Search

Combustion is a complex physical and chemical process, which is considered both in the modeling of new propulsion systems with high energy efficiency and sufficient safety, and in the modeling of explosion safety and fire extinguishing problems. Fundamental research of this process is one of the key factors responsible for the safety of current and future space flights. Modeling the behavior of chemically reacting systems is computationally complex problem. It is necessary to take into account many details and processes, such as multicomponent structure, diffusion, turbulence, chemical transformations, etc. The modeling of chemical kinetics is the most computationally complex stage. In this paper, we consider the problem of approximating chemical kinetics for modeling the detonation of a hydrogen-air mixture using neural networks. The dataset for training the neural network were prepared using the principal component analysis from the results of numerical modeling of detonation in a narrow channel. The results of the obtained neural network showed that the presented model is capable of approximating chemical kinetics processes without significant restrictions on the range of pressure, temperature or the choice of the used time step. • Space flight safety issues in combustion were treated with machine learning. • The neural network for approximating the chemical kinetics of H 2 combustion was built. • The principal component analysis was used to prepare dataset. • The resulting neural network can predict the behavior of a chemical system many steps ahead. • The time step was taken into account in the work of neural network. [ABSTRACT FROM AUTHOR]

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]

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]

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]

Combustion simulations are the key aspect for full-scale 3D simulations of modern and perspective engines for aerospace propulsion. The present work studies an opportunity to solve chemical kinetics problems using artificial neural networks. Using classical numerical methods, sets of training data were produced. Choosing among different architectures of multi-layer neural networks and tuning their parameters, a rather simple model was worked out capable to solve this problem. The neural network obtained works in a recursive mode, and it can predict the behavior of chemical multi-species dynamical system by many steps. • The kinetic mechanism is resolved using the classical numerical method. • Training datasets received. • A neural network was selected and trained. • Recursive neural network can predict the behavior of chemical kinetics. [ABSTRACT FROM AUTHOR]

In this study, a novel furnace concept for in-flight particle spheroidization is presented, characterized and evaluated. A natural gas fired burner with a continued staged air principle and internal recirculation (COSTAIR), was used to provide the required temperature for the spheroidization process. Therefore, a numerically inexpensive CFD model for the calculation of combustion and multiphase flow is proposed. Particularly for the calculation of particle trajectories and particle peak temperatures of non-spherical (chiseled and flaky) slag particles, the presented CFD model differs in two modifications from the current state-of-the-art CFD models: first, a numerically efficient combustion model with a detailed chemical reaction mechanism was used in order to calculate the temperature profile of the furnace. While in most current state-of-the-art CFD models the numerically expensive and time consuming eddy dissipation concept (EDC) model or the insufficient eddy dissipation model (EDM) are used. Second, the discrete phase model, which is based on a numerically efficient Euler-Lagrangian approach, is used for multiphase modeling. Non-spherical particles are considered by application of a suitable particle drag model from literature. Although, the assumption of spherical particles is more common in current state-of-the-art CFD models for multiphase modeling. It was concluded that the presented furnace concept is applicable for the semi-industrial scale production of spherical boiler slag particles. The numerical results show that the assumption of non-spherical particles, compared to spherical particles, is more suitable for the calculation of particle trajectories and particle peak temperatures in the presented furnace. • A novel furnace concept for particle spheroidization is presented. • Time saving combustion modelling using a flamelet approach. • Characterization of temperature and flow conditions in the furnace. • Application of a numerically inexpensive Euler-Lagrangian approach. • Comparison between CFD and measurements. [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]

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]

A numerical model was constructed by Fluent software in this study with the objective to evaluate the model performance of predicting impinging flame heat transfer. The mechanism of methane-air-2step provided by the software and a Chemkin-compatible mechanism are used. With other conditions unchanged, the flame and heat transfer characteristics of the flame were computed and compared. Both mechanisms over predicted the height and maximum temperature of the flame by referring to the experimental data. The highest temperatures were found in small hot spots which are found to influence the impingement heat transfer at large nozzle-plate distance. Overall, the Chemkin mechanism gives a more reasonable temperature field and better predicts flame height and flame structure due to the more species and reactions considered. Both mechanisms can follow the experimental heat flux variation. The Chemkin mechanism better predicts heat transfer quantitatively, while the Fluent mechanism may be preferred by engineers for a qualitative analysis with lower computational cost. [ABSTRACT FROM AUTHOR]

