hydrodynamics of collision interactions between a particle and gas-liquid interface such as dropl... more Hydrodynamics of collision interactions between a particle and gas-liquid interface such as droplet/film is of keen interest in many engineering applications. The collision interaction between a suspended liquid (water) film of thickness 3.41 ± 0.04 mm and an impacting hydrophilic particle (glass ballotini) of different diameters (1.1–3.0 mm) in low particle impact Weber number ( We = ρlvp2dp/σ) range (1.4–33) is reported. Two distinct outcomes were observed—particle retention in the film at lower Weber number and complete penetration of the film toward higher Weber number cases. A collision parameter was defined based on energy balance approach to demarcate these two interaction regimes which agreed reasonably well with the experimental outcomes. It was shown that the liquid ligament forming in the complete penetration cases breaks up purely by “dripping/end pinch-off” mechanism and not due to capillary wave instability. An analytical model based on energy balance approach was proposed to determine the liquid mass entrainment associated with the ligament which compared well with the experimental measurements. A good correlation between the %film mass entrained and the particle Bond number ( Bo = ρlgdp2/σ) was obtained which indicated a dependency of Bo1.72. Computationally, a three-dimensional CFD model was developed to simulate these interactions using different contact angle boundary conditions which in general showed reasonable agreement with experiment but also indicated deficiency of a constant contact angle value to depict the interaction physics in entirety. The computed force profiles from computational fluid dynamics (CFD) model suggest dominance of the pressure force over the viscous force almost by an order of magnitude in all the Weber number cases studied. © 2015 American Institute of Chemical Engineers AIChE J, 62: 295–314, 2016
Sadhana, 2015
In this paper, a review is presented on the experimental investigations and the numerical simulat... more In this paper, a review is presented on the experimental investigations and the numerical simulations performed to analyze the thermal-hydraulic performance of the air-cooled heat exchangers. The air-cooled heat exchangers mostly consist of the finned-tube bundles. The primary role of the extended surfaces (fins) is to provide more heat transfer area to enhance the rate of heat transfer on the air side. The secondary role of the fins is to generate vortices, which help in enhancing the mixing and the heat transfer coefficient. In this study, the annular and plate fins are considered, the annular fins are further divided into four categories: (1) plane annular fins, (2) serrated fins, (3) crimped spiral fins, (4) perforated fins, and similarly for the plate fins, the fin types are: (1) plain plate fins, (2) wavy plate fins, (3) plate fins with DWP, and (4) slit and strip fins. In Section 4, the performance of the various types of fins is presented with respect to the parameters: (1) Reynolds number, (2) fin pitch, (3) fin height, (4) fin thickness, (5) tube diameter, (6) tube pitch, (7) tube type, (8) number of tube rows, and (9) effect of dehumidifying conditions. In Section 5, the conclusions and the recommendations for the future work have been given.
Procedia Engineering, 2015
Coarse particle (typically more than 100 micrometers in diameter) flotation is adversely influenc... more Coarse particle (typically more than 100 micrometers in diameter) flotation is adversely influenced by liquid motion resulting from energy input associated with mixing of the gas and solid phases. In particular, the collected particles can become detached from the bubble as the particle-bubble aggregate passes through regions of different turbulent levels. The dynamics of particlebubble-turbulence interaction is almost impossible to visualize within a real flotation environment as the phases are in constant motion which changes with time and position. To study the phenomenon of the particle bubble detachment process the problem was mimicked in such a way as to have a bubble detaching from a stationary 3 mm diameter steel particle in the turbulent field. A bubble of known volume was firstly introduced onto the submerged particle surface via a syringe needle. Image analysis was used to determine the bubble-particle contact angle and radial position of the three phase contact line under quiescent conditions. An oscillating grid device was then used to generate turbulent liquid motion around the particle-bubble aggregate. Particle image velocimetry (PIV) was used to quantify the instantaneous velocity field around the disturbed bubble. Laser induced-fluorescence (LIF) was applied to filter out the (green) internally reflected light from the bubble so that only the (orange) light from the fluorescing seeding particles was collected. The PIV-LIF images were then analysed by firstly utilising a masking technique to eliminate spurious velocity vectors inside the bubble. The velocity data in an envelope surrounding the bubble was extracted to calculate local, instantaneous values of liquid velocity, turbulent kinetic energy and energy dissipation rate. It was found that the flow structures generated by the oscillating grids resulted in a lateral inclination of the gas-liquid interface at the three phase contact line. The subsequent change in the dynamic contact angle resulted in a reduction in the capillary (attachment) force, and at a high enough turbulence level it became less than the buoyancy (lift-off) force and detachment of the bubble from the particle surface took place. The detachment events observed in this study is analogous to what actually takes place in mineral flotation cells where the bubble-particle aggregate is in motion.
