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High-speed microjet hydrogen–air diffusion flames are investigated computationally. The focus is on the prediction of the so-called bottleneck phenomenon. The latter has been previously observed as a specific feature of the present flame class and has not yet been investigated computationally. In the configuration under consideration, the nozzle diameter is 0.5 mm and six cases with mean nozzle injection velocities (U) between 306 m/s and 561 m/s are considered. The flow in the nozzle lance is analyzed separately to obtain detailed inlet boundary conditions for the flame calculations. It is confirmed by calculation that the phenomenon is mainly determined by the transition to turbulence in the initial parts of the free jet. The transitional turbulence proves to be the biggest challenge in predicting this class of flames, as the generally available turbulence and turbulent combustion models reach the limits of their validity in transitional flows. In a Reynolds-Averaged Numerical Simulation framework, the Shear Stress Transport model is found to perform better than alternative two-equation models and is used as the turbulence model. By neglecting the interactions between the turbulence and chemistry (no-model approach), it is possible to predict the morphology of the bottleneck flame and its dependence on U qualitatively. However, the position of the bottleneck is overpredicted for U < 561 m/s. The experimental flames in the considered U range are all attached to the nozzle. This is also predicted by the no-model approach. The Eddy Dissipation Concept (EDC) used as the turbulence combustion model predicts, however, lifted flames (with increasing lift-off height as U decreases). With the EDC, no bottleneck morphology is observed for U = 561 m/s. For lower U, the EDC results for the bottleneck position are generally closer to the measurements. It is demonstrated that accuracy in predicting the bottleneck position can be improved by ad hoc modifications of the turbulent viscosity. [ABSTRACT FROM AUTHOR]

Drop-in fuels for aviation gas-turbine engines have been introduced recently to mitigate global warming. Despite their similarity to the fossil fuel Jet A-1, their combustion in traditional combustors should be thoroughly analyzed to maintain engine health and low emissions. The paper introduces criteria for assessing the impact of the chemical composition of fuels on combustion in the DEGN 380 turbofan. Based on previous emission-test results, the power functions of carbon monoxide and its emission index were adopted as the model of combustion. Based on the general notation of chemical reactions leading to the production of CO in combustion, the regression coefficients were given a physical meaning by linking them with the parameters of the kinetic equations, i.e., the reaction rate constant of CO and CO2 formation expressed as exponential functions of combustor outlet temperature and the concentration of O2 in the exhaust gas, as well as stoichiometric combustion reactions. The obtained empirical functions show that, in the entire range of engine operating parameters, synthetic components affect the values of the rate constants of CO and CO2 formation. It can be explained by the change in activation energy determined for all chain-of-combustion reactions. The activation energy for the CO formation chain changes in the range between 8.5 kJ/mol for A0 and 24.7 kJ/mol for A30, while for the CO2 formation chain between 29.8 kJ/mol for A0 and 30.8 kJ/mol for A30. The reactivity coefficient lnαiCOACODCO changes between 2.29 for A0 and 6.44 for A30, while lnαiCO2ACO2DCO2 changes between 7.90 for A0 and 8.08 for A30. [ABSTRACT FROM AUTHOR]

In the last few years, car manufacturers are moving toward electrified powertrains, like Battery Electric Vehicles (BEV) and Hybrid Electric Vehicles (HEV), to reduce the levels of CO2 in the atmosphere. A promising alternative to BEVs to reduce CO2 emissions, while maintaining an existing and well-developed technology, could be the use of hydrogen for internal combustion engines (ICEs). Particularly, the zero-carbon content of hydrogen allows for guaranteeing very clean combustion and near-zero carbon footprint. Within this context, the present study aims to deeply investigate the effects of pure hydrogen fueling of Spark Ignition (SI) engines, especially on specific fuel consumption and NOx emissions. Particularly, the benefits of pure hydrogen fueling were assessed in a production small Direct Injection (DI) SI engine. The engine is intended to operate in steady state conditions as an Auxiliary Power Unit (APU) for a series hybrid powertrain. The effects of pure hydrogen fueling were investigated at two engine speeds, namely 2000 and 3000 rpm. All tests were carried out at unthrottled conditions and lean mixture. A 1-D engine model was developed within a commercial software. Combustion, heat transfer, and NOx emissions embedded sub-models were tuned based on the experimental data, to accurately reproduce the engine behavior in a wide range of operating conditions. Both experimental and simulation results demonstrated that hydrogen fueling, together with lean mixture operation, allow for significantly improving engine thermal efficiency. The results of 1-D model simulations supported the energy management strategies of the hybrid powertrain in selecting the most suitable ICE operating condition. Particularly, the best engine operating conditions in terms of fuel consumption are achieved at 2500 rpm, with λ = 2.0, while the lowest NOx emissions are reached when λ is increased up to 2.4. [ABSTRACT FROM AUTHOR]

Thrust regulation is applied to maintain the performance of the liquid propellant rocket engine. The thrust level of a rocket engine can be readily controlled by adjusting the number of propellants introduced into the combustion chamber. In this study, a gas generator design is proposed in which thrust regulation is maintained by performing secondary combustion in the divergent section of the nozzle of a gas generator. Tangential and normal injection techniques have also been studied for better combustion analyses. A normal injection technique is used for the experiment and CFD results are validated with the experimental data. Chemical equilibrium analyses are also performed by minimizing Gibbs free energy with the steepest descent method augmented by the Nelder–Mead algorithm. These equilibrium calculations give the combustion species as obtained through the CFD results. Performance evaluation of the rocket engine, with and without secondary combustion in the gas generator, led to an increase of 42% thrust and 46.15% of specific impulse with secondary combustion in the gas generator. [ABSTRACT FROM AUTHOR]

