Mixing in a Taylor-Couette reactor in the non-wavy flow regime

The mixing in a Taylor-Couette flow cell is quantified with laser induced fluorescence (LIF). Time-resolved, two-dimensional measurements of dye concentration have been obtained in the non-wavy Taylor vortex flow regime (Re=330) and analysed in order to characterise the intervortex and intravortex mixing. The results show clear evidence of intervortex mixing especially near the inner wall region and the inflow boundaries, and demonstrate that Taylor vortex flow cannot be simply assumed as a series of well mixed tanks. Intravortex mixing is slow in relation to the mixing between adjacent vortices and is more rapid in the azimuthal direction than the meridional plane. Increasing Re towards the upper limit of the Taylor vortex flow regime (Re=950) results in enhanced mixing despite the apparent absence of an azimuthal wave. Both the intervortex and intravortex mixing times reduce substantially and the intravortex mixing in the azimuthal and meridional planes occur at similar timescales.     

High resolution experimental measurement of turbulent flow field in a high pressure homogenizer model and its implications on turbulent drop fragmentation

Particle image velocimetry is performed on a model of a high pressure homogenizer, scaled for qualitative similarity of the one phase turbulent flow field in a production scale homogenizer. Flow fields in gap entrance, gap and gap outlet chamber are obtained with high resolution. The measurements show gap flow development and formation of a turbulent wall adherent jet when exiting into the outlet chamber. Turbulent kinetic energy spectra show how the turbulent energy available for fragmentation is transported over distance along the jet centre axis.The high resolution images are also used together with a Kolmogorov–Hinze theory framework for discussing drop fragmentation together with a direct evaluation of disruptive stresses from measurements. For the turbulent inertial mechanism large drops experience high fragmenting force close to eight gap heights downstream of the gap exit whereas this occurs closer to 20 gap heights for smaller drops. The turbulent viscous mechanism is most efficient at a downstream distance of eight gap heights into the outlet chamber for all drops sizes.     

Two-phase interactions through turbulent events as described by fluid–particle correlations

An experimental investigation of a vertical upward, two-phase pipe flow was undertaken to measure kinematic parameters of the fluid and solid phases. The kinematic parameters included Reynolds stress distributions based on quadrant analyses that provided insight in understanding the behavior of two-phase kinematic correlation profiles. The data collected was based on a two-color digital particle image velocimetry (DPIV) technique that simultaneously measured the velocity fields of the fluid and solid phases.From quadrant analysis results, differences in Reynolds stress quadrant profiles between the single- and two-phase conditions were observed near the wall in the range 0.8>r/R>0.55, corresponding to wall distances between 35 and 75 viscous lengths (y+). Correlation coefficients between the two phases were then calculated, using the fluctuating velocity components of each phase. The extent of the interaction between the two phases was tracked by the changing correlation values versus distance from the wall. The correlation of the fluid and solid phase velocities was highest in the core region of the pipe (y+∼120), where the effect of turbulent events is reduced; low correlation coefficient values were found at y+<75, where differences of magnitudes, inflection points, etc. of burst and sweep event quadrant analysis profiles were observed.The extent of the influence of wall dynamic turbulent events on the solid phase was observed both by the differences in the relative Reynolds stress quadrant profiles and, more readily, by the changing values of two-phase axial and radial correlation coefficients determined from the simultaneous fluid and solid fluctuating velocities measured by the two-color DPIV methodology. These changing values of the correlation coefficients across the tube reflect the different responses of low inertia (fluid tracers) and high inertia (solid phase glass spheres) particles to the turbulent events. Similar profiles of the axial and radial correlation coefficients were observed, indicating that for the geometry and flow conditions considered, one velocity component of each phase was sufficient to track the spatial extent of turbulent event effects and their interactions with the fluid and solid phases. It is found that the two-color DPIV methodology and two-phase correlation results can give critical insight into the performance of thermal-fluid processes, as burst and sweep events have a large impact on the kinematics and dynamics of particles in the two-phase flow.     

