Directed Relation Graph Method Research Articles

Simplification of the combustion mechanism of Jatropha biodiesel surrogate fuel and reaction path analysis

The objectives of this study were to explore the evolution process of the chemical reaction pathways in the combustion of a biodiesel surrogate fuel and to reduce the calculation burden in the numerical simulation of this combustion process due to the complicated mechanism. A mixture of methyl decanoate, methyl 9-decanoate, and n-heptane was selected as the Jatropha biodiesel surrogate fuel. A simplified combustion mechanism of the biodiesel comprising 170 components and 737 reaction components was formed by systematically simplifying 3299 components and 10806 reaction mechanisms using a multistep directed relation graph method, path flux analysis, and sensitivity analysis. The simplified mechanism was compared with the detailed mechanism and experimental data to effectively verify the accuracy of the mechanism. The simplified mechanism could reproduce the detailed mechanism, and the results were in good agreement with the experimental data in terms of the ignition time of a methyl decanoate/air mixture in a shock tube, the concentrations of the components generated in the combustion process of the biodiesel in a jet stirred reactor, and the flame propagation speed of methyl decanoate and n-heptane/air mixture. The rate of production analysis method was used to explore the main reaction pathways and the evolution process of important intermediate components in the Jatropha biodiesel surrogate fuel generated at high and low temperatures. Methyl decanoate and methyl 9-decanoate formed hydrogen peroxide ketone and oxy groups mainly through dehydrogenation, oxygenation, and isomerization at low temperatures. The high-temperature stage mainly involved bond-breaking decomposition reactions, partial dehydrogenation, and isomerization reactions of low-temperature reaction products, resulting in the formation of small molecular products.

Reactive molecular dynamics simulation and chemical kinetic modeling of ammonia/methane co-combustion

Under the vision of “low carbonization”, ammonia (NH3), as a carbon-free hydrogen-rich fuel, blended with reactive additives, is an effective means to achieve low carbon emissions and reduce the consumption of traditional fossil fuels. In this work, the combustion behaviors of NH3 and CH4, the distribution of main products and the formation and reduction characteristics of NO at different temperatures, CH4 mixing ratios, and O2 equivalence ratios are explored by means of reactive molecular dynamics simulation to reveal the microscopic mechanism of NH3/CH4 co-combustion. The calculated results show that CH4 takes the lead in producing H radicals which react with O2 to generate OH radicals to promote the oxidation of NH3, and it is found that the co-combustion process of NH3/CH4 in the radical chain transfer stage is similar to the respective pyrolysis and oxidation reactions of CH4 and NH3. Accordingly, an optimized kinetic mechanism of NH3/CH4 co-combustion is constructed based on the mechanisms proposed by Glarborg et al. and Okafor et al. by incorporating the NH3 pyrolysis sub-mechanisms related to NH2, HNO, and N2H2 and the skeletal mechanism of CH4 oxidation. The optimized mechanisms of NH3/CH4 co-combustion are found to improve the prediction of the ignition delay time (IDT) and laminar burning velocity (LBV) of NH3/CH4 mixture under wide ranges of conditions compared to the original mechanisms of Glarborg et al. and Okafor et al. To decrease the computational cost of three-dimensional combustion numerical simulations, the skeletal mechanism models are established by simplification of the optimized mechanisms of NH3/CH4 co-combustion using direct relation graph method with error propagation and sensitivity analysis, which can provide nearly the same estimates of IDTs and LBVs as the optimized mechanisms mentioned above.

Numerical study on explosion limits of ammonia/hydrogen/oxygen mixtures: Sensitivity and eigenvalue analysis

The study aims to investigate explosion limit characteristics of NH3/H2/O2 mixtures and construct a compact mechanism for explosion limits prediction. To study effects of H2 addition, simulations of mixtures with varied H2 percentages were performed in an adiabatic, constant-volume reactor based on detailed mechanisms over a wide range of pressures and temperatures. Results showed that the monotonic explosion limit curves for NH3/O2 mixtures transits to non-monotonic Z-shaped curves with H2 addition, and higher H2 percentage shifts the explosion limits to lower temperatures. Moreover, sensitivity analysis was conducted to identify the dominant reactions in explosion limits. It showed that reactions involving H, NH2 and NH3 are dominant at low pressures, and with elevated pressures, the effects of oxidation reactions involving NO, NO2, HNO and H2NO are enhanced. To construct a compact mechanism for explosion limit prediction, the detailed mechanism was firstly reduced by the directed relation graph method. Then, a linear mechanism was established based on eigenvalue analysis, consisting of molecule–molecule and radical-molecule reactions in the reduced mechanism. Compared with the simulation results of the detailed mechanism, the simulation results of the linear mechanism are satisfactory at low and medium pressures, but less satisfactory at high pressures. Considering the significant effects of radical–radical reactions at high pressures, four nonlinear reactions were incorporated into the linear mechanism, finally building the compact mechanism, which is able to well reproduce the explosion limits of NH3/H2/O2 mixtures.

