电极感应熔化气雾化制粉技术中非限制式喷嘴雾化过程模拟

电极感应熔化气雾化(electrode induction melting gas atomization, EIGA)是一种制备超洁净无夹杂物的先进制粉技术,本文以粉末高温合金的氩气雾化过程为研究示例,对现有用于实际生产的国内某厂家提供的EIGA用非限制式喷嘴进行建模,采用商用计算流体力学软件FLUENT,分布采用欧拉-欧拉VOF(volume of fluid)多相流方法与欧拉-拉格朗日DPM (discrete phase model)离散相方法,对非限制式环缝喷嘴主雾化与二次雾化过程进行了数值模拟.通过对主雾化过程中多相流大涡模拟速度流场,主雾化过程中不同阶段高温熔体云图模拟以及二次雾化过程中TAB (Taylor analogy breakup)模型速度流场及TAB模型粒度分布的模拟研究,实现了对EIGA制粉技术中非限制式喷嘴雾化过程的全过程模拟,并预测了雾化后的粉末粒度分布.在此基础上,采用本文模拟使用的非限制式环缝喷嘴,设定与模拟条件一致(进气压力4 MPa,液流直径约4 mm)的实验条件,制备的粉末大部分颗粒的直径大小在100μm左右,该实验结果与模拟得到的粉末直径D50=100μm大小一致,进一步验证了模拟数据的合理性.该方法也适用于非限制式喷嘴里,其他金属或合金的雾化过的模拟研究.     

Close-coupled nozzle atomization integral simulation and powder preparation using vacuum induction gas atomization technology

We simulate the gas-atomization process of a close-coupled annular nozzle for vacuum induction gas atomization at a three-dimensional scale.Moreover,the relationship between the simulated droplet type and experimentally metallic powder is established by comparing the morphology of droplets with powders.Herein,the primary atomization process is described by the volume-of-fluid(VOF)approach,whereas the prediction of powder diameter after secondary atomization is realized by the VOF-Lagrangian method.In addition,to completely reflect the breaking and deformation process of the metallic flow,we employ the VOF model to simulate the secondary atomization process of a single ellipsoidal droplet.The results show that the primary atomization process includes the formation of surface liquid film,appearance of serrated ligaments,and shredding of ligaments.Further,gas recirculation zone plays an important role in formation of the umbrella-shaped liquid film.The secondary atomization process is divided into droplet convergence and dispersion stages,and the predicted powder diameter is basically consistent with the experiment.In general,the four main powder shapes are formed by the interaction of five different typical droplets.     

离心式喷嘴内气液两相流动的数值模拟

求解三维不可压NS方程,并应用VOF方法捕获气液分界面,计算不同压降下离心式喷嘴内的气液两相流动状况,研究了不同压降对喷嘴内流动的影响。计算很好地模拟了喷嘴内的气液两相流动,并得到了出口液膜速度、喷雾锥角以及液膜厚度等参数,数值计算所得结果同试验符合得较好,好于经验公式的结果。     

单晶生长炉全局热分析(1)——三维模型建立

晶体生长炉内高温热辐射和熔液对流十分复杂,使分析整个生长炉内传输现象的全局热分析模型一直停留在轴对称准静态假定上.本文考虑熔液对流的三维性和非定常性,构筑了三维全局热分析模型,讨论了熔液对流三维性和非定常性的影响.结果表明:本文模型预测的晶体成长界面反转临界雷诺数大大降低,更接近实际.     

Influence of the Matrix on the Atomic Absorption of a Transversely Heated Graphite Atomizer

We have investigated the influence of the sodium chloride and potassium sulfate excess in a sample on the absorption signals of Ag, Al, Au, As, Bi, Cd, Cu, Ga, Ge, In, Mn, Pb, Sb, Se, Sn, and Tl in a transversely heated graphite atomizer (THGA) of the SIMAA 6000 multielement spectrometer with a Zeeman background corrector. The maximum amounts of salts causing no change in the integral atomic absorption and no corrector errors in using the universal Pd-Mg-modifier and varying the internal argon flow have been determined. They range for different elements from a few thousand of micrograms to tens. Comparison, as to this index, with the longitudinally heated atomizer HGA has been made. In many cases, the THGA is much better than the HGA.     

Numerical investigation of hydrodynamics behaviour of melt layer during laser cutting of steel

Understanding of the melt layer hydrodynamic behaviour during laser-cutting process under gas jet assistance is of high importance for cut quality control. In the present work, a numerical model is developed to calculate the three-dimensional behaviour of the melt flow on the kerf front, while an inert gas jet interacts with the melt film. Fluent CFD code is used to solve the governing hydrodynamic equations by finite volume method. The results show that the melt flow on the kerf front reveals a strong instability, which depends on the cutting speed and on the gas jet velocity. Global flow behaviour (gas and molten metal flows) computed using a laminar model, reveals oscillations of the gas–metal liquid interface, which is assimilated to Kelvin–Helmholtz instability. The origin of this instability is discussed in terms of instabilities in thermal dynamics and hydrodynamics. Instability in thermal dynamics is related to the localized melting, while the instability in hydrodynamics is governed by forces balance between gas and resistant surface tension.     

