Effect of scale on entrainment in stirred tanks

In the present work, surface turbulence characteristics at the onset of entrainment in air water system have been investigated. For the present study, shear type entrainment in stirred tanks has been considered. Experiments have been performed in stirred tanks with different scales for different types and sizes of impellers. The results of the work reveal that radial RMS, axial RMS velocities and turbulent kinetic energy showed similar magnitudes at onset even at different scales. The RMS velocities as well as TKE magnitudes did not vary with type or size of impellers. Local energy dissipation rates have been estimated from autocorrelation function of fluctuating velocity. Very low magnitudes of local energy dissipation rates at onset have been observed.     

Flow of viscous fluid through a circular aperture

The creeping flow of a highly viscous incompressible fluid through a circular aperture located in an infinitely wide horizontal plate is analyzed by solving Navier-Stokes equations without inertia terms. Solutions for vertical and radial velocities as well as pressure have been obtained in terms of integral equations with an undetermined Kernal function. This function has been evaluated by assuming several different velocity distributions at the aperture, and the corresponding pressure drop for each case has been calculated. The results show that the pressure loss for a given flow rate goes through a minimum as the assumed velocity profile changes from flat to parabolic. Based on the minimum energy dissipation theorem of Helmholtz, the most appropriate velocity distribution is discussed. Experimental data obtained using sharp-edged orifices are compared with theoretical predictions.     

Mass transfer in two-and three-phase fluidized beds

The effects of liquid (0.03-0.12 m/s) and as (0.04-0.20 m/s) velocities, and particle size (0-8.0 mm) on the volumetric mass transfer coefficients at the grid zone have been determined in a 0.152 mI.D. x 1.8 m high Plexiglas column. The volumetric mass transfer coefficient in the grid zone increases with increasing gas velocity and particle size. However, the coefficient exhibits a maximum value at an optimum bed porosity condition. The volumetric mass transfer coefficients in terms of the Sherwood number in three-phase fluidized beds have been correlated with the Schmidt number and particle Reynolds number which is related to the energy dissipation rate in the beds based on the local isotropic turbulence theory. Also, the coefficient has been correlated with the experimental variables.     

Failure and fragmentation of ceramic target with varying geometric configuration under ballistic impact

The failure and fragmentation of monolithic bare alumina 99.5% ceramic target and energy dissipation of steel 4340 projectile have been studied in a series of ballistic experiments carried out, with the incidence velocities in a range, 122–290 m/s. The velocity drop and energy dissipation increased with incidence velocity for 10 mm thick target with damage zone extended upon the whole area of rear face at higher velocities. The ballistic results obtained with the 10 mm thick target have been compared with the ballistic performance of the 5 mm thick target used in a previous study to explore the effects of target thickness on the failure mechanism. A model for the residual velocity of projectile after perforation of the single layered ceramic target has been developed based on the Lambert Jonas model by using the experimental data available for 5 mm and 10 mm thick alumina 99.5% target against 10.9 mm projectile. The residual velocities and damage patterns were reproduced with a reasonable amount of accuracy by a three-dimensional finite element model developed on commercial ABAQUS/CAE. The effect of obliquity and projectile diameter to target thickness ratio (D/T) on ballistic performance has been determined by the numerical simulation model with impact velocity in a range of 300–500 m/s. A spatial variation of ejected fragments velocity at different time steps was plotted to develop a velocity profile for the ceramic fragments coming out of the target. A semi-empirical model has been proposed for residual velocity after perforation of a monolithic ceramic target, relating to the incidence velocity and projectile diameter to target thickness ratio. The monolithic ceramic targets have been investigated for a comparative assessment of energy dissipation by the ceramic layer to eventually design an efficient front layer of a ceramic based composite armour in future studies.     

A simple kinetic model for complex rheological fluids based on irreversible thermodynamics

In this work, we analyze the kinetics of the entanglement–disentanglement process of complex fluids coupled to a rheological constitutive equation of state within an irreversible thermodynamics framework. In the context of the coupling between the kinetics and the mechanical phenomena, we assume that the rate constants are functions of the affinities that contain the chemical potentials, which are themselves functions of the extended Gibbs free energy containing the irreversible dissipation terms. Although the derived model has a simple mathematical structure, it is able to predict complex flow behaviors, including shear-thickening, shear-thinning, and more complex flow histories such as shear-banding. As special case, we derive the constitutive equations of the Bautista–Manero–Puig (BMP) model in which the material constants have a thermodynamic basis and have been successfully used for the last two decades to predict the behavior of complex fluids such as the ones examined here.     

