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A hybrid lattice Boltzmann/finite-difference solver for low Mach thermo-compressible flows developed in earlier works is extended to more realistic and challenging configurations involving turbulence and complex geometries in the present article. The major novelty here as compared to previous contributions is the application of a more robust collision operator, considerably extending the stability of the original single relaxation time model and facilitating larger Reynolds number flow simulations. Additionally, a subgrid model and the thickened flame approach have also been added allowing for efficient large eddy simulations of turbulent reactive flows in complex geometries. This robust solver, in combination with appropriate treatment of boundary conditions, is used to simulate combustion in two configurations: flame front propagation in a 2-D combustion chamber with several obstacles, and the 3-D PRECCINSTA swirl burner. Time evolution of the flame surface in the 2-D configuration shows very good agreement compared to direct numerical and large eddy simulation results available in the literature. The simulation of the PRECCINSTA burner is first performed in the case of cold flow using two different grid resolutions. Comparisons with experimental data reveal very good agreement even at lower resolution. The model is then used, with a 2-step chemistry and multi-component transport/thermodynamics, to simulate the combustor at operating conditions similar to previously reported experimental/numerical studies for ϕ =0.83. Results are again in very good agreement compared with available large eddy simulation results as well as experimental data, demonstrating the excellent performance of the hybrid solver. [ABSTRACT FROM AUTHOR]

With the increase in computational power in the last decade and the forthcoming Exascale supercomputers, a new horizon in computational modelling and simulation is envisioned in combustion science. Considering the multiscale and multiphysics characteristics of turbulent reacting flows, combustion simulations are considered as one of the most computationally demanding applications running on cutting-edge supercomputers. Exascale computing opens new frontiers for the simulation of combustion systems as more realistic conditions can be achieved with high-fidelity methods. However, an efficient use of these computing architectures requires methodologies that can exploit all levels of parallelism. The efficient utilization of the next generation of supercomputers needs to be considered from a global perspective, that is, involving physical modelling and numerical methods with methodologies based on High-Performance Computing (HPC) and hardware architectures. This review introduces recent developments in numerical methods for large-eddy simulations (LES) and direct-numerical simulations (DNS) to simulate combustion systems, with focus on the computational performance and algorithmic capabilities. Due to the broad scope, a first section is devoted to describe the fundamentals of turbulent combustion, which is followed by a general description of state-of-the-art computational strategies for solving these problems. These applications require advanced HPC approaches to exploit modern supercomputers, which is addressed in the third section. The increasing complexity of new computing architectures, with tightly coupled CPUs and GPUs, as well as high levels of parallelism, requires new parallel models and algorithms exposing the required level of concurrency. Advances in terms of dynamic load balancing, vectorization, GPU acceleration and mesh adaptation have permitted to achieve highly-efficient combustion simulations with data-driven methods in HPC environments. Therefore, dedicated sections covering the use of high-order methods for reacting flows, integration of detailed chemistry and two-phase flows are addressed. Final remarks and directions of future work are given at the end. [ABSTRACT FROM AUTHOR]

Reactive molecular dynamics simulations are performed to investigate the combined effects of iron nanoparticles and external electrostatic fields on the combustion of n-dodecane. Results suggest that iron nanoparticle additives significantly accelerate fuel and oxidizer consumption. In particular, the decomposition of n-dodecane is initiated at the nanoparticle's surface by hydrogen abstraction and subsequent absorption of the hydrogen and carbon atoms. Products, such as H 2 and H 2 O, are formed in the nanoparticle's shell and released back into the gas phase, demonstrating a catalytic behaviour of the nanoparticle. Additionally, the application of an external electrostatic field further increases the n-dodecane consumption rate. A rise in the variety of product species is also observed when an external electrostatic field is applied due to the overall accelerated kinetics of the system. Analysis of the system's kinetic energy suggests that the presence of an external electrostatic field leads to an increase in the translational energy of the molecules. The chemical composition of the nanoparticle is also affected. The absorbed species diffuse along the surface of the nanoparticle to counteract the externally applied electric field. This species rearrangement leads to the formation of an anisotropic shell with varying chemical composition. This study suggests that the use of electrostatic fields with nanomaterial-based catalysis can offer new possibilities for the control of the reaction process as well as for the synthesis of tailored nanoparticles. [ABSTRACT FROM AUTHOR]

An adjoint-based framework is presented that measures exact sensitivity from high-fidelity simulations of turbulent reacting flows. The framework leverages and extends state-of-the-art numerical methods in a manner that is compatible with a discrete adjoint solver. To ensure energy stability and accuracy, high-order narrow-stencil finite-difference operators satisfying the summation-by-parts (SBP) property are combined with simultaneous-approximation-term boundary treatment. An adaptive SBP dissipation operator and its corresponding adjoint are formulated in a way to dampen unresolved modes and preserve scalar boundedness. A flamelet/progress variable approach is employed to handle chemical reactions using tabulated chemistry. The utility of using pre-computed lookup tables is that discrete adjoint sensitivity can be computed efficiently for arbitrary chemical mechanisms. Gradient-based optimization utilizing the sensitivity obtained from the adjoint solution is demonstrated on two configurations. First, momentum actuation is applied upstream in a spatial mixing layer to control the evolution of a passive scalar in a target region downstream. Then, the framework is used to optimize an acoustic actuator in a three-dimensional turbulent jet with the aim of anchoring an H 2 lifted flame at a target location. Both cases involve manipulating discrete space-time fields with O (10 8) − O (10 9) degrees of freedom, which would not be possible via a brute-force trial-and-error approach. [ABSTRACT FROM AUTHOR]

