Fiber-optic-coupled differential-pressure transducers are being developed for use in hot, harsh environments like those in the cores of aircraft turbine engines. The prior approach to the measurement of pressures in such engines involves the use of pressure transducers coupled to pressure taps via long tubes, which introduce delays and degrade responses to high-frequency pressure variations. In contrast, the fully developed versions of the developmental transducers would be mounted directly in the affected compressor, combustor, and turbine stages; because there would be no long pressure-coupling tubes, these pressure transducers would yield more-accurate readings in more nearly real time. The readings could help to identify conditions that must be corrected in maintenance work and could, potentially, enhance safety in flight by providing indications of impending compressor stall.
The developmental fiber-optic-coupled differential pressure transducers are denoted twin-column transducers (TCTs) for a reason that will immediately become apparent: A transducer of this type includes two tubes (T1 and T2), nominally of identical length and width. The tubes are sealed at their tips and are connected at their bases to sources of the differential pressure to be measured (see figure). Upon pressurization, each tube becomes slightly elongated — by an amount proportional to the difference between its interior pressure and the ambient pressure. Similarly, if a tube is partly or wholly evacuated, it is proportionately shortened. Thus, the differential elongation of the two tubes is proportional to the differential pressure between their interiors. The twin-column design automatically compensates for temperature; that is, any change in ambient temperature affects both tubes nearly equally and thus does not appreciably alter the differential elongation.
The tips of the tubes are capped with assemblies for input and output coupling of light via optical fibers, which are arranged for measuring the differential elongation of the tubes. Input light from a light-emitting diode (LED), provided via a source optical fiber, is aimed from the assembly on T1, across the gap to two receiver optical fibers in the assembly on T2. The positions of the fibers are adjusted so that (1) when the differential elongation of both tubes is zero (corresponding to zero differential pressure), equal amounts of light are coupled into both receiver fibers; (2) any nonzero differential elongation of the tubes gives rise to a proportional imbalance between the amounts of light coupled into the receiver fibers; and (3) the sum of amounts of light coupled into both receiver fibers remains constant as the differential elongation changes. As a result, the ratio between the difference and the sum of the receiver-fiber outputs is proportional to the differential pressure.
The LED and the photodetectors and electronic circuitry used to process the output light from the receiver fibers are located safely away from the hostile environment of the TCT. An additional optical fiber can be installed on the T1 side to provide feedback for regulating the intensity of light emitted by the LED; such feedback regulation renders the final differential-pressure reading relatively insensitive to fluctuations of temperature and voltage in the electronic circuitry.
The key components that are expected to make high-temperature TCTs possible are low-loss optical fibers capable of transmitting light between hot engine locations and cooler remote locations where electronic circuitry can be operated in safety. Sapphire optical fibers several meters long, with losses of less than 0.2 dB/km, have been fabricated experimentally. A practical design might provide for short lengths of high-quality sapphire fibers from the point of measurement, coupled to less-expensive commercial sapphire fibers of lower optical quality, and finally to low-loss, low-temperature optical fibers to cost-effectively bring signals out through zones of progressively decreasing ambient temperature.
This work was done by Charles A. Liucci of LEL Corp. for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 —8
21000 Brookpark Road
Refer to LEW-16889.