Laser spectroscopic investigation of diesel-like jet structure using C8 oxygenates as the fuel

Abstract

Di-n-butyl ether (DNBE) and n-octanol have very low sooting tendencies in diesel-like combustion, as demonstrated in previous engine studies. This finding is not fully understood for pure DNBE, because it has a very high cetane rating (∼100). In order to investigate the underlying mechanisms, the structure of diesel-type jets is analyzed by a number of optical diagnostics, such as spontaneous Raman scattering (SRS), laser-induced fluorescence (LIF), OH* luminescence imaging, Mie scattering, and shadowgraphy. Pure DNBE and a tailor-made blend of 50% DNBE and 50% n-octanol as well as neat n-heptane are used as the fuel in separate experiments. The jets are probed in a simulated engine-like environment in a high-pressure combustion vessel. In particular, the inner flame structure is analyzed by SRS and LIF. This yields information on the local temperature and the concentrations of O2, CO, and polycyclic aromatic hydrocarbons (PAH). For the first time, O2 is quantitatively detected in the core of a diesel-like flame by resonance-enhanced SRS. Thereby, air entrainment into the inner flame core is assessed. Results show that air entrainment is particularly strong for pure DNBE, explaining its high soot oxidation rate and overall low sooting tendency. High entrainment is primarily attributed to the low heat-release rate of DNBE, which is likely an effect of its high ignitability. Thus, it can be concluded that the high cetane rating of pure DNBE does not only lead to relatively poor pre-combustion mixture preparation and consequently considerable soot formation but seemingly also to particularly strong soot oxidation. Moreover, the jet structure turns out to be very similar for the DNBE/n-octanol blend and neat n-heptane, indicating that the net effect of volatility and fuel oxygenation is weak.