Turbulent Taylor Vortex Research Articles

Liquid Flow Patterns and Particle Settling Velocity in a Taylor-Couette Cell Using Particle Image Velocimetry and Particle Tracking Velocimetry

Summary Controlling the downhole pressure is an important parameter for successful and safe drilling operations. Several types of weighting agents (i.e., high-density particles), traditionally barite particles, are added to maintain the desired density of the drilling fluid (DF). The DF density is an important design parameter for preventing multiple drilling complications. These issues are caused by the settling of the dense particles, an undesired phenomenon also referred to as sagging. Therefore, there is a need to understand the settling characteristics of heavy particles in such scenarios. To this end, simultaneous measurements of liquid phase flow patterns and particle settling velocities have been conducted in a Taylor-Couette (TC) cell with a rotating inner cylinder and stationary outer cylinder separated by an annular gap of 9.0 mm. Liquid flow patterns and particle settling velocities have been measured using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques, respectively. Experiments have been performed by varying the rotational speed of the inner cylinder up to 200 rev/min, which is used in normal drilling operations. Spherical particles with diameters of 3.0 mm or 4.0 mm and densities between 1.2 g/cm3 and 3.95 g/cm3 were used. The liquid phases studied included deionized (DI) water and mineral oil, which are the basic components of a non-Newtonian DF with a shear-thinning viscosity. The DF is a mud-like emulsion of opaque appearance, which impedes the ability to observe the liquid flow field and particle settling in the TC cell. To address this issue, a solution of carboxymethyl cellulose (CMC) with a 6% weight concentration in DI water was used. This non-Newtonian solution displays shear-thinning rheological behavior and was used as a transparent alternative to the opaque DF. For water, PIV results have shown wavy vortex flow (WVF) to turbulent Taylor vortex flow (TTVF), which agrees with the flow patterns reported in the literature. For mineral oil, circular Couette flow (CCF) was observed at up to 100 rev/min and vortex formation at 200 rev/min. For CMC, no vortex formation was observed up to 200 rev/min, only CCF. The settling velocities for all particles in water matched with the particle settling velocities predicted using the Basset-Boussinesq-Oseen (BBO) equation of motion. For mineral oil and CMC, the results did not match well with the predicted settling velocities, especially for heavy particles due possibly to the radial particle migration and interactions with the outer cylinder wall.

Effect of micro-grooves on drag reduction in Taylor–Couette flow

Taylor–Couette flow with micro-grooves on the rotating inner cylinder is investigated to reveal the effect of surface structures on drag reduction. The Reynolds number (Re) ranges from 160 to 18 700. On the one hand, in the regimes of wave vortex flow (WVF, 160 < Re < 1010) and modulated wavy vortices (MWV, 1010 < Re < 1380) flow, the micro-grooves always reduce the torque, indicating drag reduction. Increasing either the size of micro-groove or Re, drag reduction will be enhanced. On the other hand, when the flow regime enters turbulent Taylor vortices (TTV, Re > 1380), drag reduction will be suppressed as Re increases and eventually turns to drag increase. The bigger the groove size, the smaller the critical Re where it turns from drag reduction to drag increase. To reveal the underlying mechanism of the effect of micro-grooves on drag reduction, particle image velocimetry measurements are conducted to observe the vortex flow structures, which demonstrates two aspects affecting the drag of Taylor–Couette flow over micro-grooved wall. First, the weakening of the large-scale Taylor vortex will lead to drag reduction. Second, the roughness effect will result in drag increase. In WVF/MWV, the former plays a dominant role, while in TTV, the latter dominates. In addition, a relationship between the micro-groove size and the predictive critical Reynolds number (Rec) is developed, providing a method for controlling the wall drag.

