Endoscopic measurement method allows quieter, more reliable jet engines
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 velocity, pressure or temperature by determining Doppler shift, total intensity and spectral distribution of light. In the case of the DLR development, a laser beam is coupled into the combustion chamber’s emission plane through a crystal fibre. The resulting Rayleigh scatter is then observed through what the Cologne team calls an "image conductor" or "image fibre". The analysis of the scattered light allows determining physical dimensions like temperature and velocity of flow simultaneously at high precision. An important aspect in developing endoscopic scattered light technology is that it can be transferred to other areas such as power generation with stationary turbines where similar problems now can also be explored, said Stockhausen.
To review the quality of the new measurement technique, the DLR scientists successfully performed additional measurements with conventional equipment. For these measurements a new combustion chamber test stand has been built. The results confirmed the quality of the approach, the institute says.
The development are part of the EU project LEMCOTEC FP7 (Low Emissions Core Engine Technologies).
More information: www.dlr.de
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