Showing papers by "Jyeshtharaj B. Joshi published in 2009"

Dynamics of Flow Structures and Transport Phenomena, 1. Experimental and Numerical Techniques for Identification and Energy Content of Flow Structures

Abstract: Most chemical engineering equipment is operated in the turbulent regime. The flow patterns in this equipment are complex and are characterized by flow structures of wide range of length and time scales. The accurate quantification of these flow structures is very difficult and, hence, the present design practices are still empirical. Abundant literature is available on understanding of these flow structures, but in very few cases efforts have been made to improve the design procedures with this knowledge. There have been several approaches in the literature to identify and characterize the flow structures qualitatively as well as quantitatively. In the last few decades, several numerical as well as experimental methods have been developed that are complementary to each other with the onset of better computational and experimental facilities. In the present work, the methodologies and applications of various experimental fluid dynamics (EFD) techniques (namely, point measurement techniques such as hot film anemometry, laser Doppler velocimetry, and planar measurement techniques such as particle image velocimetry (PIV), high speed photography, Schlieren shadowgraphy, and the recent volume measurement techniques such as holographic PIV, stereo PIV, etc.), and the computational fluid dynamics (CFD) techniques (such as direct numerical simulation (DNS) and large eddy simulation (LES)) have been discussed. Their chronological developments, relative merits, and demerits have been presented to enable readers to make a judgment as to which experimental/numerical technique to adopt. Also, several notable mathematical quantifiers are reviewed (such as quadrant technique, variable integral time average technique, spectral analysis, proper orthogonal decomposition, discrete and continuous wavelet transform, eddy isolation methodology, hybrid POD-Wavelet technique, etc.). All three of these tools (computational, experimental, and mathematical) have evolved over the past 6-7 decades and have shed light on the physics behind the formation and dynamics of various flow structures. The work ends with addressing the present issues, the existing knowledge gaps, and the path forward in this field.

CFD simulation of heat transfer in a two-dimensional vertical enclosure

Abstract: In order to predict the variation in heat transfer coefficient setup by natural convection, independent CFD simulations have been performed for various tall slender vertical geometries (100 mm H L T L ≤ 25 mm), heights (100 mm ≤ H ≤ 1000 mm) (4 ≤ AR ≤ 200) and temperature differences (20 K ≤ Δ T ≤ 90 K, 5.99E + 02 ≤ Ra ≤ 3.15E + 05) and a correlation for the estimation of Nu has been proposed. The sensitivity analysis shows %deviation of heat transfer coefficient only in the range of ±10%. A generalized correlation based on all the above results encompassing the effect of height, gap width and temperature difference has also been proposed, which can be used to accurately estimate heat losses from a vertical air gap acting as an insulation.

Dispersed phase hold-up, effective interfacial area and Sauter mean drop diameter in annular centrifugal extractors

Abstract: Annular centrifugal extractors (ACE), based on the principle of Taylor–Couette flow, offer potential advantages over the existing conventional extraction equipments in many of the engineering applications. In the present work, dispersed phase hold-up (∈D) and effective interfacial area ( a ) have been measured in 30, 75 and 250 mm rotor diameter annular centrifugal extractors over a wide range of power consumption (0.4 a , and d3,2, which are expected to be useful for practicing engineers. Power consumption was also measured for 40, 60 and 100 mm diameter rotors and an appropriate and simple correlation for power number for ACE has been proposed.

Residence Time Distribution and Flow Patterns in the Single-Phase Annular Region of Annular Centrifugal Extractor

Abstract: Flow between two concentric cylinders with high-speed rotation of the inner cylinder, also termed as turbulent Taylor−Couette flow, is an integral part of annular centrifugal extractor (ACE). The vortex motion in the annular region causes intense mixing, of the two liquids, and their separation occurs in the inner cylinder under centrifugal action. In the present work, a systematic study of residence time distribution (RTD) in the annular region of ACE has been carried out experimentally as well as using computational fluid dynamics (CFD). The effects of rotational speed (10 ≤ ω ≤ 40, r/s), aspect ratio of annulus (11 ≤ Γ ≤ 48), width of annular gap (1.5 ≤ d ≤ 6.5, mm), and the flow ratio of the immiscible fluids (0.73 ≤ FR ≤ 2.4) have been systematically investigated. Effect of flow ratio, annular gap, and rotational speed has been investigated on the RTD. It was found that the flow in ACE is near to back-mixed behavior because of the presence of counterrotating vortices. The number of vortices depends o...

