Reduced Reaction Mechanism Research Articles

Mesh convergence for turbulent combustion

Our central result is a methodology for predicting mesh convergence forthree dimensional (3D) turbulent combustion simulations,based on less expensive one dimensional (1D) and two dimensional (2D) simulations.We verify the prediction by comparison to a 3Dfinite rate chemistry simulationbased on a reduced reaction mechanism,and we further verify it by comparison to a completely independent simulation of the same problem.We validate our simulation by comparison to experiment.Additionally, we assess grid requirements for finite rate chemistrywith more detailed chemical reaction mechanism.In both cases, the test problem is an engineering scale study ofa model scramjet combustor designed by Gamba et al.We find that the mesh requirements are not feasible for finite ratechemistry simulations of engineering scale problems with detailedreaction mechanism, as expected,but these criteria are less severe than the Kolmogorov scale.

Effects of hydrogen blending mode on combustion process of a rotary engine fueled with natural gas/hydrogen blends

The highly advanced wankel rotary engine is a promising energy system because of its favorable energy to weight ratio, multi-fuel capability and large specific power output. This work aims to numerically study the performance, combustion and emission characteristics of a side-ported rotary engine fueled with natural gas/hydrogen blends under different hydrogen blending modes. Simulations were performed using multi-dimensional software FLUENT 14.0. On the basis of the software, a three-dimensional dynamic simulation model was established by writing dynamic mesh programs, choosing the RNG k-ε turbulent model, the eddy-dissipation concept (EDC) combustion model and a reduced reaction mechanism. The three-dimensional dynamic simulation model based on the chemical reaction kinetics was also validated by the experimental data. Meanwhile, further simulations were then conducted to investigate how to impact the combustion process by the coupling function between hydrogen distribution and the flow field inside the cylinder. Simulation results showed that in order to improve the combustion efficiency, the low-pressure early injection should be used as the hydrogen blending mode. The low-pressure early injection, not only allowed the hydrogen in the combustion chamber to be distributed evenly, but also resulted in the high concentration areas of hydrogen located at the front of the trialing spark plug, which can be used to increase the combustion rate. For the hydrogen low-pressure early injection, the improved combustion rate made in-cylinder pressure and the intermediate OH increase significantly. Compared with no hydrogen induction, it shows a 29% increase in the peak pressures. Meanwhile, the drawback is the increase in NO emissions.

Kinetic analysis of n-decane–hydrogen blend combustion in premixed and non-premixed supersonic flows

Numerical analysis of ignition and combustion of an n-decane–hydrogen fuel blend in a premixed supersonic flow and in a model scramjet duct is performed using a reduced reaction mechanism built especially to describe the oxidation of blended n-C10H22–H2fuel in air at the temperature T0 > 900–1000 K in the pressure range P0 = 0.1–13 atm. The developed kinetic mechanism involves the principal reactions responsible for chain mechanism development both for n-decane and for hydrogen oxidation. It has been shown that using blended n-C10H22–H2 fuel makes it possible to enhance the ignition and combustion both in premixed and in non-premixed supersonic fuel–air flows compared to burning pure hydrogen–air and n-decane–air mixtures. This allows high combustion completeness in the scramjet duct at the distance of ∼1 m even at extremely low air temperature T0 = 1000 K and pressure P0 = 0.3 atm. This is due to the interaction of kinetics of the formation of highly reactive atoms and radicals, carriers of chain mechanism, in H2–air and n-C10H22–air mixtures.

111 デカンの簡略化反応機構を用いた平面火炎に落下する噴霧燃焼の数値解析(OS1 燃焼,エネルギー変換,熱利用(4),研究討論セッション)

111 デカンの簡略化反応機構を用いた平面火炎に落下する噴霧燃焼の数値解析(OS1 燃焼,エネルギー変換,熱利用(4),研究討論セッション)

302 ノルマルトリデカンの詳細反応機構の解読による簡略化機構の構築(OS2 エンジン技術の高度化に向けた先端研究,研究討論セッション)

302 ノルマルトリデカンの詳細反応機構の解読による簡略化機構の構築(OS2 エンジン技術の高度化に向けた先端研究,研究討論セッション)

Investigation of a turbulent premixed combustion flame in a backward-facing step combustor; effect of equivalence ratio