Chemical Engineering Research and Design, 2015
ABSTRACT Vaporization of atomised feedstock is one of the critical processes in Fluid Catalytic C... more ABSTRACT Vaporization of atomised feedstock is one of the critical processes in Fluid Catalytic Cracking (FCC) risers; which is more often ignored in most of the FCC riser modelling studies. In this study, two different vaporization mechanisms of feedstock namely homogeneous mode and heterogeneous mode were studied. Different homogeneous models duly validated for various pure component droplets were applied to predict the vaporization time of the feed droplets typically expected in FCC feed vaporization zone. A new physical model for heterogeneous vaporization considering droplet-particle collision mechanics was also developed in the present study which compared well with the other existing heterogeneous modelling approaches. Comparison of the two vaporization modes indicates that under typical operating conditions of FCC riser, vaporization time of feed droplets predicted by heterogeneous mode is always lower than the homogeneous mode at least by an order of magnitude due to significant increase in heat transfer coefficient which accounts for droplet-particle contact. It is expected that actual vaporization time of feed droplets in an industrial FCC riser should lie in the range predicted by these two vaporization mechanisms which actually set the two limiting modes of vaporization. Obtained results predicted by the models could be used to aid design of the FCC feed vaporization zone.
ABSTRACT Solid-liquid fluidized beds (SLFB) are of high industrial importance due to higher heat ... more ABSTRACT Solid-liquid fluidized beds (SLFB) are of high industrial importance due to higher heat and mass transfer rates. In many industrial fluidizations, different types of particles (having different size or density or both) are involved in the process. In the design of such multiphase fluidized beds, it is important to understand the bed expansion, as well as the spatial distribution of phase volume fractions, segregation and intermixing of the two solid phases. These characteristics govern the equipment volume and the flow pattern of solid and liquid phases thereby indirectly affecting the rates of mass and momentum transfer and the reactor performance. Detailed information about the phase voidage distribution throughout the bed at different operating conditions is important for design and scale up of the system. In literature, various correlations have been published for the dispersion coefficient based on the empirical studies or theoretical framework. It was thought desirable to compare the relative predictive capabilities of so far proposed correlations. The dispersion coefficients for the phases involved are correlated with the energy dissipation rate within the system. The computational fluid dynamics (CFD) simulations of binary SLFB with particles of different size and/or density have been performed. The segregation and intermixing of the solid phases involved have been studied. It was observed that the CFD predictions show good agreement with the published experimental studies.
Chemical Engineering Journal, 2014
ABSTRACT In this study the transition from homogeneous to heterogeneous flow in a solid–liquid fl... more ABSTRACT In this study the transition from homogeneous to heterogeneous flow in a solid–liquid fluidized bed (SLFB) is examined both experimentally and numerically. The experimental apparatus comprised a refractive index-matched SLFB, comprising 5 mm diameter borosilicate glass and sodium iodine solution, which allowed for both instantaneous particle image velocimetry of the liquid flow field and solids hold-up measurements to be undertaken for superficial liquid velocities in the range of 0.06–0.22 m/s. The motion of individual, spherical steel balls (with diameters 6, 7, 8, 9 mm) was then tracked as it settled through the fluidized bed for differing superficial liquid velocities. It was observed that, for all the steel balls covered in this work, there was a change in slope in their respective classification velocity curves at a superficial liquid velocity of 0.08 m/s. This value was very close to the critical velocity of 0.085 m/s predicted from 1-D linear stability analysis; and therefore deemed to be the critical condition that marked the transition from homogeneous to non-homogenous flow. It is proposed that the change in slope of the classification velocity curve is due to the encounter of the settling foreign particles with liquid bubbles whose presence marks the onset of heterogeneous flow. Additional computational analysis, involving both Eulerian–Eulerian (E–E) and Eulerian–Lagrangian (E–L) approaches, is used to confirm the presence of liquid bubbles at a critical liquid hold-up of 0.54, which corresponds to that predicted from 1-D linear stability analysis. In summary, the study has highlighted that experimentally the transition condition for a SLFB can be obtained simply by observing the behavior of the classification velocity of a single foreign particle at different superficial liquid velocities. This transition condition was found to agree with the 1D linear stability criterion, Eulerian–Eulerian CFD (3D) and Eulerian–Lagrangian DEM (3D) approaches.
Chemical Engineering Science, 2014
ABSTRACT In a liquid fluidised bed system, the motion of each phase is governed by fluid-particle... more ABSTRACT In a liquid fluidised bed system, the motion of each phase is governed by fluid-particle and particle-particle interactions. The particle-particle collisions can significantly affect the motion of individual particles and hence the solid-liquid two phase flow characteristics. In the current work, computational fluid dynamics-discrete element method (CFD-DEM) simulations of a dense foreign particle introduced in a monodispersed solid-liquid fluidised bed (SLFB) have been carried out. The fluidisation hydrodynamics of SLFB, settling behaviour of the foreign particle, fluid-particle interactions, and particle-particle collision behaviour have been investigated. Experiments including particle classification velocity measurements and fluid turbulence characterisation by particle image velocimetry (PIV) were conducted for the validation of prediction results. Compared to those predicted by empirical correlations, the particle classification velocity predicted by CFD-DEM provided the best agreement with the experimental data (less than 10% deviation). The particle collision frequency increased monotonically with the solid fraction. The dimensionless collision frequency obtained by CFD-DEM excellently fit the data line predicted by the kinetic theory for granular flow (KTGF). The particle collision frequency increased with the particle size ratio (d(p2)/d(p1)) and became independent of the foreign particle size for high solid fractions when the fluidised particle size was kept constant. The magnitude of collision force was 10-50 times greater than that of gravitational force and maximally 9 times greater than that of drag force. A correlation describing the collision force as a function of bed voidage was developed for St(p) > 65 and d(p2)/d(p1) <= 2. A maximum deviation of less than 20% was obtained when the correlation was used for the prediction of particle collision force.
