Publications

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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.

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.

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In-cylinder reforming of injected fuel during a negative valve overlap (NVO) recompression period can be used to optimize main-cycle combustion phasing for low-load low-temperature gasoline combustion (LTGC). The objective of this work is to examine the effects of reformate composition on main-cycle engine performance. An alternate-fire sequence was used to generate a common exhaust temperature and composition boundary condition for a cycle-of-interest, with performance metrics measured for these custom cycles. NVO reformate was also separately collected using a dump-valve apparatus and characterized by both gas chromatography (GC) and photoionization mass spectroscopy (PIMS). To facilitate gas sample analysis, sampling experiments were conducted using a five-component gasoline surrogate (iso-octane, n-heptane, ethanol, 1-hexene, and toluene) that matched the molecular composition, 50% boiling point, and ignition characteristics of the research gasoline. For the gasoline, it was found that an advance of the NVO start-of-injection (SOI) led to a corresponding advance in main-period combustion phasing as the combination of longer residence times and lower amounts of liquid spray piston impingement led to a greater degree of fuel decomposition. The effect was more pronounced as the fraction of total fuel injected in the NVO period increased. Main-period combustion phasing was also found to advance as the main-period fueling decreased. Slower kinetics for leaner mixtures were offset by a combination of increased bulk-gas temperature from higher charge specific heat ratios and increased fuel reactivity due to higher charge reformate fractions.

Advanced automotive gasoline engines that leverage a combination of reduced heat transfer, throttling, and mechanical losses; shorter combustion durations; and higher compression and mixture specific heat ratios are needed to meet aggressive DOE VTP fuel economy and pollutant emission targets. Central challenges include poor combustion stability at low-power conditions when large amounts of charge dilution are introduced and high sensitivity of conventional inductive coil ignition systems to elevated charge motion and density for boosted high-load operation. For conventional spark ignited operation, novel low-temperature plasma (LTP) or pre-chamber based ignition systems can improve dilution tolerances while maintaining good performance characteristics at elevated charge densities. Moreover, these igniters can improve the control of advanced compression ignition (ACI) strategies for gasoline at low to moderate loads. The overarching research objective of the Gasoline Combustion Fundamentals project is to investigate phenomenological aspects related to enhanced ignition. The objective is accomplished through targeted experiments performed in a single-cylinder optically accessible research engine or an in-house developed optically accessible spark calorimeter (OASC). In situ optical diagnostics and ex situ gas sampling measurements are performed to elucidate important details of ignition and combustion processes. Measurements are further used to develop and validate complementary high-fidelity ignition simulations. The primary project audience is automotive manufacturers, Tier 1 suppliers, and technology startups—close cooperation has resulted in the development and execution of project objectives that address crucial mid- to long-range research challenges.

In-cylinder reforming of injected fuel during an auxiliary negative valve overlap (NVO) period can be used to optimize main-cycle auto-ignition phasing for low-load Low-Temperature Gasoline Combustion (LTGC), where highly dilute mixtures can lead to poor combustion stability. When mixed with fresh intake charge and fuel, these reformate streams can alter overall charge reactivity characteristics. The central issue remains large parasitic heat losses from the retention and compression of hot exhaust gases along with modest pumping losses that result from mixing hot NVO-period gases with the cooler intake charge. Accurate determination of total cycle energy utilization is complicated by the fact that NVO-period retained fuel energy is consumed during the subsequent main combustion period. For the present study, a full-cycle energy analysis was performed for a single-cylinder research engine undergoing LTGC with varying NVO auxiliary fueling rates and injection timing. A custom alternate-fire sequence with 9 pre-conditioning cycles was used to generate a common exhaust temperature and composition boundary condition for a cycle-of-interest, with performance metrics recorded for each custom cycle. The NVO-period reformate stream and main-period exhaust stream of the cycles-of-interest were separately collected, with sample analysis by gas chromatography used to identify the retained and exhausted fuel energy in the respective periods. To facilitate gas sample analysis, experiments were performed using a 5-component gasoline surrogate (iso-octane, n-heptane, ethanol, 1-hexene, and toluene) that matched the molecular composition, 50% boiling point, and ignition characteristics of a research gasoline. The highest total cycle thermodynamic efficiencies occurred when auxiliary injection timings were early enough to allow sufficient residence time for slow reforming reactions to take place, but late enough to prevent significant fuel spray crevice quench. Increasing the fraction of total fuel energy injected into the NVO-period was also found to increase total cycle thermal efficiencies, in part due to a modest reduction in NVO-period heat loss from a combination of fuel-spray charge cooling and endothermic fuel decomposition by pyrolysis. The effect was most pronounced at the lowest loads where larger charge mass reformate fractions increased overall specific heat ratios and main-period combustion phasing advanced closer to top dead center. These effects improved both expansion efficiency and combustion stability.

