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We illustrate how an alternating magnetic field can restrict and suppress the generation of vortex formation in ferrofluidic Couette flow. Therefore, the initial rotating outer cylinder (inner cylinder at rest) is brought to an abrupt stop, which results in the generation of more complex vortex dynamics in the system, evolving out of the initially fully laminar flow regime. The generated vortex flow structures appear to be axisymmetric Taylor vortices. Different stages during the spin-down process are described and characterised through dynamic quantities, such as the kinetic energy, cross-flow energy, and angular velocity flux. The presence of an alternating magnetic field modifies these dynamics during the spin-down, which is mainly dominated by the modulation amplitude of the alternating field. While moderate modulation amplitudes tend to minimise the vortex formation, i.e. weaken the flow dynamics, large modulation amplitudes suppress any vortex formation. The driving frequency only has a minor effect in general, but may allow to select between different flow pattern within the process. [ABSTRACT FROM AUTHOR]

• Elastic instabilities of low-molecular-weight polymers are studied in Taylor-Couette. • An elastically modified wavy vortex flow state is reported for the first time. • This state is characterised by oscillations of both inflow and outflow boundaries. • The amplitude of these oscillations increases at higher fluid elasticities. • This may contribute to the emergence of Rotating Standing Waves. We investigate the transition pathway of low elasticity fluids (E l = 0.003 − 0.008) in a Taylor-Couette configuration using low-molecular-weight polyacrylamide (PAAM) and visualisation experiments in the Reynolds range from 0 to 300. We report here for the first time an elastically modified wavy vortex flow state with altered spectral and structural characteristics, that precedes the onset of the traditional (inelastic) Newtonian wavy instability. This new wavy regime is characterised by oscillations of both the inflow and outflow boundaries, associated with a weakening of the outflow regions due to low hoop stresses. The modification of the boundaries persists at higher Reynolds numbers, where the spectral characteristics are unaltered compared to the inelastic, Newtonian case. In addition, a hysteretic behaviour is observed for increasing elasticity, as instabilities are shifted towards lower critical Reynolds numbers, confirming the importance of even vanishing elasticity on the stability of Taylor-Couette flows. At higher fluid elasticity (E l = 0.06), the amplitude of inflows/outflows oscillations increases, and momentum is transferred axially between adjacent vortices, which may contribute to the emergence of Rotating Standing Waves. [ABSTRACT FROM AUTHOR]

Direct numerical simulations in a wide-gap turbulent viscoelastic Taylor–Couette flow in the Reynolds number (R e) range of 1500 to 8000 reveals the existence of a maximum drag enhancement (MDE) asymptote. The statistical properties associated with the turbulent and polymer dynamics demonstrate that the turbulent drag enhances with the increase of the Weissenberg number (W i) and eventually saturates above a critical W i , namely, the flow reaches the MDE state. The mean velocity profile in MDE state closely follows a logarithmic-like law with an identical slope ( κ K = 2. 32) and a R e -dependent intercept. A detailed analysis of flow structures reveals that the MDE asymptote results from the creation and eventual saturation of small-scale elastic and inertio-elastic Görtler vortices in the inner- and outer-wall regions, respectively. These vortical structures arise due to competing effects of polymer-induced stresses that either suppress or promote turbulent vortices. A close examination of competing forces in the azimuthal direction shows that the ratio of polymeric to turbulent stresses reaches a large plateau, underscoring the elastic nature of the MDE state. Moreover, the energy production mechanism in the MDE state further supports: (1) the universal interplay between polymers and turbulent vortices in a broad range of curvilinear and planar turbulent flows, and (2) the fact that the elastically induced asymptotic saturation of drag modification is an inherent property of elasticity-driven and/or elasto-inertial turbulence flow states. Overall, this study provides concrete evidence for our earlier postulate that asymptotic flow states in unidirectional turbulent viscoelastic flows are of elastic nature. • A maximum drag enhancement state is discovered in viscoelastic Taylor–Couette flows. • The maximum drag enhancement asymptote results from the saturation of turbulent vortices. • Asymptotic flow states in unidirectional turbulent viscoelastic flows are of elastic nature. [ABSTRACT FROM AUTHOR]

• Up to date most numerical simulations considere ferrofluids as point size objects. • Real ferrofluids consist of a suspension of particles with a finite size. • Particle-particle interactions, agglomeration and chain formation are crucial. • Consider agglomeration effects flow structures and dynamics significant change. • Primary (supercritical) onsets of flow instabilities are altered. We study flow structure modifications due to agglomeration , aggregation and chain formation, also known as elongational flow effect for the flow of a ferrofluid in the gap between two concentric rotating cylinders. The system is subjected to either transverse, axial and oblique magnetic field. Consider elongational flow the torque is found to increase or decrease with variation of elongational flow parameter. Furthermore the 3D flow structures themselves are modified, e.g. resulting in variation of different energy portions/contents and the angular momentum transport. The detected modifications are explained by the fact that real ferrofluids consist of a suspension of particles with a finite size in an almost ellipsoid shape as well as with particle–particle interactions that tend to form chains of various lengths and also have a tendency to agglomerate. To come close to such realistic situation for ferrofluids, we consider elongational flow effects incorporated by the symmetric part of the velocity gradient field tensor, which can be scaled by a so-called transport coefficient λ 2 . The complexity of structures and modifications depend on the magnetic field configuration. In general complexer oblique fields result in stronger interactions and modifications. [ABSTRACT FROM AUTHOR]

