Endoscopic measurement method allows quieter, more reliable jet engines

December 21, 2014 //By Christoph Hammerschmidt
Endoscopic measurement method allows quieter, more reliable jet engines
A team of scientists from German aerospace research centre DLR has developed an optical measurement method that enables direct insight into the thrust chamber of a jet engine. Endoscopic probes capture images that are transferred through extremely small apertures via fibre optics, helping engineers to improve future jet generations and make them more silent, reliable and eco-friendly.

The goal of the researchers at the DLR institute for propulsion technology (Cologne, Germany) is to optimise the combustion process inside jet engines. Key element for the development of reliable and less polluting jet engines is the combustion process inside the turbines. The scientists now developed a miniaturised optical measurement technique that provides hitherto unknown insights into the combustion chamber. "This techniques allows us to gather data that help us to understand the flow field at key locations inside the jet engine", says team leader Guido Stockhausen from DLR.

In the past, experimental combustion chambers with glass walls and specific openings for measurement instruments provided important contributions to the fundamental understanding of the combustion process. This approach however allowed examining only one segment of the combustion chamber. Modern aircraft engines however contain a circular combustion chamber with 16 or more chamber segments and the same amount of burners. If all burners in such a circular design are active, they can generate oscillations that affect the combustion process and thus the performance of the jet engine; in extreme cases these vibrations can even lead to damaging the jet engine. This phenomenon is known but with existing measurement technology it was not possible to investigate and understand it. The technology developed by the DLR with its extremely miniaturised probes allows the scientists to perform their tests much closer to the real conditions inside the engine.

Another point of interest was the gas flow at the transition between combustion chamber and turbine. In this area, temperatures of more than 2000°C prevail, and pressure exceeds 20 bar. The melting point of metals lies in the range of 1500°C, depending on the alloy used. Turbine blades for this reason are equipped with internal cooling and a ceramic heat shield. Nevertheless they are highly stressed during operation.

For the measurements the researchers utilised an endoscopic implementation of the Filtered Rayleigh scattering, a technique used to measure

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