[en] A new approach for quantitative mixture fraction imaging in the turbulent mixing of a fuel jet and a high temperature oxidizer co-flow was developed by means of planar laser-induced fluorescence of nitric oxide (NO-PLIF). Unlike existing strategies, the new approach is based on seeding NO in the oxidizer stream. The method was first evaluated during the laminar mixing of methane in air and ${\text{CH}}_4/{\text{CO}}_2$ blends (synthetic biogas) in air, at room temperature. Mixture fraction measurements were validated against Rayleigh scattering imaging. Then, the measurements were extended to the turbulent mixing of a cold fuel jet issuing into a NO-seeded, hot air co-flow. By seeding the NO in the air stream, high signal-to-noise ratios were achieved at locations around the stoichiometric mixture fraction. Additionally, a stable in situ calibration region is available at every measurement location, which contributes to reduce the uncertainty. Results showed that the approach is not suitable for CH${}_4$/air mixtures due to the lack of sensitivity of the fluorescence signal to the mixture fraction. For biogas/air mixtures, the addition of CO${}_2$ increased the response of the fluorescence signal to the mixture fraction, making the measurements feasible. The stoichiometric mixture fraction can be fully resolved for each biogas blend. Two-dimensional instantaneous mixture fraction measurements were feasible with an acceptable uncertainty. The high spatial resolution of the measurement was of the same order of the smallest scale in the concentration field (i.e., Batchelor scale). Graphical abstract:

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