Fast species ranking for iterative species-oriented skeletal reduction of chemistry sets

A fast algorithm is developed for ranking the species in a chemistry set according to their importance to the modeled densities of user-specified species of interest. The species ranking can be constructed for any set of user-specified plasma conditions, but here we focus predominantly on low-temperature plasmas, with gas temperatures between 300 and 1500 K covering the typical range of ICP and CCP plasma sources. This ranking scheme can be used to acquire insight into complex chemistry sets for modeling plasma phenomena or for a species-oriented reduction of the given chemistry set. The species-ranking method presented is based on a graph-theoretical representation of the detailed chemistry set and establishing indirect asymmetric coupling coefficients between pairs of species by the means of widely used graph search algorithms. Several alternative species-ranking schemes are proposed, all building on the theory behind different flavors of the directed relation graph method. The best-performing ranking method is identified statistically, by performing and evaluating a species-oriented iterative skeletal reduction on six, previously available, test chemistry sets (including O2–He and N2–H2) with varying plasma conditions. The species-ranking method presented leads to reductions of between 10 and 75% in the number of species compared to the original detailed chemistry set, depending on the specific test chemistry set and plasma conditions.

Reduced Methane Combustion Mechanism and Verification, Validation, and Accreditation (VV&A) in CFD for NO Emission Prediction

In order to obtain a reduced methane combustion mechanism for predicting combustion field and pollutants accurately in CFD simulations with a lower computational cost, a reduced mechanism with 22 species and 65 steps of reactions from GRI-Mech 3.0 was obtained by direct relation graph method and sensitivity analysis. The ideal reactor calculation and VVA the temperature field and species can be predicted accurately in CFD simulations (RANS and LES models), and the NO prediction error of an industrial gas turbine combustor outlet is less than 2×10−6. The proposed mechanism has high engineering values.

Numerical Simulation of Laminar and Turbulent Methane/Air Flames Based on a DRG-Derived Skeletal Mechanism

Simulation of turbulent flames using detailed chemical mechanisms is still a challenge in numerical combustion due to the large number of species and the stiffness of the system of governing equations. In this sense, strategies to reduce the size of the detailed model are necessary and one of such models is the well-known directed relation graph (DRG) method. In the present work, a DRG-derived skeletal mechanism developed using only one application for methane/ air simulations is presented and validated for auto-ignition times, laminar flame speed and counterflow flames. The skeletal mechanism is tested for varying the equivalence ratio (ϕ = 0.4, to 3) and pressure (p = 1 to 150 atm). The temperature spans the range from T = 1000 K to T = 2000 K. The relative error, compared with the detailed mechanism, of our proposed model for ignition delay times and flame speed are less than 10% for most of the parameters. The skeletal mechanism is also used to simulate the piloted turbulent jet Sandia Flame D. Results show that this skeletal mechanism can reproduce the main features of laminar and turbulent methane/air flames.

A Numerical and Experimental Study on the Effects of CO2 on Laminar Diffusion Methane/Air Flames

AbstractFlue gas recirculation (FGR) is an effective method to reduce NOx emissions from the combustion of fuels, such as natural gas. Nitrogen, carbon dioxide, and water are the main components of flue gas. Nitrogen is an inert gas, and water can be condensed out of the effluent before FGR. However, recycled CO2 can alter the physical and chemical combustion characteristics of a fuel. This research investigated the effects of CO2 on CH4/air laminar diffusion flames, both experimentally and numerically. Experiments used laser-induced fluorescence to measure OH and CH distributions in the resulting flames, at different CO2 concentrations. Numerical methods were used to investigate the reaction mechanism and predict temperature and species concentration fields, as well as the NOx formation. Experiments showed that the CH fluorescence intensities decreased with the addition of CO2, while the OH fluorescence intensities increased. Both the directed relation graph method and the sensitivity analysis method were used to reduce the GRI-mech 3.0 mechanism. The chemical kinetics of methane combustion were analyzed using the reduced mechanism with the diffusion opposed-flow flame model in the chemkin 4.1 software package to determine the main reactions among the major species. Numerical simulations showed that as the amount of CO2 in the fuel increased, the concentration of CH decreased. These CFD simulations using the reduced mechanism were in agreement with the experimental data. Thus, the reduced mechanism was then used to predict NO concentrations. Numerical simulations showed that as the amount of CO2 in the fuel increased, the concentration of CH decreased, and, as a result, lower amounts of NO were predicted.

