Paper 33: Heat Transfer Effects Arising from arc Rotation in Plasma Arc Heaters of the Self-Induced Magnetic Field Type

Abstract

This paper is concerned with an analysis of the thermal behaviour of high input power (∼ 15 megawatt) arc heaters operating at values of total (bulk) enthalpy up to 2500 Btu/lb. Such heaters are currently in use to supply air for aerodynamic testing purposes, including subsonic turbulent pipe flow, the operation of propulsion tunnels with ‘full’ temperature simulation, and supersonic combustion ducts. The electrode system is essentially a ‘rail accelerator’ adapted for continuous operation by making it re-entrant, the self-induced magnetic field bringing about rapid movement of the arc roots to prevent destruction of the electrodes. The rate of rotation of the arc column (which is subject to aerodynamic drag) is shown to be a function of apparent electrode gap, current, and the stagnation temperature and pressure in the arc chamber. A distinction can also be drawn between ‘long’ and ‘short’ arcs, depending on whether the inter-electrode gap is large or small. The value of the arc rotation rate ω (which ranges between approximately 50 and 1000 per sec) has a considerable bearing on the thermal efficiency of short-gap arcs, on account of the dependence on it of the convective losses. Since the chamber gases radiate much more strongly than air uncontaminated by electrode material, convective and radiative losses are distinguished by solving for first-power and fourth-power dependence on temperature difference with the walls; the convective losses are examined in conjunction with a vortex model of the gas behaviour.