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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Revealing the interplay between “intelligent behavior” and surface reconstruction of non-precious metal doped SrTiO 3 catalysts during methane combustion

The impact of surface reconstruction of a model perovskite, SrTiO 3 (STO), on CH 4 activation for combustion and oxidative coupling was previously revealed that the reaction rate was proportional to the creation of Srterminated step sites. Doped perovskites (SrTi 1-x M x O 3 , M=metal dopant) present yet another form of reconstruction throughout the surface and the bulk, where the metal dopant can migrate in and out of the perovskite lattice, also known as "intelligent behavior". In this work, understanding the interplay between perovskite surface reconstruction (surface termination) and the "intelligent behavior" is tackled for the first time, and the catalytic consequences are probed with CH 4 combustion as a model reaction. A set of experimental techniques, including XRD, Raman spectroscopy, X-ray adsorption spectroscopy, kinetic measurements, as well as DFT calculations were used to understand the catalytic behavior of the reconstructed surfaces of Ni and Cu-doped STO for methane combustion. Here, we found that during methane oxidation, the diffusion of Ni and Cu into the lattice due to the "intelligent behavior" is accompanied by Sr enrichment on the surface of the perovskite. This Srenrichment process is reversible when Cu or Ni species exsolute as clusters/nanoparticles upon H 2 treatment. Such a surface reconstruction is found to greatly impact the catalytic activity of doped perovskites towards methane combustion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

The role of spray-enhanced swirl flow for combustion stabilization in a stratified-charge DISI engine

Implementing spray-guided stratified-charge direct-injection spark-ignited (DISI) engines is inhibited by the occurrence of misfire and partial burns. Engine-performance tests demonstrate that increasing engine speed induces combustion instability, but this deterioration can be prevented by generating swirling flow during the intake stroke. In-cylinder pressure-based heat-release analysis reveals that the appearance of poor-burn cycles is not solely dependent on the variability of early flame-kernel growth. Moreover, cycles can experience burning-rate regression during later combustion stages and may or may not recover before the end of the cycle. Thermodynamic analysis and optical diagnostics are used here to clarify why swirl improves the combustion repeatability from cycle to cycle. The fluid dynamics of swirl/spray interaction was previously demonstrated using high-speed PIV measurements of in-cylinder motored flow. It was found that the sprays of the multi-hole injector redistribute the intake-generated swirl flow momentum, thereby creating a better-centered higher angular-momentum vortex with reduced variability. The engine operation with high swirl was found to have significant improvement in cycle-to-cycle variations of both flow pattern and flow momentum. This paper is an extension of the previous work. Here, PIV measurements and flame imaging are applied to fired operation for studying how the swirl flow affects variability of ignition and subsequent combustion phases. PIV results for fired operation are consistent with the measurements made of motored flow. They demonstrate that the spark-plasma motion is highly correlated with the direction of the gas flow in the vicinity of the spark-plug gap. Without swirl, the plasma is randomly stretched towards either side of the spark plug, causing variability in the ignition of the two spray plumes that are straddling the spark plug. Conversely, swirl flow always convects the spark plasma towards one spray plume, causing a more repeatable ignition. The swirl decreases local RMS velocity, consistent with an observed reduction of early-burn variability. Broadband flame imaging demonstrates that with swirl, the flame consistently propagates in multiple directions to consume fuel–air mixtures within the piston bowl. In contrast, operation without swirl displays higher variability of flame-spread patterns, occasionally causing the appearance of partial-burn cycles.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2020

Autoignition of CRC diesel surrogates at low temperature combustion conditions: Rapid compression machine experiments and modeling

