Three-dimensional temperature gradients in premixed turbulent flamelets via crossed-plane Rayleigh imaging

A new technique for obtaining instantaneous, high-resolution, three-dimensional thermal structure data from turbulent flames, crossed-plane Rayleigh imaging is described and then demonstrated. Quantitative Rayleigh imaging measurements are made simultaneously in two orthogonal, intersecting laser-sheet illumination planes. At points along the line of intersection of the two laser sheets, instantaneous, three-dimensional temperature gradient data are measured. The technique has higher resolution than parallel plane measurement techniques, which have limited resolution in the direction orthogonal to the parallel planes. The technique is used to measure temperature gradient data for a lean, premixed, methane-air turbulent V-flame with an equivalence ratio of Φ=0.7, and normalized turbulence intensity (u′/SL0)=1, where u′ is the turbulence intensity and SL0 is the unstretched laminar flame speed. Measurements are also presented for a laminar V-flame and a laminar Bunsen flame for comparison. Finally, an unstretched laminar flame calculation is made. Quantitative estimates of the experimental uncertainty are presented. The primary source of uncertainty in the data is due to shot noise. Measured temperature gradient data for laminar flames differ from that of the unstretched laminar flame calculation, especially in the oxidation layer. Turbulent flame temperature gradient data indicate that the turbulent V-flame thermal structure is not significantly perturbed from the measured laminar V-flame structure. For the flame studied, the flamelet approximation is valid if the flamelet used is based on the measured laminar flame structure of the V-flame. Isothermal surface orientation data are presented and are close to parallel for most realizations. Isothermal surface density is calculated from the distribution of isothermal surface orientations and from conditional averages of the magnitude of the temperature gradient. Isothermal surface density does not vary for different isothermal surfaces.