Purpose and Scope
The higher temperatures of the latest engine designs require the use of thermal barrier coatings (TBC) to allow the cooling air being pumped through the blades to be minimized and thereby improve the efficiency of the engine. The use of TBCs, however, virtually prevents the use of pyrometry to measure blade and vane temperatures, since the TBCs are approximately four times less emissive than metal surfaces, and about four times more reflective of the fireball radiation being emitted immediately upstream of the turbine in the combustor.
Thermographic phosphors have been used in a number of other high-temperature applications and hold significant potential for solving our component temperature measurement problems in the engine. Again, no measurement standards currently exist for the use of thermographic phosphors. Through the use of thermographic phosphors, temperatures can be determined from pulsed-illumination-induced fluorescence of thin phosphor coatings applied to turbine engine surfaces of interest. In a similar fashion, suitable composites of thermographic phosphors integrated with thermal barrier coatings can fluoresce and yield temperature and other indications of TBC health.
The basic components consist of an excitation source (usually a laser or an LED), an optical probe that provides for illumination of the coating and collection of the fluorescence, the coating and the method for applying it, a data acquisition system, and a data analysis protocol.
Standards for comparing the various approaches and defining appropriate benchmarks are needed. Standardizing the definitions of coating durability, fluorescence efficiency, temperature range, temperature sensitivity, and methods for establishing these characteristics, as well as establishing a set of standards or best practices for the other components of a system, are needed. Establishing standards for thermographic phosphors will be a major benefit to both aircraft and power generation engine manufacturers and to sensor vendors alike.
The ISA107.2 subcommittee is currently working on a draft under the direction of Chairman Steve Allison. For more information, please contact ISA standards staff at ISA Standards.