Research in the field of Internal Combustion (IC) engines focuses on the drastic reduction of both pollutant and greenhouse gas emissions. A promising alternative to gasoline and diesel fuel is represented by the use of gaseous fuels, above all green hydrogen but also Natural Gas (NG). In previous works, the authors investigated the performance, efficiency, and emissions of a supercharged Spark Ignition (SI) engine fueled with mixtures of gasoline and natural gas; a detailed research involving the combustion process of this kind of fuel mixture has been previously performed and a lot of experimental data have been collected. Combustion modeling is a fundamental tool in the design and optimization process of an IC engine. A simple way to simulate the combustion evolution is to implement a mathematical function that reproduces the mass fraction burned (MFB) profile; the most used for this purpose is the Wiebe function. In a previous work, the authors proposed an innovative mathematical model, the Hill function, that allowed a better interpolation of experimental MFB profiles when compared to the Wiebe function. In the research work presented here, both the traditional Wiebe and the innovative Hill function have been calibrated using experimental MFB profiles obtained from a supercharged SI engine fueled with mixtures of gasoline and natural gas in different proportions; the two calibrated functions have been implemented in a zero-dimensional (0-D) SI engine model and compared in terms of both Indicated Mean Effective Pressure (IMEP) and cyclic pressure variation prediction reliability. It was found that the Hill function allows a better IMEP prediction for all the operating conditions tested (several engine speeds, supercharging pressures, and fuel mixtures), with a maximum prediction error of 2.7% compared to 4.3% of the Wiebe function. A further analysis was also performed regarding the cyclic pressure variation that affects all the IC engines during combustion and may lead to irregular engine operation; in this case, the Hill function proved to better predict the cyclic pressure variation with respect to the Wiebe function. [ABSTRACT FROM AUTHOR]

Numerical simulations of combustion processes in rocket engines requires a long run time of supercomputer systems even for a very short physical time. Therefore, creating digital twins of rocket engines needs enormous processor time, and is not very effective. This computational time surpasses the actual physical time of the process in many orders of magnitude. To speed up numerical simulations the paper presents a solution of the chemical kinetics problem using artificial neural network approach. Using the architecture of a multilayer neural network with bypass connections, namely residual network, it is possible to obtain a fast and reliable solution to the problem. The neural network is trained to predict the state of the system only one time step ahead. Using it in a recursive mode, it is possible to forecast for thousands of steps without loss of accuracy. • Simulation of a chemical kinetic problem by the neural network method was carried out. • The network was trained on a wide range of data. • A shortened neural network with preservation of accuracy was obtained. • The considered network used a small amount of RAM (Random Access Memory). [ABSTRACT FROM AUTHOR]

Estimation of combustion phasing and power production is essential to ensuring proper combustion and load control. However, archetypal control-oriented physics-based combustion models can become computationally expensive if highly accurate predictive capabilities are achieved. Artificial neural network (ANN) models, on the other hand, may provide superior predictive and computational capabilities. However, using classical ANNs for model-based prediction and control can be challenging, since their heuristic and deterministic black-box nature may make them intractable or create instabilities. In this paper, a hybridized modeling framework that leverages the advantages of both physics-based and stochastic neural network modeling approaches is utilized to capture CA50 (the timing when 50% of the fuel energy has been released) along with indicated mean effective pressure (IMEP). The performance of the hybridized framework is compared to a classical ANN and a physics-based-only framework in a stochastic environment. To ensure high robustness and low computational burden in the hybrid framework, the CA50 input parameters along with IMEP are captured with a Bayesian regularized ANN (BRANN) and then integrated into an overall physics-based 0D Wiebe model. The outputs of the hybridized CA50 and IMEP models are then successively fine-tuned with BRANN transfer learning models (TLMs). The study shows that in the presence of a Gaussian-distributed model uncertainty, the proposed hybridized model framework can achieve an RMSE of 1.3 × 10−5 CAD and 4.37 kPa with a 45.4 and 3.6 s total model runtime for CA50 and IMEP, respectively, for over 200 steady-state engine operating conditions. As such, this model framework may be a useful tool for real-time combustion control where in-cylinder feedback is limited. [ABSTRACT FROM AUTHOR]

This investigation outlines a data-assisted approach that employs random forest classifiers for local and dynamic submodel assignment in turbulent-combustion simulations. This method is demonstrated in simulations of a single-element GOX/GCH4 rocket combustor; a priori as well as a posteriori assessments are conducted to (i) evaluate the accuracy and adjustability of the classifier for targeting different quantities of interest (QoIs), and (ii) assess improvements, resulting from the data-assisted combustion model assignment, in predicting target QoIs during simulation runtime. Results from the a priori study show that random forests, trained with local flow properties as input variables and combustion model errors as training labels, assign three different combustion models – finite-rate chemistry (FRC), flamelet progress variable (FPV) model, and inert mixing (IM) – with reasonable classification performance even when targeting multiple QoIs. Applications in a posteriori studies demonstrate improved predictions from data-assisted simulations, in temperature and CO mass fraction, when compared with monolithic FPV calculations. An additional a posteriori data-assisted simulation of a modified configuration demonstrates that the present approach can be successfully applied to different configurations, as long as thermophysical behavior can be represented by the training data. These results demonstrate that this data-driven framework holds promise for dynamic combustion submodel assignments in reacting flow simulations. [ABSTRACT FROM AUTHOR]

Chemical flame structures encountered in practical turbulent combustion chambers are complex because multiple regimes such as premixed, stratified or diffusion may coexist. Combustion modeling strategies based on single flame archetype fail to predict pollutant species, such as CO. To account for multiple combustion regimes, at a reduced computational cost, a novel approach based on virtual optimized mechanisms is developed. This method consists in (i) building a kinetic scheme from scratch instead of reducing a detailed mechanism, (ii) using a reduced number of virtual reactions and virtual species that do not represent real entities and (iii) using sub-mechanisms dedicated to the prediction of given flame quantities. In the present work, kinetic rate parameters of the virtual reactions and virtual species thermodynamic properties are optimized through a genetic algorithm to properly capture the flame temperature as well as CO formation in hydrocarbon/air flames. The virtual optimized chemistry approach is first applied to the derivation of methane/air reduced kinetic schemes. The flame solutions obtained with the virtual optimized mechanisms are subsequently compared to the reference flame library showing good predictive capabilities for both premixed and non-premixed flame archetypes. Analysis of the impact of the reference database demonstrated that the quality of CO formation modeling depends on the reference flame library used to train the optimized kinetic scheme. The virtual mechanisms are also tested on 2-D partially-premixed burners showing good agreement between the reference mechanism and the reduced virtual schemes. Finally, the virtual optimized chemistry approach is used to derive virtual optimized mechanisms for heavy fuels oxidation. [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]