Nanothermites are interesting energetic systems as their combustion driven by the oxidation of the metallic fuel associated with the reduction of the oxidizer, can produce extremely fast burning rates exceeding hundreds of m s − 1. In addition, by changing the reactant (composition, stoichiometry) geometry and compaction conditions, the control of the burning rate can be achieved, allowing the designer to customize the chemical energy for each application. To date, only rough combustion models exist, most restricting the combustion mechanisms to only condensed phase processes, thus providing an approximative prediction of structure-combustion performance relationships. This work presents a tri-phasic model for the combustion of Al/CuO powder considering 9 gaseous species (Al, Cu, O 2 , O, Al 2 O, Al 2 O 2 , AlO, AlO 2 , N 2) and 4 condensed species (Al, Cu, CuO, Al 2 O 3) that can be liquid or solid. The reactionnal scheme involves 12 heterogeneous reactions and 2 phase changes based on diffusional kinetics, while gaseous reactions are considered through a chemical equilibrium. A detailed description of the theoretical formulation and numerical method is presented, followed by a discussion of a closed-bomb simulation. This work highlights the great impact of the Al particles initial diameter on the pressure development in the chamber. After the initiation stage, the decomposition of CuO releases gaseous O 2 , which is spontaneously absorbed on the surface of submicronic Al particles and diffuses through alumina to react with pure Al. At high temperature, gaseous copper, aluminum sub-oxides and aluminum condense on both particle types. By contrast, Al particle micron-size limits the quantity of O 2 absorption and gaseous species surface condensation, leading to the formation of a pressure pre-peak 4 times higher than the final chamber pressure. [ABSTRACT FROM AUTHOR]

The likelihood of steady burning in microgravity is examined with Burning Rate Emulation (BRE) using a gas burner with a flat 25 mm diameter porous surface with two embedded heat flux sensors. The data are greatly expanded from a previous work. The fuel mixture used in this analysis are ethylene and ethylene diluted with nitrogen resulting in a range of heats of combustion from 12 to 47.2 kJ/g. An analytical solution of these flames was utilized to compute their steadiness, temperatures at flame sheet, and sizes. 103 flames were ignited in an ambient with nominal oxygen mole fractions of 0.21, 0.265, 0.34 and 0.40. 49 of those flames burned for the entire duration of the test before the fuel supply was terminated, while the remainder self-extinguished. The flames attained at least 90% of their asymptotic heights for all 49 fuel terminated flames. An estimated critical steady flame temperature ranging from 1100 to 1200 K was observed segregating the sustained and self-extinguished flames. Flame temperatures for the 49 terminated flames were above the critical temperature range indicating sustained steady burning, whereas below they self-extinguished. The measured flame sizes, and radiative fractions are reported at the endpoint of the terminated tests. An empirical correlation of the radiative fraction based on radiation theory with Ω, a dimensionless parameter, was used in prediction for real fuels at steady burning in microgravity. A flammability diagram, as a plot of the emulated fuel heat of gasification with its burning mass flux, for the range of steady data, is presented comparing how real fuels might burn steadily in normal and microgravity conditions. The results suggest that fuel mass burning rates would be about 3 to 4-times higher in 1-g. The heats of gasification and combustion of some real fuels are presented and compared against the burner flames. [ABSTRACT FROM AUTHOR]

During the combustion of biomass in drop-tube furnace (DTF) systems, the released alkali metal (e.g., potassium, K) inevitably reacts with reactor tube at high temperatures, affecting the experimental results on the emission of particulate matter with aerodynamic diameters of <10 μm (PM 10). This study reports the interactions between K vapor and tube reactors made of silicon carbide, corundum, and mullite and their impacts on PM 10 emission. Demineralized wood samples loaded with potassium chloride (KCl) or ion-exchanged K respectively were combusted in a DTF at 1300 °C under air or oxy-fuel atmosphere. Another series of experiments was conducted to collect and analyze the PM 10 from the combustion of KCl-loaded wood, K-exchanged wood, and two typical biomass samples (cotton stalk and wheat straw) in the three reactor tubes under air atmosphere. Experimental results show that 4.1‒72.5% of K is retained in the three tubes when burning the KCl-loaded wood in air, and the combustion in oxy-fuel atmosphere slightly increases the K retention. For K-exchanged wood combustion in air, only 3.7‒23.6% of K is released from the reactor tubes. In all conditions, the reactivity of the reactor tubes with K vapor follows a sequence of mullite > corundum > silicon carbide. The retained K is unstable, 49.0‒64.8% of which can be re-released during polyvinyl chloride combustion. In addition, the results demonstrate that, compared with silicon carbide tube, the use of corundum and mullite tubes leads to a 16.2‒54.3% decrease in PM 1 yields and a significant drop in fine mode peaks in PM 10 during the combustion of biomass samples in air, while the PM 1–10 yields and the coarse mode peaks remain largely unchanged. These are attributed to the enhanced retentions of alkali metals in corundum and mullite tubes, which reduce the yields of Na, K, and Cl in PM 10 , but has negligible effect on those of refractory elements such as Mg and Ca. [ABSTRACT FROM AUTHOR]