The effects of the number of surface grooves on the hydraulic resistance are experimentally investigated in the case of fully turbulent Taylor-Couette flow at a fixed radius ratio η = 0.9375 and for Reynolds numbers Re i from 2 × 10 4 to 13 × 10 4 . Three configurations of inner cylinder having 6 , 12 and 24 grooves of the same rectangular shape and regularly distributed are compared to a reference smooth cylinder case. Such configurations are common in electric motors of high power density and of very high rotation rate, for which better understanding and knowledge of the flow patterns and aerodynamic losses are essential to enhance their design and to develop appropriate cooling systems. Torque and pointwise velocity measurements are performed. The effects of the flow modulations induced by the grooves on the friction coefficient and the flow features are examined. The results show significant difference in the average flow pattern, the presence of the grooves suppressing the Taylor Vortices. An increase of the friction coefficient with the groove number is reported. However, the individual drag-increasing contribution of each groove is reduced when increasing the groove number. It is also found that the local scaling exponent of the friction coefficient as a function of the Reynolds number is insensitive to the present surface roughness, suggesting that the bulk flow contribution is similar for all the four configurations. [ABSTRACT FROM AUTHOR]

• Dynamics and heat transfer properties of Taylor-Couette flow with magnetic field are analyzed. • Temperature distribution exhibit anisotropy with the magnetic field. • Vortex in the slit region expands and changes numbers with the magnetic field. • The increase in heat transfer properties is evaluated from the magnetic field mechanism. • Entropy generation decreases with the increasing magnetic field intensity. An applied magnetic field could effectively regulate the fluid heat transmission after adding magnetic particles, and it has a wide application perspective in various fields. In the current research, the flow behavior and heat transfer characteristics of Iron (II, III) oxide (Fe 3 O 4) microparticle suspension in a coaxial cylindrical annular gap with a magnetic field have been performed numerically. Lorentz force and Joule heat are evaluated to elucidate the mechanism of the flow and heat transfer properties regulated by a magnetic field. The entropy generation method is applied for analyzing energy loss inside the annular gap. The results demonstrate that enhancing the Reynolds number (Re) and Hartmann number (Ha) improves the heat transfer capability of the Taylor vortex flow. The maximum improvement in heat transfer performance compared with the case at no magnetic field is 8.09%, under the Re 2663∼4009 and Ha 0∼186. The applied magnetic field causes the velocity and temperature distributions to exhibit anisotropy, and it gets more prominent with the increase of Hartmann number. Lorentz force derived from the magnetic field promotes particle motion and facilitates the base fluids and particles to exchange momentum and energy, and the Joule heat effect also regulates heat transmission. When Re =2663, the S F increase by 21.72% but S T decrease by 17.15% as Ha increases from 0 to 186. It demonstrates the changes in frictional and thermal entropy generation (S F and S T) are positive and negative correlated with the enhancement of Ha. Additionally, the effect of volume fraction on entropy production is less than that of magnetic field intensity which indicates the advantage of a magnetic field in reducing irreversibility. The analysis presented herewith provides a new means to enhance heat transfer in Taylor-Couette flow for utilizing magnetic fields. [Display omitted] [ABSTRACT FROM AUTHOR]

Taylor-Couette flow of aqueous dispersions of nanometric Laponite platelets was studied by dynamic light scattering and direct observation. Despite high rotation rates, gel growth was observed starting from the stator, and filling almost the whole gap after some period of time depending on the angular velocity of the rotor. SEM microscopy observations of the flowing liquid showed the presence of spheres in the micrometer range. A mechanism for gel growth is proposed, where lumps of gel are formed in the sheared liquid and form spherical aggregates which eventually adhere to the side of the gel, with a progressing gelling front. [ABSTRACT FROM AUTHOR]

Air cavities, i.e. air layers developed behind cavitators, are seen as a promising drag reducing method in the maritime industry. Here we utilize the Taylor–Couette (TC) geometry, i.e. the flow between two concentric, independently rotating cylinders, to study the effect of air cavities in this closed setup, which is well-accessible for drag measurements and optical flow visualizations. We show that stable air cavities can be formed, and that the cavity size increases with Reynolds number and void fraction. The streamwise cavity length strongly depends on the axial position due to buoyancy forces acting on the air. Strong secondary flows, which are introduced by a counter-rotating outer cylinder, clearly decrease the stability of the cavities, as air is captured in the Taylor rolls rather than in the cavity. Surprisingly, we observed that local air injection is not necessary to sustain the air cavities; as long as air is present in the system it is found to be captured in the cavity. We show that the drag is decreased significantly as compared to the case without air, but with the geometric modifications imposed on the TC system by the cavitators. As the void fraction increases, the drag of the system is decreased. However, the cavitators itself significantly increase the drag due to their hydrodynamic resistance (pressure drag): In fact, a net drag increase is found when compared to the standard smooth-wall TC case. Therefore, one must first overcome the added drag created by the cavitators before one obtains a net drag reduction. [ABSTRACT FROM AUTHOR]

This article presents a continuous counter-current extraction column with a high throughput that relies on a modified Taylor–Couette vortex flow in a centrifugal force field. The extraction column consists of a top settler, bottom settler, stationary outer cylinder connecting the two settlers, and rotating inner cylinder with a spiral guide channel. The organic and aqueous phases flow continuously and counter-currently through the annulus between the outer and inner cylinders. The high-speed inner cylinder breaks up the organic phase into small droplets in the continuous aqueous phase, and three flow patterns, i.e., a disordered droplet flow, banded droplet flow, and full droplet flow, can be formed at different rotating speeds. The spiral rotating inner cylinder was proved to facilitate the formation of the banded droplet flow, which enhances the mass transfer and increases the operating flexibility. A high back-extraction efficiency of approximately 95.6% was achieved using a spiral rotating inner cylinder of 25 cm in contact height with a rotating speed of 800 rpm, even at a high total throughput of up to 7.3 L h −1  cm −2 . [ABSTRACT FROM AUTHOR]

