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J B Joshi - One of the best experts on this subject based on the ideXlab platform.
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relation between flow pattern and blending in stirred tanks
Industrial & Engineering Chemistry Research, 1999Co-Authors: Ashwin W Patwardhan, J B JoshiAbstract:In the present work, the relationship between the flow pattern and blending has been investigated. The flow patterns generated by around 40 axial flow Impellers have been examined. The Impellers differed in blade angle, blade twist, blade width, Impeller Diameter, Impeller location, and pumping direction. The mean-flow and turbulence characteristics generated by all of the Impellers have been measured using laser doppler velocimetry (LDV). On the basis of available LDV data, the flow pattern throughout the vessel was established by employing computational fluid dynamics (CFD) and subsequently used for the simulation of the blending process. The predicted mixing times were found to be in excellent agreement with the experimental measurements. It has been shown that the dimensionless mixing time (θ) varies inversely with the secondary flow number of the Impeller. Comparison of the Impellers on the basis of equal power consumption per unit mass has shown that θmix ∝ NP1/3T2/3/NQS. The present CFD model has ...
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comparison of axial flow Impellers using a laser doppler anemometer
Industrial & Engineering Chemistry Research, 1992Co-Authors: Vinayak V Ranade, V P Mishra, V S Saraph, G B Deshpande, J B JoshiAbstract:Influence of shapes of eight axial flow Impellers on flow in agitated vessels was studied using a laser Doppler anemometer. The tank Diameter was 500 mm with a flat bottom and provided with four standard (width = T/10) baffles. In all cases the tank to Impeller Diameter ratio was 3 and the Impellers were centrally located. The flow generated by different axial Impellers has been compared in terms of mean velocities, turbulent kinetic energy, pumping effectiveness, and hydraulic efficiency. The measured flow data near the Impeller have been presented in the form suitable for specifying the boundary conditions to the numerical model. The two-equation (k-e) turbulence model has been shown to be adequate for predicting the bulk flow in the case of all Impellers.
Suzanne M Kresta - One of the best experts on this subject based on the ideXlab platform.
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air entrainment in baffled stirred tanks
Chemical Engineering Research & Design, 2007Co-Authors: D. Hébert, Suzanne M KrestaAbstract:Abstract The Impeller speed at which air is first entrained from the surface of a stirred tank ( N E ) is an operational limit. Where air entrainment is desirable, it is a lower limit, but where air entrainment is detrimental it is an upper limit. This study (1) determines parameters which affect N E and (2) develops a mechanistic model of air entrainment. Experiments were conducted to determine the effect of Impeller submergence, Impeller Diameter, baffle geometry, and the physical properties of the fluid on N E for an up-pumping (PBTU), and a down-pumping pitched blade turbine (PBTD). Mean and RMS velocity profiles were measured for selected cases using laser Doppler velocimetry (LDV). Using this data, air entrainment in stirred tanks and at other free surfaces is compared and is found to depend on the balance between gravity, surface tension and surface turbulence. There must be sufficient turbulence at the surface to overcome surface tension and form bubbles. The entrained bubble size is determined by the mean flow below the surface, which acts to pull the bubbles into the tank. It is shown that Impeller variables, such as the power number, Impeller speed and Diameter, cannot predict the point of air entrainment at the surface. The key predicting variable is the ratio of u , the RMS velocity at the surface, to the mean downward velocity U . At the point of air entrainment, this velocity ratio just balances the physical properties of the fluid.