In the present study, LES (large-eddy simulation) is utilized to analyze lean-premixed propane-air flame stability in a backward-step combustor over a range of equivalence ratio. The artificially thickened flame approach coupled with a reduced reaction mechanism is incorporated for modeling the turbulence–combustion interactions at small scales. Simulation results are compared to high-speed PIV (particle image velocimetry) measurements for validation. The results show that the numerical framework captures different topological flow features effectively and with reasonable accuracy, for stable flame configurations, but some quantitative differences exist. The RZ (recirculation zone) is formed of a primary eddy and a secondary eddy and its overall size is significantly impacted by the equivalence ratio. The temperature distribution inside the recirculation zone is highly non-uniform, with much lower values observed close to the backward step and the bottom wall. The mixture distribution inside the RZ is also non-uniform because of mixing with reactants and heat loss to the walls. The flame is stabilized closer to the backward step as the equivalence ratio increases. At lower fuel fractions, the flame lifts off the step starting at equivalence ratio of 0.63 and the lift off distance is increased while the equivalence ratio is lowered.

A Genetic Algorithm–Based Method for the Optimization of Reduced Kinetics Mechanisms

ABSTRACTThis paper describes an automatic method for the optimization of reaction rate constants of reduced reaction mechanisms. The optimization technique is based on a genetic algorithm that aims at finding new reaction rate coefficients that minimize the error introduced by the preceding reduction process. The error is defined by an objective function that covers regions of interest where the reduced mechanism may deviate from the original mechanism. The mechanism's performance is assessed for homogeneous reactor or laminar‐flame simulations against the results obtained from a given reference—the original mechanism, another detailed mechanism, or experimental data, if available. The overall objective function directs the search towards more accurate reduced mechanisms that are valid for a given set of operating conditions. An optional feature to the objective function is a penalty term that permits to minimize the change to the reaction coefficients, keeping them as close as possible to the original value. This means that the penalty function can be used to constrain the reaction rates modifications during the optimization if needed. It is demonstrated that the penalty function is successful and can be combined with predefined uncertainty bounds for each reaction of the mechanism. In addition, the penalty function can be modified to achieve a further reduction of the mechanism. The algorithm is demonstrated for the optimization of a previously reduced variant of the GRI‐Mech 3.0, a tert‐butanol combustion mechanism by Sarathy et al. (Combust. Flame, 2012, 159, 2028–2055) and a hydrogen mechanism by Konnov (Combust. Flame, 2008, 152, 507–528), for which the complete uncertainty vector is known. The method has shown to be, robust, flexible, and suitable for a wide range of operating conditions by using multiple criteria simultaneously.

Development and validation of a reduced chemical kinetic model for dimethyl ether combustion

A new, reduced reaction mechanism for DME combustion in internal combustion engines is proposed. The new mechanism is based on a detailed DME mechanism suitable for low to high temperature ranges. The reduced model consists of 29 species and 66 reactions and contains a detailed H2/CO mechanism and reduced C1–C2 chemistry. The reduced mechanism inherits the major reaction pathways of the detailed mechanism, which ensures its predictive capability when coupled into CFD simulations. The performance of the reduced mechanism was compared to simulation results of the detailed mechanism, ignition delay times from a shock tube and a rapid compression machine, and DME engine combustion data. Overall, the reduced mechanism shows a good balance between accuracy and computational efficiency necessary for use in combustion system design applications.

Optimized Reduced Chemistry and Molecular Transport for Large Eddy Simulation of Partially Premixed Combustion in a Gas Turbine

A methodology is discussed to automatically determine the parameters of closed budget equations for chemical species mass fractions and energy, in order to simulate spatially filtered flames as required in large eddy simulation (LES). The method accounts for the effects of LES filtering on chemistry and transport by simultaneously optimizing, for a reduced number of species, the Arrhenius reaction rates and a correction to mixture-averaged molecular diffusion coefficients. The objective is to match, for a given filter size, spatially filtered canonical one-dimensional flames simulated with detailed chemistry solutions. This approach is designed for quite well-resolved LES, in which most of the unresolved fluctuations result from flame thickening due to spatial filtering, thus featuring weak levels of sub-grid scale flame wrinkling. Methane-air partially premixed combustion is addressed. A four-step reduced reaction mechanism involving seven species is developed along with mass and heat molecular transport properties. The optimization is performed at atmospheric pressure and at 3 bar, for ranges of fresh gas temperatures [300–650 K] and equivalence ratios [0.4–1.2]. Comparisons with the filtered detailed chemistry solution of a planar propagating front show that the laminar flame speed, the adiabatic flame temperature, the species profiles in the reaction zone, and the flow chemical composition and temperature at equilibrium are adequately predicted. The new sub-grid scale modeling approach is then applied to three-dimensional LES of an industrial gas turbine burner. Good agreement is found between the quantities predicted with LES and experimental data, in terms of flow and flame dynamics, axial velocities, averaged temperatures, and some major species concentrations. Results are also improved compared to previous simulations of the same burner.

