Reduced-Order Chemical Mechanisms for Oxygen-Enriched Combustion of Methane and N-decane Fuel (original) (raw)
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Automatic Analysis and Reduction of Reaction Mechanisms for Complex Fuel Combustion
2001
This work concentrates on automatic procedures for simplifying chemical models for realistic fuels using skeletal mechanism construction and Quasi Steady-State Approximation (QSSA) applied to detailed reaction mechanisms. To automate the selection of species for removal or approximation, different indices for species ranking have thus been proposed. Reaction flow rates are combined with sensitivity information for targeting a certain quantity, and used to determine a level of redundancy for automatic skeletal mechanism construction by exclusion of redundant species. For QSSA reduction, a measure of species lifetime can be used for species ranking as-is, weighted by concentrations or molecular transport timescales, and/or combined with species sensitivity. Maximum values of the indices are accumulated over ranges of parameters, (e.g. fuel-air ratio and octane number), and species with low accumulated index values are selected for removal or steady-state approximation. In the case of QSSA, a model with a certain degree of reduction is automatically implemented as FORTRAN code by setting a certain index limit. The code calculates source terms of explicitly handled species from reaction rates and the steady-state concentrations by internal iteration. Homogeneous-reactor and one-dimensional laminar-flame models were used as test cases. A staged combustor fuelled by ethylene with monomethylamine addition is modelled by two homogeneous reactors in sequence, i.e. a PSR (Perfectly Stirred Reactor) followed by a PFR (Plug Flow Reactor). A modified PFR model was applied for simulation of a Homogeneous Charge Compression Ignition (HCCI) engine fuelled with four-component natural gas, whereas a two-zone model was required for a knocking Spark Ignition (SI) engine powered by Primary Reference Fuel (PRF). Finally, a laminar one-dimensional model was used to simulate premixed flames burning methane and an aeroturbine kerosene surrogate consisting of n-decane and toluene. In general, detailed calculations of temperature, pressure, concentration and flame velocity show excellent agreement with measurements. Skeletal mechanisms for PRF were constructed for the SI engine case, reproducing autoignition well on removal of reactions pertaining to 15% of the species. QSSA reduction was tested on the staged combustor and the engines, using pure and weighted lifetime indices. Monitoring NO concentrations in the staged combustor and ignition timing in the engines, good reproduction is possible while approximating about 70% of the species. However, some species have to be manually retained for accuracy and numerical stability. For improved ranking, sensitivity was added to the index applied to the premixed flames, in addition to necessary molecular transport information. The maximum atomic mass fraction occupied by a certain molecular species was also constrained to limit the mass and energy deficiency caused by QSSA. For methane, the laminar flame velocities as well as concentration profiles are well predicted by the most strongly reduced mechanism with five global reaction steps. For the kerosene surrogate mechanism, QSSA involving 50% of the species was successfully attempted.
CH4/NOx Reduced Mechanisms Used for Modeling Premixed Combustion
2012
This study has identify useful reduced mechanisms that can be used in computational fluid dynamics (CFD) simulation of the flow field, combustion and emissions of gas turbine engine combustors. Reduced mechanisms lessen computa-tional cost and possess the ability to accurately predict the overall flame structure, including gas temperature and spe-cies as CH4, CO and NOx. The S-STEP algorithm which based on computational singular perturbation method (CSP) is performed for reduced the detailed mechanism GRI-3.0. This algorithm required as input: the detailed mechanism, a numerical solution of the problem and the desired number of steps in the reduced mechanism. In this work, we present a 10-Step reduced mechanism obtained through S-STEP algorithm. The rate of each reaction in the reduced mechanism depends on all species, steady-state and non-steady state. The former are calculated from the solution of a system of steady-state algebraic relations with the point relaxation algorithm. Ba...
