High bandwidth unsteady temperature measurements of gas flows inside turbomachinery are required as part of a general engineering objective of increasing the efficiency of gas turbines. When combined with unsteady pressure data, temperature measurements allow the unsteady losses occurring in turbomachinery to be derived and used to validate CFD codes of the unsteady flow.
We describe the development of fiber optic sensors to measure heat flux and unsteady temperature in wind tunnel experiments for turbomachinery applications. The sensors are intrinsic Fabry-Perot interferometers fabricated from single-mode optical fiber. The optical path length within the interferometer fiber is sensitive to temperature. We present results from three sensors embedded as calorimeter gauges in a ceramic nozzle guide vane end wall model exposed to a transient heat flux in wind tunnel experiments and validated by comparison with previous data from platinum thin film resistance gauges. The optical sensors exhibit high spatial resolution (approximately 5 micrometers ), high heat transfer resolution (approximately 1 kWm<SUP>-2</SUP>), and wide temperature measurement bandwidth (100 kHz) with intrinsic calibration. No electrical connections to the measurement volume are required and multiplexing is possible. Very short length (< 60 micrometers ) fiber sensors have been constructed and demonstrated as fast response thermometers suitable for measuring gas total temperature fluctuations in unsteady flow fields. We show results from a vortex shedding experiment from a heated bluff body in continuous flow generating temperature oscillations at 3 kHz.
The design, construction, operation, and preliminary evaluation of miniature fiber Fabry-Perot (FFP) interferometers used as heat transfer gauges is described. These gauges are being developed for a particular application where heat transfer data is currently obtained using conventional platinum thin-film resistance thermometers. The specifications that the sensors must exceed are: (1) temperature resolution of 25 mK over a 50 K range; (2) temporal response of 10 microsecond(s) ; (3) an ability to operate as a calorimetric heat-transfer gauge. The sensor consists of a short length of single-mode optical fiber (approximately equals 3 mm) to which low- reflectivity coatings have been applied at each end. It is illuminated and interrogated by an arbitrary length of addressing fiber. A laser diode is used as the source and the authors have exploited the facility to frequency modulate the diode in a novel signal processing scheme. To determine the performance of the sensor, short duration heat pulses derived from a pulsed Nd:YAG laser were applied to one end of the FFP. The response time was found to be 8 microsecond(s) and the sensor operation as a calorimeter was verified.