Numerical study of combustion characteristics of ammonia as a renewable fuel and establishment of reduced reaction mechanisms

With its high hydrogen density and already existing infrastructure, ammonia (NH3) is believed to be an excellent green fuel that can be used in energy generation and transportation systems. Combustion of ammonia has certain challenges (associated with its low flame speed and fuel bond NOx emissions) that need to be addressed before its widespread use in practical systems. The primary objective of this study is to develop a reduced reaction mechanism for the combustion of ammonia which can be used to expedite the design of effective ammonia combustors through numerical simulations of realistic combustor geometries with accurate kinetics models. First we have investigated the combustion characteristics of NH3/H2/air mixtures at elevated pressure and lean conditions which are encountered in practical systems such as gas turbine combustors. Laminar premixed freely propagating flame model is used to calculate the combustion properties. The results of sensitivity study of total NOx formation with respect to the equivalence ratio indicates the possibility of localized rich combustion as an effective way to reduce the NOx concentration down to levels that are the same order as the modern gas turbine engines. In the second part of the study, by considering a wide range of conditions in terms of pressure, fuel mixture, and equivalence ratio we have developed two reduced mechanisms based on the Konnov mechanism. The reduced mechanisms are capable of predicting the total NOx emission level and the laminar flame speed at an acceptable accuracy over a wide range of conditions. Evaluating the performance of the reduced mechanisms with respect to the full mechanism and experimental data shows that the mechanisms are able to predict the combustion properties almost at the same accuracy level as the Konnov mechanism, but at a nearly five times less CPU time expense.

A combustion model for multi-component fuels using a physical surrogate group chemistry representation (PSGCR)

A combustion model to simulate the oxidation of multi-component fuels has been developed and applied to model autoignition combustion processes. The model is called the Physical Surrogate Group Chemistry Representation (PSGCR) method, which combines a multi-component fuel model to describe the physical properties of the fuel with reduced reaction mechanisms to represent the chemistry of the fuel components. A group of physical surrogate (PS) components are used to describe the fuel’s physical properties, such as the distillation profile, specific gravity, lower heating value and hydrogen-to-carbon (H/C) ratio. The oxidation processes of the fuel are modeled using skeletal reaction mechanisms for a group of chemical surrogate (CS) components that employ generic reactions as well as detailed reaction kinetics. The generic reactions were built to model unavailable reaction pathways from physical surrogates to the base CS components or their intermediate species. The model includes the 6 major chemical classes of typical hydrocarbon fuels, i.e., n-paraffins, iso-paraffins, olefins, naphthenes, aromatics and oxygenates, and 65 representative hydrocarbon components are included in a data base for the physical surrogate components and 43 chemical surrogate components are considered for the group chemistry representation. Six types of generic reaction sets are proposed to model the oxidation of 15 physical surrogate components that are called extended-CS (chemical surrogate) components. The present generic reactions are modeled such that the major characteristics of the oxidation process of the fuel components are included in a consistent manner.The final version of the PSGCR reaction mechanism consists of 264 species and 1292 reactions. Validation of the reaction mechanisms was performed by comparing predicted ignition delay times with experimental measurements and/or predictions from comprehensive mechanisms available in the literature. The model was also validated against HCCI experimental data of the FACE fuels. Then the model was applied to simulate practical diesel fuel (F76) spray combustion in a constant volume chamber, and the predicted ignition delay times of the surrogate model fuel were compared with measured values. The results show that the present PSGCR method captures the combustion characteristics of complex multi-component fuels, giving reliable performance for combustion predictions, as well as computational efficiency improvements for multi-dimensional CFD simulations through the use of reduced mechanisms.

LES of a methanol spray flame with a stochastic sub-grid model

This paper describes the Large Eddy Simulation (LES) of a methanol/air turbulent nonpremixed spray flame. An Eulerian stochastic field method is employed for the turbulence-chemistry interaction of the gas phase while a Lagrangian formulation is used for the liquid phase. A reduced reaction mechanism (18 species and 14 reactions) is adopted and stochastic models are used to account for the influence of sub-grid scale (sgs) motions on droplet dispersion and evaporation. Comparisons of the predicted gas phase and droplet statistics with measurements show a good agreement confirming that the droplet dispersion and evaporation models used in this work are adequate. The general features of the spray flame such as the occurrence of external group combustion and its development into separate combusting islands are well captured.