Abstract Resistance to extinction by stretch and and laminar flame speed are important properties of any combustible mixture. Recent work has shown that extinction by stretch controls the overall structure of several important types of methane-based turbulent flames. The parameter used to quantify this phenomena, Extinction Strain Rate (ESR), is numerically studied here for methane-based flames across a range of pressures relevant to gas turbines and internal combustion engines, 1-40 atm. The pressure trends are compared with those of laminar flame speed which is historically better studied. Current kinetic models agree that ESR of lean flames is a non-monotonic function of pressure and that ESR of rich flames increases significantly with pressure, but are found to differ significantly in their numerical predictions of ESR, particularly at higher pressures. To better identify the source of model prediction differences and what governs the overall accuracy of the ESR predictions, various model sensitivity analyses were conducted. Pressure-dependent kinetics are shown to be vital to determining ESR pressure trends as are molecular collision efficiencies. Yet, reactions sensitivities for ESR largely mirror those for laminar flame speed calculations. Sensitivity to the transport parameter, Lennard Jones diameter, significantly exceeds reaction sensitivities for the fuel, oxidizer and bath gas. Thermodynamic parameter ESR sensitivities vary widely with pressure, but at least for enthalpy, appear insignificant when uncertainties are considered. This study informs and motivates further efforts to understand the phenomena of flame extinction by stretch at elevated pressures. [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]

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]

Abstract The paper presents a detailed numerical investigation of a pulsed detonation combustor (PDC) coupled with a transonic axial turbine stage. The time-resolved numerical analysis includes detailed chemistry to replicate detonation combustion in a stoichiometric hydrogen–air mixture, and it is fully coupled with the turbine stage flow simulation. The PDC–turbine performance and flow variations are analyzed for different power input conditions, by varying the system purge fraction. Such analysis allows for the establishment of cycle averaged performance data and also to identify key unsteady gas dynamic interactions occurring in the system. The results obtained allow for a better insight on the source and effect of different loss mechanisms occurring in the coupled PDC–turbine system. One key aspect arises from the interaction between the non-stationary PDC outflow and the constant rotor blade speed. Such interaction results in pronounced variations of rotor incidence angle, penalizing the turbine efficiency and capability of generating a quasi-steady shaft torque. [ABSTRACT FROM AUTHOR]

The current political scenario and the concerns for global warming have pushed very harsh regulations on conventional propulsion systems based on the use of fossil fuels. New technologies are being promoted, but their current technological status needs further research and development for them to become a competitive substitute for the ever-present internal combustion engine. Hydrogen-fueled internal combustion engines have demonstrated the potential of being a fast way to reach full decarbonization of the transport sector, but they still have to face some limitations in terms of the operating range of the engine. For this reason, the present work evaluates the potential of reaching full load operation on a conventional diesel engine, assuming the minimum modifications required to make it work under H 2 combustion. This study shows the methodology through which the combustion model was developed and then used to evaluate a multi-cylinder engine representative of the medium to high duty transport sector. The evaluation included different strategies of dilution to control the combustion performance, and the results show that the utilization of EGR brings different benefits to engine operation in terms of efficiency improvement and emissions reduction. Nonetheless, the requisites defined for the needed turbocharging system are harsher than expected and result in a potential non-conventional technical solution. • A new methodology for numerical modeling of hydrogen combustion is proposed. • The boosting system requirements for hydrogen high load operation are identified. • The benefits and limitations of using EGR for combustion control are determined. [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]

