Impeller Plane Research Articles

Experimental investigation of the effect of sail geometry on the flow around the SUBOFF submarine model inspired by the dolphin’s dorsal fin

ABSTRACT The geometric shape of the submarine sail has a significant effect on the flow uniformity amount in the submarine impeller plane. In the present study, using a Hotwire and a five-hole probe, the downstream flow field for the three sail in the SUBOFF submarine model was investigated. Two of the sails have the same NACA0018 cross-section and height, one of which has a fillet on the leading-edge. The third sail has the NACA0018 cross-section which is shaped like a dolphin’s dorsal fin. In this study, the rotational field of the vortex downstream of the sail has been studied. The presence of fillets at the leading-edge of the sail significantly reduces the amount and intensity of the vortex. The sail with a dolphin’s dorsal fin geometry also has a significant effect on improving the flow quality. In addition, as the distance from the sail increases downstream, the wake becomes weaker.

Hydrodynamics of solid and liquid phases in a mixing tank containing high solid loading slurry of large particles via tomography and computational fluid dynamics

The main goal of this study was to assess the turbulent and hydrodynamics of the mixing of high solid loading slurries of large particles. To determine the state of homogeneity in the slurry tank, electrical resistance tomography was adopted. A computational fluid dynamics model was developed by employing the Eulerian-Eulerian multiphase model and the sliding mesh technique to explore the suspension of the large particles in the highly concentrated slurry tank. The influences of the solid loading, particle size, and impeller clearance on the homogeneity, just-suspended speed, power consumption, axial and radial velocities of the solid phase, and turbulent energy dissipation were assessed. It was found that the radial and axial velocities of the solid phase attained below the impeller plane at the just-suspended impeller speed were promoted by decreasing the particle size and increasing the solids loading while they were reduced with an increase in the impeller clearance.

Vortex shape and gas‐liquid hydrodynamics in unbaffled stirred tank

AbstractThe present study investigated the effect of impeller speed and vortex ingestion on vortex shape, gas holdup, and bubble size distribution in an unbaffled stirred tank using optical probe measurements. Further, the ability of the volume of the fluid model to predict vortex shape was examined. Without vortex ingestion, an increase in impeller speed resulted in a significant variation in vortex shape, whereas it had a negligible effect on vortex shape with ingestion. This suggests that when vortex ingestion occurred, most of the energy was consumed for the dispersion of gas rather than the deformation of the gas‐liquid interface. It was observed that a large number of gas bubbles were entrained into the vortex core around the impeller region, which led to a lower gas holdup at the top axial locations. An increase in the impeller speed also resulted in the formation of larger bubbles. The absence of baffles limits shear for bubble break up, resulting in larger bubbles above the impeller plane.

Role of blade rotational angle on energy performance and pressure fluctuation of a mixed-flow pump

The role of blade rotational angle in the energy performance and pressure fluctuation of a mixed-flow pump is investigated through an experimental measurement and numerical simulation. The mixed-flow pump head increases at a blade rotational angle of 4° and decreases at a blade rotational angle of −4° compared with a blade rotational angle of 0°. Meanwhile, the highest efficiency decreases by 0.3% at a blade rotational angle of 4° and increases by 0.8% at a blade rotational angle of −4°. The pressure fluctuation characteristics in the mixed-flow pump at different blade rotational angles are also revealed. The dominant frequencies of pressure fluctuations in the impeller are the axis rotation frequency or six times this frequency corresponding to six guide vanes. The dominant frequencies of pressure fluctuations at the middle plane of impeller and guide vane are the blade-passing frequencies or twice this frequency. The maximum amplitude of pressure fluctuation in the impeller at a blade rotational angle of −4° is greater than that of blade rotational angle 0° and 4° because of strong vortex intensity. The maximum amplitude of pressure fluctuation at the middle span of the impeller and vane occurs at a blade rotational angle of 4° because of the largest pressure gradient.