Reduced Detailed Mechanism for Methane Combustion
Energy and Power Engineering, 2012
Simulated results from a detailed elementary reaction mechanism for methane-containing species in flames consisting of nitrogen (NO x), C 1 or C 2 fuels are presented, and compared with reduced mechanism; this mechanism have been constructed with the analysis of the rate sensitivity matrix f (PCAF method), and the computational singular perturbation (CSP). The analysis was performed on solutions of unstrained adiabatic premixed flames with detailed chemical kinetics described by GRI 3.0 for methane including NO x formation. A 9-step reduced mechanism for methane has been constructed which reproduces accurately laminar burning velocities, flame temperatures and mass fraction distributions of major species for the whole flammability range. Many steady-state species are also predicted satisfactorily. This mechanism is especially for lean flames. This mechanism is accurate for a wide range of the equivalence ratio (1, 0.9, 0.8, and 0.7) and for pressures as high as 40 atm to 60 atm. For both fuels, the CSP algorithm automatically pointed to the same steady-state species as those identified by laborious analysis or intuition in the literature and the global reactions were similar to well established previous methane-reduced mechanisms. This implies that the method is very well suited for the study of complex mechanisms for heavy hydrocarbon combustion.
Analysis and reduction of chemical kinetics for combustion applications
2021
Combustion of fossil fuels has been used for decades for all kinds of purposes, from generating electricity to make air planes fly but they are also the main source of pollution leading to climate change. New sustainable, less polluting fuels must be studied in order to diminish as much as possible the human impact on the planet. Combustion is a very complex process combining fluid dynamics, thermodynamics and chemistry with hundreds of species involved. In order to be able to use all the tools the numerical simulation has to offer with increasing complexity, from canonical cases to 3D Large Eddy Simulations (LES) with two-phase flows, analysing the relevant chemical pathways and reducing the reaction mechanisms describing this chemistry is necessary. Analytically Reduced Chemistry (ARC) is a way of reducing the size and the complexity of chemical mechanisms where only the species and reactions relevant to given conditions are kept while keeping a physically coherent mechanism. ARC ...
Validation of a reduced combustion mechanism for light hydrocarbons
Clean Technologies and Environmental Policy, 2012
Due to the tremendous costs and difficulties associated with flare measurements, computational fluid dynamics (CFD) simulation could be a viable approach to predict the combustion efficiency as well as VOC/NOx emissions from industrial flaring activities. However, consisting of a large number of reactions and species, most of the detailed kinetic mechanisms for the speciation study of flaring events are too complicated to use in the CFD simulation of industrial-scale flares. A reduced combustion mechanism will lead to improved computational efficiency; however, its fidelity must be validated. This study uses 2D CFD simulations and 1D Chemkin simulations to validate a reduced mechanism developed for the combustion of light hydrocarbons up to C1–C3. This mechanism, consisting of 50 species and 337 reactions, is applicable to C1–C3 hydrocarbons and can be used to predict the combustion efficiency and fate of pollutants released from industrial flares composed of C1–C3 waste gases. In this article, experimental data reported in the literatures have been used to validate the reduced mechanism. The key performance indicators used for comparison are laminar burner-stabilized flames, laminar flame speeds, adiabatic flame temperatures, ignition delay tests, and temperature and concentration profiles of the critical species. The software package CHEMKIN 4.1.1 was used to verify the computational results of laminar flame speeds, adiabatic flame temperatures, and ignition delays. The axial profiles of various critical species are simulated using the commercial CFD software package FLUENT. It is demonstrated that simulation results using this reduced mechanism are in good agreement with reported experimental results.
Combustion and Flame, 2007
Large-scale simulations of multidimensional unsteady reacting flows with detailed chemistry and transport can be computationally extremely intensive even on distributed computing architectures. With the development of computationally efficient reduced chemical kinetic models, the smaller number of scalar variables to be integrated can lead to a significant reduction in the computational time required for the simulation with limited loss of accuracy in the results. A general MATLAB-based automated procedure for the development of reduced reaction models is presented. Based on the application of the quasi-steady-state (QSS) approximation for certain chemical species and on the elimination of selected fast elementary reactions, any complex starting reaction mechanism (detailed or skeletal) can be reduced with minimal human intervention. A key feature of the reduction procedure is the decoupling of the QSS species appearing in the QSS algebraic relations, enabling the explicit solution of the QSS species concentrations, which are needed for the evaluation of the elementary reaction rates. In contrast, previous approaches mainly relied on an implicit solution, requiring computationally intensive inner iterations. The automated procedure is first tested with the generation of an implicit 5-step reduced reaction model for CH 4 /air flame propagation. Next, two explicit robust reduced reaction models based on ignition data (18-step) and on flame propagation data (15-step) are systematically developed and extensively validated for ignition delay time, flame propagation, and extinction predictions of C 2 H 4 /air, CH 4 /air, and H 2 /air systems over a wide range of equivalence ratios, initial temperatures, pressures, and strain rates. In order to assess the computational advantages of the explicit reduced reaction models, comparisons of the computational time required to evaluate the chemical source terms as well as for the integration of unsteady nonpremixed flames for each model are also presented.
