Unbaffled Tank Research Articles

Straight-line braking dynamic analysis of a partly filled baffled and unbaffled tank truck

The straight-line braking performance characteristics of a partly filled tank truck with and without transverse baffles are investigated in the presence of transient fluid slosh within the tank. The dynamic interactions of the floating cargo with the vehicle are evaluated by integrating a dynamic fluid slosh model of the partly filled tank with and without transverse baffles with the seven-degrees-of-freedom pitch plane model of a tridem truck. The dynamic fluid slosh within the tank containing equally spaced four curved single-orifice baffles with flow equalizer is modelled using three-dimensional Navier—Stokes equations coupled with the volume-of-fluid equation and analysed using the FLUENT software. The coupled tank—vehicle model is subsequently analysed to determine the straight-line braking properties for different fill volumes, magnitudes of braking treadle pressure, and road surface adhesion coefficients, while the cargo load is considered to be constant. The results show interactions of the sloshing cargo with the vehicle. A degradation of the braking performance of the partly filled tank truck is evident in the presence of transient fluid slosh, particularly in the absence of baffles. The braking performance, however, is highly dependent upon the fill volume, presence of baffles, and severity of braking input. For a clean-bore tank truck, the stopping distance increases monotonically with decreasing fill volume, while the addition of transverse baffles in general results in considerably shorter stopping distance. The baffled tank truck, however, reveals a relatively shorter stopping distance under an intermediate fill condition (approximately 52.1 per cent), compared with the lower and higher fill volumes. Although the analyses are limited to conventional single-orifice baffles, the proposed coupled vehicle—tank model could serve as an important tool for exploring alternative baffle designs and layouts.

Effect of baffles on a partially filled cubic tank: Numerical simulation and experimental validation

In this paper, sloshing waves have been analyzed for baffled and un-baffled tanks. Numerical simulations were carried out based on volume of fluid (VOF) techniques with arbitrary-Lagrangian–Eulerian (ALE) formulation which adopts the displacement of solid, the pressure and displacement in the fluid as variables to model the coupled system. The response of the coupled system is obtained by using the well-known software ADINA, which offers efficient fully coupled fluid–structure interaction capabilities by finite element method. The results obtained are compared with the available experimental data to demonstrate the reduction of sloshing effects in fluid model.

The sensitivity of reconstructed images and process engineering metrics to key choices in practical electrical impedance tomography

This paper explores, from an experimental perspective, the dramatic effect that measurement strategy, reconstruction algorithm and reconstruction parameters can have on electrical impedance tomography images. Measurement data, from a stirred tank and jet mixer, have been acquired using two tomographs from the University of Cape Town and Industrial Tomography Systems Ltd respectively. Simulations consider conductively contrasting objects that are placed strategically in the vessel. The adjacent and opposite measurement strategies are employed to interrogate an unbaffled mixing tank. The superior sensitivity of the opposite strategy in the centre of the vessel is verified. For a central inclusion, simulated results suggest 4% ‘image error’ compared to 6% for the adjacent strategy. Five reconstruction algorithms: Linear Back Projection, Landweber, Conjugate Gradients, Generalized Singular Value Decomposition (GSVD) and Nonlinear Gauss Newton, have been considered. A measure of ‘image error’ is typically below 10%, but values as high as 30% are not unusual with algorithms such as Linear Back Projection. For a homogeneous step change in conductivity ‘image error’ is seen to vary from 3% for Nonlinear Gauss Newton to 150% for Linear Back Projection. Corresponding measures of the coefficient of variation range from 25% to 44%. Overall it is suggested that the GSVD algorithm provides the best balance of attributes for identifying discrete objects and for homogeneous step changes. The dramatic effect of regularization parameters is illustrated by considering the GSVD. The use of the discrete Picard condition to determine optimum values is demonstrated. Mixing lengths have been calculated from the reconstructed image data and this is seen to vary dramatically with the regularization parameter.

