Abstract
Recuperation increases the efficiency of a gas turbine engine by extracting heat from the exhaust gas stream and using it to pre-heat the compressor discharge air. High temperature oxidation and creep are major concerns, necessitating the use of heat-resistant alloys for the recuperator panels. Most current recuperator designs specify austenitic stainless steel foil as the material of construction. Water vapor, present in the exhaust gas as a by-product of combustion, has been shown to be detrimental to the elevated temperature oxidation resistance of common stainless steels. Increasing the amount of chromium and nickel in heat-resistant alloys appears to alleviate the risk of breakaway oxidation in air containing water vapor. Alloy 625 (UNS N06625), a common wrought nickel-base superalloy, is a candidate material for recuperators. Long-term oxidation testing using 0.10 mm (0.004 inches) thick samples showed no indication of breakaway oxidation in both ambient and humidified air at 704-815°C (1300-1500°F). Significantly lower overall weight gains were observed for the samples exposed in humidified air. The experimentally observed oxidation kinetics can be explained by a model combining the simultaneous growth and evaporation of an oxide layer. The oxidation test data and observed microstructural features agree generally well with this model.
