The Gas Turbine is seen by many as a technological marvel. How can a machine made up of over 2000 individual high performance parts run effortlessly when subjected to extreme environments both internally and externally and often running under near relentless operating conditions?
The answer is found in some seriously comprehensive engineering design, excruciating testing and maintenance regimes and as near as dammit constant monitoring both in-service and between operations.
Through the design, manufacture and operation of a Gas Turbine there are a few key factors that are under scrupulous assessment:
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The pressure within different sections of the turbine
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The performance and degradation of parts within the turbine
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Fuel consumption
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Operating conditions
Temperature has a big influence on each of these factors so it is incredibly important to get the most reliable, accurate and responsive readings of temperature at all times. The ability to do this with the smallest possible margin for error in readings, would offer manufacturers and operators the potential to save millions of pounds on maintenance and fuel consumption by allowing improved engine optimization and refining maintenance programmes.
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A temperature sensor developed by a team of researchers at University of Cambridge might well have met that challenge. The team have designed a new type of sensor that could improve the efficiency, control and safe operation of high-temperature engines by minimising drift-degradation of the sensor itself. A typical problem associated with temperature sensors placed in such extreme environments that can result in faulty temperature readings that have a potential knock-on effect when reviewing operation and assessing degradation of operational parts.
The new thermocouple (sensor) has been shown to reduce drift by 80% at temperatures of 1200 degrees C and 90% at 1300 degrees C thus, potentially doubling the lifespan of these components when compared to current sensor designs. The results of these test can be found in the Sept 13 edition of the Journal of Engineering for Gas Turbines and Power.
As the pressure for gas turbines to become more fuel efficient and to pro-long the life of components to ultimately reduce 'down-time' and repair and overhaul 'turn-around-time' pushes material science and engineering to its limits, more accurate sensing and analysis becomes even more important.
With the hot section of modern Gas Turbines operating at temperatures that far exceed the viable long term operational range for existing sensor designs, temperature sensors are typically placed further from the 'hot section' of the turbine, thus resulting in the need to extrapolate data to estimate peak temperature data for the hottest points in the combustion chamber. A more precise and stable temperature sensor, placed closer to the peak has the potential to provide far more reliable data allowing engineers to optimise engine operation by being able to more accurately predict engine part performance and degradation over the longer term.
“A more stable temperature sensor provides several advantages – a better estimation of temperature can increase the lifetime of engine components and decrease maintenance costs to manufacturers, without any reduction in safety,” said Dr Michele Scervini, from the Department of Materials Science and Metallurgy, who developed the new thermocouple.
Traditional thermocouple designs are not suitable for such high temperature applications, as the elements tend to oxidise when subjected to heat in excess of 800 degrees, which then increases the amount of drift and thus potentially increasing the margin for error in readings. Since the 1970's engineers have attempted to overcome this issue by coating the thermocouple components in an oxidation resistant sheath but even though this does reduce oxidisation, contamination of the thermocouple wires from the sheath material means the issue of drift returns when temperatures exceed 1000 degrees C.
The team at the University of Cambridge have developed a new thermocouple design that can withstand both oxidisation and the issue of contamination. The new design from Dr Michele Scervini and Dr Cathie Rae is made from an outer wall of a conventional oxidisation-resistant nickel alloy (to withstand high temperatures) and an innovative inner wall of an impurity-free nickel alloy which eradicate contamination from the outer wall while also reducing drift.
Initial results from a prototype device has shown significant improvements in performance and a large reduction in drift at temperatures in excess of the current limitation of 1000 degrees C. Meaning these Nickel alloy thermocouples could operate at temperatures in excess of 1200 degrees. Of course, there are other materials that can potentially withstand and operate at such high temperatures but, materials like Platinum which have been used in thermocouples are often uneconomical for such applications.
"Nickel is an ideal material for these applications as it is a good compromise between cost and performance, but there is a gap in the market for applications above 1000 degrees," said Scervini. "We believe our device could see widespread usage across a range of industries."
The team at the University of Cambridge are currently focusing on commercialising their new thermocouple design with the help of Cambridge Enterprise and are said to have attracted interest from a diverse range of industries.
The research leading to the new thermocouple has been funded by the European Community as part of the HEATTOP project. Additional funding from the European Community has been granted to the University of Cambridge to develop further the new thermocouple, as part of the STARGATE project.