Highlights • Increased mixing tube diameter increases aeration throughout firing range. • Decreased injector diameter increases aeration for high firing rates. • Increased burner surface permeability increases aeration for high firing rates. • Increased combustion chamber height increases aeration for low firing rates. • Increased secondary aeration reduces primary aeration for low firing rates. Abstract This work is about a model for the prediction of air entrainment in a full-premix, atmospheric, ejector-pump like gas burner, used for domestic water heating purposes. The model improves current knowledge by including the effects of combustion and buoyancy on the air entrainment mechanism. It predicts the λ ratio (excess of air in relation to stoichiometry) as a function of ambient conditions, burner geometry, type of hydrocarbon fuel and burner firing rate. A prototype burner with variable geometry was assembled to calibrate and validate the model. The combustion products were analyzed with a CO 2 sensor and the λ ratio was compared with model's predictions, showing good accuracy. A parametric study was done to ascertain the impact of key geometric parameters on burner aeration. [ABSTRACT FROM AUTHOR]

Abstract The key factors for efficient in-flight particle heating in a combusting flow were investigated within this paper for the development of a novel boiler slag bead production furnace. A natural gas fired industrial burner with a thermal input of 1.2 MW was thus evaluated using Computational Fluid Dynamics (CFD). The steady laminar flamelet model (SFM) and a detailed chemical reaction mechanism, considering 25 reversible chemical reactions and 17 species were used to account for the steady-state gas phase combustion. Measurements of gas temperature and flow velocity within the furnace were found to be in good accordance with the numerical results. In the second step, sintered bauxite beads were injected into the furnace as an experimental material and heated up in flight. The particle heating characteristics were investigated using the Discrete Phase Model (DPM). The computational results of the particle laden flow raised the issue that convective heat transfer is a key factor for efficient particle heating. At the burner chamber outlet, the temperature of a particle which had been injected into the burner flame was 178 K higher compared to a particle, which trajectory led through zones with lower gas temperatures. Highlights • Calculations and experiments were performed on a 1.2 MW industrial furnace. • In-flame temperature measurements and numerical results are compared. • The main influence factors for particle heating were evaluated. • Numerical calculation of convective and radiative particle heat transfer. • Convection plays an important role for in-flight heating of particles. [ABSTRACT FROM AUTHOR]

An implementation and validation of the coupled Σ-Y ADF model is presented in this work for reacting Diesel spray CFD simulations under a RANS turbulence modeling approach. An Approximated Diffusion Flamelet (ADF) model Michel et al. (2008) implemented in the OpenFOAM CFD open-source library by Winklinger (2014) is fed with the spray description, i.e. mixing formation process, provided by the Σ-Y Eulerian atomization model Garcia-Oliver et al. (2013). In the present investigation, the Engine Combustion Network Spray A reference configuration is used for validation. Specifically, the model can provide accurate predictions of typical reacting spray metrics, such as the ignition delay and the lift-off length. Moreover, the internal structure is also fairly reproduced in terms of quasi-steady spatial distribution of formaldehyde and OH, related with low and high temperature reactions respectively. Additionally, modeling results have been compared to recent Particle image velocimetry (PIV) measurements Garcia-Oliver et al. (2017) under both inert and reacting conditions. Flow response to heat release is quantitatively predicted by the model, both in terms of local velocity increase as well as radial dilation. The model has been used to understand combustion-induced reduction in entrainment, in particular around the lift-off length location. Flow confinement does not seem to influence the global flame behaviour, even though some changes in the local flow hint can be observed when moving from an open to a closed domain. [ABSTRACT FROM AUTHOR]

In the two limits of a well-mixed charge (e.g. homogenous charge compression ignition (HCCI)) engine, and a well separated charge (e.g. conventional diesel combustion (CDC)) engine, the CHEMKIN coupled computational fluid dynamics (CFD) codes approach has been proved to work well in predicting both chemistry controlled combustion and mixing controlled combustion. In this study, it is shown that for some certain cases in the new combustion mode reactivity controlled compression ignition (RCCI) engine where flame propagation exists, the CHEMKIN approach could fail to predict the combustion characteristics. To extend the capability of the existing CHEMKIN models in combustion, a flame propagation model (FPM) accounting for combustion in partially premixed mixtures was proposed and coupled into the CFD framework KIVA4. In this FPM model, the turbulent flame speed was calculated and the heat release and species conversion rate in the turbulent flame brush was solved with detailed chemical kinetics. A NOx sub-mechanism and a nine-step phenomenological soot model were also coupled into the current integrated model for emission prediction. This model was validated in 3 different dual-fuel experimental engines by comparing the in-cylinder pressure, heat release rate (HRR), NOx emissions and soot emissions. The CHEMKIN and CHEMKIN-FPM were also compared in terms of their capability of predicting the combustion in different combustion regimes. The results show that under certain operating conditions when CHEMKIN failed in the combustion prediction, the current CHEMKIN-FPM shows a superior ability in predicting combustion characteristics. [ABSTRACT FROM AUTHOR]

