Pyrometry for turbine blade development

Introduction A IRFOIL temperature distributions under actual engine pressure levels, gas stream turbulence, combustor radial profile, and external boundary-layer heat transfer coefficient are a critical aspect of any turbine development program. The optical pyrometer has proven to be a valuable tool for obtaining experimental substantiation without the expense of prolonged engine durability testing. The conventional measurement technique using embedded thermocouples requires grooving the thin airfoil sidewall, creating undesirable stress concentrations. Furthermore, durability at high temperature is limited, and signal transmission using a slipring or telemetry is expensive and complex. Other techniques such as thermal paints have been used successfully but do not offer the versatility of pyrometers. Pyrometers in jet engines are subject to reflection from the combustor and emission from burning particles in the field of view. Engine data indicate good agreement between thermocouples and reflection-corrected data using the Dual Spectral Area technique. The error due to burning particles in the field of view is corrected under engine steady-state conditions by discarding the affected data and replacing it in subsequent revolutions. Thermal barrier coatings (composites of ceramic and metallic underlayers) are translucent and have low emittance in the wavelength region currently being used and become opaque in the 8-12 IJL range. These characteristics require a new measurement approach to obtain the surface temperature of coated turbine blades.