Transient Phenomena in d.c. Plasma‐Spray Torches

Abstract

ABSTRACT: D.c. plasma torches have been involved for many years in industrial applications such as welding, cutting or deposit elaboration by plasma spraying or plasma assisted vapor deposition. These techniques use specific properties of plasmas produced by d.c. arcs where high temperatures (up to 25000 K) and high flow velocities (up to few km/s) are available. A considerable amount of experimental work was devoted, in the past, to characterize the properties of arc jets as a function of arc current, torch design and configuration, plasma forming gas thermophysical properties and flowrate. In the same time models approached more and more accurately the behavior of arc jets, testifying of a real advance in both knowledge and understanding of the different implicated processes and mechanisms.Nevertheless, and mainly because of experimental difficulties, transient phenomena due to arc instabilities have been less intensively studied and their interest has only been underlined the last decade, although they were mentioned more than thirty years ago.Due to the extraordinary progress accomplished in the field of instrumentation, new diagnostic techniques have become available, accompanied by powerful signal processing methods which are now easy to apply thanks to the development of computers and associated software. It has been experimentally demonstrated in plasma torches that the arc is submitted to gas dynamic forces which give rise to two different modes of instability among which two major cases can be distinguished. The first one is the so‐called “take‐over” mode, characterized by rather low fluctuations of arc voltage compared to the mean value, the second one being the “restrike” mode, for which the fluctuations of arc voltage are about the same in amplitude than the mean value. The restrike mode is clearly identified as the result of the stretching of the arc by the cold gas flow, followed by electrical breakdown of the cold gas layer which gives rise to a new arc‐anode attachment. The stretching‐rearcing sequences are repeated at a frequency which is governed by experimental conditions and give a sawtooth shaped voltage signal, the analysis of which provides clear information about the arc dynamics. The fluctuations of arc length generate strong voltage variations which are correlated to the electrical power supplied to the gas and, as a consequence, to the variations of the arc jet temperature and velocity. The amplitude and frequency of voltage fluctuations can be related to the torch working and geometrical parameters and dimensional analysis allow to establish reliable semi‐empirical correlations. Temperature and velocity distributions are strongly correlated to the torch working conditions and the resulting fluctuations. It will be illustrated for different operating conditions of a d.c. spray torch.