Numerical Simulation of Liquid−Solid Flow in an Unbaffled Stirred Tank with a Pitched-Blade Turbine Downflow

Computational fluid dynamics (CFD) provides an efficient method for investigating highly complex fluid flows in mechanically stirred tanks, but less attention was given to unbaffled stirred tanks generated with axial impeller, which are frequently used in process industries. The present study is intended to evaluate the CFD predictions of key properties related to the mixing against measurements and to provide a detailed insight into the process. A three-blade 70° PBTD (Pitched Blade Turbine Downflow) was used to generate axial flows in a stirred tank, of which the local solid concentration profiles were numerically simulated. The liquid−solid system consisted of spherical silica beads in water. A method using “vector distance” in a Eulerian reference frame was proposed to resolve the impeller and the bottom of the tank of complex geometry. The predicted results of two-phase flows at various impeller speeds and solid holdups are presented. In addition, various Schmidt numbers and laminar viscosity coefficients of dispersion phase were tested to improve the predictions. In the experiments, an optical fiber probe was used to measure solid particle concentration, and the effects of impeller speed on solid holdup were examined. The solid particle distribution was affected with the impeller speed in the range of 113−173 rpm in the stirred tank. The radical concentration of solids particle presents different profiles at different axial cross sections. Good agreement between the experiments and simulations was observed.

Scaling Up of the Geometrically Similar Unbaffled Circular Tank Surface Aerators

AbstractThe present investigation is concerned with the development of scale‐up equations for the oxygen transfer coefficient, k, and the power number, PO, for unbaffled circular tank surface aerators under geometrically similar conditions, with the purpose of designing energy efficient aerators. It has been found for the present aerators that k and PO are uniquely related to a parameter, X, governing the theoretical power per unit volume and which is defined as Fr4/3Re1/3, where Fr and Re are the impellers' Froude and Reynolds numbers, respectively. Empirical correlations between k and X, as well as PO with X are developed for the range of experiments conducted. Based on such experimental results, procedures to design energy efficient aeration systems have been demonstrated and it was found that smaller sized aerators are more energy efficient and economical when compared to bigger sized tanks, while aerating the same volume of water, by maintaining the same input power to the tanks irrespective of their size. It has been also demonstrated that substantial amounts of energy can be saved if the aeration tanks run at higher input power.

Macro- and micromixing studies in an unbaffled vessel agitated by a Rushton turbine

Macromixing characteristics, power number and visual observation of the vortex behaviour and micromixing in an unbaffled tank agitated with a Rushton turbine are reported. The latter has also been compared in detail with earlier results from an identical tank containing baffles. The maximum mean specific energy dissipation rate, ε ¯ T , in the unbaffled tank that can be utilised without severe air incorporation is ∼ 0.18 W / kg compared to ∼ 1.2 W / kg with baffles. However, at this lower ε ¯ T , the micromixing efficiency is always significantly greater without baffles except when addition is made onto the top of the liquid or into the trailing vortex very close to the impeller. In these latter cases, they are approximately the same but even a small submergence of the feed tube below the liquid surface greatly enhances micromixing in the unbaffled case whilst it is still very poor with baffles. This good micromixing performance of the unbaffled vessel was very unexpected. Furthermore, an established method of estimating the local ε T gave values of ε T / ε ¯ T = φ ⪢ 1 at every feed position where measurement was undertaken. Since the spatially averaged value of φ = 1 , this result suggests the possibility that the accepted concept of micromixing being totally controlled by the local ε T at the feed point may not be valid for such swirling flows.