Influence of the lift force in direct numerical simulation of upward/downward turbulent channel flow laden with surfactant contaminated microbubbles

In this work, we employ direct numerical simulation of turbulence one-way coupled to Lagrangian tracking to investigate microbubble distribution in upward and downward channel flow. We consider a closed channel flow at Re_τ=150 and a dispersion of microbubbles characterized by a diameter of . Bubbles are assumed contaminated by surfactants (i.e., no-slip condition at bubble surface) and are subject to drag, gravity, pressure gradient forces, Basset history force and aerodynamic lift.Our results confirm previous findings and show that microbubble dispersion in the wall region is dominated by the action of gravity combined with the lift force. Specifically, in upward flow, bubble rising velocity in the wall region generates a lift force which pushes bubbles to the wall. In downward flow, bubble rising velocity against the fluid generates a lift force which prevents microbubbles from reaching the viscous sublayer.In the wall region, we observe bubble preferential segregation in high-speed regions in the downflow case, and non-preferential distribution in the upflow case. This phenomenon is related to the effect of the lift force. Compared to experiments, the current lift force model produces larger consequences, this effect being overemphasized in the upflow case in which a large number of bubbles is segregated near the wall. In this case, the resulting bubble wall-peak of concentration outranges experimental results.These results, so deeply related to the lift force, underline the crucial role of current understanding of the fluid forces acting on bubbles and help to formulate questions about available force models, bubble-bubble interactions and two-way coupling which can be crucial for accurate predictions in the region very near the wall.     

Reynolds number scaling of flow in a Rushton turbine stirred tank. Part I—Mean flow, circular jet and tip vortex scaling

We consider scaling of flow within a stirred tank with increasing Reynolds number. Experimental results obtained from two different tanks of diameter 152.5 and 292.1 mm, with a Rushton turbine operating at a wide range of rotational speeds stirring the fluid, are considered. The Reynolds number ranges from 4000 to about 78,000. Phase-locked stereoscopic PIV measurements on three different vertical planes close to the impeller give phase-averaged mean flow on a cylindrical surface around the impeller. The scaling of θ- and plane-averaged radial, circumferential and axial mean velocity components is first explored. A theoretical model for the impeller-induced flow is used to extract the strength and size of the three dominant elements of the mean flow, namely the circumferential flow, the jet flow and the pairs of tip vortices. The scaling of these parameters with Reynolds number for the two different tanks is then obtained. The plane-averaged mean velocity scales with the blade tip velocity above a Reynolds number of about 15,000. However, parameters associated with the jet and tip vortices do not become Reynolds number independence until Re exceeds about 10⁵. The results for the two tanks exhibit similar Reynolds number dependence, however, a perfect collapse is not observed, suggesting a sensitive dependence of the mean flow to the finer details of the impeller.     

Study of bubble induced flow structure using PIV

An experimental investigation of the flow structure induced by a chain of gas bubbles was carried out in a rectangular bubble column using particle image velocimetry (PIV). It is observed that the bubble rising trajectory changes from one dimension to three dimension as liquid viscosity reduces. The variation of bubble rising trajectory associates with the alternation of bubble motions—with or without oscillatory and rotational motion depending the bubble rising trajectory is 3-D or 1-D. The different behaviors of gas bubbles introduce various instantaneous and averaged liquid flow structures. In general, complex fluid velocity fields present in liquid system of low viscosity where free vortex, cross flow, and irregular circular flow can be observed. The liquid pseudo-turbulence measured in terms of turbulence intensity and Reynolds stress is more intense in liquid of low viscosity. The turbulence is also enhanced by the frequency of bubble formation.     

Flow patterns in the wake of a Taylor bubble rising through vertical columns of stagnant and flowing Newtonian liquids: An experimental study

The flow in the wake and near-wake regions of individual Taylor bubbles rising through stagnant and co-current vertical columns of Newtonian liquids was studied, employing simultaneously particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST). Experiments were made with water and aqueous glycerol solutions covering a wide range of viscosities , in an acrylic column of 32 mm ID.Different wake structures (laminar, transitional and turbulent) are identified, in both stagnant and co-current flow conditions. In stagnant liquids, the wake flow pattern is only dependent on the dimensionless group N_f. The different types of wakes obtained are in accordance with the critical N_f numbers proposed in previous works. For co-current flow conditions, the flow patterns in the wake depend on the Reynolds number based on the relative (to the bubble) average velocity of the upward liquid flow, the laminar-transitional and transitional-turbulent limits being for the first time experimentally determined.The wake flow patterns are quantified by means of instantaneous and average flow fields. Values for the wake length and wake volume are also presented and compare well with correlations found in literature. Study of the flow in the near-wake zone enabled determination of the distance needed to recover the undisturbed liquid velocity profile.The detailed study of the flow in the wake and near-wake regions is an important contribution to better understanding the interaction and coalescence mechanisms between Taylor bubbles.The data reported are relevant to the validation of numerical simulation codes in the vertical slug flow regime.     