As federal programs require increasingly stringent engine emissions and fuel economy standards, these ambitions can only be met if next-generation combustion technology is developed focusing on high-efficiency and low-emissions engines. Recent research has indicated the need to operate engines at higher compression ratios and with low temperature combustion (LTC) to achieve the needed gains in engine efficiency and reductions in emissions. Because there is a lack of understanding of the chemistry of diesel fuel components and their mixtures at these LTC conditions, this limits the ability to develop predictive chemical kinetic models that can be used to optimize engine combustion. The current study aims to fill in gaps in fundamental combustion data on surrogate fuel mixtures relevant to diesel fuels. Specifically, four multicomponent diesel surrogates formulated by the Coordinating Research Council (CRC) to emulate an ultra-low-sulfur research-grade #2 certification diesel fuel (CFA), namely V0a (4 components), V0b (5 components), V1 (8 components), and V2 (9 components), have been investigated in a rapid compression machine (RCM) through determination of total and first-stage ignition delay times. Autoignition characteristics of lean to rich fuel/O 2 /N 2 mixtures, for the four CRC surrogates and CFA, have been measured using an RCM at LTC relevant pressures and temperatures, in the ranges of 10–20 bar and 650–1000 K, respectively. The equivalence ratios have been varied by independently changing the oxygen mole fraction and the fuel mole fraction in the test mixtures, thereby illustrating the individual effects of oxygen concentration and fuel loading on diesel autoignition. Autoignition results of these four CRC surrogates are compared among them and with those of CFA. Some degree of agreement in autoignition response between each CRC surrogate and CFA is observed, while discrepancies are also identified and discussed. In addition, a detailed chemical kinetic model for diesel surrogates has been developed and validated against these newly-acquired RCM data. We find that this model shows reasonable agreement with the overall ignition delay time results of the current RCM experiments. Chemical kinetic analyses of the developed model were further conducted to help identify the reactions controlling the autoignition processes and the consumption of fuel components in CRC surrogates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Automatically generated model for light alkene combustion

Light alkenes are common combustion intermediates for a variety of fuels. Therefore, understanding their oxidation and pyrolysis chemistry is key to building detailed mechanisms for heavier fuels. This work was focused on the development and evaluation of a detailed kinetic mechanism suitable for the combustion of light alkenes up to C 4 without the use of tuned parameters, instead, the parameter values come from first principles or direct measurements. The generated mechanism accurately estimates the laminar burning velocity (S u ) and ignition delay time (IDT) of light alkenes available in the literature, which represent fundamental combustion properties at a wide range of conditions. Because each parameter is thought to have a physically realistic value, not tuned to these measurements, the new model could be used as a sub-mechanism in models for other applications. The reaction network was generated with the open-source Reaction Mechanism Generator (RMG) software. Sensitivity analyses were performed under wide ranges of temperatures and pressures, allowing for the identification of the most impactful species and reactions. Based on these, a comprehensive thermochemistry database, including calculations on 550 molecules performed in this work at the CBS-QB3 level of theory, and a kinetic library, including theoretically-derived reaction rates retrieved from the literature, were built and used in the mechanism generation. The developed mechanism was compared against several existing detailed kinetic mechanisms for ethene, propene, 1-butene, 2-butene, and isobutene. Here, the newly generated model is the most accurate among the ones analyzed, in terms of fractional bias and normalized mean square error. Hence, this new model was used to analyze the chemistry of alkene combustion. Key rate coefficients were compared, to identify the cause of deviations between the models and possible areas for further improvements.

09 BIOMASS FUELS↗
DOE OSTI2018

Application of Bound-to-Bound Data Collaboration approach for development and uncertainty quantification of a reduced char combustion model