Direct-Write Fabrication of Pb(Nb,Zr,Ti)O3 Devices: Influence of Paste Rheology on Print Morphology and Component Properties

Lead niobium zirconate titanate (PNZT) pastes with tailored rheological properties have been developed for direct-write fabrication of thick-film capacitor elements in highly integrated, multifunctional electroceramic devices. Such pastes exhibited pseudoplastic behavior with a low shear apparent viscosity of roughly 1 × 10⁶ cP. On aging, the degree of shear thinning and the low shear apparent viscosity decreased. Pastes prepared from as-received powders attained printable, steady-state viscosities of ∼2 × 10⁵ cP after 50 days of aging. In contrast, pastes prepared from dispersant-coated powders showed no measurable rheological changes after 1 day of aging. Square elements were patterned on dense alumina substrates or Teflon sheets. Leveling behavior as a function of time for single line prints, and the resulting surface topographies of dried PNZT films were measured by laser profilometry. PNZT layers sintered at varying temperatures between 950° and 1050°C for 5 h in either air or a lead-rich atmosphere yielded porous microstructures as revealed by scanning electron microscopy (SEM). Such layers exhibited dielectric constants ( K ) of 1400–1570 at 1 kHz with dissipation factors ( D ) of less than 4.1%.     

A novel coalescence model for binary collision of identical wet particles

A coalescence model that incorporates both capillary and viscous contributions of the liquid binder, along with the liquid bridge volume effects, has been developed. The objective is to predict the coalescence of two spherical particles coated with a thin liquid film approaching with equal initial velocities from opposite directions. The model is based on an overall coefficient of restitution that is determined with the aid of the approximate analytical values of the maximum possible energy dissipation and a critical value that depends on the total initial kinetic energy of particles. The maximum possible energy dissipation accounts for the energy loss due to the viscous and capillary effects and inelastic collision. The proposed simplified method to determine the critical velocity has been compared with the numerical solution of the general equation of motion, and an excellent agreement has been found. The coalescence model has been investigated at the limiting conditions by neglecting either the capillary effect or the viscous effect, respectively. Finally, comparisons have been made with experimental data, and a reasonable agreement has been found.     

Determination d'un champ de vitesses par anemometrie a films chauds dans des domaines a turbulence elevee application a des installations de decantation et d'Agitation

The authors present a study of experimental methods which permit the determination of the turbulent hydrodynamic characteristics of the flow within settling and agitation units. The measurements have been made by thermal anemometry implying one and three hot films. The processing of the experimental data obtained on-line in real time permits the average velocities, the turbulent fluctuations, the turbulent lengths and the dissipation rate to be obtained.     

水力旋流器湍流场数值模拟

<正> 引言 水力旋流器作为一种简便、易行和高效率的分离、分级和离心沉降设备,已被广泛应用于化工、冶金、石油等众多工业领域中.以往的水力旋流器设计主要是根据大量物理模型试验得出的经验准数方程来求出旋流器的几何结构参数和操作参数.然而,随着水力旋流器应用范围的迅速扩大和人们对其分离(级)性能指标的要求日益提高,传统的按经验或半经验公式进行旋流器设计方法的局限性越来越明显,以及物模试验的耗时费钱,已促使人们开始采用数值模拟的方法,通过对旋流器内部流体运动的深入研究,弄清旋流器的分离机理,以便为提高水力旋流器的分离效率和分级准确度予以理论指导.本文采用了适于水力旋流器液相(水)流场的K-ε湍流数学模型,对水力旋流器内的湍流运动规律进行了数值模拟并根据激光实测结果对部分模型常数进行了修正.     