Volume fraction gradient-induced flow patterns in a two-liquid phase mixing layer

This paper presents experimental and numerical results related to the dynamic behavior of a two-liquid phase mixing layer induced by a gradient of phase fraction at the flow inlet. A particle image velocimetry-based technique has been used together with a refractive index matching method in order to measure the volume phase fraction and the continuous phase velocity in the two-dimensional (2D) flow. The analysis of experimental results reveals the strong unsteady feature of the flow and the development of large-scale coherent structures. Experimental data have been compared to numerical simulations obtained using both a two-fluid model and a single-fluid mixture model (where only the density difference is accounted for). The similarities and discrepancies between numerical and experimental results provide an understanding of the relative importance of variable density effects compared to the two-phase interfacial exchanges in the momentum and the turbulent transport.     

Flow in the nose region and annular film around a Taylor bubble rising through vertical columns of stagnant and flowing Newtonian liquids

The flow in the nose region and in the annular film around individual Taylor bubbles rising through stagnant and co-current vertical columns of liquid were studied, employing particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST) at the same time. The combined techniques enabled simultaneous determination of the bubble shape and the velocity profiles in the liquid film. Experiments were performed with water and aqueous glycerol solutions in a wide range of viscosities , in an acrylic column of 32 mm ID.Values for the distance ahead of the nose in which the flow is disturbed by the presence of the bubble are presented for the conditions studied. The bubble shapes in the nose region are compared with Dumitrescu's shape for potential flow. The velocity profiles show that after the nose region the liquid begins to accelerate downwards, and at a certain distance from the bubble nose the velocity profile and the liquid film thickness stabilise. The liquid film acquires characteristics of a free-falling film. Values of the developing length and film thickness are reported for the experimental conditions studied. Average velocity profiles in the fully developed film are also presented. A critical Reynolds number of around 80 (based on the mean absolute velocity in the liquid film and on the film thickness) is reported for the transition from laminar to turbulent regime. Shear stress profiles (in the fully developed film) are also provided.The data reported are relevant for the validation of numerical codes in slug flow.     

Liquid re-circulation in turbulent vertical pipe flow behind a cylindrical bluff body and a ventilated cavity attached to a sparger

The turbulent flow field (Re=60024) in the wake of a cylindrical bluff body in a 0.105 m internal diameter pipe with an area blockage ratio of 82% in turbulent single-phase flow was studied using laser Doppler velocimetry (LDV). The results for the time-averaged velocity showed a toroidal vortex below the bluff body. The axial location below the bluff body where both the time-averaged radial and axial velocity components were zero (eye of the vortex) was found at approximately 0.72D. The end of the re-circulation region as defined by a stagnation point on the centreline of the pipe was found at an axial location below the bluff body of approximately 1.3D. These two locations did not change when altering the liquid superficial velocity confirming that the geometry (i.e., size) of the toroidal vortex is not dependent on the superficial liquid velocity or the speed of the vortex.Similar measurements using LDV were taken in the wake of a ventilated cavity in a vertical 0.105 m internal diameter pipe, with an area blockage ratio of 80%. The flow beneath the cavity was turbulent two-phase bubbly flow and the liquid-only flow ahead of the cavity was turbulent (Re=45618). The cavity was attached to a (central) sparger, which is a scale-up of the design used by Bacon (1995). The average gas void fraction in the wake of the cavity was 7%. The results for the time-averaged velocity confirmed the formation of a toroidal vortex remarkably similar to the vortex formed below the bluff body. The eye of the vortex and the end of the re-circulation region were found at an axial location below the ventilated cavity of 0.78 and 1.35D, respectively, i.e., almost identical to the results for the bluff body.The LDV results of the cylindrical bluff body and the ventilated cavity were compared with the fully predictive model of the velocity distribution in the vortex proposed by Thorpe et al. (2001) and good agreement was found in both cases. The model also agreed well with the data of van Hout et al. (2002) for a Taylor bubble rising in stagnant liquid in a 0.025 m internal diameter pipe. The CFX simulations of Thorpe et al. (2001) carried out for a 0.050 m internal diameter pipe, agreed well with the experimental data of the cylindrical bluff body, the ventilated cavity and the data obtained by van Hout et al. (2002) when correlating the results in the appropriate dimensionless form. Our analysis showed that the maximum axial re-circulation velocity in the centre of the vortex ring was directly proportional to the mean velocity in the annulus at the base of the cylindrical bluff body, the ventilated cavity or the Taylor bubble. The proportionality constant for all cases was found to be approximately 0.38 confirming the value proposed by Thorpe et al. (2001).     