The development of efficient industrial oxy-coal boilers can be significantly aided by Computational Fluid Dynamics (CFD) tools, as far as fidelity in modeling coal combustion is also complemented by feasible computational costs. Reduced and predictive models are the most suitable for this application scale. Reduced models feature predictivity when they are validated against a broad range of experiments and targeted by Uncertainty Quantification (UQ) procedures. This work proposes a numerical procedure that uses Bound-to-Bound Data Collaboration (B2B-DC) to derive a reduced char combustion model describing transport phenomena and reactions between char carbon and O 2 , CO 2 and H 2 O, in both conventional and oxy-conditions. The approach determines the consistency between a numerical model and an experimental dataset. The latter is made up of the experiments carried out in an optically accessible laminar entrained flow reactor, operated by Sandia National Laboratories. The procedure follows five steps towards predictive modeling capability, namely: quantification of the uncertainty in the experiments, via instrument verification and modeling; development of a physics model and continuous improvement of its fidelity, via model-form uncertainty; identification of the uncertain and most sensitive parameters and of their prior bounds; sampling of the initial uncertain parameter space and training of a surrogate model; validation of the physics model via inference from the data. The last step, also known as inverse problem, is performed by applying the Bound-to-Bound Data Collaboration approach. A char combustion model is found consistent with the experimental data and its validity stands for conventional and oxy-combustion conditions. It accounts for heterogeneous reactions at the particle surface, mass transport of species in the particle boundary layer, pore diffusion and surface area changes. The consistent reduced model overcomes the differences in mass transport and kinetics observed in the experimental campaign. In conclusion, a reduction of the initial degree of uncertainty in both model and experiments is achieved.

42 ENGINEERING↗
DOE OSTI2019

Role of fuel properties and piston shape in influencing soot oxidation in heavy-duty low swirl diesel engine combustion

The presented study evaluated effects on soot emissions of important diesel fuel properties (e.g. volatility T95, hydrogen to-carbon ratio H/C and oxygen content O/C) in a heavy-duty, low-swirl, direct injection diesel engine operated with cooled exhaust gas recirculation to control the emissions of nitrogen oxides. Variation of injection pressure was conducted in three representative operation points in the single-cylinder engine including conventional and wave piston bowl types. Additionally, detailed optical combustion studies were performed in a high pressure/high temperature combustion chamber with an engine-like flame-wall interaction geometry. The fuel properties mainly influenced parameters relating to events near the nozzle, such as the ignition delay, flame lift-off distance and spray air entrainment. Oxygenation of the fuel reduced soot formation (a known effect of spray-jet dilution), but the fuels’ H/C and T95 also influenced soot emissions. Viscosity, heating value and density, treated as secondary fuel parameters, affected the injected fuel’s kinetic energy, and hence mean flow field and local turbulent mixing late in the combustion cycle. The results suggested that fuel effects on soot oxidation during late phases of combustion are mainly governed by parameters related to the fuel injection profile under the specific cylinder gas conditions. With an oxygenated fuel, flow structures (local turbulent side vortices) which are typical for the investigated engine type were found to form in a similar way as with a reference fuel. Hence, improved internal flow using a wave piston in combination with a low-sooting oxygenated fuel gave additional reductions in net soot emissions.

42 ENGINEERING↗
DOE OSTI2020

Assessment of various tar and soot treatment methods and a priori analysis of the steady laminar flamelet model for use in coal combustion simulation

Soot formed in the fuel-rich regions of coal flames is known to have a significant impact on the radiative heat transfer characteristics of the flame. It is therefore important to account for the effect of soot radiation when simulating coal combustion systems. Several recent coal combustion studies have used acetylene as a surrogate for devolatilized tar and neglect soot formation altogether, while others have utilized empirical models for tar and soot or even chemical mechanisms which include tar and soot as species. In this work, we investigate several methods of treating coal-derived tar and soot and the impact each treatment has on predicted furnace temperature, particle mass loss, and other quantities of interest. Furthermore, we perform a priori comparisons of simulation results to calculations from a steady laminar flamelet model in order to assess the accuracy of a flamelet approach for each tar and soot treatment considered. Data used as a basis for the study is generated through One-Dimensional Turbulence (ODT) simulation of a coal combustion system in which high- fidelity models are used for gas phase chemistry and devolatilization, where char combustion is neglected.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2021

Numerical analysis of particle dispersion and deposition in coal combustion using large-eddy simulation

