Axial Flow Impeller Research Articles

Using tomograms to assess the local solid concentrations in a slurry reactor equipped with a Maxblend impeller

The solid-liquid mixing operation has extensive applications in many industrial processes. The aim of this investigation was to assess the mixing performance of a Maxblend impeller in a slurry reactor. The Maxblend impeller is one of the most efficient types of the new generation of the close clearance impellers with a unique geometry. The mixing efficiency of the Maxblend impeller for solid-liquid mixing operation was compared to those measured for the A200 (an axial-flow impeller) and the Rushton turbine (a radial-flow impeller). Electrical resistance tomography (ERT) as a non-intrusive/non-invasive visualisation technique was employed to quantify the degree of solid suspension within the vessel. The signals received from four ERT sensor planes (each plane containing sixteen electrodes) were used for the reconstruction of the tomograms through the non-iterative linear back projection algorithm. The tomography images were employed to assess the particles' distribution inside the slurry reactor. The particle concentration profiles were utilized to evaluate the extent of homogeneity and mixing index for the suspensions of the particles. The tomography data enabled us to examine the effects of several significant parameters such as impeller speed, particle size, solid loading, use of baffles, and impeller off-bottom clearance on the mixing performance of the Maxblend impeller. The results from this study showed that the use of baffles enhanced the extent of homogeneity attained by the Maxblend impeller. The extent of homogeneity and mixing index in the system enhanced with a raise in the agitation speed. Nonetheless, after attaining the maximum homogeneity level, further increment in the agitation speed had an adverse impact on the extent of homogeneity and mixing index. Hence, the assessment of the optimum impeller speed is extremely important to enhance the local mixing quality in the mixing vessel. The results also demonstrated that the maximum attainable homogeneity for the Maxblend impeller increased with a raise in solid loadings. The maximum homogeneity attained by the Maxblend impeller was higher than those of the A200 and the Rushton impellers.

Mass and heat transfer behaviour of catalytic and electrochemical stirred tank reactors employing metallic screens lining as a reaction surface

AbstractLiquid‐solid mass transfer behaviour of a rectangular stirred tank reactor lined with single screens and stacks of closely packed screens was studied in relation to catalytic and electrochemical reactor design by the electrochemical technique. Variables studied were impeller rotation speed, mesh number, and wire diameter of the screen, as well as physical properties of the solution, number of screens per stack, and impeller geometry. The rate of mass transfer increased with increasing impeller rotation speed and decreased with increasing screen mesh number and number of screens per stack. Radial flow turbine impellers produced higher rates of mass transfer than axial flow impellers. The data were correlated by dimensionless mass transfer equations. A comparison between the volumetric mass transfer coefficient at a stirred single‐screen electrode and at a stirred flat plate electrode shows that the volumetric mass transfer coefficient at the single screen is higher than that at the flat plate by a factor ranging from 7.3–22.5, depending on the operating conditions. For screen stacks, the ratio between the stack volumetric mass transfer coefficient and the flat plate value ranges from 2–46 depending on the operating conditions.The importance of the present results in the design and operation of catalytic and electrochemical reactors used to conduct diffusion‐controlled reactions was highlighted. The possibility of using screens as turbulence promoters to enhance the rate of heat transfer between the reactor wall and the surrounding cooling jacket was noted.