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the effect of Impeller and tank geometry on power number for a pitched blade turbine
Chemical Engineering Research & Design, 2002Co-Authors: D Chapple, Suzanne M Kresta, A Wall, Arti AfacaAbstract:Previous studies of the Rushton turbine have shown that the power number is sensitive to the details of Impeller geometry, and in particular to the blade thickness, but is independent of the Impeller Diameter to tank Diameter ratio. In this paper, a similar study is reported for the pitched blade Impeller. The results show that the power number is independent of blade thickness, but dependent on the Impeller to tank Diameter ratio. This is exactly the opposite result to that observed for the Rushton turbine. Physical explanations are given for the differences in behaviour between the two Impellers. For the Rushton turbine, power consumption is dominated by form drag, so details of the blade geometry and flow separation have a significant impact (30%) on the power number. For the pitched blade Impeller, form drag is not as important, but the flow at the Impeller interacts strongly with the proximity of the tank walls, so changes in the position of the Impeller in the tank can have a significant impact on the power number.
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impact of tank geometry on the maximum turbulence energy dissipation rate for Impellers
Aiche Journal, 1996Co-Authors: Genwe Zhou, Suzanne M KrestaAbstract:The maximum turbulence energy dissipation rate per unit mass, emax, is an important variable in dispersion systems, particularly for drop breakup and coalescence, and for gas dispersion. The effect of tank geometry (number of baffles, Impeller Diameter, and off-bottom clearance) on emax for four Impellers (the Rushton turbine, RT; the pitched blade turbine, PBT; the fluidfoil turbine, A310; and the high-efficiency turbine, HE3) is examined. Mean and fluctuating velocity profiles close to the Impellers were measured in a cylindrical baffled tank using laser doppler velocimetry. Local and maximum turbulence energy dissipation rates in the Impeller region were estimated using e = Av3/L with A = 1 and L = D/10 for all four Impellers. Factorial designs were used to test for the effects of single geometric variables under widely varying conditions and interactions between variables. Several factorial designs were used to ensure that real effects were separated from effects that appeared as an artifact of the experimental design. Results show that the tank geometry has a significant effect on emax, primarily with respect to variations in Impeller Diameter and interactions between the off-bottom clearance and Impeller Diameter. For the same power input and tank geometry, the RT consistently produces the largest emax and/or emax scaled with N3D2.
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the effects of Impeller and tank geometry of circulatory time distributions in stirred tanks
Chemical Engineering Research & Design, 1995Co-Authors: R M Roberts, M R Gray, Suzanne M KrestaAbstract:The Lagrangian experience of a fluid particle in a stirred tank is one of the factors affecting process results ranging from drop size distributions to cell growth kinetics. To examine the Lagrangian experience of a fluid particle with respect to the Impeller plane, the mean circulation time (MCT) and the circulation time distribution (CTD) in a stirred tank were studied using the magnetic flow follower method. Two-level factorial designs were used to examine two Impellers, a pitched-blade turbine (PBT), and a Rushton turbine (RT). Four geometric variables were varied for each Impeller: the baffle width, the number of baffles, the off-bottom clearance of the Impeller, and the Impeller Diameter. All four variables, and the interaction between the number of baffles and the off-bottom clearance, had a statistically significant effect (5%) on the MCT for the RT. The significant factors for the PBT were the off-bottom clearance, the Impeller Diameter and the interaction between the clearance and the Diameter (30%). The CTDs were mostly bi-modal. Changes in the shape of the distribution were observed for different geometric configurations. We show that the Lagrangian experience of a fluid particle depends on the type of Impeller used, and on the tank geometry
A W Nienow - One of the best experts on this subject based on the ideXlab platform.
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a new mathematical model to predict cavern Diameters in highly shear thinning power law liquids using axial flow Impellers
Chemical Engineering Science, 1998Co-Authors: A Amanullah, S A Hjorth, A W NienowAbstract:An axial force model is described for predicting cavern Diameters in highly shear thinning liquids using axial flow Impellers. The model has been verified experimentally by measuring cavern sizes in viscous and shear thinning Carbopol solutions with SCABA 3SHP1 Impellers. The proposed force model considers the total momentum imparted by the Impeller as the sum of both tangential and axial components and assumes a torus-shaped cavern. It combines torque and axial force measurements with the simple power-law equation to predict the cavern Diameter with the cavern boundary defined by a limiting velocity. The proposed new model is capable of predicting the measured cavern Diameters for Re > 20 and is valid for sizes greater than the Impeller Diameter but less than the vessel Diameter. This approach is also shown to be superior to the traditional Elson and Nienow yield stress model in extremely shear thinning fluids whose flow curve can be well-fitted by the power-law equation. In principle, the model can also be applied to caverns generated by radial flow Impellers.
