Temperature monitoring of nozzle guide vanes using ultrasonic guided waves

Abstract

This thesis explores the use of ultrasonic guided waves for the online temperature monitoring of nozzle guide vanes (NGVs). These components are found within the turbine section of jet engines, operated at up to 1800°C. A literature review covering the currently used methods of temperature monitoring for NGVs has identified the potential of a new online sensor. Ultrasonic guided waves can propagate through thin structures such as NGVs, where different modes of propagation exhibit differing properties in respect to their sensitivity to temperature, amplitude, and dispersiveness. The complex geometry of NGVs represents a challenge to the implementation of such a system. Cooling hole arrays and multi-layered thermal barrier coatings are likely to have a considerable impact on wave propagation. In this study the effect of temperature, cooling hole structures, and thermal barrier coatings on wave propagation has been investigated through dispersion curve prediction, experimental measurement, and finite element simulation. An experimental test system has been developed to target modes of interest, and analyse the effect of temperature on wave propagation. The temperature sensitivities of individual modes have been measured successfully. Results are in good agreement with predicted values extracted from dispersion curves, despite the range of errors identified. A finite element model mimicking the experimental setup has been developed and validated against experimental results. The temperature range was extended up to 1000°C for an Inconel 718 plate, which is a Nickel-based super alloy typically used for jet engine components. Results continue to align with predictions at 1000°C. The effect of cooling hole structures on wave propagation are investigated through experimentation and COMSOL simulation. In general, dense hole arrays at the leading edge of a vane limit pulse-echo operation, however pitch-catch operation is still viable with careful mode and wavelength selection. In less dense areas away from the leading edge, temperature hotspot detection is possible in both configurations, although pulse-echo operation is likely to be more applicable. Sensors may need to be operated from both sides of the vane to effectively cover the whole array. Even under the favourable conditions of these models (with limited additional reflections, environmental noise, etc.) identifying changes in temperature at multiple locations is challenging, and the extent to which reflections from cooling holes can be used for this application is highly dependant on the specific geometry of the vane. The effect of thermal barrier coatings on wave propagation has been investigated through the generation of dispersion curves and the use of COMSOL simulations. The range of materials, make up of the layered structure, and the types of application methods typically used have been considered, and the effect of temperature on the system has been evaluated. Dispersion curves generated for the multi-layered composite show how the higher order modes increase in complexity in comparison with the response in a single material. Through-thickness displacement varies across the thickness as material properties vary, with the top coat often exhibiting considerably larger displacement than then other layers. Although the work carried out on a single material (Aluminium or Inconel 718) looked to have promising advantages to working at higher order modes, the application of TBCs complicates signal propagation, causing a greater number of modes to propagate with similar wave velocities, which limits the ability to target a single mode. Operating at a lower frequency where there only the two fundamental modes are present is likely to be the most effective method of targeting/identifying single modes.

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Identifiers

ORCID for Lawrence Yule: orcid.org/0000-0002-0324-6642

ORCID for Martyn Hill: orcid.org/0000-0001-6448-9448

ORCID for Nicholas Harris: orcid.org/0000-0003-4122-2219

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