Two-dimensional tomography for gas concentration and temperature distributions based on tunable diode laser absorption spectroscopy

Abstract

A tomography system is presented that uses wavelength-scanned direct absorption of two transitions of a target species (NH3 in the demonstration experiment) to determine the distributions of gas concentration and temperature. The absorption measurements are performed simultaneously from four platforms that each rotate a beam from a single laser through an 11° arc, acquiring a data set from all four laser platforms in 100 ms to enable observation of dynamic flow events. The laser is wavelength scanned through two absorption transitions with different internal energy producing two sets of equations with species mole fraction and temperature as independent variables. The mole fraction and temperature distributions are reconstructed using the algebraic reconstruction technique (ART) for this set of incomplete projections. A numerical simulation is used to evaluate the measurement accuracy for measurements of an NH3 mixture escaping from an open pipe. This phantom distribution is then realized in the laboratory and the measurement strategy is demonstrated using a tunable diode laser absorption spectroscopy (TDLAS) measurement using a single laser near 1.5 µm to scan adjacent transitions in NH3. The reconstruction of NH3 concentration and gas temperature is compared with independently determined values to illustrate the fidelity of the tomographically reconstructed distributions for the NH3 mole fraction assuming a fixed temperature and for unknown mole fraction and temperature. Potential extensions of this research in the future include evaluation of other reconstruction algorithms and investigation of the dynamic distribution of various gases for combustion diagnostics.