Effects of geometry and position of the metal delivery tube on ultrasonic gas atomization

The effects of the geometry and the position of the metal delivery tube on the pressure condition in the gas-metal interaction zone were studied. The measurements were performed under conditions which simulated ultrasonic gas atomization experiments, but at low gas atomization pressures (50–200 psig, 345–1380 kPa). Low gas atomization pressures are used in spray atomization and deposition processes such as liquid dynamic compaction (LDC), and Osprey. Depending on the experimental conditions, either underpressure or overpressure in the metal delivery tube was detected. The magnitude of the underpressures and overpressures was found to increase with the gas atomization pressure; the maximum pressure differences with respect to the atomization tank pressure were about 3 psi (21 kPa) when argon was used as an atomization gas with a pressure of 200 psig (1380 kPa). Underpressure or overpressure effects of such magnitude have a large effect on the metal flow rate during gas atomization. The large pressure differences measured in the present work result from using an atomizer with gas jet diameters larger than those used in previous investigations. Using a large ultrasonic atomizer, argon gas flow rates of about 5 kg/min can be obtained for atomization pressures of 200 psig (1380 kPa).     

Experimental investigation on the effect of melt delivery tube position on liquid metal atomization

Metal powders are often made by gas atomization of liquid metal. During the process, liquid metal which flows from a melt delivery tube (MDT) is atomized by high speed gas discharging from a gas nozzle. In this work, the effect of the melt delivery tube position on atomization outcomes such as the yield, mass median diameter, and spread of the particle size distribution, is studied experimentally. A melt atomization setup (pilot-scale) is used to produce tin powder by gas-atomization. Three MDT positions, namely, intruded, extruded and flush with respect to the gas nozzle, are chosen for this study. Three pressure regimes (atmospheric, aspiration and pressurization) are established by varying the relative distance between the MDT and the gas nozzle exit for the three positions. Experimental investigations revealed that the intruded position produces powder with lower mean particle sizes and lower spread than the extruded configuration. The intruded position also gives a significantly higher yield compared to the extruded and flush positions at low gas flow rates, and hence appears to be the most suited for metal atomization using a free-fall configuration.     

A 1.6-mm,metal tube ultrasonic motor

A miniaturized metal tube ultrasonic motor, the dimensions of which are 1.6 mm in diameter and 6 mm in length, was developed. Two flattened surfaces with 90 degrees were ground on the outer surface of the stator. Two PZT-based piezoelectric ceramics were bonded onto these flat surfaces. The asymmetrical surface of the stator developed the split of the two degenerated orthogonal bending modes, resulting in a wobble motion. The working frequency of the 1.6-mm motor with 6 mm in length was 130 kHz. A torque of 0.5 mNm was reached at a maximum power of 45 mW with a speed of 45 rad/sec. The maximum efficiency was 16%.     

Pressure driven rarefied gas flow through a slit and an orifice

The flow of a monatomic gas through a slit and an orifice due to an arbitrarily large pressure difference is examined on the basis of the nonlinear Bhatnagar-Gross-Krook (BGK) model equation, subject to Maxwell diffuse boundary conditions. The governing kinetic equation is discretized by a second-order control volume scheme in the physical space and the discrete velocity method in the molecular velocity space. The nonlinear fully deterministic algorithm is optimized to reduce the computational effort by introducing memory usage optimization, grid refinement and parallelization in the molecular velocity space. Results for the flow rates and the macroscopic distributions of the flow field are presented in a wide range of the Knudsen number for several pressure ratios. The effect of the various geometric and physical parameters on the flow field is examined. Comparison with previously reported corresponding Direct Simulation Monte Carlo (DSMC) results indicates a very good agreement, which clearly demonstrates the accuracy of the kinetic algorithm and furthermore the reliability of the BGK model for simulating pressure driven flows.     

