Mixture fraction imaging in turbulent non-premixed flames with two-photon LIF of krypton

The use of a noble gas as an inert tracer for mixing studies in combustion systems is investigated. Simultaneous two-photon laser-induced fluorescence (LIF) of krypton and Rayleigh scattering are used for imaging measurements of mixture fraction and temperature in turbulent non-premixed jet flames. The turbulent flames investigated in this study include a piloted CH4/air flame (Sandia flame D) and a CH4/H2/N2 flame (DLR-B flame). These flames are well-documented in the literature and enable an evaluation of krypton as a tracer in different fuel mixtures with varying degrees of differential diffusion. Krypton is excited from the ground state to the 5p[3/2]2 state using 215 nm laser radiation, and the fluorescence decay to the metastable state, 5s[3/2]2, is detected at 760 nm. Single-shot krypton LIF and Rayleigh scattering images are analyzed in an iterative routine to determine mixture fraction and temperature. Measurements of the temperature- and species-dependent quenching rates for Kr-LIF are incorporated into this routine. The resulting average radial profiles of mixture fraction and temperature for both flames agree well with previously published measurements. The use of a noble gas as a chemically inert tracer has potential applications for mixing studies in a broad range of combustion environments.