Kinetic Simulation of Combustion Process in the Combustor of the Aero-Engine

The use of detailed chemical reaction mechanisms of aviation fuels is still very limited in analyzing the combustion process concerning complex multidimensional fluid dynamics issues. In this work, a reduced chemical kinetic mechanism containing 210 elemental reactions (including 92 reversible reactions and 26 irreversible reactions) and 50 species was introduced and studied in the kinetic simulation of the ignition and combustion of n-decane (n-C10H22) at various combustion regimes. Moreover, it was validated with the experimental results obtained from both premixed flame of a flat-flame burner and n-C10H22 shock-tube ignition. The results show that the calculated ignition delay time of fuel n-C10H22 in the shock-tube under typical conditions viz. a pressure of 12 or 50 bar and corresponding equivalence ratio at 1.0 or 2.0, as well as the calculated mole fractions of the main reactants and products of fuel n-C10H22 in the premixed combustion process, agree very well with the experimental data. The combustion processes in the single flame tube of a tube annular combustor were simulated through coupling this reduced reaction mechanism of surrogate fuel n-C10H22 and one step reaction mechanism of surrogate fuel C12H23 into the computational fluid dynamics (CFD) software, and this combustion process was also experimentally studied. It is found that the reduced reaction mechanism exhibits clear advantages in simulating the combustion processes and certain species formation in the single flame tube over the one step reaction mechanism. In particular, the temperature radial distribution at the outlet of the single flame tube computed by using reduced reaction mechanism of surrogate fuel n-C10H22 is found to be more reasonable and agreed better with the experimental data than that computed by one step reaction mechanism of surrogate fuel C12H23.

Chemical kinetic simulation of kerosene combustion in an individual flame tube

The use of detailed chemical reaction mechanisms of kerosene is still very limited in analyzing the combustion process in the combustion chamber of the aircraft engine. In this work, a new reduced chemical kinetic mechanism for fuel n-decane, which selected as a surrogate fuel for kerosene, containing 210 elemental reactions (including 92 reversible reactions and 26 irreversible reactions) and 50 species was developed, and the ignition and combustion characteristics of this fuel in both shock tube and flat-flame burner were kinetic simulated using this reduced reaction mechanism. Moreover, the computed results were validated by experimental data. The calculated values of ignition delay times at pressures of 12, 50bar and equivalence ratio is 1.0, 2.0, respectively, and the main reactants and main products mole fractions using this reduced reaction mechanism agree well with experimental data. The combustion processes in the individual flame tube of a heavy duty gas turbine combustor were simulated by coupling this reduced reaction mechanism of surrogate fuel n-decane and one step reaction mechanism of surrogate fuel C12H23 into the computational fluid dynamics software. It was found that this reduced reaction mechanism is shown clear advantages in simulating the ignition and combustion processes in the individual flame tube over the one step reaction mechanism.

LES combustion modeling for high Re flames using a multi-phase analogy

In this work we propose a novel model for Large Eddy Simulations (LESs) of high Reynolds moderate Damkohler number turbulent flames. The development is motivated by the need for more accurate and versatile LES combustion models for engineering applications such as gas turbines. The model is based on the finite rate chemistry approach in which the filtered species equations of a reduced reaction mechanism are solved prior to closure modeling. The modeling of the filtered reaction rate provides the challenge: As most of the chemical activity, and thus also most of the exothermicity occurs on the subgrid scales, this model needs to be based on the properties of fine-scale turbulence and mixing and Arrhenius chemistry. The model developed here makes use of the similarities with the mathematical treatment of multi-phase flows together with the knowledge of fine-scale turbulence and chemistry obtained by Direct Numerical Simulation (DNS) and experiments. In the model developed, equations are proposed for the fine-structure composition and volume fraction that are solved together with the LES equations for the resolved scales. If subgrid convection can be neglected the proposed model simplifies to the well-known Partially Stirred Reactor (PaSR) model. To validate the proposed LES model comparisons are made with DNS data of a planar turbulent flame in homogenous isotropic turbulence and experimental data for an axisymmetric dump combustor.