Chemistry reduction of complex CO2 chemical kinetics: application to a gliding arc plasma

A gliding arc (GA) plasma has great potential for CO2 conversion into value-added chemicals, because of its high energy efficiency. To improve the application, a 2D/3D fluid model is needed to investigate the CO2 conversion mechanisms in the actual discharge geometry. Therefore, the complex CO2 chemical kinetics description must be reduced due to the huge computational cost associated with 2D/3D models. This paper presents a chemistry reduction method for CO2 plasmas, based on the so-called directed relation graph method. Depending on the defined threshold values, some marginal species are identified. By means of a sensitivity analysis, we can further reduce the chemistry set by removing one by one the marginal species. Based on the so-called flux-sensitivity coupling, we obtain a reduced CO2 kinetics model, consisting of 36 or 15 species (depending on whether the 21 asymmetric mode vibrational states of CO2 are explicitly included or lumped into one group), which is applied to a GA discharge. The results are compared with those predicted with the full chemistry set, and very good agreement is reached. Moreover, the range of validity of the reduced CO2 chemistry set is checked, telling us that this reduced set is suitable for low power GA discharges. Finally, the time and spatial evolution of the CO2 plasma characteristics are presented, based on a 2D model with the reduced kinetics.

A skeletal n-butane mechanism with integrated simplification method

A new skeletal mechanism of n-butane is developed for describing its ignition and combustion characteristics applicable over a wide range of conditions: initial temperature 690–1430 K, pressure 1–30 atm, and equivalence ratio 0.5–2.0. Starting with a detailed chemical reaction kinetic model of 230 species and 1328 reactions (Healy et al., Combust. Flame, 2010), the directed relation graph method is applied as the first step to derive a semi-detailed mechanism with 134 species. Then, the reaction path analysis in conjunction with temperature sensitivity analysis is used to remove the redundant species and reaction paths simultaneously under the condition of low-temperature and moderate-to-high temperatures, respectively. Finally, a skeletal n-butane mechanism consisting of 86 species and 373 reactions can be obtained. Mechanism validation indicates that the new developed skeletal mechanism is in good agreement with the detailed mechanism in predicting the global ignition and combustion characteristics. The new skeletal mechanism is further validated using extensive available literature data including rapid pressure machine ignition delay time, shock-tube ignition delay time, laminar flame speed, and jet-stirred reaction oxidation, covering a large range of temperatures, pressures, and equivalence ratios. The comparison results demonstrate that a satisfactory agreement between predictions and experimental measurements is achieved.

Investigation of the structure and propagation speeds of n-heptane cool flames

Low temperature combustion of n-heptane was investigated through an experimental and numerical study of laminar cool flame structure and propagation speeds. Lean and stoichiometric n-heptane cool flames of equivalence ratios 0.4–1.0 were stabilized using a Hencken burner at sub-atmospheric pressures. Characterization of cool flame lift-off heights in burner-stabilized and freely propagating regimes was performed using broadband chemiluminescence and formaldehyde planar laser-induced fluorescence (CH2O PLIF), and the adiabatic cool flame propagation speed was estimated at the intersection of these two regimes. Temperature measurements of cool flames were performed and temperature profiles were employed in fixed temperature numerical simulations. Reduction of a chemical kinetics model was conducted using the Directed Relation Graph method along with reaction rate sensitivity analysis, and a reduced model covering the range of experimentally investigated equivalence ratios was achieved. The resultant propagation speeds from freely propagating flame simulations using the reduced model were compared with experimentally measured values. Additionally, CH2O PLIF intensity profiles over a range of reactant flow rates were compared to assess flame structures in freely propagating and burner-stabilized regimes. Finally, mole concentrations of major species at quasi-equilibrium conditions downstream of the flame were compared to numerical simulations using the full and reduced models, and reasonable agreement between the experiments and simulations was observed.