Combustion and Flame, 2013
In this study a 5-step reduced chemical kinetic mechanism involving 9 species is developed for combustion of Blast Furnace Gas (BFG), a multi-component fuel containing CO/H 2 /CH 4 /CO 2 , typically with low hydrogen, methane and high water fractions, for conditions relevant for stationary gas-turbine combustion. This reduced mechanism is obtained from a 49-reaction skeletal mechanism which is a modified subset of GRI Mech 3.0. These skeletal and reduced mechanisms are validated for laminar flame speeds, ignition delay times and flame structure with available experimental data, and using computational results with a comprehensive set of elementary reactions. Overall, both the skeletal and reduced mechanisms show a very good agreement over a wide range of pressure, reactant temperature and fuel mixture composition.
Comprehensive Chemical Kinetics, 1997
Chemical mechanisms have been employed for many years in hydrocarbon combustion . Initially they were used as a means of understanding the underlying phenomenology of the combustion process in terms of the elementary reactions of individual species. This could be at a very schematic level, such as in thermal explosions where the process was modelled by a single reaction , or at a more complex level, exemplified by the peroxy radical mechanism of autoignition . As understanding developed, mechanisms were required to play a more quantitative role. Greater demands were made on their agreement with experiment both at the macroscopic level, and in the simulation of minor products and, more recently, radical intermediates [4]. These requirements have coincided with the existence of expanding databases of rate parameters for elementary reactions (see Chapter 3), so that the construction of ever more detailed "complete" mechanisms is becoming both increasingly feasible and complex. To this development has been added the need to embed chemical mechanisms in computational fluid dynamic codes, so that the understanding developed in studying homogeneous chemical kinetics can be employed in the reactive flow conditions found in real combustion devices (see Chapter 7). The present limitations of computer hardware mean that complete chemical mechanisms cannot be incorporated in computational fluid dynamics (CFD) codes, and for turbulent combustion systems these limitations are especially pronounced. In general, some approximations have to be made which allow the size of the mechanism to be drastically reduced.
Evaluation of Reduced Chemical Kinetic Mechanisms Used for Modelling Mild Combustion for Natural Gas
Thermal Science, 2009
A numerical and parametric study was performed to evaluate the potential of reduced chemistry mechanisms to model natural gas chemistry including NO x chemistry under mild combustion mode. Two reduced mechanisms, 5-step and 9-step, were tested against the GRI-Mech3.0 by comparing key species, such as NO x, CO 2 and CO, and gas temperature predictions in idealized reactors codes under mild combustion conditions. It is thus concluded that the 9-step mechanism appears to be a promising reduced mechanism that can be used in multi-dimensional codes for modelling mild combustion of natural gas. http://www.doiserbia.nb.rs/Article.aspx?id=0354-98360903131H
A detailed natural gas mechanism developed at Galway University (NUIG NGM, 228 species) for gas turbine relevant conditions was reduced by the combined application of Simulation Error Minimization Connectivity Method and iterative species-removal sensitivity method. Approximately 40,000 scenarios describing the ignition of fuels containing 40−100% methane, 0−30% ethane and 0−30% propane at lean, stoichiometric and rich conditions in air were investigated. An iterative worst-case scenario selection procedure was devised which made the development of reliable reduced mechanisms possible for such a huge number of cases. Four robust skeletal mechanisms with 50, 53, 61 and 75 species were developed. They could reproduce ignition delays for more than 99% of the cases within 20%, 13%, 10% and 5.5% of errors. The reduced mechanisms could also reproduce adiabatic flame temperatures and flame speeds reliably for lean and stoichiometric conditions.