Blade runner

A laser sensor for more efficient turbine blade construction could help the aerospace industry develop low-weight jet engines with non-metallic parts.

Developed by Technical University of Dresden (TU Dresden) and the German Aerospace Centre (DLR), the system measures the clearance between the tips of the turbine blade and the casing, and the vibrations caused by the blades breaking the sound barrier as they rotate.

Tip clearance is a particularly important parameter in turbine design, because it has a very strong influence on efficiency. The lower the clearance — that is, the smaller the distance between the tip of the blade and the case — the more efficient the turbine.

But because of the expansion of the materials as they heat up, and the mechanical movement of the blades as they spin, the clearance varies constantly by a matter of micrometres.

Traditionally, tip clearance is measured using electrical sensors on the turbine casing to monitor the change in capacitance as the size of the gap between the two metal bodies fluctuates. However, this form of measurement can be imprecise; moreover, it only works with metal.

And as the aerospace industry seeks to reduce weight by investigating ceramics and engineering polymers as turbine materials, a new tip measurement method is needed.

The TU Dresden team, led by Jürgen Czarske of the department of metrology — part of the faculty of electrical engineering — uses the Doppler effect to measure the clearance. Most familiar from the perceived change in pitch of ambulance sirens as they go past, the effect uses the change in received frequency from a wave reflected from a moving surface to derive the speed of the object.

Czarske's system uses two laser wavelengths which are sent via a fibreoptic cable to a measuring head mounted on the outside wall of the turbine. The beams pass through a window on to the tips of the blades, which reflect the beams back to the detector. As the speed of the blades can be set precisely, the signal can be processed to derive the distance from turbine wall to blade tip.

The team used a turbine set at the DLR's Institute of Drive Engineering, with a rotation speed of 20,000 rpm. Using standard measuring techniques, the uncertainty in the measurement was 50-100µm, but the laser Doppler measurement achieved uncertainties of 20µm — a new record.

The vibration measurement is a lucky side-effect of the measurement, said Czarske. because the precise position of each blade is known, the changes in position over time can also be measured accurately.

The design of the measuring head was a crucial factor. Czarske's team built a robust system which could withstand the vibration, and also incorporated a cooling system, enabling it to take measurements even when the casing's temperature exceeded 300^oC.

The system can also be used for machining, said Czarske. Measuring the absolute radius and shape of an object in process allows tighter control.

The team is currently seeking an industrial partner to help commercialise the research.