Modeling of the break-up of deformable particles in developed turbulent flow

Dynamic behavior of the drops and bubbles in developed turbulent flow depend on turbulent length scale (λ), Morton (Mo), Weber (We) and Reynolds (Re_a) numbers. In the present work, in order to calculate the maximum stable size of drops and bubbles, the A factor of break-up, Ay (Ay=ωa/U), that is the ratio of the break-up rate in developed turbulent flow to the mean velocity of the flow has been introduced and the effect of the pipe roughness on this factor has also been given. Comparison of all the results obtained in this study with those taken from the literature for the range of Mo?7, We?10 and Re_a?100 showed a good agreement.     

Experimental studies of particle flow dynamics in a two-dimensional spouted bed

In this paper, both time-averaged and fluctuating behaviors of granular solids in a two-dimensional spouted bed (2DSB) were investigated by particle image velocimetry (PIV). A self-developed algorithm for the high-gradient granular flow field was employed to measure particle velocity sequences together with power spectral density, mean particle velocity and granular temperature. The incoherent spout was characterized as an ‘X’ geometry marked with a periodic upwardly moving neck consisting of particle clusters. In the annulus, particles move periodically as a process of acceleration-deceleration-stagnation that has the same domain frequency as the pressure drop of 2DSB. The time-averaged downward velocities have a maximum at a certain position between the spout wall and conical wall. In the spout, the longitudinal profiles of vertical particle velocities along the axis exhibit a fast acceleration followed by a long flat peak, while the normalized lateral profiles at all bed levels tend to collapse into a third polynomial curve with an inflection point. A mushroom-like distribution of the granular temperature exists in 2DSB. The peaks of granular temperature occur not only near the spout-annulus interface, but also at the corner zone between the annulus and the fountain.     

Turbulent mixing in a confined rectangular wake

Liquid-phase turbulent transport in a confined rectangular wake was investigated for a Reynolds number of 37,500 based on bulk velocity and the hydraulic diameter of the test section and a Schmidt number of 1250 using particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). The velocity and concentration field data were analyzed for flow statistics such as the mean velocity, Reynolds stresses, turbulent kinetic energy, turbulent dissipation rate, mixture-fraction mean, mixture-fraction variance and one-point composition probability density functions (PDF). Computational fluid dynamics (CFD) models, including a two-layer k-ε turbulence model, a scalar gradient-diffusion model and a scalar dissipation rate model were validated against PIV and PLIF data collected at six downstream locations. Low-Reynolds-number effects on turbulent transport were taken into consideration through the mechanical-to-scalar time-scale ratio. The experimental and computational results were found to be in satisfactory agreement.     

Biofluid mechanical investigations in sequencing batch reactor (SBR)

In the present contribution experimental and numerical investigations of multiphase flow in a sequencing batch reactor (SBR) are presented. In the bioreactor the formation and growth of granular activated sludge (GAS) with diameter up to 5 mm occurs. In order to experimentally analyse multiphase flow patterns in a mixture of water, air and granules in the SBR, optical in situ techniques are applied. Particle image velocimetry (PIV) and particle tracking velocimetry (PTV) are employed to observe the velocity fields of fluid and granules. For the three-dimensional numerical simulation of the flow problem the Euler-Euler approach is used. The comparison of experimental and numerical results shows a lot of similarities. The characteristic flow patterns can be observed in three zones of the SBR. It can be shown that effect of normal strain rate up to and shear strain rate up to , besides biochemical activities have a major influence on the formation, shape and size of the granules in the SBR under aerobic conditions.     

