Analysis of the Stability of DC Plasma Gun Operating with Ar‐He‐H₂ Gas Mixtures

Abstract

The plasma‐forming gas composition is one of the parameters of a plasma spray process which can be adjusted by the operator to obtain deposits with the required properties. The gas composition affects the heat and momentum transfer to the particles injected in the flow, but also the working and stability of the plasma gun. This paper presents an investigation of the effect of gas composition and flow rate, and arc current on arc instabilities. The study was carried out for various ternary gas mixtures of argon, helium and hydrogen. The movement of the arc root attachment spot on the anode wall was characterized from the time‐evolution of arc voltage and the fluctuations of the plasma jet from the velocity fluctuations. The experiments were carried out with a new and a worn anode. The characteristic frequency of arc fluctuations was correlated in terms of the primary parameters of the spray process and gas mixture properties.

Three-Dimension and Transient D.C. Plasma Flow Modeling

“…This zone, which spreads from the cathode tip to the anode wall, is divided into two volumes V 1 and V 2 . V 1 near the cathode tip depends on the radius r(x) and has a length l 1 ; V 2, inside the nozzle, can be described as a truncated cornet at one side: it has a variable length l 2 (t), depending on the motion of the electrical arc root on the anode wall (higher the arc voltage, higher the length), and its exit side presents an a-inclined plan; a remains constant at 30°by default for restrike mode because of the high plasma column constriction due to hydrogen presence [2]. The 30°value arbitrary choice comes from the plasma column constriction which is assumed major and constant and depending only on the quiteregulated input gas composition.…”

“…Recently, Mariaux et al [9] have presented comparisons of plasma flow calculations based on similar assumptions in a time-transient configuration. The present work deals with next developments in a three-dimension case with time-dependent fluctuations reproducing the restrike mode plasma flow [2]. It is devoted to three main purposes:…”

“…The reasons of these motions come from struggles between hydrodynamic and electromagnetic forces upon the gas column flowing in the cold boundary layer, both forces resulting in upstream or downstream displacement of the electrical attachment point at the anode wall. Three fluctuation modes [1] have been firstly identified: stagnation one (same position in length for the arc attachment), take-over (regular periodic variation), restrike mode (saw tooth shape) before Janisson et al [2] have highlighted an intermediate mode, mixing the characteristics of the two last ones. These modes depend strongly on the thickness of the cold gas boundary layer developed at the anode wall [3], which is linked to the operating parameters (current intensity, nature and flow rate of plasma gas...) [2].…”

“…Three fluctuation modes [1] have been firstly identified: stagnation one (same position in length for the arc attachment), take-over (regular periodic variation), restrike mode (saw tooth shape) before Janisson et al [2] have highlighted an intermediate mode, mixing the characteristics of the two last ones. These modes depend strongly on the thickness of the cold gas boundary layer developed at the anode wall [3], which is linked to the operating parameters (current intensity, nature and flow rate of plasma gas...) [2]. The velocity and temperature field measurements are generally time-average values.…”

“…Constant for k-e model r e PRANDTL turbulent number for e l t Turbulent viscosity (kg/m s) P m Average input power (W) I Arc current intensity (A) g Thermal efficiency rate (%) V 2 Second heating zone volume (m 3 ) P 2 Electrical power generated in V 2 volume (W) l 2 (t)…”