Capacitance tip timing techniques in gas turbines

The vibration of turbomachinery blades is an important phenomenon to understand, observe and predict and is the reason for developing a tip timing measurement system. Vibration leads to High Cycle Fatigue (HCF), which limits blade durability and life. HCF can result in blade failure, having expensive consequences for the engine involved. The traditional method for monitoring blade vibration under test conditions is to use blade mounted strain gauges. However, strain gauges are costly and time consuming to install. They have a limited operating life as they are subjected to the harsh on-engine conditions. Only a limited number of blades can be monitored with strain gauges as the number that can be used is limited by the number of channels in the slip ring or telemetry. They can also interfere with the assembly aerodynamics. Consequently non-intrusive alternative techniques such as tip timing are sought. Capacitance probe based clearance measurement systems see widespread use in turbomachinery applications to establish rotor blade tip clearance. This thesis reports investigations into an alternative and additional use in aero-engine rotor blade tip timing measurement for these commercially available systems. Tip clearance is of great importance in the gas turbine industry; this is clear from the fact that gas turbine efficiency has an inverse relationship with tip clearance. Large tip clearance leads to large leakage flows, hence low efficiency, thus the common use of the capacitance probe clearance measurement technique in monitoring turbomachinery. Optical systems have been successfully used to measure rotor blade tip timing on test rigs with several optical probes mounted equally spaced around the turbomachine casing. However, there are practical problems associated with mounting such monitoring systems on in-service jet engines. Optical probes require high maintenance to keep the lenses clean, probably incorporating a purge air system to keep the lenses from fouling. Such impracticalities and added weight make it unlikely that an optical probe based tip timing system will be fitted on an in-service engine in the foreseeable future. In this thesis the scope for a dual use sensor to measure both turbomachinery tip clearance and tip timing is investigated. Since it is impractical to measure blade tip clearance with an optical probe, then the obvious choice for such a sensor is a capacitance probe. Therefore, a commercially available FM capacitance probe based blade tip clearance measurement system is used in a series of tip timing practical investigations. The equipment and instrumentation designed, assembled and produced to facilitate this investigation is documented. These include the development of an optical once per revolution sensor and the design of an independent vibration measurement system based on blade mounted strain gauges. Through an extensive body of experimental work the practicalities in this alternate use of the tip clearance measurement equipment have been assessed. System responses pertaining to tip timing measurement have been investigated, characterised and quantified. The accuracy by which tip timing can be measured using the system has been reported through the findings of an experimental programme carried out on a full-sized, low-speed compressor. Specifically, dual capacitance probe tip timing derived vibration amplitudes have been compared to those derived from blade mounted strain gauge signals. Sources of error have been identified and quantified. Amplitudes were found to agree within the calculated error bands. Instantaneous resonant blade vibrations measured through single capacitance probe tip timing have been correlated with strain gauge derived vibration levels. This has also been done as the rotor traverses blade resonant speed. In this case the vibration phase change across resonance expected from theory was successfully detected through tip timing. Also, the accuracy by which blade time of arrival can be determined by using capacitance probe tip timing has been assessed using a precision OPR sensor and a non-vibrating compressor rotor blade. The characteristics of a DC capacitance probe based clearance measurement system's response to movement in 3D space in proximity to a blade tip have been mapped. Detection of small vibrations have also been investigated in a series of static impulse tests.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Tip timing techniques for turbomachinery HCF condition monitoring

High Cycle Fatigue (HCF) has been established as the major common failure mode in the US Air Force large fleet of aero-engines. Corrective measures for this failure mode in themselves deliver additional technical, managerial and cost pressures. Two responses are in place to address this problem; risk mitigation through accelerated engine development fixes and technology transition through targeted and focussed R&D studies. It is the latter that is of interests and is discussed in this paper. Aero-engine blade vibrations of sufficient amplitude cause High Cycle Fatigue, which reduces blade life. In order to observe this vibration a non-intrusive monitoring system is sought. The vibration can be detected by measuring blade tip timing since in the presence of vibration the blade timing will differ slightly from the passing time calculated from rotor speed. Work done to investigate the suitability of a commercially available capacitance probe tip clearance measurement system for application as a non-intrusive turbomachinery blade tip timing measurement device is reported. Capacitance probe results are correlated with simultaneously measure strain gauge results and the performance of the capacitance system in measuring blade vibration is analysed. The growing interest in blade high cycle fatigue within the aerospace industry, and an approach to monitoring their condition are discussed as an extension to the above study. The suggested approach is based upon the tip-timing method, using non-contact optical probes located around the engine’s casing. Two current tip-timing techniques are suggested for the purpose. The techniques are summarised, the experimental validation of both methods outlined, and the approach taken to investigate the potential use as a condition monitoring tool described. The paper is concluded with a discussion of the future use of tiptiming as a condition monitoring tool