A correlated dynamic adaptive chemistry and transport (CO-DACT) method is developed to accelerate numerical simulations with detailed chemistry and transport properties in a reactive flow. Different sets of phase parameters, which govern the transport properties and chemical reaction pathways, respectively, are proposed to identify the correlated groups for transport properties and reaction pathways in both temporal and spatial coordinates. The correlated transport properties and reduced chemical mechanisms in phase space are dynamically updated by different user-specified threshold values. For the calculation of detailed transport properties, the mixture-averaged diffusion model is employed. For the on-the-fly generation of reduced chemistry, the multi-generation path flux analysis (PFA) method is used. In the present method, the chemical reduction and transport properties calculation are only conducted once for all the computation cells in the same correlated group within the pre-specified thresholds. Therefore, without sacrificing accuracy within the range of uncertainty of mechanisms and transport properties, the CO-DACT method can eliminate all redundant chemistry reductions and transport properties calculations in temporal and spatial coordinates when the transport properties and chemical reaction pathways are correlated due to the similarities in phase space. The CO-DACT method is further integrated with the hybrid multi-timescale (HMTS) method to achieve efficient integration of chemistry. Simulations of outward propagating spherical premixed flames and one dimensional (1D) diffusion ignitions of a jet fuel surrogate mixture, as well as an unsteady spherical propagating diffusion flame of a DME/air mixture are conducted to validate the present algorithm. The impact of the selection of threshold values as well as the dependence of numerical errors on pressure and equivalent ratio are also examined. The results demonstrate that the CO-DACT method can increase the computation efficiency for transport properties by at least two-order of magnitudes. Moreover, it is robust, accurate, and improves the overall computation efficiency involving a large kinetic mechanism. [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]

A genetic algorithm (GA) was proposed and validated for the optimal extraction of a sub-mechanism for H 2 /CO/CH 4 /CO 2 mixtures from the detailed Aramco1.3 chemical kinetics mechanism (Metcalfe et al., 2013), which was developed for C 1 – C 5 hydrocarbons and oxygenated fuels. Ninety ignition delay time data involving mixtures containing H 2 , CO, CH 4 , CO 2 , N 2 , and H 2 O at wide range of experimental conditions were chosen as optimization targets to guide the GA, so that the final reduced mechanism was able to fully describe the combustion characteristics of syngas and biogas fuel mixtures. The final reduced mechanism for H 2 /CO/CH 4 /CO 2 fuel mixtures comprised of 72 species and 290 reactions (reduced from 325 species and 2067 reactions), and it was extensively validated against experimental results, such as measured ignition delay times and laminar flame speeds, over a wide range of operating conditions. The excellent agreement between the reduced mechanism and Aramco1.3 mechanism in predicting the combustion properties of H 2 /CO/CH 4 /CO 2 mixtures with maximum relative error values of only 0.9% and 2.75%, respectively for the ignition delay time and laminar flame speed results, indicates that the proposed reduced mechanism can be used for predicting the combustion characteristics of biogas and syngas fuel mixtures. Furthermore, it was observed that the reduced mechanism shows excellent agreement with the Aramco1.3 mechanism in predicting the ignition delay time of mixtures with added ethane and propane. Therefore, the proposed reduced mechanism represents the most up-to-date detailed chemical kinetics mechanism for biogas and syngas fuel mixtures, and it can also be used for predicting the combustion properties of natural gas with impurities such as ethane and propane. The reduced mechanism agreed so well with the Aramco1.3 mechanism in predicting the combustion properties of H 2 /CO/CH 4 /CO 2 mixtures, where both mechanisms performed identically in over predicting the ignition delay time for H 2 /CO/CO 2 , CO 2 , H 2 /CO/CH 4 , and H 2 /CO/CH 4 /CO 2 /H 2 O mixtures at several experimental conditions. These observations were also reported by Lee et al. (2015), where they proposed two new rate constants for H + O 2 (+CO 2 ) = HO 2 (+CO 2 ) and CH 4 + OH = CH 3 + H 2 O to reconcile the discrepancies observed. These two new rate constants were assessed in this study by incorporating the modified rate constants into the 290Rxn mechanism (referred as 290Rxn-V1), where it was found that the modified rate constants did improve the ignition delay time predictions. However, the two proposed rate constants did not improve the predictions of the laminar flame speed for mixtures with a high CO 2 content at high equivalence ratios ( ϕ > 1.2). Therefore, optimization of the rate constants in the 290Rxn mechanism is highly recommended to further improve its agreement with experimental data for biogas and syngas fuel mixtures. [ABSTRACT FROM AUTHOR]

