Abstract :

Institute of Combustion Technology, RWTH Aachen Mixing processes are central to turbulent combustion, and multi-dimensional measurements of mixing are needed to improve understanding of turbulence-chemistry interactions in turbulent non premixed flames. In non-premixed combustion, the state of mixing between the fuel and oxidizer streams is quantified by the conserved scalar (mixture fraction), and the associated scalar dissipation rate. Laser-based measurements of mixture fraction in flames are a challenge because of the variations in temperature and chemical composition. Previous mixture fraction measurements using Raman Scattering, multi-scalar imaging, polarization/depolarization imaging, etc., have provided immense insights into the chemistry-turbulence interaction; however, these techniques are applicable to relatively narrow range of fuel mixtures, away from the wall, and at non-sooting conditions. Motivated by the need for a technique that does not possess the above drawbacks, we explore the use of an inert gas (krypton) seeded into the fuel stream to make mixture fraction measurements. Simultaneous mixture fraction and temperature imaging was performed in turbulent non-premixed jet flames (TNF DLR-A and -B) by using two-photon PLIF of Krypton and planar Rayleigh scattering, respectively. The motivation is to study the scalar dissipation ( z c ) and thermal dissipation ( T c ) rates characteristics of the flames. The experiments were performed at an axial distance of 10 tube diameters from the jet exit. The mixture fraction field was calculated from the Kr PLIF and the temperature images using a combination of the state relationship and an iterative technique. The mean and RMS radial profiles compared very well with those of the point measurements made by previous researchers. The resolution requirements for the accurate determination of the scalar dissipation were explored by comparing the measured scalar dissipation spectrum with the model spectrum suggested by Pope (2000). The mixture fraction and thermal dissipation field were highly correlated at locations close to the jet center line; this correlation decreased considerably close to the reaction zone. The implication of these results and further application of the Kr-PLIF technique to supersonic flows would also be discussed in the talk.