Michael Yianneskis

An experimental study of the steady and unsteady flow characteristics of stirred reactors

Liquid flow in a baffled stirred reactor vessel driven by a six-blade disk impeller has been investigated experimentally. Laser-slit photography provided an overview of the flows which were quantified by measurements of velocity characteristics, obtained with a laser-Doppler anemometer, for an impeller rotational speed of 300 r.p.m. and for three impeller clearances from the bottom of the vessel. The mean flow results show an inclination of the impeller stream and the formation of ring vortices above and below the impeller, which depend on the clearance; the flow was strongly three-dimensional with large regions having circumferential velocities in a direction opposite to that of the impeller rotation. Impeller-induced torque measurements show that the Power number is invariant with clearance for turbulent-flow Reynolds numbers ([ges ] 40000) and increases with impeller diameter. The flow structure was controlled mainly by convection and pressure forces with turbulent mixing important in the impeller region.

Assessment of Sliding Mesh CFD Predictions and LDA Measurements of the Flow in a Tank Stirred by a Rushton Impeller

LDA measurements and CFD predictions of the flow in a vessel stirred by a Rushton impeller are reported. The predictions employed a rotating mesh around the impeller and a stationary mesh in the remainder of the vessel. The experimental and numerical results are compared and discussed. It is shown that the mean velocity field is well reproduced quantitatively by the CFD model; the turbulence field is well predicted qualitatively across the whole vessel and quantitatively in the bulk flow region but differences between experiment and calculation are found near the impeller blades. Both the measurement and prediction methods are assessed.

Numerical Simulation of Turbulent Flow Characteristics in a Stirred Vessel Using the LES and RANS Approaches with the Sliding/Deforming Mesh Methodology

Numerical predictions using the large eddy simulation (LES) and RANS methods have been performed on a mixing vessel stirred by a Rushton impeller at Re = 40,000. The finite volume method in an unstructured mesh consisting of around 490,000 computational cells was used. For the LES method, the subgrid-scale effects were modelled using the standard Smagorinsky model, while for the RANS method the standard k-ɛ model was used. The interaction between the rotating impeller and the static baffles was accounted for using the sliding–deforming mesh methodology. Comparisons were made between the predictions and previously reported phase-resolved LDA measurements of mean and rms velocities, as well as turbulent kinetic energy. The results obtained with the LES approach capture well both the overall as well as the detailed flow features (such as the trailing vortices) and show very substantial improvement in comparison to RANS predictions with the standard k-ɛ model. The global turbulent energy dissipation rate ( ɛ ) across the vessel volume was also well predicted by the LES, to within 15% of the measured value; in contrast, ɛ was underpredicted by 45% with the RANS model. The local values of ɛ in the impeller stream also compare well with measured values.

On the origin, frequency and magnitude of macro-instabilities of the flows in stirred vessels

Abstract The mean flow and turbulence fields in a fully baffled vessel stirred by a Rushton impeller at three clearances and a pitched-blade turbine at one clearance have been investigated with laser-Doppler anemometry (LDA) to characterise the macro-instabilities (MIs) present in such flows. Time-resolved velocity measurements were made and the frequency content of the velocity recordings was analysed with FFT techniques. The study aims to throw light into the frequency, magnitude, nature and origin of such flow variations, especially in view of the different findings that have been reported in the published literature. The frequency of the MIs was found to be linearly related to the rotational speed of the impeller and to be essentially independent of impeller design. A single fundamental frequency, around 0.015–0.02 N (Hz), where N is the impeller rotational speed, was present for all configurations, together with harmonic frequencies, the prominence of which depended on impeller geometry and/or clearance. The LDA data and direct observations made with laser-sheet flow visualisation indicated clearly that the macro-instability stems from a precessional motion about the vessel axis, similar to the precession encountered in most swirling flows. The results show that MIs, as a mean flow motion superimposed on the flow pattern in the vessel, can result in a broadening of the measured turbulence levels by up to 25%, and a mean velocity variation of up to 0.3 V tip . The findings indicate therefore that it might be necessary and indeed it could be advantageous for improved process prediction to take MI into account in models of the flows in stirred vessels.

Bimodal vortex shedding in a perturbed cylinder wake

Cylinder wakes display distinct modes of vortex shedding when perturbed by appropriate means. By investigating experimentally the wake of a circular cylinder perturbed by a periodic fluctuation imposed on the inflow velocity, it is shown that bimodal behavior is possible. During a given experiment, the wake switches back and forth between two different vortex shedding modes, more specifically, a 2S↔2P transition is observed. No discernible change in the timing of vortex formation is found to accompany the transition. Modal decomposition of the velocity field is employed to exemplify the interaction of the imposed symmetrical perturbation and the intrinsic antisymmetrical instability of the near wake.