Suspension of solid particles in vessels agitated by Rushton turbine imperllers

The “just suspended” condition is an important criterion for the design of agitators in solid−liquid mixing processes. This criterion is met when all the particles are in motion on the base of the vessel but not necessarily uniformly distributed throughout the liquid. Grenville et al. (2015) have proposed a new correlation for predicting the just suspension speed of axial flow impellers in standard, baffled vessels with dished bases. It is based on the physical model proposed by Davies (1986) and correlates data taken at three scales over a wide range of physical properties.Many previous workers have measured the just suspension speed for Rushton turbines, which produce a radial primary flow, and forced their data to the correlation proposed by Zwietering (1958) but without testing the statistical fit. The applicability of the Davies (1986) model and Grenville et al. (2015) correlation to radial flow impellers was recognized as an outstanding question and it is addressed in this paper.Chapman (1981) measured the just suspension speed for Rushton turbines at five vessel scales in standard baffled vessels with flat bases under gassed and ungassed conditions. The ungassed data have been successfully fitted to the Grenville et al. (2015) correlation but with a change in geometrical dependencies. The just suspension speed is independent of impeller clearance above the vessel base but strongly dependent on the impeller to vessel diameter ratio. These geometric effects are very different to the ones observed for axial impellers in dished base vessels and this suggests that further work should be carried out to reconcile these differences.

Improved Mixing in a Magnetite Iron Ore Tank via Swirl Flow: Lab‐Scale and Full‐Scale Studies

AbstractSmall‐scale studies were conducted in laboratories with the aim to solve a difficult mixing problem in a life magnetite iron ore slurry tank. The existing 4‐bladed axial flow impeller located close to the bottom can be made to provide improved off‐bottom suspension and dramatically better mixing at approximately same power input when the tank baffles are removed. To maintain the same power input after removal of baffles, extending the impeller diameter or increasing speed was used. This result was implemented at LKAB's full‐scale slurry tank in Sweden, achieving a successful outcome of more consistent blending, which allowed ∼ 50 % increase in the tank surge capacity. The large full‐scale tank provided an opportunity to assess scaling up from the laboratory measurements. This scale‐up confirms that laboratory measurements are in approximate agreement with full‐scale observation based on using the Zwietering correlation.

Hydraulic Performance Comparison for Axial Flow Impeller and Mixed Flow Impeller with Same Specific Speed

An axial flow impeller and a mixed flow impeller with same specific speed were experimentally investigated, and the suction performance was studied with the help of CFD simulations. The results show that the axial impeller is roughly better than the mixed flow one. Especially under the design condition and a low flow rate condition range near the designed one, the axial flow impeller is more stable and therefore more suitable to be used in a water jet propulsion, while under these conditions the mixed flow impeller displays significant discrepancies. On the other hand, though its efficiency at the best efficiency point is lower than that of the axial flow one, the mixed flow impeller has a larger range of high efficiency conditions and is more convenient to be controlled to satisfy the irrigation and drainage systems that ought to be adjusted to varied flow rate conditions under a fixed head. In addition, the numerical investigation at the rated point shows that the axial impeller has a much better suction performance than the mixed flow impeller, which contradicts with the experience knowledge and therefore details need to be further studied.

Swarm intelligence based on modified PSO algorithm for the optimization of axial-flow pump impeller

This paper presents a multi-objective optimization of the impeller shape of an axial-flow pump based on the Modified particle swarm optimization (MPSO) algorithm. At first, an impeller shape was designed and used as a reference in the optimization process then NPSHr and η of the axial flow pump were numerically investigated by using the commercial software ANSYS with the design variables concerning hub angle βh, chord angle βc, cascade solidity of chord σc and maximum thickness of blade H. By using the Group method of data handling (GMDH) type neural networks in commercial software DTREG, the corresponding polynomial representation for NPSHr and η with respect to the design variables were obtained. A benchmark test was employed to evaluate the performance of the MPSO algorithm in comparison with other particle swarm algorithms. Later the MPSO approach was used for Pareto based optimization. Finally, the MPSO optimization result and CFD simulation result were compared in a re-evaluation process. By using swarm intelligence based on the modified PSO algorithm, better performance pump with higher efficiency and lower NPSHr could be obtained. This novel algorithm was successfully applied for the optimization of axial-flow pump impeller shape design.