High-fidelity modeling provides a useful approach to investigate the particle dispersion and deposition mechanism in pulverized coal combustion. To be able to analyze these detailed mechanisms, this work couples detailed multiphysics models for poly-dispersed turbulent, particle-laden flow, particle and gas phase reaction chemistry, convective, conductive and radiative heat transfer, ash formation and deposition with a Large-Eddy Simulation (LES) approach to numerically simulate coal combustion in a downfired self-sustained oxy-fuel combustor (OFC). It is necessary to explicitly capture all but the highest frequency dynamics of the turbulence and its coupling with each of the other physical phenomena using LES. Effects of subgrid-scale unresolved turbulence on the particle motions are also analyzed by a novel Stokes number analysis. Due to the inherent relation ship between coal combustion and ash deposition, this study integrates the improved ash deposition model into the numerical simulation. Overall simulation results are compared with experimentally measured data from the OFC. The simulation and measured data for the averaged gas temperature and deposition rates agree with 5% and 28%. This study shows that high-fidelity LES coupled with other detailed multiphysics models running on a exascale computing facility can provide a good representation of complex coal combustion and deposition in a laboratory-scale furnace.

multiphysics model↗
DOE OSTI2020

Effects of Zr substitution on soot combustion over cubic fluorite-structured nanoceria: Soot-ceria contact and interfacial oxygen evolution

Ceria is widely used as a catalyst for soot combustion, but effects of Zr substitution on the reaction mechanism is ambiguous. The present work elucidates effects of Zr substitution on soot combustion over cubic fluorite-structured nanoceria. The nanostructured CeO 2 , Ce 0.92 Zr 0.08 O 2 , and Ce0.84Zr0.16O2 composed of 5–6 nm crystallites display Tm-CO 2 (the temperature at maximum CO 2 yield) at 383, 355, and 375°C under 10 vol.% O 2 /N 2 , respectively. Additionally, the size of agglomerate decreases from 165.5 to 51.9–57.3 nm, which is beneficial for the soot-ceria contact. Moreover, Zr increases the amount of surface oxygen vacancies, generating more active oxygen (O2- and O-) for soot oxidation. Thus, the activities of Ce 0.92 Zr 0.08 O 2 and Ce 0.84 Zr 0.16 O 2 in soot combustion are better than that of CeO2. Although oxygen vacancies promote the migration of lattice O 2- , the enriched surface Zr also inhibits the mobility of lattice O 2- . Therefore, the Tm-CO 2 of Ce 0.84 Zr 0.16 O 2 is higher than that of Ce 0.92 Zr 0.08 O 2 . Based on reaction kinetic study, soot in direct contact with ceria preferentially decomposes with low activation energy, while the oxidation of isolated soot occurs through diffusion with high activation energy. The obtained findings provide new understanding on the soot combustion over nanoceria.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2017

One-step solution combustion synthesis of pure Ni nanopowders with enhanced coercivity: The fuel effect

In this paper, we report a new modified one-step combustion synthesis technique for production of Ni metal nanoparticles. The main unique feature of our approach is the use of microwave assisted foam preparation. Also, the effect of different types of fuels (urea, citric acid, glycine and hexamethylenetetramine) on the combustion process and characteristics of resultant solid products were investigated. It is observed that the combination of microwave assisted foam preparation and using of hexamethylenetetramine as a fuel allows producing pure ferromagnetic Ni metal nanoparticles with enhanced coercivity (78 Oe) and high value of saturation magnetization (52 emu/g) by one-step solution combustion synthesis under normal air atmosphere without any post-reduction processing. - Highlights: • The new approach in one-step SCS rote for Ni nanoparticles preparation is proposed. • Ni particles were synthesized with microwave-assistance and under air atmosphere. • The effect of different type of fuels on the combustion process was investigated. • Using of hexamethylenetetramine allows obtaining pure Ni nanoparticles. • Ni nanoparticles have enhanced coercivity and high saturation magnetization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