Error-controlled kinetics reduction based on non-linear optimization and sensitivity analysis

Efficient reduction techniques are necessary for the construction of compact chemical kinetic mechanisms to enable the application to large-scale simulations. In this work, a new formulation is proposed to derive skeletal mechanisms by species elimination. The proposed method relies on a gradient-based non-linear optimization (NLO) method, allowing for direct error control on user-defined quantities of interest (QoIs). The key idea consists in formulating the species elimination by relaxing an integer-optimization problem into a continuous optimization problem that is solved iteratively. A species-targeted sensitivity analysis (SA) formulation is presented that can be combined with the optimization method to reduce the overall computational complexity of the procedure. After illustrating its principle, the NLO procedure is applied to different fuels of increasing chemical complexity, including methane, n-dodecane, and a four-component kerosene surrogate. Direct comparisons are performed with the directed relation graph method with error propagation (DRGEP) and SA. For the reduction of a detailed methane mechanism, consistency with DRGEP and SA is demonstrated, and the NLO procedure is shown to generate smaller mechanisms compared to the other two methods. In application to larger hydrocarbon and multicomponent transportation fuels, it is shown that significantly smaller mechanisms are obtained with the NLO procedure compared to the other approaches.

Characterization of ozone-enhanced propane cool flames at sub-atmospheric pressures

A recently developed experimental setup for the study of cool flames was employed to investigate low temperature combustion of propane. Cool flames were stabilized though ozone activation using a laminar flat flame Hencken burner at sub-atmospheric pressures. Propane cool flame stability was observed to be highly sensitive to pressure, and a pressure stability map is presented for a range of equivalence ratios (ϕ) from 0.17 to 1.0 in 0.17 increments and a range of reactant flow rates. Based on the stability map, a propane cool flame of ϕ = 0.17 at 17.3 kPa was chosen for further study. Flame lift-off height above the burner surface was measured and two operating regimes of burner-stabilized and freely propagating flames were observed. The intersection of these two regimes was used to estimate the cool flame propagation speed. Temperature measurements of the cool flame were taken and used in fixed-temperature numerical flame simulations. Additionally, flame temperatures and propagation speeds are presented for all equivalence ratios and pressures where stable, freely propagating flames were observed in the stability map. In order to enable non-fixed temperature, freely propagating cool flame simulations to be performed, reduction of a chemical kinetics model using the Directed Relation Graph method (DRG) in conjunction with reaction rate sensitivity analysis was performed. Experimentally determined propagation speed, temperature and species concentrations at partial equilibrium were compared to numerical simulations with reasonable agreement and results are discussed.

A simplified chemical reaction mechanism for two-component RP-3 kerosene surrogate fuel and its verification

The simplification of the dynamic model of aviation kerosene oxidation is one of the important problems in numerical simulation of combustion chamber. It plays a decisive role in CFD numerical calculation and for reducing the stiffness of convergence, as well as for introducing chemical reactions to more complex turbulent combustion problems. This study aims to present a two-component surrogate fuel model for chemical reactions. Furthermore, the simplified mechanism of the optimization of the ignition delay time and experimental value is adopted using the direct relation graph method (DRG). As the primary originality for the paper, the eclectic simplified mechanisms with certain precision and fewer components and elementary reactions were concluded. And the simplified mechanism was introduced into FLUENT to simulate the precast evaporation flame of the Bunsen burner, which is a RP-3 fuel. Numerical results are in good agreement with the experimental results. The existing simplified mechanism for the two-component RP-3 aviation kerosene alternative fuel can be applied to the practical engineering problems of the numerical forecast.

Detailed OMCTS/H2/O2 chemical kinetics mechanism and its application in the fused silica glass synthesis

The application of OMCTS/H2/O2 combustion process for the fused silica glass synthesis can prevent the halide emission compared to the conventional SiCl4/H2/O2 system. Detailed chemical kinetics of OMCTS/H2/O2 is critical for the prediction of flame characteristics, vapor nucleation, particle growth and deposition in the synthesis process. In this work, a detailed OMCTS/H2/O2 mechanism consisting of 55 species and 239 elementary reactions is formulated, which accounts for the oxidation, pyrolysis and hydrolysis of OMCTS, the combustion processes of H2/O2 and low hydrocarbon intermediate species. The dominant reaction pathways at representative thermo-chemical conditions for the fused silica glass synthesis are further analyzed through the directed relation graph method and a reduced mechanism of 24 species and 39 reactions is developed. The fused silica glass synthesis processes with OMCTS/H2/O2 and SiCl4/H2/O2 are then simulated respectively with the proposed mechanisms to reveal the differences in the flame, radicals and SiO2 formation characteristics. It is shown that although the two systems yield similar flame characteristics such as temperature and OH distributions, the SiO2 formation characteristics is different. The OMCTS/H2/O2 system has significant larger SiO2 concentration and a wider spread of SiO2 near the silica glass ingot and consequently a higher SiO2 formation rate. It is also shown that one new peak appears in the SiO distribution between the nozzle and SiO2 peak because the OMCTS pyrolysis process is important and it yields a larger amount of SiO. The effects of hydrogen inflow mass flow rate on the flame and SiO2 formation characteristics in the OMCTS/H2/O2 system are also analyzed.