The ability of flamelet models to reproduce turbulent combustion in devices such as diesel engines or gas turbines has enhanced the usage of these approaches in Computational Fluid Dynamics (CFD) simulations. The models based on turbulent look-up tables generated from counterflow laminar diffusion flames (DF model) permit drastic reduction of the computational cost of the CFD calculation. Nevertheless, for complex molecular fuels, such as n-heptane, the oxidation process involves hundreds of species and the calculation of the transport equations together with the ODE system that models the chemical kinetics for the DF solution becomes unaffordable for industrial devices where hundreds of flamelets are required. In this context, new hypotheses have to be introduced in order to reduce the computational cost maintaining the coherence of the combustion process. Recently, a new model known as Approximated Diffusion Flamelet (ADF) has been proposed with the aim of solving the turbulent combustion for complex fuels in a reduced time. However, the validity of this model is still an open question and has to be verified in order to justify subsequent CFD calculations. This work assesses the ADF model and its ability to reproduce accurately the combustion process and its main parameters for three fuels with different chemical complexity and boundary conditions by its comparison with the DF model. Results show that although some discrepancies arise, the ADF model has the ability to correctly describe the ignition delay and the combustion structure in the auto-ignition zone that is the most relevant one for industrial processes. [ABSTRACT FROM AUTHOR]

The blending of ammonia with alternative fuel alcohols has recently received increasing interest and it has been shown that their cross-reactions significantly influence the ignition process. In the present work, the detailed electronic structure calculations and kinetic studies for the reactions of CH 3 OH/C 2 H 5 OH with NH 2 were performed. The rate constants were calculated by Master Equation/Rice–Ramsperger–Kassel–Marcus (ME/RRKM) over a pressure range of 10−1–105 Torr for the pressure-dependent effect caused by reactant complex. The multistructural torsional anharmonicity effects of the reaction were analyzed and the high-pressure limiting rate constants were calculated over a wide temperature range of 250–2000 K by using multistructural variational transition state theory. All calculations considered the tunneling effect. The results show that the CH 3 OH/NH 2 system exhibits pressure-dependent below 550 K, while the pressure-dependent of the C 2 H 5 OH/NH 2 system can be safely neglected. Besides, the multistructural torsional anharmonicity effect is pronounced for the C 2 H 5 OH/NH 2 system, especially at intermediate and high temperatures. The channel for the formation of CH 2 CH 2 OH is underestimated and the branching ratio of this channel exceeds that of CH 3 CH 2 O and CH 3 CHOH under the multistructural influence at high temperatures. Thus, we updated the kinetic model and further investigation on ignition delay times and sensitivity analysis. The discrepancies in rate constants caused by the kinetic methods mentioned above seems to have a non-negligible effect on the combustion modeling. [ABSTRACT FROM AUTHOR]

The utilization of low-dimensional manifold combustion models for large-eddy simulation (LES) of turbulent reactive flows introduces model simplifications that represent sources of uncertainties in addition to those arising from turbulent closure models and numerical discretization. The ability to quantitatively assess these uncertainties in the absence of measurements or reference results is vital for reliable and predictive simulations of practical combustion devices. This paper is concerned with the extension of the manifold drift term to LES to examine the compliance of a particular combustion model in describing a quantity of interest (QoI) with respect to the underlying flow-field representation. This drift term was previously introduced as a key component of the Pareto-efficient combustion (PEC) framework. The behavior of the drift term is examined in a series of test cases. To this end, large-eddy simulations of a partially-premixed turbulent pilot flame with inhomogeneous inlet streams are performed, in which the non-premixed flamelet/progress-variable (FPV) model and the premixed filtered tabulated chemistry LES (F-TACLES) formulation are employed. The drift term is shown to be capable of identifying chemically sensitive regions with respect to user-specific QoIs. With this, a species-specific combustion regime indicator is derived by computing the relative magnitude of the drift terms for different combustion models. Comparisons with commonly employed flame indicators suggests that the flame index and other indicators that are solely based on major species and flame topology are insufficient in describing complex physical processes in multi-regime combustion. [ABSTRACT FROM AUTHOR]

Various aspects of fast cook-off simulations are explored with a spatially one-dimensional, fully transient numerical model. The model is applied to HMX and consists of equations for the solid (condensed) phase, the gas phase, and the surrounding steel container. The condensed phase HMX decomposition reactions are described by distributed kinetics. The gas phase description includes a detailed chemistry model for the combustion of HMX. Results are presented that outline considerations for simulating fast cook-off ignition. It is found that small changes in the heat path, such as air gaps, significantly change the predicted ignition time for fast cook-off, but have minimal effect on slow cook-off predictions. Simulations indicate that an average heat flux can be used in place of the temporally varying heat flux created by a pool fire. A method is outlined to predict time-to-ignition using lab-scale empirical data instead of directly implementing decomposition kinetics of an energetic material into a simulation. [ABSTRACT FROM AUTHOR]

In this paper we study the stability of fronts in a reduction of a well-known PDE system that is used to model the combustion in hydraulically resistant porous media. More precisely, we consider the original PDE system under the assumption that one of the parameters of the model, the Lewis number, is chosen in a specific way and with initial conditions of a specific form. For a class of initial conditions, then the number of unknown functions is reduced from three to two. For the reduced system, the existence of combustion fronts follows from the existence results for the original system. The stability of these fronts is studied here by a combination of energy estimates and numerical Evans function computations and nonlinear analysis when applicable. We then lift the restriction on the initial conditions and show that the stability results obtained for the reduced system extend to the fronts in the full system considered for that specific value of the Lewis number. The fronts that we investigate are proved to be either absolutely unstable or convectively unstable on the nonlinear level. [ABSTRACT FROM AUTHOR]

The selection of an appropriate combustion model for the numerical prediction of reacting flows remains an outstanding issue. Often, expert knowledge or experimental data is required to make an informed decision in selecting a suitable model. Furthermore, the computational cost that is associated with the application of a certain combustion model introduces another constraint in the selection process. By addressing these issues, the objective of this work is to develop a Pareto-efficient combustion (PEC) framework for application to complex chemically reacting flows under consideration of user-specific input about quantities of interest, desired simulation accuracy and computational cost, and a set of combustion models. PEC utilizes a Pareto efficiency, and introduces a manifold drift term as a measure for determining the adequacy of using a certain combustion-manifold model to predict selected quantities of interest. Since underlying model assumptions are encoded in the manifold, PEC restricts the application of submodels within its intended use. Further, the proposed approach for evaluating the manifold drift provides a rigorous method for combining different combustion models – as long as they can be described by a manifold. As such, this formulation represents a general description for the selection of combustion models, thereby overcoming potential limitations of flame-topology indicators and regime-specific combustion models. The capability of the PEC-framework is demonstrated in application to a tribrachial flame. By considering combustion models from the class of reaction-transport manifolds (inert mixing, equilibrium, flamelet/progress variable, and flame-prolongation in ILDM) and chemistry manifolds (using detailed and skeletal mechanisms), it is shown that PEC locally adapts the submodel fidelity within the user-defined threshold for selected quantities of interest. A parametric analysis is conducted to illustrate the dynamic range of the PEC-framework in accommodating Pareto-efficient submodel arrangements. [ABSTRACT FROM AUTHOR]