CFD simulation of stirred tanks: Comparison of turbulence models. Part I: Radial flow impellers

A critical review of the published literature regarding the computational fluid dynamics (CFD) modelling of single‐phase turbulent flow in stirred tank reactors is presented. In this part of review, CFD simulations of radial flow impellers (mainly disc turbine (DT)) in a fully baffled vessel operating in a turbulent regime have been presented. Simulated results obtained with different impeller modelling approaches (impeller boundary condition, multiple reference frame, computational snap shot and the sliding mesh approaches) and different turbulence models (standard k ? ε model, RNG k ? ε model, the Reynolds stress model (RSM) and large eddy simulation) have been compared with the in‐house laser Doppler anemometry (LDA) experimental data. In addition, recently proposed modifications to the standard k ? ε models were also evaluated. The model predictions (of all the mean velocities, turbulent kinetic energy and its dissipation rate) have been compared with the experimental measurements at various locations in the tank. A discussion is presented to highlight strengths and weaknesses of currently used CFD models. A preliminary analysis of sensitivity of modelling assumptions in the k ? ε models and RSM has been carried out using LES database. The quantitative comparison of exact and modelled turbulence production, transport and dissipation terms has highlighted the reasons behind the partial success of various modifications of standard k ? ε model as well as RSM. The volume integral of predicted energy dissipation rate is compared with the energy input rate. Based on these results, suggestions have been made for the future work in this area.     

Physical properties of La-doped NiO sprayed thin films for optoelectronic and sensor applications

Lanthanum-doped nickel oxide NiO:La thin films were deposited onto glass substrates at 450 °C, by the spray pyrolysis technique using nickel and lanthanum chlorides as precursors. These films belonging to cubic structure, crystallize preferentially along (111) plane. First, Raman study shows the presence of bands corresponding to NiO structure. The same study confirms the presence of both Ni(OH)₂ and LaNiO₃ as secondary phases. Moreover, using SEM observations, all samples exhibit porous microstructures with rough surfaces and spherical nanoparticles of about 40 nm as size. Second, NiO:La films present a direct band gap energy value lying in the range of 3.63–3.84 eV. Also, the effect of the La incorporation in NiO matrix on the disorder is studied in terms of Urbach energy. Some optical constants (refractive index, extinction coefficient, dielectric constants, and dispersion parameters) are reached. On the other hand, the photoluminescence spectroscopy reveals the presence of peaks related to the electronic transition of the Ni²⁺ ions and others confirming the presence of some defects in NiO matrix in terms of La content. Finally, it has been found that La doping allows the improvement of the electrical conductivity as well as Haacke’s figure of merit of NiO sprayed thin films by at least, three orders of magnitude.     

Characterization of flow phenomena induced by ultrasonic horn

Mean flow and turbulence parameters have been measured using laser Doppler anemometer (LDA) in ultrasound reactor. The effects of the ultrasonic power have been investigated over a power density (P/V) range of 15-. The liquid circulation velocities are dominant in the zone nearer to the source of energy and are substantially low at the walls and at the bottom of the reactor. The levels of turbulence kinetic energy and dissipation rate are high near the horn and decrease rapidly with increasing distance from the horn. Average turbulent normal stresses are larger than the turbulent shear stresses. However, they are much lower than stirred reactors when compared at the same power consumption per unit mass. Comparisons of LDA measurements and computational fluid dynamics (CFD) predictions have been presented. The good agreement indicates the validity of the CFD model. The flow information has been extended for the prediction of mixing time. For uniform mixing in ultrasound-assisted reactors, optimum power density and diameter of the vessel is needed, yet it is far less effective than conventional stirred vessel. The possibility of optimization has been suggested in terms of power dissipation and the vessel size.     

Jet momentum dissipation at a grid of a large gas fluidized bed

Rational scale-up of fluidized beds to commercial size requires an understanding of the operation of the grid support plate. One aspect of grid behavior has been studied experimentally; namely, the momentum dissipation of vertical air jets within an 11 inch diameter bed of fluidized cracking catalyst. Nozzle diameters were varied from ¼ to 1½ in. and nozzle velocities from 50 to 300 ft./sec. The data have been found to fit a general correlation relating the normalized axial momentum of the jet to a modified Froude number. The significance of the correlation to the process and design engineer is briefly discussed.     