More than half of all electrical energy is consumed by motors and generators in an industrialized country. About 5-25% of this energy is lost and converted to heat. This heat produced by the losses has adverse effect on the lifetime and performance of a machine. A machine has to be operated at a given temperature to achieve maximum efficiency, therefore heat transfer study of machines is of special interest to rotating machines manufacturers. In this paper we investigate the heat transfer in the air-gap between the rotor and the stator of a simplified induction motor using Computational Fluid Dynamics. We consider three different air-gap widths and rotation speeds to explore the change in air-gap heat transfer when changing the air-gap width and the rotation speed. The simulated average heat transfer coefficients for all the models are in good agreement with the correlations from published literature. The Taylor-Couette vortical flow pattern is observed in the air-gap in our simulation results for the models with large air-gaps. The numerical results show that the presence of Taylor-Couette vortices in the air-gap enhance the heat transfer. The heat transfer coefficient increases with the increase in the rotation speed and decreases with the decrease in the air-gap width. [ABSTRACT FROM AUTHOR]

• This study performs LES to reveal the flow field of TC flow in multiwedge clearance with three lobes. • The coherent structure evolution and the Taylor vortex pairings are highly associated with the variable cross-section in the multiwedge clearance. • The end leakage has less effect on the shearing flow in the multiwedge clearance than circular clearance. • The multi-wedge clearance can stabilize the momentum and energy transport of the shearing flow. With the increasing demands of load capacity and dynamic response, a new generation of gas foil bearings (GFBs), assembled from preloaded elastic multileaf foils, has been developed. Its performance has become more sensitive to shearing flow induced by the multiwedge clearance, which is distinct from the classic circular clearance. In this study, large-eddy simulation (LES) reveals the instantaneous and time-averaged Taylor–Couette flow field in a three-lobe multiwedge clearance. The results have shown that the evolution of the coherent structures and the Taylor vortex pairings are highly associated with the variable cross-section in the multiwedge clearance. Near the maximum clearance, the large-scale coherent structures form stripes around the circumference and gradually become distorted and broken as they move downstream. The additional turbulence intensity has been incited by the joints between lobes, in which the flow channel first contracts and then expands. Compared with the shearing flow in the circular clearance, which is more easily affected by the end leakage, the Taylor vortex pairings are steadier and more complete in the multiwedge clearance through secondary flow, which handicaps the end leakage effect. The multiwedge clearance has guaranteed efficient momentum and energy transport of the shearing flow compared with the circular clearance, by inducing more turbulence near the joints and ensuring the operational stability beyond this region, which is also beneficial to its adaption of the GFBs. The results obtained in this study are significant because they describe the physical mechanism of the multiwedge Taylor–Couette flow and can be used for guidance in the GFBs optimization. [ABSTRACT FROM AUTHOR]

• Anti-cyclonic rotation can break the Reynolds analogy between heat and momentum transfer in plane Couette and Taylor Couette flow. • The heat transfer can be much faster than momentum transfer in Couette flows subject to anti-cyclonic rotation. • The turbulent Prandtl number can be much less than unity in Couette flows subject to anti-cyclonic rotation. Rotation influences flows and transport processes in many engineering applications, however, even in canonical flow cases, the effects of rotation are not fully understood. Here, we present the results of direct numerical simulations of heat transfer plane Couette and Taylor-Couette flows subject to rotation about the spanwise and axial axis, respectively. Temperature is a passive scalar since buoyancy is neglected. The Reynolds number R e and the rotation rate R Ω are systematically varied to thoroughly investigate the influence of rotation on heat and momentum transfer and the Reynolds analogy. We find that with increasing anti-cyclonic rotation, the Nusselt numbers for the momentum transfer N u m and heat transfer N u h both increase at first before declining and approaching unity at rapid rotation rates when the flow becomes fully laminar. The Reynolds analogy factor R A = N u h / N u m is near unity for non-rotating Couette flows, but it grows significantly with increasing rotation rate. R A reaches a maximum of approximately 2 at low R e up to 6 and 8 near R Ω = 1 at higher R e in plane Couette and Taylor-Couette flow, respectively. The simulations thus show that the Reynolds analogy between heat and momentum transfer breaks down and that the heat transfer can become much faster than moment transfer when plane Couette and Taylor-Couette flows are subject to anti-cyclonic rotation. This happens at low R e as well as higher R e when the flows are fully turbulent. The turbulent Prandtl becomes much smaller than unity and the mean velocity and temperature profiles differ when the Reynolds analogy breaks down. We also present empirical models for N u m and R A , which agree reasonably well to very well with the data within a limited R Ω range. [ABSTRACT FROM AUTHOR]

When a polymer solution is sheared between concentric cylinders, with the inner rotating and the outer fixed, the torque on the inner cylinder is modified compared to a Newtonian fluid of the same viscosity revealing the different flow patterns that emerge above the linear stability threshold for circular Couette flow. Here, mixtures of relatively short and long linear polymers in dilute and semi-dilute concentrations were considered. Their shear viscosity and extensional relaxation time are quantified. The stability of the flow is monitored through torque measurements and flow visualisations for a constant rate of acceleration and deceleration of the inner cylinder in a wide range of polymer concentrations. The torque exhibits an hysteretic behaviour, typical of subcritical transition. For large concentrations, six different numbers of steady solitary pairs of vortices, called diwhirls, were observed depending on the deceleration rate and their torque contribution is reported. [ABSTRACT FROM AUTHOR]

A comprehensive Eulerian approach for integrating a three-phase CFD model, a sophisticated detailed model for radiation transport, and a transport equation for algal growth kinetics is developed and utilized to predict the performance of a Taylor vortex algal photobioreactor. Simulation predictions are compared with corresponding experimental data and with simulation predictions obtained using the more commonly employed Lagrangian particle tracking method. The Eulerian simulations correctly predict the experimental trend that biomass productivity increases with increased rates of mixing, and they also suggest that there are limits to these productivity increases as the mixing rate becomes very large. Simulation over-prediction of biomass productivity at high azimuthal Reynolds numbers can be attributed to the fact that at high biomass loadings most radiation is absorbed near illuminated reactor surfaces, and it becomes increasingly important, but also more difficult, to properly resolve the thinning hydrodynamic and radiative boundary layers. [ABSTRACT FROM AUTHOR]