• 1-D engine model with predictive TJI combustion is developed and validated. • Benefits of passive TJI on combustion and efficiency are assessed. • The effect of mixture leaning is assessed through model simulations. • The impact of prechamber geometry on combustion and efficiency is assessed. The increasing concern about Greenhouse Gas (GHG) emissions led the European Union to introduce increasingly stringent limits to the CO 2 from road vehicles, with an impact on the sales of passenger cars. Vehicles equipped with Spark-Ignition (SI) engines became more numerous than those equipped with Compression-Ignition (CI) engines, due to the expensive aftertreatment system needed to comply with the restrictive European emission standards. However, SI engines provide lower efficiency than CI engines, due to the low compression ratio and the operation with a stoichiometric air–fuel ratio. Lean combustion can be a solution to increase SI engines' efficiency. Nevertheless, extremely lean mixtures lead to an increase in Cycle-to-Cycle Variability (CCV). The prechamber ignition concept, also known as Turbulent Jet Ignition (TJI), is an attractive solution for lean combustion, without its drawbacks. There are two ways to implement this concept: active TJI and passive TJI. In active TJI, there is an additional fuel supply system inside the prechamber, while passive TJI operates without additional injection. Therefore, passive TJI offers advantages in terms of simplicity, easy packaging, and low cost. In this work, the effects of passive TJI on combustion and performance are investigated by simulation analyses. Particularly, a 1-D engine model was developed to simulate the TJI combustion and validated against the experimental data. Furthermore, model simulations were carried out to assess how the prechamber geometry, in terms of A/V ratio, affects the jet performance, main chamber combustion, and fuel consumption, for different λ values. The analysis was conducted in a medium-to-high speed and load operating condition, namely 4500 rpm and 13 bar of Indicated Mean Effective Pressure (IMEP), under both stoichiometric and lean mixture. Simulation results demonstrated that the best jet performance and the highest engine efficiency are obtained for medium-to-high values of prechamber volume and large diameters, both in stoichiometric and lean-burn conditions, defining a common optimum prechamber design regardless of the λ level. [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 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]

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]

This research describes numerical and experimental studies on the co-combustion characteristics of hydrothermally treated municipal solid waste (HT MSW) with coal by utilizing a bubbling fluidized bed (BFB) reactor. The developed model is supported by some experimental test on co-combustion of coal with HT MSW to simulate the combustion process in a BFB and to evaluate the possibility of co-combustion application in a BFB reactor with different MSW blending ratios. HT MSW mixing ratios of 10, 20, 30, and 50% are chosen and examined at 700, 800, and 900 °C to determine at which temperature coal could be substituted with the HT MSW regarding emissions, such as CO, SO 2 and HCl. Emissions of NO, N 2 O, NH 3 and HCN from the mixtures are measured, simulated, and contrasted with the results of only combustion of coal. Hydrodynamics and heat and mass transfer, along with reactions during combustion, e.g., coal/waste devolatilization, volatile combustion and char combustion, are taken into account. The results obtained in this part of the study ensure the possibility of accepting the mixing ratio of the hydrothermally treated MSW, co-combusted with coal up to 30% (wt.%) without major modification of the coal-fired BFB reactor. The simulation results are compared with experimental data, which show that the gas species versus time at different heights from the fluidized bed is reasonably simulated. This indicates that the numerical model presented is valid and provides a promising way to simulate the combustion of solid waste in a BFB, which is the predominant technology for co-combustion of waste. The advantage of the theoretical study lies in its ability to reveal features of the detailed structure of the combustion process inside a solid bed. [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]