Numerical simulation on dynamic behaviors of bubbles flowing through bifurcate T-junction in microfluidic device

Based on the volume of fluid(VOF) method, a numerical model of bubbles splitting in a microfluidic device with T-junction is developed and solved numerically. Various flow patterns are distinguished and the effects of bubble length,capillary number, and diameter ratio between the mother channel and branch are discussed. The break-up mechanism is explored in particular. The results indicate that the behaviors of the bubbles can be classified into two categories: break-up and non-break. Under the condition of slug flowing, the branches are obstructed by the bubbles that the pressure difference drives the bubbles into break-up state, while the bubbles that retain non-break state flow into an arbitrary branch under bubbling flow condition. The break-up of the short bubbles only occurs when the viscous force from the continuous phase overcomes the interfacial tension. The behavior of the bubbles transits from non-break to break-up with the increase of capillary number. In addition, the increasing of the diameter ratio is beneficial to the symmetrical break-up of the bubbles.     

A level set simulation of dendritic solidification with combined features of front-tracking and fixed-domain methods

A method combining features of front-tracking methods and fixed-domain methods is presented to model dendritic solidification of pure materials. To explicitly track the interface growth and shape of the solidifying crystals, a front-tracking approach based on the level set method is implemented. To easily model the heat and momentum transport, a fixed-domain method is implemented assuming a diffused freezing front where the liquid fraction is defined in terms of the level set function. The fixed-domain approach, by avoiding the explicit application of essential boundary conditions on the freezing front, leads to an energy conserving methodology that is not sensitive to the mesh size. To compute the freezing front morphology, an extended Stefan condition is considered. Applications to several classical Stefan problems and two- and three-dimensional crystal growth of pure materials in an undercooled melt including the effects of melt flow are considered. The computed results agree very well with available analytical solutions as well as with results obtained using front-tracking techniques and the phase-field method.     

Ion coupling efficiency for an extraction applied-B ion diode onthe HELIA linear-induction adder in positive polarity

Initial experiments to investigate coupling of the four-stage HELIA linear-induction accelerator to a uniformly insulated applied-B ion diode in planar extraction geometry are reported. Results describing the efficient operation of an applied-B extraction ion diode coupled to the HELIA linear induction accelerator operated in positive polarity are reported. Operation of a close-coupled, undermatched, applied-B diode on HELIA was consistent with magnetically insulated transmission line (MITL) electron flow intermediate between locally emitted flow and generalized flow, rather than with full-gap flow. Peak ion coupling efficiencies of 60-70% and peak ion power levels of 0.3-0.4 TW have been achieved     

Impact mechanism of gas temperature in metal powder production via gas atomization

This paper aims at studying the influence mechanism of gas temperatures(300 K, 400 K, 500 K, and 600 K) on gas atomization by simulating the integral atomization process of the close-coupled nozzle in vacuum induction gas atomization(VIGA). The primary atomization is simulated by the volume of fluid(VOF) approach, and the second atomization is studied by the discrete phase model(DPM) combined with the instability breakage model. The results show that, at an increased gas temperature, the influences of gas–liquid contact angle and gas temperature in the recirculation zone on the primary atomization are virtually negligible. However, increasing the gas temperature will increase the gas–liquid relative velocity near the recirculation zone and decrease the melt film thickness, which are the main reasons for the reduced mass median diameter(MMD, d50) of primary atomized droplets. During the secondary atomization, increasing the gas temperature from 300 K to 600 K results in an increase in the droplet dispersion angle, which is beneficial to the formation of spherical metal powder. In addition, increasing the gas temperature, the positive effect of gas–liquid relative velocity increase on droplets refinement overweighs the negative influence of the GMR decrease, resulting in the reduced MMD and diameter distribution interval. From the analysis of the atomization mechanism, the increase in atomization efficiency caused by increasing the temperature of the atomizing gas, including primary atomization and secondary atomization, is mainly due to the increase in the gas drag force difference between the inner and outer sides of the annular liquid film.     