Some industrial applications of gamma-ray tomography

Abstract: Various tomographic techniques such as electrical capacitance, electrical resistivity, MRI, X-ray/gamma-ray tomography have been reviewed with special emphasis on applicability for the industrial set-ups are discussed. It was found that gamma-ray tomography is most suitable technique which could be used for industrial set-ups. Gamma-ray tomography was also applied for variety of industrial equipment, such as paddle bed dryer, packed distillation column, to explore the possibility of their use for diagnosis of unidentified problems. From the preset work it is recommended to undertake tomographic scanning of critical equipments periodically to enhance the efficiency and reduce possible downtime.

Hydrodynamic and mass transfer characteristics of asymmetric rotating disc extractors

Abstract: Asymmetric rotating disc contactor (ARDC) offers potential advantages in having comparable mass transfer coefficient ( k C a _ ) and lower back mixing over the conventional rotating disc contactor (RDC). In the present work, measurements of dispersed phase hold-up (∈D), effective interfacial area ( a _ ), continuous phase mass transfer coefficient ( k C a _ ) and axial dispersion coefficient for the dispersed as well as continuous phase have been performed in 80 mm and 150 mm i.d. ARDC over a wide range of operating variables and physical properties. From these measurements, the values of slip velocity and sauter mean drop diameter (d3,2) have been estimated. Suitable correlations have been proposed for ∈D, VS, k C a _ , a _ , kC, d3,2, PeC and PeD. The results on true mass transfer coefficient (kC) have been compared with those published in the literature.

Design of ring and spider type spargers for bubble column reactor: Experimental measurements and CFD simulation of flow and weeping

Abstract: In case of low H/D ratio bubble columns, sparger design governs the performance of bubble column reactors. Therefore, a comprehensive study was undertaken for single ring spargers, multiple ring spargers and spiders covering a wide range of hole diameters (2–6 mm), ring diameter (0.4–1 m), number of holes (19–90), number of rings (1–4), number of arms (3–5) and arm length (0.4–1 m). The flow distribution and critical weep point velocity was related to geometric and operating parameters. CFD simulations (ANSYS CFX-10.0) have been performed for all these cases and very good agreement was observed between the CFD predictions and experimental values of pressure, hole velocity and the critical weep point velocity. A step-wise design procedure for all types of sparger is presented along with a worked example. Further a new sparger (Wheel type) is proposed which would be superior to the existing spargers.

Dynamics of Flow Structures and Transport Phenomena, 2. Relationship with Design Objectives and Design Optimization

Abstract: There have been several approaches in the literature to identify and characterize flow structures qualitatively as well as quantitatively. In the first part of this review, the methodologies and applications of various experimental fluid dynamics and computational fluid dynamics techniques, as well as mathematical techniques, have been discussed. Their chronological developments, and relative merits and demerits, have been presented to allow readers to make a judgment as to which techniques to adopt. In the present part of the review series, a stepwise procedure is suggested for the design of equipment using flow structure knowledge. An attempt has been made to relate the structure properties (such as age, penetration depth, size, shape, and energy content distribution) to the design parameters (such as mixing time, heat- and mass-transfer coefficient, drag coefficient, dissipation rate, etc.). This understanding of flow structures has brought improvements in the formulations of heuristic models of mass and heat transfer. This review makes an effort in developing insights into the views of earlier established analytic and heuristic theories of heat and mass transfer. The recently revealed dynamics of flow structures (as uncovered through the use of various techniques) has helped in furthering the efforts of developing new theories of heat, mass, and momentum transfer. Such an understanding between the structure dynamics and the transport phenomena has helped in the optimization of flow pattern (for instance, maximization of ratios of heat and mass transfer, as well as mixing, with respect to energy input). In this direction, some success stories have been described that have already been implemented in industry and have good potential for implementation.

Computational and Experimental Fluid Dynamics of Jet Loop Reactor

Abstract: A computational analysis using standard k-e model, RSM and LES has been carried out for jet loop reactors (JLR) to investigate the mean and turbulence quantities. These simulations have revealed that the flow in JLR was different from the self-similar round jets. RSM and LES showed better agreement with PIV measurements compared with standard k-e model. The modeled turbulence production and transport in k-e model overpredicted those estimated from LES data. To reduce the limitations, modified k-e models have been evaluated for JLR. Also, a hybrid k-e model has been suggested, which was found to perform better than other modified k-e models. This model was also found to hold for stirred tank reactors (STRs). Mixing time analysis has been carried out for JLR and STR at same power consumption. It has been shown that JLR can be inferior to STR if proper nozzle diameter is not selected.