Predictive high-fidelity simulations of turbulent spray combustion must capture the combined effects of complex chemistry, multiphase evaporating flow and spray-flame interactions to achieve physical accuracy. Finite-rate chemistry (FRC) combined with a realistic chemical mechanism is a combustion model well-suited for this purpose, but has a high computational cost due to the large number and stiffness of transported chemical species. In contrast, flamelet-based models achieve lower cost by transporting a small number of quantities of reduced stiffness, but assumptions regarding local flame topology, boundary conditions and inter-phase coupling limit their physical accuracy. Recently, the Pareto-efficient combustion (PEC) framework was developed to dynamically assign combustion models based on local cost and accuracy metrics in gas-phase reacting flows. In this work, we extend this PEC framework to spray combustion through the rigorous analysis of the multiphase coupling terms in the governing equations. The derivation shows that spray evaporation causes errors in the prediction of species mass fractions for flamelet-based models due to the sensitivity of the local thermo-chemical state to changes in composition caused by fuel vaporization across combustion regimes present in practical spray combustion devices. Sub-model assignment is formulated as a multiple-choice knapsack problem, where computational cost is directly controlled through the fraction of the domain assigned to the FRC sub-model. The extended PEC formulation is applied to the simulation of a realistic rich-quench-lean gas turbine combustor at steady-state conditions, as well as transient operation resulting in lean blow-out (LBO). Analysis of transient simulations during LBO demonstrates the extended PEC formulation's capacity to dynamically adapt to changing conditions within the combustor. Transient combustor dynamics are shown to approach convergence with limited increases in computational cost, while retaining substantial computational cost reduction compared to monolithic FRC simulations. Through PEC simulations with increasing fractions of the domain assigned to FRC, monolithic flamelet simulations are shown to over-predict flame stability during LBO. The extended PEC formulation is thus shown to overcome deficiencies of monolithic models by controlling modeling error for multiphase combustion modeling. [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]

The transient behavior of confined HMX in response to an external heat flux has been simulated using a detailed kinetic model. The model is spatially one-dimensional, fully transient, and consists of equations for modeling the solid (condensed) phase HMX, the gas phase, and the surrounding container for fast cook-off conditions. The condensed-phase decomposition reactions are described by semi-global, distributed kinetics. The gas phase description includes a detailed chemistry model for the combustion of HMX. A nonlinear, numerical solution of the partial differential equations results in predictions of temperature, pressure, velocity, and species fractions as a function of position and time. Predicted time-to-ignition yields good agreement with experimentally measured fast cook-off times. Temperature at ignition is found to be a function of heating rate. The ignition process for fast cook-off is consistent with laser ignition of bare HMX samples. Simulation results depend on accurate boundary conditions applied to the steel container. Pressurization rates are calculated and insight is given into the fast cook-off behavior of HMX. The main decomposition pathway is the conversion of HMX into CH 2 O and N 2 O. [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]

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]

The laser-induced ignition response of HMX has been investigated using a detailed numerical model. The model is one-dimensional, fully transient, and solves the conservation equations for both the condensed and gas phases. The condensed phase representation includes radiation absorption, solid-phase transitions, melting, evaporation, and distributed semi-global decomposition kinetics. The gas phase utilizes a detailed kinetic mechanism to predict species formation and destruction. Ignition occurs in the gas phase and the flame propagates back toward the surface of the HMX in what is known as the ‘snap-back’ effect. The model then transitions to steady-state combustion. Calculations were performed in which the solid HMX is irradiated with heat fluxes ranging from 50 to 1600 W/cm 2 . Results are compared to empirical data for the laser-induced ignition of HMX. Good agreement with these data and other steady-state data (burning rate, surface temperature, melt thickness) provide the necessary validation of the developed model. [ABSTRACT FROM AUTHOR]

Abstract: Laser-based OH, CO, CH3, and H2O species histories during 3-pentanone oxidation were measured behind reflected shock waves over the temperature range of 1277–1678K at pressures around 1.6atm, and for equivalence ratios of 0.5, 1.0 and 1.5, complementing recent work also conducted in this laboratory (Lam et al., 2012). These data are used to develop an improved detailed kinetic mechanism for 3-pentanone pyrolysis and oxidation. A 3-pentanone submechanism consisting of 13 species and 61 reactions is superimposed upon the H2/CO/C1–C4 base mechanism of JetSurF 2.0 (Wang et al., 2010), with several reactions updated in order to obtain better agreement with shock tube oxidation data. The improved mechanism shows satisfactory agreement with the wealth of shock tube data presented here and previously published, although there remain some conditions where the model can benefit from further improvements. Major rate parameter uncertainties in the model are also discussed and will be used in a companion work addressing model optimization and experimental design of shock tube studies using data obtained with the Stanford shock tube. [Copyright &y& Elsevier]