The use of circular surface aerators in wastewater treatment tanks

AbstractAeration experiments were conducted in different sized baffled and unbaffled circular surface aeration tanks to study their relative performance on oxygen transfer process while aerating the same volume of water. Experiments were carried out with the objective of ascertaining the effect of baffle on oxygen transfer coefficient k. Simulation equations govern the oxygen transfer coefficient with the theoretical power per unit volume, X and actual power per unit volume, PV. It has been found that, for any given X, circular tanks with baffle produce higher values of k than unbaffled circular tanks, but in terms of actual power consumption unbaffled tanks consume less power when compared to baffled circular tanks to achieve the same value of k. It has been found that in terms of energy consumption, ε, baffled tanks consume more energy than unbaffled tanks at any value of X. This suggests that the unbaffled circular tank gives a better performance as far as energy consumption is concerned and hence better economy. An example illustrating the energy conservation to aerate the same volume of water in both types of aerators is given. Copyright © 2007 Society of Chemical Industry

Sloshing in a three-dimensional rectangular tank: Numerical simulation and experimental validation

Pressure variations and three-dimensional effects on liquid sloshing loads in a moving partially filled rectangular tank have been carried out numerically and experimentally. A numerical algorithm based on the volume of fluid (VOF) technique is used to study the non-linear behavior and damping characteristics of liquid sloshing. A moving coordinate system is used to include the non-linearity and avoid the complex boundary conditions of moving walls. The numerical model solves the complete Navier–Stokes equations in primitive variables by using of the finite difference approximations. In order to mitigate a series of discrete impacts, the signal computed is averaged over several time steps. In order to assess the accuracy of the method used, computations are compared with the experimental results. Several configurations of both baffled and unbaffled tanks are studied. Comparisons show good agreement for both impact and non- impact type slosh loads in the cases investigated.

Effect of the shaft eccentricity on the hydrodynamics of unbaffled stirred tanks

The aim of this work is to investigate the effect of the shaft eccentricity on the hydrodynamics of unbaffled stirred vessels. The difference between coaxial and eccentric agitation is studied using a combination of experiments carried out by particle image velocimetry, that provide an accurate representation of the time-averaged velocity, and computational fluid dynamics simulations, that offer a complete, transient volumetric representation of the three-dimensional flow field, once a proper modelling strategy is devised. The comparison of the experimental and simulated mean flow fields has demonstrated that calculations based on Reynolds-averaged Navier–Stokes equations are suitable for obtaining accurate results. Depending on the position of the shaft, steady-state or transient calculations have to be chosen for predicting the correct flow patterns. Care must be exerted in the choice of turbulence models, as for the unbaffled configurations the results obtained with the Reynolds stress model are superior to that of the k – ε model.

Screening Tool to Evaluate the Levels of Local Anisotropy of Turbulence in Stirred Vessels

A new simplified method to evaluate levels of local anisotropy of turbulence in mixing vessels has been proposed. The method has been tested both in a baffled and an unbaffled tank stirred with a Rushton turbine, by means of a detailed comparison with levels of local anisotropy of turbulence determined with the analysis of the invariants of the anisotropy stress tensor. In both configurations three-dimensional laser Doppler anemometry measurements have been performed throughout the whole vessel. The results indicated that turbulence deviates significantly from the isotropic state not only in the impeller region but also near the bottom of the vessel, for both baffled and unbaffled configurations. Larger regions with high local anisotropy of turbulence are present in the unbaffled vessel with respect to the baffled vessel. Finally, the simplified method was also tested and applied to a magnetically driven impeller that has been poorly investigated in the published literature but largely employed for industrial mixing processes. The flow field generated by this impeller is also shown. Advantages, drawbacks, and applicability of the simplified method are discussed.

Experimental investigation of pressure distribution on a rectangular tank due to the liquid sloshing

Pressure variations and 3D effects on liquid sloshing loads in tanks have been carried out experimentally. Recently, considerable advances have been made in the development of numerical techniques for studying the liquid sloshing on large structures. However, there is a lack of high quality experimental data that may be used for validating the analytical and numerical solutions obtained. This paper describes the results of the research project conducted at the Hydroulics Laboratory of Civil Engineering Faculty of Istanbul Technical University. The primary objective of the project is to investigate the pressure distributions at different locations and 3D effects on liquid sloshing. In this purpose, an experimental setup is designed to study the non-linear behavior and damping characteristics of liquid sloshing in partially filled 3D rectangular tank. Both several configurations of baffled and unbaffled tanks are studied.

