Modern jet engines can reach temperatures as high as 1,500�C so accurate temperature readings are vital

New heat sensor could save jet designers millions

A highly-accurate temperature sensor could save jet and turbine manufacturers millions in maintenance costs and fuel consumption.

The device developed by researchers at the University of Cambridge could vastly improve the efficiency, control and safety of high-temperature engines such as jet engines and large gas turbine engines like those used in nuclear reactors.

The sensor, or thermocouple, minimises drift – degradation of the sensor which results in faulty temperature readings and reduces the longevity of engine components.

In results published in the September issue of the Journal of Engineering for Gas Turbines and Power, the sensor has been shown to reduce drift by 80 per cent at temperatures of 1,200°C and by 90 per cent at 1,300°C, potentially doubling the lifespan of engine components.

"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, a postdoctoral researcher in the Department of Materials Science and Metallurgy, who developed the new thermocouple.

Generally, the hotter a jet engine burns, the more power it generates, improving fuel efficiency, range and thrust. However, an accurate temperature reading is critical, as when temperatures get too high, the mechanical integrity of engine components could be at risk and a temperature error of just ten degrees can trigger engine failure.

Modern jet engines can reach temperatures as high as 1,500°C, but drift in the nickel-based thermocouples used to measure temperature increases to unacceptable levels at temperatures above 1,000°C so they are placed away from the hottest part of the engine, and the maximum temperature is extrapolated from that point.

The inaccuracy resulting from this form of measurement means that the engine temperature, and therefore efficiency, has to be set below maximum in order to leave a safety margin for the survival of engine components.

In its simplest form, a thermocouple consists of two bare wires of two different metals joined together at their ends, with a voltmeter incorporated into the circuit. The difference between the two ends of the thermocouple is measured by the voltmeter and used to determine the temperature.

This type of thermocouple is not suitable for high-temperature applications as the elements oxidise above 800°C, increasing the amount of drift, so thermocouples sheathed in oxidation-resistant materials were introduced in the 1970s.

While this configuration addressed the issue of oxidation, the sheath contaminated the wires at temperatures above 1,000°C, increasing drift.

This prompted Scervini, along with Dr Cathie Rae, to develop a thermocouple made of an outer wall of a conventional oxidisation-resistant nickel alloy which can withstand high temperatures, and an inner wall of a different, impurity-free nickel alloy which prevents contamination while reducing drift.

Results from tests on a prototype device showed a significant reduction in drift at temperatures of 1,200°C and 1,300°C, meaning that a double-walled thermocouple can be used at temperatures well above the current limitation of 1,000°C.

There are platinum-based thermocouples which can withstand higher temperatures, but their extremely high cost means that they are not widely used.

"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 1,000°C," said Scervini. "We believe our device could see widespread usage across a range of industries."

The team are currently commercialising their invention with the assistance of Cambridge Enterprise, the University's commercialisation arm, and have attracted interest from a range of industries. Tests on new prototypes are on-going.

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