Sadhana, 2013
The gas-liquid separation equipments are aimed to be designed for maximum efficiency of phase sep... more The gas-liquid separation equipments are aimed to be designed for maximum efficiency of phase separation. In order to maximize their capacity the flow rates are required to be optimized for the capital cost of equipment. This leads to the situation where the gas phase leaves the separation interface with high velocities and carry liquid phase along with it in the form of droplets reducing the equipment efficiency. This is known as entrainment or carryover. Depending on the nature of the separation interface i.e., turbulence intensity, bubble dynamics, the size and velocity distribution of liquid fragments, droplets at the separation interface varies. This is the main source of empiricism involved in the analysis of such equipments. The mechanics of motion of the dispersed liquid phase in bulk of gas is relatively well studied. In the present paper the various experimental, analytical and numerical investigations carried out to address the issues of entrainment/carryover are carefully analyzed. Further, a critical review has been presented for bringing out a coherent theme and a current status of the subject under reference.
Sadhana, 2013
Nuclear power is currently the fourth largest source of electricity production in India after the... more Nuclear power is currently the fourth largest source of electricity production in India after thermal, hydro and renewable sources of electricity. Currently, India has 20 nuclear reactors in operation and seven other reactors are under construction. Most of these reactors are indigenously designed and built Heavy Water Reactors. In addition, a 300 MWe Advanced Heavy Water Reactor has already been designed and in the process of deployment in near future for demonstration of power production from Thorium apart from enhanced safety features by passive means. India has ambitious plans to enhance the share of electricity production from nuclear. The recent Fukushima accident has raised concerns of safety of Nuclear Power Plants worldwide. The Fukushima accident was caused by extreme events, i.e., large earthquake followed by gigantic Tsunami which are not expected to hit India's coast considering the geography of India and historical records. Nevertheless, systematic investigations have been conducted by nuclear scientists in India to evaluate the safety of the current Nuclear Power Plants in case of occurrence of such extreme events in any nuclear site. This paper gives a brief outline of the safety features of Indian Heavy Water Reactors for prevention and mitigation of such extreme events. The probabilistic safety analysis revealed that the risk from Indian Heavy Water Reactors are negligibly small.
Environmental Forensics, 2014
ABSTRACT The behavioral distribution of the atmospheric turbulence flow over the terrain with cha... more ABSTRACT The behavioral distribution of the atmospheric turbulence flow over the terrain with changes in a rough surface has become one of the most important topics of air pollution research, among such other topics as transportation and dispersion pollutants. In this study, a computational model on atmospheric turbulence flow over a terrain hill shaped with rough surface was investigated under neutral atmospheric conditions. The flow was assumed to be 2D and modeled using computational fluid dynamics (CFD) models, which were numerically solved using Reynolds-averaged Navier-Stokes equations. Rough surface conditions were modeled using a number of windbreak fences regularly spaced on the hill. The mean velocity and turbulent structures such as turbulence intensity and turbulent kinetic energy were investigated in the upwind and downwind regions over the hill, and the numerical models were validated against the wind-tunnel results to optimize the turbulence model. The computational results agreed well with the results obtained from the wind tunnel experiments. The computational results indicate that the mean velocity was observed to increase dramatically around the crest of the upwind slope of the hill. A thick internal boundary layer was observed with a fence on the crest and downwind region of the hill. The reversed flow and recirculation zone were formed in the wake region behind the hill. It was thus determined that turbulent kinetic energy decreases as the mean velocity increases.
CHEMECA 2013, Brisbane, Oct 2, 2013
In the present work, collision behaviour of a solid particle on an unconfined gas-liquid interfac... more In the present work, collision behaviour of a solid particle on an unconfined gas-liquid interface e.g. droplet was studied at low Weber number range. A glass ballotini particle (1.17 mm) was impacted on a stationary water droplet (3.15 mm) at the Weber number range of 0.2 to 3.6 and the collision process was captured by high speed imaging technique. It was observed, at lower to intermediate impact velocity range, that the particle was partially submerged into the droplet and slide along the convex interface whilst at higher impact velocity, the complete penetration was observed. Based on the forces acting on the particle at the interface, a simple model is proposed providing a satisfactory agreement with the experimental observations. Of all the forces involved, surface tension force was found to dominate the collision process in all the cases investigated. A 3D CFD model has also been developed incorporating the dynamic meshing technique with multiphase Volume of Fluid method whic...
h i g h l i g h t s " Axial mixing in annular centrifugal extractors was studied for different ro... more h i g h l i g h t s " Axial mixing in annular centrifugal extractors was studied for different rotor sizes. " Effect of rotor speed and flow rate on axial mixing was quantified. " The effect of helical baffles was studied on axial mixing. " Helical baffles were found to impart a plug flow behavior. " For application in scale-up, correlations were proposed for Peclet number.
