N112-110 TITLE: High-Temperature Sensor System for Turbine Clearance Measurement

TECHNOLOGY AREAS: Air Platform, Sensors

ACQUISITION PROGRAM: F-35, Joint Strike Fighter

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OBJECTIVE: Develop and validate a sensor system to measure turbine tip clearance in high-temperature environments.

DESCRIPTION: Turbine tip clearance is a critical parameter affecting the performance of propulsion engines. To operate at top efficiency, a turbine�s tip clearance must be minimized under the constraint of positive clearance to the turbine case. A turbine�s tip clearance varies throughout different operating conditions because of differential thermal expansion, manufacturing tolerances, stresses, creep, and erosion. Currently, the sensors that measure turbine tip clearance are not robust enough to withstand the high temperatures of the turbine environment. In addition, the present sensors suffer from deterioration and mechanical failure, often resulting in domestic object damage issues in the turbine.

The goal of this project is to develop and validate a production-quality sensor system that can be used in the harsh environment of propulsion engines to quantify turbine tip clearance of individual blades around the circumference of one or more turbine stages. This technology is intended to be installed in a fleet of aircraft for engine health management and not in a development test configuration. Involvement with an engine original equipment manufacturer (OEM) is strongly suggested. Accuracy and repeatability of a turbine tip clearance sensor system are of paramount importance. Turbine tip clearance can range from 0 to more than 0.125 inch. Clearance measurement accuracy must be 0.001 inch or better.

Environmental conditions in a gas turbine are challenging. Temperatures can range from 2500 degrees Fahrenheit to 4000 degrees Fahrenheit and static gas path pressures can range from ambient to approximately 30 atmospheres. Chemically, the turbine gas path constituents include air, intermediate species and products of hydrocarbon combustion, soot, calcium-magnesium alumino silicate (CMAS), and chloride and sulfate salts. Any sensor feature that is exposed to the gas path must be resistant both to chemical attack by these compounds and to deposition of the solids onto exposed surfaces. The sensor should have a service life of at least 10,000 operating hours in this environment and should have an unlimited shelf life. Innovation should be present in the design of the measurement system, and accuracy, reliability, and repeatability of the sensor measurement results must be demonstrated. In addition, durability will be considered a key parameter, as in any sensor system, as will a low propensity to generate domestic object damage.

In many applications, space limits accessibility to the turbine, and adding sensors to an existing instrumentation package may not be possible. Therefore, it would be beneficial if the turbine�s tip clearance sensor could acquire information in addition to clearance, such as temperature, pressure, tip timing, vibration, and so on. Quite often, the limiting factors in the application of a newly developed sensor are not related to the sensor itself but rather to the packaging and mounting of the sensor.

During the engine design phase, provisions for mounting sensors onto the turbine case can be provided with relative ease. However, after the engine has been fielded, a redesign of major engine modules to accommodate new sensors is rarely a justifiable exercise. An objective is to produce a sensor that can be retrofitted onto existing turbine hardware without major modifications. Also, the use of cooling and purge air would not be acceptable due to engine weight gain, efficiency loss, and other penalties. Weight of any aircraft-mounted system must be minimized to limit impact on aircraft performance and balance. The total weight of a production-representative turbine clearance measurement system should be less than 10 pounds (lb). Additionally, the system should consume less than 5 watts (W) of electrical power.

The sensor system must also be proven to perform robustly within the environment of an aircraft in order to be approved for service. Representative requirements include:

� Electromagnetic interference characteristics per MIL-STD-461E Elements CE102, CS114 (Navy aircraft internal criteria), CS115, CS116 (Navy aircraft level), RE102 (Navy aircraft level), RS103 (Navy aircraft internal level), and, depending on the design, CE106, CS103, CS104, and CS105
� Environmental performance per MIL-STD-810F with the following criteria:
� Minimum pressure 1 pounds per square inch absolute (psia)
� Minimum temperature negative 78 degrees Fahrenheit, maximum air temperature around signal conditioning electronics 250 degrees Fahrenheit
� Vibration: Levels for aircraft-mounted hardware per MIL-STD-810F Figure 514.5C-8 and for engine-mounted hardware per Figure 514.5C-16
� Acoustic noise to 150 decibels (dB)
� No nutritive materials that support fungus growth
� Immunity to salt fog, sand and dust, and fluids encountered in the vicinity of an engine
� Water proofness in the presence of spraying water
� Sustained acceleration of 15g
� Shock: Withstand a 48-inch fall
� Explosion proof

The sensor system must also be able to indicate a failure that causes the measurement outputs to exceed tolerance specifications. This characteristic can be accomplished through self-diagnostics or through design features that permit a connected diagnostic system to identify a fault within the tip clearance measurement system.

The turbine tip clearance measurement system must be compatible with aircraft electrical systems. Aircraft electrical power characteristics and compatibility requirements are defined in MIL-STD-704.

PHASE I: Develop and demonstrate the feasibility of a sensor that can measure turbine tip clearance in environments with temperature ranges on the order of 1800 �F, with a plan to extend the temperature range in Phase II.

PHASE II: Demonstrate the sensor system to the 2500 �F to 4000 �F temperature range. Run the sensor in an application that produces the typical pressure, temperature, and chemical environment (such as a rig) to enable reaching Technology Readiness Level 5.

PHASE III: Mature the sensor package and signal conditioning module, if applicable, to meet the service qualification requirements of electromagnetic interference characteristics, environmental criteria, weight limit, power consumption limit, and aircraft electrical power compatibility. Integrate the sensor system into an OEM development program where demonstration of durability, accuracy, reliability, and repeatability can be verified.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Commercialize the technology by integrating the developed sensor into multiple engine manufacturers� military engine development efforts to reduce engine development cycle time and cost by enabling turbine clearances to be understood and optimized to achieve improved engine fuel efficiency and durability in a more efficient manner. Also integrate into engine manufacturer�s future turbine engines to attain improved engine fuel efficiency on future advanced military or commercial engine development programs or on upgrades to current fielded systems.

REFERENCES:
1. Propulsion Instrumentation Working Group. (n.d.) Blade tip clearance. Retrieved from http://www.piwg.org/sensor/sensor_tclearance.html

2. Kempe, A., Schlamp, S., Rosgen, T., & Haffner, K. (2006, June). Spatial and Temporal High-Resolution Optical Tip-Clearance Probe for Harsh Environments. Paper presented at the 13th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal.

3. Overton, G. (2006). Laser Doppler Velocimetry: Fiberoptic probe measures turbine tip clearance. Laser Focus World, 42(6). Retrieved from http://www.optoiq.com/index/photonics-technologies-applications/lfw-display/lfw-article-display/257213/articles/laser-focus-world/volume-42/issue-6/world-news/laser-doppler-velocimetry-fiberoptic-probe-measures-turbine-tip-clearance.html

KEYWORDS: sensor; gas turbine engine; clearance; high-temperature probes; turbine tip-clearance measurement system; size weight and power (SWAP)

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