IMPINGEMENT CASTING PROCESS OF IMMISCIBLE ALLOYS

Experimental work on impingement casting process of Pb-Zn, Pb-Zn-Sn and Al-Pb systems was carried out. Rapid mixing of immiscible liquid metals was achieved in an arrow-shaped mixing chamber due to vigorous turbulence. The particle size distribution in the castings fitted lognormal curve well. Pb-Zn-Sn system showed the smallest geometric average particle diameter and geometric standard deviation due to low interfacial tension. Various mechanisms of agglomeration were examined and related to the particle sizes. Gravitational sedimentation was shown to be a major parameter governing the agglomeration and hence the final microstructure. The geometric connectivity effect of volume fraction was studied by graphical simulation. When the volume fraction of the dispersed phase was above 30 percent, major portion of the particles were interconnected.     

Gas-particle turbulent flow of foursquare tangential jets in a simplified pulverized-coal firing boiler

An experimental cold-model of a simplified tangential firing boiler was established to investigate the mesoscale turbulent flow behaviors, including gas vortex structures, particle motions and interactions between two phases. A modified PIV technology, employing two pairs of lasers and cameras, was applied to measure the velocity and velocity gradient of turbulent flow in foursquare tangential jets alternatively. At a given initial gas velocity and particle mass loading, the interaction between gas and particles was studied at three different particle sizes. It was found that two main coherent vortex structures, circular eddy and hairpin eddy, distributed mainly in low speed area and heavy impingement area, respectively. The characteristics of particle motion in foursquare tangential jets correlated with gas turbulence dissipation, particle size, particle concentration and particle density. Small particles were easily entrained by gas vortex, so that they consumed more turbulence energy and attenuated the gas turbulence intensity. On the contrary, large particles had more inertia and led to heavier impingement in the chamber center, resulting in particle random distribution and complex momentum transfer between gas and particles. Moreover, large particles stretched the coherent vortex to be narrow and long, while small particles pulled down the vortices rotation intensity.     

Particle Adhesion and Detachment in Turbulent Flows Including Capillary Forces

The effect of capillary forces on particle adhesion and removal mechanism in turbulent flows is studied. Different detachment theories are used and the increase of adhesion force by the capillary effect is included in the analysis. The criteria for incipient rolling and sliding detachments are evaluated. The sublayer and burst models, which account for the structure of turbulent near-wall flows, are used for evaluating the air velocity condition near the substrate. The critical shear velocities for removing particles of different sizes under different conditions are evaluated, and the results are compared with those obtained in the absence of the capillary force. Comparisons of the model predictions with the available experimental data are also presented.     

A Higher-Order Asymptotic Theory for Fully Developed Turbulent Flow in Smooth Pipes

A complete second-order asymptotic theory for fully developed turbulent flow in smooth pipes at high turbulent Reynolds numbers is presented in the paper. The theory is based on Prandtl's mixing-length hypothesis involving a fourth-order polynomial representation for the mixing length and taking into account its dependence on the Reynolds number. Two main contributions with respect to the existing literature have been achieved:(a) the friction law is obtained by asymptotic evaluation of an integral, completely independently of the velocity field, and(b) an axis layer (in addition to the wall layer and the outer layer) has to be included in the analysis in order to remove a nonuniformity appearing in the second-order solution for the velocity fieldClosed-form analytic expressions for all constants and wake functions appearing up to the second-order solution in both the friction law and the velocity field are obtained. The results are in a very good agreement with experiments.     

Turbulent effect of SF6 thermal plasmas inside a puffer-assisted self-blast chamber

In this study, we calculated the whole arcing history of SF₆ thermal plasmas happened inside a puffer-assisted self-blast chamber during the alternative current interruption process with two different turbulence models. It is very difficult to correctly realize all phenomena happened inside the interrupter using the computational schemes; therefore, we have been trying to simplify the physics related in the problem. Because most flows encountered in the process are turbulent, the ability to predict turbulence for this application is invaluable for the engineer to design the chamber reliably. The objective of this paper is to model turbulence by averaging the unsteadiness of the turbulence, which are called Reynolds averaged Navier-Stokes models, and to compare the results with pressure and temperature field distributions during the whole arcing history. It was found that the two-equation turbulent model predicts bigger mixing of momentum, heat and species on the arc discharge of this interrupter than the zero-equation model does.     

