Scalar dissipation rate measurements in flames: A method to improve spatial resolution by using nitric oxide PLIF

Abstract

Scalar dissipation rate (χ) previously has been identified as one of the most important parameters inturbulent non-premixed flames. A new method to image χ is demonstrated which is a variation of the method of Strner, Bilger, Frank, and Long; nitric oxide (NO) is added to the jet fluid to mark the fuel, replacing previous fuel markers such as acetone. Planar laser-induced fluorescence of NO is combined with temperature images from Rayleigh scattering to measure mixture fraction and scalar dissipation rate with greatly improved signal-to-noise ratio (SNR) and spatial resolution. Calibration measurements of mixture fraction (f) and χ in a laminar flame and qualitative images of a turbulent flame are presented. The method has three advantages. (1) Differential diffusion problems (that have limited the use of acetone) are eliminated, since the diffusivity of NO is equivalent to that of the methane/nitrogen fuel mixture. (2) NO is an excellent marker of the fuel (unlike acetone) since it properly disappears from centerline to the flame boundary due to rapid reactions with H and CH radicals. A small correction (typically 5% of the centerline value) must be applied to account for slight leakage of NO through the CH layer, but this correction can be made entirely from the experimental data. (3) NO can be added in large amounts to achieve ecceptional SNR values exceeding 150 when the spatial resolution equals a typical Taylor scale of 400 μm. Radial profiles of mixture fraction and scalar dissipation rate in the calibration flame compare favorably with previous measurements.