Electrical Characteristics of the Oxyfuel Preheat Flame: 3D Computational Model Subject to Electric Bias Voltages

Abstract

A three-dimensional (3D) computational model is presented in this paper that illustrates the detailed electrical characteristics, and the current-voltage (i-v) relationship throughout the preheating process of premixed methane-oxygen oxyfuel cutting flame subject to electric bias voltages. As such, the equations describing combustion, electrochemical transport for charged species, and potential are solved through a commercially available finite-volume Computational Fluid Dynamics (CFD) code. The reactions of the methane-oxygen (CH4 – O2) flame were combined with a reduced mechanism, and additional ionization reactions that generate three chemi-ions, H3O+, HCO+, and e−, to describe the chemistry of ions in flames. The electrical characteristics such as ion migrations and ion distributions are investigated for a range of electric potential, V ∈ [−5V, +5V]. Since the physical flame is comprised of twelve Bunsen-like conical flame, inclusion of the third dimension imparts the resolution of fluid mechanics and the interaction among the individual cones. It was concluded that charged ‘sheaths’ are formed at both torch and workpiece surfaces, subsequently forming three distinct regimes in the i-v relationship. The i-v characteristics obtained out of the current study have been compared to the previous experimental and two-dimensional (2D) computational model for premixed flame. In this way, the overall model generates a better understanding of the physical behavior of the oxyfuel cutting flames, along with a more validated i-v characteristics. Such understanding might provide critical information towards achieving an autonomous oxyfuel cutting process.

A model for a Langmuir sheath in a stagnating dense plasma with secondary ion formation

“…The electrical mobility of the positive ion has been approximated to be μ i e ≈ 10 −3 m 2 /Vs [17], and based solely on their dissimilar masses the electron mobility is at least 200 times that. Velocities in the outer cone of the oxyfuel flame have been approximated to be on the order of 50m/s [7]. Under these conditions, the practical maximum for the ionic Reynolds number is about 2.…”

“…Though the original model includes some simple treatment of the sheath at the metal surface, it ignores secondary ion generation and the non-uniform velocity distribution above the work surface. A preliminary numerical investigation also predicts enhanced current signals with surface reactions [7], but even with computational tools, it is difficult to thoroughly study the effect without a theoretical basis to reduce the parameter space into dimensionless groups. The present study is entirely devoted to the sheath at the metal surface with the goals of (1) establishing dimensionless groups, and (2) establishing corresponding predictions for the phenomena observed in the sheath.…”