A process of dispersion of droplets in a developed turbulent Taylor-Couette flow is modeled. An apparatus, where the inner cylinder rotates whereas outer one is immobile, is considered. To exclude the effect of Taylor vorticities on a turbulent flow pattern, only flow regimes characterized by relatively high Reynolds numbers (>13,000) are investigated. Relatively dilute dispersions are modeled: the maximum droplet volume fraction does not exceed 10%. The time-dependent droplet size distribution in a Taylor-Couette device is described by an advection-diffusion equation containing a population balance term. Both breakup and coalescence of droplets are taken into account. An experimentally verified engineering model of a turbulent Taylor-Couette flow, based on the Prandtl mixing length theory, is used for calculating velocity, eddy diffusivity and turbulence energy dissipation rate distributions across a Taylor-Couette device gap. The known breakup and coalescence models of Coulaloglou and Tavlarides are employed for population balance modeling. However, the breakup model has been modified to account more accurately for turbulence – droplet interactions. The governing model equations are solved numerically. The computed droplet size distributions are compared with those obtained in the laboratory Taylor-Couette device in the presence of emulsion stabilizing surfactants for different rotation speeds and stirring durations. The dispersion process in a Taylor-Couette flow in both the presence and absence of surfactants is illustrated by numerical examples. The results of simulations and experiments are critically analyzed. [ABSTRACT FROM AUTHOR]

During crystal growth, the fluid flow plays an important role on the quantity and distribution of impurities in the crystal. This study examines with modelling and experiments the removal of impurities from the solidification front for the case where a turbulent fluid flow increases the effective diffusivity. The theoretical model is based on the properties of turbulent boundary layers. It predicts a transition between the diffusive regime (no segregation) and the convective regime (efficient segregation) when the stirring velocity is higher than a threshold depending on the solidification rate. Efficient segregation is generally obtained when the stirring velocity is 10 5 times higher than the solidification rate. Silicon solidification experiments in a Taylor-Couette flow showed that concentrations in the crystal are close to the thermodynamic minimum for stirring velocities above the threshold. This result shows that the turbulent fluid flow efficiently removes impurities from the solidification front and validates the theoretical model. [ABSTRACT FROM AUTHOR]

A comprehensive quantitative model incorporating the effects of fluid flow patterns, light distribution, and algal growth kinetics on biomass growth rate is developed in order to predict the performance of a Taylor vortex algal photobioreactor for culturing Chlorella vulgaris . A commonly used Lagrangian strategy for coupling the various factors influencing algal growth was employed whereby results from computational fluid dynamics and radiation transport simulations were used to compute numerous microorganism light exposure histories, and this information in turn was used to estimate the global biomass specific growth rate. The simulations provide good quantitative agreement with experimental data and correctly predict the trend in reactor performance as a key reactor operating parameter is varied (inner cylinder rotation speed). However, biomass growth curves are consistently over-predicted and potential causes for these over-predictions and drawbacks of the Lagrangian approach are addressed. [ABSTRACT FROM AUTHOR]

Taylor-Couette flow in the annulus was investigated by numerical simulation. The stability of Taylor-Couette flow and its heat transfer process of four different models were studied. By comparing the result in four different models, it is found that the Nusselt number of the model with slit aspect ratio of 0.75 is larger than the result of other models, with the further increasing of aspect ratio, the heat transfer performance of the fluid in the annular gap is not enhanced significantly. The Nusselt number of the model with aspect ratio of 0.75 increases as the Reynolds number increases. Additionally, the temperature gradient has the inhibition effect on the transition from laminar Couette flow to Taylor vortex flow, and as the temperature gradient increasing, this effect becomes more obvious. Finally, the fitting empirical formulas of the model with aspect ratio of 0.75 were obtained at different conditions. [ABSTRACT FROM AUTHOR]

A novel transition sequence to elastically dominated turbulence (EDT) in Taylor–Couette flow of dilute polymeric solutions at elasticity number E l = W i / R e = 0. 15 that signifies the ratio of elastic to inertial forces, has been elucidated via direct numerical simulation. Specifically, as the Reynolds number (R e) and Weissenberg number (W i) are progressively increased, the primary instability causes a flow transition from an azimuthal Taylor–Couette flow (AZI) to a flow composed of rotating standing wave (RSW), which in turn transitions to disordered oscillations (DO) followed by an intriguing modulated traveling wave (MTW) flow pattern and eventually to EDT (AZI → RSW → DO → MTW → EDT). The MTW pattern is found to be a spiral mode superimposed on a RSW-like mode. The experimentally observed transition hysteresis is examined by detailed examination of the different flow patterns obtained as the flow transitions from MTW → EDT and the reverse transition, namely from EDT → MTW. Furthermore, distinct flow regions have been identified in the EDT state, namely, an elasticity-dominated outer-wall region in which the polymeric stress shares the same patterns with the radial velocity and an inertia-dominated inner-wall region in which no similarity between these two patterns is observed. Detailed examination of turbulent statistics demonstrates that the intense merging and splitting events of the large-scale vortices adjacent the outer-wall regions give rise to large, highly localized and fluctuating polymeric stresses. Moreover, in the EDT flow state the elastic production of turbulent kinetic energy is mostly derived from interactions of fluctuating elastic shear stresses and shear strain fluctuations. • A novel transition route to elastically dominated turbulence has been elucidated. • An intriguing modulated traveling wave flow pattern is observed. • Mechanistic understandings of elastically dominated turbulence are provided. [ABSTRACT FROM AUTHOR]