Conditional Source-term Estimation (CSE) is applied to three turbulent ethanol spray flames (EtF3, EtF6, and EtF8) in Large Eddy Simulation (LES). The objectives of this paper are to include the heat losses due to spray evaporation and gas radiation in the chemistry tabulation, assess the impact of these changes on the temperature and droplet statistics, and evaluate the performance of LES-CSE for the selected flames. The profiles of gas temperature, spray velocity, velocity root mean square (rms) and droplet size distribution are well reproduced in the simulations compared to available experimental data. Temperature underpredictions near the centreline are observed, in particular, at locations closer to the jet exit for flames with lower jet velocity. A wider flame is predicted in EtF8 compared to the experiment and regions of local extinction are visible. The use of non-adiabatic chemistry library results in a noticeable improvement in the temperature predictions near the peak locations, especially for flames with higher velocity and closer to the jet exit. The heat losses due to evaporation are larger than those from radiation, confirming the importance of including the evaporation effects in the chemistry tables. The droplet velocity is well predicted, except for EtF8 where an underprediction is observed far downstream. The velocity rms is slightly underpredicted at some locations, probably due to the simple stochastic model used. Overall, LES-CSE with non-adiabatic chemistry tables successfully captures the gas-spray quantities in the selected flames. [ABSTRACT FROM AUTHOR]

A Fourier transform infrared (FTIR) spectra-based strategy was developed for estimating three important combustion properties of jet fuels and their constituent hydrocarbon species: ignition delay time (IDT), net heat of combustion (NHC), and derived cetane number (DCN). This approach leverages the strong sensitivity of combustion behavior to the different infrared absorption features of hydrocarbon fuels. Gas-phase FTIR spectra of pure hydrocarbons and jet fuels in the 2–15.38 μ m wavelength range were used to train elastic-net regularized linear models, with the model parameters optimized separately for each property. The results from these models were compared with the results from previous models, which utilized only a limited spectral region in the wavelength range 3.3–3.55 μ m. The new, optimized models developed in this work show significant improvement in predictive performance compared to the previous models for all three properties. The models' prediction errors on test data (blends of conventional and alternative jet fuels) are found to be comparable to the standard property measurement uncertainties, demonstrating the value of infrared spectral analysis as a low-volume prescreening tool for accurate property estimation of both conventional and alternative jet fuels. [ABSTRACT FROM AUTHOR]

Degenerate chain-branching mechanisms of n -alkanes are centered on the formation of hydroperoxy-alkyl radicals (̇QOOH), formed via ̇R + O 2 reactions, and the ensuing competition between unimolecular decomposition and second-O 2 -addition. Quantitative measurements of partially oxidized intermediates formed via reactions of ̇QOOH provide critical constraints that are required for accurate modeling of combustion chemistry. To examine the influence of temperature and oxygen concentration on intermediates from unimolecular decomposition of ̇QOOH, isomer-resolved speciation measurements were conducted on n -butane oxidation at 835 Torr in a jet-stirred reactor (JSR) from 500 – 900 K. Resulting from negative-temperature coefficient behavior, cyclic ether formation peaked at two temperatures, 650 K and 800 K, which were selected for separate experiments to quantify the O 2 -dependence of species profiles using O 2 concentrations of 4.2 · 1017 – 1.1 · 1019 molecules cm–3. Utilizing vacuum-ultraviolet absorption spectroscopy and electron-impact mass spectrometry, cyclic ether isomers were quantified separately, including explicit resolution of cis - and trans - isomers of 2,3-dimethyloxirane. Stereoisomers of 2-butene were also quantified explicitly. For all cyclic ethers, a common trend in O 2 -dependence emerged: species concentrations reach a maximum near 3.0 · 1018 molecules cm–3 (equivalence ratio of 0.5). Although quantitative disparities are evident, chemical kinetics modeling qualitatively reproduces the O 2 dependence of species at 650 K. However, at 800 K, weak dependence on O 2 is predicted, which is in contrast with the measurements. Two carbonyls, diacetyl and methyl vinyl ketone, were also quantified and follow similar dependence on [O 2 ] and temperature as the cyclic ethers, which indicates some fraction forms via ̇QOOH-mediated reactions. The discrepancies between the measured and model-predicted species profiles indicate that sub-mechanisms for important intermediates may require additional elementary reactions, including stereochemical-specific reactions, to improve the fidelity of n -alkane combustion modeling. [ABSTRACT FROM AUTHOR]

Direct injection spark ignition (DISI) engines have been widely used in passenger cars due to their lower fuel consumption, better controllability, and high efficiency. However, DISI engines are suffering from wall wetting, imperfect mixture formation, excess soot emissions, and cyclic variations. Applying a new fuel atomization technique and using biofuels with their distinctive properties can potentially aid in improving DISI engines. In this research, the effects of isobutanol and 2-butanol and their blends with Toluene Primary Reference Fuel (TPRF) on spray characteristics, DISI engine combustion, and particle number (PN) emissions are investigated for conditions with and without flash boiling of the injected fuel. Spray characteristics are investigated using a constant volume chamber. Then, the combustion, flame propagation, and PN emissions are examined using an optical DISI engine. The fuel temperature is set to 298 K and 453 K for liquid injection and flash boiling injection, respectively. The tested blending ratio is 30 vol% butanol isomers and 70 vol% TPRF. The results of the spray test reveal that liquid fuel plumes are distinctly observed, and butanol blends show a slightly wider spray angle with lower penetration length compared to TPRF. However, under flash boiling injection, the sprays collapse towards the injector axis, forming a more extended single central vapor jet due to the plumes' interaction. Meanwhile, butanol blends yield a narrow spray angle with more extended penetration compared to TPRF. The flame visualization test shows that the flash boiling injection reduced yellow flames compared to liquid fuel injection, reflecting the improvements in mixture formation. Thus, improvements were noted in the heat release and PN emissions. Butanol addition reduced the PN emissions by 43% under regular liquid injection. Flash boiling injection provided an additional 25% reduction in PN emissions. [ABSTRACT FROM AUTHOR]

