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Simulations are presented for a generic, round supersonic combustor. Turbulence is modeled in the combustor using a wall-modeled large eddy simulation approach. Combustion is modeled using a small quasi-global mechanism and a more detailed skeletal mechanism. Both mechanisms are used in conjunction with two variations of the partially-stirred reactor model for sub-grid turbulence chemistry interactions. Sensitivity of the solutions to grid resolution is investigated. It is found that in order to achieve reasonable grid convergence in the mean wall pressure, the model constant that appears in the partially-stirred reactor model must be a function of both the chemistry mechanism and the grid resolution. Most of the combinations of mechanism and turbulent combustion model tested can be tuned in order to predict the location of the pre-combustion shock train and the peak mean pressure in the combustor. It is found that while the different models are able to reproduce the mean wall pressure, there are significant differences in the mean temperature and heat release rate fields. The sensitivity of the different combinations of mechanisms and partially-stirred reactor formulation is quantified and some combinations are found to be more prone to blowout. Two of the tuned models were tested across several fuel equivalence ratios with a single value of the partially-stirred reactor model constant. One model produced reasonable predictions of shock location and peak mean pressure for each equivalence ratio. The second model captured the global trends in the mean wall pressure, but was unable to quantitatively predict the shock location and peak mean pressure for all equivalence ratios tested. [ABSTRACT FROM AUTHOR]

In this study, the effects of intake port injection of methane and direct of injection diesel on emissions and the combustion of a light-duty RCCI engine were numerically researched. In this way, AVL Boost software was used for 1-dimensional simulation of the combustion process and emission estimation. Higher octane number methane gas was mixed with air from the intake port, while lower octane number diesel fuel was injected directly into the combustion chamber during the compression stroke. Methane gas was injected at a rate of 65% and diesel fuel at a rate of 35%. The diesel injected directly into the combustion chamber was sprayed at a rate of 5% between 0 and 35 °CA with 8 different injection timings after the main injection. In the model engine, 50% high EGR rate was applied in all combustion modes. The results showed that these parameters have significant effects on performance and emissions. The results in summary; NOx emission reduced at all engine speeds with delayed diesel post-injection timing. The maximum drop of NOx emission was 57.34% with Post15. Although the addition of methane slightly increased the soot emission, it was significantly reduced by the simultaneous addition of methane and application of post-injection strategies of diesel fuel. The soot emission was reduced by 58.85% with the Post35 injection strategy. [ABSTRACT FROM AUTHOR]

The probabilty density function (PDF) of the mixture fraction is of integral importance to a large number of combustion models. Here, a novel modelling approach for the PDF of the mixture fraction is proposed which employs dissipation elements. While being restricted to the commonly used mean and variance of the mixture fraction, this model approach individually considers contributions of the laminar regions as well as the turbulent core and the turbulent/non-turbulent interface region. The later region poses a highly intermittent part of the flow which is of high relevance to the non-premixed combustion of pure hydrocarbon fuels. The model assumptions are justified by means of the gradient trajectory based analysis of high fidelity direct numerical simulation (DNS) datasets of two turbulent inert configurations and a turbulent non-premixed jet flame. The new dissipation element based model is validated against the DNS datasets and a comparison with the beta PDF is presented. [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]

Abstract Resistance to extinction by stretch is a key property of any flame, and recent work has shown that this property controls the overall structure of several important types of turbulent flames. Multiple definitions of the critical strain rate at extinction (ESR) have been presented in the literature. However, even if the same definition is used, different experiments report different extinction strain rates for flames burning the same fuel-air mixture at very similar temperatures using similarly constructed opposed-flow instruments. Here we show that at extinction, all these flames are essentially identical, so one would expect that each would be assigned the same value of a parameter representing its intrinsic resistance-to-stretch-induced-extinction, regardless of the specifics of the experimental apparatus. A similar situation arises in laminar flame speed measurements since different apparatuses could result in different strain rate distributions. In that instance, the community has agreed to report the unstretched laminar flame speed, and methods have been developed to translate the experimental (stretched) flame speed into a universal unstretched laminar flame speed. We propose an analogous method for translating experimental measurements for stretch-induced extinction into an unambiguous and apparatus-independent quantity (ESR ∞) by extrapolating to infinite opposing burner separation distance. The uniqueness of the flame at extinction is shown numerically and supported experimentally for twin premixed, single premixed, and diffusion flames at Lewis numbers greater than and less than one. A method for deriving ESR ∞ from finite-boundary experimental studies is proposed and demonstrated for methane and propane experimental diffusion and premixed single flame data. The two values agree within the range of ESR differences typically observed between experimental measurements and simulation results for the traditional ESR definition. [ABSTRACT FROM AUTHOR]