Experimental investigation of the bubble behavior in an aerated coaxial mixing vessel through electrical resistance tomography (ERT)

To enhance the efficiency of the aerated mixing vessel, it is critical to investigate the influence of the flow hydrodynamics generated by the impeller on the bubble size and gas holdup. In the current work, the global and local gas holdup values and the Sauter mean bubble diameter for an aerated coaxial mixing vessel equipped with a central pitched blade turbine and an anchor impeller were estimated for the viscous corn syrup solution. The main characteristics of the bubbles including the number of bubble size classes and the contribution of each class of bubble in the overall gas holdup were measured by using the dynamic gas disengagement theory coupled with the electrical resistance tomography (ERT) data. The impacts of the speed ratio and the gas flow rate on the bubble behavior (size and dispersion) were also explored. Bubbles generated at the central impeller plane were smaller than those generated beneath and above the impeller. It was shown that the speed ratio had no effect on the number of the bubble classes existed in the predefined liquid levels of the aerated coaxial mixing vessel. However, the gas holdup related to each class of bubble was affected by changing the speed ratio. The minimum overall gas holdup was obtained at the speed ratio of 10 for all studied gas velocities. In addition, the effect of the speed ratio on the bubble breakup through anchor blades was demonstrated. Gas holdup was correlated to the speed ratio, aerated power per liquid volume, and gas velocity.

Mixing dynamics in uncovered unbaffled stirred tanks

Abstract The present work is aimed at providing experimental information on mixing rates in an unbaffled vessel under free surface vortexing conditions. The planar laser induced fluorescence (PLIF) technique was used for measuring the dispersion dynamics of a passive tracer over a vertical section of the vessel. In agreement with the quite scant literature information available for these systems, results confirm the existence of two well defined, partially segregated, zones that give rise to a double mixing dynamics behavior. A suitable mixing time definition is proposed and applied to a number of experimental runs with different stirrer geometries and agitation speeds. Results confirm that unbaffled vessels are indeed poorer mixers than baffled tanks. In fact, when compared on the basis of same power input, i.e. in terms of mixing efficiency, mixing times are found to be 2–3 times larger than those pertaining to baffled tanks. However, in the 0.19 m diameter vessel employed, the observed mixing times typically fell in the range between 10 and 100 s, which makes them fully compatible with many applications involving relatively slow processes. Moreover, when free-surface vortex bottom approached the impeller plane, mixing efficiencies became practically identical to those of baffled tanks. This is a remarkable finding, as it considerably widens the potential application field of unbaffled vessels.

Optimization of a seven-stage centrifugal compressor by using a quasi-3D inverse design method

This paper focuses on performance improvement of a centrifugal compressor. An inverse design method for 3D design approaches is formulated to address this concern. The design procedure encompasses two major steps. First, with the use of ball spine algorithm, which is an inverse design algorithm, on the meridional plane of impeller, the hub and shroud of impeller are computed based on a modified pressure distribution along them. Second, an original and progressive algorithm is developed for design of blade camber line profile on the blade-to-blade planes of impeller based on blade loading improvement. Full 3D analysis of the current and designed compressor is accomplished by using a Reynolds-averaged Navier-Stokes equations solver. A comparison between the analysis results of the current and designed compressor shows that the total-to-total isentropic efficiency and pressure ratio of the designed compressor under the same operating conditions are enhanced by more than 4.5% and 5%, respectively.

Particle distribution in dilute solid liquid unbaffled tanks via a novel laser sheet and image analysis based technique

The availability of experimental information on particle distribution inside stirred tanks is a topic of great importance for many industrial applications such as catalysis and polymerization. The measurement of solid particle distribution is not simple and the development of suitable measuring techniques is still an open problem in chemical engineering research.In this work, a non-intrusive optical technique for measuring particle concentration fields in solid–liquid systems is discussed. The “Laser Sheet Image Analysis” (LSIA) technique described here makes use of a laser sheet, a digital camera for image acquisition and a suitable procedure for post-processing. It is able to provide solid concentration distribution maps on the laser sheet plane.The technique developed is applied to the case of an unbaffled vessel stirred by a Rushton turbine and provided with a top cover in order to avoid vortex formation. Visual observation of the system reveals that particles tend to concentrate in two tori located above and below the impeller plane, respectively. The phenomenon is more marked with heavy (either larger or denser) particles, that practically concentrate exclusively in the two tori, leaving the rest of the vessel substantially free from particles. These features are fully confirmed, as well as quantitatively assessed, by the results obtained via the LSIA technique.

