Authors: Ronald K. Hanson
Source:  (article, 2023-06-26)
Field: Chemistry — Spectroscopy

Laser-Based Sensing for Energy and Propulsion Sciences In this lecture I review several laser-based sensor and spectroscopic techniques developed at Stanford for the advancement of energy and propulsion technologies. These techniques are largely based on absorption or fluorescence methodologies, and make use of lasers throughout the ultraviolet, visible and infrared regions of the optical spectrum. These methods all require knowledge of the absorption spectrum of relevant species, over a range of conditions, often including high temperatures. Hence a key component of our work has been to build a database for spectrally resolved absorption of numerous species that impact modern variants of energy conversion and propulsion. Such data at high temperatures is particularly lacking and yet critically needed in application of laser diagnostics to make quantitative measurements and thereby aid development of new and improved energy and propulsion systems. This lecture will review some of the experimental methods developed at Stanford and provide examples of the fundamental data obtained, particularly absorption cross-section and spectral lineshapes. A particular strength of this work has been the application of wavelength-tunable laser sources to probe samples of gases over a very wide range of well-controlled temperature (300-12,000K) produced in a shock tube. The diagnostic techniques enabled by such spectral information will be highlighted. Example diagnostic methods include tunable-wavelength absorption spectroscopy, using a variety of laser sources that allow application in the ultraviolet for rovibronic transitions, the visible and near-infrared for rovibronic and rovibrational transitions, and the infrared for rovibrational transitions. Results have been obtained for a variety of species present in energy conversion and propulsion systems. With regard to energy systems, emphasis has been placed on building chemical kinetic models for reactive systems, such as hydrocarbon pyrolysis and oxidation, needed for quantitative modeling of new energy systems, including transformation of methane and ammonia to hydrogen. The diagnostic targets in such work include time-resolved speciation and thermometry, which can be measured with high accuracy using the methods developed at Stanford. In the area of propulsion, new techniques have been developed to probe a multiplicity of key properties, including time-resolved temperature, pressure, species concentrations, velocity and mass fluxes, all of which are needed for evaluation of new propulsion system concepts and characterization of propulsion performance. Highlights include measurement in rotating detonation engines and in modern internal combustion engines. Some of these same diagnostic methods are now being applied in fundamental studies of supersonic and hypersonic systems, with flow velocities up to 5 km/sec. Lastly I plan to present new work on the use of infrared spectroscopy to infer the physical and combustion properties of new fuels, such as biofuels and sustainable aviation fuels, from machine-learning interpretation of the absorption spectra of these fuels. Rotational–vibrational spectroscopy ; Laser ; Absorption (acoustics) ; Infrared ; Ultraviolet ; Materials science ; Spectroscopy ; Propulsion ; Absorption spectroscopy ; Optics ; Optoelectronics ; Aerospace engineering ; Physics ; Spectral line ; Engineering Spectroscopy and Laser Applications ; Laser Design and Applications ; Atmospheric Ozone and Climate