ISOTHERMAL PREDICTION OF PARTICLE AND GAS FLOW IN A COAL-FIRED REACTOR

The steady, turbulent gas flow with entrained coal particles in a laboratory-scale axisymmetric coal gasification reactor is numerically analyzed. The reactor is designed to provide rapid mixing and heatup of the coal in a configuration which results in a nearly isothermal and uniform flow in the main reaction chamber so as to allow controlled study of coal gasification. A detailed knowledge of the reactor dynamics is required in order to interpret experimental results. The nonreacting, isothermal flow pattern is first presented as a base case. Calculations are performed with an iterative, implicit scheme suitable to the elliptic nature of the gas flow equations in an Eulerian frame-work. The turbulent motion is resolved using the eddy-viscosity concept with the standard k-ε turbulence model. Coal particle trajectories are then calculated using the Lagrangian form of the momentum equations. The influence of solid particles on the gas phase is neglected. Particle trajectories and residence time distributions are presented for a variety of particle sizes and particle inlet locations. The influence of the inlet conditions, turbulent diffusion, and gravity on the particle motion, are investigated. Implications of the predictions, with respect to the design of the reactor, are discussed.     

Impingement cooling of gas turbine components

Impingement cooling heat transfer data were obtained for test geometries relevant to full coverage gas turbine combustion chamber wall cooling with the full flame tube pressure loss across the impingement plate. An impingement hole pitch to diameter ratio (X/D) of between 10 and 13 is appropriate for this application and two test geometries within this range were studied. The impingement gap to hole diameter ratio (Z/D) was the main parameter studied together with the hole Reynolds number. A significant influence of Z/D on the heat transfer was found and a general correlation equation derived. Evidence of enhanced impingement heat transfer for Z/D less than unity was found.     

Deposition of titanium/titanium oxide clusters produced by magnetron sputtering

Titanium clusters of nanometer sizes are produced by magnetron sputtering with subsequent aggregation in an argon gas flow. The produced Ti clusters are directed and deposited on a silicon substrate. Deposited films are analyzed by X-ray photoelectron spectroscopy in order to obtain the chemical composition and by atomic force microscopy and X-ray reflection methods to obtain information about the film structure. Experiments were carried out at different temperatures of the walls of the magnetron chamber. The size and the flux of clusters from the magnetron chamber are obtained by the analysis of the substrate surface with deposited clusters. It is found that the cluster parameters strongly depend on the temperature of the magnetron chamber walls. Molecules of titanium oxides may be nuclei of condensation and accelerate the nucleation process. A theoretical analysis based on experimental results is presented. It allows us to describe various stages of cluster evolution from their formation up to the deposition on the substrate and provides estimations for parameters of the processes involving clusters.     

Predicting the Particle Deposition Characteristics Using a Modified Eulerian Method on a Tilted Surface in the Turbulent Flow

This study uses a v2-f turbulence model with a two phase Eulerian approach. The v2-f model can accurately calculate the near wall fluctuationsm which mainly represent the nonisotropic nature of turbulent flow near the walls. The Eulerian method was modified based on considering the most important mechanisms in the particle deposition rate when compared to the experimental data. The model performance is examined by comparing the rate of particle deposition on a vertical surface with the experimental and numerical data in a turbulent channel flow available in the literature. The model takes into account the effects of lift, turbophoretic, electrostatic, gravitational, and Brownian forces together with turbulent diffusion on the particle deposition rate. Electrostatic forces due to mirror charging and due to charged particles under the influence of an electric field were considered. The influence of the tilt angle on the particle deposition rate was investigated. The results show that, using the modified model with v2-f model predicts the rate of deposition with reasonable accuracy. It is shown that considering the turbophoretic force as the only inertia force and neglecting the lift force, leads to reasonable accuracy in predicting particle deposition rate. It is also observed that when the mirror charging and electric field are present, the electrostatic force has the dominant effect in a wider range of particles’ size. Furthermore, the results show that increasing the Reynolds number at a given tilt angle decreases the rate of particle deposition and the tilt angle has insignificant impact on the particle deposition rate in high shear velocity or high Reynolds number.     

Fracture and fatigue analysis of an agitator shaft with a circumferential notch

This paper is concerned with the fracture analysis of an agitator shaft of a large vessel and predicting its high cycle fatigue life. The agitator shaft has a circumferential notch around it and is subjected to remote bending and torque created by the mixing operation. The problem is comprised (i) the analyses of the bending force and torque acting on the agitator by using the analytical method, (ii) calculation of stress intensity factors under mode I and III loading conditions by using finite element method and, (iii) fatigue analysis of the agitator shaft failed in service.An agitator model is set up and data obtained from the agitator are processed to make more realistic approximations for bending forces, since they form a base for stress analysis, in which mode I stress intensity factors are evaluated. Mode I stress intensity factors obtained by finite element analysis are compared with the results provided by using the body force method.     