Investigation of turbulent fluid flows in stirred tanks using a non-intrusive particle tracking technique

Fully turbulent fluid flows in a laboratory-scale stirred tank (ST) equipped with a radial flow impeller (Rushton turbine; RT) or an axial flow impeller (pitched blade turbine; PBT) were analyzed using the radioactive particle tracking (RPT) technique. The present study covered the Eulerian and Lagrangian descriptions of fluid motions. The RPT measurement of the turbulent flow field in a tank agitated by an RT was benchmarked with CFD simulations of RANS-based turbulence models and laser-based measurements. There was good agreement between all the methods for the measured and predicted 3D mean velocity profiles at all locations in the ST. The RPT technique was used to measure the turbulent flow field in a tank agitated by a PBT for the first time. The behavior of the wall jet was investigated. There was close agreement between our results and those of previous studies for both systems. Lagrangian mixing measurements showed that particle trajectories can be used to generate Poincaré maps, which in turn can be used as a tool to visualize the 3D flow structure inside mixing systems. Two mixing indices, one based on the concept of stochastic independence and the other on the statistical concept of memory loss in mixing processes, were used to measure mixing times using RPT results. The present study showed that the RPT technique holds great promise for investigating turbulent flows and the mixing characteristics of STs, and for assessing the adequacy of numerical models.

Strength Analysis of Axial-Flow Pump Impeller Based on FSI

It’s necessary to calculate and analyze the strength of the pump impeller for the safe operation of the pump. In this paper, the impeller strength of a two-way full-adjust horizontal axial-flow pump in a domestic pump station under forward pumping conditions was calculated by using the unidirectional fluid-structure interaction method; it means loading the blade surface water pressure calculating from CFD software CFX as structure surface loads to the blade, and then calculating the strength of the impeller using finite element software ANSYS ; the strength of the impeller was calculated under different blade rotating angle conditions. Through the calculation, we has got static stress and deformation distribution in the impeller. The results show that under each blade rotating angle, the maximum static stress always increases with lift increasing; the maximum static stress occurs at the junction of the blade and hub; the stress concentration also occurs in there prone to cause fatigue failure; maximum deformation of the blade occurs in the leading edge close to the rim; the maximum static stress is far less than yield strength of the material that the static stress can not cause cracks.

Suspension of solid particles in vessels agitated by axial flow impellers

In the design of agitators for solid–liquid mixing processes there are two important criteria. The first is “just suspended” where all the particles are in motion on the base of the vessel but not necessarily uniformly distributed. The second is “uniform distribution” where the particles are distributed throughout the liquid phase so that, at each axial location within the slurry, the solids concentration is very close to the calculated average.The correlation proposed by Zwietering (1958. Chem. Eng. Sci. 8, 244) has been widely used for estimating the “just suspension” speed, NJS, and is recommended for agitator design in the Handbook of Industrial Mixing (Atiemo-Obeng et al., 2004. Solid-liquid mixing. In: Paul, Atiemo-Obeng, Kresta (Eds.), The Handbook of Industrial Mixing. John Wiley & Sons, Hoboken, NJ, p. 558). However, there is significant evidence that the correlation does not properly account for the effect of key variables, including liquid viscosity, density difference, and scale. This is likely related to the correlation's lack of physical basis and development using simple dimensional analysis, along with the fact that it was based on limited data taken over a narrow range of physical properties and scales. Another problem with the correlation is that in order to use it the value of a geometric specific constant must be known.This paper reports data taken at three vessel scales and over a wide range of liquid phase viscosities using axial and mixed flow impellers in standard baffled vessels with torispherical or dished bases. The model used to correlate the data is based on the work of Davies (1986. Chem. Eng. Proc. 20, 175). Analysis shows that there are two regimes; one where the large and medium sized particles interact with turbulent eddies in the inertial sub-range and a second where smaller particles interact with eddies approaching the viscous sub-range. The correlation developed for the inertial sub-range mechanism can be rearranged to show that the appropriate scale-up rule for the just suspension speed is constant power input per unit mass.