On the formation of Mo{sub 2}C nanocrystals by a novel system through microwave assisted combustion synthesis

This research is devoted to microwave assisted combustion synthesis of Mo{sub 2}C nanoparticles. The ternary system of MoO{sub 3}–Zn–C was used as a novel approach for the in-situ synthesis of Mo{sub 2}C in which the zincothermic reduction of MoO{sub 3} was responsible for the combustion to take place. Results showed that the formation of Mo{sub 2}C was assisted by the zincothermic reaction, although further microwave heating up to 6 min was necessary to complete the reaction. The effects of the microwave heating and mechanical activation on the reaction progress were investigated. X-ray powder diffraction was used to examine the synthesis progress. Final products (Mo{sub 2}C and ZnO) were successfully separated from each other and the synthesized carbide was characterized by transmission electron microscopy (TEM), showing the formation of Mo{sub 2}C hexagonal nanocrystals during combustion process. - Highlights: • Hexagonal β-Mo{sub 2}C nanocrystals were successfully synthesized. • MoO{sub 3}–Zn–C powder mixture was selected as the initial mixture for the in-situ synthesis of Mo{sub 2}C. • 30 min of mechanical activation was necessary for the carbide formation to be completed. • The zincothermic reduction of MoO{sub 3} by Zn was responsible for the combustion. • The final products included of Mo{sub 2}C and ZnO in which ZnO was removed by acid leaching.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Combustion synthesized CeO{sub 2} as an anodic material in dye sensitized solar cells

Highlights: • One-step combustion synthesis of CeO{sub 2}. • Application of synthesized material as an anodic material for DSSC. • DSSC using CeO{sub 2} displayed an efficiency of 1.84%. - Abstract: Cerium oxide (CeO{sub 2}) has received a considerable attention due to its capacity to absorb visible light, low cost, high stability and environmental friendly catalytic applications. Among the various techniques employed to synthesize CeO{sub 2}, solution combustion synthesis is a one-step, economic and effective process. In this work, we report application of combustion synthesized CeO{sub 2} as an anodic material in the dye sensitized solar cell (DSSC). The synthesized material were characterized using standard analytical techniques such as XRD, SEM, BET, UV-DRS, and TEM. DSSC was fabricated using the synthesized material and characterized for incident photon to current conversion efficiency (IPCE), current density-voltage (J-V) measurements and electrochemical impedance spectroscopy. The power conversion efficiency of solar cell was found to be 1.84%. This is the first study to report use of combustion synthesized CeO{sub 2} for DSSC application.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of propylene glycol/nitrates ratio on microwave-assisted combustion synthesis of CuO-ZnO-Al2O3 nanocatalyst: Structural and catalytic properties toward hydrogen production from methanol

Highlights: • Applicability of propylene glycol in synthesis of CuO/ZnO/Al{sub 2}O{sub 3} via combustion method. • Addressing the influence of fuel to nitrates ratio on catalytic properties and performance. • Synthesis of stable and active CuO/ZnO/Al{sub 2}O{sub 3} nanocatalyst in methanol to hydrogen. • Achievement of complete conversion at 260 °C by choosing appropriate amount of fuel. • Decrease of CO production by applying suitable fuel/nitrates ratio. - Abstract: A series of CuO-ZnO-Al{sub 2}O{sub 3} nanocatalysts were fabricated by microwave-assisted combustion method to investigate the role of propylene glycol/nitrates ratio. XRD, FESEM, FTIR, EDX, BET and TG techniques were applied to study the physicochemical properties of prepared nanocatalysts. It was found that increasing of fuel/nitrates ratio decreased the specific surface area due to incomplete combustion reaction. TG and FTIR results proved that some combustion residuals such as nitrate species and propylene glycol remained in fabricated samples and their amount increased by increasing of fuel/nitrates ratio. Also, higher dispersion of CuO crystallites as the major active sites of steam methanol reforming achieved by CZA(PG/N = 1). So, it reached the complete conversion of methanol at lower temperature compared to the other samples. The synthesized sample with propylene glycol to nitrates ratio of 1 showed high stability during 1200 min. Moreover, the deactivation study was performed on best and worst synthesized nanocatalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Characteristics of the sub-micron ash aerosol generated during oxy-coal combustion at atmospheric and elevated pressures