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High-voltage (20 kV peak), single-pulse, nanosecond, low-temperature plasma discharges were examined in nitrogen-diluted desiccated air (15.9% oxygen) with addition of 1%, 2%, and 3% carbon dioxide or water for a pin-to-pin electrode configuration in an optically accessible spark calorimeter at elevated density (2.9 kg/m3). Discharge behavior was characterized through pressure-rise calorimetry, direct imaging of excited-state atomic oxygen, and high-speed schlieren. Increasing carbon dioxide or water concentration led to an increased likelihood of surface discharges rather than the desired streamer discharge between the pin electrodes. For streamer discharges, carbon dioxide addition decreased the electrical-to-thermal conversion efficiency, while minimal impact was observed for water. Both carbon dioxide and water addition led to faster pressure rise rates. Carbon dioxide addition decreased excited state atomic oxygen signal, while water addition led to negligible changes. Finally, increased streamer branching was observed in the schlieren images when carbon dioxide or water was added to the gas mixture.

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A computational fluid dynamics (CFD) analysis of a natural gas vehicle experiencing a mechanical failure of a pressure relief device on a full CNG cylinder was completed to determine the resulting amount and location of flammable gas. The resulting overpressure if it were to ignite was also calculated. This study completes what is discussed in Ekoto et al. which covers other related leak scenarios. We are not determining whether or not this is a credible release, rather just showing the result of a possible worst case scenario. The Sandia National Laboratories computational tool Netflow was used to calculate the leak velocity and temperature. The in - house CFD code Fuego was used to determine the flow of the leak into the maintenance garage. A maximum flammable mass of 35 kg collected along the roof of the garage. This would result in an overpressure that could do considerable damage if it were to ignite at the time of this maximum volume. It is up to the code committees to decide whet her this would be a credible leak, but if it were, there should be preventions to keep the flammable mass from igniting.

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For next-generation engines that operate using low-temperature gasoline combustion (LTGC) modes, a major issue remains poor combustion stability at low-loads. Negative valve overlap (NVO) enables enhanced main combustion control through modified valve timings to retain combustion residuals along with a small fuel injection that partially reacts during the recompression. While the thermal effects of NVO fueling on main combustion are well understood, the chemical effects of NVO reactions are less certain, especially oxygen-deficient reactions where fuel pyrolysis dominates. To better understand NVO period chemistry details, comprehensive speciation of engine samples collected at the end of the NVO cycle was performed by photoionization mass spectroscopy (PIMS) using synchrotron generated vacuum-ultraviolet light. Two operating conditions were explored: 1) a fuel lean condition with a short NVO fuel injection and a relatively high amount of excess oxygen in the NVO cycle (7%), and 2) a fuel-rich condition with a longer NVO fuel injection and low amount of NVO-cycle excess oxygen (4%). Samples were collected by a custom dump-valve apparatus from a direct injection, single-cylinder, automotive research engine operating under low-load LTGC and fueled by either isooctane or an 88-octane research certification gasoline. Samples were stored in heated stainless steel cylinders and transported to the Lawrence Berkeley National Laboratory Advanced Light Source for analysis using a Sandia National Laboratories flame sampling apparatus. For all isooctane fueled conditions, NVO cycle sample speciation from the PIMS measurements agreed well with previously reported GC sample measurements if the sum total of all isomer constituents from the PIMS measurements were considered. PIMS data, however, provides richer speciation information that is useful for validation of computational modeling approaches. The PIMS data also revealed that certain species for the GC diagnostic were either misidentified during the calibration process or not identified at all. Examples of unidentified species include several classes of oxygenates (e.g., ketenes, aldehydes, and simple alcohols) and simple aromatics (e.g., benzene and toluene). For the gasoline fueled NVO cycles, performance characteristics were well matched to corresponding isooctane fueled NVO cycles. However, significant PIMS cross-talk from a wide range of gasoline components restricted the sampling analysis to a handful of species. Nonetheless, it was confirmed that for fuel-lean NVO operation there was a comparable increase in acetylene with NVO injection timing retard that is attributed to the prevalence of locally-rich, piston-surface pool fires caused by fuel spray impingement.

