Yuanjie Shi

Numerical analysis of the arc in pulsed current gas tungsten arc welding using a boundary-fitted coordinate

A two-dimensional transient model is introduced to describe the heat transfer and fluid flow in pulsed current gas tungsten welding arc (GTA). In the model, a boundary fitted coordinate system is adopted to describe precisely the cathode shape. The current-continuity equation has been solved with the combined arc plasma-cathode system, independent of the assumption of the current density distribution on the cathode surface, which was essential in previous studies of the arc plasma. The temperature distribution in pulsed GTA welding has been described and the transition processes of temperature contours and arc pressures at the anode center have been studied. Moreover, the effects of pulsed welding parameters on the dynamic processes of the arc pressure at the anode center have been studied using the developed model, the results being compared with experimental data measured by the micro-pressure sensor.

Mathematical model of arc in pulsed current gas tungsten arc welding

A two-dimensional transient model is introduced to describe the heat transfer and fluid flow in pulsed gas tungsten arc (GTA) welding. The current continuity equation has been solved with the combined arc plasma - cathode system, independent of the assumption of the current density distribution on the cathode surface which was essential in previous studies of the arc plasma. The temperature distribution in pulsed GTA welding has been described, and the transition processes of temperature contours and current densities at the anode centre have been studied. Moreover, the effects of pulsed welding parameters on the dynamic process of current density at the anode centre are studied using the developed model. The results have been compared with experimental data measured by the probe method.

Study of wetting and spontaneous motion of droplets on microstructured surfaces with the lattice Boltzmann method

The influence of arrangement of micro-pillars on wetting and self-motion of droplets is numerically studied via a three-dimensional lattice Boltzmann model for multiphase flow. Substrates textured with identical pillar array density but different arrangements are considered. The results show that the prediction of Cassie-Baxter model can be improved with the modified roughness factors. In addition, transportation of droplets on the stepwise gradient structured substrates is investigated. A simple model, taking account of pillar arrangement, is developed to estimate droplet velocity. The results show that it is critical to restrain droplet spreading in the lateral direction to achieve higher velocity and longer transportation distance.

Simulation of three-component fluid flows using the multiphase lattice Boltzmann flux solver

In this work, we extend the multiphase lattice Boltzmann flux solver, which was proposed in 1 for simulating incompressible flows of binary fluids based on two-component Cahn-Hilliard model, to three-component fluid flows. In the present method, the multiphase lattice Boltzmann flux solver is applied to solve for the flow field and the three-component Cahn-Hilliard model is used to predict the evolution of the interfaces. The proposed method is first validated through the classical problem of simulation of partial spreading of a liquid lens between the other two components. Numerical results of interface shapes and contact angles agree well with theoretical solutions. After that, to further demonstrate the capability of the present method, several numerical examples of three-component fluid flows are presented, including a bubble rising across a fluid-fluid interface, single droplet falling through a fluid-fluid interface, the collision-coalescence of two droplets, and the non-contact collision of two droplets. It is shown that the present method can successfully handle complex interactions among three components. We extend the multiphase lattice Boltzmann flux solver to three-component fluid flows.The three-component Cahn-Hilliard model is used to capture fluid-fluid interface.The present model is validated with simulation of partial spreading of a liquid lens.Simulations including bubble rising and droplet collision-coalescence are presented.

The equation of state and ionization equilibrium of dense aluminum plasma with conductivity verification

The equation of state, ionization equilibrium, and conductivity are the most important parameters for investigation of dense plasma. The equation of state is calculated with the non-ideal effects taken into consideration. The electron chemical potential and pressure, which are commonly used thermodynamic quantities, are calculated by the non-ideal free energy and compared with results of a semi-empirical equation of state based on Thomas-Fermi-Kirzhnits model. The lowering of ionization potential, which is a crucial factor in the calculation of non-ideal Saha equation, is settled according to the non-ideal free energy. The full coupled non-ideal Saha equation is applied to describe the ionization equilibrium of dense plasma. The conductivity calculated by the Lee-More-Desjarlais model combined with non-ideal Saha equation is compared with experimental data. It provides a possible approach to verify the accuracy of the equation of state and ionization equilibrium.