Dimethyl ether (DME) is promising as an additive for co-firing with ammonia (NH 3) to solve the problem of its low reactivity combustion. However, the combustion modeling of blended fuels is challenging, and previous efforts primarily relied on the adjustment of rate constants for key reactions to bridge the gap between experiments and simulations. In the present work, ab initio kinetic studies were performed for the key reactions of OH/HO 2 /NH 2 with DME in blended combustion. High-level theoretical calculations were carried out for all saddle points and a benchmark was implemented for accurate evaluation of the barrier heights. The direct dynamics calculations used variational transition state theory with interpolated single-point energies method. The kinetic studies took into account the small-curvature tunneling, low-pressure limit or pre-equilibrium model and the multistructural torsional anharmonicity effect. The calculated rate constants are in very good agreement with the available experimental kinetic data. The mechanism of Xiao et al. (Combust. Flame 245 (2022) 112372) was updated based on this kinetic study and the simulations of the ignition delay times with modified rate constants better agree with the measurements than with the original mechanism. Especially, the present model avoids decreasing the cross-reaction rate constants to improve the simulation results. This work provides detailed theoretical insights and updated models with good predictive performance based on ab initio kinetics, which have more rigorous theoretical significance. [ABSTRACT FROM AUTHOR]

The formation and destruction pathways of the formyl radical (HCO) occupy a pivotal role in the conversion of fuel molecules (and their intermediates) to eventual products CO and CO 2 , and therefore, HCO has been a prescient indicator for heat release in combustion. In this work, we have characterized the impact of including non-equilibrium effects for HCO, i.e. “prompt” dissociation of HCO to H + CO, in simulations of laminar flame speeds for archetypal hydrocarbon and oxygenated molecules relevant to combustion. Prompt dissociation probabilities for HCO were systematically applied to all elementary reactions that included this radical (as either a product or reactant) in literature combustion kinetics models. Simulations with the prompt HCO dissociation corrected models predicted a 7–13% increase in laminar flame speeds at 1 atm for the fuels characterized here (CH 4 , n-C 7 H 16 , CH 3 OH, CH 3 OCH 3 ) relative to the predictions using the original models. It is evident that simulations of other fuel-air flames at 1 atm will be similarly impacted, suggesting the indispensability of incorporating these non-equilibrium effects for predictive flame modeling. Simulations of higher pressure (10 atm) heptane-air flames predicted a more modest effect (< 5%) of incorporating these non-equilibrium effects. Additionally, species profiles in low-pressure (0.03 atm) flames of CH 2 O and auto-ignition delay simulations (1.4 atm) for CH 2 O O 2 Ar mixtures were also impacted to a noticeable extent. Lastly, it is also worth noting that prompt dissociations are a ubiquitous feature of all weakly-bound radicals; the kinetics of many of which (C 2 H 3 , C 2 H 5 , CH 3 O, CH 2 OH, etc.) are central to our current understanding of combustion chemistry. Theory/modeling studies are in progress to address the relevance of prompt dissociations in these weakly-bound radicals to combustion modeling. [ABSTRACT FROM AUTHOR]