Numerical analysis of a flywheel energy storage system for low carbon powertrain applications

Flywheel energy storage has emerged as a viable energy storage technology in recent years due to its large instantaneous power and high energy density. Flywheel offers an onboard energy recovery and storage system which is durable, efficient, and environmentally friendly. The flywheel and the housing surface temperatures can be considerably influenced by the friction induced windage losses associated with non-vented airflows in the air-gap of a high-speed rotating flywheel. Many engineering applications have been interested in the features of radial and axial air-gap flows. The flow within the annulus of a flywheel is extremely complicated. This study has developed a numerical technique using ANSYS Fluent solver to model turbulent Taylor vortices formation and oscillation for thermal performance evaluation, and windage loss prediction of high-speed flywheel storage systems, operating under atmospheric and partial vacuum conditions. The numerical model has been experimentally validated with good accuracy. Several rotational speeds and pressures were investigated experimentally and numerically. The results demonstrated that a 40 % reduction in the operating pressure can reduce the flywheel surface temperature and windage loss by 20 % and 30 %, respectively. Consequently, a partial vacuum environment can achieve better energy conversion efficiencies provided an appropriate bearing seal is achieved to maintain the pressure inside the housing. The investigated flywheel energy storage system can reduce the fuel consumption of an average light-duty vehicle in the UK by 22 % and decrease CO2 emission by 390 kg annually.

Aspect-ratio dependence of heat and angular momentum transport in turbulent Taylor-Couette flows with axial thermal forcing

• Scaling law of transports determined in finite systems can be extrapolated to large-scale flows. • Coherent turbulent structures in form of Taylor vortices promote heat and angular transports. • Magnitudes of both heat and angular momentum transport depend on domain aspect-ratio. We report numerical studies of the effects of geometry factors, i.e. the aspect ratio Γ and radius ratio η , on the axial heat transport N u and the radial angular momentum transport N ω by turbulent Taylor-Couette flows that are subjected to a vertically destabilizing temperature gradient. For a given Reynolds number R e the magnitudes of both N u and N ω exhibit a pronounced aspect-ratio dependence. The angular momentum transport is reduced in long cylinders, since endwall effects are weakened as Γ increases. For the heat transfer in large- Γ cylinders, a significant N u -reduction appears for low- R e regime, whereas N u can be greatly enhanced in the regime of turbulent Taylor vortex (TV). The former N u -reduction is associated with the process that the thermal plumes are swept away by the Ekman vortices near the boundaries, and the fluid exchange is severely restricted between adjacent vortices in the bulk region. The latter N u -enhancement is attributed to the strengthened shears arising in large- Γ cylinders, resulting in a more turbulent boundary layer. Moreover, the pronounced inter-mixing of turbulent TVs continuously pumps the hot (cold) fluids into the bulk, intensifying the transport processes. It is found that although the constraints from the endwalls impact the global heat and angular momentum transport in the low- R e regime, they hardly affect their scaling properties in the high- R e regime with large Γ . We thus expect that the scaling relationship of N u ( R e ) and N ω ( R e ) determined with finite Γ can be extrapolated to large-scale, and even vertically unbounded industrial and geophysical flows.

Strong alignment of prolate ellipsoids in Taylor–Couette flow

We report on the mobility and orientation of finite-size, neutrally buoyant, prolate ellipsoids (of aspect ratio $\varLambda =4$ ) in Taylor–Couette flow, using interface-resolved numerical simulations. The set-up consists of a particle-laden flow between a rotating inner and a stationary outer cylinder. The flow regimes explored are the well-known Taylor vortex, wavy vortex and turbulent Taylor vortex flow regimes. We simulate two particle sizes $\ell /d=0.1$ and $\ell /d=0.2$ , $\ell$ denoting the particle major axis and $d$ the gap width between the cylinders. The volume fractions are $0.01\,\%$ and $0.07\,\%$ , respectively. The particles, which are initially randomly positioned, ultimately display characteristic spatial distributions which can be categorised into four modes. Modes (i) to (iii) are observed in the Taylor vortex flow regime, while mode (iv) encompasses both the wavy vortex and turbulent Taylor vortex flow regimes. Mode (i) corresponds to stable orbits away from the vortex cores. Remarkably, in a narrow $\textit {Ta}$ range, particles get trapped in the Taylor vortex cores (mode (ii)). Mode (iii) is the transition when both modes (i) and (ii) are observed. For mode (iv), particles distribute throughout the domain due to flow instabilities. All four modes show characteristic orientational statistics. The focus of the present study is on mode (ii). We find the particle clustering for this mode to be size-dependent, with two main observations. Firstly, particle agglomeration at the core is much higher for $\ell /d=0.2$ compared with $\ell /d=0.1$ . Secondly, the $\textit {Ta}$ range for which clustering is observed depends on the particle size. For this mode (ii) we observe particles to align strongly with the local cylinder tangent. The most pronounced particle alignment is observed for $\ell /d=0.2$ at around $\textit {Ta}=4.2\times 10^5$ . This observation is found to closely correspond to a minimum of axial vorticity at the Taylor vortex core ( $\textit {Ta}=6\times 10^5$ ) and we explain why.

