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Clean-burning oxygenated and synthetic fuels derived from renewable power, so-called e-fuels, are a promising pathway to decarbonize compression–ignition engines. Polyoxymethylene dimethyl ethers (PODEs or OMEs) are one candidate of such fuels with good prospects. Their lack of carbon-to-carbon bonds and high concentration of chemically bound oxygen effectively negate the emergence of polycyclic aromatic hydrocarbons (PAHs) and even their precursors like acetylene (C2H2), enabling soot-free combustion without the soot-NOx trade-off common for diesel engines. The differences in the spray combustion process for OMEs and diesel-like reference fuels like n-dodecane and their potential implications on engine applications include discrepancies in the observed ignition delay, the stabilized flame lift-off location, and significant deviations in high-temperature flame morphology. For CFD simulations, the accurate modeling and prediction of these differences between OMEs and n-dodecane proved challenging. This study investigates the spray combustion process of an OME3 − 5 mixture and n-dodecane with advanced optical diagnostics, Reynolds-Averaged Navier–Stokes (RANS), and Large-Eddy Simulations (LESs) within a constant-volume vessel. Cool-flame and high-temperature combustion were measured simultaneously via high-speed (50 kHz) imaging with formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) representing the former and line-of-sight OH* chemiluminescence the latter. Both RANS and LES simulations accurately describe the cool-flame development process with the formation of CH2O. However, CH2O consumption and the onset of high-temperature reactions, signaled by the rise of OH* levels, show significant deviations between RANS, LES, and experiments as well as between n-dodecane and OME. A focus is set on the quality of the simulated results compared to the experimentally observed spatial distribution of OH*, especially in OME fuel-rich regions. The influence of the turbulence modeling is investigated for the two distinct ambient temperatures of 900 K and 1200 K within the Engine Combustion Network Spray A setup. The capabilities and limitations of the RANS simulations are demonstrated with the initial cool-flame propagation and periodic oscillations of CH2O formation/consumption during the quasi-steady combustion period captured by the LES. [ABSTRACT FROM AUTHOR]

With aviation's dependence on the high volumetric energy density offered by liquid fuels, Sustainable Aviation Fuels (SAFs) could offer the fastest path toward the decarbonization of aircrafts. However, the chemical properties of SAFs present new challenges, and research is needed to better understand their injection, combustion, and emission processes. While efforts such as the United States National Jet Fuel Combustion Program (NJFCP) that investigated several aspects in detail, certain processes were unfortunately beyond the reach of this program. One of them in particular is about droplet evaporation at relevant pressures and temperatures, and this represents the focus of the present paper. To address this gap, we characterized the evaporation and mixing of spray droplets injected into well-controlled thermodynamic environments at conditions relevant to modern and next-generation aero-engine combustors. We tested three fuels from the NJFCP, namely, an average Jet A fuel (A-2), an alcohol-to-jet fuel containing highly branched dodecane and hexadecane type components (C-1), and a blend made of 40% C-1 and 60% isoparaffins ranging from 9 to 12 carbon atoms (C-4). We also tested a single component normal alkane: n-dodecane, as well as an advanced bioderived cyclo-alkane fuel: bicyclohexyl. The time evolution of fuel droplets was monitored using high-speed long-distance microscopy in a specific configuration that enabled sharp images to be acquired at these extreme conditions. The collected images were processed using a purposely developed and trained machine learning (ML) algorithm to detect and characterize the droplets' evaporation regime. The results revealed different evaporation regimes, such as classical and diffusive. In agreement with previous studies, evaporation regimes appear to be controlled by ambient pressure, temperature, and fuel type. The measurements demonstrate that diffusive evaporation is relevant at high-pressure conditions, such as takeoff combustor pressures for modern commercial aircraft engines. However, classical evaporation mostly controls mixing at lower pressure, such as cruise altitude conditions. The ML analysis emphasized that multiple evaporation regimes co-existed at the same operating condition and no significant relationship was found between droplet size and evaporation regime. The findings of this work constitute a database for validating spray and droplet models that are necessary for implementing lower emissions fuels in aero-engines. [ABSTRACT FROM AUTHOR]

This work investigates the low- and high-temperature ignition and combustion processes, applied to the Engine Combustion Network Spray A flame, combining advanced optical diagnostics and large-eddy simulations (LES). Simultaneous high-speed (50 kHz) formaldehyde ( C H 2 O) planar laser-induced fluorescence (PLIF) and line-of-sight OH* chemiluminescence imaging were used to measure the low- and high-temperature flame, during ignition as well as during quasi-steady combustion. While tracking the cool flame at the laser sheet plane, the present experimental setup allows detection of distinct ignition spots and dynamic fluctuations of the lift-off length over time, which overcomes limitations for flame tracking when using schlieren imaging [ Sim et al. Proc. Combust. Inst. 38 (4) (2021) 5713–5721 ]. After significant development to improve LES prediction of the low-and high-temperature flame position, both during the ignition processes and quasi-steady combustion, the simulations were analyzed to gain understanding of the mixture variance and how this variance affects formation/consumption of C H 2 O. Analysis of the high-temperature ignition period shows that a key improvement in the LES is the ability to predict heterogeneous ignition sites, not only in the head of the jet, but in shear layers at the jet edge close to the position where flame lift-off eventually stabilizes. The LES analysis also shows concentrated pockets of C H 2 O , in the center of jet and at 20 mm downstream of the injector (in regions where the equivalence ratio is greater than 6), that are of similar length scale and frequency as the experiment (approximately 5–6 kHz). The periodic oscillation of C H 2 O match the frequency of pressure waves generated during auto-ignition and reflected within the constant-volume vessel throughout injection. The ability of LES to capture the periodic appearance and destruction of C H 2 O is particularly important because these structures travel downstream and become rich premixed flames that affect soot production. [ABSTRACT FROM AUTHOR]

Electronic and optical properties of flame-generated carbon nanoparticles were modeled based on quantum confinement and amorphous semiconductor theory of monodispersed particles. The ionization energies and optical band gaps of polydispersed particles 4–23 nm in volume median diameter (Liu et al., Proc. Natl. Acad. Sci. U.S.A. 116 (2019) 12692–12697) were re-analyzed by deconvolution to obtain particle-size specific properties. The complex refractive index was derived from optical absorption and the Kramers–Krönig relations over the wavelength range of 185–1400 nm. Dependence of the refractive index on the primary particle size is shown for the first time. The real component of the refractive index is weakly dependent on the particle size, but the imaginary component is found to be particularly sensitive to the particle size. The refractive indices of relatively large soot particles are found to be in close agreement with literature values in the visible spectrum; those of smaller particles (〈 ∼ 15 nm in diameter) show a significant difference from the literature values. The current result highlights the need for accounting for the size effect in the refractive index in laser diagnostics of soot in flames. It also suggests that some of the earlier extinction and scattering measurements of flame soot may have to be re-evaluated, especially for lightly sooting flames and during the early stage of soot growth. [ABSTRACT FROM AUTHOR]