Generation of Highly Dissociated Oxygen Flows Using a Constrictor-Type Arc Heater

Abstract

I N developing thermal protection systems (TPS) for reentry vehicles, a constrictor-type arc heater is widely used to simulate such high enthalpyflows because it has a simple and rugged structure and long operational time; moreover, it requires little maintenance after several hours’ operation [1,2]. Recently, atomic oxygen has been found to play important roles through heat-flux enhancement by catalytic effects and active–passive oxidation of TPS surfaces [3,4]. Figure 1a shows that, in constrictor-type arc heaters, oxygen is usually injected at the constrictor part to prevent the cathode’s oxidation, whereas an inert gas such as argon or nitrogen is supplied from the cathode base. In our previous studies, number density distributions of atomic oxygen in argon–oxygen flows generated by constrictor-type arc heaters were evaluated using laser absorption spectroscopy (LAS) and CFD analysis [5,6]. Results showed that oxygen was insufficiently mixed with argon and then only slightly dissociated in the constrictor region.Although the oxygenwasmixed gradually in the plume, the dissociation rate was quite low because of the low temperature after nozzle expansion, engendering the low degree of dissociation in oxygen. Figure 1b shows that for the enhancement of oxygen dissociation, a hollow cathode arc heater was developed to supply oxygen through the cathode tip into the high temperature cathode-jet region [7]. Consequently, the degree of dissociation in oxygen increased. However, severe cathode erosion up to 1:5 10 3 g=s limited the operation time to less than 20 min. Next, we tested a premixed gas injection of oxygen and argon from the cathode base. As a cathode material, zirconium was used instead of conventional thoriated tungsten to reduce cathode erosion. Zirconium reacts with oxygen and forms an oxide ceramic (zirconia) layer on the cathode surface. This zirconia has a highmelting point of 3000 K and lower vapor pressure than oxide tungsten [8,9]. In this study, the erosion rate of the zirconium cathode was measured. In addition to the degree of dissociation in oxygen, the specific enthalpy was evaluated using LAS.