Experimental investigation on the energy deposition and morphology of the electrical explosion of copper wire in vacuum

This paper presents the experimental results of the electrical explosion of copper wires in vacuum using negative nanosecond-pulsed current with magnitude of 1–2 kA. The 20 μm-diameter copper wires with different lengths are exploded with three different current rates. A laser probe is applied to construct the shadowgraphy and interferometry diagnostics to investigate the distribution and morphology of the exploding product. The interference phase shift is reconstructed from the interferogram, by which the atomic density distribution is calculated. Experimental results show that there exist two voltage breakdown modes depending on the amount of the specific energy deposition. For the strong-shunting mode, shunting breakdown occurs, leading to the short-circuit-like current waveform. For the weak-shunting mode with less specific energy deposition, the plasma generated during the voltage breakdown is not enough to form a conductive plasma channel, resulting in overdamped declining current waveform. The infl...

Experimental investigation on the energy deposition and expansion rate under the electrical explosion of aluminum wire in vacuum

Experimental investigations on the electrical explosion of aluminumwire using negative polarity current in vacuum are presented. Current pulses with rise rates of 40 A/ns, 80 A/ns, and 120 A/ns are generated for investigating the influence of current rise rate on energy deposition. Experimental results show a significant increase of energy deposition into the wire before the voltage breakdown with the increase of current rise rate. The influence of wire dimension on energy deposition is investigated as well. Decreasing the wire length allows more energy to be deposited into the wire. The energy deposition of a 0.5 cm-long wireexplosion is ∼2.5 times higher than the energy deposition of a 2 cm-long wireexplosion. The dependence of the energy deposition on wire diameter demonstrates a maximum energy deposition of 2.7 eV/atom with a diameter of ∼18 μm. Substantial increase in energy deposition is observed in the electrical explosion of aluminumwire with polyimide coating. A laser probe is applied to construct the shadowgraphy, schlieren, and interferometry diagnostics. The morphology and expansion trajectory of exploding products are analyzed based on the shadowgram. The interference phase shift is reconstructed from the interferogram. Parallel dual wires are exploded to estimate the expansion velocity of the plasma shell.

The calculation of electron chemical potential and ion charge state and their influence on plasma conductivity in electrical explosion of metal wire

The electron chemical potential and ion charge state (average ion charge and ion distribution) are important parameters in calculating plasma conductivity in electrical explosion of metal wire. In this paper, the calculating method of electron chemical potential and ion charge state is discussed at first. For the calculation of electron chemical potential, the ideal free electron gas model and Thomas-Fermi model are compared and analyzed in terms of the coupling constant of plasma. The Thomas-Fermi ionization model, which is used to calculate ion charge state, is compared with the method based on Saha equation. Furthermore, the influence of electron degenerated energy levels and ion excited states in Saha equation on the ion charge state is also analyzed. Then the influence of different calculating methods of electron chemical potential and ion charge state on plasma conductivity is discussed by applying them in the Lee-More conductivity model.

Factors affecting the exploding characteristics of tungsten wires with negative-polarity current

This paper presents the experimental results of electrical explosion of tungsten (W) wires in a vacuum using a negative-polarity current pulse with a magnitude of 1–2 kA. Uniform wire explosions with deposited energy sufficient for full vaporization are achieved for 0.5 cm-long W wires with the help of dielectric coating and soldering. Experimental results show that the dielectric coating can give 2–3 times more energy deposition by delaying the voltage breakdown. In order to improve the electrical contact, the metallic oxide on the surface of the electrodes is removed with sandpaper, and then the W wire is soldered to the electrodes. It is found that the improved electrical contact does not have an obvious influence on increasing the energy deposition. However, from the optical images, it can be seen that the uniformity of the wire explosion is greatly improved, especially for the W wire with 2 μm polyimide coating. Therefore, the dielectric coating and good electrical contact are the two key factors to ...

Characteristics of the electrical explosion of fine metallic wires in vacuum

The experimental investigations on the electrical explosion of aluminum, silver, tungsten and platinum wires are carried out. The dependence of the parameters related to the specific energy deposition on the primary material properties is investigated. The polyimide coatings are applied to enhance the energy deposition for the exploding wires with percent of vaporized energy less than unit. The characteristics of the exploding wires of different materials with and without insulating coatings are studied. The effect of wire length on the percent of vaporization energy for exploding coated wires is presented. A laser probe is employed to construct the shadowgraphy, schlieren and interferometry diagnostics. The optical diagnostics demonstrate the morphology of the exploding products and structure of the energy deposition. The influence of insulating coatings on different wire materials is analyzed. The expansion trajectories of the exploding wires without and with insulating coatings are estimated from the s...