Computational simulation of mixing flow of shear thinning non-Newtonian fluids with various impellers in a stirred tank

The main features of the flow generated by three axial flow impellers installed on the side of cylindrical tanks filled with pseudoplastic solutions exhibiting yield stress were exposed using computational fluid dynamics (CFD). The numerical results were evaluated using velocity vector maps obtained from particle image velocimetry (PIV) experiments. The models were able to predict macroscale flow structures and global mixing parameters under different operating conditions. However, limitations of the model to predict the symmetric flow observed during the experiments were identified at high rotational speeds. The operating conditions included angular speeds from 327rpm to 684rpm, and yield stresses and viscosity levels provided by three carbopol solutions (0.075, 0.09 and 0.1w/w%). These conditions create mainly laminar flow inside the mixing domain. The results indicated that the operating conditions and the blade angle establish the structure of the impeller discharge, which in turn defines the shear rate distribution, some physical cavern properties, and the formation of segregated regions.

Three-dimensional laser flow measurements of a patient-specific fontan physiology with mechanical circulatory assistance.

Mechanical assistance of the Fontan circulation is hypothesized to enhance ventricular preload and improve cardiac output; however, little is known about the fluid dynamics. This study is the first to investigate the three-dimensional flow conditions of a blood pump in an anatomic Fontan. Laser measurements were conducted having an axial flow impeller in the inferior vena cava. Experiments were performed for a physiologic cardiac output, pulmonary arterial flows, and pump speeds of 1000-4000 rpm. The impeller had a modest effect on the flow conditions entering the total cavopulmonary connection at low pump speeds, but a substantial impact on the velocity at higher speeds. The higher speeds of the pump disrupted the recirculation region in the center of the anastomosis, which could be advantageous for washout purposes. No retrograde velocities in the superior vena cava were measured. These findings indicate that mechanical assistance is a viable therapeutic option for patients having dysfunctional single ventricle physiology.

Improvement of glucoamylase production using axial impellers with low power consumption and homogeneous mass transfer

Significantly influenced by complex cell morphology, glucoamylase fermentation using Aspergillus niger is characterized by high apparent viscosity and shear-thinning rheology. In this study, the influence of liquid flow field patterns on morphology, broth rheology, mass transfer and glucoamylase production was investigated by applying two different configurations with radial (Ruston Turbine, RT) and axial (Wide-blade hydrofoil upward-pumping, WHu) flow impellers. It was found that empirical correlations for averaged quantities, such as the mass transfer coefficient and viscosity, cannot reasonably explain the observations. Therefore, numerical simulation was carried out to study the detailed characteristics of local field in lab-scale bioreactors. The results showed, under similar glucose and oxygen uptake rates, that the WHu configuration formed relatively homogeneous viscosity and mass transfer fields, while the RT configuration was accompanied with significant heterogeneities. Under these conditions, the fraction of active mycelia in pellets could be highly correlated with enzyme production, and a novel parameter (Active Part Percentage, APP) was defined to introduce the effects of flow field on pelletized morphology. The WHu impellers facilitated the formation of pellets and hairy structures, with a higher APP of the pellets. As a result, the culture with the axial flow impeller configuration exhibited a larger glucoamylase production rate (+25%) and product yield on sugar (+23%) and yield on energy (+60%) in comparison to the radial flow impeller. Computational fluid models were proposed to in-depth understand such results based on local mass transfer and viscosity values, since the average values are similar over the entire fermentation processes.

Micro-bubble aeration in turbulent stirred bioreactors: Coalescence behavior in Pluronic F68 containing cell culture media