Laboratory-scale experiments pertinent to pulverised fuel (PF) combustion are often carried out in drop-tube furnaces (DTFs) at air-fuel equivalence ratios and cooling rate for quenching flue gas that are much higher than those in PF boilers. This paper reports the effect of flue gas cooling conditions on the properties of PM with aerodynamic diameter of <10 µm (PM 10) from biomass combustion. This study considers four cooling rates (1000, 2000, 6000 and 20,000 °C/s) and two biomass feeding rates (0.05 and 0.25 g/min) that represents flue gases with significantly-different concentrations of inorganic vapours. The PSDs of PM 10 have a bimodal distribution with a fine mode within PM with aerodynamic diameter of <1 µm (PM 1) and a coarse mode within PM with aerodynamic diameter of 1–10 µm (PM 1–10). All experimental conditions produce PM 10 with similar PM 1 and PM 1–10 yields (∼0.8 and ∼1.6 mg/g_biomass, respectively) and similar coarse mode diameters (i.e. 6.863 µm). However, at a biomass feeding rate of 0.05 g/min, the fine mode diameter shifts from 0.022 to 0.077 µm when the cooling rate decreases from 20,000 to 1000 °C/s, indicating more profound heterogeneous condensation at a lower cooling rate. As the biomass feeding rate increases to 0.25 g/min, the fine mode diameter further shifts to 0.043 µm and at 20,000 °C/s but remained at 0.077 µm at 1000 °C/s though a clear shift of PSD to larger diameters is evident. These are attributed to enhanced heterogeneous condensation and coagulation of small particulates resulting from increased particle population density in hot flue gas. Chemical analyses show PM 1 contains dominantly volatile elements (i.e. Na, K and Cl) while PM 1–10 consists of mainly Ca. Similar trends are also observed for elemental PSDs and yields. It is also observed that slow cooling of hot flue gas leads to an increased yield of Cl in PM 1–10 due to enhanced chlorination of Ca species. [ABSTRACT FROM AUTHOR]

Understanding the lower stability limit of gas turbine combustors is imperative for safe operation as well as for the approval of novel sustainable aviation fuels. Previous results have indicated that at relatively low temperatures and pressures (T air = T fue l = 238 and 258 K, P = 101 kPa, Δ P / P = 2%), the lean blowout (LBO) limits correlated to fuel physical properties, while at relatively high temperatures and pressures (T air = 394 K, T fuel = 322 K, P = 207 kPa, Δ P / P = 3%) chemical properties were highly correlated. This suggests that a transition occurs between physical and chemical property dependency. In the present effort, a single nozzle swirl-stabilized combustor was used to study the LBO for fuels with varying chemical and physical properties as well as operating conditions to study a transition from physical property dependence at low pressures and temperatures to chemical property dependence at higher temperatures and pressures. Four fuels with a wide range of physical and chemical properties were tested at combustor inlet conditions of T air = T fuel = 305–355 K, P = 107 kPa, and ΔP/ P = 2–6%. It was found that the transition of LBO dependence from fuel physical to chemical properties does not happen at a single point. Rather, a transition range of conditions was observed for changes made to the pressure drop across the combustor dome (Δ P) as well as changes made to air and fuel temperature. Changes in ΔP/P were found to change the relative LBO performance across fuels and demonstrated a change in physical to chemical property dependence as ΔP/P increased. Analysis via random forest regression using dimensionless parameters such as an approximate Weber number and Ohnesorge number further highlighted the transition from physical to chemical property dependency with increasing Δ P / P. [ABSTRACT FROM AUTHOR]

Due to the urgent needs to reduce anthropogenic carbon dioxide emissions there is an increasing interest in the use of alternative fuels. For this reason, there is a need for new knowledge on how to design and adapt existing heat and power plants to biogenic and waste-derived fuels. This work relates to co-firing of biomass and coal and the sulfation of alkali chlorides in coal-fired flames doped with chemical additives. We aim to examine the global time scales of alkali sulfation and chlorination based on combustion experiments that were conducted in a 30-kW coal flame. Temperature, gas and particle composition measurements were conducted. Both experiments and modelling support that the apparent alkali sulfation kinetics are fast in a coal-fired flame and that it is dominated entirely by the presence of SO 2. The availability of oxygen and carbon monoxide, or hydrocarbons, is also critical to sustain the sulfation reaction cycle; low concentrations are sufficient. For industrial boilers this implies that sulfur addition, in combination with reburning, should constitute an efficient strategy to mitigate alkali-chlorination and the related high temperature corrosion. [ABSTRACT FROM AUTHOR]