The preferential accumulation and the settling velocity of small heavy particles in Taylor–Couette flows

ABSTRACT In a gap between two concentric cylinders, we discover that the small heavy particle settling velocity (Vs) is greater than its associated terminal velocity (Vt) in featureless turbulence that is generated by counter-rotating inner and outer cylinders simultaneously. The peak enhancement of Vs occurs at St × (Vt/u′) ∼ 1, where u′ and St are, respectively, the r.m.s. turbulence fluctuation velocity and the Stokes number defined as the ratio between the particle response time to the Kolmogorov turbulence time. The present (Vs − Vt)/u′ data as a function of St measured by the high-speed particle imaging velocimetry are compared with previous experimental results obtained from different turbulent flow configurations in an attempt to gain further understandings of the complicated interactions between small heavy particles and featureless turbulence with negligible mean velocities. Moreover, it is found that small heavy particles are preferentially accumulated around the edges of an array of counter-rotating turbulent Taylor vortices with the inner cylinder rotation alone when a time ratio between particle response and Taylor vortices is near unity. These experimental results are interesting that may deserve to disseminate in our fluid mechanics community.

Effect of hydrodynamic heterogeneity on particle dispersion in a Taylor-Couette flow reactor with variable configurations of inner cylinder

BackgroundEffect of hydrodynamic heterogeneity on particle dispersion in a Taylor-Couette flow (TC) reactor with variable configurations of inner cylinder has been investigated using CFD modelling. MethodsParticle dispersion was tracked based on the Eulerian-Lagrangian approach, where the reactant solution phase was solved in the Eulerian reference frame, while the particle dispersion was calculated by tracking a large number of particles with consideration of the hydrodynamic forces acting on particles and adopting actual particle properties measured from the particle synthesis experiments. Significant FindingsThe simulation reveals that particle dispersion is significantly enhanced by increasing the inner cylinder rotational speed, characterized by particle distribution for both circular inner cylinder Taylor-Couette flow reactor (CTC) and lobed cross-section inner cylinder Taylor-Couette flow reactor (LTC). Particle trajectories or dispersion are influenced by the turbulent Taylor vortices. Particle radial dispersion affects the particle classification by presenting different particle axial velocities in radial direction, while particle axial dispersion can be seen as an indicator for global mixing occurring in the TC reactor, which is enhanced at high rotational speed, especially in the LTC. The calculated dispersion coefficient is found to be similar to the shape of particle size distribution found in the experiments.

High-efficiency biodiesel production using rotating tube reactor: New insight of operating parameters on hydrodynamic regime and biodiesel yield

Transesterification is a preferred reaction in biodiesel production using oil and alcohol as feedstocks. The major limitation of biodiesel production is the immiscibility of reactants resulting in the slow reaction rate. The development of an intensification reactor is the main purpose to eliminate the limitation and to increase the production capacity. The present research explored the state of the art for the hydrodynamic regime inside the rotating tube reactor (RTR) to enhance the biodiesel production rate using alkali-catalyzed transesterification based on the design specification. The influence of operating conditions in terms of methanol-to-oil molar ratio, NaOH loading, total flowrate, rotational speed using two dimensionless numbers, i.e. rotating Reynolds number (Rer) and Taylor number (Ta), as well as torque, were originally proposed to determine the optimal operating condition for biodiesel production in this RTR. This was interesting to report that the modulated wavy vortex flow is required to promote biodiesel yield in the RTR. The increment values of Rer, Ta and torque associated with the turbulent Taylor vortex flow regime allowed the high reaction temperature which can generate excessive heat raising the methanol vaporization rate, leading to reduced biodiesel yield. Therefore, the optimal operating condition of this RTR was found to be 6:1 methanol-to-oil molar ratio, total flowrate of 30 mL/min, rotational speed of 1000 rpm at room temperature giving the highest yield of 97.5% with yield efficiency of 3.75×10−3 g/J and the quality of biodiesel met the ASTM specification.

