Tomography

Tomography is a field of measurement concerned with the imaging of internal parameters of a subject based on measurements taken around its periphery in response to an externally applied stimulus. From the measurements, image reconstruction methods are used to reconstruct the internal distribution of the parameter of interest. Then, information about the imaged process can be inferred. Measurement strategies are typically engineered for a given system arrangement and target parameters, resulting in a wide range of imaging modalities exploiting the many different characteristics of a given process. Consequently, tomography has found application in many fields including medical imaging and industrial processing.

Chemical Species Tomography (CST) is an optical tomographic technique concerned with imaging the distribution of molecules in industrial processes. The basic principle of CST is that an optical source and a series of detectors are situated around a region of interest, within which there is a substance with certain light absorption and/or emission parameters. The optical source, typically a laser, is used to project electromagnetic radiation with a particular wavelength across the region. The detectors receive the transmitted or emitted light. Many receive-signals are measured from sources in many positions around the periphery. The source-detector pairs are often arranged in several subsets of parallel or fan arrangements, each subset then being called a “projection”. The complete measurement data set comprises the multiple projections across the region of interest. The inverse problem of converting from the measured data to the distribution of the parameter of interest is solved by applying image reconstruction algorithms to the measurement data set, either directly or iteratively, to approximate a solution.

Previous research into CST has exploited two aspects of a substance’s response to an applied source: transmission, which considers the measured amount of energy absorbed as the light travels through a substance from source to detector; and fluorescence, which considers the measured emission of light from a substance based on the amount of light absorbed from the applied source. Therefore it is essential that the source and detectors in a CST system are carefully chosen to exploit the necessary absorption characteristics of a given substance. Current research focuses on using lasers in the near-to-mid IR range of the electromagnetic spectrum, thus making CST a characteristically hard-field tomographic imaging technique. Examples of applied research into CST in the literature have focused on hydrocarbon distribution and the fuel mix in internal combustion engines using Near-IR Absorption Tomography (NIRAT). The imaging of commercial gasoline using Optical Fluorescence Auto-Project Tomography (OFAPT) has been considered, but its practical application has had to await the arrival of more powerful lasers. Other combustion-related species have been considered, such as water and carbon monoxide.

In the FLITES project, tomographic research will build upon the knowledge base of OFAPT and NIRAT by applying CST to produce 2D images of a range of chemical species across the plane transverse to the axis of a jet engine exhaust plume. Research will examine:

• the imaging of soot using multi-path Continuous-Wave Laser-Induced Incandescence (CW-LII) in the near-IR range using an all-fibre laser system at around 1060nm;
• imaging of CO₂ using near-IR absorption tomography (NIRAT) using an all-fibre laser system at around 1997nm; and
• imaging of Unburnt Hydro-Carbons (UHC) at around 100ppm using mid-IR absorption tomography (MIRAT) using an all-fibre laser system at around 3400nm.

Although single-path measurements of NO using mid-IR absorption will be obtained, tomographic methods will not be applied to these measurements.

The key tomographic research challenges for the FLITES project are to apply tomographic principles to multi-path CW-LII for imaging 2D soot concentrations across the exhaust plume, addressing the hardware and spatial challenges presented by a 120+ sensor NIRAT system for imaging of CO₂, and to address the technical challenges presented by Mid-IR Absorption Tomography (MIRAT) for imaging of UHCs.

Through this research FLITES aims to establish the fundamental engineering principles for CST as applied to imaging chemical species in jet engine exhaust plumes.