In this paper, the interaction between different organic and inorganic K/S/Cl compounds in the solid structure of biomass is studied and a model is presented to predict the temporal release of K g , HCl, CH 3 Cl, KCl, KOH, K 2 SO 4 and SO 2 from biomass devolatilization. Four types of pulverized biomass are chosen from literature, two of which have no chlorine content and two with chlorine content in lower stoichiometry to potassium. The results of the model are compared with the experimental measurements. In the presence of chlorine, KCl, HCl and K g were found to be the dominant chlorine and potassium species. In the absence of chlorine, K g dominates the release of potassium. KOH and K 2 SO 4 release into the gas phase towards the end of devolatilization due to the overlapping with char combustion. SO 2 is the main sulfur species released into the gas phase. The model is coupled with a CFD solver where the gas phase chemistry of the K/S/Cl system can be studied using available chemical mechanisms for these species. [ABSTRACT FROM AUTHOR]

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH 3 in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH 3 dissociation was varied from 0 to 25% (0 to 20%v/v H 2 in the fuel mixture with a fixed H 2 /N 2 ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH 3 in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH 3 dissociation. At 25% NH 3 dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH 3 in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH 3. The production of key radicals including OH, H, O, and NH 2 was enhanced in the presence of H 2 and thus the conversion of NH to NO and then NO to N 2 near LFL and the conversion of NH 2 and NO to N 2 near UFL leading to wider flammability limits. [ABSTRACT FROM AUTHOR]

The present work shows the first application of the PC-transport approach in the context of Large Eddy Simulation (LES) of turbulent combustion. Detailed kinetic mechanisms, together with advanced computational tools, are needed to advance our knowledge of turbulent reacting systems. However, the cost related to high-fidelity simulations of turbulent reacting flows is still prohibitive for realistic configurations. Therefore, there is a need to reduce the complexity of the problem by identifying low-dimensional manifolds. To this end, the potential offered by Principal Component Analysis (PCA) in parameterizing the thermo-chemical state-space is very appealing. The present paper extends the PC-transport framework to three-dimensional Large Eddy Simulation (LES), coupling PCA with Gaussian Process Regression (GPR). To demonstrate the potential of the method, LES simulations of Sandia flames D, E and F are shown. Results show the great potential of the PC-GPR model, as indicated by the accuracy of the simulation results when compared with experimental data, using only 2 principal components. The sensitivity to the kinetic mechanism and subgrid closure model is also investigated. [ABSTRACT FROM AUTHOR]

Given the constraints on typical bond energies and the commonality of final products produced from combustion of C H N O based energetic materials, the possibilities for further increases in stored potential energy and thermodynamic performance from these classes of materials are limited. Thus, modulating the energy release to achieve efficiency and effectiveness for desired applications is of great value. Investigation of nanomaterials as energetic materials began more than twenty years ago with much of the interest to increase reaction rates and reduce sensitivity. During this period, research on energetic nanoparticles was devoted to reducing the loss of energy density with metallic materials due to the naturally occurring oxide passivating layer, managing their high surface area preventing high loadings in solids, minimizing particle-particle interactions making dispersion in the gas-phase difficult, and understanding combustion mechanisms. As an outcome, novel synthesis methods of producing nanocomposites, and new fields of applications, such as micro-pyrotechnics, have developed. Yet, the research community is only beginning to understand how to manipulate and build energetic materials at the nanoscale, and what designs are optimal for desired functions. Furthermore, recognizing the difficulties for increased energy density and reduced sensitivity, the development of multifunctional and smart nanoenergetic materials is currently being researched to enable control of energy release rates and material sensitivity on demand. This research is being advanced by assembly of nanoengineered energetic materials to bulk scales by additive manufacturing, the development and application of combustion diagnostics that resolve nanometer and micron scales, and ab initio quantum chemistry and molecular dynamics calculations. The challenges that have been confronted and the directions of continuing research on nanoenergetics are presented and discussed. [ABSTRACT FROM AUTHOR]

Combustion involves chemical reactions that are often highly exothermic. Combustion systems utilize the energy of chemical compounds released during this reactive process for transportation, to generate electric power, or to provide heat for various applications. Chemistry and combustion are interlinked in several ways. The outcome of a combustion process in terms of its energy and material balance, regarding the delivery of useful work as well as the generation of harmful emissions, depends sensitively on the molecular nature of the respective fuel. The design of efficient, low-emission combustion processes in compliance with air quality and climate goals suggests a closer inspection of the molecular properties and reactions of conventional, bio-derived, and synthetic fuels. Information about flammability, reaction intensity, and potentially hazardous combustion by-products is important also for safety considerations. Moreover, some of the compounds that serve as fuels can assume important roles in chemical energy storage and conversion. Combustion processes can furthermore be used to synthesize materials with attractive properties. A systematic understanding of the combustion behavior thus demands chemical knowledge. Desirable information includes properties of the thermodynamic states before and after the combustion reactions and relevant details about the dynamic processes that occur during the reactive transformations from the fuel and oxidizer to the products under the given boundary conditions. Combustion systems can be described, tailored, and improved by taking chemical knowledge into account. Combining theory, experiment, model development, simulation, and a systematic analysis of uncertainties enables qualitative or even quantitative predictions for many combustion situations of practical relevance. This article can highlight only a few of the numerous investigations on chemical processes for combustion and combustion-related science and applications, with a main focus on gas-phase reaction systems. It attempts to provide a snapshot of recent progress and a guide to exciting opportunities that drive such research beyond fossil combustion. [ABSTRACT FROM AUTHOR]