Prediction of flame structure and pollutant formation of Sandia flame D using Large Eddy Simulation with direct integration of chemical kinetics

Large Eddy Simulation (LES) with direct integration of reduced chemical kinetics including NO chemistry is performed on the Sandia flame D. This approach allows a detailed analysis of the flame structure and pollutant formation. The Analytically Reduced Chemistries (ARCs) are obtained using Directed Relation Graph method with Error Propagation (DRGEP) and Quasi-Steady-State (QSS) approximation. Two ARCs containing both 22 species are derived for methane–air oxidation, from GRI 2.11 and GRI 3.0 detailed mechanisms. They correctly predict fuel consumption speed, as well as NO and CO concentrations in laminar premixed and non-premixed flames at atmospheric conditions. It is found that the NO production strongly depends on the detailed mechanism, being significantly higher with GRI 3.0 in rich premixed flames and in diffusion flames. The two ARCs are then used in highly-resolved LES of the Sandia flame D. The numerical results are in very good agreement with the experiment in terms of aerodynamics, mixture fraction and temperature profiles. The CO concentration is also well predicted with the two ARCs. For NO, a satisfactory agreement with the measurements is obtained with the ARC based on GRI 2.11, while a significant over-prediction is obtained with the GRI 3.0-based ARC, consistently with the differences observed in laminar cases between the two GRI versions. A detailed investigation of the flame structure including a comparison with reference laminar flames reveals that the flame structure is essentially non-premixed. The presence of the pilot jet alters the mixing process, leading to a flame structure that falls between two extreme non-premixed combustion regimes corresponding to the interaction of the rich central jet with either the hot gases from the pilot, or the coflow of fresh air. The associated laminar diffusion flamelets indicate that this particular flame structure influences the formation of pollutants, with a strong impact on CO production.

Comprehensive Validation of Skeletal Mechanism for Turbulent Premixed Methane–Air Flame Simulations

A new skeletal mechanism, consisting of 16 species and 72 reactions, has been developed for lean methane–air premixed combustion from the GRI-Mech 3.0. The skeletal mechanism is validated for elevated unburnt temperatures (800 K) and pressures up to 4 atm, thereby addressing realistic gas turbine conditions. The skeletal mechanism is obtained by applying the directed relation graph method and performing sensitivity analysis on the detailed mechanism. The mechanism has been validated for flame speed and flame structure in a wide range of conditions and configurations. A good agreement between the skeletal mechanism and GRI-3.0 was obtained. The configurations considered include one-dimension laminar premixed flames, laminar non-premixed counterflow burners, and two- and three-dimensional unsteady configurations with variations of temperature, pressure, and composition. The skeletal mechanism allows for the inclusion of accurate finite rate chemistry in large-scale direct numerical simulations of lean turbulent premixed flames. In a large-scale direct numerical simulation, the use of the skeletal mechanism reduces the memory requirements by more than a factor of 3 and accelerates the simulation by a factor of 7 compared with the detailed mechanism. The skeletal mechanism is suitable for unsteady three-dimensional simulations of methane turbulent premixed, non-premixed, and globally lean partially premixed flames and is available as supplementary material.

Development of a new reduced hydrogen combustion mechanism with NOx and parametric study of hydrogen HCCI combustion using stochastic reactor model

