A new method of measuring turbine tip clearance.
With the ever-increasing use of non-metallic aerospace parts,
a new method of measuring turbine tip clearance is needed.
German researchers believe laser sensors hold the key.
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 20~tm 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°C.
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.
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A development by Technical University of Dresden (TU Dresden) and
the German Aerospace Centre (DLR).
April 2006