This paper is concerned with the effect of pressure on the particle size distribution and the size-segregated composition of the sub-micron ash aerosol created during oxy-coal combustion under near practical self-sustaining combustion conditions. The problem is important because pressurized oxy-coal combustion has been proposed as one promising technology to minimize CO 2 emissions. Sub-micron ash plays a major role in ash deposition mechanisms, which, in turn, can control boiler performance. In this work, the same bituminous coal was burned at pressures of 1, 8 and 15 bar in O 2 /CO 2 environments. Here, tests employed a 100 kW (rated) oxy-fuel combustor (OFC) operated at atmospheric pressure (1 bar) and a 300 kW (rated) entrained-flow pressurized reactor (EFPR) at elevated pressures (8 and 15 bar). Although these tests were conducted under near practical combustion conditions, confounding effects of peak flame temperature variations were minimized for the 1 bar and 15 bar tests, allowing the role of elevated pressure to be isolated. For the EFPR tests, a specially designed sampling system was used to sample sub-micron ash aerosols from the pressurized combustor and is described in detail. Results showed that at the same peak temperature but higher pressure, the fractions of ash aerosol partitioned into the PM 0.6 and PM 1 size fractions were greatly diminished. Moreover, elevated pressures caused significant changes in the composition of the (size-segregated) sub-micron aerosol, especially in its alkali content, which increased significantly. Examination of fractions of each element that ended up in the sub-micron fume suggested that, at constant temperature, the effect of pressure on vaporization of semi-volatile metals was very different from that on the release into the gas phase of non-volatile metals and could not be explained by equilibrium.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Uniform Pt/Pd Bimetallic Nanocrystals Demonstrate Platinum Effect on Palladium Methane Combustion Activity and Stability

Bimetallic catalytic materials are in widespread use for numerous reactions, as the properties of a monometallic catalyst are often improved upon addition of a second metal. In studies with bimetallic catalysts, it remains challenging to establish clear structure–property relationships using traditional impregnation techniques, due to the presence of multiple coexisting active phases of different sizes, shapes, and compositions. Here, a convenient approach to prepare small and uniform Pt/Pd bimetallic nanocrystals with tailorable composition is demonstrated, despite the metals being immiscible in the bulk. By depositing this set of controlled nanocrystals onto a high-surface-area alumina support, we systematically investigate the effect of adding platinum to palladium catalysts for methane combustion. At low temperatures and in the absence of steam, all bimetallic catalysts show activity nearly identical with that of Pt/Al 2 O 3 , with much lower rates in comparison to that of the Pd/Al 2 O 3 sample. BUt, unlike Pd/Al 2 O 3 , which experiences severe low-temperature steam poisoning, all Pt/Pd bimetallic catalysts maintain combustion activity on exposure to excess steam. These features are due to the influence of Pt on the Pd oxidation state, which prevents the formation of a bulk-type PdO phase. Despite lower initial combustion rates, hydrothermal aging of the Pd-rich bimetallic catalyst induces segregation of a PdO phase in close contact to a Pd/Pt alloy phase, forming more active and highly stable sites for methane combustion. Altogether, this work unambiguously clarifies the activity and stability attributes of Pt/Pd phases which often coexist in traditionally synthesized bimetallic catalysts and demonstrates how well-controlled bimetallic catalysts elucidate structure–property relationships.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Characterization of LiFePO4 samples obtained by pulse combustion under various conditions of synthesis

