The flow field produced by a pitched blade turbine : characterization of the turbulence and estimation of the dissipation rate

The influence of the agitation conditions on the morphology of Penicillium chrysogenum (freely dispersed and aggregated forms) was examined using radial (Rushton turbines and paddles), axial (pitched blades, propeller, and Prochem Maxflow T), and counterflow impellers (Intermig). Culture broth was taken from a continuous fermentation at steady state and was agitated for 30 min in an ungassed vessel of 1.4-L working volume. The power inputs per unit volume of liquid in the tank, P/V(L), ranged from 0.6 to 6 kW/m(3). Image analysis was used to measure mycelial morphology. To characterize the intensity of the damage caused by different impellers, the mean total hyphal length (freely dispersed form) and the mean projected area (all dispersed types, i.e., also including aggregates) were used. [In this study, breakage of aggregates was taken into account quantitatively for the first time.]At 1.4-L scale and a given P/V(L), changes in the morphology depended significantly on the impeller geometry. However, the morphological data (obtained with different geometries and various P/V(L)) could be correlated on the basis of equal tip speed and two other, less simple, mixing parameters. One is based on the specific energy dissipation rate in the impeller region, which is simply related to P/V(L) and particular impeller geometrical parameters. The other which is developed in this study is based on a combination of the specific energy dissipation rate in the impeller swept volume and the frequency of mycelial circulation through that volume. For convenience, the function arising from this concept is called the "energy dissipation/circulation" function.To test the broader validity of these correlations, scale-up experiments were carried out in mixing tanks of 1.4, 20, and 180 L using a Rushton turbine and broth from a fed-batch fermentation. The energy dissipation/circulation function was a reasonable correlating parameter for hyphal damage over this range of scales, whereas tip speed, P/V(L), and specific energy dissipation rate in the impeller region were poor. Two forms of the energy dissipation/circulation function were considered, one of which additionally allowed for the numbers of vortices behind the blades of each impeller type. Although both forms were successful at correlating the data for the standard impeller designs considered here, there was preliminary evidence that allowing for the vortices would be valuable. (c) 1996 John Wiley & Sons, Inc.

Dependence of mycelial morphology on impeller type and agitation intensity.

On a analyse le courant de decharge et l'ecoulement principal produits par une turbine a aubes inclinee dans un reacteur agite a chicanes, en combinant des techniques de visualisation de l'ecoulement et de mesure anemometrique laser Doppler. Deux profils de circulation globale distincts ont ete observes, l'un pour des degagements dans le fond du reservoir importants, l'autre pour des petits degagements aux parois. On montre que les profils de circulation ont un effet marque sur le courant de decharge, meme a proximite des aubes de la turbine. Ces decouvertes indiquent que l'action de la turbine n'est pas independante de la geometrie du reservoir et qu'elle subit une influence non negligeable de l'ecoulement global. Cette conclusion a des implications importantes pour la modelisation et la prediction des champs d'ecoulement dans les reservoirs agites

The mean flow field produced by a 45° pitched blade turbine: Changes in the circulation pattern due to off bottom clearance

Overview of clinical flow cytometry data analysis: recent advances and future challenges

Theories of cake filtration and consolidation and implications to sludge dewatering

Very low frequency fluctuations in the velocity signal from several seconds to several minutes in scale are usually observed in agitated vessels. It is shown in this work that these fluctuations result from the abrupt replacement of the flow pattern in the vessel, even in part. Pattern recognition studies of the flow in the impeller region of a paddle were carried out, using high-speed VTR. The results showed that the flow between blades is characterized into five patterns which are replaced one after another. They are classified into the discharge and the cross-pass types of flow. The latter means that flow passes between impeller blades, upwardly or downwardly. The lifetime of the discharge flow type was three times that of the cross-pass flow type. The velocity distributions of the discharge and cross-pass flow types estimated from visualized pictures were quite different from each other and were different from the average distributions obtained by LDV. When the cross-pass pattern had developed over the whole region between blades, a horizontal figure-eight pattern of flow developed in the entire vessel with abrupt reduction of the torque.

Pattern recognition in flow visualization around a paddle impeller

Unstable flow phenomena in a mixing vessel with a marine propeller were investigated experimentally using flow visualization and LDV techniques. It was found that the instantaneous bulk flow profiles were usually asymmetric with respect to the impeller shaft and they fluctuated in the large time scale. The circulation flow patterns in the space between two adjacent baffles were classified into quick-return flow (QR), full-circulation flow (FC) and intermediate-circulation flow (IC), which were replaced randomly one after another with time. Their lifetimes were widely distributed from about half a second to several minutes. Connected with the circulation flow patterns, the impeller stream was directed vertically downward or obliquely outward to the vessel wall and held a high level of angular momentum when pattern FC took place, while the impeller stream was directed obliquely inward to the impeller shaft with a low level of angular momentum when pattern QR took pace.

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Unstable phenomenon of flow in a mixing vessel with a marine propeller

Fluid motion and mixing in a gas-liquid contactor with turbine agitators

Etude experimentale de la structure des cavites autour des pales de l'agitateur, en fonction des caracteristiques de puissance

Power characteristics and cavity formation in aerated agitations

Object of this work is to analyse experimentally the rheological behavior of the consolidated suspension layer. The experiment has started when the consolidated particle bed expanded visco-elastically after loading on the bed was reduced. To confirm the visco-elastic expansion of the suspension, and uniaxial compression test (UCT) was applied for a highly filled suspension (HFS) at a constant concentration of the same material as the consolidation experiment. Stress hardening was observed after loading on the UCT. A three-element model (Voigt model plus an elastic element) was found to be simple and sufficient to represent the response of the creep test after stress hardening was achieved. The expansion experiment of the consolidated HFS bed showed that real expansion appeared after loading on the bed was reduced to some limited one, FB, from the original load, FA. It was confirmed from the experiment that the load difference (FA-FB) corresponded to the friction between the cylinder wall and HFS. The visco-elastic expansion of the HFS bed on the load region less than FB showed the strain approached the predicted strain curve from the UCT experiment.

Yoichi Nagase

Papers

Pattern recognition in flow visualization around a paddle impeller

Unstable phenomenon of flow in a mixing vessel with a marine propeller

Fluid motion and mixing in a gas-liquid contactor with turbine agitators

Power characteristics and cavity formation in aerated agitations

Consolidation Analysis. Mechanical Characteristics of a Particle Bed.