Impact of process flow conditions on particle morphology in metal powder production via gas atomization

Additive manufacturing processes as for instance selective laser melting or electron beam melting are becoming more common and just turning into standard manufacturing processes for metal components. Nevertheless, these processes are still new compared to classic powder metallurgy manufacturing routes such as pressing and sintering. Hence not all necessary requirements for the powders in use are fully known yet. This makes an increase in control of the powder properties a crucial task to achieve. To reach this goal one must understand the different influences on the powder production process from the beginning of the whole production route. In this work, the influence of the spray chamber flow on the particle morphology is examined. The nozzle system used to produce the metal powders is a close-coupled atomization system with a convergent-divergent gas nozzle configuration. The particle morphology as well as the particle size distribution have been analyzed to examine the influence of the atomization gas flow compared to an additional use of a coaxial gas flow. To review the changes of the flow patterns, computational fluid dynamic simulations have been performed. The particle trajectories were calculated to assess the change in particle behavior as well. Atomization experiments have been conducted with an AISI 52100 (1.3505) steel in a small batch atomization plant to evaluate the influence of the change in flow on the particle size distribution and circularity. The experimental results show that a use of additional coaxial gas leads to an increase in particle circularity up to 10% for relevant particle sizes. An approach for the quantification of satellite occurrence is given by examination of the shift of the particle size distribution to smaller diameters.     

超音速气体雾化高硅铝合金粉末冷却速度计算

通过快速凝固技术制备合金材料,可以大幅度细化合金组织,其技术的冷却速度.本文通过对流换热原理对超音速气体雾化高硅铝合金粉末的冷却速度进行了理论计算,结果表明其冷却速度大约在104～107K/s之间,说明利用超音速气体雾化制备高硅铝合金粉末可以达到很高的冷却速度;另外,通过测定合金粉末内部凝固枝晶间距,并利用冷却速度和枝晶间距之间的经验关系,确定合金冷却速度,其结果与理论计算基本相符.     

Experimental study on the design of orifice pulse tube refrigerator

An experimental study on the design of a single-stage orifice pulse tube refrigerator (OPTR) was carried out. It was shown experimentally that there exists an optimum operating frequency which increases with decreasing pulse tube volume. For a fixed pulse tube volume, increasing the pulse tube diameter will improve the performance. The experimental results are used to derive a correlation for the performance of OPTR which correlates the net cooling capacity with the operating conditions and the dimensions of the OPTR.     

Effects of process parameters on surface morphology of metal powders produced by free fall gas atomization

There are a number of process parameters which affect the characteristics of metal powders produced by free fall gas atomization. In the following work effects of various process parameters like apex angle of atomizer, focal length of atomizer, number of nozzles, diameter of nozzles, diameter of liquid metal delivery tube, superheat of liquid metal and type of metal etc. were studied on their surface morphology. It was observed that shape of powder particles depends on apex angle, superheat of liquid metal, type of metal and particle size range within a powder collective. Other parameters like focal length of atomizer, number of nozzles, diameter of nozzles and diameter of liquid metal delivery tube were found to have no effect on the shape of powder particles. However, Surface porosity and solidification shrinkage were observed on almost all types of metal powders.     

Gas flow measurement by the thin orifice and the classical Venturi tube

Many applications of vacuum technology require the generation and measurement of known gas flows. For this purpose, one may use ducts with constrictions. At a constriction, the flow-dependent pressure drop occurs. The gas throughput can be obtained from the inlet and outlet pressures at the constriction, provided its characteristics are known. In the present study, the characteristics of constrictions with basic geometry, i.e. the (infinitely) thin circular orifice and the standardised (DIN 1952) classical Venturi tube were investigated. Experimental methods for measuring the gas flow through a duct are described. The characteristics of individual constrictions were carefully measured and the data quantitatively compared to theoretical calculations. The results are discussed in order to provide a better understanding of the flow phenomena and to make them applicable to thin orifices and to Venturi tubes of any size and arbitrary gas. Over the whole flow range from molecular to viscous, the thin orifice can be used. However, it causes permanent pressure loss and it only has a small aperture in relation to the duct dimensions. In the viscous range, the Venturi tube can be used successfully. Thus, it is possible to establish stable secondary flow standards if the proper constriction is chosen.     

旋转流场作用下金属熔体初始雾化过程数值模拟

为探究初始雾化破碎情况,采用数值模拟的方法,结合龙卷风形成原理对金属熔体初始雾化进行理论分析,模拟不同流动形式以及气液比的初始雾化过程。结果表明:与无旋转流动形式相比,旋转流场涡旋作用更容易使熔体形成液膜,气体利用率高;在相同熔体流量下,存在临界气体质量流量0.003 8 kg/s,使液流由喷泉式破碎突变成经典的伞状液膜破碎形式。     

Preparation of spherical fine ZnO particles by the spray pyrolysis method using ultrasonic atomization techniques

An ultrasonic atomizer was used in the spray pyrolysis method to prepare fine, spherical and uniform ZnO particles. Almost spherical particles were obtained successfully which had a mean particle size of 0.15 m and had a very narrow particle size distribution. By using alcohol as the solvent, it was found that the particles do not have hollow shell layers which could usually be observed in the spray pyrolysis process by using water as the solvent. The morphology of the ZnO particles was strongly affected by the concentration of the starting solution.     