• Taylor-Couette flow in the narrowest gap width of 0.5 mm ever before. • Occurrence of an ideal slug Taylor-Couette flow without liquid film in the CTCR. • Gas-liquid slug flows with ideal plug-flow mixing characteristics in the CTCR. • Similar transitional flow properties of slug flow to those of single-phase flow. This work experimentally investigated flow characteristics and mixing properties of the two-phase flow in a continuous Taylor-Couette flow reactor (CTCR) with narrow gap width. In the device with a gap width of 0.5 mm, to the authors' knowledge, the narrowest gap width device ever studied in CTCR systems, the present study revealed that slug flows with alternating gas and liquid phases, as observed in microchannels, appeared in a certain flow regime. This slug flow appeared mainly in the region of low rotational Reynolds and axial Weber numbers. The non-dimensional effective diffusion coefficient D eff / u L [-] of slug flow was also measured by a tracer response method. D eff / u L in the slug flow was on the order of 10 − 6 and it has been successfully found that ideal plug flow mixing characteristics could be achieved under this flow condition. Since the intensified and controllable mixing in the liquid phase due to Taylor vortex motion induced by the inner cylinder rotation is expected to be much better than the mixing in the liquid phase of slug flows in microchannels, this flow is expected to be one of the process intensification techniques that will outperform the performance of the conventional microreactors with microchannels. [Display omitted] [ABSTRACT FROM AUTHOR]

Flow characterization in a Taylor Couette system was made by investigating the radial velocity component with Ultrasonic Doppler Velocimetry based flow mapping. With the technique presented in this work, it is possible to measure the radial velocity components for variable axial position in a Couette cell within Taylor vortex flow (TVF), wavy vortex flow (WVF), modulated vortex flow (MVF) as well as spiral vortex domains in a conical shaped gap. The resulting maps for the different flow states show the location of vortices in the annular gap between the inner and outer cylinder. Cylindrical and conical concentrically rotating inner bodies were applied and respective flow patterns were analyzed. The method uses a stroboscopic triggering to synchronize flow measurements and rotational motion. The oscillation frequency f of unsteady motion in WVF, MVF, and spirals can be obtained from the power spectrum of velocity. The UVP transducer was preferably positioned in radial direction, perpendicular to the surface of the inner rotating body for measuring the radial velocity component. At the same time, the transducer was moved with constant velocity vertically along the outer cylinder height. [ABSTRACT FROM AUTHOR]

Different methods of nanofluid preparation and further preserving of their properties have been developed so far. One of them is a so called centrifugal method. During the nanofluid preparation in centrifugal devices, centrifugal instability of different nature may arise, which must be taken into account. Therefore, this paper focuses on a theoretical study of instability of Taylor–Couette flows of nanofluids in a curved channel formed by two concentric cylindrical surfaces. The flow is driven by rotation of the inner cylindrical surface. The collocation method is used to find numerically critical values of the Taylor number, which plays the role of the instability criteria. Effects of the following parameters on the critical values of the Taylor number were studied: the ratio of the radii of the concave and convex walls, the dimensionless parameters that describe the temperature gradient, the relative density of the nanoparticles, the ratio of Brownian diffusion and thermophoretic diffusion, as well as the Prandtl and the Schmidt numbers. It was demonstrated that an increase in the relative density, the ratio of Brownian diffusion and thermophoretic diffusion and Schmidt numbers incurs loss of the flow stability under both positive and negative temperature gradients. In the same time, increasing Prandtl numbers stabilize flow under the negative temperature gradient and destabilize it in the conditions of the positive temperature gradient. The theoretical results obtained in the paper help understand mechanisms of centrifugal instability in nanofluids and thereby optimize the functionality of centrifugal devices used to prepare nanofluids. [ABSTRACT FROM AUTHOR]

Despite the significance of Taylor Couette flow over a slit wall in industries, little is known about the impact of Iron (II, III) oxide (Fe 3 O 4) microparticle on the dynamics and heat transfer properties of Taylor-Couette flow. This study aims to investigate the effect of solid particles on heat transfer performance of Taylor-Couette flow over a slit wall. The trajectory of particles in the flow field is described utilizing Eulerian-Lagrangian method in this report. The Fe 3 O 4 -water suspension with particle size d p = 1 μm , 5 μm and particle volume fraction φ = 0.5 % , 1.0 % , 1.5% are considered. Results reveal that the heat transfer performance of Taylor vortex is remarkable enhanced by adding the solid particles inside the annular gap compared to pure water. Solid particles in the annular gap show the movement of migration from inner to outer cylinders due to the interaction with fluid, which causes more disordered flow field around the inner cylinder. The addition of particles could significantly thin the velocity and temperature boundary layers. Frictional and thermal entropy generation display a reverse tendency with increasing particle volume fraction and size. Thermal entropy generation appears obviously higher than frictional entropy generation. Suspension with φ = 1.5% and d p = 1 μm makes optimal performance of Taylor vortex because of the minimum entropy generation and boundary layers. [ABSTRACT FROM AUTHOR]

CO 2 gas is physically absorbed in liquid methanol in bubbly Taylor–Couette absorber. The experiments are performed in the turbulent regime with rotational Reynolds numbers ranging from 1.9 × 10 4 to 19.2 × 10 4 . The volumetric mass transfer coefficients are obtained for the counter-current operation. Methanol is utilized as the base absorbent. In addition, with the purpose of enhancing CO 2 absorption performance, Al 2 O 3 nanoparticles are combined with methanol to produce the absorbent. The results show an increase in the volumetric mass transfer coefficient that reaches a maximum value at 4 × 10 4 rotational Reynolds number for both pure methanol and the nanoabsorbents, after which the absorption rate declines almost linearly with the rotational speed. The reasons for the reduction in the absorption performance are discussed. The maximum enhancement in the volumetric mass transfer coefficient is estimated at 20% for 4 × 10 4 rotational Reynolds number and up to 27% for methanol and Al 2 O 3 nanoparticles at a concentration of 0.1 vol%. [ABSTRACT FROM AUTHOR]