Abstract: Tabulated chemistry is a popular technique to account for detailed chemical effects with an affordable computational cost in gaseous combustion systems. However its performances for spray combustion have not completely been identified. The present article discusses the chemical structure modeling of spray flames using tabulated chemistry methods under the hypothesis that the chemical subspace accessed by a two-phase reactive flow can be mapped by a collection of gaseous flamelets. It is shown that tabulated chemistry methods based either on pure premixed flamelets or on pure non-premixed flamelets fail to capture the structure of spray combustion. The reason is the complexity of the chemical structure of spray flames which exhibits both premixed-like and non-premixed-like reaction zones. To overcome this issue, a new multi-regime flamelet combustion model (called Partially-Premixed Flamelet Tabulation 2PFT) is presented in this paper. Information from premixed, partially-premixed and diffusion flames are stored in a 3-D look-up table parametrized as a function of the progress variable Y c , describing the progress of the reaction, the mixture fraction Y z , denoting the local equivalence ratio, and the scalar dissipation χ ∗, which identifies the combustion regime. The performances of the 2PFT method are evaluated on counterflow laminar spray flames for different injection conditions of droplet diameter, liquid volume fraction and velocity. The 2PFT tabulation method better describes the chemical structure of spray flames compared to the classical techniques based on single archetypal flamelets. These results also confirm that the chemical structure of laminar spray flame can be modeled by a multi-regime flamelet combustion model based on gaseous flamelets. [Copyright &y& Elsevier]

Abstract: Chemistry tabulation techniques such as flamelet models are a popular way to account for detailed chemistry effects in numerical simulation. These techniques are based on the identification of a low dimensional manifold in chemical space that accurately represents chemical evolutions associated to a specific combustion regime. During the last years, several authors used the Principal Component Analysis (PCA) to identify low dimensional manifold for combustion problems. However, full coupling between this manifold and flow solver has not yet been performed to the authors knowledge. The present paper introduces a new approach called Manifold Generated by a Local PCA or MG-L-PCA, which fully couple the manifold identified by a PCA and a DNS flow solver. The first part of the paper presents the PCA approach. Then, the coupling between this manifold and a DNS solver is presented. The MG-L-PCA approach is finally validated against a DNS simulation of flame vortex interaction using both detailed mechanism and a FPI manifold. Unlike FPI, the MG-L-PCA reproduces the dispersion in the chemical space induced by the flame-vortex interaction both for the species and the source terms. [Copyright &y& Elsevier]

Abstract: Reactions of hydrocarbon radicals with oxygen atoms are important in combustion, and both the rate coefficient and product branching have to be known for an accurate combustion modeling. In this work, the primary product formation in the reaction of the simplest open-chain secondary alkyl radical, iso-propyl (2-C3H7), with oxygen atoms in the gas phase was studied at room temperature and a pressure of 4mbar. 2-C3H7 radicals were generated from diisopropylketone ((2-C3H7)2CO) and isopropyliodide (2-C3H7I), and O atoms were produced from SO2 by laser-flash photolysis at λ =193nm, respectively. The reactants and products were detected by quantitative FTIR spectroscopy. The combined product analysis in the experiments with the different precursors leads to the following relative branching fractions: 2-C3H7 +O→CH3CHO+CH3 (40%), CH3COCH3 +H (36%), C3H6 +OH (24%). The channel branching of the iso-propoxy (2-C3H7O) radical formed from the 2-C3H7 +O reaction was modeled using statistical rate theory with molecular and transition state data from G3MP2B3 calculations. The absolute rate of reaction was studied at room temperature and a pressure of 5.8mbar. Laser-induced fluorescence (LIF) was used for the specific detection of the OH (v =0,1) radicals, and the rate coefficient of the 2-C3H7 +O reaction was derived from the OH (v =1) LIF-time profile leading to k(2-C3H7 +O)=(1.14±0.15)×1014 cm3/(mol·s) at 298K. The OH-forming direct abstraction route and the channel to CH3CHO+CH3 may influence the flame speed of a propane flame. This is revealed, when the rate coefficient and channel branching of the 2-C3H7 +O reaction is incorporated in a suitable detailed reaction mechanism and target experiments are modeled in absence and presence of the title reaction. [Copyright &y& Elsevier]

Abstract: The combustion processes of both gasoline and diesel engines are becoming similar, as a result of the application of direct fuel injection to the former and the reduction of the compression ratio of the latter. A novel time-scale interaction (TI) combustion model has been developed for simulating combustion phenomena with high accuracy, ranging from premixed charge combustion to diffusion combustion. This model is based on reasonable combustion regimes, taking into account the characteristic time scales of the chemical reactions and turbulence eddy break-up. The accuracy of the TI combustion model was validated by applying it to gasoline and diesel IC engines under a wide range of operating conditions. Comparisons of measured and calculated heat release rate patterns and cylinder pressure histories showed good agreement for both gasoline and diesel IC engines. [Copyright &y& Elsevier]