A gas-inducing impeller enables efficient recycling of gas from the headspace into the liquid. Hi... more A gas-inducing impeller enables efficient recycling of gas from the headspace into the liquid. Historically, these impellers were used for the first time in froth flotation machines. The various designs of gas-inducing impellers (including those used in froth flotation) could be classified into three categories, depending on the flow pattern coming into and leaving the impeller zone. These are denoted as type 11, type 12, and type 22 systems. The critical impeller speed for the onset of gas induction (N CG) is governed by a balance between the velocity head generated by the impeller and the hydrostatic head above the impeller. A number of correlations (for types 11 and 22) are based on this balance (Bernoulli's equation). The rate of gas induction (Q G) for the type 11 system can be accurately determined by equating the pressure difference (between the impeller zone and the headspace) generated by the impeller and the pressure drop required for the flow of gas. For type 22 systems, the correlations for Q G are mainly empirical in nature. Correlations for the power consumption, fractional gas holdup, mass-transfer coefficient, and so forth are also available in the literature, although these studies on are not comprehensive. A process design algorithm has been presented for the design of gas-inducing impellers. The algorithm consists of the determination of the rate-controlling step, selection of geometry and the operating conditions, and an economic analysis to choose the optimum design. Guidelines have been given about the desired geometry of gas-inducing impellers for achieving different design objectives such as heat transfer, mass transfer, mixing, solid suspension, froth flotation, and so forth. It has been shown that the use of a gas-inducing impeller in a conventional stirred vessel can lead to a substantial increase in the productivity. It has been shown that the optimum geometry may not correspond to the maintenance of equal power consumption per unit volume, or equal tip speed on scale-up. Suggestions have been made for future work in this area.
A computational fluid dynamics (CFD)-code has been developed using finite volume method in Euleri... more A computational fluid dynamics (CFD)-code has been developed using finite volume method in Eulerian framework for the simulation of axisymmetric steady state flows in bubble columns. The population balance equation for bubble number density has been included in the CFD code. The fixed pivot method of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization-I. A fixed pivot technique. Chemical Engineering Science 51, 1311-1332] has been used to discretize the population balance equation. The turbulence in the liquid phase has been modeled by a k. model. The novel feature of the framework is that it includes the size-specific bubble velocities obtained by assuming mechanical equilibrium for each bubble and hence it is a generalized multi-fluid model. With appropriate closures for the drag and lift forces, it allows for different velocities for bubbles of different sizes and hence the proper spatial distributions of bubbles are predicted. Accordingly the proper distributions of gas holdup , liquid circulation velocities and turbulence intensities in the column are predicted. A survey of the literature shows that the algebraic manipulations of either bubble coalescence or break-up rate were mainly guided by the need to obtain the equilibrium bubble size distributions in the column. The model of Prince and Blanch [1990. Bubble coalescence and break-up in air-sparged bubble columns. A.I.Ch.E. Journal 36, 1485-1499] is known to overpredict the bubble collision frequencies in bubble columns. It has been modified to incorporate the effect of gas phase dispersion number. The predictions of the model are in good agreement with the experimental data of Bhole et al. [2006. Laser Doppler anemometer measurements in bubble column: effect of sparger. Industrial & Engineering Chemistry Research 45, 9201-9207] obtained using Laser Doppler anemometry. Comparison of simulation results with the experimental measurements of Sanyal et al. [1999. Numerical simulation of gas-liquid dynamics in cylindrical bubble column reactors. Chemical Engineering Science 54, 5071-5083] and Olmos et al. [2001. Numerical simulation of multiphase flow in bubble column reactors: influence of bubble coalescence and breakup. Chemical Engineering Science 56, 6359-6365] also show a good agreement for liquid velocity and gas holdup profiles.
Bubble dynamics of a single condensing vapor bubble in a subcooled pool boiling system with a cen... more Bubble dynamics of a single condensing vapor bubble in a subcooled pool boiling system with a centrally heated cylindrical tank has been studied in the Rayleigh number range 7.9 × 10 12 < Ra < 1.88 × 10 13. The heat source in the system is steam condensing inside a vertical tube. The tube was placed in the center of the tank (300 mm i.d., 300 mm height) which is well filled with water. Experimental investigation has been carried out with High Speed Camera while Computational Fluid Dynamics (CFD) investigation has been performed using Volume of Fluid (VOF) method. The heat source has been modeled using simple heat balance. The rise behavior of condensing bubbles (change in size during rise and path tracking) was studied and the CFD model was validated both quantitatively and qualitatively.
Expansion behaviour of a binary solid-liquid fluidised bed (SLFB) system with different initial m... more Expansion behaviour of a binary solid-liquid fluidised bed (SLFB) system with different initial mass of solids was studied both experimentally and numerically. Three different sizes (3, 5 & 8 mm diameter) of borosilicate glass beads of equal density (2230 kgm-3) were used as fluidised particles. Three different combinations of particle size pairs of both equal and unequal mass ratios were used using a constant liquid (water) superficial velocity of 0.17 ms-1 in all the cases. Numerically, a two dimensional Eulerian-Eulerian (E-E) CFD model incorporating kinetic theory of granular flow (KTGF) was developed to predict the bed expansion behaviour. It was observed that complete bed segregation occurred when the difference between the solid particle diameters was higher while lower difference in particle diameters led to partial bed segregation. The CFD model also predicted these behaviours which were in good agreement with the experimental data.