Micro-bubble aeration in stirred bioreactors represents a suitable method that allows sufficient oxygen transfer without the necessity of intense stirring. However, understanding of micro-bubble behavior in real culture media remains a crucial factor in the interpretation of possible cell/bubble interactions and the scale up of bioprocesses. The present study focuses the impact of various stirrer configurations, sparging rates, as well as 2 different bioreactor scales on micro-bubbles in SF-900 II insect culture medium, which represents a suitable example for Pluronic F68 supplemented culture media commonly applied in cell culture processes. Obtained results for the first time demonstrate a coalescence determined increase in bubble Sauter diameter d32 from 187 to 211µm with increasing stirring intensities. The development of an empirical model reveals d32∝V0.4(P/V)0.4, were P represents the stirrer induced power input and V the medium volume. The results were interpreted a consequence of micro-bubble entrapment in turbulent flow structures. Additional studies on the volumetric oxygen transfer coefficient kLa demonstrate a strong impact of axial flow impellers and indicate that increases in kLa can be attributed to an improvement of the gas–liquid oxygen transfer rather than an increased specific gas surface area due to bubble breakup.

The generation of 3D flows in a combined current and wave tank

One means of producing a 3D current in a circular tank is by using groups of conditioned axial flow impellers arranged around the perimeter to collectively create a sea representative bulk flow in a laboratory setting. Unfortunately to achieve the required bulk flow neighbouring impellers have to operate at different speeds resulting in steps in the plan view velocity profile. Therefore the underlying situation that governs tank behaviour is that of two fluid streams at different speeds combining, leading to a turbulent mixing layer which then dissipates and develops. Here a simulation of this flow is created using a 2D Reynolds Averaged Navier–Stokes method and then validated with physical experiments. The implications for accuracy and computational costs of various turbulence models, boundary conditions setups, and geometry representations are assessed. These findings are then used to produce a simplified 2D numerical model of the plan view flows in a 3D test tank which is then employed to demonstrate how a satisfactory device test zone might be generated from groups of stepped inputs. This finding helps prove that a combined current and wave tank can be created using the described configuration with the model providing a useful means of testing control scenarios.

Application of KHX Impeller in a Low-shear Stirred Bioreactor

In our previous work, a low-shear stirred bioreactor was explored. With a pitched blade turbine impeller downflow (PBTD) used, the shear stress generated is high compared with that in some low shear axial flow impellers. KHX impeller is an efficient axial flow impeller, which provides large onflow diffusivity and low shear force. In this work, the KHX impeller was applied in a lower-shear bioreactor and the performance of this reactor was evaluated and compared with that of the PBTD impeller. The experimental results show that the KHX impeller can disperse gas at lower power consumption and gives greater gas–liquid volumetric mass transfer coefficients than PBTD at the same power consumption. An empirical correlation for evaluating the mass transfer coefficient of the KHX impeller in the bioreactor is presented to provide reference for its industrial application.

Comparison of solids suspension criteria based on electrical impedance tomography and visual measurements

Different approaches have been adopted to quantify the performance of stirred vessels in suspending sinking solids into liquid phase. In this study we used electrical impedance tomography (EIT) to estimate the solids distribution in a lab-scale stirred vessel with a diameter of 362mm. Also visual measurements were performed to determine the cloud height and just suspended impeller speed. Quartz sand with a density of 2650kg/m3 was employed as the solid phase with different particle size fractions from 50 to 180µm and solids volume fractions of 7.5% and 15%. The effect of impeller type was studied by using two axial flow impellers, a pitched blade turbine and a hydrofoil impeller.Two different states—partial and homogeneous suspensions—were defined from the EIT data in addition to visual measurement of complete off-bottom suspension and cloud height. Partial suspension was determined from the EIT data, and it was reached at relatively low agitation rates. Visual measurements and data from the literature also support this observation, and EIT was proved to be a suitable method to quantify a repeatable partial suspension criterion. Complete off-bottom suspension was measured visually by determining the agitation rate at which there were no stationary solid particles at the vessel bottom for longer than 2s. However, the applicability of this widely used criterion was questioned in the case of dense suspensions of small particles. Homogeneous suspension was estimated from the EIT data, and it was reached by approximately doubling the impeller revolution rate from the partial suspension criterion. The hydrofoil impeller reached all states of suspension with lower power consumption compared to the pitched blade turbine.

