Effects of Nozzle Fluid Dynamics on the Dynamic Characteristics of a Plasma Spray Torch

Abstract

This paper is devoted to the investigation of the dynamics of the plasma jet, which is produced by a subsonic DC spray torch operated with an argon-helium mixture as the plasma gas. It focuses on studying the effect of some parameters which influence the arc attachment inside the anode nozzle. For this purpose, the paper uses different means of gas injection, that is straight flow injection and vortex flow injection, and anodes which have experienced different degree of wear. A heavily eroded anode is characterized by large voltage fluctuations at relatively low frequencies, while straight gas injection at high current levels led to a low average voltage with small fluctuations and to low burner performance. The results are interpreted by assuming changes in the thickness of the cold interface between the arc and the anode, and conclusions are drawn as to the voltage characteristics indicative of good torch operation. Paper includes a German-language abstract.

The anode region of electric arcs: a survey

“…A correlation has been found between the attachment residence time and the arc current and the mass flow rate using a nondimensional analysis. Duan et al [56,57] used the analysis of the voltage traces to characterize the different modes of the anode attachment: the 'stable' mode indicated by a steady voltage, the 'take-over' mode characterized by random voltage fluctuations and the 'restrike' mode with the sawtooth shaped voltage trace. By using the values for the amplitude of the voltage fluctuation, and for the slopes of the voltage traces during voltage increases and decreases, an algorithm was formulated describing the anode attachment stability for any arc operating conditions, including 'mixed' modes, i.e.…”

Influence of Helmholtz oscillations on arc voltage fluctuations in a dc plasma spraying torch

“…The frequency evolution presented in figure 9 must be interpreted in terms of thermal properties, in the framework of the above theory. Introducing the Rayleigh correction, C R , and the factor, K H , which takes into account the geometrical characteristics of the nozzle, ( 5) and (6) give…”

“…If t i is the time at which a breakdown occurs, the ith voltage jump, δ Vi , is defined so that δ Vi = U(t i − ε) − U(t i + ε), where U(t i − ε) is the voltage sampled 'just before' restrike and ε is of a few µs. (14), C R : correction factor (6). When the arc column is well stabilized and occupies the central part of the channel, the cold layer shows a rather steady and uniform thickness and the arc length oscillates regularly between the two situations shown in figures 1(a) and (b).…”

“…Arc jets produced by dc plasma spraying torches present non-stationary features which may be responsible for nonuniform heating and transport of the injected material used for coating elaboration [1,2]. Due to their specific origin, the instabilities are characterized by very different time scales, ranging from a few µs for the anodic arc root movement inside the nozzle [2][3][4][5][6][7][8] up to tens of hours for the slow drift of the torch performance which appears as a consequence of the nozzle 1 This paper is dedicated to the memory of D Rigot. erosion [7,9,10]. For the short time scale, three different modes have already been identified [6,11,12] which are, by increasing the amplitude of voltage fluctuations, the steady mode, the takeover mode and the restrike mode.…”

“…This kind of instability is found for plasma spray torches working most of the time with argon-hydrogen mixtures or nitrogen. However, this mode of instability can be encountered whatever the plasma gas is [6,13], for example, with pure argon or argonhelium mixtures, provided the current is sufficiently low, the nozzle diameter sufficiently large or the flow rate high enough.…”