Direct numerical simulation (DNS) of freely-propagating premixed flames of a multi-component fuel is performed using a skeletal chemical mechanism with 49 reactions and 15 species. The fuel consists of CO, H2, H2O, CH4, and CO2in proportions akin to blast furnace gas or a low calorific value syngas. The simulations include low and high turbulence levels to elucidate the effect of turbulence on realistic chemistry flames. The multi-component fuel flame is found to have a more complex structure than most common flames, with individual species’ reaction zones not necessarily overlapping with each other and with a wide heat releasing zone. The species mass fractions and heat release rate show significant scatter, with their conditional average however remaining close to the laminar flame result. Probability density functions of displacement speed, stretch rate, and curvature are near-Gaussian. Five different mean reaction rate closures are evaluated in the RANS context using these DNS data, presenting perhaps the most stringent test to date of the combustion models. Significant quantitative differences are observed in the performance of the models tested, especially for the higher turbulence level case. [ABSTRACT FROM AUTHOR]

This paper analyzes published works on the emission models of diesel and BD 1 1 Biodiesel. fuels. To the best our knowledge, this is the first comprehensive survey that reviews various modeling aspects of soot emitted from the combustion of diesel and BD fuels. The pros and cons of past and recent soot models, the chronological advancement of diesel combustion chemistry, and soot modeling approaches are highlighted in this review. Soot models are divided into three main groups of empirical, semi-empirical, and detailed soot model. Phenomenological model is also explored as a soot model which is one of the most extensively investigated soot models in recent years. Soot formation mechanism is discussed with an emphasis on their molecular structure. In a vast majority of the papers reviewed, acetylene was used as a soot precursor, and also as a reactant for soot mass growth and aromatics formation in diesel soot modeling studies. Thus, it is recommended that the formation and consumption of acetylene and aromatic compounds should be included in the diesel soot modeling. For BD, aromatic compounds are found at very low concentrations during the combustion, so the contribution of aromatic compounds to soot formation may be reduced or excluded in BD soot modeling. Unlike diesel, oxygen in BD fuels is found very important in soot oxidation, thus, formation and consumption of oxygen molecules, radicals and OH 2 2 Hydroxide bonds. should be incorporated in the soot modeling as well. Finally, regardless of their structures, simple molecules such as MB 3 3 Methyl butanoate. and MD 4 4 Methyl decanoate. are found practical as BD surrogates in many modeling papers. [ABSTRACT FROM AUTHOR]

A one-dimensional transient model for simulating the conversion of solid biomass fuels in grate boilers is presented. The model considers drying; pyrolysis; and char oxidation; gas flow through the pore space of the bed; conductive, convective, and radiative heat transfer; and shrinkage of the fuel bed. It allows to study the influence of fuel properties and operating parameters on the solid fuel conversion, the resulting grate coverage, and the gas profiles. Experimental results from a 1.2 MW grate boiler at optimum conditions are used to adjust the moving bed velocity in the model. Calculations with increased fuel humidity then result in a prolonged drying time and unburned carbon in the ash. Adopted model runs further reveal that ideal conditions can be recovered either by increasing the excess air, by air pre-heating, or by reducing the boiler capacity. Hence, the model is useful to predict the influence of parameters on the fuel conversion, to evaluate measures for combustion optimization, and to develop control strategies. [ABSTRACT FROM PUBLISHER]

MILD (Moderate or Intense Low-oxygen Dilution) combustion is a novel technology to reduce NO X emission from combustion. With the development of this technology, it is expected to combine this technology with oxy-fuel combustion to jointly control the pollutants from fossil fuel combustion. The present paper investigates the combustion characteristics of coal under oxy-MILD operation with the aid of CFD simulations. A seven-step global reaction mechanism is used for modeling coal combustion involving gaseous volumetric reaction and particle surface reaction. The predicted results using the adopted models for the reference case agree well with the experimental results. It is revealed that, the combustion temperature level increases with the enhancement of initial oxygen concentration because of the reduced injection momentum and promoted heat release. The in-furnace CO formation increases but the final CO emission reduces when enhancing the oxygen concentration under oxy-MILD combustion. Moreover, oxy-MILD combustion shows a greater potential on NO X formation reduction than air-MILD combustion, and this potential is promoted at higher initial oxygen concentration. [ABSTRACT FROM AUTHOR]

Highlights: [•] The paper presents an evaporation model for Heavy Fuel Oil (HFO) combustion studies. [•] HFO is considered as a mixture of a heavy and a light fuel component. [•] CFD simulations in spray chambers and in a large marine engine are performed. [•] A modified characteristic time combustion model is utilized. [•] The results are in good agreement with experimental data. [ABSTRACT FROM AUTHOR]

Highlights: [•] Variation in hazardous waste quantities or composition affect incinerator throughput. [•] Novel combustion balance modeling isolates uncertainty from combustion invariants. [•] A new dimensionless waste fingerprint determines corrective manipulations’ direction. [•] Throughput sensitivity with respect to operating parameters is explicitly expressed. [•] Throughput loss under waste uncertainty and capacity constraints is averted. [ABSTRACT FROM AUTHOR]