Jet breakup regimes in liquid–liquid Taylor vortex flow

Optical experiments were used to identify breakup regimes for a liquid jet injected into in a second immiscible liquid undergoing turbulent Taylor vortex flow. Four jetting breakup behaviors were observed and these were compared with analogous jetting regimes in quiescent liquid-liquid systems as well as liquid-gas systems with a gaseous crossflow. Several jet breakage regime maps for liquid–liquid Taylor vortex flow were generated using jet Reynolds number, Ohnesorge number, crossflow Weber number, and momentum flux ratio as organizing parameters. However, selection of the breakup mechanism was found to depend primarily on the jet Reynolds number and was largely independent of parameters characterizing the fluid crossflow, such as the azimuthal Reynolds number and crossflow Weber number. This finding is consistent with previous observations that the downstream droplet size distribution in Taylor vortex flow is relatively insensitive to parameters other than the jet Reynolds number. Lastly, the transitions between the four identified jet breakage regimes were found to be coincident with changes in the mean droplet size observed downstream from the inlet jet.

Angular momentum transport and flow organization in Taylor-Couette flow at radius ratio of η=0.357

Experimental and numerical investigations find that in a wide-gap turbulent Taylor-Couette flow, $N\phantom{\rule{0}{0ex}}{u}_{\ensuremath{\omega}}$ features nonconstant effective scaling with $R\phantom{\rule{0}{0ex}}{e}_{S}$ due to the curvature-dependent limited capacity of the outer cylinder to emit plumes. For counterrotation, strengthened turbulent Taylor vortices cause a torque maximum at ${\ensuremath{\mu}}_{m\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}x}$=-0.123.

Statistics, plumes and azimuthally travelling waves in ultimate Taylor–Couette turbulent vortices

In this paper, we experimentally study the influence of large-scale Taylor rolls on the small-scale statistics and the flow organization in fully turbulent Taylor–Couette flow for Reynolds numbers up to $Re_{S}=3\times 10^{5}$. The velocity field in the gap confined by coaxial and independently rotating cylinders at a radius ratio of $\unicode[STIX]{x1D702}=0.714$ is measured using planar particle image velocimetry in horizontal planes at different cylinder heights. Flow regions with and without prominent Taylor vortices are compared. We show that the local angular momentum transport (expressed in terms of a Nusselt number) mainly takes place in the regions of the vortex in- and outflow, where the radial and azimuthal velocity components are highly correlated. The efficient momentum transfer is reflected in intermittent bursts, which becomes visible in the exponential tails of the probability density functions of the local Nusselt number. In addition, by calculating azimuthal energy co-spectra, small-scale plumes are revealed to be the underlying structure of these bursts. These flow features are very similar to the one observed in Rayleigh–Bénard convection, which emphasizes the analogies of these systems. By performing a complex proper orthogonal decomposition, we remarkably detect azimuthally travelling waves superimposed on the turbulent Taylor vortices, not only in the classical but also in the ultimate regime. This very large-scale flow pattern, which is most pronounced at the axial location of the vortex centre, is similar to the well-known wavy Taylor vortex flow, which has comparable wave speeds, but much larger azimuthal wavenumbers.