Observations on the Distribution of Energy Dissipation in Stirred Vessels

The distribution of the rate of dissipation of the turbulence energy, ɛ , in a vessel stirred by a Rushton turbine has been obtained from CFD predictions of the flow employing a sliding mesh technique. Volume-weighted average values of ɛ for different regions in the vessel have been calculated, and are compared with values previously reported in the literature obtained from CFD predictions and/or LDA and other measurements. It is shown that in general values of ɛ obtained from estimates of the macro length scales of turbulence from measurements may overestimate the amount of energy dissipated in the impeller swept region and in the impeller stream, while the present CFD results, as well as recent LDA experiments where ɛ has been measured directly, indicate that a substantial amount of energy might be dissipated in the bulk of the flow and near the wall and baffles.

The timing of vortex shedding in a cylinder wake imposed by periodic inflow perturbations

The interaction of vortex shedding from a circular cylinder with an inflow which has low-amplitude periodic velocity oscillations (perturbations) superimposed upon it, was investigated experimentally by means of particle image velocimetry. The experiments were made at three perturbation frequencies across the lock-on range in which the vortex shedding frequency is synchronized with the subharmonic of the imposed frequency. The basic wake pattern in this range is antisymmetric vortex shedding, i.e. the familiar 2S mode. The timing of vortex shedding is defined with respect to the cross-flow oscillation of the wake which is found to play a critical role. Quantitative analysis of the phase-referenced patterns of vorticity distribution in the wake shows that a vortex is actually shed from the cylinder when the cross-flow oscillation of the wake is strongest, marked by a sudden drop in the computed vortex strength. At the middle of the lock-on range, shedding occurs near the minimum inflow velocity in the cycle or, equivalently, during the forward stroke of a cylinder oscillating in-line with the flow. It is argued that the imposed timing of vortex shedding relative to the cylinder motion induces a negative excitation from the fluid, which might explain why the in-line response of a freely vibrating cylinder exhibits two positive excitation regions separated by the lock-on region found in forced oscillations.

Semibatch Emulsion Polymerization of Methyl Methacrylate with a Neat Monomer Feed

Semibatch emulsion polymerization of methyl methacrylate (MMA) with neat monomer feed in the presence of sodium lauryl sulphate as emulsifier and potassium persulphate as initiator was investigated. The effects of the monomer feed rate (R a ), the emulsifier concentration, and the monomer distribution ratio between the initial charge and feed on the kinetic features of MMA emulsion polymerization in a semibatch reactor were studied. Under monomer‐starved conditions particles did not undergo an appreciable growth during polymerization and as a result a large number of particles were formed. The number of particles increased significantly as R a decreased. The number of polymer particles formed under starved conditions showed an exponent of − 1.98 against R a , which is larger than the value of − 0.67 that is theoretically obtained for the styrene monomer. The average molecular weights decreased and the molecular weight distribution became narrower with decreasing R a . The steady state rate of polymerization (R pss ) was confined to the correlation of the type R pss ≈ R a when a high concentration of the emulsifier was used. The rate of polymerization under monomer‐starved conditions was found to depend on the size of polymer particles formed. The experimental results suggest that models, which can take account of more specified size‐dependent kinetic parameters, should be developed for prediction of kinetic behavior of MMA in semibatch reactors.

Experiments and predictions of the transition of the flow pattern with impeller clearance in stirred tanks

Abstract In the present work, the double- to single-loop flow pattern transition in a stirred vessel equipped with a Rushton turbine is investigated by Laser Doppler anemometry (LDA). In particular, the clearance at which such transition occurs is assessed by comparing axial velocity profiles underneath the impeller. Computational fluid dynamics (CFD) simulations of the same system are carried out, by employing the ‘inner–outer’ fully predictive computation strategy. The comparison of predicted results with the experimental data collected shows that the transition is well reproduced by simulations. A good agreement on the mean velocities is also observed but for the impeller discharge stream angle to the horizontal in the single-loop flow pattern configuration. Finally the predicted turbulent fluctuations are underestimated, a finding in agreement with those of most earlier CFD predictions.

Spectral and wavelet analysis of the flow pattern transition with impeller clearance variations in a stirred vessel

Abstract The double- to single-loop pattern transition in stirred vessels stemming from a change in the off-bottom clearance of a Rushton turbine has been investigated by laser doppler anemometry. Time-resolved data showed the transition occurring within a range of clearance values and allowed the distinction of three types of flow: the double-loop regime, the single-loop regime and an unstable one termed “transitional state”. Experiments of up to 3– 4 h duration showed that both the onset and the lifetimes of these types of flow were random; however, in the transitional state, the flow varied between the two circulation patterns in a periodic manner, with a frequency linearly related to the impeller rotational speed. The results have important implications for mixing process and vessel design as well as CFD predictions of the flows which are discussed.