Highlights: [•] A model has been developed to predict oxy-coal combustion in fluidized beds. [•] Model comprises fluid-dynamics, coal combustion, sulfur capture and heat transfer. [•] Validation is accomplished for three coals, under a variety of operating conditions. [•] Pressure, temperature and emissions patterns are simulated. [Copyright &y& Elsevier]

Abstract: Combustion simulations at the micro and meso-scales are coupled to give a more theoretically based and accurate description of AP (ammonium perchlorate)/HTPB (hydroxyl-terminated polybutadiene) composite propellant combustion. One and two-dimensional micro-scale combustion models of AP and AP/HTPB, which include detailed kinetics and species transport, are utilized to define semi-global kinetics for a meso-scale propellant combustion model capable of representing the complex morphology of an actual propellant. The AP/HTPB flame structure predicted with the micro-scale models is separated into four flames. Each flame is represented with a single reaction, thus a new four-step kinetic mechanism is implemented into the meso-scale model. Physical and kinetic parameters are determined with the micro-scale models. The meso-scale model is calibrated and validated with the micro-scale models to ensure the correct flame structure, as a function of pressure and AP particle sizes, is recreated with the four-step mechanism. The primary focus of this work is to outline the methodology used to couple the two numerical scales. Results of the meso-scale model are compared with the previous empirically-parameterized meso-scale model results and experimental data. Predictions are within 10% of experimental values for a range of AP/HTPB propellants. [Copyright &y& Elsevier]

In the present study, a theoretical investigation is accomplished to model the combustion of a single iron particle by virtue of a novel thermophysical approach. It is assumed that a spherical iron particle falls freely in the gaseous medium and preheats and then burns heterogeneously on its external surface in this hot oxidizing environment and oxygen diffuses inward to the particle. A new physical parameter for thermal radiation is introduced. By solving the energy conservation equation for the iron particle, the temperature distribution of the iron particle during the preheating and combustion processes is calculated analytically. Also, by solving the oxygen mass conservation equation, the variation of oxygen concentration within the iron particle during the combustion stage is estimated. In the proposed model, the effect of thermal radiation and dynamic behavior of burning iron particle are considered. Because of high thermal conductivity and micro size of iron particle, the Biot number is negligibly small. The non-homogeneous partial differential equations of energy and mass species resulted from the modeling of combustion are solved by utilizing the method of separation of variables. The assumptions applied in the modeling are such that do not violate the actual combustion phenomenon. Also, the numerical solution of energy equation in the combustion stage is presented and compared with the obtained analytical solution. Also, the burnout time of iron particle is evaluated in this article. This investigation is one of the first performed efforts for analyzing and modeling of single iron particle combustion. [ABSTRACT FROM PUBLISHER]

Abstract: In the present study, combustion of a single iron particle is modeled by virtue of a novel thermophysical procedure. The temperature of iron particle during combustion is studied analytically and numerically. In the proposed model, the effect of thermal radiation from the external surface of burning particle, and alterations of density of iron particle with temperature are considered. Iron particle burns heterogeneously in air. Because of high thermal conductivity and micro size of iron particle, the Biot number is negligibly small, and the lumped system analysis can be utilized for the combustion modeling. The nonlinear energy equation resulted from modeling is solved by using homotopy perturbation method. The assumptions applied in the modeling are such that do not violate the actual combustion phenomenon. Also, the numerical solution of nonlinear differential equation is presented and compared with the analytical solution obtained from homotopy method. It is the first model presented for analyzing the combustion of single iron particle. [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: Modeling NO x emissions from coal fired utility boiler is critical to develop a predictive emissions monitoring system (PEMS) and to implement combustion optimization software package for low NO x combustion. This paper presents an efficient NO x emissions model based on support vector regression (SVR), and compares its performance with traditional modeling techniques, i.e., back propagation (BPNN) and generalized regression (GRNN) neural networks. A large number of NO x emissions data from an actual power plant, was employed to train and validate the SVR model as well as two neural networks models. Moreover, an ant colony optimization (ACO) based technique was proposed to select the generalization parameter C and Gaussian kernel parameter γ. The focus is on the predictive accuracy and time response characteristics of the SVR model. Results show that ACO optimization algorithm can automatically obtain the optimal parameters, C and γ, of the SVR model with very high predictive accuracy. The predicted NO x emissions from the SVR model, by comparing with the BPNN model, were in good agreement with those measured, and were comparable to those estimated from the GRNN model. Time response of establishing the optimum SVR model was in scale of minutes, which is suitable for on-line and real-time modeling NO x emissions from coal-fired utility boilers. [Copyright &y& Elsevier]

Abstract: Probability density function (PDF) methods are especially suited for turbulent combustion calculations, since the averaging of the reaction source term in the governing equation poses no closure problem. Molecular mixing and computing thereof in the presence of chemical kinetics, however, remains a major modeling challenge; not only for PDF methods. In this work we present a new model for partially premixed combustion based on the joint statistics of mixture fraction, scalar dissipation rate and a burning indicator, which is used to evolve notional fluid particles in mixture fraction-reactive scalar space (e.g., enthalpy). Therefore, the parameterized scalar profile (PSP) mixing model was extended for reactive scalars. As in the PSP mixing model for inert scalars, each fluid particle is associated with a representative profile in physical space. Different than in the standard PSP model, the reactive profiles get modified by chemical reactions. The decision whether a fluid particle is reacting or not is based on a burning indicator, the local scalar dissipation rate and the profile boundaries. The burning indicator accounts for the flame propagation in the partially premixed environment and controls extinction and re-ignition; together with the scalar dissipation rate. The solutions of the reaction–diffusion equation for different scalar dissipation rates is used to determine the state of the embedded flame associated with a reacting particle, which allows to construct reactive scalar profiles and on the other hand pure diffusion is assumed for extinct particles. The new combustion model was validated with the Sandia flame F data and comparisons demonstrate its ability to account for the relevant phenomena in partially premixed flames. [Copyright &y& Elsevier]