Aliphatic amines are an important class of nitrogen-containing compounds present in renewable fuels such as bio-oils. Conversion of this fuel-bound nitrogen can lead to the formation of HCN and NH 3 , as well as NO x emissions. In this work, the combustion chemistry of small aliphatic amines is investigated via a combination of quantum chemical calculations, chemical kinetic modeling and experimental validation. The influence of the degree of substitution on the nitrogen atom and the alkyl chain length on the reactivity and product distribution is studied via three model compounds, i.e. ethylamine (EA, CH 3 CH 2 NH 2), dimethylamine (DMA, (CH 3) 2 NH) and diethylamine (DEA, (CH 3 CH 2) 2 NH). A detailed kinetic model containing 258 species and 2274 reactions is developed to describe their combustion over a wide range of conditions. The proposed model captures the trends in ignition delay times and species concentrations over a temperature range from 500 to 2000 K and pressures from 4 to 170 kPa. The ignition delay data are predicted with an average deviation of 10%. The difference between experimental and simulated species concentrations from laminar premixed flames is for the major species on average 50%, while the agreement for the JSR is even better, with an average deviation of 10%. The dominant decomposition pathway under all the studied conditions is a set of hydrogen abstractions from the C α and N positions followed by β-scission of the fuel radicals. Among the unimolecular decomposition pathways, the homolytic C C and C N scissions and four-centered elimination play a minor role. HCN is the main intermediate and at temperatures above 1100 K the amines are completely converted to N 2 and NO. [ABSTRACT FROM AUTHOR]

Recent theoretical studies have shown that termolecular chemistry can be facilitated through reactions of flame radicals (H, O, and OH) or O 2 with highly-energized collision complexes (either radical or stable species) formed in exothermic reactions. In this work, radical-radical recombination reaction induced termolecular chemistry and its impact on combustion modeling was studied. Two recombination reactions, H + CH 3 + M → CH 4 + M and H + OH + M → H 2 O + M, were analyzed using ab-initio master equation analyses guided by quasiclassical trajectory results. The dynamics results and the master equation calculations indicate that CH 4 ⁎ and H 2 O⁎ (formed in the two radical-radical reactions outlined above) react rapidly with flame radicals and O 2 at rates that are competitive with collisional cooling. The addition of these processes into conventional combustion modeling requires two modifications: the inclusion of the new nonthermal termolecular reaction rates and the simultaneous reduction of the competing recombination reaction rates. The former is described with newly derived Arrhenius expressions based on quasiclassical trajectories, and the latter is achieved by perturbing the recombination reaction rate during the simulation. Kinetic modeling was used to gauge the impact of including this nonthermal chemistry for H 2 /CH 4 -air laminar flames speeds. Inclusion of this nonthermal chemistry has a noticeable impact on simulated flame speeds. The procedure developed here can be utilized to properly quantify the effects of such nonthermal reactions in macroscopic kinetic models. [ABSTRACT FROM AUTHOR]

Abstract Higher engine efficiency and ever stringent pollutant emission regulations are considered as the most important challenges for today's automotive industry. Fast evaporation and combustion technique has caused unprecedented attention due to its potential to solve both of the above challenges. Flash boiling, which features a two-phase flow that constantly generates vapor bubbles inside the liquid spray is ideal to achieve fast evaporation and combustion inside direct-injection (DI) gasoline engines. In this study, three spray conditions, including liquid, transitional flash boiling and flare flash boiling spray were studied for comparison under cold start condition in a spark-ignition direct-injection (SIDI) optical gasoline engine. Optical access into the combustion chamber includes a quartz linear and a quartz insert on the piston. In separate experiments, we recorded the crank angle resolved spray morphology using laser scattering technique, and distribution of fuel before ignition employing laser induced fluorescence technology, as well as time-resolved color images of flame with high-speed camera. The spray morphology during the intake stroke shows stronger plume-plume and plume-air interaction under flash boiling condition, as well as smaller penetration. Then around the end of compression (before ignition), the fuel distribution is also shown to be more homogeneous with less cyclic variation under flash boiling. Finally, from the color images of the flame, it was found that with the increase of superheat degree, the diffusion rate of blue flame (generated by excited molecules) is higher, which is considered to be related with the larger fractal dimension of the flame front. Also, the combustion is more complete with less yellow flame under flash boiling. [ABSTRACT FROM AUTHOR]

Abstract A late-injection strategy is typically adopted in stratified-charge direct injection spark ignition (DISI) engines to improve combustion stability for lean operation, but this may induce wall wetting on the piston surface and result in high soot emissions. E30 fuel, i.e. , gasoline with 30% ethanol, is a potential alternative fuel that can offer a high Research Octane Number. However, the relatively high ethanol content increases the heat of vaporization, potentially exacerbating wall-wetting issues in DISI engines. In this study, the Refractive Index Matching (RIM) technique is used to measure fuel wall films in the piston bowl. The RIM implementation uses a novel LED illumination, integrated in the piston assembly and providing side illumination of the piston-bowl window. This RIM diagnostics in combination with high-speed imaging was used to investigate the impact of coolant temperature on the characteristics of wall wetting and combustion in an optical DISI engine fueled with E30. The experiments reveal that the smoke emissions increase drastically from 0.068 FSN to 1.14 FSN when the coolant temperature is reduced from 90 °C to 45 °C. Consistent with this finding, natural flame luminosity imaging reveals elevated soot incandescence with a reduction of the coolant temperature, indicative of pool fires. The RIM diagnostics show that a lower coolant temperature also leads to increased fuel film thickness, area, and volume, explaining the onset of pool fires and smoke. [ABSTRACT FROM AUTHOR]