In this paper we introduce a simplified variant of the well-known Taylor–Couette flow. The aim is to develop and investigate a model problem which is as simple as possible while admitting a wide range of behaviour, and which can be used for further study into stability, transition and ultimately control of flow. As opposed to models based on ordinary differential equations, this model is fully specified by a set of partial differential equations that describe the evolution of the three velocity components over two spatial dimensions, in one meridian plane between the two counter-rotating coaxial cylinders. We assume axisymmetric perturbations of the flow in a narrow gap limit of the governing equations and, considering the evolution of the flow in a narrow strip of fluid between the two cylinders, we assume periodic boundary conditions along the radial and axial directions, with special additional symmetry constraints. In the paper, we present linear stability analysis of the first bifurcation, leading to the well known Taylor vortices, and of the secondary bifurcation, which, depending on the type of symmetries imposed on the solution, can lead to wave-like solutions travelling along the axial direction. In addition, we show results of numerical simulations to highlight the wide range of flow structures that emerge, from simple uni-directional flow to chaotic motion, even with the restriction placed on the flow. [ABSTRACT FROM AUTHOR]

The forced convective heat transfer in the gap of a flywheel is affects the safety of reactor coolant pump (RCP), such as the operation temperature limit of the bearing, therefore it is necessary to analyze the heat transfer in the annular gap flow of a flywheel. The current study provides evidence that the convective heat transfer depends on the flow structure. When the Taylor number of this flow is about 1E12, the flow structure has a special pattern likes Herringbone near the wall, and consists of many smaller Görtler vortices. Although the earliest experimental studies found the Görtler vortex with laser-induced fluorescence (LIF), these vortices are difficult to study by numerical analysis with direct numerical simulation (DNS) or the large eddy simulation (LES), and the Reynolds-averaged Navier–Stocks (RANS) cannot capture the smaller scale vortices in the wall boundary layers with overestimated eddy viscosity. In this paper, we adopt the partially-averaged Navier–Stokes (PANS) model to observe the smaller turbulent motions in the annular gap flow of a flywheel, and analyze the effect of Görtler vortices on convective heat transfer. Compared with k – ε , PANS simulation suggests that there are larger temperature gradients at the Görtler vortices region, and two temperature mixing processes occur at the boundaries of a pair of Taylor vortices. [ABSTRACT FROM AUTHOR]

The last decades have seen a number of liquid metal experiments on the interaction of magnetic fields with the flow of electrically conducting fluids. The opaqueness of liquid metals requires non-optical methods for inferring the velocity structure of the flow. Quite often, such experiments are carried out in the presence of high electrical currents to generate the necessary magnetic fields. Depending on the specific purpose, these currents can reach several kiloamperes. The utilized switching mode power supply can then influence seriously the measurement system by electromagnetic interference. A recent experiment on the azimuthal magnetorotational instability (AMRI) has shown that a hydrodynamically stable Taylor–Couette flow becomes unstable under the influence of a high azimuthal magnetic field. An electrical current along the axis of the experiment with up to 20 kA generates the necessary field to destabilize the flow. We present experimental results of this AMRI experiment carried out at the PROMISE facility with an enhanced power supply. For this setup, we discuss the elaborate measures that were needed to obtain a reasonable signal-to-noise ratio of the ultrasonic Doppler velocimetry (UDV) system. In dependence on various parameter variations, some typical features of the observed instability, such as the energy content, the wavelength, and the frequency are analyzed and compared with theoretical predictions. [ABSTRACT FROM AUTHOR]

In the present work, the effect of non-affine motion of the constituents in a complex fluid system (say, a polymeric liquid) is theoretically investigated on the centrifugal instability of circular Couette flow. To achieve this goal, use was made of the linearized Phan-Thien/Tanner (LPTT) model thanks to its allowing non-affine deformation for the polymer strands in a temporary network of junctions through invoking a slip parameter. Knowing the basic-flow velocity and stress fields from the literature, they were subjected to infinitesimally-small, normal-mode perturbations and their time-evolution was monitored using a linear stability analysis for both the axisymmetric and non-axisymmetric modes. An eigenvalue problem was obtained this way which was solved numerically using the pseudo-spectral, Chebyshev-based, collocation method. Based on the results obtained in this work, it is concluded that the non-affine motion can have a stabilizing or destabilizing effect on circular Couette flow depending on the Weissenberg number and the sign/magnitude of the angular velocity ratio . [ABSTRACT FROM AUTHOR]

The paper focuses on mixing properties of different Taylor–Couette flow regimes and their consequence on axial dispersion of a passive tracer. A joint approach, relying both on targeted experiments and numerical simulations, has been used to investigate the interaction between the flow characteristics and local or global properties of mixing. Hence, the flow and mixing have been characterized by means of flow visualization and simultaneous PIV (Particle Imaging Velocimetry) and PLIF (Planar Laser Induced Fluorescence) measurements, whereas the axial dispersion coefficient evolving along the successive flow states was investigated thanks to dye Residence Time Distribution measurements (RTD). The experimental results were complemented, for each flow pattern, by Direct Numerical Simulations (DNS), allowing access to 3D information. Both experimental and numerical results have been compared and confirmed the significant effect of the flow structure (axial wavelength of Taylor vortices and azimuthal wavenumber) on axial dispersion. [ABSTRACT FROM AUTHOR]

Recently it has been demonstrated that algal biomass yield can be enhanced using fluid flow patterns known as Taylor vortices. It has been suggested that these growth rate improvements can be attributed to improved light delivery as a result of rapid transport of microorganisms between light and dark regions of the reactor. However, Taylor vortices also strongly impact fluid mixing and interphase (gas–liquid) mass transport, and these in turn may also explain improvements in biomass productivity. To identify the growth-limiting factor in a Taylor vortex algal photobioreactor, experiments were performed to determine characteristic time scales for mixing and mass transfer. By comparing these results with the characteristic time scale for biomass growth, it is shown that algal growth rate in Taylor vortex reactors is not limited by fluid mixing or interphase mass transfer, and therefore the observed biomass productivity improvements are likely attributable to improved light utilization efficiency. [ABSTRACT FROM AUTHOR]

Linear transient growth of perturbation energy is studied in co-rotating Taylor-Couette flow of a Bingham fluid. The effects of yield stress on transient growth and structure of the optimal perturbation are mainly considered. For the wide gap case, the peak of the amplitude of optimal perturbation is shifted towards the inner cylinder with increasing yield stress. The anti-lift-up mechanism found in a range of inner cylinder Reynolds number Re 1 is substituted by the classical lift-up view as strong yield stress is imposed. While for the narrow gap case, the peak value of the optimal perturbation is shifted towards the outer cylinder as the yield stress is increased, and the lift-up mechanism always exists. [ABSTRACT FROM AUTHOR]