Combustion simulations of the fuels for advanced combustion engines in a homogeneous charge compression ignition engine

Numerical simulations of combustion have been performed for a homogeneous charge compression ignition engine operating on multi-component reference diesel fuels. Two surrogate representation methods were used to describe the nine fuels for advanced combustion engines. The first method of surrogates, denoted as physical-surrogate components, was formulated to describe the fuel’s physical properties by matching its distillation profile, specific gravity, lower heating value, hydrogen-to-carbon ratio, and cetane index with measured data. The second method of surrogates, denoted as chemical-surrogate components, was introduced to represent the chemistry of the fuel components using a new group chemistry representation model. In the model, the fuel components in the physical-surrogate components and chemical-surrogate components are related through the classification of chemical structures of the components, i.e. the component in the physical-surrogate components are grouped based on their chemical classes and the chemistry of each group is calculated using a chemical kinetics mechanism (MultiChem) that represents the combustion characteristics of its chemical class. This MultiChem mechanism includes reduced reaction mechanisms for the four main surrogate hydrocarbon chemistry components of diesel fuel, n-paraffins, iso-paraffins, naphthenes, and aromatics, with two n-paraffins to provide the ability to mimic molecular weight effects. The computations were performed using a multi-dimensional computational fluid dynamic code, KIVA-ERC-CHEMKIN, and the results show that the predictions of the present multi-component combustion models are in good agreement with experimental measurements. The combustion characteristics of the fuels for advanced combustion engines are well represented and the present models are well suited for practical engine computations.

A new reduced reaction mechanism of a surrogate fuel for kerosene

AbstractThe introduction of detailed chemical reaction mechanisms for aviation fuels into complex multidimensional fluid dynamics problems is not practical at the present time. Simplified reaction mechanisms that have been thoroughly evaluated must be developed to address specific issues arising in realistic combustor configurations. A reduced chemical kinetic mechanism features 210 elemental reactions (including 92 reversible reactions and 26 irreversible reactions) and 50 species for the ignition and combustion of n‐decane was compiled and validated for a wide range of combustion regimes. Validations were performed using experimental measurements on a premixed flame of Jet‐A1, O2 and N2, stabilised at 1 atm on a flat‐flame burner, as well as from n‐decane shock‐tube ignition experiments. Numerical calculations were performed using this reduced mechanism and the detailed mechanism respectively for n‐decane surrogate fuel. The calculated values of ignition delay times at pressures of 12, 50 bar and equivalence ratio is 1.0, 2.0, respectively and the main reactants and main products mole fractions agree well with experimental data. The present study shows that this reduced mechanism for the n‐decane surrogate can be employed to predict premixed combustion of kerosene. © 2012 Canadian Society for Chemical Engineering

Numerical Simulation of the Two-Phase Turbulent Combustion Flow in the Multicomponent Propellant Rocket Engine

The numerical simulation, based on computational fluid dynamics methodology, has been performed to study the two-phase turbulent combustion flow in rocket engine using non-metallized multicomponent propellant. A reduced reaction mechanism is developed for modelling combustion of fuel droplets in the absence of metal. Gas governing equations are two dimensional axisymmetric N-S equations in Eulerian coordinates. The trajectory model is adopted to analyse the droplet-phase including the droplet collision, breakup and evaporation. The gas flow is influenced by the droplets by adding source term to N-S equations. The reliability of the simulation programme is validated by comparing numerical simulation result with engine test data.

Numerical Simulation of the Two-Phase Turbulent Combustion Flow in the Multicomponent Propellant Rocket Engine

The numerical simulation, based on computational fluid dynamics methodology, has been performed to study the two-phase turbulent combustion flow in rocket engine using non-metallized multicomponent propellant. A reduced reaction mechanism is developed for modelling combustion of fuel droplets in the absence of metal. Gas governing equations are two dimensional axisymmetric N-S equations in Eulerian coordinates. The trajectory model is adopted to analyse the droplet-phase including the droplet collision, breakup and evaporation. The gas flow is influenced by the droplets by adding source term to N-S equations. The reliability of the simulation programme is validated by comparing numerical simulation result with engine test data.

Development of reduced reaction mechanisms for hydrogen and methanol diffusion flames

Biofuels, such as biohydrogen, biomethanol, bioethanol and biodiesel may be the future of energy for transportation. The methanol can be used directly for powering Otto engines or fuel cells; it is commonly used in biodiesel production for its reactivity. Biohydrogen is a replacement for fossil and biorenewable liquid fuels. Based on a mechanism composed by 20 reversible elementary reactions among 8 reactive species, we propose a reduction process to obtain the mechanism of hydrogen, and from 129 reversible elementary reactions among 23 reactive species, we obtain a reduced mechanism for the methanol. Therefore, the aim of this work is the development of an algorithm consisting of 7 steps, which eliminates the fastest reactions of the reaction mechanism since the slowest reactions are rate-determining. The main advantage of the strategy is the decrease of the work needed to solve the system of chemical equations.