Axial Force at the Vessel Bottom Induced by Axial Impellers

This paper deals with the axial force affecting the flat bottom of a cylindrical stirred vessel. The vessel is equipped with four radial baffles and is stirred with a four 45° pitched blade impeller pumping downwards. The set of pressure transducers is located along the whole radius of the flat bottom between two radial baffles. The radial distribution of the dynamic pressures indicated by the transducers is measured in dependence on the impeller off-bottom clearance and impeller speed.It follows from the results of the experiments that under a turbulent regime of flow of an agitated liquid the mean time values of the dynamic pressures affecting the bottom depend not on the impeller speed but on the impeller off-bottom clearance. According to the model of the flow pattern of an agitated liquid along the flat bottom of a mixing vessel with a pitched blade impeller, three subregions can be considered in this region: the liquid jet streaming downwards from the impeller deviates from its vertical (axial) direction to the horizontal direction,  the subregion of the liquid flowing horizontally along the bottom and, finally, the subregion of the liquid changing direction from the bottom upwards (vertically) along the wall of the cylindrical vessel, when the volumetric flow rates of the liquid taking place in the downward and upward flows are the same.

Study of wheel-rail adhesion during braking maneuvers

The present work aims to better understand the phenomenon of adhesion under degraded conditions during railway braking maneuvers with the aim of optimizing the anti-slip algorithms in order to reduce damage to the profiles of wheels and rails and to minimize the braking distance. The proposed approach is based on the analysis of experimental data acquired during braking tests carried out on track, considering different types of vehicles and different types of contaminants, able to reproduce the typical degraded adhesion conditions occurring during normal operation. The work describes a numerical model that allows to evaluate the dynamics of the vehicle during the braking operation and to correlate the pressures to the brake cylinder, which are related to the braking forces, and the angular velocities measured on the axles of the vehicle, with the adhesion coefficient

Liquid homogenisation characteristics in vessels stirred with multiple Rushton turbines mounted at different spacings: CFD study and comparison with experimental data

The purpose of this work is to investigate the capability of the currently available CFD tools to correctly forecast the homogenisation process in a baffled vessel stirred with sets of identical Rushton turbines mounted with different spacings. The results of the simulations are compared with experimental data for the validation of the computational procedure. The simulations provide a good prediction of the time evolution of tracer dispersion inside the vessel volume for all the geometrical configurations. The differences in mixing performance due to flow variations occurring as a result of the modification of the impeller number and spacing are correctly forecasted

Modelling of solids distribution in stirred tanks: analysis of simulation strategies and comparison with experimental data

The predictive capabilities of CFD techniques as applied to solid\u2013liquid stirred vessels are investigated. The distribution of solid particles was simulated in three baffled stirred tanks agitated with single and multiple impellers. Suspensions of glass beads of different diameters and average concentration up to 6 vol. % in water were studied. The simulations of solid\u2013liquid suspensions in the stirred vessels were performed by using fully predictive approaches. Eulerian multiphase models were adopted for modelling the solid\u2013liquid flow, coupled with three different extensions of the standard k-1 model to the case of multiphase flows. The simulated particle axial concentration profiles are compared with experimental data and critically discussed. The most successful simulation strategy and one possible implementation are described

Distribution of solid particles in stirred suspensions

In this work, an in-house technique for the measurement of local particle concentration in solid-liquid suspensions based on an fibre-optical probe is presented. The technique is preliminary evaluated on the basis of the detailed flow field information gathered by means of RANS based CFD simulations, which allow to identify the best operating mode for minimizing the experimental uncertainties. Afterwards, the experimental and computed radial and axial solid concentration profiles in a pilot stirred reactor are compared and a very good agreement is found. This study allows to point out that, once proper computational strategies are selected, the usefulness of CFD simulations is twofold: they can be adopted for obtaining reliable predictions of various parameters in solid-liquid systems and used for gaining a better understanding of the complex fluid flow features of turbulent multiphase flows

Dispersion coefficients and settling velocities of solids in slurry vessels stirred with different types of multiple impellers

Equipment dealing with stirred suspensions are broadly used in the process industry. Out of the various aspects affecting their performance, solids distribution is addressed in this paper. The behaviour of a tank of high aspect ratio stirred with multiple impellers of three types is analysed. The purpose is to acquire fundamental information on parameters useful for their description, namely the axial dispersion coefficient of the solids phase and the particle settling velocity. Both transient and steady-state experiments were performed with a variety of dilute suspensions. The data were analysed with the axial sedimentation-dispersion model. The axial dispersion coefficient of the solid phase was found to differ from that of the liquid by less than 15% for all the impellers and conditions tested. The effective particle settling velocity in the stirred medium was then determined. As happens in equipment stirred with multiple radial turbines, the value of this parameter was found to be different, in general, from the terminal settling velocity. It is shown that a correlation among these parameters and Kolmogoroff length microscale and particle size obtained previously for a specific geometric configuration is actually independent of the impeller type

Role of CFD techniques in discriminating experimental solids concentration data in stirred suspensions and modelling of the solids concentration profiles in a pilot reactor

In this work, the suitability of an optical probe for studying local solid particle distribution in agitated systems and of Computational Fluid Dynamics (CFD) as a means to discriminate among the experimental data is investigated. Solids concentrations are determined in a high-aspect-ratio baffled tank stirred with three Rushton turbines by means of a fibre-optical probe and by CFD simulations. The local analysis of the simulated liquid flow field allows to better understand the source of differences in the measured solid concentrations that are obtained by changing the angular orientation of the probe in a few specific positions and, therefore, to identify the best operating mode for the measurements. The predicted radial and axial solid concentration profiles are then compared with the experimental data obtained with the most reliable probe orientation and very good agreement is found. This study allows to point out that, once proper computational strategies are selected, the usefulness of CFD simulations is twofold: they can be adopted for obtaining reliable predictions of various parameters in solid-liquid systems and used for gaining a better understanding of the complex fluid flow features of turbulent multiphase flows