Heat Transfer Mechanisms in Arcs of Various Gases at Atmospheric Pressure

Abstract

This work expands findings about the dominant heat transfer mechanisms in argon and helium arcs at atmospheric pressure for monoatomic (Ar, He, 50% Ar–50% He), diatomic (air, {hbox{N}}_{2}, {hbox{O}}_{2}, {hbox{F}}_{2}, {hbox{Cl}}_{2}), and triatomic ({hbox{CO}}_{2}) gases. The objective is to understand the dominant mechanisms in atmospheric plasmas through validated numerical modeling for GTAW welding process. Arcs of all gases have lengths of 10 mm and 200 A current. Five heat transfer mechanisms are considered: Joule heating, convection, radiation, conduction, and Thomson effect. Results indicate that the general structure of the arcs and dominant mechanisms are qualitatively similar for all gases; sizes change depending on the gas. The dominant energy input near the cathode is Joule heating, while that near the anode is convection. The dominant energy output always follows the same sequence: Thomson effect next to the cathode followed by convection, radiation in the arc column, and conduction near the anode. Joule heating is the most significant in Ar, while in He, it has the lowest significance. This is due to differences in electric conductivity of He (higher up to 21,000 K and lower beyond 21,000 K than other gases) and high viscosity of He, which creates a small Joule heating versus a large convection region. He transfers the most heat towards the anode by convection while {hbox{N}}_{2} has the lowest; due to the high enthalpy and viscosity of He compared to {hbox{N}}_{2}. Finally, Ar has the most significant radiative emission and He the smallest due to their net emission coefficient.