CFD simulation of solid–liquid stirred tanks

Solid liquid stirred tanks are commonly used in the minerals industry for operations like concentration, leaching, adsorption, effluent treatment, etc. Computational Fluid Dynamics (CFD) is increasingly being used to predict the hydrodynamics and performance of these systems. Accounting for the solid–liquid interaction is critical for accurate predictions of these systems. Therefore, a careful selection of models for turbulence and drag is required. In this study, the effect of drag model was studied. The Eulerian–Eulerian multiphase model is used to simulate the solid suspension in stirred tanks. Multiple reference frame (MRF) approach is used to simulate the impeller rotation in a fully baffled tank. Simulations are conducted using commercial CFD solver ANSYS Fluent 12.1. The CFD simulations are conducted for concentration 1% and 7% v/v and the impeller speeds above the “just suspension speed”. It is observed that high turbulence can increase the drag coefficient as high as forty times when compared with a still fluid. The drag force was modified to account for the increase in drag at high turbulent intensities. The modified drag is a function of particle diameter to Kolmogorov length scale ratio, which, on a volume averaged basis, was found to be around 13 in the cases simulated. The modified drag law was found to be useful to simulate the low solids holdup in stirred tanks. The predictions in terms of velocity profiles and the solids distribution are found to be in reasonable agreement with the literature experimental data. Turbulent kinetic energy, homogeneity and cloud height in the stirred tanks are studied and discussed in the paper. The presence of solids resulted in dampening of turbulence and the maximum deviation was observed in the impeller plane. The cloud height and homogeneity were found to increase with an increase in impeller speed. The work provides an insight into the solid liquid flow in stirred tanks.

Effect of eccentricity on laminar mixing in vessel stirred by double turbine impellers

Laminar mixing is often conducted in industrial processes, for example in polymerization reactors or in biotechnological processes. The laminar flow conditions caused problems of inefficient mixing due to some mixing anomalies like occurrence of the isolated mixing regions (IMR), segregation or compartmentalization phenomena. In this paper, flow visualization experiments are used to examine the size, positions and structure of the IMR regions as a function of Reynolds number and eccentricity ratio in the vessel equipped with double turbine impellers. It was found that the eccentricity brings deformation and reduction of the IMR volume. Moreover another benefit of using eccentrically located impeller systems is an improvement of axial flow. Two types of IMR regions are found: undulated IMR (UIMR) and ribbon-like IMR (RIMR). The structure of IMR depends on the eccentricity ratio defined as E/ R. At the low eccentricity values the structure of single filament wrapped around core of the IMR is found. Additionally, the IMR region is inclined to the impeller plane.

Surfactant-induced effects on turbulent swirling flows

We employ digital particle imaging velocimetry (DPIV) to investigate the influence of a drag reducing cationic surfactant additive, cetyltrimethyl-ammonium chloride (CTAC), on turbulent swirling flows generated in a cylindrical vessel either by a rotating disk or a rotating disk fitted with vertical flat blades. The largest concentration of CTAC used in this study (0.05 ≤ C ≤ 0.5 mmol/l) is an order of magnitude smaller than those used in experimental investigations of surfactant induced drag reduction in turbulent pipe/channel flows. Even for such dilute systems, a number of dramatic and intriguing effects are observed. In the case of disk-driven flow, it is shown that the surfactant has a non-monotonic influence on turbulence intensity: both radial and axial root mean square velocity fluctuations first increase with increasing surfactant concentration C, reach a maximum and decrease upon further increase in C. Moreover, the maximum intensity is attained at a concentration that is practically independent of the angular frequency Ω of the disk. For the flow driven by bladed impeller, the introduction of the surfactant leads to flow reversal at the impeller plane for low concentrations. Enhancement in the radial and azimuthal mean velocities is also observed. For relatively larger concentrations (=0.5 mmol/l), a mean flow field that consists of multiple transient mixing pockets emerges as Ω exceeds a critical value. Plausible mechanisms are proposed to explain these observations.

SOLIDS DISTRIBUTION IN STIRRED SLURRY REACTORS: INFLUENCE OF SOME MIXER CONFIGURATIONS AND LIMITS TO THE APPLICABILITY OF A SIMPLE MODEL FOR PREDICTIONS

The features of solids concentration distribution were investigated in baffled and unbaffled tanks of high aspect ratio, stirred with multiple radial, mixed-flow or axial impellers. In the baffled tanks the average profiles can be predicted with the sedimentation-dispersion model regardless of impeller type - in spite of slight systematic departures from the average trend at the impeller plane and/or midway between the impellers. This is hardly possible for the unbaffled tanks due to lack of physical foundation of this simple model. When the unbaffled tank is stirred with hydrofoil impellers, an inverted profile is even obtained. The mentioned departures and this last anomalous behaviour are qualitatively discussed with reference to particle-turbulence interaction.

