It is possible for researchers to capture images of carbon dioxide emissions from commercial aircraft engines using specialized imaging techniques such as infrared imaging or laser-based remote sensing. These techniques can detect the specific wavelengths of infrared light emitted by CO2 molecules, allowing researchers to “see” the plumes of CO2 coming from the engines of aircrafts. Additionally, the researchers could also use aircrafts outfitted with sensors that can directly measure the CO2 emissions to produce images. These images can help researchers to better understand the impact of aviation on the environment and to develop strategies for reducing aircraft emissions.
Researchers used a novel near-infrared light imaging technique to capture the first cross-sectional images of carbon dioxide in a commercial jet engine exhaust plume. This cutting-edge technology could help accelerate turbine combustion research aimed at developing more environmentally friendly engines and aviation fuels.
“We call this approach chemical species tomography because it provides real-time spatially resolved information for carbon dioxide emissions from a large-scale commercial engine,” said Michael Lengden of the University of Strathclyde in the United Kingdom. “This information has never been available at this industrial scale before, and it represents a significant improvement over the current industry-standard emissions measurement, which involves transferring gas from the exhaust to a gas analyzer system in a different location.”
The new findings are published in the Optica Publishing Group journal Applied Optics. Chemical species tomography is similar to X-ray-based CT scans used in medicine, except that it uses near-infrared laser light tuned to the absorption wavelength of a target molecule and requires extremely fast imaging speeds to capture the dynamic processes of combustion.
The very refined measurement methodology we used demanded an exquisite knowledge of carbon dioxide spectroscopy and the electronics systems that provide very precise data. Also, a very sophisticated mathematical method had to be developed to compute each chemical species image from the measured absorptions of the 126 different beams we used.
Michael Lengden
“Because the aviation industry contributes significantly to global CO2 emissions, turbine and fuel technologies must improve dramatically,” said Lengden. “By providing fully validated emissions measurements, our new method could assist the industry in developing new technology that reduces the environmental impact of aviation.”
Imaging emissions from airplane engines
Until now it has been impossible to image turbine combustion on test rigs containing a large airplane engine. To solve this problem, four instrumentation research groups in the U.K. came together to combine their knowledge in gas species measurement in harsh environments, chemical species tomography, and optical source development. These teams worked with industrial partners to develop technology that would be practical for industrial research and development
“The teams saw an opportunity to develop world-class instrumentation for the aerospace industry, as well as to better understand emissions and performance improvements from large-scale engines,” Lengden explained. “We can now begin to ‘see’ the chemical detail of combustion in a real production airplane engine using chemical species tomography.”
After years of fine-tuning signal-to-noise ratios, data acquisition methods, imaging techniques, and optical sources, the researchers developed the first facility capable of measuring industrial emissions on the large scale of a commercial airplane engine.
To perform chemical species tomography, 126 beams of near-infrared laser light are shone through the gas from all around the side at many angles in a way that doesn’t disturb the gas flow. Adequately capturing the exhaust from a commercial airplane engine requires imaging an area up to 1.8 m in diameter. To capture this, the imaging components were mounted on a 7-m diameter frame located just 3 m from the exit nozzle of the engine. 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 demanded an exquisite knowledge of carbon dioxide spectroscopy and the electronics systems that provide very precise data,” said Lengden. “Also, a very sophisticated mathematical method had to be developed to compute each chemical species image from the measured absorptions of the 126 different beams we used.”
Capturing combustion on a large scale
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 combustor with 18 fuel injectors arranged in a circle. For the tests, researchers recorded data at frame rates of 1.25 Hz and 0.3125 Hz while the engine was operated over the full range of thrust.
The resulting images revealed that a ring structure of high carbon dioxide concentration was present in the engine’s central region at all thrust levels. A raised region in the middle of the plume was also present, which was most likely caused by the engine’s shape.
The researchers are now modifying the new instrument to allow quantitative measurement and imaging of other chemicals produced by turbine combustion in both the aerospace and industrial power generation sectors, as well as to capture temperature images. Engineers and scientists working on new turbines and fuels will be able to better understand the combustion process for current and future technologies.
