The effect of flame position on the Kelvin-Helmholtz instability in non-premixed jet flames

The influence of heat release and position of a diffusion flame on the characteristics of the Kelvin-Helmholtz (K-H) instability was investigated using a coflowing axisymmetric jet flame configuration. The flame location with respect to the center of the jet shear layer was varied by dilution of the fuel jet and the coflowing oxidizer stream. The investigation focused on two particular fuel mixtures: 100% methane issuing into coflowing air (flame located outside the jet shear layer) and 13% methane/87% nitrogen issuing into coflowing pure oxygen (flame located in the shear layer). The characteristics of these two jet flames were compared with the corresponding non-reacting cases. The flame location was determined with OH planar laser-induced fluorescence, temperature profiles were obtained with a type R thermocouple, and velocity profiles were obtained with laser Doppler anemometry (LDA). A loudspeaker upstream of the jet exit was used to excite K-H waves of different frequencies at low amplitude. Their spatial growth rates in the linear region, phase speeds, and mode shapes were obtained from phase-averaged LDA at different downstream locations. For the experimental parameters investigated, the measurements show that the K-H instability can be completely damped when the flame is located in the jet shear layer. The adequacy of linear stability theory, assuming a locally parallel base flow, for predicting the effect of combustion on the K-H instability; has been assessed, and it has been found that a viscous linear model qualitatively agrees with our experimental data.