Lithium iron phosphate (LiFePO4, LFP) is one of the widely used cathode materials for rechargeable lithium ion batteries. LFP batteries are widely used for electric vehicles and backup power due to their important advantages such as low cost, lifetime, efficiency, and reliability. There are still several technical challenges that need to be addressed: The increase of energy density or further reduction of their final cost. This paper concerned with the characterization of carbon coated LiFePO4 nanopowder cathode materials produced under different conditions by pulse combustion for providing energy to the reactor for the synthesis. The reactor was built according to the principles of the thermoacoustic burner on the basis of the Helmholtz resonator. The investigated nanopowders are synthesized by complete and incomplete combustion and calcined at 700 °C. The obtained samples were characterized by X-ray diffraction, Fourier transform infrared, Raman, and Mössbauer spectroscopy. Observed low-Temperature magnetic phase transitions definitively identified the crystal phases. The morphology of samples was controlled by scanning electron microscopy. The aim of this work is to show that it is possible to achieve a desired crystal phase by pulse combustion in a relatively cheap and fast way. The extremely rapid synthesis of almost pure phase material is possible due to the reduction in size of interacting particles and to an enormous number of collisions between them as a result of strong turbulent flow associated with explosive combustion. © 2019 Author(s).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Revealing the structure of a catalytic combustion active-site ensemble combining uniform nanocrystal catalysts and theory insights

Supported metal catalysts are extensively used in industrial and environmental applications. To improve their performance, it is crucial to identify the most active sites. This identification is, however, made challenging by the presence of a large number of potential surface structures that complicate such an assignment. Often, the active site is formed by an ensemble of atoms, thus introducing further complications in its identification. Being able to produce uniform structures and identify the ones that are responsible for the catalyst performance is a crucial goal. Here, we utilize a combination of uniform Pd/Pt nanocrystal catalysts and theory to reveal the catalytic active-site ensemble in highly active propene combustion materials. Using colloidal chemistry to exquisitely control nanoparticle size, we find that intrinsic rates for propene combustion in the presence of water increase monotonically with particle size on Pt-rich catalysts, suggesting that the reaction is structure dependent. We also reveal that water has a near-zero or mildly positive reaction rate order over Pd/Pt catalysts. Theory insights allow us to determine that the interaction of water with extended terraces present in large particles leads to the formation of step sites on metallic surfaces. These specific step-edge sites are responsible for the efficient combustion of propene at low temperature. This work reveals an elusive geometric ensemble, thus clearly identifying the active site in alkene combustion catalysts. These insights demonstrate how the combination of uniform catalysts and theory can provide a much deeper understanding of active-site geometry for many applications

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Next-Cycle Optimal Dilute Combustion Control via Online Learning of Cycle-to-Cycle Variability Using Kernel Density Estimators

Dilute combustion using exhaust gas recirculation (EGR) presents a cost-effective method for increasing the efficiency of spark-ignition (SI) engines. However, the maximum amount of EGR that can be used at a given condition is limited by a rapid increment of cycle-to-cycle variability (CCV). This study describes a methodology to design a model-based stochastic optimal controller to adjust the cycle-to-cycle fuel injection quantity in order to reduce CCV and further extend the dilute limit. Given the complexity and chaotic nature of combustion events, the controller was enhanced with online learning in order to identify the statistical properties of combustion efficiency, which are needed to generate predictions for next-cycle events. This study showed that a kernel density estimator (KDE) can be used to learn the combustion properties in real time and can be incorporated into the feedback policy in order to calculate the optimal control command. Experimental results suggested that the dilute limit can be extended from 18.5% to 21% EGR fraction at an operating condition relevant for highway cruising. Additionally, the proposed controller can achieve a large CCV reduction with less fuel enrichment compared to previous methods, overall contributing to an increase in 0.2% indicated fuel conversion efficiency.

33 ADVANCED PROPULSION SYSTEMS↗