Numerical and Experimental Investigations on the Ignition Behavior of OME

Abstract

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.

LES and RANS Spray Combustion Analysis of OME3-5 and n-Dodecane

“…After analyzing the global combustion behavior for both fuels and operating points, the spatial distribution of the high-temperature flame morphology will be discussed in detail. The results in Figures 5 and 6 The comparison between the two fuels for OP1 (900 K) in Figure 5 demonstrates significant differences in the spatial distribution of the high-temperature reaction zone, already observed in [17,18]. For n-dodecane (Figure 5a), the highest intensity is measured and simulated in the shear layer of spray and ambient air, which are also the locations of the first ignition kernels.…”

“…In that case, only five frame counts were set as thresholds. As the liquid phase of OME penetrates further into the combustion chamber than n-dodecane, as shown in [17], part of the CH 2 O formation when using OME as fuel at the 1200 K operating point cannot be captured experimentally. For both fuels, the flame stabilization in Figure 4b occurs upstream of the maximum concentration of CH 2 O, which differs from the 900 K case.…”

“…Figure 2 visualizes the spray-box mesh with its refinement areas and a cell count of 966,000. The mesh and the models for the liquid and gaseous phase for the RANS calculations were validated extensively in [17,18]. The validation included a grid independence study, which was conducted for non-reacting as well as reacting conditions.…”

“…All fuels proved to have very similar mass distributions, and the differences in equivalence ratio distributions were solely a consequence of the different air requirements for stoichiometric mixing of the oxygenated fuels. Non-reacting as well as ignition characteristics of the OME 3−5 fuel were studied in [17] within a constant-pressure vessel experimentally and numerically, employing Reynolds-Averaged Navier-Stokes (RANS) calculations. Significant differences to n-dodecane were reported concerning the distribution of high-temperature reaction zones characterized by OH* chemiluminescence.…”

A CFD Modelling Approach of Fuel Spray under Initial Non-Reactive Conditions in an Optical Engine

“…Desantes et al [25] proposed a combined phenomenological and CFD model to characterise diesel spray under evaporative conditions. Other work focused on studying the flame lift-off length using CFD models [26,27], while others studied the fuel spray under reactive conditions [28]. But there are few studies of fuel spray under non-reactive but evaporative conditions [29].…”