OPTICAL STRUCTURES FOR HIGH SPEED GAS TEMPERATURE MEASUREMENT

This paper reports on recent work to produce a sensor based on optical waveguides. The geometry of the structure will be described and its response to changes in gas temperature will be modelled. The validity of the modelling will be examined by comparing with actual - results taken with preliminary flowing gas/heat gun experiments, and by using the technique of heating the sensor with a laser and measuring the decay time, and examining its causes. The response of a bulk sapphire sensor of the sort currently suited to installation in a gas turbine will be similarly examined. This will show the advantage of the waveguide sensor in terms of speed and the pros and cons of each type will be evaluated for a gas turbine, using published data for gas flows and convection coefficients in a gas turbine representative location. The requirements for high speed of operation (i.e. rapid response to changes in surrounding gas temperature) and low conduction to the sensor package/mounting will be examined and the arrival at a waveguide based structure will be justified. A diaphragm structure is proposed, modelled and fabricated. It is found that there are tradeoffs in the design for speed of response against conduction crosstalk, and limitations on the gas flows that will yield accurate results. These will be examined theoretically and experimentally. Due to the use of fibre optic interconnects between the sensor and the remote interrogator, EMI (electromagnetic interference) issues are well controlled. The packaging scheme used to package such a sensor including its optical connection will be described, and its performance in harsh environments such as an engine exhaust will be demonstrated. Finally, the performance of such a sensor in a reciprocating engine manifold will be presented, together with an interpretation of the results and conclusions therefrom.