Abstract: We develop and solve a novel model for the combustion of reactive solutions impregnated into a cellulose carrier. This procedure has been shown to be effective in the synthesis of metallic oxides with a nanoscale microstructure, which are suitable for catalyst applications. The model involves three reactions, (i) combustion of the carrier matrix, (ii) an endothermic reaction related to the decomposition or gasification of the synthesis reaction precursors and (iii) the exothermic oxide synthesis reaction. This model is shown to provide qualitative agreement with experimental observations. A parametric study of the model demonstrates that increasing the heat released via the leading cellulose burning reaction (i) is most favorable in terms of increasing the reaction yield and providing conditions for smaller size of the synthesized material. [Copyright &y& Elsevier]

Abstract: This review examines current approximations and approaches that underlie the evaluation of transport properties for combustion modeling applications. Discussed in the review are: the intermolecular potential and its descriptive molecular parameters; various approaches to evaluating collision integrals; supporting data required for the evaluation of transport properties; commonly used computer programs for predicting transport properties; the quality of experimental measurements and their importance for validating or rejecting approximations to property estimation; the interpretation of corresponding states; combination rules that yield pair molecular potential parameters for unlike species from like species parameters; and mixture approximations. The insensitivity of transport properties to the intermolecular forces is noted, especially the non-uniqueness of the supporting potential parameters. Viscosity experiments of pure substances and binary mixtures measured post 1970 are used to evaluate a number of approximations; the intermediate temperature range 1 < T∗ < 10, where T∗ is kT/ε, is emphasized since this is where rich data sets are available. When suitable potential parameters are used, errors in transport property predictions for pure non-polar substances and their binary mixtures are less than 5% when they are calculated using the approaches of Kee et al.; Mason, Kestin, and Uribe; Paul and Warnatz; or Ern and Giovangigli. Recommendations stemming from the review include (1) revisiting the supporting data required by the various computational approaches, and updating the data sets with accurate potential parameters, dipole moments, and polarizabilities; (2) characterizing the range of parameter space over which the fit to experimental data is good, rather than the current practice of reporting only the parameter set that best fits the data; (3) looking for improved combining rules, since existing rules were found to under-predict the viscosity of mixtures in most cases; (4) performing more transport property measurements for mixtures that include radical species, an important but neglected area; (5) using the TRANLIB approach for treating polar molecules; (6) continuing to evaluate whether a different parameterization is required for the intermolecular potential for T∗ > 10; (7) performing more accurate measurements of the molecular parameters used to evaluate the molecular heat capacity and the rotational relaxation collision number, since they affect thermal conductivity; and (8) using the EGLIB approach and computer program with improved supporting data to evaluate transport properties. EGLIB uses the TRANLIB methodology for collision integral evaluation. [Copyright &y& Elsevier]

Abstract: Detailed modeling of complex combustion kinetics remains challenging and often intractable, due to prohibitive computational costs incurred when solving the associated large kinetic mechanisms. The Graphics Processing Unit (GPU), originally designed for graphics rendering on computer and gaming systems, has recently emerged as a powerful, cost-effective supplement to the Central Processing Unit (CPU) for dramatically accelerating scientific computations. Complex scientific computations are now being performed on the GPU in several research fields, such as quantum chemistry, molecular dynamics, and atmospheric modeling. Here, we present methods for exploiting the highly parallel structure of GPUs for combustion modeling. This paper outlines simple algorithm revisions that can be applied to the majority of existing combustion modeling algorithms for GPU computations. Significant simulation acceleration and predictive capability enhancements were obtained by using these GPU-enhanced algorithms for reaction rate evaluation and in ODE integration. For the demonstrations, we implemented the rate evaluation revisions in CHEMKIN and the ODE integration revisions in DASAC and DVODE and we tested the performance for simulating constant-volume ignition using SENKIN. The simulations using the revised algorithms are more than an order of magnitude faster than the corresponding CPU-only simulations, even for a low-end (double-precision) graphics card. Additionally, the computational time scales less than quadratically with the number of chemical species in the kinetic mechanism when using the GPU, as compared to the super-quadratic scaling normally seen with CPU-only chemical kinetics computations; and the GPU-based revisions do not involve approximations to the detailed kinetics. An analysis of the growth rates of combustion mechanism sizes versus computational capabilities of CPUs and GPUs further reveals the important role that GPUs are expected to play in the future of combustion modeling. Finally, we briefly outline practical steps for effectively transitioning from CPU-only to GPU-enhanced combustion modeling. [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: This paper is devoted to premixed combustion modeling in turbulent flow. First, we briefly remind the main features of the Self-Similar Turbulent Flame model that was more extensively developed in a former paper. Then, we carefully describe some improvements of the model. The determination of the turbulent flame velocity is based on the observed self-similarity of the turbulent flame and uses the local flame brush width as a fundamental parameter, which must be retrieved. With respect to the former version, we now derive more rigorously how the density variation has to be taken into account in the width retrieving function. We reformulate the diffusion term as a classical flux divergence term. We enforce the compatibility of the model for the limit of weak turbulence. We include a contracting effect of the source term, thus allowing to give a stationary mono-dimensional asymptotic solution with a finite width. We also include in a preliminary form, a stretch factor, which proves to be useful for controlling the flame behavior close to the flame holder and near the walls. The model implementation in the Star-CD CFD code is then tested on three different flame configurations. Finally, we shortly discuss the model improvements and the simulation results. [Copyright &y& Elsevier]

Combustion is an important subject of internal combustion engine studies. To reduce the air pollution from internal combustion engines and to increase the engine performance, it is required to increase combustion efficiency. In this study, effects of air/fuel ratio were investigated numerically. An axisymmetrical internal combustion engine was modeled in order to simulate in-cylinder engine flow and combustion. Two dimensional transient continuity, momentum, turbulence, energy, and combustion equations were solved. The k-e turbulence model was employed. The fuel mass fraction transport equation was used for modeling of the combustion. For this purpose a computational fluid dynamics code was developed by using the finite volume method with FORTRAN programming code. The moving mesh was utilized to simulate the piston motion. The developed code simulates four strokes of engine continuously. In the case of laminar flow combustion, Arrhenius type combustion equations were employed. In the case of turbulent flow combustion, eddy break-up model was employed. Results were given for rich, stoichiometric, and lean mixtures in contour graphs. Contour graphs showed that lean mixture (λ = 1.1) has longer combustion duration. [ABSTRACT FROM AUTHOR]