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Radiative heat fluxes from small to medium-scale hydrogen jet flames (<10 m) compare favorably to theoretical predictions provided the product species thermal emittance and optical flame thickness are corrected for. However, recent heat flux measurements from two large-scale horizontally orientated hydrogen flames (17.4 and 45.9 m respectively) revealed that current methods underpredicted the flame radiant fraction by 40% or more. Newly developed weighted source flame radiation models have demonstrated substantial improvement in the heat flux predictions, particularly in the near-field, and allow for a sensible way to correct potential ground surface reflective irradiance. These updated methods are still constrained by the fact that the flame is assumed to have a linear trajectory despite buoyancy effects that can result in significant flame deformation. The current paper discusses a method to predict flame centerline trajectories via a one-dimensional flame integral model, which enables optimized placement of source emitters for weighted multi-source heat flux prediction methods. Flame shape prediction from choked releases was evaluated against flame envelope imaging and found to depend heavily on the notional nozzle model formulation used to compute the density weighted effective nozzle diameter. Nonetheless, substantial improvement in the prediction of downstream radiative heat flux values occurred when emitter placement was corrected by the flame integral model, regardless of the notional nozzle model formulation used.

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Sandia National Laboratories has worked with stakeholders and original equipment manufacturers (OEMs) to develop scientific data that can be used to create risk-informed hydrogen codes and standards for the safe operation of indoor hydrogen fuel-cell forklifts. An important issue is the possibility of an accident inside a warehouse or other enclosed space, where a release of hydrogen from the high-pressure gaseous storage tank could occur. For such scenarios, computational fluid dynamics (CFD) simulations have been used to model the release and dispersion of gaseous hydrogen from the vehicle and to study the behavior of the ignitable hydrogen cloud inside the warehouse or enclosure. The overpressure arising as a result of ignition and subsequent deflagration of the hydrogen cloud within the warehouse has been studied for different ignition delay times and ignition locations. Both ventilated and unventilated warehouses have been considered in the analysis. Experiments have been performed in a scaled warehouse test facility and compared with simulations to validate the results of the computational analysis. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights.

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Analytic methods used to establish thermal radiation hazard safety boundaries from ignited hydrogen plumes are based on models previously developed for hydrocarbon jet fires. Radiative heat flux measurements of small- and mediumscale hydrogen jet flames (i.e., visible flame lengths < 10 m) compare favorably to theoretical calculations provided corrections are applied to correct for the product species thermal emittance and the optical flame thickness. Recently, Air Products and Chemicals Inc. commissioned flame radiation measurements from two larger-scale hydrogen jet flames to determine the applicability of current modeling approaches to these larger flames. The horizontally orientated releases were from 20.9 and 50.8 mm ID pipes with a nominal 60 barg source pressure and respective mass flow rates of 1.0 and 7.4 kg/s. Care was taken to ensure no particles were entrained into the flame, either from the internal piping or from the ground below. Radiometers were used to measure radiative heat fluxes at discrete points along the jet flame radial axis. The estimated radiant fraction, defined as the radiative energy escaping relative to chemical energy released, exceeded correlation predictions for both flames. To determine why the deviation existed, an analysis of the data and experimental conditions was performed by Sandia National Laboratories' Hydrogen Safety, Codes and Standards program. Since the releases were choked at the exit, a pseudo source nozzle model was needed to compute flame lengths and residence times, and the results were found to be sensitive to the formulation used. Furthermore, it was thought that ground surface reflection from the concrete pad and steel plates may have contributed to the increased recorded heat flux values. To quantify this impact, a weighted multi source flame radiation model was modified toinclude the influence of planar surface radiation. Model results were compared to lab-scale flames with a steel plate located close to and parallel with the release path. Relative to the flame without a plate, recorded heat flux values were found to increase by up to 50% for certain configurations, and the modified radiation model predicted these heat fluxes to within 10% provided a realistic steel reflectance value (0.8) was used. When the plate was heavily and uniformly oxidized, however, the reflectance was sharply attenuated. Model results that used the surface reflectance correction for the larger-scale flames produced good agreement with the heat flux data from the smaller of the two flames if an estimated reflectance of 0.5 was used, but was unable to fully explain the under predicted heat flux values for the larger flame.Copyright © 2012 by ASME.

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We, the Postdoc Professional Development Program (PD2P) leadership team, wrote these postdoc guidelines to be a starting point for communication between new postdocs, their staff mentors, and their managers. These guidelines detail expectations and responsibilities of the three parties, as well as list relevant contacts. The purpose of the Postdoc Program is to bring in talented, creative people who enrich Sandia's environment by performing innovative R&D, as well as by stimulating intellectual curiosity and learning. Postdocs are temporary employees who come to Sandia for career development and advancement reasons. In general, the postdoc term is 1 year, renewable up to five times for a total of six years. However, center practices may vary; check with your manager. At term, a postdoc may apply for a staff position at Sandia or choose to move to university, industry or another lab. It is our vision that those who leave become long-term collaborators and advocates whose relationships with Sandia have a positive effect upon our national constituency.

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