We present a computationally efficient reconstruction method for the limited-data chemical species tomography problem that incorporates projection of the unknown gas concentration function onto a low-dimensional subspace, and regularization using prior information obtained from a simple flow model. In this context, the contribution of this work is on the analysis of the projection-induced data errors and the calculation of bounds for the overall image error incorporating the impact of projection and regularization errors as well as measurement noise. As an extension to this methodology, we present a variant algorithm that preserves the positivity of the concentration image.

A novel opto-electronic scheme for line-of-sight Near-IR gas absorption measurement based on direct absorption spectroscopy (DAS) is reported. A diode-laser-based, multiwavelength system is designed for future application in nonintrusive, high temporal resolution tomographic imaging of H2O in internal combustion engines. DAS is implemented with semiconductor optical amplifiers (SOAs) to enable wavelength multiplexing and to induce external intensity modulation for phase-sensitive detection. Two overtone water transitions in the Near-IR have been selected for ratiometric temperature compensation to enable concentration measurements, and an additional wavelength is used to account for nonabsorbing attenuation. A wavelength scanning approach was used to evaluate the new modulation technique, and showed excellent absorption line recovery. Fixed-wavelength, time-division-multiplexing operation with SOAs has also been demonstrated. To the best of our knowledge this is the first time SOAs have been used for modulation and switching in a spectroscopic application. With appropriate diode laser selection this scheme can be also used for other chemical species absorption measurements.

mechanisms and heat transfer processes [1,2]. They are also used as important combustion devices in development and calibration of optical diagnostic techniques [3][4][5][6][7]. Generally, the premixed flame of the flat-flame burner is axisymmetric and assumed to be flat in a radial direction. In other words, distributions of temperature and mole fractions of chemical species on a cross section of the flame are assumed to be uniform in the radial direction. The flame flatness is one of the most critical factors in evaluating the performance of the flat-flame burner. On the one hand, when calibrating optical diagnostic instruments, a clear separation is required between the region of interest, i.e., the core flame, and the regions which are not of interest, i.e., surrounding air. On the other hand, to obtain accurate flame parameters from equilibrium calculations, temperature and mole fractions of chemical species are required to be as constant as possible throughout the core flame. Therefore, the flat-flame burner should be well designed to ensure the flame as flat as possible.To produce a flat flame, the burner plug is always ringed with shrouding nitrogen to isolate core flame from air disturbance [4,8]. However, the effects of heat transfer between the core flame and the cold boundary of air are inevitable, which lead to non-uniform distributions of temperature and mole fractions of chemical species in the radial direction. For instance, due to the diffusion of the unburned fuel into surroundings, temperature-uniform area was reduced at sub-atmospheric pressures, and the laminar burning velocities dropped down [9]. In addition, the material of the burner plug plays an important role in affecting the flame flatness. For rich premixed flame produced by the McKenna burner with a stainless steel plug, the flame is not as flat as that with a bronze plug, which is mainly caused by the difference of thermal conductibility for two cases [10]. Therefore, to examine the performance of the Abstract Flame flatness is one of the most critical factors in evaluating the performance of a flat-flame burner. In this paper, the flame flatness of a flat-flame burner is validated using a resolution-doubled one-dimensional wavelength modulation spectroscopy tomography (1D-WMST) technique that only uses one view of multiple parallel laser beams. When the interval of two neighboring parallel laser beams is Δr, a designed novel geometry of the parallel laser beams realizes a doubled tomographic resolution of Δr/2. Using the proposed technique, the distributions of temperature and H 2 O mole fraction in an axisymmetric premixed flame are simultaneously reconstructed and hence the flame flatness of a flat-flame burner can be validated. The flatness factor is quantitatively described by the similarity between the reconstructed and expected distributions of H 2 O mole fraction. For flat and non-flat flames, the experimental results agree well with the CFD simulation results, denoting that the resolution-doubled 1D-WMST technique provides...

A feasibility study is presented and the achieved key design milestones towards the first application of Chemical Species Tomography by Near-IR Absorption Tomography on a heavy duty, large-bore marine engine to visualise relative mixture strength are described. The engine is equipped with pre-chamber ignition and operates using Liquefied Natural Gas with > 88.9 % methane content. Operation of the engine under maximum-load conditions is a key design requirement, with peak cylinder pressure and mean temperature exceeding 127,510 Torr (170 bar) and 850 K respectively. The near-IR spectroscopic behaviour of methane is examined for suitable absorption and reference regions for the above application. In particular, the spectroscopic absorption around the 2ν3 transition region near 1,666 nm is approximated by simulation using data from the HITRAN database under worst-case conditions. The simulation results are compared with methane spectra experimentally acquired at high temperature and ambient pressure. Interference from other chemical species as well as measurement linearity is also investigated. A 31-laser beam tomographic imaging array is proposed, which has been optimised to achieve higher spatial resolution performance in the vicinity of the pre-chamber’s orifices. To enable optical access, a novel, minimally-intrusive method is presented, utilising standard fibreoptics and collimator

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