Abstract
This paper describes the development of an infrared laser absorption tomography system for the 3D volumetric imaging of chemical species and temperature in reacting flows. The system is based on high-resolution near-infrared tunable diode laser absorption spectroscopy (TDLAS) for the measurement of water vapour above twin, mixed fuel gas burners arranged with an asymmetrical output. Four parallel laser beams pass through the sample region and are rotated rapidly in a plane to produce a wide range of projection angles. A rotation of 180° with 0.5° sampling was achieved in 3.6 s. The effects of changes to the burner fuel flow were monitored in real time for the 2D distributions. The monitoring plane was then moved vertically relative to the burners enabling a stack of 2D images to be produced which were then interpolated to form a 3D volumetric image of the temperature and water concentrations above the burners. The optical transmission of each beam was rapidly scanned around 1392 nm and the spectrum was fitted to find the integrated absorbance of the water transitions and although several are probed in each scan, two of these transitions possess opposite temperature dependencies. The projections of the integrated absorbances at each angle form the sinogram from which the 2D image of integrated absorbance of each line can be reconstructed by the direct Fourier reconstruction based on the Fourier slice theorem. The ratio of the integrated absorbances of the two lines can then be related to temperature alone in a method termed, two-line thermometry. The 2D temperature distribution obtained was validated for pattern and accuracy by thermocouple measurements. With the reconstructed temperature distribution, the temperature-dependent line strengths could be determined and subsequently the concentration distribution of water across the 2D plane whilst variations in burner condition were carried out. These results show that the measurement system based on TDLAS can be used for 2D temporal or 3D volume imaging of temperature and chemical species concentration in reacting flows.
Highlights
The ability to measure non-invasively the spatial distribution of chemical species concentrations and temperature distributions in reacting flows is of importance in a range of application areas but in combustion and plasma science and engineering where it can contribute to designing systems to improve energy efficiency and reduce pollutant species by increased understanding of chemical kinetics, fluid dynamics as well as heat and mass transfer
With the reconstructed temperature distribution, the temperaturedependent line strengths could be determined and subsequently the concentration distribution of water across the 2D plane whilst variations in burner condition were carried out. These results show that the measurement system based on tunable diode laser absorption spectroscopy (TDLAS) can be used for 2D temporal or 3D volume imaging of temperature and chemical species concentration in reacting flows
View-1 in Fig. 18 shows the ten slices stacked over each other, each slice set at 75% transparency to simulate how it may look as a translucent volumetric space
Summary
The ability to measure non-invasively the spatial distribution of chemical species concentrations and temperature distributions in reacting flows is of importance in a range of application areas but in combustion and plasma science and engineering where it can contribute to designing systems to improve energy efficiency and reduce pollutant species by increased understanding of chemical kinetics, fluid dynamics as well as heat and mass transfer. For practical applications it could be beneficial for the detection of faulty or blocked burners in multi-burner engines such as in some aero-engines. Cavity-enhanced spectroscopic methods such as cavity-ring down spectroscopy (CRDS) and cavityenhanced absorption spectroscopy (CEAS) [6] have enabled trace chemical species to be measured
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