3D Analysis of Crystal/Melt Interface Shape and Marangoni Flow Instability in Solidifying Liquid Bridges

Solidification of gallium (Pr=0.02) in liquid bridges in zero-gravity conditions is investigated by numerical solutions of the three-dimensional and time-dependent flow-field equations. A single region (continuum) formulation based on the enthalpy method is adopted to model the phase-change problem. This paper analyzes the influence of the azimuthally asymmetric and steady first bifurcation of the Marangoni flow on the shape of the solid/melt interface during the crystal growth process. The numerical results show that this interface is distorted in the azimuthal direction. The distortion is related to the sinusoidal three-dimensional temperature disturbances due to the instability of the Marangoni flow. The three-dimensional flow field organization, related to the wave number, changes during the solidification process; this behavior is explained according to the variation of the aspect ratio of the solidifying liquid bridge. A correlation law is found for the azimuthal wave number of the instability as function of the melt zone aspect ratio.     

Electrothermal atomization of a substance with fractional condensation of the element being determined on a probe

This paper describes a method of electrothermal atomization with a fractional condensation of the elements being determined on a refractory probe with the aim of decreasing the matrix influences on the atomic-absorption signal. In the course of primary atomization of the sample, the probe is placed over the dosing port of a tubular atomizer. The internal argon flow directs the vapor to the probe for the condensation of the elements being determined. The matrix vapors volatilize. Then the probe is inserted into the atomizer for evaporation of the elements and analytical signal recording. It has been shown that this technique makes it possible to decrease the influence of sodium chloride and potassium sulfate on the absorption of Ag, Cd, Pb, and Au by a factor of 50–20,000 as compared to the atomization from the atomizer wall. In the case of Au, this decrease is comparable to the level attained under the conventional conditions of a stabilized temperature furnace with a platform, a modifier, and a background corrector based on the Zeeman effect, while for the other elements its efficiency is 1.5–40 times higher.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 124–128, January–February, 2005.     

Electrical and transient atomization characteristics of a pulsed charge injection atomizer using electrically insulating liquids

Charge injection atomizers are energy efficient devices that can be used in order to promote the atomization of dielectric liquids, and a potential application of such devices is fine spray delivery in small internal combustion engines. The operation of a pulsed charge injection atomization system operating at practical engine frequencies under a high voltage pulse train has not been well recorded in the literature. This initial investigation defines the electrical and transient global atomization performance of a charge injection atomizer operating under a steady flow regime, but with a typical high voltage pulse train. Results show that voltage-current characteristics follow similar trends to that of a steady flow, steady voltage system, and observation of the data also reveals that output current waveforms depend on the input pulse train frequency. No degradation in charging efficiency was observed at higher frequencies, which suggests that a charge injection atomizer can operate efficiently at practical engine speeds. Photographs also confirmed the high voltage pulse train injects charge that produces sections of primary atomization on the continuous liquid jet.     

Flow-induced oscillations of a cantilevered pipe conveying fluid with base excitation

It is known that a plain cantilevered pipe conveying fluid loses its stability by a Hopf bifurcation, leading to either planar or non-planar flutter for flow velocities beyond the critical flow velocity for Hopf bifurcation. If an external mass is attached to the end of the pipe (an end-mass), the resulting dynamics become much richer, showing 2D and 3D quasiperiodic and chaotic oscillations at high flow velocities. In this paper, a cantilevered pipe, with and without an end-mass, subjected to a small-amplitude periodic base excitation is considered. A set of three-dimensional nonlinear equations is used to analyze the pipe?s response at various flow velocities and with different amplitudes and frequencies of base excitation. The nonlinear equations are discretized using the Galerkin technique and the resulting set of equations is solved using Houbolt?s finite difference method. It is shown that for a plain pipe (with no end-mass), non-planar post-instability oscillations can be reduced to planar periodic oscillations for a range of base excitation frequencies and amplitudes. For a pipe with an end-mass, similarly to a plain pipe, three-dimensional period oscillations can be reduced to planar ones. At flow velocities beyond the critical flow velocity for torus instability, the three-dimensional quasiperiodic oscillations can be reduced to two-dimensional quasiperiodic or periodic oscillations, depending on the frequency of base excitation. In all these cases, a low-amplitude base excitation results in reducing the three-dimensional oscillations of the pipe to purely two-dimensional oscillations, over a range of excitation frequencies. These numerical results are in agreement with the previous experimental work.     