AIChE Journal, 2015
Hydrodynamics of collision interactions between a particle and gas-liquid interface such as dropl... more Hydrodynamics of collision interactions between a particle and gas-liquid interface such as droplet/film is of keen interest in many engineering applications. The collision interaction between a suspended liquid (water) film of thickness 3.41 ± 0.04 mm and an impacting hydrophilic particle (glass ballotini) of different diameters (1.1–3.0 mm) in low particle impact Weber number ( We = ρlvp2dp/σ) range (1.4–33) is reported. Two distinct outcomes were observed—particle retention in the film at lower Weber number and complete penetration of the film toward higher Weber number cases. A collision parameter was defined based on energy balance approach to demarcate these two interaction regimes which agreed reasonably well with the experimental outcomes. It was shown that the liquid ligament forming in the complete penetration cases breaks up purely by “dripping/end pinch-off” mechanism and not due to capillary wave instability. An analytical model based on energy balance approach was proposed to determine the liquid mass entrainment associated with the ligament which compared well with the experimental measurements. A good correlation between the %film mass entrained and the particle Bond number ( Bo = ρlgdp2/σ) was obtained which indicated a dependency of Bo1.72. Computationally, a three-dimensional CFD model was developed to simulate these interactions using different contact angle boundary conditions which in general showed reasonable agreement with experiment but also indicated deficiency of a constant contact angle value to depict the interaction physics in entirety. The computed force profiles from computational fluid dynamics (CFD) model suggest dominance of the pressure force over the viscous force almost by an order of magnitude in all the Weber number cases studied. © 2015 American Institute of Chemical Engineers AIChE J, 62: 295–314, 2016
Sadhana, 2015
In this paper, a review is presented on the experimental investigations and the numerical simulat... more In this paper, a review is presented on the experimental investigations and the numerical simulations performed to analyze the thermal-hydraulic performance of the air-cooled heat exchangers. The air-cooled heat exchangers mostly consist of the finned-tube bundles. The primary role of the extended surfaces (fins) is to provide more heat transfer area to enhance the rate of heat transfer on the air side. The secondary role of the fins is to generate vortices, which help in enhancing the mixing and the heat transfer coefficient. In this study, the annular and plate fins are considered, the annular fins are further divided into four categories: (1) plane annular fins, (2) serrated fins, (3) crimped spiral fins, (4) perforated fins, and similarly for the plate fins, the fin types are: (1) plain plate fins, (2) wavy plate fins, (3) plate fins with DWP, and (4) slit and strip fins. In Section 4, the performance of the various types of fins is presented with respect to the parameters: (1) Reynolds number, (2) fin pitch, (3) fin height, (4) fin thickness, (5) tube diameter, (6) tube pitch, (7) tube type, (8) number of tube rows, and (9) effect of dehumidifying conditions. In Section 5, the conclusions and the recommendations for the future work have been given.
Procedia Engineering, 2015
Coarse particle (typically more than 100 micrometers in diameter) flotation is adversely influenc... more Coarse particle (typically more than 100 micrometers in diameter) flotation is adversely influenced by liquid motion resulting from energy input associated with mixing of the gas and solid phases. In particular, the collected particles can become detached from the bubble as the particle-bubble aggregate passes through regions of different turbulent levels. The dynamics of particlebubble-turbulence interaction is almost impossible to visualize within a real flotation environment as the phases are in constant motion which changes with time and position. To study the phenomenon of the particle bubble detachment process the problem was mimicked in such a way as to have a bubble detaching from a stationary 3 mm diameter steel particle in the turbulent field. A bubble of known volume was firstly introduced onto the submerged particle surface via a syringe needle. Image analysis was used to determine the bubble-particle contact angle and radial position of the three phase contact line under quiescent conditions. An oscillating grid device was then used to generate turbulent liquid motion around the particle-bubble aggregate. Particle image velocimetry (PIV) was used to quantify the instantaneous velocity field around the disturbed bubble. Laser induced-fluorescence (LIF) was applied to filter out the (green) internally reflected light from the bubble so that only the (orange) light from the fluorescing seeding particles was collected. The PIV-LIF images were then analysed by firstly utilising a masking technique to eliminate spurious velocity vectors inside the bubble. The velocity data in an envelope surrounding the bubble was extracted to calculate local, instantaneous values of liquid velocity, turbulent kinetic energy and energy dissipation rate. It was found that the flow structures generated by the oscillating grids resulted in a lateral inclination of the gas-liquid interface at the three phase contact line. The subsequent change in the dynamic contact angle resulted in a reduction in the capillary (attachment) force, and at a high enough turbulence level it became less than the buoyancy (lift-off) force and detachment of the bubble from the particle surface took place. The detachment events observed in this study is analogous to what actually takes place in mineral flotation cells where the bubble-particle aggregate is in motion.
