On the path towards climate-neutral future mobility, the usage of synthetic fuels derived from renewable power sources, so-called e-fuels, will be necessary. Oxygenated e-fuels, which contain oxygen in their chemical structure, not only have the potential to realize a climate-neutral powertrain, but also to burn more cleanly in terms of soot formation. Polyoxymethylene dimethyl ethers (PODE or OMEs) are a frequently discussed representative of such combustibles. However, to operate compression ignition engines with these fuels achieving maximum efficiency and minimum emissions, the physical-chemical behavior of OMEs needs to be understood and quantified. Especially the detailed characterization of physical and chemical properties of the spray is of utmost importance for the optimization of the injection and the mixture formation process. The presented work aimed to develop a comprehensive CFD model to specify the differences between OMEs and dodecane, which served as a reference diesel-like fuel, with regards to spray atomization, mixing and auto-ignition for single- and multi-injection patterns. The simulation results were validated against experimental data from a high-temperature and high-pressure combustion vessel. The sprays’ liquid and vapor phase penetration were measured with Mie-scattering and schlieren-imaging as well as diffuse back illumination and Rayleigh-scattering for both fuels. To characterize the ignition process and the flame propagation, measurements of the OH* chemiluminescence of the flame were carried out. Significant differences in the ignition behavior between OMEs and dodecane could be identified in both experiments and CFD simulations. Liquid penetration as well as flame lift-off length are shown to be consistently longer for OMEs. Zones of high reaction activity differ substantially for the two fuels: Along the spray center axis for OMEs and at the shear boundary layers of fuel and ambient air for dodecane. Additionally, the transient behavior of high temperature reactions for OME is predicted to be much faster.
In order to be able to use the full potential of regenerative fuels, a comprehensive characterization is necessary to identify the differences between conventional fuels and regenerative fuels. In the current work, we compare OME3−5 and 1-Octanol with diesel-like Dodecane in terms of mixture formation under ECN Spray A conditions for single and multi-injection. To determine the mixtures, i.e., the mass distribution and the resulting air-fuel equivalence ratio, Naber and Siebers’ model as well as Musculus and Kattke’s model are used, which are based on experimental data. For this work, the mass flow rates and also the liquid and gaseous penetration depths of the fuel spray are measured. Results show that the mass ratios for the quasi-steady state of a single main injection for all three fuels are nearly the same, whereas the air-fuel equivalence ratios are very different. In addition, multiple injections are used to show that the fuel influences the opening and closing behavior of the injector. In the transient case of multiple injections, completely different mixtures result. In summary, it can be stated that OME3−5 and also 1-Octanol show a clearly different physio-chemical behavior from Dodecane and cannot simply be used as a drop-in fuel. Therefore, a simple exchange is not possible without major adaptations.
Momentum conservation is a principle rule that affects the behaviour of vapour jet and liquid spray penetration. The air entrainment and mixture formation processes are dominated by the momentum transferred from the fuel to the ambient gas. Thus, it is a significant factor in the development of spray and jet penetration. This mixture formation process is well described for small-scale passenger car injectors; however, it has to be investigated in more detail for large-scale injector nozzles. The current work provides qualitative and quantitative results of spray and jet parameters in a constant volume combustion chamber (CVC). Two optical methods have been utilized to evaluate spray and jet details: Schlieren photography as a method to visualize the jet penetration and cone angle as well as Mie scattering for the phase change evaluation and the determination of liquid spray parameters. The temperature and pressure of inert gas and fuel inside a CVC are set to exemplify engine conditions. The chamber temperature is ranged between 873 to 973K, the chamber pressure is increased from 5 to 7 MPa and the injection pressure is changed between 50 to 150 MPa. Four fuels are selected in order to shed light upon the effects of fuel properties on spray and jet parameters. As a part of this evaluation, n-dodecane and two types of bio-diesel fuel generations, RME (1 st Generation) and HVO (2 nd Generation) are dissected to expand their influences on mixture formation, which can be connected to the emission production in diesel engines. Finally, two largescale injector nozzles with an outlet hole diameter of 300 µm (cylindrical and conical nozzles) cover the effects of geometry parameters on spray and jet development. The results accentuate the fact that the liquid spray parameters are effectively fuel-dependent, while total jet parameters are mostly affected by nozzle geometry. Liquid spray length is varied from n-dodecane as a low-boiling fuel to RME as a less-volatile fuel. The conical nozzle results in less cavitation, which is effectively influential on liquid spray and total jet penetrations.
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