Abstract: Tetra-ol glycidyl azide polymer (GAP) is one of the best candidates for the solid fuel of gas hybrid rocket system because of self-combustibility, better mechanical property and high heat of formation, and comprehensive understanding of combustion phenomena is indispensable for such an application. Combustion model of GAP, which is one-dimensional three-phase mode combustion model, was developed by Beckstead et al. and they applied it to tri-ol GAP successfully. We have applied this model to tetra-ol GAP as an initial attempt, and numerical simulation showed that maximum temperatures in the gas phase exceeded those of experimental results significantly, and calculated burning rates were much higher than strand burner data, thus, modification of the model taking account of combustion incompleteness was found to be necessary. Combustion residues of GAP were investigated, and those were found to be composed of soot (black color), and high viscosity residue (HVR), and yellow powder which was only observed at high pressures. These residues were analyzed by means of SEM and FTIR, and mass balance was also measured. Modifications of combustion model were made taking the residue analysis results into account as Blow Off Mechanism. Simulated final temperature in the gas phase and burning rate are lowered effectively and coincide well with experimental data adjusting kinetic parameters. [Copyright &y& Elsevier]

Abstract: Nitromethane was investigated in this study due to the push for higher performance and reduced toxicity monopropellant. A comprehensive detailed model for its flame structure and linear regression rate was developed and validated with experimental data. The model considered one-dimensional behavior with surface vaporization and detailed gas-phase kinetics based on the RDX mechanism of Yetter et al. combined with the nitromethane decomposition mechanism of Glarborg, Bendtsen, and Miller, resulting in a mechanism consisting of 47 species and 250 elementary reactions. The predictive model was implemented using a custom FORTRAN code wrapping the CHEMKIN 4 PREMIX gas-phase solver coupled with the condensed-phase solution. Predicted burning rates using the model showed good agreement with measured rates up to 15MPa. Calculated species and temperature profiles showed three distinct regions based upon the appearance and consumption of certain species. The first region was marked by decomposition of nitromethane, the second region by consumption of all intermediate species except CH4 and NO, and the third region by the rise to final temperature and species concentrations near the equilibrium values. Among the intermediate species, CH4 and NO had higher concentrations than those of CH2O, N2O, HNO, and HONO. CH4 served as fuel species and NO provided a portion of the oxidizer for the third-region reactions to reach equilibrium composition. Sensitivity analysis identified the importance of two elementary reactions involving HNO to the temperature profile, and therefore the burning rate. Although the absolute level of NH and HCO were low, they served as an important intermediate species transporting nitrogen and carbon, respectively, between other higher-concentration species. The calculated flame zone thickness is consistent with that measured by microthermocouples. [Copyright &y& Elsevier]

Abstract: Spray-guided spark-ignited direct-injection (SG-SIDI) engines are likely to be the next generation of gasoline engines. Compared to current wall-guided (WG-SIDI) engines, they offer improved fuel economy and substantially reduced hydrocarbon and soot emissions. Two stages of stratified combustion (partially premixed flame propagation and mixing-controlled combustion of rich products left from partial combustion of initially fuel-rich mixtures) are investigated here in an experimental SG-SIDI engine using detailed CFD modeling, high-speed spectrally resolved combustion luminosity imaging, and cylinder pressure analysis. The CFD model reproduces the spatial and temporal distribution of both combustion stages. Correlation of CFD calculations and experimental combustion measurements demonstrates that optimum ignition timing occurs for somewhat rich equivalence ratios (0.9–1.6) and relatively low flow velocities (&#60;15m/s) at the spark location. Both model and experiment show: (1) for fixed injection time, increased rich burning occurs for earlier ignition times and (2) higher in-cylinder swirl leads to asymmetric flame propagation and increased rich burning. Calculated and measured overall heat-release rates agree quite well over a wide range of engine operating conditions. These experiment-model comparisons provide a detailed picture of combustion processes in spray-guided SIDI engines and a sound scientific basis for engine design optimization. [Copyright &y& Elsevier]