Chemical Engineering Research and Design, 2015
ABSTRACT Vaporization of atomised feedstock is one of the critical processes in Fluid Catalytic C... more ABSTRACT Vaporization of atomised feedstock is one of the critical processes in Fluid Catalytic Cracking (FCC) risers; which is more often ignored in most of the FCC riser modelling studies. In this study, two different vaporization mechanisms of feedstock namely homogeneous mode and heterogeneous mode were studied. Different homogeneous models duly validated for various pure component droplets were applied to predict the vaporization time of the feed droplets typically expected in FCC feed vaporization zone. A new physical model for heterogeneous vaporization considering droplet-particle collision mechanics was also developed in the present study which compared well with the other existing heterogeneous modelling approaches. Comparison of the two vaporization modes indicates that under typical operating conditions of FCC riser, vaporization time of feed droplets predicted by heterogeneous mode is always lower than the homogeneous mode at least by an order of magnitude due to significant increase in heat transfer coefficient which accounts for droplet-particle contact. It is expected that actual vaporization time of feed droplets in an industrial FCC riser should lie in the range predicted by these two vaporization mechanisms which actually set the two limiting modes of vaporization. Obtained results predicted by the models could be used to aid design of the FCC feed vaporization zone.
ABSTRACT Solid-liquid fluidized beds (SLFB) are of high industrial importance due to higher heat ... more ABSTRACT Solid-liquid fluidized beds (SLFB) are of high industrial importance due to higher heat and mass transfer rates. In many industrial fluidizations, different types of particles (having different size or density or both) are involved in the process. In the design of such multiphase fluidized beds, it is important to understand the bed expansion, as well as the spatial distribution of phase volume fractions, segregation and intermixing of the two solid phases. These characteristics govern the equipment volume and the flow pattern of solid and liquid phases thereby indirectly affecting the rates of mass and momentum transfer and the reactor performance. Detailed information about the phase voidage distribution throughout the bed at different operating conditions is important for design and scale up of the system. In literature, various correlations have been published for the dispersion coefficient based on the empirical studies or theoretical framework. It was thought desirable to compare the relative predictive capabilities of so far proposed correlations. The dispersion coefficients for the phases involved are correlated with the energy dissipation rate within the system. The computational fluid dynamics (CFD) simulations of binary SLFB with particles of different size and/or density have been performed. The segregation and intermixing of the solid phases involved have been studied. It was observed that the CFD predictions show good agreement with the published experimental studies.
Chemical Engineering Journal, 2014
ABSTRACT In this study the transition from homogeneous to heterogeneous flow in a solid–liquid fl... more ABSTRACT In this study the transition from homogeneous to heterogeneous flow in a solid–liquid fluidized bed (SLFB) is examined both experimentally and numerically. The experimental apparatus comprised a refractive index-matched SLFB, comprising 5 mm diameter borosilicate glass and sodium iodine solution, which allowed for both instantaneous particle image velocimetry of the liquid flow field and solids hold-up measurements to be undertaken for superficial liquid velocities in the range of 0.06–0.22 m/s. The motion of individual, spherical steel balls (with diameters 6, 7, 8, 9 mm) was then tracked as it settled through the fluidized bed for differing superficial liquid velocities. It was observed that, for all the steel balls covered in this work, there was a change in slope in their respective classification velocity curves at a superficial liquid velocity of 0.08 m/s. This value was very close to the critical velocity of 0.085 m/s predicted from 1-D linear stability analysis; and therefore deemed to be the critical condition that marked the transition from homogeneous to non-homogenous flow. It is proposed that the change in slope of the classification velocity curve is due to the encounter of the settling foreign particles with liquid bubbles whose presence marks the onset of heterogeneous flow. Additional computational analysis, involving both Eulerian–Eulerian (E–E) and Eulerian–Lagrangian (E–L) approaches, is used to confirm the presence of liquid bubbles at a critical liquid hold-up of 0.54, which corresponds to that predicted from 1-D linear stability analysis. In summary, the study has highlighted that experimentally the transition condition for a SLFB can be obtained simply by observing the behavior of the classification velocity of a single foreign particle at different superficial liquid velocities. This transition condition was found to agree with the 1D linear stability criterion, Eulerian–Eulerian CFD (3D) and Eulerian–Lagrangian DEM (3D) approaches.