Assessment of k–ε models using tetrahedral grids to describe the turbulent flow field of a PBT impeller and validation through the PIV technique

In turbulence modeling, the RNG and Realizable models have important improvements in the turbulent production and dissipation terms in comparison to the Standard. The selection of the appropriate turbulence model has an impact on the convergence and solution in STRs, and they are used in mixing, multiphase modeling or as starting solution of transient models as DES and LES. Although there are several studies with the pitched blade turbine(PBT) impeller, most of them used the Standard model as representative of all k–ε models, using structured hexahedral grids composed of low number of cells, and in some cases under axial symmetry assumptions.Accordingly, in this work the assessment of the Standard, RNG and Realizable models to describe the turbulent flow field of this impeller, using the Multiple Reference Frame(MRF) and Sliding Mesh(SM) approaches with tetrahedral domains in dense grids, is presented. This kind of cell elements is especially suitable to reproduce complex geometries. Flow velocities and turbulent parameters were verified experimentally by PIV and torque measurements. The three models were capable of predicting fairly the pumping number, the power number based on torque, and velocities. Although the RNG improved the predictions of the turbulent kinetic energy and dissipation rate, the Realizable model presented better performance for both approaches. All models failed in the prediction of the total dissipation rate, and a dependence of its value on the number of cells for the MRF was found.     

Peel Adhesion of Solid Films-The Surface and Bulk Effects

The peel strength of rubber and paint films has been measured over a range of peeling velocities using a dead weight method. At low peel rates the peel force is fairly constant but rises rapidly at higher peeling speeds.

Experiments show that the peel strength is a function both of the energy of interfacial bonds which must be broken as peeling proceeds and of bulk energy losses in a viscoelastic peeling material.

The interfacial effect has two components: an equilibrium surface force which accounts for the peel strength at low velocities, and a viscous peeling force which depends on the peeling rate. This viscous interfacial force explains the increase in peel strength of purely elastic films at higher peeling velocities.

The energy loss in the bulk of the peeling film introduces two additional effects: a magnification of the peel strength in steady peeling over a certain velocity range, and a slowing down or stopping of peeling as transient relaxation occurs shortly after the application of the peel force.     

Synthesis and Characterization of Low-Dielectric Silica Aerogel Films

Thin films of silica aerogel on a silicon wafer have been synthesized via supercritical drying of wet gel films that were obtained by spin coating the polymeric silica sol, followed by aging in an isopropanol (IPA) or tetraethoxysilane/isopropanol (TEOS/IPA) solution. The viscosity of the sol and the spin rate required to form uniform coating layers were optimized. The film thickness and microstructures could be controlled by adjusting the concentration of the sol, the spin rate, and the aging conditions. The porosities and the dielectric constants of the thin films were in the range of 76%–90% (densities of 0.59–0.22 g/cm³) and 2.0–1.5, respectively. The degree of planarization in the aerogel thin film was ∼100%, and the gap-filling capability on a 0.2 μm tungsten patterning wafer was excellent. In particular, aging the wet gel film in the TEOS/IPA solution was very effective in improving the properties of the aerogel films.     

Peel Adhesion of Solid Films-The Surface and Bulk Effects

The peel strength of rubber and paint films has been measured over a range of peeling velocities using a dead weight method. At low peel rates the peel force is fairly constant but rises rapidly at higher peeling speeds.

Experiments show that the peel strength is a function both of the energy of interfacial bonds which must be broken as peeling proceeds and of bulk energy losses in a viscoelastic peeling material.

The interfacial effect has two components: an equilibrium surface force which accounts for the peel strength at low velocities, and a viscous peeling force which depends on the peeling rate. This viscous interfacial force explains the increase in peel strength of purely elastic films at higher peeling velocities.

The energy loss in the bulk of the peeling film introduces two additional effects: a magnification of the peel strength in steady peeling over a certain velocity range, and a slowing down or stopping of peeling as transient relaxation occurs shortly after the application of the peel force.     

Structural and Optical Characterization of Chemically Deposited PbS Thin Films

PbS thin films were deposited on glass substrates by a chemical bath deposition method. The effect of varying the film thickness on the structural and optical properties has been investigated. XRD analysis reveals the crystallinity of the deposited PbS films with (200) preferred crystal orientation. Increasing the film thickness enhances the crystallinity of the films as well as decreases the strain and dislocation density. The surface morphology features were dramatically changed from small spherical grains to bead-like shape. The absence of impurities in the deposited films was confirmed by energy dispersive x-ray spectrometry (EDX) measurements. The optical constants of the deposited films were calculated and a small decrease in the band gap energy was observed with increasing the film thickness.     

Determination of the Velocity of Constrained Motion of Nonspherical Particles in Liquid

A semi-empirical equation has been derived based on the variation principle of the minimum of the energy dissipation intensity, which describes the ratio of the velocities of the constrained and free motion of nonspherical solid particles and is in good agreement with the experimental data.