Researchers successfully capture the first images of carbon dioxide emissions in aircraft engine

The first cross-sectional photos of carbon dioxide in a jet engine exhaust plume were taken by researchers using brand-new near-infrared light imaging technology.

As claimed in the statement, the development of more ecologically friendly engines and aviation fuels could be sped up with the aid of this brand-new cutting-edge technology for turbine combustion.

The research was published in Applied Optics' 28th issue.

“This approach, which we call chemical species tomography, provides real-time spatially resolved information for carbon dioxide emissions from a large-scale commercial engine,” said research team leader Michael Lengden from the University of Strathclyde located in Scotland.

“This information has not been available before at this industrial scale and is a big improvement over the current industry-standard emissions measurement, which involves taking gas from the exhaust to a gas analyzer system in a different location."

Similar to the X-ray-based CT scans used in medicine, chemical species tomography uses near-infrared laser light adjusted to a target molecule's absorption wavelength and requires extremely quick imaging rates to capture the dynamic processes of combustion.

“The aviation industry is a major contributor to global carbon dioxide emissions so there is a need for turbine and fuel technologies to improve radically,” said Lengden.

“By providing fully validated emissions measurements, our new method could help the industry develop new technology that reduces the environmental impact of aviation.”

Now it is possible

Until now, imaging turbine combustion on test rigs containing a large airplane engine was impossible. To address this issue, four instrumentation research groups in the United Kingdom collaborated to pool their expertise in gas species measurement in harsh environments, chemical species tomography, and optical source development.

These teams collaborated with industrial partners to create technology that could be used in industrial research and development.

“The teams saw an opportunity to develop world-leading instrumentation for the aerospace industry and to understand emissions and performance improvements from large-scale engines,” said Lengden.

“With chemical species tomography, we can now start to ‘see’ the chemical detail of combustion in a real production airplane engine.”

The researchers developed the first facility capable of measuring industrial emissions at the massive scale of a commercial aviation engine after years of experimenting with different signal-to-noise ratios, data gathering systems, imaging techniques, and optical sources.

126 near-infrared laser light beams are shined through the gas in chemical species tomography in a manner that doesn't disrupt the gas flow from all sides and at all 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 put on a frame with a 7-m diameter that was just 3 m from the engine's exit nozzle in order to capture this. In the middle of the engine exhaust, the researchers employed 126 laser beams to reach a spatial resolution of roughly 60 mm.

“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

In order to perform chemical species tomography on the carbon dioxide produced by combustion in a contemporary Rolls-Royce Trent gas engine turbine, the researchers utilized this elaborate set-up.

These engines have a combustor with 18 fuel injectors arranged in a circle, which are commonly used in long-haul airplanes. For the testing, the engine was run through its entire range of thrust while data were gathered at frame rates of 1.25 Hz and 0.3125 Hz.

The resulting images showed that, at all thrust levels, a ring structure of high carbon dioxide concentration was present in the central region of the engine.

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.

Abstract

We report here the first implementation of chemically specific imaging in the exhaust plume of a gas turbine typical of those used for propulsion in commercial aircraft. The method used is chemical species tomography (CST) and the target species is CO2, absorbing in the near-infrared at 1999.4 nm. A total of 126 beams propagate transverse to the plume axis, along 7 m paths in a coplanar geometry, to probe a central region of diameter ≈1.5m. The CO2 absorption spectrum is measured using tunable diode laser spectroscopy with wavelength modulation, using the second harmonic to first harmonic (2f/1f) ratio method. The engine is operated over the full range of thrust, while data are recorded in a quasi-simultaneous mode at frame rates of 1.25 and 0.3125 Hz. Various data inversion methodologies are considered and presented for image reconstruction. At all thrust levels, a persistent ring structure of high CO2 concentration is observed in the central region of the measurement plane, with a raised region in the middle of the plume assumed to be due to the engine’s boat tail. With its potential to target various exhaust species, the CST method outlined here offers a new approach to turbine combustion research, turbine engine development, and aviation fuel research and development.