Gas–liquid Taylor–Couette flow devices have attracted interest for use as chemical and biological reactors, and consequently the accurate prediction of interphase mass transfer coefficients is crucial for their design and optimization. However, gas–liquid mass transport in these systems depends on many factors such as the local velocity field, turbulent energy dissipation rate, and the spatial distribution and size of bubbles, which in turn have complicated dependencies on process, geometric, and hydrodynamic parameters. Here we overcome these problems by employing a recently developed and validated Eulerian two-phase CFD model to compute local values of the mass transfer coefficient based upon the Higbie theory. This approach requires good estimates for mass transfer exposure times, and these are obtained by using a novel approach that automatically selects the appropriate expression (either the penetration model or eddy cell model) based upon local flow conditions. By comparing the simulation predictions with data from corresponding oxygen mass transfer experiments, it is demonstrated that this adaptive mass transfer model provides an excellent description for both the local and global mass transfer of oxygen in a semibatch gas–liquid Taylor–Couette reactor for a wide range of azimuthal Reynolds numbers and axial gas flow rates. [ABSTRACT FROM AUTHOR]

Taylor-Couette devices with a rotating filtration membrane are utilized to separate cellular components and to prepare platelet (thrombocytes, PLT) concentrate from whole blood. Due to the complicated nature of the working fluids (e.g. blood), systematic analysis is conducted to enhance the separation efficiency. Flow visualization and cell extraction techniques are applied to understand the cell distributions and classification in different flow regimes in the devices. Blood cells are found to be classified by Taylor vortices: erythrocytes (red blood cells, RBCs) and leukocytes (white blood cells, WBCs) moved toward the periphery of the vortices, while platelet remained close to the center of the vortices due to different sizes for these blood cells. The blood filtration efficiencies are evaluated using two devices at different radius ratios with a series of Reynolds number and flow rates. Operated at similar Reynolds numbers, the device can have higher filtration throughput by making a smaller inner cylinder, for comparable PLT recovery rates. An approximately 90 % recovery rate for PLT was obtained within 40 % volumetric ratio, while the ones for RBC and WBC were 75 % and 85 % respectively. [ABSTRACT FROM AUTHOR]

We theoretically investigate the electrical double layer-driven alteration in the dynamics of Taylor–Couette flows in which the annular space between the two concentric cylinders is occupied by an electrorheological fluid. The physical phenomenon under consideration is perceived as a competing effect of the hydrodynamic shear and the spatially varying electrorheological effects as a consequence of the electrical field within the electrical double layer. Such considerations are further shown to dramatically alter the extent of existence of a shear free region within the annular space. We arrive at a mathematical model of the above phenomenon in order to predict the stress transfer mechanism. The consequent possibility of a relative augmentation in the torque transfer capacity as compared to that in Newtonian fluids finds emerging applications in several practical scenarios, such as in clutches and bearings. [ABSTRACT FROM AUTHOR]

The linear stability of flows with wall slip in microbearings is examined. The analytic expression of the basic flow for a narrow gap and the linearized differential equation governing flow instability for the case of axisymmetric disturbances are derived by using the Navier slip model. The generalized matrix eigenvalue problem of the flow stability is solved based on a Chebyshev collocation method. Neutral stability curves with wall slip are obtained. It is shown that, when the slip coefficient representing the degree of slip increases, the stable region expands in the instability diagram, showing the stabilizing effect of slip on the flow stability. [ABSTRACT FROM AUTHOR]

Despite time-dependent boundary conditions being ubiquitous in natural and industrial flows, to date the influence of such temporal modulations (e.g. with driving frequency Ω H) has been given minor attention. The present problem addresses ferrofluidic Couette flow in between counter-rotating cylinders in a spatially homogeneous magnetic field subject to time-periodic modulation. Such a modulation can lead to a significant inner Reynolds number (R e i ) enhancement for both, either helical and toroidal flow structures. Using a modified Niklas approximation, the effect of low- and high-frequency modulation onto the primary instabilities, stability boundaries as well as on the non-linear oscillations that may occur is investigated. Focusing on bi-stable co-existing solutions, around their stability thresholds quite complex non-linear system response and flow dynamics is detected. For the system remaining supercritical always a single solution is selected by Ω H , while crossing the bifurcation thresholds within a modulation period the triggered system response is more complex, reaching from alternation between different solutions towards the appearance of intermittent behavior. • Natural and industrial flows subjected to time-dependent boundary conditions are ubiquitous. • Main focus of this work is the study of ferrofluidic Couette flow subject to time-periodic modulation introduced by an external applied alternating magnetic field. • Such a modulation is found to enhance the critical Reynolds number on the primary stabilization threshold in the system of various flow structures. • Further a variation in the modulation frequency allows to trigger different non-linear system response. • The latter reaching from alternation of different solutions, either between two supercritical solutions or between supercritical and subcritical solutions, and eventually the appearance of intermittent behavior. • These findings provide a framework to study other types of magnetic flows driven by time-dependent forcing. [ABSTRACT FROM AUTHOR]

Highlights: [•] Linear stability of Taylor–Couette flow of polymeric solutions is assessed. [•] The thermal sensitivity leads to a drastic reduction in Critical Weissenberg, Wi c . [•] A reduction in Critical Weissenberg, Wi c is observed as gap ratio is enhanced. [•] In general, lower chain extensibility gives rise to higher Wi c . [ABSTRACT FROM AUTHOR]