Abstract The dynamics of a premixed ethylene-air jet injected transverse to a vitiated cross-flow were investigated using high-repetition rate particle image velocimetry (PIV). Both non-reacting and reacting jets were found to be characterized by a dominant frequency associated with the jet wake vortex system. For the isothermal jet, increasing the momentum flux ratio (J) has only a slight effect on the frequency of the oscillation but significantly increases its magnitude. The reacting jet was found to exhibit different behavior, with a monotonic increase in the dominant frequency with J. The jet equivalence ratio (ϕ j) was found to have little effect on the rate of wake vortex shedding but affects the overall magnitude of the oscillation. Comparison with data reported in the literature suggests the relationship between the wake Strouhal number (St w) and J is fuel dependent. Application of a vortex detection algorithm shows a stark difference in the location of the wake vortices under non-reacting and reacting conditions. Under isothermal conditions, the vortices are found close to the jet centerline and dissipate relatively quickly. Reaction confines the vortices to a narrow shear layer until a farther distance downstream and the vortices convect through the entire area of interest. Additionally, the vortex circulation strength was found to increase with J. Proper orthogonal decomposition (POD) analysis of the non-reacting and reacting jets demonstrates the dominance of the wake vortex structures in the oscillating flow fields. In both cases, the temporal information extracted from the most energetic modes is identical to the dominant frequencies measured in the flow fields. The primary effect of heat release is to reduce the overall amount of coherence and to delay the appearance of elevated coherence levels until a larger streamwise distance from the jet exit. [ABSTRACT FROM AUTHOR]

Abstract Apparent char kinetic rates are commonly used to predict pulverized coal char burning rates. These kinetic rates quantify the char burning rate based on the temperature of the particle and the oxygen concentration at the external particle surface, inherently neglecting the impact of variations in the internal diffusion rate and penetration of oxygen. To investigate the impact of bulk gas diffusivity on these phenomena during Zone II burning conditions, experimental measurements were performed of char particle combustion temperature and burnout for a subbituminous coal burning in an optical entrained flow reactor with helium and nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in cooler char combustion temperatures than in equivalent N 2 environments. Measured char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given char combustion temperature. To augment the experimental measurements, detailed particle simulations of the experimental conditions were conducted with the SKIPPY code. These simulations also showed a 60% higher burning rate in the helium environments for a given char particle combustion temperature. To differentiate the effect of enhanced diffusion through the external boundary layer from the effect of enhanced diffusion through the particle, additional SKIPPY simulations were conducted under selected conditions in N 2 and He environments for which the temperature and concentrations of reactants (oxygen and steam) were identical on the external char surface. Under these conditions, which yield matching apparent char burning rates, the computed char burning rate for He was 50% larger, demonstrating the potential for significant errors with the apparent kinetics approach. However, for specific application to oxy-fuel combustion in CO 2 environments, these results suggest the error to be as low as 3% when applying apparent char burning rates from nitrogen environments. [ABSTRACT FROM AUTHOR]

Abstract This paper as the first time in the field reports the direct experimental evidence for demonstrating the important role of cooling in ash cenosphere fragmentation using a simple but unique combustion system. The combustion system used pulverised pyrite (38–45 µm) for combustion in drop-tube furnace under designed conditions (gas temperature: 1000 °C; residence time: 1.2 s), which produced dominantly ash cenosphere particles or fragments. The combustion products were quenched under various cooling conditions (represented by nominal cooling rates of 6400–11,800 °C/s) for sampling. The results show that increasing cooling rate from 6400 to 11,800 °C/s substantially intensifies ash cenosphere fragmentation. Such enhanced ash cenosphere fragmentation leads to a significant shift in the particle size distribution of ash collected in the cyclone (>10 µm) to much smaller sizes. It also produces considerably more particulate matter (PM) with aerodynamic sizes less than 10 µm (i.e., PM 10) that consists of dominantly PM with aerodynamic sizes between 1 and 10 µm (i.e., PM 1 – 10) and some PM with aerodynamic sizes less than 1 µm (i.e., PM 1). It is further noted that the PM 1 is mainly PM with aerodynamic sizes between 0.1 and 1 µm (i.e., PM 0.1 – 1) and to a considerably lesser extent PM with aerodynamic sizes less than 0.1 µm (i.e., PM 0.1). Chemical analyses further show that both ash and PM samples contain only Fe 2 O 3 , indicating that complete consumption of sulphur and full oxidation of iron have been achieved during pulverised pyrite combustion under the conditions. [ABSTRACT FROM AUTHOR]

Abstract Fluorescence interference in Raman spectrum is a big barrier for rapid and precise analysis of coal structures by Raman spectroscopy. Dealing with fluorescence interference suitably is one of the key tasks before efficient application of Raman spectroscopy in coal assessment. In this study, Raman spectra and coal combustion characteristics of 32 kinds of Chinese coals were respectively obtained in a micro-Raman spectrometer and Thermal Gravimetric Analyzer. The degree of fluorescence interference in Raman spectrum was firstly defined and quantified as the drift coefficient α using a simple method without curve-fitting the spectrum. The correlations between the degree of fluorescence interference and coal property, coal combustion characteristics were set up and multivariable analysis was done. The results indicate that the degree of fluorescence interference is well related to the coal structures, and it is synthetically determined by volatile, moisture and ash content in coal. With the increase of volatile, moisture content in coal, the fluorescence interference increases continuously, and it can be reduced but not eliminated by drying the moisture in coals. Significant mathematical relations between the drift coefficient α and volatile, moisture content, coal combustion characteristic temperatures have been found. Coal with more evident fluorescence interference in Raman spectrum usually has lower degree of coalification, more polar functional groups, and burns at a lower temperature. The drift coefficient α can act as an efficient probe for coal property and coal combustion characteristics. This study provided a new and simple approach for evaluating coal property and coal combustion characteristics by fluorescence interference in Raman spectrum. [ABSTRACT FROM AUTHOR]