Optimum Solids Concentration in an Agitated Vessel

Particle suspension in high-concentration slurries has been studied using radial-, mixed-, and axial-flow impellers. Impeller power measurements in this study were linked to the mass of solids suspended in the agitation system rather than the suspension volume. This approach was based on the consideration that the rate of dissolution or reaction depends to a large extent on the exposed surface area or mass of solids and might not be affected by the suspension volume, once off-bottom suspension is achieved. It was found that the specific power, based on the mass of solids, can be minimized by operating the system at relatively higher solids concentrations in the range of 0.20–0.35 (v/v) for the solids, impeller types, and geometrical conditions used in this work. Overall, improved energy efficiency can be achieved by using higher-power-number impellers under unbaffled conditions over a range of solids concentrations. A case example is illustrated to demonstrate the benefits of adopting some of the optimiza...

Experimental Measurements of Energy Augmentation for Mechanical Circulatory Assistance in a Patient-Specific Fontan Model

A mechanical blood pump specifically designed to increase pressure in the great veins would improve hemodynamic stability in adolescent and adult Fontan patients having dysfunctional cavopulmonary circulation. This study investigates the impact of axial-flow blood pumps on pressure, flow rate, and energy augmentation in the total cavopulmonary circulation (TCPC) using a patient-specific Fontan model. The experiments were conducted for three mechanical support configurations, which included an axial-flow impeller alone in the inferior vena cava (IVC) and an impeller with one of two different protective stent designs. All of the pump configurations led to an increase in pressure generation and flow in the Fontan circuit. The increase in IVC flow was found to augment pulmonary arterial flow, having only a small impact on the pressure and flow in the superior vena cava (SVC). Retrograde flow was neither observed nor measured from the TCPC junction into the SVC. All of the pump configurations enhanced the rate of power gain of the cavopulmonary circulation by adding energy and rotational force to the fluid flow. We measured an enhancement of forward flow into the TCPC junction, reduction in IVC pressure, and only minimally increased pulmonary arterial pressure under conditions of pump support.

Determination of Flow Structure in a Gold Leaching Tank by CFD Simulation

Experimental residence time distribution (RTD) measurement and computational fluid dynamics (CFD) simulation are the best methods to study the hydrodynamics of process flow systems. However, CFD approach leads to better understanding of the flow structure and extent of mixing in stirred tanks. In the present study, CFD models were used to simulate the flow in an industrial gold leaching tank. The objective of the investigation was to characterize the flowfield generated within the tank after process intensification. The flow was simulated using an Eulerian-Eulerian multi-fluid model where the RANS standard kmixture model and a multiple reference frame approach were used to model turbulence and impeller rotation respectively. The simulated flowfield was found to be in agreement with the flow pattern of pitched blade axial-flow impellers that was used for mixing. The leaching tank exhibited good “off-bottom suspension” which reveals minimum deposition of gold ore particles on the bottom of the leaching tanks. Simulation results were consistent with experimental results obtained from a radioactive tracer investigation. CFD approach gave a better description of the flow structure and extent of mixing in a leaching tank. Hence it could be a preferred approach for flow system analysis where the cost of experimentation is high.

Effect of mixer geometry and operating conditions on flow mixing of shear thinning fluids with yield stress

Flow mixing of a non‐Newtonian fluid in a stirred tank equipped with a side‐entry impeller was observed using particle image velocimetry (PIV). The effects of some geometrical parameters including the mixer shape and impeller type and position on the flow pattern were studied on velocity fields obtained at different locations inside the mixing domain. The different flow structures revealed that the ratio of inertial and viscous forces largely defines the flow pattern. Dead zones were observed inside the tank due to the rheological properties of the fluid. The size of the dynamic regions and the average velocity near the impeller were enhanced by increasing the suction area. Likewise, large pitch ratios were found to improve the active mixing zone and the axial discharge. Curves for the power and pumping numbers are reported for different axial flow impellers. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1156–1167, 2014