Abstract: Methane Thermal Partial Oxidation (TPOX) within a small scale Inert Porous Media (IPM) based reactor was investigated numerically in order to explore the operating conditions and possible procedures for maximizing the reforming efficiency and minimizing the soot formation. A quasi-1D model of the TPOX reactor was validated and further used to study the process. The model considers detailed chemistry and solves the energy balances for both gas and solid phases, including radiative heat transfer in the solid phase. The parametric results of the reactor operation show that the optimal air–fuel ratio is a compromise between soot formation and reforming efficiency. Moreover, a high preheating temperature of the reactants is found to be always beneficial for the process, and the effect of power input is negligible for the reforming efficiency. The numerical investigations also suggest that shorting the IPM length, as well as mixing small amounts of water vapor with the reactants, appear to be effective procedures for improving the operation performance of the TPOX reactor. [Copyright &y& Elsevier]

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]

Abstract: Conversion under O2/N2 and O2/CO2 atmospheres of coal particles of sizes typically used in fluidized bed combustion is studied by means of experiments and modeling. Experiments are conducted for six different fuels in a thermo-gravimetric oven, for different gas atmospheres and temperatures, providing empirical data on devolatilization and char conversion. A model is developed to predict coal conversion (drying, devolatilization, and char conversion), coupling mass and heat transfer and accounting for primary and secondary fragmentation. The validation of the model yields satisfactory agreement between experimental and modeled mass loss curves and conversion rates during devolatilization and char conversion. It is shown that O2/CO2 mixtures give slightly longer devolatilization times and a decrease of char reactivity compared to O2/N2 mixtures. [Copyright &y& Elsevier]

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]

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: In order to accurately simulate the fuel spray, atomization, combustion and emission formation processes of a diesel engine fueled with biodiesel, adequate knowledge of biodiesel’s physical properties is desired. The objective of this work is to do a detailed physical properties prediction for the five major methyl esters of biodiesel for combustion modeling. The physical properties considered in this study are: normal boiling point, critical properties, vapor pressure, and latent heat of vaporization, liquid density, liquid viscosity, liquid thermal conductivity, gas diffusion coefficients and surface tension. For each physical property, the best prediction model has been identified, and very good agreements have been obtained between the predicted results and the published data where available. The calculated results can be used as key references for biodiesel combustion modeling. [Copyright &y& Elsevier]

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: Simulating multi-dimensional combustion with detailed kinetics often requires solving a large number of ordinary differential equation (ODE) problems at each global time step. In many cases, the ODE integrations account for the bulk of the total wall-clock time for the simulation. This paper introduces CHEMEQ2-GPU – a new explicit stiff ODE solver (based on the existing CHEMEQ2 solver) that exploits the parallel architecture of the modern graphics processing unit (GPU) to accelerate ODE integration in multi-dimensional combustion simulations. We also demonstrate efficient application of the CPU and GPU as co-processors, for further speedup. We describe a hybrid explicit/implicit ODE solver approach that combines the strengths of both solver types running simultaneously on the GPU and CPU, respectively. A dynamic load balancing scheme was used to assign the kinetics ODE integrations over all grid points to either the CPU-based implicit solver DVODE (which is the more efficient solver for highly stiff grid points) or CHEMEQ2-GPU (more efficient for moderately stiff or non-stiff grid points). We demonstrate CHEMEQ2-GPU and the hybrid approach in 3-D simulations of homogeneous charge compression ignition (HCCI) engines. The test cases applied two different n-heptane reaction mechanisms (a large detailed model and a small skeletal model) and three different mesh sizes. Engine simulations were performed using KIVA-CHEMKIN. CHEMEQ2 was about 2–3 times faster than DVODE, with similar prediction accuracy. The CHEMEQ2-GPU speedup relative to CHEMEQ2 increased linearly with the number of grid points for the range of meshes tested in this work. Assuming ideal linear scaling of simulation time with number of processors, the speed of CHEMEQ2-GPU on the Tesla C2050 GPU was equivalent to CHEMEQ2 running on approximately 13 parallel 2.8GHz CPU processors for the finest mesh; and the hybrid solver approach was equivalent to CHEMEQ2 on ∼15 such CPU processors. In summary, CHEMEQ2-GPU provided the additional computing power of 14 parallel CPU processors (for the finest mesh tested) and the hybrid solver approach demonstrated a method to efficiently apply these additional co-processors with existing CPU cores for combustion simulations. CHEMEQ2-GPU scales favorably with the number of grid points and is available by request to the authors. This work presents opportunities for further development, particularly in CPU/GPU load balancing algorithms. [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]