Robust Tip Gap Measurements: A Universal In-Situ Dynamic Calibration & Demonstration In A Two-Stage High-Speed Turbine

Abstract Tip clearance monitoring is essential for the active health monitoring of turbomachinery and their development towards more efficient systems. Proper sensor calibration is paramount to this purpose, frequently being a time-consuming process. This paper introduces a novel in-situ dynamic calibration routine for high-frequency capacitance sensor measurements for tip clearance. The method predicts the calibration curve based on a single clearance measurement, the evolution of the acquired signal through various operational conditions, and the dimensional features of the multi-rim squealer-tip passing blades. The experimental data was obtained at 2MHz in a state-of-the-art two-stage high-speed turbine operated by the Purdue Experimental Turbine aerothermal Lab. A description of the empirical setup is provided, emphasizing the capacitance probes, the conditioning and acquisition systems, the metrology instruments used, and other ancillary instrumentation relevant to the calibration procedure. The prior filtering and data identification from the raw signal is detailed. The step-by-step development of the algorithm is presented, including justification of the curves imposed by the method. The resulting calibrations are provided, achieving accuracies of a few microns. The results are compared against previously used calibration techniques, emphasizing the potential advantages of the presented routine. Finally, the time-resolved tip clearance is analyzed against high frequency aerothermal data within the gap region, identifying relationships between the tip gap, unsteady pressure, and heat flux on the shroud.