INFINA (EX INFIP)   05545
INSTITUTO DE FISICA INTERDISCIPLINARIA Y APLICADA
Unidad Ejecutora - UE
capítulos de libros
Título:
On the Double-Arcing Phenomenon in a Cutting Arc Torch
Autor/es:
PREVOSTO, LEANDRO; KELLY, HÉCTOR; MANCINELLI ,BEATRIZ
Libro:
Numerical Simulations / Book 2
Editorial:
INTECH Open Access Publisher
Referencias:
Año: 2010; p. 1 - 23
Resumen:
The aim of this chapter is presenting a comprehensive study of the double-arcing phenomenon which is one of the main drawbacks that put limits to increasing capabilities of the plasma arc cutting process. Surprisingly, relatively little has been done to explore and understand this process, keeping in mind that the double-arcing is common not only to arc cutting but to other industrial processes as well. Section 2 presents some hypothesis suggested in the literature on the physical mechanism that triggers the double-arcing in cutting torches. In Section 3, an experimental study carried out by the authors is presented. The starting point for such a study is the analysis and interpretation of the nozzle current-voltage characteristic curve. Since there is no comprehensive theory for interpreting the electron branch in highly collisional plasmas, only the ion branch is analyzed. The influence of the collisions on the physical structure of the plasma-nozzle transition has been considered under the typical assumption of constant ion mean free path. Within this assumption and using an approximate analytical solution for the sheath thickness, the value of the ion flux to the wall is related to the nozzle voltage and the electron temperature and density at the plasma boundary. To describe the plasma composition an appropriate non-equilibrium two-temperature statistical model was used. A physical interpretation on the origin of the double-arcing phenomenon is presented, that explains why the double-arcing (that it is established when the space-charge sheath adjacent to the nozzle wall breaks-down) appears for example at low values of the gas mass flow.  A complementary numerical study of the space-charge sheath formed between the plasma and the nozzle wall of a cutting torch is also reported in Section 4. The numerical study corresponds to a collision-dominated model (ion mobility-limited motion) for the hydrodynamic description of the sheath adjacent to the nozzle wall inside of a cutting torch. The model does not assume cold ions so drift-diffusion type equations are used. Also an improved expression for the ion-neutral momentum transfer is employed, instead of the classical ion collision approximations (constant ion mean free path, and constant ion collision frequency). The ion and electron densities, electrostatic potential and ion velocity distributions are calculated inside the sheath. Boundary conditions for the numerical solutions within this sheath are based on experimental plasma data previously obtained by the authors. A physical explanation on the origin of the transient double-arcing (the so called non-destructive double-arcing) in cutting torches is reported in Section 5. Against to the proposed hypothesis (Colombo et al., 2009; Nemchinsky 2009) which assumes a transient arc voltage rise due to dielectric films deposited on the nozzle surface (which are later either carried away by the gas flow or are burned out); the experimental observations suggest that such a phenomenon is related with a strong dynamics of the space-charge sheath contiguous to the nozzle due to the arc power source ripple. Conclusions are summarized in Section 6.