Characterization of Oxygenated-Fuel Combustion by Quantitative Multiscalar SRS/LIF Measurements in a Diesel-Like Jet

Due to experimental challenges, combustion of diesel-like jets has rarely been characterized by laser-based quantitative multiscalar measurements. In this work, recently developed laser diagnostics for combustion temperature and the concentrations of CO, O₂, and NO are applied to a diesel-like jet, using a highly oxygenated fuel. The diagnostic is based on spontaneous Raman scattering (SRS) and laser-induced fluorescence (LIF) methods. Line imaging yields multiscalar profiles across the jet cross section. Measurements turn out to be particularly accurate, because near-stoichiometric combustion occurs in the central region of the jet. Thereby, experimental cross-influences by light attenuation and interfering emissions are greatly reduced compared to the combustion of conventional, sooting diesel fuel jets. This is achieved by fuel oxygenation and enhanced premixing. The second stage of ignition occurs relatively close to the end of fuel injection, so that the flame is similar to partially premixed, low-load, single-injection, low-temperature combustion (LTC) in heavy-duty engines. Since exhaust-gas recirculation (EGR) is not applied in this work, relatively high NO concentrations (~500 ppm) are found in the downstream jet. The origin of enhanced NO formation is investigated in more detail by the other measured scalars. The local equivalence ratio is estimated based on both the O₂ and CO profiles. Temperature measurements yield further information for the interpretation of NO signals. Rapid leaning caused by the end of injection is indicated by the CO and O₂ data, but the corresponding effect on temperature and NO appears to be weak.