Effects of anode nozzle geometry on thermal plasma characteristics generated by non-transferred torches for material processing

Abstract

This paper outlines the theory of hypervelocity plasma spraying with railgun system and describes the laboratory system designed and built at the University of Texas at Austin, which successfully validated the method of hypervelocity plasma sprayer and conducted proof of principle experiments for hypervelocity plasma-powder deposition, using and railgun system.The bore of the railgun is filled with an ionizable gas, and a radio frequency-excited cavity provides a line source of plasma. Fast, high-energy electrical pulses from the magnetic flux compression device expand the plasma line into a planar arc driven by Lorentz forces. The arc is an efficient ''snowplow'' sweeping the gas to velocities of 4 to 5 W s , which is twice the desired powder velocity producing coatings of superior quality in an almost continuous manner, compared to the existing plasma sprayers as shown by state-of-the art Hadland CCD framing cameras. The paper describes how the Rayleigh-Taylor instabilities (Kruskal-Schwarzschild __ for magnetohydrodynamic configurations) are circumvented and the role and the interplay of the Hartmann flow (two-dimensional).For a technology to be viable, the process must be driven on a continuous basis. Then the power source must be also an energy storage device capable of a large number of shots, repeated at a relatively high frequency. The process requires an average power of 250 kW, but in successive pulses of 100 ms duration at a peak power level of 80,000 kW. In order to operate without interruption, novel concepts of repetitive pulsed electrical machines were developed. One refers to an actively compensated compulsatorembedding a capacitive pulse forming network.The second is a pair of series-excited dc generators operating in tandem.The thermal plasma characteristics produced by the nontransferred torches with various anode nozzle geometries, such as a tubular nozzle and a stepped nozzle, operating at an atmospheric pressure are calculated and measured by a numerical simulation and an optical emission spectroscopy, respectively. On the basis of the assumption of local thermodynamic equilibrium (LTE) and optically thin plasmas, the temperature distributions of argon thermal plasmas are determined by the Abel inversion and Boltzmann plot methods for the measured intensity of Ar I lines.On the other hand, their velocity distributions are deduced from the measured temperatures by means of power balance equations. For the numerical simulation, two-dimensional magnetohydrodynamic (MHD) equations are employed with a Kepsilon turbulence model. As a numerical scheme, the finite volume discretization and SIMPLE-like pressure correction algorithm are adopted in an unstructured triangular grid system for reflecting the complicated nozzle geometry. The thermal plasma propenies produced with different nozzle types and sizes, such as tubular nozzles of various diameters and stepped nozzles of different step positions and diameters, are compared between the experimental and numerical results. Furthermore, from the obtai...

Numerical Simulation of Helium Arc at High Pressure and Low Current

“…Thereafter the time step is set to 1 μs up to a physical time of 50 ms. This time step is much lower than the typical value of 5μs used for plasma modelling [14,15]. A lower time step was used to ensure convergence of the simulation.…”