Hydrogen is a potential alternative and renewable fuel for homogenous charge compression ignition (HCCI) engine to achieve higher efficiency and zero emissions of CO, unburned hydrocarbons as well as other greenhouse gases such as CO2 and CH4. In this study, a detailed hydrogen oxidation mechanism with NOx was developed by incorporating additional species and NOx reactions to the existing hydrogen combustion mechanism (10 species and 40 reactions). The detailed hydrogen combustion mechanism used in this study consists of 39 species and 311 reactions. A reduced mechanism consisting 30 species and 253 reactions was also developed by using directed relation graph (DRG) method from detailed mechanism. Developed mechanisms were validated with experimental data by HCCI engine simulation using stochastic reactor model. Sensitivity analysis was performed to identify the most important reactions in hydrogen combustion and NOx formation in HCCI engine. Pathway analysis was also performed to analyze the important reaction pathways at different temperatures. Results revealed that H2+HO2 [=] H+H2O2 and O2+NNH [=] N2+HO2 are the most significant reactions in the hydrogen HCCI combustion and NOx formation respectively. Detailed parametric study of HCCI combustion was conducted using developed chemical kinetic model. Numerical simulations are performed at different engine operating condition by varying engine speed (1000–3000rpm), intake air temperature (380–460K), and compression ratio (16–18) at different relative air fuel ratios (λ). The HCCI operating range was determined for different compression ratios and results show that operating range expands with increase in compression ratio. The effect of intake temperature, engine speed and equivalence ratio on cylinder pressure and heat release rate were investigated. Maximum thermal efficiency of 46% and maximum combustion efficiency of 98% was observed among all the test conditions. Parametric study of NOx emissions was also conducted and it was found that NOx emissions decrease exponentially from higher to lower engine loads.

Prediction Accuracy and Efficiency of the n-Heptane Mechanism at Different Reduction Levels

The detailed combustion mechanism can be coupled with the computational fluid dynamics (CFD) software to simulate the combustion process in a practical engine. However, the computing time may be unfeasibly long. To improve the efficiency of the simulations, a reduced mechanism is preferable. However, the combustion characteristics and prediction accuracy will be influenced by the reduction of the combustion mechanism. In this work, the effects of the reduction of the detailed n-heptane mechanism on the prediction accuracy and efficiency were investigated theoretically. The reduction was based on the directed relation graph method without revising the original kinetic parameters. The results indicated that the reduced combustion mechanism at 1/2 size (1/2 mechanism) of the detailed mechanism performed well and the 1/4 mechanism showed some deviation from the experimental data as a result of the removal of some low-temperature reactions. The 1/8 mechanism performed even worse. An ideal combustion mechanism for coupling with CFD simulations should be of the size between that of the 1/2 mechanism and 1/4 mechanism.

Comparison of Different CH4-Air Combustion Mechanisms in a Perfectly Stirred Reactor with Oscillating Residence Times Close to Extinction

ABSTRACTTransient simulations of the methane-air mixture combustion in a Perfectly Stirred Reactor (PSR) with global one-step and two-step, reduced (by directed relation graph method) and detailed chemical reaction mechanisms, have been conducted. The simulation conditions were carefully selected to observe the dynamic of a PSR with an oscillating residence time close to the extinction. The effect of the oscillation amplitude and frequency have been investigated over a wide range of values. The temperature response of the global and reduced mechanism has been compared to that of the detailed reaction mechanisms, the GRI Mech 3.0. The discussion of results points out limits and problems associated with the adoption of global reaction mechanisms. The results of the comparison indicate that the adoption of a global mechanism is not adequate in order to obtain an accurate description of methane-air combustion in a PSR with an oscillating residence time. This comparison underlines that only a little benefit is added by the adoption of the selected two-step mechanism. Instead, the reduced mechanism shows an excellent agreement with the detailed reaction mechanism.

Numerical investigation of ethanol fuelled HCCI engine using stochastic reactor model. Part 1: Development of a new reduced ethanol oxidation mechanism

Ethanol is considered a potential biofuel for internal combustion engines. In this study, homogeneous charge compression ignition (HCCI) simulations of ethanol engine experiments were performed using stochastic reactor model (SRM). Detailed ethanol oxidation mechanism is developed by including NOx reaction in existing detailed oxidation mechanism with 57 species and 383 reactions. Detailed ethanol mechanism with NOx used in this study contains 76 species and 495 reactions. This mechanism was reduced by direct relation graph (DRG) method, which was validated with the experimental results. Existing Lu’s 40-species skeletal mechanism with NO formation were also compared with detailed and reduced mechanisms for predicting maximum cylinder pressure, maximum heat release rate and crank angle position of maximum cylinder pressure in HCCI engine. Reduced mechanism developed in this study exhibited the best resemblance with the experimental data. This reduced mechanism was also validated by measured engine cylinder pressure curves and measured ignition delays in constant volume reactors. The results showed that reduced mechanism is capable of predicting HCCI engine performance parameters with sufficient accuracy. Sensitivity analysis was conducted to determine the influential reactions in ethanol oxidation. Results also show that detailed and reduced mechanism was able to predict NOx emission in good agreement with the corresponding experimental data.