Experimental verification of blade elongation and axial rotor shift in steam turbines

The experimental research of static and dynamic characteristics of rotating parts of turbomachines is a very important part of the design and verification of the machine parameters as well as optimization of their operational regimes. The described method of measuring dynamic and static characteristics, especially blade elongation and rotor axial shift, are based on the utilization of the non-contact magnetoresistive sensors developed in the Institute of Thermomechanics AS CR. These sensors exhibit positive properties for this purpose

Advanced expander test bed program

The Advanced Expander Test Bed (AETB) is a key element in NASA's Chemical Transfer Propulsion Program for development and demonstration of expander cycle oxygen/hydrogen engine technology component technology for the next space engine. The AETB will be used to validate the high-pressure expander cycle concept, investigate system interactions, and conduct investigations of advanced missions focused components and new health monitoring techniques. The split-expander cycle AETB will operate at combustion chamber pressures up to 1200 psia with propellant flow rates equivalent to 20,000 lbf vacuum thrust

HIGH-SPEED ROTOR TIP CLEARANCE MEASUREMENTS IN A TRANSONIC COMPRESSOR

Performance of a gas turbine compressor is directly dependent on the size of the region between the rotor blade’s tips and the surrounding casing, the tip clearance, which dynamically changes with rising rotor speed due to rotor blade radial growth from centrifugal loading. Too large a tip clearance introduces disruptive air flow that will lower compressor efficiency and lead to stall conditions, whereas too small a tip clearance will increase the risk of blade tip rubbing with the casing inner wall and may lead to catastrophic failure. This experiment is a part of a program of research that characterizes the Naval Postgraduate School Military Fan (NPSMF) in the Turbopropulsion Lab’s (TPL) Transonic Compressor Rig (TCR). This study involves the design, creation, and use of two benchtop rigs with a capacitive proximity probe blade tip clearance measurement system to develop mathematical methods to post-process capacitive probe output signals for calibration and tip clearance measurements. The mathematical methods developed in this study are validated against the tip clearance measurement system manufacturer’s method, showing improvement. A comparison of the different calibration rigs’ resulting calibration curves is discussed. The post-process method is then applied to high-speed tip clearance measurements of the NPSMF in the TCR and the results are compared to a model.Office of Naval Research, Arlington, VAOutstanding ThesisLieutenant, United States NavyApproved for public release. Distribution is unlimited

Upgraded automotive gas turbine engine design and development program, volume 2

Results are presented for the design and development of an upgraded engine. The design incorporated technology advancements which resulted from development testing on the Baseline Engine. The final engine performance with all retro-fitted components from the development program showed a value of 91 HP at design speed in contrast to the design value of 104 HP. The design speed SFC was 0.53 versus the goal value of 0.44. The miss in power was primarily due to missing the efficiency targets of small size turbomachinery. Most of the SFC deficit was attributed to missed goals in the heat recovery system relative to regenerator effectiveness and expected values of heat loss. Vehicular fuel consumption, as measured on a chassis dynamometer, for a vehicle inertia weight of 3500 lbs., was 15 MPG for combined urban and highway driving cycles. The baseline engine achieved 8 MPG with a 4500 lb. vehicle. Even though the goal of 18.3 MPG was not achieved with the upgraded engine, there was an improvement in fuel economy of 46% over the baseline engine, for comparable vehicle inertia weight

A probabilistic approach to analyse Blade Tip Timing data of non-synchronous vibrations under constant rotor speed

Blades are among the most critical components of turbomachines, their monitoring and characterization undergoing working conditions are fundamental for the insiders, both for preventing eventual breakage and for optimising future development. Two approaches are possible for monitoring rotor blade vibrations: a traditional one based on the use of strain gauges and another one called Blade Tip Timing (BTT). BTT is an indirect, non-intrusive simple and robust measurement method, but the processing of such data is not easy because they are often subsampled with respect to the Nyquist limit and the ordering of the samples is not unique. In this work the focus is on multi component non-synchronous vibrations, typical for example of flutter, measured at constant rotor speed by a BTT system. These data are organized into batches of fixed length called snapshots and they are interpreted as members of a random vector. When the signal contains only one harmonic component the frequency can be determined using a method here described and called Harmonic Matching (HM). While for the analyses of multi harmonic component vibrations a probabilistic approach capable of separating and identify the components using Principal Component Analysis (PCA) and Independent Component Analysis (ICA) is proposed. For the development of data processing methods, the possibility of having controllable and repeatable data is fundamental, for this reason two test rigs of increasing complexity have been developed and are here described