The first cross-sectional images of carbon dioxide in an exhaust plume from a commercial jet engine have been taken by researchers using a novel near-infrared light imaging technique. This cutting-edge idea could speed up research on turbine combustion, which aims to make engines and aviation fuels that are better for the environment. According to research team leader Michael Lengden of the University of Strathclyde in the UK, “this approach, which we call chemical species tomography, provides real-time spatially resolved information for carbon dioxide emissions from a large-scale commercial engine.” This information was not available on this large of an industrial scale before, and it is a big improvement over the current industry standard for measuring emissions, which is to take exhaust gas to a different place to analyze it.
In the journal Applied Optics from the Optica Publishing Group, the researchers report their latest findings. Chemical species tomography is similar to X-ray-based CT scans used in medicine. It uses near-infrared laser light tuned to the absorption wavelength of a target molecule and very fast imaging speeds to capture the dynamic combustion processes.
According to Lengden, “the aviation industry is a significant contributor to global carbon dioxide emissions, so there is a need for radical improvements in turbine and fuel technologies.” “Our new method could help the industry develop new technology that reduces the impact of aviation on the environment by giving fully validated measurements of emissions.”
Imaging emissions from airplane engines
On test stands with a sizable airplane engine, turbine combustion has previously been impossible to visualize. Four instrumentation research groups in the UK joined forces to address this issue by combining their expertise in optical source development, chemical species tomography, and gas species measurement in challenging environments. These teams collaborated with business partners to create technology that would be useful for industrial R&D.
According to Lengden, “the teams saw an opportunity to develop industry-leading instrumentation and to comprehend emissions and performance enhancements from large scale engines.” Thanks to chemical species tomography, we can now begin to “see” the chemical details of combustion in a real production airplane engine.
The researchers developed the first facility capable of collecting industrial emission measurements on the large scale of a commercial airplane engine after years of work to fine-tune signal-to-noise ratios, data acquisition methods, imaging techniques, and optical sources.
126 near-infrared laser light beams are shone through the gas in chemical species tomography in a manner that doesn’t disrupt the gas flow from all sides and at various angles. Imaging an area up to 1.8 m in diameter is required to capture the exhaust from a commercial airplane engine in an adequate manner. The imaging components were mounted on a frame with a 7-m diameter that was just 3 m from the engine’s exit nozzle in order to capture this. The researchers used 126 optical beams to get a resolution of about 60 mm in the middle of the engine exhaust.
According to Lengden, “the very precise measurement methodology we used required an exquisite knowledge of carbon dioxide spectroscopy and the electronics systems that provide very precise data. A very complex mathematical technique 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 elaborate set-up in order to perform chemical species tomography on the carbon dioxide emissions produced by combustion in a contemporary Rolls-Royce Trent gas engine turbine. These engines have a combustor with 18 fuel injectors arranged in a circle and are typically used on long-haul aircraft. For the tests, the engine was run through its entire range of thrust while data was gathered at frame rates of 1.25 Hz and 0.3125 Hz.
The resulting images demonstrated that a ring-structure with a high carbon dioxide concentration was present in the engine’s center at all thrust levels. The shape of the engine may have contributed to the raised area in the middle of the plume.
The researchers are currently modifying the new instrument to make it possible to quantitatively measure and image other chemicals produced by turbine combustion in the aerospace and industrial power generation sectors, as well as to take pictures of temperature. This will make it possible for engineers and scientists working on new fuels and turbines to have a better understanding of how combustion works in both present and future technologies.