Residence Time Distributions in a Stirred Tank: Comparison of CFD Predictions with Experiment

Residence time distributions (RTD) are measured for both a baffled and an unbaffled laboratory reactor of the same size with several internal pipes and a Rushton turbine operating at different feed flow rates and impeller rpms. Ideal behavior as determined by the mean and the variance of the RTD was observed at an impeller Reynolds number of 2327 for the baffled tank and 3878 for the unbaffled tank both in the turbulent transition range. The experimental results for the baffled tank are compared to computational fluid dynamics (CFD) predictions of the RTD using the k−e turbulence model in Fluent for transitional flow regime in the tank, i.e., impeller Reynolds number between 10 and 10 000. All the qualitative aspects of the predicted RTDs are similar to those measured experimentally. The mean residence times as well as the variances of the residence time are accurately predicted by CFD in the transition flow regime.

Steady‐state and decay dynamics for impellers of varying aspect ratio in unbaffled tanks

AbstractHydrodynamic torque measurements on impellers of square cross section were conducted for 33 different impellers in 3 different cylindrical unbaffled tanks. The tanks differed in volume by two orders of magnitude, and the Reynolds number range of the data was 104–105. A wide range of impeller sizes and aspect ratios was investigated, focusing on the decay regime where the tank walls impacted the torque measurement, and the steady state where the torque is constant on average. The number of revolutions required for spin‐up to steady state is a function of the tank radius and height and the impeller radius and height, and an expression is presented that describes the spin‐up time in terms of these variables and is valid for all of the data. At steady state and for a Reynolds number of 105, the torque coefficient or power number depended primarily on the height of the impeller and tank, but not on the impeller radius. During the decay regime, the measured torque coefficient decreases with a power‐law relation and is proportional to the number of revolutions raised to the k power. The k value did not depend on the height of the tank, but only on the impeller radius, impeller height, and tank radius. Expressions relating the steady‐state torque coefficient and the decay exponent, k, to the geometric variables model all of the data quite well. With these relations, the effects of the varying impeller aspect ratios on the energy required for mixing of momentum, and on the time required for mixing to occur, can be quantified.

SOLIDS DISTRIBUTION IN STIRRED SLURRY REACTORS: INFLUENCE OF SOME MIXER CONFIGURATIONS AND LIMITS TO THE APPLICABILITY OF A SIMPLE MODEL FOR PREDICTIONS

The features of solids concentration distribution were investigated in baffled and unbaffled tanks of high aspect ratio, stirred with multiple radial, mixed-flow or axial impellers. In the baffled tanks the average profiles can be predicted with the sedimentation-dispersion model regardless of impeller type - in spite of slight systematic departures from the average trend at the impeller plane and/or midway between the impellers. This is hardly possible for the unbaffled tanks due to lack of physical foundation of this simple model. When the unbaffled tank is stirred with hydrofoil impellers, an inverted profile is even obtained. The mentioned departures and this last anomalous behaviour are qualitatively discussed with reference to particle-turbulence interaction.

Two-Equation Turbulence Modeling for Impeller Stirred Tanks

In this study, the predictive performance of six different two-equation turbulence models on the flow in an unbaffled stirred tank has been investigated. These models include the low Reynolds number k-ε model of Rodi, W., and Mansour, N. N., “Low Reynolds Number k-ε Modeling With the Aid of Direct Simulation Data,” J. Fluid Mech., Vol. 250, pp. 509–529, the high and low Reynolds number k-ω models of Wilson, D. C., 1993, Turbulence Modeling for CFD, DCW Industries, La Canada, CA., the RNG k-ε model, and modified k-ω and k-ε models which incorporate a correction for streamline curvature and swirl. Model results are compared with experimental laser Doppler velocimetry (LDV) data for the turbulent velocity field in an unbaffled tank with a single paddle impeller. An overall qualitative agreement has been found between the experimental and numerical results with poor predictions observed in some parts of the tank. Discrepancies in model predictions are observed in the anisotropic regions of the flow such as near the impeller shaft and in the impeller discharge region where the model overpredicts the radial velocity component. These results are discussed and a strategy for improving two-equation models for application to impeller stirred tanks is proposed.