Abstract: Annular centrifugal extractors (ACE) based on Taylor–Couette flow have been studied for axial mixing and mass transfer with chemical reaction. The residence time distribution (RTD) was measured in three size extractors having 50, 150 and 250mm rotor diameter. The power consumption per unit mass in the annular region was in the range of 25–750W/kg. Alkaline hydrolysis of a chloro-substituted ester was carried out in ACE-50 at various rotor speeds. The effect of helical baffles on axial mixing and in turn on chemical reaction was observed by conducting two separate sets of experiments, viz., in the absence and in presence of helical baffles. An excellent agreement has been observed between the experimental results and the CFD predictions of the extents of axial mixing and chemical reaction. [Copyright &y& Elsevier]

Highlights: [•] Carbon dioxide (CO2) absorption experiments are performed in a vertical annular contactor (AC). [•] Methanol is used as solvent and Al2O3, SiO2 and TiO2 nanoparticles are combined with the methanol. [•] The highest rotation effectiveness is found in the T-AC for Al2O3 with 24.2%. [•] Methanol-based nanofluid will be a promising candidate for CO2 absorption enhancement. [ABSTRACT FROM AUTHOR]

Taylor-Couette flows between two concentric cylinders have great potential applications in chemical engineering. They are particularly convenient for two-phase small scale devices enabling solvent extraction operations. An experimental device was designed with this idea in mind. It consists of two concentric cylinders with the inner one rotating and the outer one fixed. Moreover, a pressure driven axial flow can be superimposed. Taylor-Couette flow is known to evolve towards turbulence through a sequence of successive hydrodynamic instabilities. Mixing characterized by an axial dispersion coefficient is extremely sensitive to these flow bifurcations, which may lead to flawed modelling of the coupling between flow and mass transfer. This particular point has been studied using experimental and numerical approaches. Direct numerical simulations (DNS) of the flow have been carried out. The effective diffusion coefficient was estimated using particles tracking in the different Taylor-Couette regimes. Simulation results have been compared with literature data and also with our own experimental results. The experimental study first consists in visualizing the vortices with a small amount of particles (Kalliroscope) added to the fluid. Tracer residence time distribution (RTD) is used to determine dispersion coefficients. Both numerical and experimental results show a significant effect of the flow structure on the axial dispersion. [ABSTRACT FROM AUTHOR]

Abstract: An alternative method is presented for solving the eigenvalue problem that governs the stability of Taylor–Couette and Dean flow. The eigenvalue problems defined by the two-point boundary value problems are converted into initial value problems by applying unit disturbance method developed by Harris and Reid [27] in 1964. Thereafter, the initial value problems are solved by differential transform method in series and the eigenvalues are computed by shooting technique. Critical wave number and Taylor number for Taylor–Couette flow are computed for a wide range of rotation ratio (μ), −4⩽ μ ⩽1 (first mode) and −2⩽ μ ⩽1 (second mode). The radial eigenfunction and cell patterns are presented for μ =−1, 0, 1. Also, we have computed critical wave number and Dean number successfully. [Copyright &y& Elsevier]

Abstract: The effect of an axial flow on the stability of the Taylor–Couette flow is explored for pseudoplastic fluids. The fluid is assumed to follow the Carreau–Bird model and mixed boundary conditions are imposed while the axial flow can be independent of rotational flow. The four-dimensional low-order dynamical system, resulted from Galerkin projection of the conservation of mass and momentum equations, includes additional non-linear terms in the velocity components originated from the shear-dependent viscosity. In absence of axial flow the base flow loses its radial flow stability to the vortex structure at a lower critical Taylor number, as the pseudoplasticity effects increases. The emergence of the vortices corresponds to the onset of a supercritical bifurcation which is also seen in the flow of a linear fluid. However, unlike the Newtonian case, pseudoplastic Taylor vortices lose their stability as the Taylor number reaches a second critical number corresponding to the onset of a Hopf bifurcation. Existence of an axial flow, induced by a pressure gradient appears to further advance each critical point on the bifurcation diagram. Complete flow field together with viscosity maps are given for stability regions in the bifurcation diagram. [Copyright &y& Elsevier]

Abstract: Annular centrifugal extractors (ACEs) based on Taylor–Couette flow, have found various applications, some of which include liquid–liquid extraction and as bio and polymerization reactors. The residence time distribution (RTD) was measured in three size extractors having 50, 150 and 250mm rotor diameter. The rotational speed was varied in the range of 10–30r/s so that the power consumption per unit mass in the annular region was in the range of 25–750W/kg. The extent of axial mixing was substantially reduced by providing helical baffles in the annular region. The effect of pitch was also studied. All the geometries were simulated using computational fluid dynamics (CFDs). A good agreement was obtained between the CFD predictions and the experimental measurements. Also, correlations have been proposed for Peclet numbers in the absence and presence of helical baffles. [Copyright &y& Elsevier]

Drop size distributions have been measured in a 39 mm annular centrifugal extractor using phase Doppler particle analyzer (PDPA). The power consumption and the interfacial tension have been varied in the range of 10-450 kW/m³ and 13--33 mN/m, respectively. The two liquid phases have been selected with different density and viscosity ratios. The results have been analyzed using Rosin-Rammler distribution function. A correlation has been developed for the drop size distribution. The same has been compared with the correlations in the literature. [ABSTRACT FROM AUTHOR]

Abstract: The paper is concerned with experimental investigations of the Taylor–Couette flow between two finite length large aspect ratio concentric cylinders, with asymmetric end disturbances. The fluid sample is Newtonian and the working conditions are isothermal. The applied experimental protocol consists of a constant inner cylinder angular velocity ramp (the outer cylinder is at rest), followed by a period of 40 min with constant velocity. During this experimental time, the dynamics of the wavy regime is investigated by means of the average azimuthal amplitude, number of vortices, wave frequency and power spectra time dependences. The dimensionless ramp rate is chosen above critical rates for quasi-stationary flows used in similar studies. The results feature transient patterns of unstable wavy vortices that return in time to the simple Taylor regime or settle for two preferential states within the wavy regime. The investigation of WVF dynamics is of great importance due to the applications such as Taylor–Couette mixing-reaction devices. [Copyright &y& Elsevier]