Better understanding of fuel-oxidizer mixing in combustion chambers is a critical step towards the development of highly efficient, low-emission engines for transportation, power generation and propulsion applications. In a wide range of engines such as gas turbines, diesel and gasoline engines, fuel and air mixing primarily controls engine efficiency, combustion emissions, and in extreme cases engine failure. The objective of this research is to investigate the fundamentals of fuel-air mixing using high-speed laser diagnostics that are capable of capturing spatio-temporal dynamics in mixing in the turbulent flows associated with these combustion devices. In particular, we use cutting-edge, ultrashort-pulse (femtosecond-duration) two-photon laser-induced fluorescence (fs-TPLIF) imaging technique for high-speed imaging of characteristic turbulent jets. TPLIF technique, when employed with an inert gas tracer such as Kr can provide images of mixing flow fields of gas jets. Specifically, in the current study, atomic krypton is excited from the 4p~6(~1S_o) state to Sp'[3/2]_2 state by using 204.1-nm radiation, and the fluorescence signal is detected from 5p'[3/2]_2 state to 5s'[3/2]_2 state near 826-nm. The effects of Kr seed concentration, total pressure and laser pulse energy were investigated, as well as the signal dependence on various quenching partners. Furthermore, high-speed imaging measurements of Kr-PLIF show its potential in capturing the spatial and temporal dynamics of mixing process in turbulent flow fields.
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