Particle motion and separation in a laminar tube flow with downstream enlargement

Particle classification becomes difficult when the difference in density between particle and fluid is low or negligible and the fluid is viscous. For such applications, a process capable of separating the particles according to their size is needed. Such applications are, e.g. found in biological systems for cell separation or in the removal of gel particles from polymer melts. Particle transport in laminar tube flows at low but non zero Reynolds numbers leads to accumulation of large particles near the tube center and forms a particle free zone near the wall. Small particles find their position on their equilibrium radius. Downstream widening of the flow enhances segregation between large and small particles. Large particles can be collected in a centered collector tube downstream, whereas small particles follow their streamlines around the collector tube and can be removed with the remaining flow. The said particle migration is observed when the ratio of particle to tube diameter is 0.2<d/D<0.51 and the tube Reynolds number is in between 0.2<Re<40. CFD simulations reveal the shape of the streamlines in the downstream enlargement with different tube Reynolds number. The efficiency of the classification process is characterized. Particles need a sufficient transportation length in the tube for proper demixing. This effect is analyzed by a laser sheet illuminated system within an acrylic glass tube.     

Use of PIV to measure turbulence modulation in a high throughput stirred vessel with the addition of high Stokes number particles for both up- and down-pumping configurations

A refractive index matching technique combined with particle image velocimetry (PIV) was used to measure turbulent properties of solid–liquid suspensions in a small high throughput scale cylindrical vessel of 45 mm diameter agitated with a 45° pitched blade turbine (PBT) for up-pumping (U) and down-pumping (D) configurations. This study analyses the effect of large 1.5 mm diameter particles (Stokes number>1), on liquid mean velocities, turbulent kinetic energy (TKE) and energy dissipation (ε) at particle concentrations of 0%, 1.5% and 5% by volume. Only small changes in the time-averaged liquid velocities were observed with increasing particle concentration. However, maximum TKE near the impeller decreased up to 40% with increasing particle concentration for both configurations. The Smagorinsky SGS method was used to estimate local energy dissipation rate near the impeller and the maximum value was found to decrease by 50% between 0% and 5% concentration for the (U) configuration. A lesser but still significant drop of 30% was observed for the (D) configuration. These data confirm that large Stokes number particles can suppress turbulence, in agreement with some previous experimental studies, but in contradiction with existing theories.     

Direct numerical simulation of a three-dimensional particle laden plane mixing layer considering inter-particle collisions

To investigate the behavior of inter-particle collision and its effects on multiphase flow, the direct numerical simulation of a three-dimensional gas–solid two-phase plane mixing layer is conducted. The flow is assumed to be temporally evolving and incompressible. The particle trajectories are traced by the one-way or two-way coupled Lagrangian method separately. The deterministic hard-sphere model is used to describe the inter-particle collision. Calculations are performed for a particle Stokes numbers of 3. The results show that the preferential concentration phenomenon of particles is found after the beginning of the rolling up of the large-scale vortex structures due to the influence of the vortex. It is also found that the inter-particle collision occurs frequently in the local regions with higher particle concentration of the flow field. The evolution of inter-particle collision can be divided into 3 stages under the influence of the growth of the vortex and the particle dispersion. The results under the two-way coupling show that the particle distribution is more uniform. The modifications of the mixed fluid thickness, the Reynolds stresses, and the mean stream-wise velocity of two phases due to inter-particle collision are quantitatively investigated.     

PIV study of the flow field generated by a sawtooth impeller

Stereoscopic and high-speed particle image velocimetry (PIV) techniques have been employed to study the flow field induced by a sawtooth (EkatoMizer) impeller, operated in the fully turbulent flow regime at an impeller speed of 1500 rpm. Ensemble-averaged mean flow fields and turbulence quantities were calculated for a region close to the impeller blades. The flow was found to be anisotropic near the impeller and exhibited return-to-isotropy behaviour further away from it. Macro-instabilities were found to have a high probability of occurrence in the discharge stream. All three velocity components from the stereo-PIV measurements were used to estimate the dissipation rate, by adopting a large eddy simulation (LES) analogy. Spurious vectors distorting the dissipation rate calculation were identified, and various standard deviation filters were applied for vector validation. By evaluating the filtered dissipation rate profiles against the multifractal intermittency model of Meneveau and Sreenivasan (1991), the global standard deviation filter was found to be the most suitable type. The ratio of the maximum to the mean dissipation rate for the EkatoMizer discharge stream was found to be similar to that reported for Rushton disk turbine and pitched-blade turbine impellers in the literature, raising questions about the reported high-shear advantage of sawtooth impellers. However it should be noted that these PIV experiments were conducted outside the sawtooth blades and it is possible that the maximum dissipation rate occurs within the impeller swept volume, where could be significantly higher.