Bubbly Jet Impingement in Different Liquids

The impingement of bubbly jets in distilled water and ethanol has been experimentally studied on ground. An experimental apparatus for the study of jet impingement on ground and in microgravity has been designed. The opposed-jet configuration with changeable orientation is used in order to study which is the better disposition to achieve an efficient mixing process. The impact angle between jets that can be changed from 0° (frontal collision) up to 90° (perpendicular collision). The impinging jets are introduced into a test tank full of liquid by means of two bubble injectors. The bubble generation method, insensitive to gravity level for low Bond numbers, is based on the creation of a slug flow inside a T-junction of capillary tubes of 0.7 mm of diameter. Bubble velocities at the injector outlet and generation frequencies can be controlled by changing gas and liquid flow rates. Individual bubble properties and coalescence events, as well as the whole jet structure are analyzed from the images recorded by a high speed camera. Bubble velocities are compared with the velocity field of a single-phase jet. Rate of coalescence between bubbles is found higher in ethanol than in water, creating a higher dispersion in bubble sizes.     

Design and Numerical Simulation of a Gas Mixer

A high precision gas mixer used to mix gases of small flow rapidly and uniformly was proposed in this paper. Nine simulation schemes were proposed based on orthogonal test. There were four factors including dilute gas flow rate, the length of mixing tube, the diameter of the mixing chamber and the width of the mixing chamber in orthogonal test and each factor had three levels. The numerical simulation was carried out to explore the relationship of the flow field and the four factors and to calculate the concentration of carbon monoxide at the mixer’s outlet. The primary factor that affected the mixing effectiveness was found out by means of range analysis. The heterogeneous degree of the distribution of carbon monoxide concentration at the mixer’s outlet was smaller than 0.002 under different conditions. The results reached uniform micromixing in engineering and met the requirement of measurement and detection.     

射流冲击凸台时传热与流动的数值模拟研究

针对冲击射流应用中经常会遇到不平整表面的情况(如各种电子元件),采用RNG的κ-ε模型,对处在半封闭板内凸台的冲击传热和流动进行了数值模拟.研究了冲击凸台表面、侧面及平板上的传热特性,分析了冲击高度H/D、流动Re数等参数对传热和流动的影响.结果表明,冲击高度较小时凸台上表面的传热Nu数沿径向的分布有一个先抑后扬的特征,在凸台边缘处达到最大.数值模拟还较好地给出了射流冲击凸台后流体分离、再次冲击平板等复杂的流动特征.     

Analysis and optimisation of particle mixing performance in fluid phase resonance mixers based on Euler/Lagrange calculations

A novel mixing principle utilising oscillating liquid columns was analysed numerically with regard to particle dispersion characteristics. For producing fluid oscillations a pipe (diameter 100 mm) was immersed centrally into a vessel (diameter 450 mm) filled with liquid (filling height 700 mm) and periodically pressurised (frequency 1.2 Hz). The outlet geometry of the central pipe, just ending near the vessel bottom, has a strong effect on mixing and was optimised in this study. The principle of a FPR-mixer does not require rotating stirrers and in the turbulent regime it has power numbers comparable to propellers. The numerical calculations were conducted by a Euler/Lagrange approach neglecting two-way coupling as well as inter-particle collisions for clarity in order to only focus on the effect of interfacial forces on particle dispersion. The continuous phase was calculated in an unsteady way based on the Reynolds-averaged equations combined with the k-ω-SST (shear stress transport) turbulence model. Lagrangian tracking was conducted considering all relevant forces; drag, gravity/buoyancy, fluid inertia, added mass, Basset force and transverse lift forces due to shear and particle rotation. The importance of these forces was analysed with respect to the turbulent particle Stokes number (considered range 0.004 < St < 10.0) and particle/liquid density ratio (i.e. 1.05, 1.5 and 2.5). Finally, the significance of Basset force and shear-rotation lift force (i.e. Magnus effect) on the dispersion process was quantified by mixing parameters.     

Turbulent shear flow of micropolar fluids

Fundamental equations of turbulent flow of micropolar fluids are obtained employing the regular averaging technique. Turbulent stresses and couple stresses are discussed. The transfer of energy between the basic flow and turbulence is studied and various terms are identified. Prandtl theory of mixing length is extended to relate the turbulent stresses and couple stresses to the mean flow field. The special case of turbulent shear flow is treated in detail.     

