Wave-Phire takes the heat

A micro-machined sapphire pressure sensor that works at temperatures higher than 1,000ºC can monitor pulsations inside gas turbine combustion chambers — an area too hot for other sensors on the market.

The Wave-Phire sensor, from Oxsensis, an Oxfordshire-based optical instrumentation developer, is claimed to help operators monitor combustion dynamics in real time and detect serious problems as they happen.

The new resilient sensor can be flush-mounted to the wall of a gas turbine combustion chamber, whereas conventional sensors must be located outside the chamber in a protective, heat-resistant tube.

Remote sensors, however, are less sensitive to pressure pulsations inside the chamber and the sound energy of the signal decreases as it travels down the tube.

John Drake, engineering director at Oxsensis, said the Wave-Phire sensor can operate inside the high-temperature combustion chamber because its head is fabricated from single crystal sapphire, which has a high melting point of 2,053ºC. It is also resistant to chemical attack and corrosion. ‘Conventional pressure sensors are based on piezoelectric materials, which means they create an electrical charge when squashed,’ he said. ‘These materials lose their properties at high temperatures.’

Drake said the Wave-Phire is optical rather than electrical and measures pressure change with an interferometric technique. The basic element of the sensor is an interferometer cavity. One part is a sapphire diaphragm, which deflects with the change in pressure in the combustion chamber. As the diaphragm moves, it changes the length and characteristic of the cavity.

A laser beam, delivered through a rugged fibre optic cable, is constantly shone through the cavity and it is reflected back. The light intensity of the returning beam depends on the length and characteristics of the cavity.

The reflected optical signal travels back through the same fibre optic cable that transmitted it. The signal is fed to a control box, on the outside of the turbine, which converts the intensity of the signal into a pressure reading.

Industry’s need for such on-the-spot pressure-measuring sensors is more important than ever due to the development of dry low nitrogen oxide (NOx) combustion systems. These gas turbine combustors burn a leaner mixture of fuel and air and lower the peak flame temperature. While this reduces NOx formation, it leaves the flame inside the chamber prone to instability.

Combustion chambers are therefore more vulnerable to large pressure pulsations and have the potential to experience problems such as acoustic resonances, which can destroy components in the combustion system and cause millions of pounds worth of damage. The Wave-Phire sensors are designed to help operators detect any flame instabilities before they become costly problems.

Drake said the Wave-Phire gas turbine sensors are being developed for both land-based power generation and aero engines. The company has been programme partners with Rolls-Royce and Siemens and has conducted trials and tests with them, among others.

Drake said in future Oxsensis plans to develop a sensor that can measure pressure and temperature simultaneously. That technology will rely on the same sensor head, he said, but it will use a slightly different technique to obtain information from it.

Siobhan Wagner