Hydrodynamic education with rheoscopic fluid

We present a educational poster supporting the subject „Mechanics of fluids I“, which the students evaluate to be difficult mainly due to abstractness. Our goal is to show in vivo the behavior, especially the non-linearity, of various flows transiting into turbulence. The fluid motion is visualized by using the rheoscopic fluid, which consist of water and the dust of mica, whose particles are longitudinal and shiny resulting into easily observable reflections, when the particles coherently orient along the maximum stress. This happens mainly in shear layers, e.g. at the boundary between vortex core and envelope. An example of flow transiting into turbulence is the Taylor-Couette flow between two concentric cylinders, which with increasing Taylor number passes through various regimes from fully laminar bearing flow through the Taylor vortex flow (TVF) and later Wavy vortex flow (WVF) up to Turbulent Taylor vortices regime (TTV) and, finally, the regime of featureless turbulence.

Experiments on skin friction reduction induced by superhydrophobicity and Leidenfrost phenomena in a Taylor-Couette cell

Film boiling and superhydrophobic surfaces may each decrease the skin drag by creating a vapor/air layer encapsulating the body. However, film boiling requires enormous power, and air plastron on superhydrophobic surfaces is not stable. The combined utilization of superhydrophobicity and film boiling is promising as on superhydrophobic surfaces the film boiling (Leidenfrost) temperature is only a few degrees higher than the boiling point. In a recent literature, a Taylor-Couette (T-C) cell was proposed to measure the combined effect of superhydrophobicity and film boiling on skin drag. However, rotation of the inner cylinder of the TC cell creates turbulent Taylor vortices, makes the vapor layer unstable and creates uncertainty in the results. Whereas if the outer cylinder rotated, flow pattern would become azimuthal laminar with weak Ekman vortices. We modified the T-C cell, accordingly; and found that by decreasing the surface wettability (i.e. apparent contact angle), Leidenfrost temperature, and the minimum heat flux to reach the film boiling decrease as well. Furthermore, by increasing the shear Reynolds number from 0.8×104 to 3.2×104, for both heated and unheated superhydrophobic surfaces, skin drag reduces. The heated one results in 67% decrease in skin drag. It is worth mentioning that the heat flux to create this 67% skin drag reduction is less than 2W/cm2, which is 25 times less than the minimum heat flux to create film boiling on a regular aluminum surface.

Rough-wall turbulent Taylor-Couette flow: The effect of the rib height

.In this study, we combine experiments and direct numerical simulations to investigate the effects of the height of transverse ribs at the walls on both global and local flow properties in turbulent Taylor-Couette flow. We create rib roughness by attaching up to 6 axial obstacles to the surfaces of the cylinders over an extensive range of rib heights, up to blockages of 25% of the gap width. In the asymptotic ultimate regime, where the transport is independent of viscosity, we emperically find that the prefactor of the Nu_{omega} propto Ta^{1/2} scaling (corresponding to the drag coefficient C_{f}(Re) being constant) scales with the number of ribs N_r and by the rib height h^{1.71}. The physical mechanism behind this is that the dominant contribution to the torque originates from the pressure forces acting on the rib which scale with the rib height. The measured scaling relation of N_{r} h^{1.71} is slightly smaller than the expected N_{r} h^{2} scaling, presumably because the ribs cannot be regarded as completely isolated but interact. In the counter-rotating regime with smooth walls, the momentum transport is increased by turbulent Taylor vortices. We find that also in the presence of transverse ribs these vortices persist. In the counter-rotating regime, even for large roughness heights, the momentum transport is enhanced by these vortices.Graphical abstract

Axial mixing and vortex stability to in situ radial injection in Taylor–Couette laminar and turbulent flows

Taylor–Couette flows have been widely studied in part due to the enhanced mixing performance from the variety of hydrodynamic flow states accessible. These process improvements have been demonstrated despite the traditionally limited injection mechanisms from the complexity of the Taylor–Couette geometry. In this study, using a newly designed, modified Taylor–Couette cell, axial mass transport behaviour is experimentally determined over two orders of magnitude of Reynolds number. Four different flow states, including laminar and turbulent Taylor vortex flows and laminar and turbulent wavy vortex flows, were studied. Using flow visualization techniques, the measured dispersion coefficient was found to increase with increasing $Re$, and a single, unified regression is found for all vortices studied. In addition to mass transport, the vortex structures’ stability to radial injection is also quantified. A dimensionless stability criterion, the ratio of injection to diffusion time scales, was found to capture the conditions under which vortex structures are stable to injection. Using the stability criterion, global and transitional stability regions are identified as a function of Reynolds number, $Re$.

