Publications

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With aviation’s dependence on the high volumetric energy density offered by liquid fuels, Sustainable Aviation Fuels (SAFs) could offer the fastest path towards 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 manuscript. 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 % iso-paraffins ranging from 9 to 12 carbon atoms (C-4). We also tested a single component normal alkane: n-dodecane, as well as an advanced bio-derived 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 take-off 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.

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Pre-chamber ignition has demonstrated capability to increase internal combustion engine in-cylinder burn rates and enable the use of low engine-out pollutant emission combustion strategies. In the present study, newly designed passive pre-chambers with different nozzle-hole patterns - that featured combinations of radial and axial nozzles - were experimentally investigated in an optically accessible, single-cylinder research engine. The pre-chambers analyzed had a narrow throat geometry to increase the velocity of the ejected jets. In addition to a conventional inductive spark igniter, a nanosecond spark ignition system that promotes faster early burn rates was also investigated. Time-resolved visualization of ignition and combustion processes was accomplished through high-speed hydroxyl radical (OH*) chemiluminescence imaging. Pressure was measured during the engine cycle in both the main chamber and pre-chamber to monitor respective combustion progress. Experimental heat release rates (HRR) calculated from the measured pressure profiles were used as inputs for two different GT-Power 1D simulations to evaluate the pre-chamber jet-exit momentum and penetration distance. The first simulation used both the calculated main-chamber and pre-chamber HRR, while the second used only the main chamber HRR with the pre-chamber HRR modeled. Results show discrepancies between the models mainly in the pressurization of the pre-chamber which in turn affected jet penetration rate and highlights the sensitivity of the simulation results to proper input selection. Experimental results further show increased pressurization, with an associated acceleration of jet penetration, when operating with nanosecond spark ignition systems regardless of the pre-chamber tip geometry used.

The effects of passive pre-chamber (PC) geometry and nozzle pattern as well as the use of either conventional spark or non-equilibrium plasma PC ignition system on knocking events were studied in an optically-accessible single-cylinder gasoline research engine. The equivalence ratio of the charge in the main chamber (MC) was maintained equal to 0.94 at a constant engine speed of 1300 rpm, and at constant engine load of 3.5 bar indicated mean effective pressure for all operating conditions. MC pressure profiles were collected and analyzed to infer the amplitude and the frequency of pressure oscillations that resulted in knocking events. The combustion process in the MC was investigated utilizing high-speed excited methylidyne radical (CH*) chemiluminescence images. The collected results highlighted that PC volume and nozzle pattern substantially affected the knock intensity (KI), while the use of the non-equilibrium plasma ignition system exhibited lower KI compared to PC equipped with a conventional inductive ignition system. It was also identified that knocking events were likely not generated by conventional end gas auto-ignition, but by jet-related phenomena, as well as jet-flame wall quenching. The relation between these phenomena and PC geometry, nozzle pattern, as well as ignition system has been also highlighted and discussed.

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