Laserspektroskopische Analyse von selbstzündenden motorischen Einspritzstrahlen alternativer Biokraftstoffe

• Laser spectroscopic analysis of self-igniting engine fuel injection jets of alternative biofuels

Raffius, Thomas Michael; Pischinger, Stefan (Thesis advisor); Grünefeld, Gerd (Thesis advisor)

1. - Aachen : Wissenschaftlicher Verlag Mainz GmbH (2020)
Book, Dissertation / PhD Thesis

In: Aachener Beiträge zur Technischen Thermodynamik 27
Page(s)/Article-Nr.: 1 Online-Ressource (X, 96 Seiten) : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2020

Abstract

Temperature is an important factor in the formation of pollutants during combustion. However, quantitative spatially resolved measurements of temperature and species in combusting diesel jets, especially inside the combustion, are missing. Major problems are light attenuation and interfering light emissions. However, these factors are reduced in non-sooting diesel-like jets, as demonstrated in the present work, because light is not attenuated by soot and interfering LIF (laser-induced fluorescence) from PAHs (polycyclic aromatic hydrocarbons) is substantially lower. The work is divided into three areas. First, the methodology is evaluated on the basis of an almost soot-free fuel. The fuel n-heptane used in the ECN (Engine Combustion Network) is used to test the measurement technique for stronger soot. Last but not least, TMFB (TailorMade Fuels from Biomass) fuels (di-n-butylether (DNBE) and n-octanol and a blend thereof) which, according to earlier engine studies, show a low soot tendency in diesellike combustion are investigated. This finding is not fully understood for pure DNBE as it has a very high cetane number (∼ 100). The current results show that thermometry by SRS (spontaneous Raman scattering) excited by a UV (ultraviolet) laser is therefore feasible even in the core of a non-sooting diesel-like jet in a combustion vessel. Two diagnostic approaches are assessed. The first one is based on the spectral band shape of the Stokes (red-shifted) ro-vibrational SRS from N2, whereas the ratio of integrated ro-vibrational Stokes to anti-Stokes (blue-shifted) N2-SRS bands is exploited in the second one. The data of the second procedure are taken from [2]. It turns out that the first method is advantageous in terms of light attenuation by molecular species, the influence of interfering emissions, and resulting single-shot capability. Furthermore, the recorded spectra indicate that additional quantitative species measurements by SRS are feasible in the non-sooting jet. For instance, the mole fraction of CO is quantified in this work for the first time in the jet core. During the investigation of the fuel n-heptane, effects of the inhomogeneity of the precombustion mixture are observed, especially at lower injection pressure (700 bar). Strong deviations from the adiabatic equilibrium conditions with respect to temperature and CO mole fraction can be observed in particularly fuel-rich packages. The formation of CO there is obviously influenced by turbulent mixing. Relatively low temperatures are measured in the area of the flame lift-off region, especially for the highest injection pressure (1500 bar), which indicates a turbulence-chemistry interaction. For the fuel DNBE, the internal flame structure in particular is analysed by SRS and LIF. This yields information on the local temperature and the concentrations of O2, CO, and polycyclic aromatic hydrocarbons (PAH). A comparison of the O2 quantified in [2, 3] and the qualitative O2-LIF is undertaken. Using O2, CO and PAK, the air input in the inner flame core is evaluated. Thereby, air entrainment into the inner flame core is assessed. Results show that air entrainment is may be particularly strong for pure DNBE, explaining its high soot oxidation rate and overall low sooting tendency. High entrainment is primarily attributed to the low heatrelease 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.

Institutions

• Maßgeschneiderte Kraftstoffe aus Biomasse [080026]
• Chair and Institute of Technical Thermodynamics [412110]