Chemical Engineering Science, 2014
ABSTRACT In a liquid fluidised bed system, the motion of each phase is governed by fluid-particle... more ABSTRACT In a liquid fluidised bed system, the motion of each phase is governed by fluid-particle and particle-particle interactions. The particle-particle collisions can significantly affect the motion of individual particles and hence the solid-liquid two phase flow characteristics. In the current work, computational fluid dynamics-discrete element method (CFD-DEM) simulations of a dense foreign particle introduced in a monodispersed solid-liquid fluidised bed (SLFB) have been carried out. The fluidisation hydrodynamics of SLFB, settling behaviour of the foreign particle, fluid-particle interactions, and particle-particle collision behaviour have been investigated. Experiments including particle classification velocity measurements and fluid turbulence characterisation by particle image velocimetry (PIV) were conducted for the validation of prediction results. Compared to those predicted by empirical correlations, the particle classification velocity predicted by CFD-DEM provided the best agreement with the experimental data (less than 10% deviation). The particle collision frequency increased monotonically with the solid fraction. The dimensionless collision frequency obtained by CFD-DEM excellently fit the data line predicted by the kinetic theory for granular flow (KTGF). The particle collision frequency increased with the particle size ratio (d(p2)/d(p1)) and became independent of the foreign particle size for high solid fractions when the fluidised particle size was kept constant. The magnitude of collision force was 10-50 times greater than that of gravitational force and maximally 9 times greater than that of drag force. A correlation describing the collision force as a function of bed voidage was developed for St(p) > 65 and d(p2)/d(p1) <= 2. A maximum deviation of less than 20% was obtained when the correlation was used for the prediction of particle collision force.
Sadhana, 2013
The gas-liquid separation equipments are aimed to be designed for maximum efficiency of phase sep... more The gas-liquid separation equipments are aimed to be designed for maximum efficiency of phase separation. In order to maximize their capacity the flow rates are required to be optimized for the capital cost of equipment. This leads to the situation where the gas phase leaves the separation interface with high velocities and carry liquid phase along with it in the form of droplets reducing the equipment efficiency. This is known as entrainment or carryover. Depending on the nature of the separation interface i.e., turbulence intensity, bubble dynamics, the size and velocity distribution of liquid fragments, droplets at the separation interface varies. This is the main source of empiricism involved in the analysis of such equipments. The mechanics of motion of the dispersed liquid phase in bulk of gas is relatively well studied. In the present paper the various experimental, analytical and numerical investigations carried out to address the issues of entrainment/carryover are carefully analyzed. Further, a critical review has been presented for bringing out a coherent theme and a current status of the subject under reference.
Sadhana, 2013
Nuclear power is currently the fourth largest source of electricity production in India after the... more Nuclear power is currently the fourth largest source of electricity production in India after thermal, hydro and renewable sources of electricity. Currently, India has 20 nuclear reactors in operation and seven other reactors are under construction. Most of these reactors are indigenously designed and built Heavy Water Reactors. In addition, a 300 MWe Advanced Heavy Water Reactor has already been designed and in the process of deployment in near future for demonstration of power production from Thorium apart from enhanced safety features by passive means. India has ambitious plans to enhance the share of electricity production from nuclear. The recent Fukushima accident has raised concerns of safety of Nuclear Power Plants worldwide. The Fukushima accident was caused by extreme events, i.e., large earthquake followed by gigantic Tsunami which are not expected to hit India's coast considering the geography of India and historical records. Nevertheless, systematic investigations have been conducted by nuclear scientists in India to evaluate the safety of the current Nuclear Power Plants in case of occurrence of such extreme events in any nuclear site. This paper gives a brief outline of the safety features of Indian Heavy Water Reactors for prevention and mitigation of such extreme events. The probabilistic safety analysis revealed that the risk from Indian Heavy Water Reactors are negligibly small.
Environmental Forensics, 2014
ABSTRACT The behavioral distribution of the atmospheric turbulence flow over the terrain with cha... more ABSTRACT The behavioral distribution of the atmospheric turbulence flow over the terrain with changes in a rough surface has become one of the most important topics of air pollution research, among such other topics as transportation and dispersion pollutants. In this study, a computational model on atmospheric turbulence flow over a terrain hill shaped with rough surface was investigated under neutral atmospheric conditions. The flow was assumed to be 2D and modeled using computational fluid dynamics (CFD) models, which were numerically solved using Reynolds-averaged Navier-Stokes equations. Rough surface conditions were modeled using a number of windbreak fences regularly spaced on the hill. The mean velocity and turbulent structures such as turbulence intensity and turbulent kinetic energy were investigated in the upwind and downwind regions over the hill, and the numerical models were validated against the wind-tunnel results to optimize the turbulence model. The computational results agreed well with the results obtained from the wind tunnel experiments. The computational results indicate that the mean velocity was observed to increase dramatically around the crest of the upwind slope of the hill. A thick internal boundary layer was observed with a fence on the crest and downwind region of the hill. The reversed flow and recirculation zone were formed in the wake region behind the hill. It was thus determined that turbulent kinetic energy decreases as the mean velocity increases.
CHEMECA 2013, Brisbane, Oct 2, 2013
In the present work, collision behaviour of a solid particle on an unconfined gas-liquid interfac... more In the present work, collision behaviour of a solid particle on an unconfined gas-liquid interface e.g. droplet was studied at low Weber number range. A glass ballotini particle (1.17 mm) was impacted on a stationary water droplet (3.15 mm) at the Weber number range of 0.2 to 3.6 and the collision process was captured by high speed imaging technique. It was observed, at lower to intermediate impact velocity range, that the particle was partially submerged into the droplet and slide along the convex interface whilst at higher impact velocity, the complete penetration was observed. Based on the forces acting on the particle at the interface, a simple model is proposed providing a satisfactory agreement with the experimental observations. Of all the forces involved, surface tension force was found to dominate the collision process in all the cases investigated. A 3D CFD model has also been developed incorporating the dynamic meshing technique with multiphase Volume of Fluid method whic...
h i g h l i g h t s " Axial mixing in annular centrifugal extractors was studied for different ro... more h i g h l i g h t s " Axial mixing in annular centrifugal extractors was studied for different rotor sizes. " Effect of rotor speed and flow rate on axial mixing was quantified. " The effect of helical baffles was studied on axial mixing. " Helical baffles were found to impart a plug flow behavior. " For application in scale-up, correlations were proposed for Peclet number.