Numerical Simulation of Fluid Flow and Mixing in Gas-Stirred Ladles

In this work air injection into a water physical model of an industrial steel ladle was simulated. Calculations were developed based on a multiphase Eulerian fluid flow model involving principles of conservation of mass, momentum, and chemical species for both phases to predict turbulent flow patterns and mixing times for centric and eccentric injections. Effects of gas flow rate, injector position, number of injectors, and geometry of ladle on mixing time were analyzed. Optimum injection conditions are: single injector at 2/3 of radius and high gas flow rates. Quantitative correlation of mixing time as a function of main process variables was obtained.     

小型粘性物料灌装混拌机搅拌桨结构优化设计

目的针对2种粘稠液体物料混拌后定量灌装充填的机械设备,设计使混合效果更加快速和均匀的搅拌桨结构。方法应用有限元方法分析灌装到混拌器中的流体物料三维流场特性,研究不同搅拌桨对相应物料推送、搅拌和灌装的影响。比较各种工况下的连续速度场、压力场、混合效果。结果桨叶的搅拌运动使物料在较短时间内快速均匀混合。轴向错列向前倾斜的推进式桨叶对流体具有混合作用,同时增加输送能力,减低内部流体的压力,降低工艺和结构设计的复杂程度。实测结果验证了模拟计算的合理性。结论获得了搅拌桨优化设计结果预测的有效方法。该研究成果能为小型粘性物料灌装混拌机升级换代提供理论依据和参考。     

Multidroplet Impact Model for Prediction of Residual Stresses in Water Jet Peening of Materials

In this article, a multidroplet impact model, proposed for predicting residual stresses induced on materials subjected to water jet peening, is presented. This approach considers the impact pressure distribution due to high-velocity droplets impinging on the material surface instead of stationary pressure distribution for prediction of residual stresses on water jet-peened surfaces. It makes use of Reichardt's theory for predicting the velocity distribution of droplets and liquid impact theory for predicting the impact pressure and duration of impact of high-velocity droplets. For predicting residual stresses on the surface and subsurface of material subjected to water jet peening, finite element modeling approach was adopted by using transient elastoplastic finite element analysis by considering an impingement of a set of droplets in succession to one another over a certain time period after which this pressure is released. The effectiveness of the proposed approach was demonstrated bv comparing the predicted residual stresses with those predicted by using the single set of droplets approach proposed by Rajesh et al. [6]. Finally, the practical relevance of the proposed approach was shown by comparing the predicted results with the experimental results obtained by water peening of 6063-T6 aluminium alloy.     

THE CALCULATION OF PARTICLE DEPOSITION FROM A TURBULENT FLOW IN A PARALLEL VERTICAL PLATE

The formation of ash deposits may cause slagging and fouling problems in furnaces. The difficulties of predicting particle depositions are caused by the complexity of two-phase flow, which includes the particle size effects in a turbulent flow and the interparticle force between particles. Although some models were proposed to predict the particle deposition, few attempts were made for larger particles in the region of the dimensionless relaxation time (τ⁺) greater than 20. Thus, a reliable deposition model experimental results are needed for model verification, In this study, the modified turbulent intensity and apparent turbulent viscosity of the fluid were used to describe particles in suspension flow. And, an isothermal flow model was developed for calculating particle deposition rate in a parallel vertical plate, and for comparison with the experimental data. The predicted particle deposition rates under selected conditions are found to be in good qualitative agreement with available experimental results.     

Thermodynamics of concentrated solid solution alloys

This paper reviews the three main approaches for predicting the formation of concentrated solid solution alloys (CSSA) and for modeling their thermodynamic properties, in particular, utilizing the methodologies of empirical thermo-physical parameters, CALPHAD method, and first-principles calculations combined with hybrid Monte Carlo/Molecular Dynamics (MC/MD) simulations. In order to speed up CSSA development, a variety of empirical parameters based on Hume-Rothery rules have been developed. Herein, these parameters have been systematically and critically evaluated for their efficiency in predicting solid solution formation. The phase stability of representative CSSA systems is then illustrated from the perspectives of phase diagrams and nucleation driving force plots of the σ phase using CALPHAD method. The temperature-dependent total entropies of the FCC, BCC, HCP, and σ phases in equimolar compositions of various systems are presented next, followed by the thermodynamic properties of mixing of the BCC phase in Al-containing and Ti-containing refractory metal systems. First-principles calculations on model FCC, BCC and HCP CSSA reveal the presence of both positive and negative vibrational entropies of mixing, while the calculated electronic entropies of mixing are negligible. Temperature dependent configurational entropy is determined from the atomic structures obtained from MC/MD simulations. Current status and challenges in using these methodologies as they pertain to thermodynamic property analysis and CSSA design are discussed.