Large-scale chemical-specific emissions information is poised to aid in the development of greener aircraft engines and fuels
Researchers used a new near-infrared light imaging technique to capture the first cross-sectional images of carbon dioxide in the exhaust plume of a commercial jet engine. This cutting-edge new technology could help accelerate turbine combustion research aimed at developing more environmentally friendly aviation engines and fuels.
“This approach, which we call chemical species tomography, provides real-time, spatially resolved information on carbon dioxide emissions from a large-scale commercial engine,” said research team leader Michael Lengden. from the University of Strathclyde in the UK. “This information has not been available before on this industrial scale and is a vast improvement over the current industry standard emissions measurement, which involves feeding the gas from the exhaust to a system for analyzing gas located in a different place.
Researchers report the new research in the journal Optica Publishing Group Applied Optical. Chemical species tomography works much like X-ray CT scans used in medicine, except it uses near-infrared laser light tuned to the absorption wavelength of a target molecule and requires imaging speeds very fast to capture dynamic combustion processes.
“The aviation industry is a major contributor to global carbon dioxide emissions, so there is a need for radical improvements in turbine and fuel technologies,” Lengden said. “By providing fully validated emissions measurements, our new method could help industry develop new technology that reduces the environmental impact of aviation.”
Imaging of aircraft engine emissions
Until now, it was impossible to image the combustion of a turbine on test benches containing a large aircraft engine. To solve this problem, four instrumentation research groups in the UK have come together to combine their knowledge of measuring gaseous species in harsh environments, tomography of chemical species and developing optical sources. These teams worked with industrial partners to develop technology that would be practical for industrial research and development.
“Teams saw an opportunity to develop state-of-the-art instrumentation for the aerospace industry and understand large-scale engine emissions and performance improvements,” Lengden said. “With chemical species tomography, we can now begin to ‘see’ the chemical details of combustion in an actual production aircraft engine.”
After years of work to refine signal-to-noise ratios, data acquisition methods, imaging techniques and optical sources, researchers have created the first facility capable of acquiring large-scale industrial emissions measurements. scale of a commercial aircraft engine.
To perform chemical species tomography, 126 beams of near-infrared laser light pass through the gas from the entire side at many angles so as not to disturb the gas flow. Proper capture of commercial aircraft engine exhaust requires imaging of an area up to 1.8 m in diameter. To capture this, the imaging components were mounted on a 7m diameter frame located just 3m from the engine exit nozzle. The researchers used 126 optical beams to achieve a spatial resolution of about 60 mm in the central region of the engine exhaust.
“The very refined measurement methodology we used required a deep understanding of carbon dioxide spectroscopy and electronic systems that provide very precise data,” Lengden said. “In addition, a very sophisticated mathematical method had to be developed to calculate the image of each chemical species from the measured absorptions of the 126 different beams that we used.”
Capturing large-scale combustion
The researchers used this large-scale setup to perform chemical species tomography of carbon dioxide produced by combustion in a modern Rolls-Royce Trent gas engine turbine. These engines are typically used on long-haul aircraft and contain a combustion chamber with 18 fuel injectors arranged in a circle. For the tests, the researchers recorded data at frame rates of 1.25 Hz and 0.3125 Hz while the motor was running through the entire thrust range.
The resulting images showed that at all thrust levels, an annular structure with a high concentration of carbon dioxide was present in the central region of the engine. There was also a raised area in the middle of the plume, probably due to the shape of the engine.
Researchers are now working to adapt the new instrument to enable quantitative measurement and imaging of other chemicals produced by turbine combustion in the aerospace and industrial power generation sectors, and to capture temperature images. This will allow engineers and scientists developing new turbines and new fuels to better understand the combustion process for current and future technologies.
The project team includes the universities of Strathclyde, Edinburgh, Manchester, Southampton, Loughborough and Sheffield; aircraft engine manufacturer Rolls-Royce; industrial gas turbine engine manufacturer Siemens; laser instrument manufacturer OptoSci Ltd. ; and imaging system makers M Squared Lasers and Tracerco.
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