Abstract The engine combustion network (ECN) Spray A is modelled using the Reynolds-averaged Navier–Stokes-transported probability density function (RANS-TPDF) approach to validate the application of a new multiple mapping conditioning (MMC) mixing model to multiphase reactive flows. The composition TPDF equations are solved using a Lagrangian stochastic approach and the spray is modelled with a discrete particle approach. The model is first validated under non-reacting conditions (at 900 K) using experimental mixture-fraction data. Reactive simulations are then performed for three different ambient temperatures (800, 900, 1100 K) and oxygen concentrations (13, 15, 21%) at an ambient density of 22.8 kg/m3. The MMC mixing model is compared with the interaction by exchange with the mean (IEM) mixing model. The ignition delay predictions are not sensitive to the mixing model and are predicted well by both the mixing models under all the tested ambient conditions. The IEM model overpredicts the flame lift-off length (FLOL) at high temperature and high oxygen conditions with a mixing constant C ϕ = 2. The MMC model with C ϕ = 2 and a target correlation coefficient r t = 0.935 between the mixture fraction and a reference variable used to condition mixing predicts good FLOL under all the conditions except 800 K. It is demonstrated that the lift-off length is controllable by changing the target correlation coefficient, while C ϕ and therefore the mixing fields are held fixed. In comparison to the MMC model, the IEM model predicts a higher variance of temperature conditioned on mixture fraction near the flame base owing to its lacking the property of localness. The mixing distance between the notional TPDF particles in the composition space is also higher with the IEM model and it is demonstrated that by changing r t , different levels of mixing locality can be achieved. [ABSTRACT FROM AUTHOR]

Abstract This work presents a new method of measuring the CO/CO 2 ratio at the surface of carbon particles during combustion. This thermogravimetric method deduces the ratio of CO to CO 2 by comparing the rate of consumption of carbon with the rate of oxidation of an external reference material with fast oxidation kinetics, in this case Cu. The method is useful when combustion is controlled by external mass transfer, commonly encountered in large-scale processes. The viability of this method has been demonstrated experimentally with graphite and a lignite char. It was found that in an atmosphere of ∼ 1% O 2 , the graphite produced CO 2 between 700 and 900 °C whilst the lignite char produced a mixture of CO and CO 2 between 700 and 800 °C with the proportion of CO increasing with temperature, and above 850 °C, only CO was produced. It was also found that for this particular lignite char, the ratio of CO/CO 2 increased with decreasing p O 2 in the environment. [ABSTRACT FROM AUTHOR]

Abstract The oxidation properties of propane have been investigated by conducting experiments in a laminar flow reactor at a pressure of 100 bar and temperatures of 500–900 K. The onset temperature for reaction increased from 625 K under oxidizing conditions to 725 K under reducing conditions. A chemical kinetic model for high pressure propane oxidation was established, with particular emphasis on the peroxide chemistry. The rate constant for the important abstraction reaction C 3 H 8 + HO 2 was calculated theoretically. Modeling predictions were in satisfactory agreement with the present data as well as shock tube data (6–61 bar) and flame speeds (1–5 bar) from literature. [ABSTRACT FROM AUTHOR]

Abstract A computational investigation into the kinetics of the NO + CH 2 CCH reaction is presented. The stationary points on the C 3 H 3 N 1 O 1 potential energy surface are analyzed using the compound method ANL0, with key regions of the potential energy surface computed using multi-reference methods. The temperature- and pressure-dependent rate constants are computed using the RRKM/Master Equation. The dominant bimolecular products are HCN + CH 2 CO, CH 2 CNH + CO, and CH 3 CN + CO. Additional calculations for the thermal decomposition of an unimolecular intermediate, isoxazole, are in excellent agreement with the available experimental data. The new rate constants are implemented in a detailed chemical kinetic mechanism for the oxidation of C 2 H 4 by O 2 + NO. Analysis of a constant temperature, constant pressure batch reaction suggests that NO + CH 2 CCH could be an important pathway for both NO reduction and CH 2 CCH oxidation in reburn chemistry. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we consider the impact of polar curved polycyclic aromatic hydrocarbons (cPAH) on the process of soot formation by employing electronic structure calculations to determine the earliest onset of curvature integration and the binding energy of curved homodimers. The earliest (smallest size) onset of curvature integration was found to be a six ring PAH with at least one pentagonal ring. The σ bonding in the presence of pentagons led to curvature, however, the π bonding strongly favored a planar geometry delaying the onset of curvature and therefore the induction of a flexoelectric dipole moment. The binding energies of cPAH dimers were found to be of similar magnitude to flat PAH containing one or two pentagons, with an alignment of the dipole moments vectors. For the more curved structures, steric effects reduced the dispersion interactions to significantly reduce the interaction energy compared with flat PAH. Homogeneous nucleation of cPAH at flame temperatures then appears unlikely, however, significant interactions are expected between chemi-ions and polar cPAH molecules suggesting heterogeneous nucleation should be explored. [ABSTRACT FROM AUTHOR]