NOx formation in methyl ester, alcohol, and alkane droplet autoignition and combustion: PLIF measurements and detailed kinetic modeling

Abstract

Numerous studies have shown that diesel engines fueled by fatty-acid methyl ester biodiesel often exhibit slightly increased production of oxides of nitrogen (NOx) in comparison to petroleum diesel. A number of explanations for this increase have been proposed. One theory, which has been supported by optical engine test results, suggests that the presence of oxygen atoms in the methyl ester fuel molecule results in a leaner premixed autoignition zone, thereby increasing in-cylinder temperatures and promoting Zel'dovich NOx production. Other experiments have suggested that the unsaturated methyl esters in biodiesel cause an increase in CH radical production (and/or other potential precursors such as C2O) which in turn increases Fenimore NOx formation. In this work, these hypotheses are explored experimentally and computationally by considering autoignition and combustion of single, isolated methyl ester, alcohol and alkane droplets. Experiments were conducted in which the planar laser-induced fluorescence (PLIF) spectroscopy technique was applied to burning liquid fuel droplets in free-fall. A monodisperse stream of droplets was generated by a piezoelectric device and passed through a resistively heated ignition coil. A pulsed laser beam from a Nd:YAG-pumped dye laser (10 Hz, 10 ns width) was formed into a sheet and passed through the droplet flame. The dye laser was tuned to excite hydroxyl (OH) at 282.9 nm and nitric oxide (NO) at 226.0 nm. The resulting fluorescence was imaged by a Cooke Corporation DiCam Pro ICCD digital camera. Band pass filters were utilized to reject laser light scattering while admitting fluorescence wavelengths. Due to the small fluorescence signal, many fluorescence images were averaged together to create a useful average image; approximately 250 and 1000 images were averaged for OH and NO spectroscopy, respectively. Finally, pixel intensity of the averaged fluorescence image was integrated about the droplet center to create qualitative radial profiles of OH and NO concentration. Profiles were generated for a number of oxygenated fuels and one pure hydrocarbon: methanol, ethanol, 1-propanol, methyl butanoate, methyl decanoate, and n-heptane. To quantitatively interpret the contribution of Zel'dovich and Fenimore NOx mechanisms on NOx formation in the vicinity of igniting liquid droplets, detailed numerical droplet combustion simulations were conducted. The transient, spherically symmetric droplet combustion modeling featured detailed gas-phase kinetics, spectrally resolved radiant heat transfer, and multicomponent gas transport. New chemical kinetic mechanisms were created by appending NOx chemical kinetics to existing detailed methanol, methyl butanoate, and n-heptane mechanisms. In the computations, non-oxygenated (heptane) and oxygenated (methyl butanoate, methanol) fuel droplets are introduced into a hot (1150 K) air ambient whereupon the liquid vaporizes, thus producing a stratified fuel/air mixture that thermally autoignites after an ignition delay period. The computational results suggest that NOx formation in stratified fuel/air mixture in the vicinity of a cold liquid droplet is influenced greatly by the detailed full NOx chemistry (Fenimore, Zel'dovich and N2O) and cannot be fully explained by considering only the Zel'dovich NOx route. The computations also suggest, however, that the stoichiometry of the premixed autoignition zone in the laminar gas phase surrounding a spherical droplet differs from that observed in turbulent diesel spray ignition. In single droplets, irrespective of the fuel used, autoignition always initiates in the relatively hot lean region far from the droplet. In diesel sprays, depending on the thermodynamic conditions and fuel reactivity, ignition can occur in lean or rich regions by virtue of turbulent transport of heat and mass. In large molecular weight fuels like n-heptane or petroleum diesel fuel, this is often in mixtures which are quite rich (Φ ≈ 3). To underscore the difference between turbulent spray ignition and ignition of a single droplet, the most reactive mixture fraction and critical scalar dissipation rate were derived for the case of turbulent ignition The results show that for a turbulent non-premixed flame to ignite, two requirements must be met: (1) the fuel/air mixture fraction must be equal or similar to the most reactive mixture fraction, (2) the local scalar dissipation rate must be less than the critical scalar dissipation rate. Due to the effect of scalar dissipation rate on transport and mixing in turbulent, non-premixed flames, it is concluded that, at least as far as autoignition is concerned, autoignition of spherically-symmetric isolated fuel droplets has limitations as physical model for ignition of diesel sprays in compression ignition engines. However, the computations clearly show that transient NOx formation in presence of thermal and concentration gradients cannot be adequately described by the Zel'dovich NOx mechanism, which has consequences with regards to capability of computational engine simulations to accurately predict NOx formation.