Measurements of the detailed flame structure in turbulent H 2-Ar jet diffusion flames with Line-Raman/Rayleigh/LIPF-OH technique

Abstract

The instantaneous as well as conditionally averaged flame structure of a 78% H 2 -22% Ar jet diffusion flame is investigated with the Line-Raman/Rayleigh/LIPF-OH technique. Radial profiles of major species concentration, flame temperature, and OH concentration are measured along a line 10,5 mm long at two exit velocities and several axial positions. Statistical distributions of mixture fraction and one-dimensional scalar dissipation rate at different turbulence levels can be obtained to validate current model assumptions. The difference between upstream flamelets and downstream connected reaction zones can be inferred from the simultaneously measured multipoint scalar profiles in the mixture fraction space. Comparison with flamelet calculations shows that in the near field close to the nozzle region, flamelet behavior is qualitatively preserved. At positions farther downstream, the root mean square (rms) of the instantaneous mixture fraction in the mean reaction zone = st becomes smaller than the reaction zone width. Connected reaction zones are therefore a more appropriate conceptual model. Preferential molecular diffusion is found to be unimportant under strong small-scale turbulent mixing inside the jet flame, which justified the Lc =1 assumption for calculation of hydrogen diffusion flamelets at high strain rates. The probability density function of the measured one-dimensional scalar dissipation rates is lognormal at downstream positions but becomes more skewed when moving upstream. The mean scalar dissipation rate scales inversely with the streamwise distance in the axial direction, showing a rather slower decay than that in the cold flows. In addition, a local minimum of the radial scalar dissipation rate near the instantaneous reaction zone position is found at all the measuring stations and can be attributed to the thermal expansion effect and low turbulence level of the surrounding air.