Sensing for Controls and Propulsion Health Management in Turbine Engines

New methods can mitigate deleterious effects on turbine engine safety and performance.

Advances in engine performance and reliability require sensor components that operate reliably under extreme engine operating conditions (e.g., takeoff, max thrust) and in harsh environments (e.g., high temperature and radiation). The design of advanced controls and Propulsion Health Man agement (PHM) also depend on the use of components with increased susceptibility to atmospheric radiation. Current and future engine operating temperature environments that provide major challenges in sensor design for control and propulsion health management are being explored.

High-Temperature Regimes for Sensors with aerospace applications.
Eliminating the life-reducing effects of high cycle fatigue (HCF), combustion instabilities, compressor surge, and combustion instability control will require the use of sensors with high-temperature tolerance and high-frequency response. This requirement cannot be met by currently available technology. As a result, accurate, high-frequency sensors capable of operating in harsh environments are being developed. The need for accurate high-temperature dynamic pressure measurements is an integral step to the implementation of active surge control methodologies. For high-frequency applications, it is generally necessary to reduce the distance from the sensor to the environment to a minimum.

Typical aerospace pressure transducers are limited to about 500°F, while in today’s large gas turbine engines, compressor exit temperatures can be on the order of 1,200 to 1,400°F, meaning that current pressure transducers are not sufficient for measuring these conditions. The normal technique used to handle this temperature environment is to cool the transducer or locate the transducer in a benign environment.

Modern turbines have Full Authority Digital Engine Controls (FADECs) that provide safe and stable engine operation. These FADECs govern and limit operation of the combustion system. To minimize emissions of carbon monoxide and nitric oxides (NOx), and ensure design life, combustion systems may include control scheduling algorithms that receive input measurements of the exhaust temperature of the turbine and the actual compressor operating pressure ratio.

In a turbine engine control system, the fuel control uses a fuel metering valve assembly that is responsive to electrical signals generated by the FADEC. The FADEC response depends on sensors that measure turbine speed, pressure, and temperature, indicative of the operator thrust request. A fuel bypass valve provides a means for returning excess (unmetered) fuel from the main pump to the inlet low pressure supply. Sensors are also required to measure compressor discharge pressure for operating bypass valves to control pressure fluctuations. A high-response sensor is needed to measure differential pressure for controlling the main fuel-metering valve to achieve a rate of metered fuel flow corresponding to compressor discharge pressure. The figure shows the high-temperature regimes for the sensors needed for future aerospace applications.

The future challenges for turbine engine sensors and controls are implementation of specific technologies for diagnostics, stability management, and reconfiguration for damage tolerance. These challenges include tip clearance control, active combustion control, data fusion, integration of the turbine engine with the flight control, and high-frequency analysis for turbine engine controls. These technologies will prolong the life of the engine, increase reliability, as well as reduce lifecycle cost. In all techniques, the objective is to reduce engine wear and tear and obtain maximum life from the components.

This work was done by Alireza Behbahani and Kenneth Semega of the Air Force Research Laboratory. AFRL-0122



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Sensing for Controls and Propulsion Health Management in Turbine Engines

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