Quantitative nitrogen oxide measurements by laser-induced fluorescence in diesel-like n-heptane jets with enhanced premixing

Maintaining low NOx emissions over the operating range of diesel engines continues to be a major issue. However, quantitative measurements of nitric oxide (NO) are lacking especially in the core of diesel jets even if optical measurement techniques like laser-induced fluorescence (LIF) are used. NO may be present in the jet core due to enhanced fuel/air premixing. Close to the flame axis, severe light attenuation occurs. However, the diesel-like jets investigated in this study are largely non-sooting, so that light attenuation is reduced. n-heptane is used as diesel-surrogate fuel in the current investigations, which are performed in a combustion vessel. Quantitative NO measurements are conducted based on the knowledge of attenuation, temperature, the estimated equivalence ratio, and a calibration procedure. Attenuation turns out to be the most important factor in terms of measurement uncertainty. To avoid interfering LIF emissions by PAHs (polycyclic aromatic hydrocarbons) and O2, line imaging with a relatively high spectral resolution is conducted. Thereby, a higher accuracy can be achieved than by state-of-the-art planar imaging set-ups. Results show that NO initially forms throughout the jet cross-section. Quasi-steady NO concentrations measured in the jet core are compared to detailed kinetic simulations, taking the residence time of the fluid parcels in the main reaction zone into account. The residence-time “corrected” NO concentrations seemingly show a significant amount of prompt NO and are essentially consistent with simulation results for a corresponding equivalence ratio that was previously determined by Raman scattering.