Modelling of the break up mechanism in gas atomization of liquid metals Part II. The gas flow model

An analytical model for the gas flow produced by a close coupled atomization assembly is described. The algorithm is based on physical arguments and it fully accounts for the expansion of compressible gas from cylindrical gas jets and the convergence/divergence of the subsequent turbulent flow. A critical input parameter of the model, the radius of the convergence area of the individual jet flows, established elsewhere [G. Antipas, PhD Thesis, University of Surrey, UK, 1995] to be 3 mm by high speed photography, is in tight agreement with the predictions of the model. Pitot tube gas velocity measurements compared well with model predictions.     

Output of an ultrasonic wave-driven nanogenerator in a confined tube

The output of an ultrasonic wave-driven nanogenerator (NG) has been found to depend on the excitation conditions and geometry. Incidence angle tests indicate that the effective area of an NG determines the amount of power that can be generated. The output power of an NG is also directly related to its distance from the ultrasonic source. A sinusoidal profile of the electrical output was observed when an NG was moved inside a long tube filled with water with the ultrasonic source located at one end. This is due to the oscillation of the wave intensity inside the tube as a function of the distance from the excitation source. This article is published with open access at Springerlink.com     

互相关算法在气体超声波流量计中的应用

针对当前大口径气体超声波流量计存在抗干扰能力差和测量精度低的问题,设计了一种利用互相关算法提高测量精度的气体超声波流量计.即以STM32F407作为核心控制器,采用时差法为测量方法,利用互相关波形匹配算法对信号进行定位处理.通过STM32主控芯片和TDC_GP22计时芯片测量的时间值与互相关算法得到的时间值进行对比参照,得到了精确的测量时间.进而搭建了试验样机并进行了试验测试和数据仿真.结果表明,该算法提高了流量计测量精度,增强了稳定性和抗干扰能力.     

An alternative observation on the orifice pulse tube cryocooler and analysis through the unified model of cryocoolers

A new observation of the orifice pulse tube (OPT) cryocooler is proposed and analyzed on the ground of the unified model of cryocoolers. The elementary temperature reduction mechanism is modeled and verified through the measurements of Mikulin and others. This analysis refers to the upper limit performance for zero phase shift between the pulses of pressure and flow rate at the cold end. The lowest attainable temperature is formulated and pointed out as central characteristics of an OPT cooler. It dominates the closed form expressions for cooler's cooling capacity, COP and FOM as function of the compression ratio, the size of regenerator, the species of coolant and average flow rate. The so obtained upper limit of COP and FOM is about half of that in literature. Performance is also compared with Solvay and Gifford-MaMahon cryocoolers.     

Pressure losses in a conduit with a venturi tube

We present the experimental relationships for the coefficient of kinetic flow energy and static pressure as a function of conduit length following a Venturi tube. A formula is proposed for the determination of the coefficient of the hydraulic resistance of the Venturi tube as a function of its structural parameters, as well as a function of the length of the straight segment of the conduit behind the Venturi tube.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 16, No. 4, pp. 731–736, April, 1969.     

Changes in the NPL orifice conductance on a transition from molecular gas flow to transitional flow

The conductance of an NPL orifice in the molecular regime is constant and can be exactly calculated from the geometric dimensions. For smaller Knudsen numbers the conductance increases and correction functions are employed to reduce the uncertainty in this range of pressures. The conductance is also constant in the viscous regime during flow into a vacuum and can also be calculated. A suitable function has been chosen with one free parameter, which is constant for both very low and sufficiently high pressures and the parameter was determined on the basis of the experimentally measured course of the conductance at the borderline between molecular and transitional flow. The function fits the experimental data very well and can be used to calculate the conductance of the orifice up to Knudsen number ≈1.     

Velocity-dependent characteristics of modulated ultrasonic signal in gas flow

Natural variations of pressure, density, and velocity occur in streaming fluid modulated ultrasonic signals. The characteristics of modulated ultrasonic signals in gas flow are strongly dependent on velocity. This paper describes a way to find a velocity-dependent characteristic quality detected in the modulated signal and to determine the flow rate on the basis of signal analysis of only one ultrasonic barrier.