Ka-Lok Lee

Numerical simulations of the dependency of flow pattern on impeller clearance in stirred vessels

The flow and turbulence fields in a fully baffled vessel stirred by a Rushton turbine have been simulated by means of computational fluid dynamics (CFD) techniques. The simulation techniques adopted (the “Sliding-grid” and “Inner–outer” methods) are fully predictive and require no experimental data as boundary or initial conditions. The effect of clearance (C) on the flow patterns in stirred vessels was simulated as the flow field transition observed can be considered a particularly stringent benchmark for the testing of CFD techniques. The results are compared with previously obtained LDA data and show that the double- to single-loop transition experimentally observed when impeller clearance from the vessel bottom is suitably reduced, can be well reproduced by the CFD simulations. The single-loop flow structure present for C/T=0.15 is compared in detail with the experimental data and good overall agreement is shown between the experiment and simulation. The periodic component of the kinetic energy is well predicted, but the random component is underestimated, a finding also observed in earlier investigations. The mean flow in most of the vessel is also well predicted but the angle to the horizontal of the impeller discharge flow is overestimated. Predictions are also reported with grid refinements and different turbulence models, in an effort to identify means of improving agreement with the measurements. The findings indicate that further improvements in turbulence modelling might be necessary.

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.

Dynamic dead space in face masks used with noninvasive ventilators: a lung model study

The aim of this study was to determine what the influence of different designs of face masks and different noninvasive ventilator modes would be upon total dynamic dead space. Using a spontaneous breathing model, total dynamic dead space was measured when using 19 commercially available face masks and a range of ventilators in various ventilation modes. Total dynamic dead space during spontaneous ventilation was increased above physiological dead space from 32% to 42% of tidal volume by using face masks. The use of noninvasive ventilation modes such as bilevel and continuous positive airway pressure, with continuous pressure throughout the expiratory phase, reduced total dynamic dead space to approach physiological dead space with most face masks. Pressure assist and pressure support ventilation decreased total dynamic dead space to a lesser degree, from 42% to 39% of tidal volume. Face masks with expiratory ports over the nasal bridge resulted in beneficial flow characteristics within the face mask and nasal cavity, so as to decrease total dynamic dead space to less than physiological dead space from 42% to 28.5% of tidal volume. Exhaust ports over the nasal bridge in face masks effect important decreases in dynamic dead space provided positive pressure throughout the expiratory phase is used.

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.

Lck is a key target of imatinib and dasatinib in T-cell activation

childhood ALL, and it is remarkable that the identical SNP is penetrant for ALL risk in both racial groups. It is noteworthy that this single locus explains approximately 30% of the racial difference in ALL incidence between blacks and whites. Nevertheless, even after accounting for the racial differences in frequency of the risk allele at rs10821936, there remains a lower incidence of B-lineage ALL in blacks. These unexplained racial differences could be due to racial differences in allele frequencies at additional inherited loci, in environmental exposure to leukemogens, or in immune function. In addition, B-lineage ALL without known cytogenetic abnormalities could include a heterogeneous group of cryptic genomic variations such as chromosomal gains or losses. It is possible that germline polymorphisms are associated with subgroups of ALL cases with cryptic gains or losses. Future systematic cataloging of race-specific lesions in ALL patients should shed new light on the etiology of this disease.

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.

A Liquid Crystal Thermographic Technique for the Measurement of Mixing Characteristics in Stirred Vessels

A liquid crystal thermographic technique has been developed and employed to determine the transient mixing characteristics in a stirred vessel of 100mm diameter. The techniquemakes use of temperature as a passive scalar and shows promise for the rapid and accurate determination of mixing time and temperature distributions in stirred vessels. Characteristic results obtained in the 100mm vessel stirred by one and two Rushton impellers are presented and compared with previously reported measurements obtained with other techniques.

A Novel Profiled Blade Impeller for Homogenization of Miscible Liquids in Stirred Vessels

A novel axial flow impeller with profiled blades has been designed and manufactured using CAD/CAM techniques. The impeller was of diameter D = T/3 and located at a clearance of C = T/3. Its mixing characteristics were measured in two vessels of 100 mm and 290 mm diameter. The impeller power number is 0.22 and its mixing mumber is 48. The flow number and the circulation flow number were determined by LDA measurements to be 0.52 and 1.00 respectively. Comparisons with other axial flow impellers are made and it is shown that the impeller can provide a very efficient means for the homogenisation of liquids in stirred vessels.

Rapid Bidirectional Reorganization of Cortical Microcircuits

Mature neocortex adapts to altered sensory input by changing neural activity in cortical circuits. The underlying cellular mechanisms remain unclear. We used blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to show reorganization in somatosensory cortex elicited by altered whisker sensory input. We found that there was rapid expansion followed by retraction of whisker cortical maps. The cellular basis for the reorganization in primary somatosensory cortex was investigated with paired electrophysiological recordings in the periphery of the expanded whisker representation. During map expansion, the chance of finding a monosynaptic connection between pairs of pyramidal neurons increased 3-fold. Despite the rapid increase in local excitatory connectivity, the average strength and synaptic dynamics did not change, which suggests that new excitatory connections rapidly acquire the properties of established excitatory connections. During map retraction, entire excitatory connections between pyramidal neurons were lost. In contrast, connectivity between pyramidal neurons and fast spiking interneurons was unchanged. Hence, the changes in local excitatory connectivity did not occur in all circuits involving pyramidal neurons. Our data show that pyramidal neurons are recruited to and eliminated from local excitatory networks over days. These findings suggest that the local excitatory connectome is dynamic in mature neocortex.

On the Trailing Vortices Around Hydrofoil Impeller Blades

The trailing vortices around the profiled blades of a novel axial flow impeller were investigated with angle-resolved laser-Doppler anemometry techniques in a stirred vessel of diameter T = 100 mm. The impeller was of diameter D = T /3 and located at a clearance of C = T /3. The measurements showed that a single vortex is formed behind each blade; the axes of the vortices are inclined at 13° to the horizontal and remain at a constant radius of r/T =0.14. The kinetic energy of turbulence, k, reaches maxima of 0.0175 V 2 tip in the vortices, while away from the blades klV 2 tip values do not exceed 0.0025. The effect of the vortices on the mean flow and turbulence structure is discussed and the implications of the results for mixing process design are assessed.