Oxidation Resistant Heating Elements for High Temperature Wind Tunnels

ABSTRACT: High temperature wind tunnels are needed to replicate the environment vehicles will experience during high Mach number flight. Of particular interest are sustained airflow temperatures of 2800 F. Although this level of performance has been obtained by two of NASA's wind tunnel facilities (i.e. High Temperature Tunnel (HTT) at Langley Research Center and Hypersonic Tunnel Facility (HTF) at Glenn Research Center), the period of performance at such a high level is short, and the quality of the air flow suffers significantly from undesirable combustion media and/or foreign object debris (FOD). In addition, the current heating configurations require excessive downtime for repair and maintenance. Therefore, an improved heating element/configuration is needed that possess excellent high temperature oxidation resistance and will resist spallation that results in FOD generation. For thermal shock resistance, a high thermal conductivity, which is typically associated with metallic materials, is required. Of the high temperature metallic materials, only iridium has both the high melting temperature (~4500 F) and the oxidation resistance to be considered. Recent work has shown the advantages of iridium based materials for use in high temperature, oxidizing environments such as iridium lined radiatively cooled rocket engines using highly oxidizing fuels and iridium electrodes for the processing of molten oxides. During this effort, an oxidation resistant, high temperature wind tunnel heating element will be demonstrated based on iridium. However for extended service life in high temperature wind tunnels, improvements in the oxidation resistance of an iridium will be needed. Therefore, the use of rhodium alloy additions and the formation of a stable hafnium oxide layer for additional oxidation protection will be evaluated. BENEFIT: This proposed technology is applicable to leading edges of hypersonic vehicles, rockets nozzles, combustion liners, jet vanes, furnaces, igniters, spark plugs, and other applications that may experience extremely high thermal gradients.