The influence of wall roughness on bubble drag reduction in Taylor–Couette turbulence

We experimentally study the influence of wall roughness on bubble drag reduction in turbulent Taylor–Couette flow, i.e. the flow between two concentric, independently rotating cylinders. We measure the drag in the system for the cases with and without air, and add roughness by installing transverse ribs on either one or both of the cylinders. For the smooth-wall case (no ribs) and the case of ribs on the inner cylinder only, we observe strong drag reduction up to DR$=33\,\%$ and DR$=23\,\%$, respectively, for a void fraction of $\unicode[STIX]{x1D6FC}=6\,\%$. However, with ribs mounted on both cylinders or on the outer cylinder only, the drag reduction is weak, less than DR$=11\,\%$, and thus quite close to the trivial effect of reduced effective density. Flow visualizations show that stable turbulent Taylor vortices – large-scale vortical structures – are induced in these two cases, i.e. the cases with ribs on the outer cylinder. These strong secondary flows move the bubbles away from the boundary layer, making the bubbles less effective than what had previously been observed for the smooth-wall case. Measurements with counter-rotating smooth cylinders, a regime in which pronounced Taylor rolls are also induced, confirm that it is really the Taylor vortices that weaken the bubble drag reduction mechanism. Our findings show that, although bubble drag reduction can indeed be effective for smooth walls, its effect can be spoiled by e.g. biofouling and omnipresent wall roughness, as the roughness can induce strong secondary flows.

Velocity profiles, flow structures and scalings in a wide-gap turbulent Taylor–Couette flow

Fully turbulent Taylor–Couette flow between independently rotating cylinders is investigated experimentally in a wide-gap configuration ($\unicode[STIX]{x1D702}=0.5$) around the maximum transport of angular momentum. In that regime turbulent Taylor vortices are present inside the gap, leading to a pronounced axial dependence of the flow. To account for this dependence, we measure the radial and azimuthal velocity components in horizontal planes at different cylinder heights using particle image velocimetry. The ratio of angular velocities of the cylinder walls $\unicode[STIX]{x1D707}$, where the torque maximum appears, is located in the low counter-rotating regime ($\unicode[STIX]{x1D707}_{max}(\unicode[STIX]{x1D702}=0.5)=-0.2$). This point coincides with the smallest radial gradient of angular velocity in the bulk and the detachment of the neutral surface from the outer cylinder wall, where the azimuthal velocity component vanishes. The structure of the flow is further revealed by decomposing the flow field into its large-scale and turbulent contributions. Applying this decomposition to the kinetic energy, we can analyse the formation process of the turbulent Taylor vortices in more detail. Starting at pure inner cylinder rotation, the vortices are formed and strengthened until $\unicode[STIX]{x1D707}=-0.2$ quite continuously, while they break down rapidly for higher counter-rotation. The same picture is shown by the decomposed Nusselt number, and the range of rotation ratios, where turbulent Taylor vortices can exist, shrinks strongly in comparison to investigations at much lower shear Reynolds numbers. Moreover, we analyse the scaling of the Nusselt number and the wind Reynolds number with the shear Reynolds number, finding a communal transition at approximately $Re_{S}\approx 10^{5}$ from classical to ultimate turbulence with a transitional regime lasting at least up to $Re_{S}\geqslant 2\times 10^{5}$. Including the axial dispersion of the flow into the calculation of the wind amplitude, we can also investigate the wind Reynolds number as a function of the rotation ratio $\unicode[STIX]{x1D707}$, finding a maximum in the low counter-rotating regime slightly larger than $\unicode[STIX]{x1D707}_{max}$. Based on our study it becomes clear that the investigation of counter-rotating Taylor–Couette flows strongly requires an axial exploration of the flow.

Taylor-Couette flow with radial fluid injection.