Turbulent flow in closed and free-surface unbaffled tanks stirred by radial impellers

The three-dimensional turbulent flow field in unbaffled tanks stirred by radial impellers was numerically simulated by a finite-volume method on body-fitted, co-located grids. Simulations were run, with no recourse to empirical input, in the rotating reference frame of the impeller. Free-surface problems were also simulated, in which the profile of the central vortex was computed as part of the solution by means of an iterative technique. Predicted velocity and turbulence fields in the whole vessel and power consumptions were assessed against available literature data; free-surface profiles were also compared with original experimental data obtained in a model tank. Both the eddy-viscosity k− ε turbulence model and a second-order differential stress (DS) model were used and compared: satisfactory results were obtained only by using the latter model. The need for including source terms arising from fluctuating Coriolis forces in the Reynolds stress transport equations is highlighted.

Flow induced by an impeller in an unbaffled tank—I. Experimental

Flow induced by an impeller in an unbaffled cylindrical tank was studied. In Part I, both mean and turbulent (root-mean-square) velocities in all three directions were measured using a laser Doppler anemometer (LDA). The effects of the impeller rotational speed and clearance of tank bottom on flow characteristics were investigated. It is found that the flow turbulence is approximately homogeneous in the bulk liquid except in the impeller stream. Near the gas—liquid interface the turbulence is strongly damped by the surface tension, and is fairly anisotropic. The mean and rms velocities in dimensionless form were observed to be independent of the impeller rotational speed. It was also found that the impeller clearance to the tank bottom affects the pumping capacity of the impeller, i.e. the rate, of flow ejected by the impeller blades, but does not influence the circulation capacity of flow in a vertical plane.

Flow induced by an impeller in an unbaffled tank—II. Numerical modelling

Experimental data for the flow induced by an impeller in a cylindrical tank were presented and analyzed in Part I (Dong ., 1994, Chem. Engng Sci. 49, 549–560). In Part II, a Coriolis computer model is developed and employed to predict flow characteristics in a 3D calculation in conjunction with the standard k-ε turbulence model. The computer model involves a coordinate transformation in which the computational coordinates rotate with the impeller, allowing the impeller to be treated as a stationary blade in a rotating tank. Mean velocities and turbulent energy in the bulk liquid predicted by the computer model are observed to be in satisfactory agreement with the measurements. The main discrepancies between the prediction and the experimental data are analyzed.

Transporting Adult and Larval Gulf Menhaden and Techniques for Spawning in the Laboratory

Abstract Fifty-five adult gulf menhaden (Brevoortia patronus) were transported from Gulf Breeze, Florida, to Beaufort, North Carolina, in 30 h without mortality. The life-support features during transport included a 600-L cylindrical, unbaffled fish tank, a continuous-duty pump, a filter using crushed oyster shells and activated carbon, tris-buffered seawater, oxygen, and monitors for temperature, oxygen, and pH. Spawning, induced by temperature, photoperiod, and injections of human chorionic gonadotropin and carp pituitary, occurred following transport and occurred nine times from November 1982 through March 1982. Larvae from two spawns were transported to the Gulf of Mexico for shipboard feeding experiments.

Transferts de chaleur a la paroi d'une cuve mecaniquement agitee

Local heat transfer coefficient between the agitated liquid contained in a tank and the wall of the tank has been determined for baffled and unbaffled tanks. The diameter of the six blade-turbine used was one third of the tank diameter. The overall coefficient has been calculated by integration of the local values. Results show the influence of the position of the turbine and of the baffles.