Present technologies of diesel engine combustion involve various operating conditions that can lead to a broad spectrum of combustion regimes. Existing combustion models for diesel engines usually lack universality across combustion regimes. In this study, a mixed-mode combustion model was developed to cover the major regimes pertaining to diesel engine combustion. The model uses kinetically controlled, quasi-steady homogeneous, quasi-steady flamelet, and partially premixed combustion modes. The combustion regime is identified locally by two combustion indices based on the local chemical and mixing conditions. The combustion model takes advantage of mixing details provided by large-eddy simulation. The model was tested over a wide range of engine conditions including conventional diesel-type and low-temperature combustion operating conditions. The model results compared well with experimental data and exhibited strength in both universality and computational efficiency. [ABSTRACT FROM AUTHOR]

Abstract: The complexities of the flame structure above a composite propellant containing 86% ammonium perchlorate (AP) and 14% hydroxy-terminated-polybutadiene (HTPB) have been elucidated using a two-dimensional, detailed gas-phase kinetic mechanism diffusion flame model. The model utilizes a vorticity formulation of the transport equations, which essentially eliminates the pressure field calculation and speeds convergence. Mass and energy coupling between the condensed and gas phases are achieved through iteration with one-dimensional, premixed combustion models to dynamically update the inlet boundary conditions. The model uses a detailed gas-phase kinetic mechanism consisting of 37 species and 127 reactions. Numerical studies have been performed to examine the influence of particle size on the flame structure above the AP/HTPB propellant. Three different combustion zones, based on AP particle size, were predicted: the AP monopropellant limit, the diffusion flame region, and the premixed limit. The modeled flame structure changed dramatically with particle size and was found to be qualitatively similar to the Beckstead–Derr–Price Model. Mechanistic insights are presented to explain AP’s unique ability to modify a propellant burning rate based on particle size alone. The premixed limit at which decreasing the size of the AP particles no longer influences the burning rate was also predicted. Results show promise in predicting formulistic effects using fundamental calculations. [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: A common technique in computational fluid dynamics (CFD) modeling of fire is to assume single step, infinitely fast combustion, in which case the transport equations of all gas species can be combined into one for a single conserved scalar called the mixture fraction. While this approach is adequate for many engineering applications, for fire scenarios that require predictions of CO formation or flame extinction, this approach is inadequate. This paper describes a method of extending the mixture fraction concept to address two-step chemistry. The two-step chemistry allows for flame extinction and the prediction of CO formation and destruction. The mixture fraction is decomposed into components representing the states of the two-step chemistry. The new model is demonstrated with two test cases: a slot burner using direct numerical simulation (DNS) and a reduced scale enclosure using large eddy simulation (LES). Results of the new model are compared with experimental data and simulations using single-step chemistry. [Copyright &y& Elsevier]

Abstract: The ever increasing cost of hydrocarbon fuels and more stringent emission standards may resolve challenges in producing hydrogen and using it as an alternative fuel in industries. Internal combustion engines are well-established technology and hydrogen fuel in such engines is considered as an attractive choice in exploiting clean, efficient and renewable hydrogen energy. This work presents an improved thermo-kinetics model for simulation of hydrogen combustion in SI engines. The turbulent propagating flame is modeled using turbulent burning velocity model. During combustion the charge is divided into three zones containing unburned charge, flame and burned gas. The adiabatic flame is assumed to be in thermodynamic equilibrium while the detailed chemical kinetics scheme is considered for burned and unburned zones. The results were first validated against published experiments. Good agreements were obtained between simulation and experiment for varying equivalence ratio, ignition timing and compression ratio. Detailed analysis of engine NO X emission was performed afterward. The lean-burn and EGR strategies'' potentials were examined by the current model. The effects of different amounts of cooled dry EGR and hot wet EGR on the NO X emission, engine power output and indicated thermal efficiency were investigated and compared theoretically. [Copyright &y& Elsevier]

Abstract: Techniques for modeling energetic-material combustion and ignition have evolved tremendously in the last two decades and have been successfully applied to various solid-propellant ingredients. There has been a paradigm shift in the predictive capability of solid-propellant combustion models as the field has advanced from a simple and global-kinetics approach to a detailed approach that employs elementary reaction mechanisms in the gas phase, and accommodates thermal decomposition and subsequent reactions in the condensed phase. The detailed models not only allow calculation of propellant burning-rate characteristics, such as pressure and temperature sensitivities, but also of the surface conditions and entire combustion-wave structure, including the spatial variations in temperature and species concentrations. This paper provides a comprehensive review of recent advances in the modeling and simulation of various solid-propellant ingredients over a wide range of ambient conditions. The specific materials of concern include nitramines (RDX, HMX), azides (GAP), nitrate esters (NG, BTTN, TMETN), ADN, and AP monopropellants, as well as homogeneous mixtures representing binary (RDX/GAP, HMX/GAP, and AP/HTPB) and ternary (RDX/GAP/BTTN) pseudo-propellants. Emphasis is placed on the steady-state combustion and laser-induced ignition of nitramines. The capabilities and deficiencies of existing approaches are addressed. In general, the detailed gas-phase reaction mechanisms developed so far represent the chemistry of monopropellants and associated mixtures consistently well, and help understand the intricate processes of solid-propellant combustion. The reaction mechanisms in the condensed phase, however, still pose an important challenge. Furthermore, the current knowledge of the initial decomposition of molecules emerging from the propellant surface is insufficient to render the models fully predictive. Modeling is thus not yet a predictive tool, but it is a useful guide. In the near future, it is likely that detailed combustion models can assist in the formulation of advanced solid propellants meeting various performance and emission requirements. [Copyright &y& Elsevier]