Process modeling gas atomization of close-coupled ring-hole nozzle for 316L stainless steel powder production

The paper aims at modeling and simulating the atomization process of the close-coupled ring-hole nozzle in vacuum induction gas atomization(VIGA) for metallic powder production. First of all, the primary atomization of the ring-hole nozzle is simulated by the volume of fluid(VOF) coupled large eddy simulation(LES) model. To simulate the secondary atomization process, we use the method of selecting the droplet sub-model and the VOF model. The results show that the ring-hole nozzle forms a gas recirculation zone at the bottom of the delivery tube, which is the main reason for the formation of an annular liquid film during the primary atomization. In addition, the primary atomization process of the ring-hole nozzle consists of three stages: the formation of the serrated liquid film tip, the appearance and shedding of the ligaments, and the fragmentation of ligaments. At the same time, the primary atomization mainly forms spherical droplets and long droplets, but only the long droplets can be reserved and proceed to the secondary atomization. Moreover,increasing the number of ring holes from 18 to 30, the mass median diameter(MMD, d50) of the primary atomized droplets decreases first and then increases, which is mainly due to the change of the thickness of the melt film. Moreover, the secondary atomization of the ring-hole nozzles is mainly in bag breakup mode and multimode breakup model, and bag breakup will result in the formation of hollow powder, which can be avoided by increasing the gas velocity.     

The effect of two-dimensional shear flow on the stability of a crystal interface in a supercooled melt

A model is developed based on the time-related thermal diffusion equations to investigate the effects of twodimensional shear flow on the stability of a crystal interface in the supercooled melt of a pure substance.Similar to the three-dimensional shear flow as described in our previous paper,the two-dimensional shear flow can also be found to reduce the growth rate of perturbation amplitude.However,compared with the case of the Laplace equation for a steady-state thermal diffusion field,due to the existence of time partial derivatives of the temperature fields in the diffusion equation the absolute value of the gradients of the temperature fields increases,therefore destabilizing the interface.The circular interface is more unstable than in the case of Laplace equation without time partial derivatives.The critical stability radius of the crystal interface increases with shearing rate increasing.The stability effect of shear flow decreases remarkably with the increase of melt undercooling.     

The effect of two-dimensional shear flow on the stability of a crystal interface in the supercooled melt

A model is developed based on the time-related thermal diffusion equations to investigate the effects of two-dimensional shear flow on the stability of a crystal interface in the supercooled melt of pure substance. Similar to the three-dimensional shear flow as described in our previous paper, the two-dimensional shear flow can also be found to reduce the growth rate of perturbation amplitude. However, compared with the case of Laplace equation for steady state thermal diffusion field, due to the existence of time partial derivatives of the temperature fields in diffusion equation the absolute value of the gradients of the temperature fields increases, therefore destabilizing the interface. The circular interface is more unstable than in the case of Laplace equation without time partial derivatives. The critical stability radius of the crystal interface increases with shearing rate increasing. The stability effect of shear flow decreases remarkably with the increase of melt undercooling.     

Investigations into ultrasound induced atomization

The present work deals with measurements of the droplet size distribution in an ultrasonic atomizer using photographic analysis with an objective of understanding the effect of different equipment parameters such as the operating frequency, power dissipation and the operating parameters such as the flow rate and liquid properties on the droplet size distribution. Mechanistic details about the atomization phenomena have also been established using photographic analysis based on the capture of the growth of the instability and sudden ejection of droplets with high velocity. Velocity of these droplets has been measured by capturing the motion of droplets as streaks. It has been observed that the droplet size decreases with an increase in the frequency of atomizer. Droplet size distribution was found to change from the narrow to wider range with an increase in the intensity of ultrasound. The drop size was found to decrease with an increase in the fluid viscosity. The current work has clearly highlighted the approach for the selection of operating parameters for achieving a desired droplet size distribution using ultrasonic atomization and has also established the controlling mechanisms for the formation of droplet. An empirical correlation for the prediction of the droplet size has been developed based on the liquid and equipment operating properties.     

Direct Determination of Mercury at 0.2 ppm Level by Flame AA Using a Thermospray Atomizer and a Circular Burner

A circular flame atomizer system using a quartz T flame adapator has been developed for the direct determination of mercury. A thermospray nebulizer was used as a sample introduction device to the atomizer. This design gave a sensitivity (1% absorption) of 0.20 ppm and detection limits of 10 ppb at 1.0 mL/min solvent flow at 253.7 nm. The absolute sensitivity was 25 ng and absolute detection limits was 1.5 ng.     

一种新型喷嘴的提出及流量特性的研究

本文在对各种气动喷嘴及其雾化机理分析基础上提出了一种新型的气动雾化喷嘴-"旋转型气-液雾化喷嘴"。在此喷嘴中,油与气分别从不同的槽道切向进入混合室,且油与气一一对应,油与气互相混合、旋转后从喷口喷出。其气液比在热态实验时为4%-6%(用压缩空气雾化),雾化状态良好。本文中对其流量系数及雾化角进行了系统的研究。主要考虑了喷嘴的结构参数,气液比(ALR),液体粘度等因素对流量系数的影响。通过实验测量与拟合,最后得到了喷嘴的流量系数和雾化角的表达式,可以用来指导喷嘴的设计。