A gas-inducing impeller enables efficient recycling of gas from the headspace into the liquid. Hi... more A gas-inducing impeller enables efficient recycling of gas from the headspace into the liquid. Historically, these impellers were used for the first time in froth flotation machines. The various designs of gas-inducing impellers (including those used in froth flotation) could be classified into three categories, depending on the flow pattern coming into and leaving the impeller zone. These are denoted as type 11, type 12, and type 22 systems. The critical impeller speed for the onset of gas induction (N CG) is governed by a balance between the velocity head generated by the impeller and the hydrostatic head above the impeller. A number of correlations (for types 11 and 22) are based on this balance (Bernoulli's equation). The rate of gas induction (Q G) for the type 11 system can be accurately determined by equating the pressure difference (between the impeller zone and the headspace) generated by the impeller and the pressure drop required for the flow of gas. For type 22 systems, the correlations for Q G are mainly empirical in nature. Correlations for the power consumption, fractional gas holdup, mass-transfer coefficient, and so forth are also available in the literature, although these studies on are not comprehensive. A process design algorithm has been presented for the design of gas-inducing impellers. The algorithm consists of the determination of the rate-controlling step, selection of geometry and the operating conditions, and an economic analysis to choose the optimum design. Guidelines have been given about the desired geometry of gas-inducing impellers for achieving different design objectives such as heat transfer, mass transfer, mixing, solid suspension, froth flotation, and so forth. It has been shown that the use of a gas-inducing impeller in a conventional stirred vessel can lead to a substantial increase in the productivity. It has been shown that the optimum geometry may not correspond to the maintenance of equal power consumption per unit volume, or equal tip speed on scale-up. Suggestions have been made for future work in this area.
A computational fluid dynamics (CFD)-code has been developed using finite volume method in Euleri... more A computational fluid dynamics (CFD)-code has been developed using finite volume method in Eulerian framework for the simulation of axisymmetric steady state flows in bubble columns. The population balance equation for bubble number density has been included in the CFD code. The fixed pivot method of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization-I. A fixed pivot technique. Chemical Engineering Science 51, 1311-1332] has been used to discretize the population balance equation. The turbulence in the liquid phase has been modeled by a k. model. The novel feature of the framework is that it includes the size-specific bubble velocities obtained by assuming mechanical equilibrium for each bubble and hence it is a generalized multi-fluid model. With appropriate closures for the drag and lift forces, it allows for different velocities for bubbles of different sizes and hence the proper spatial distributions of bubbles are predicted. Accordingly the proper distributions of gas holdup , liquid circulation velocities and turbulence intensities in the column are predicted. A survey of the literature shows that the algebraic manipulations of either bubble coalescence or break-up rate were mainly guided by the need to obtain the equilibrium bubble size distributions in the column. The model of Prince and Blanch [1990. Bubble coalescence and break-up in air-sparged bubble columns. A.I.Ch.E. Journal 36, 1485-1499] is known to overpredict the bubble collision frequencies in bubble columns. It has been modified to incorporate the effect of gas phase dispersion number. The predictions of the model are in good agreement with the experimental data of Bhole et al. [2006. Laser Doppler anemometer measurements in bubble column: effect of sparger. Industrial & Engineering Chemistry Research 45, 9201-9207] obtained using Laser Doppler anemometry. Comparison of simulation results with the experimental measurements of Sanyal et al. [1999. Numerical simulation of gas-liquid dynamics in cylindrical bubble column reactors. Chemical Engineering Science 54, 5071-5083] and Olmos et al. [2001. Numerical simulation of multiphase flow in bubble column reactors: influence of bubble coalescence and breakup. Chemical Engineering Science 56, 6359-6365] also show a good agreement for liquid velocity and gas holdup profiles.
Bubble dynamics of a single condensing vapor bubble in a subcooled pool boiling system with a cen... more Bubble dynamics of a single condensing vapor bubble in a subcooled pool boiling system with a centrally heated cylindrical tank has been studied in the Rayleigh number range 7.9 × 10 12 < Ra < 1.88 × 10 13. The heat source in the system is steam condensing inside a vertical tube. The tube was placed in the center of the tank (300 mm i.d., 300 mm height) which is well filled with water. Experimental investigation has been carried out with High Speed Camera while Computational Fluid Dynamics (CFD) investigation has been performed using Volume of Fluid (VOF) method. The heat source has been modeled using simple heat balance. The rise behavior of condensing bubbles (change in size during rise and path tracking) was studied and the CFD model was validated both quantitatively and qualitatively.