Spatially Resolved Diagnostics Using Tunable Laser Absorption Spectroscopy

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Accurate measurements of temperature and species concentrations are important for characterization of high temperature gas flows, particularly in the area of combustion. Optical measurements are desirable since they are nonintrusive, nonperturbing, and the probe is not subject to a hostile environment. An absorption measurement provides a simple quantitative definition of the concentration of the absorbing specie along a line-of-sight; this is sometimes adequate information, but more often spatially resolved measurements are needed. In principle, two-dimensional temperature and concentration fields should be recoverable from a limited number of line-of-sight measurements using tomographic reconstruction, a method widely used in medical imaging. However, accurate quantitative reconstructions of temperature and concentration fields have been difficult to achieve because reconstruction algorithms are very sensitive to errors in the input data. The principal objective of this program is to measure temperature and concentration fields simultaneously using a reconstruction technique that exploits the tuning capability of infrared diode lasers to greatly reduce the sensitivity of the reconstruction methods to both random and systematic errors in the input data. The study will begin with reconstruction of radial temperature and concentration profiles (CO and CO2) in an axisymmetric flow, with subsequent extension of the procedures to reconstruction of two-dimensional sections of three-dimensional flows. The application of quantitative laser diagnostics to industrial scenarios has been hampered by the typical high cost of equipment involved and by the user sophistication required for achievement of quantitative results. Success with the these lower-cost, more user-friendly systems may well lead to wider usage in industry; the software and hardware developed should enable implementation of tomographic absorption diagnostic techniques in a wide variety of applications. Potentially, further development of this technology, with use of inexpensive laser diode sources, may allow instrumentation packages cheap and robust enough for use in in-plant process control systems.