Use of ultrasound in petroleum residue upgradation

Abstract: Conventional processes for the upgradation of residual feedstocks, viz., thermal cracking and catalytic cracking are carried out in the temperature range of 400–520°C. Such high temperatures can in principle be substituted by acoustic cavitation. In the present work, two vacuum residues, namely, Arabian mix vacuum residue (AMVR) and Bombay high vacuum residue (BHVR) and one asphalt, viz., Haldia asphalt (HA) were subjected to acoustic cavitation for different reaction times from 15 min to 120 min at ambient temperature and pressure. An attempt has been made to seek a performance comparison of two devices of acoustic cavitation, namely, ultrasonic bath and ultrasonic horn with regard to their ability to upgrade the petroleum residues to lighter, more value-added products mainly the hydrocarbons boiling in the range of gas oil fraction. Another attempt has been made to study the effect of ultrasound on the upgradation of the residue when it is emulsified in water with the help of different surfactants. For all the cases, a kinetic model has been developed based on the constituents of the residue so as to get an insight into the reaction mechanism. The study revealed that ultrasonic horn is more effective in bringing about the upgradation than ultrasonic bath and that the acoustic cavitation of the aqueous emulsified hydrocarbon mixture could reduce the asphaltenes content to a greater extent than the acoustic cavitation of non-emulsified hydrocarbon mixture. The reduction in asphaltenes content of BHVR was found to be more followed by AMVR followed by HA. The variation in the rate constants was found to be feed specific and the rate constants for the conditions of maximum conversion of asphaltenes to gas oil for AMVR, BHVR and HA were found to be 0.29 × 10−4 s−1, 1.4 × 10−4 s−1 and 0.23 × 10−4 s−1, respectively. Les procedes traditionnels visant a enrichir la concentration des matieres premieres residuelles, c.-a-d. le craquage thermique et le craquage catalytique, se font a des temperatures variant entre 400 et 520°C. Des temperatures aussi elevees peuvent, en principe, etre substituees par la cavitation acoustique. Dans le cadre de cette etude, deux residus sous vide, a savoir le melange arabe de residus sous vide (MARSV) et le residu de vide pousse de Bombay (RVPB) et un asphalte, c.-a-d. l'asphalte Haldia (AH), ont ete soumis a la cavitation acoustique selon differents temps de reaction, variant entre 15 et 120 minutes, a la temperature et la pression ambiantes. Une tentative a ete faite pour chercher a obtenir une comparaison du rendement des deux dispositifs de cavitation acoustique, a savoir un bain a ultrasons et une sonde a ultrasons, en ce qui a trait a leur capacite a enrichir la concentration des residus de petrole de facon a obtenir des produits plus legers a valeur ajoutee accrue, principalement les hydrocarbures en ebullition dans la gamme de fractions gaz-petrole. Une autre tentative a ete faite pour etudier l'effet des ultrasons sur l'enrichissement de la concentration du residu lorsqu'il est en emulsion dans l'eau avec l'aide de differents agents de surface. Dans tous les cas, on a elabore un modele cinetique fonde sur les elements constitutifs du residu de facon a obtenir un apercu du mecanisme de reaction. L'etude a revele que la sonde a ultrasons est plus efficace pour provoquer l'enrichissement de la concentration que le bain a ultrasons et la cavitation acoustique du melange d'hydrocarbures en emulsion aqueuse pouvait reduire le contenu en asphaltenes dans une plus grande mesure que la cavitation acoustique d'un melange d'hydrocarbures non en emulsion. La reduction du contenu en asphaltenes du RVPB s'est averee etre la plus grande, suivie du MARSV et de l'AH. La variation des constantes de vitesse de reduction s'est averee etre propre a la charge et les vitesses de reduction concernant les conditions de la conversion maximale des asphaltenes en gaz-petrole du MARSV, du RVPB et de l'AH se sont averees etre de 0,29 × 10−4 s−1, 1,4 × 10−4 s−1 et 0,23 × 10−4 s−1, respectivement. Mots-cles: ultrason, cavitation, residu de petrole, agent de surface, emulsion, cinetique.

Solid−Liquid Circulating Multistage Fluidized Bed: Hydrodynamic Study

Abstract: The solid-liquid circulating multistage fluidized bed (SLCMFB) was constructed with the principal components being the riser column and the multistage column. The riser column was made up of 50 mm i.d. and 2 m long glass pipe while the multistage column was made up of seven glass stages of 100 mm i.d. and 100 mm length. The ion-exchange resin was used as solid phase and water as fluidizing medium. The flow characteristics of SLCMFB were investigated for 0.365, 0.605, and 0.725 mm particle sizes (dry basis). The voidage in the riser and multistage column was measured using γ-ray tomography (GRT). In the riser column, the voidage was found to be maximum at the center and minimum near the wall. However, in the multistage column, voidage was found to be uniform over cross section of the column. The solid particle velocity was measured using ultrasonic velocity profiler (UVP) in the riser column. The particle velocity profiles show similar trend as the voidage, confirming the existence of the radial non-uniformity in the riser column. Further, the non-uniformity was analyzed using the drift flux model (DFM) and the effect of particle size and superficial liquid velocity on the non-uniformity was also investigated.

