Development of a 2D Temperature Measurement Technique for Combustion Diagnostics using 2-Line Atomic Fluorescence

Sammanfattning: The present thesis is concerned with the development and application of a novel planar laser-induced fluorescence (PLIF) technique for temperature measurements in a variety of combusting flows. Accurate measurement of temperature is an essential task in combustion diagnostics, since temperature is one of the most fundamental quantities for the characterization of combustion processes. The technique is based on two-line atomic fluorescence (TLAF) from small quantities of atomic indium (In) seeded into the fuel. It has been developed from small-scale experiments in laboratory flames to the point where practical combustion systems can be studied. The technique is conceptually simple and reveals temperature information in the post-flame regions. The viability of the technique has been tested in three extreme measurement situations: in spark ignition engine combustion, in ultra-lean combustion situations such as lean burning aero-engine concepts and, finally, in fuel-rich combustion. TLAF was successfully applied in an optical SI engine using isooctane as fuel. The wide temperature sensitivity, 700 – 3000 K, of the technique using indium atoms allowed measurements over the entire combustion cycle in the engine to be performed. In applications in lean combustion a potential problem caused by the strong oxidation processes of indium atoms was encountered. This limits measurement times due to deposits of absorbing indium oxide on measurement windows. The seeding requirement is a disadvantage of the technique and can be a limitation in some applications. The results from experiments performed in sooting flames are very promising for thermometry measurements in such environments. Absorption by hydrocarbons and other native species was found to be negligible. Since low laser energies and low seeding concentrations could be used, the technique did not, unlike most other incoherent optical thermometry techniques, suffer interferences from LII of soot particles or LIF from PAH. Furthermore, the technique is not affected by fluorescence quenching, which is an important feature compared to other techniques, since local quenching rates are difficult to assess, especially in turbulent combustion applications. All these facts make In-TLAF very attractive for thermometry applications in fuel-rich and turbulent combustion processes. In-TLAF may prove to be a good candidate for future measurements in diesel engines where temperature information is of crucial interest. The advantages of and problems associated with the technique are discussed, and critical comparisons with other techniques are made. Imaging processing software has been developed to automate the temperature evaluation process, and problems in assessing errors are discussed.