Gas sparging in vessels agitated by mixed flow impellers

The influence of the location of gas introduction on the gas dispersion performance of upward and downward pumping 45° pitched blade disc impellers has been studied. Ring spargers of various diameters are used, at positicus both below and above the impeller plane. The adequacy of the two phase dispersion and the aerated power consumption of each imperaller is found to be strongly influenced by the location of the sparger. For the arrangements studied, the geometry that provides the best gas dispersion performance is found to vary for each turbine. One common feature, however, is that the sparger configurations that place the point of gas introduction away from the centre and into the discharge stream from the impeller, result in a reduced influence of the aeration rate on power consumption, and the susceptibility of the turbine to flooding, while still providing good gas dispersion. This observation has important implications for the design of equipment for gas-liquid-solid mixing.

Local solids concentration measurement in a slurry mixing tank

The local solids concentration in a mixing tank was measured using both sample withdrawal and a new conductivity probe. The conductivity probe was used to assess the errors associated with various sample withdrawa! techniques and to measure solids concentration profiles in the mixing tank. The effects of sampling tube design (tip shape, face angle and inside diameter), sampling position, bulk solids concentration and particle size on the sampling errors were examined in detail. Solids concentration profiles were also measured as a function of particle size, bulk solids concentration and mixer rotational speed. The experimental results indicated that on the impeller plane, the sample withdrawal techniques (tapered sample tube) gave a lower solids concentration than that in the tank at a sampling velocity ratio of unity. These results suggest that the flow at the impeller plane was three-dimensional. The errors associated with sampling techniques depended on the sample tube shape and location in tank, and were significant for the coarse sand particles of 1000 μm. When sampling at right angle to the flow, sampling errors increased with particle size, especially when a small diameter sample tube was used. The data provided in the present study can be employed to correct for the errors associated with the sampling withdrawal techniques over a wide range of parameters. Solids concentration profiles in the mixing tank were found to be a function of the particle size, bulk solids concentration and mixer rotational speed. Solids concentration varied with the radial position, except when fine sand particles of 82 μm were used. A strong variation in solids concentration with the axial position was observed at the impeller plane for the sand particles examined in the present study. This variation increased with the particle mean size and the mixer rotational speed.

Model of vortex flow of charge in a vessel with turbine impeller

A theoretical model is described of the mean two-dimensional flow of homogeneous charge in a flat-bottomed cylindrical tank with radial baffles and six-blade turbine disc impeller. The model starts from the concept of vorticity transport in the bulk of vortex liquid flow through the mechanism of eddy diffusion characterized by a constant value of turbulent (eddy) viscosity. The result of solution of the equation which is analogous to the Stokes simplification of equations of motion for creeping flow is the description of field of the stream function and of the axial and radial velocity components of mean flow in the whole charge. The results of modelling are compared with the experimental and theoretical data published by different authors, a good qualitative and quantitative agreement being stated. Advantage of the model proposed is a very simple schematization of the system volume necessary to introduce the boundary conditions (only the parts above the impeller plane of symmetry and below it are distinguished), the explicit character of the model with respect to the model parameters (model lucidity, low demands on the capacity of computer), and, in the end, the possibility to modify the given model by changing boundary conditions even for another agitating set-up with radially-axial character of flow.

SHEAR STRESSES ON THE WALLS OF AGITATED VESSELS

Abstract Characteristics of flow near the walls of baffled vessels equipped with a six blade Rushton type turbine were measured by an electrochemical method. A directionally sensitive probe was used for determination of velocity gradient vector fields. It was found that the flow conditions near the wall could be described by a meridional profile of the friction coefficient. An extensive set of experimental data obtained by the limiting current technique showed that the profile depends on the Reynolds number, on the flow index of the (power-law) fluids and on the d/D simplex. The maximum values of the friction coefficient characterizing flow near the impeller plane can be correlated by a simple relation of the laminar boundary layer type.