Development of Unified Correlations for Volumetric Mass-Transfer Coefficient and Effective Interfacial Area in Bubble Column Reactors for Various Gas−Liquid Systems Using Support Vector Regression

Abstract: The objective of this study was to develop a unified correlation for the volumetric mass-transfer coefficient (k L a) and effective interfacial area (a) in bubble columns for various gas-liquid systems using support vector regression (SVR-) based modeling technique. From the data published in the open literature, 1600 data points from 27 open sources spanning the years 1965-2007 for k L a and 1330 data points from 28 open sources spanning the years 1968-2007 for a were collected. Generalized SVR-based models were developed for the relationship between k L a (and a) and each design and operating parameters such as column and sparger geometry, gas-liquid physical properties, operating temperature, pressure, superficial gas velocity, and so on. Further, these models for k L a and a are available online at http://www.esnips.com/web/UICT-NCL. The proposed generalized SVR-based correlations for k L a and a have prediction accuracies of 99.08% and 98.6% and average absolute relative errors (AAREs) of 7.12% and 5.01%, respectively. Also, the SVR-based correlation provided much improved predictions compared to those obtained using empirical correlations from the literature.

Development of correlations for overall gas hold-up, volumetric mass transfer coefficient, and effective interfacial area in bubble column reactors using hybrid genetic algorithm-support vector regression technique: viscous newtonian and non-newtonian liquids

Abstract: The objective of this study was to develop hybrid genetic algorithm−support vector regression (GA-SVR)-based correlations for overall gas hold-up (ϵG), volumetric mass-transfer coefficient (kLa), and effective interfacial area (a) in bubble column reactors for gas−liquid systems employing viscous Newtonian and non-Newtonian systems as the liquid phase. The hybrid GA-SVR is a novel technique based on the feature generation approach using genetic algorithm (GA). In the present study, GA has been used for nonlinear rescaling of attributes. These, exponentially scaled, are eventually subjected to SVR training. The technique is an extension of conventional SVR technique, showing relatively enhanced results. For this purpose an extensive literature search was done. From the published literature, 1629 data points for viscous Newtonian and 845 data points for viscous non-Newtonian systems for ϵG, 500 data points for viscous Newtonian and 556 data points for viscous non-Newtonian systems for kLa, and 208 data poin...

Effect of Flow Structures on Heat Transfer in Single and Multiphase Jet Reactors

Abstract: High frequency experimental measurements by hot film anemometry (HFA) of liquid velocities and temperature in the region of vapor−liquid (VL) and solid−liquid (SL) interfaces for two important reac...

Evaluation of Local Turbulent Energy Dissipation Rate using PIV in Jet Loop Reactor

Abstract: In the present work, experimental analysis has been carried out for jet loop reactors to investigate the mean and turbulence parameters. Measurements have been made using particle image velocimetry (PIV), laser Doppler velocimetry (LDV), and hot film anemometry (HFA). The major aim of this work is to evaluate the local value of turbulent dissipation rate (e) using PIV. LDV has been used as a benchmark for analyzing the mean flow quantities, while HFA has been mainly used for the evaluation of energy spectra. Thus, the relative strength of these instruments was harnessed for better understanding of the flow pattern. A method has been proposed for the evaluation of e from the experimental data from PIV and HFA based on structure function relationships and 3D energy spectrum. Overall energy balance has been satisfied implicitly to make the method of e estimation more robust. As an application, the values of e obtained from the CFD predictions (Large eddy simulations) have been compared with that of the experimental values showing opportunity for improved design of turbulence based on local e variations.

Stereo PIV/Shadowgraphy used to study bubble characteristics in turbulent flows

Abstract: A combined PIV/Shadowgraphy setup has been used to investigate liquid and bubble motion in a grid-generated turbulent flow field. Simultaneous measurement of the liquid velocity flow field and bubble shape and rise velocity were performed for single- and multi-bubble systems at different turbulent energy inputs. Experiments were carried out for bubbles up to an equivalent spherical diameter of 10mm in both clean water and with surfactants present. It was found that the motion (rise velocity) of the bubbles was influenced by the level of turbulence and also by the presence of the surfactant. Further study is required with finer PIV resolution and higher recording rate to discern exact effect of the enhanced interface rigidity and the alteration of the vorticity field by the incident turbulence.