Taylor-Couette cells have been shown to improve a number of industrial processes due to the wide variety of hydrodynamic flow states accessible. Traditional designs, however, limit the ability to introduce new fluids into the annulus during device operation due to geometric confinement and complexity. In this paper, a co- and counter-rotating Taylor-Couette cell with radial fluid injection has been constructed. The incorporation of 16 ports in the inner cylinder enables radial fluid injection during rotation of both cylinders. The design is also capable of continuous axial flow, enabling large injection volumes. The new inner cylinder design does not modify the critical Re for flow instabilities and can precisely inject a desired mass at a desired flow rate. A range of injection rates and masses were explored to quantify the effect of radial injection on the stability of the turbulent Taylor vortex structure. Only the highest injection rate and total mass studied (5.9 g/s, 100 g) modified the turbulent Taylor vortex structure after injection for a sustained period. The post-injection vortices remained larger than the pre-injection vortices, whereas at lower injection rates or masses, the vortex structure quickly returned to the pre-injection structure. This new system allows for in situ study of hydrodynamic effects on fluid-fluid (gas and liquid) mixing and multiphase complexation, growth, and structure. We demonstrated this new design's potential for studying the flocculation of bentonite using cationic polyacrylamide for enhancing water treatment operations.

Taylor-Couette flow and transient heat transfer inside the annulus air-gap of rotating electrical machines

Losses in an electric motor amount to between 4–24% of the total electrical power, and are converted to heat. The maximum hot spot temperature is one of the design constraints of thermal and electrical performance. Several studies have explored flow and thermal characteristics inside the air-gap between two concentric rotating cylinders such as those found in electric motors, however the transient flow and thermal effects still remain a challenge. This study uses Computational Fluid Dynamics to predict the thermal behaviour of a machine rotating at the kind of speed usually encountered in motors. The Reynolds Averaged Navier-Stokes model together with the realizable k-ε turbulence model are used to perform transient simulations. Velocity profiles and temperature distribution inside the air-gap are obtained and validated. The transient flow features and their impact on thermal performance are discussed. The numerical results show turbulent Taylor vortices inside the air-gap that lead to a periodic temperature distribution. When compared to correlations from published literature, the simulated average heat transfer coefficient at the rotor surface shows overall good agreement. The transient effects introduce local impacts like oscillations to the Taylor-Couette vortices. These flow oscillations result in oscillations of the hotspots. However, this transient oscillatory behaviour does not show any additional impact on the global thermal performance.

Pharmaceutical crystallization in Couette-Taylor crystallizer: A case study of polymorphism of amino acid L-glutamic acid

The influence of intensity Taylor vortex flow in Couette-Taylor crystallizer on the crystallization of polymorphic amino acid L-glutamic acid was investigated in cooling crystallization. Here, the L-glutamic acid was chosen as the model crystal product, where it has two kinds of polymorphism including the unstable phase α-form and stable phase β-form crystal. In cooling crystallization, the α-form crystal transformed to the β-form crystal corresponding to the phase transformation of α-form to β-form crystal. The present study found that the selective polymorphism of α-form and β-form crystal as well as the phase transformation significantly depended on the intensity of turbulent Taylor vortex flow in Couette-Taylor crystallizer. Here, the selective β-form nucleation and phase transformation were remarkably promoted as increasing the rotation speed of inner cylinder in Couette-Taylor crystallizer. By comparison with the conventional stirred tank (ST) crystallizer, the Couette-Taylor (CT) crystallizer was at least 2.0 times more effective as regards the selective β-form polymorphism and phase transformation time. The advantage of CT crystallizer over the conventional ST crystallizer was explained in terms of the high shear stress and mass transfer of turbulent Taylor vortex flow in CT crystallizer. Here, the shear stress of Taylor vortex flow in CT crystallizer was at least 23.0 times higher than that of fluid motion in conventional ST crystallizer, whereas the mass transfer of Taylor vortex flow in CT crystallizer was at least 1.2 times higher than that of fluid motion in conventional ST crystallizer. As such, the high turbulent shear stress of Taylor vortex flow was expected to promote the β-form nucleation via the effective molecules alignment, whereas the high mass transfer of Taylor vortex flow facilitated the dissolution rate of α-form and growth rate of β-form crystal, resulting in an acceleration of phase transformation rate.