Two-Line Atomic Fluorescence for Thermometry in Reactive Flows

Abstract: Advances in the field of laser-based combustion diagnostics over the past decades have allowed for detailed characterisation, modelling and increased understanding of the complex combustion process. However; many combustion phenomena are still unexplained and there is a continued need for development and application of diagnostic tools to further the understanding of the combustion process. One of the governing physical properties in the combustion process is the temperature due to its exponential effect on the chemical reaction rates. Hence, the work reported throughout this thesis deals with the development and extension of a thermometric technique for reactive flows called two-line atomic fluorescence (TLAF). In TLAF an atomic species with a suitable electronic structure, that of a three-level lambda-system, is seeded to the flame and the two lower levels are consecutively probed with light. The ratio of the emitted laser-induced fluorescence intensities is governed by the temperature-dependent Boltzmann distribution and used to infer the temperature of the system. TLAF offers several beneficial features such as being independent on the gas composition, strong fluorescence signals and insensitivity to elastic scattering.The thesis reports on the application of the thermometric technique in a wide range of combustion environments, from low-pressure flat flames, atmospheric jet flames to sooty and particulate laden flames of burning biomass pellets. Two variations of the TLAF technique were performed with external-cavity diode lasers (ECDL): 1) Line shape resolved TLAF where the absorption profile of the two excited levels are recorded as the lasers are tuned and 2) fixed wavelength TLAF where the lasers are stabilized to the peak of the absorption profile. The accuracy and precision, being figures of merit for any quantitative technique, have been measured and estimated for all the applied cases. An accuracy in the order of 2-3% at flame temperatures around 1800 K is typical for the TLAF technique and the precision is for many cases below 1% for averaged measurements. Even with low-power ECDLs imaging and temporally resolved temperature measurements have been demonstrated. A versatile seeding system being able to seed a wide range of burners with an adjustable and constant concentration of the necessary atomic species is also presented.