Bhramara Panitapu - One of the best experts on this subject based on the ideXlab platform.
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effect of axial gap between diffuser inlet and Impeller on efficiency and flow pattern of centrifugal fans
Applied Mechanics and Materials, 2013Co-Authors: Mojtaba Gholamian, Bhramara PanitapuAbstract:Inlet is one of the basic elements of squirrel cage fan that can have great effect on performance and losses, especially between inlet exit and first section of Impeller width. In this paper the effect of axial gap between inlet diffuser and Impeller on performance and flow pattern is considered. Three diffuser inlet sizes with respect to Impeller size (smaller, nearly same and bigger than inner Impeller Diameter) and three axial gaps within the available dimensions of the casing and Impeller were chosen. Numerical simulations were performed to find the effect of this axial gap on flow pattern, performance and efficiency. From the simulation of each case study, flow pattern and its mechanism and the causes that affecting the efficiency and performance due to axial gap are analyzed and presented.
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effect of axial gap between inlet nozzle and Impeller on efficiency and flow pattern in centrifugal fans numerical and experimental analysis
Case Studies in Thermal Engineering, 2013Co-Authors: Mojtaba Gholamian, Bhramara PanitapuAbstract:Abstract Inlet is one of the basic elements of squirrel cage fan that can have great effect on performance and losses, especially between inlet nozzle exit and first section of Impeller width. But, enough research has not been done on its parameters. In this paper the effect of axial gap between inlet nozzle and Impeller on performance and flow pattern is considered. Four inlet nozzle sizes with respect to Impeller size (two smaller, nearly same and bigger than inner Impeller Diameter) and three axial gaps within the physical dimensions of the casing and Impeller were chosen. Numerical simulations with different turbulence models, special geometry and mesh pattern were performed to find the effect of this axial gap on flow pattern, performance and efficiency. For the validation of numerical results, some experiments were done and all the performance parameters were compared with that of numerical simulations. These results show good matching between experimental and numerical results. From the simulation of each case study, flow pattern and its mechanism and the causes affecting the efficiency and performance due to axial gap are analyzed and presented.
Zhipeng Li - One of the best experts on this subject based on the ideXlab platform.
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piv experiments and large eddy simulations of single loop flow fields in rushton turbine stirred tanks
Chemical Engineering Science, 2011Co-Authors: Zhipeng LiAbstract:Abstract The single-loop flow fields in Rushton turbine stirred tanks with clearance C =0.15 T ( T is tank Diameter) were investigated by using particle image velocimetry (PIV) experiments and large eddy simulation (LES) methods. The velocity and turbulent kinetic energy (TKE) were carefully measured and resolved with high resolution camera. The regions with high TKE are affected by the movement of the trailing vortices generated behind the Impeller blades. The effects of both geometrical configuration and Reynolds number were discussed. It is found that the Reynolds number has little effect on the mean flow for the configuration of Impeller Diameter D = T /3, C =0.15 T . However, the single-loop flow pattern is changed into a double-loop one if D is increased from T /3 to T /2. The LES results were compared with the PIV experiments and the laser Doppler anemometry (LDA) data in the literature. The effect of the grid was validated, and the levels of local anisotropy of turbulence near the Impeller discharge regions were investigated. Both the phase-averaged and phase-resolved LES results are in good agreement with the PIV experimental data, and are better than the predictions of the k – e model. The agreement shows that the LES method can be used to simulate the complex flow fields in stirred tanks.
