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SENSOR CALIBRATION SYSTEM AND METHODOLOGY FOR TIP CLEARANCE MEASUREMENTS IN TURBOMACHINES
With increasingly tighter tip clearances in modern turbomachinery, it is essential to precisely measure this parameter during turbomachinery characterization. Benefits from measuring tip clearances include monitoring the structural integrity of the machine and estimating aerodynamic losses incurred due to leakage flows. At present, capacitance probes are one of the most commonly used sensors for tip clearance measurements in turbomachines as they are accurate and robust. One of the main challenges when using capacitance probes is properly calibrating the sensors, which usually involves complex positioning systems and blade representative targets. This manuscript describes in detail the development of a methodology for in-house calibration of capacitance probes for tip clearance measurements. A novel calibration procedure that does not involve rotating components is investigated and compared against established calibration methods. First, a calibration bench was developed to demonstrate the static and dynamic performance of the acquisition system and perform quasi-static as well as dynamic calibrations in a controlled environment. An in-situ methodology was then developed to calibrate the sensors once installed in a two-stage rotating turbine rig. The proposed methodology does not require complex positioning systems and a regression analysis using a least squares scheme resulted in a coefficient of determination of 0.9998. The calibration was validated using specially designed instrumentation at various speeds that span the operating envelope of the rig. A Bayesian model that was developed to estimate measurement uncertainties for each method showed that uncertainties as low as ± 5μm can be achieved with the proposed system. The proposed methodology was used in a two-stage turbine rig. Measurements taken at three different circumferential locations were subsequently used to map the spatial distribution of tip clearances throughout the speed operational envelope of the turbine. Finally, a reduced order rotor displacement model was developed and fitted to capacitance probes data. The work presented in this thesis lays the foundation for high fidelity tip clearance measurement capabilities at the Purdue Experimental Turbine Aerothermal Laboratory and can be implemented into any rotating rig.