Fiber-optic sensors for long-wavelength pyrometry and thermometry in gas turbines

Abstract

Radiation pyrometry and thermometry combined with fiber optics provide sensors for determining surface and gas temperatures in otherwise inaccessible locations. However several applications require that temperature measurements be made in the long-wavelength infrared (LWIR) where widely used silica fibers are nontrasmissive. This study involved the development and testing of hollow sapphire fiber-optic radiometric temperature sensors that operate in the LWIR. Hollow sapphire waveguides are suitable for use at high temperatures and transmit in the LWIR: no currently available solid-core fiber offers this combination of features. A prototype fiber-optic radiometric temperature sensor was assembled for the LWIR spectral region, and several experiments were performed to demonstrate the feasibility of developing a system that could be used in gas turbine applications. Open-tip and closed-tip hollow sapphire waveguides were used as the sensor probes and were coupled to an FT-IR spectrometer using hollow glass waveguides. Both types of sensors were tested in laboratory facilities and the closed-tip configuration was also used in a jet engine test rig. The principal findings can be summarized as follows: (1) Open-tip hollow sapphire waveguides were demonstrated to be useful sensors for remote pyrometric temperature measurements in the LWIR. Analysis of calibration data predicted an error level of less that 1% of the reading at 800°C. (2) Closed-tip hollow sapphire waveguides were demonstrated to be useful sensors for radiometric gas temperature measurements in the LWIR. (3) It was determined that interference ence caused by hot H 2 O and CO 2 emission in the open-tip configuration could be eliminated through careful selection of the analyzed spectral region. (4) It was found that interference caused by heating of the waveguide walls in either configuration could be eliminated by limiting the throughput of the optical system.