Methanol Oxidation Induction Times Using the Rate-Controlled Constrained-Equilibrium Method

2003 ◽  
Author(s):  
Sergio Ugarte ◽  
Mohamad Metghalchi ◽  
James C. Keck
Keyword(s):  
Methanol Oxidation ◽  
Ignition Delay ◽  
Rate Equations ◽  
Oxygen Mixture ◽  
Free Oxygen ◽  
Equilibrium Method ◽  
Ignition Delay Times ◽  
Delay Times ◽  
Constrained Equilibrium ◽  
Rate Controlled

Methanol oxidation has been modeled using the Rate-Controlled Constrained-Equilibrium method (RCCE). In this method, composition of the system is determined by constraints rather than by species. Since the number of constraints can be much smaller than the number of species present, the number of rate equations required to describe the time evolution of the system can be considerably reduced. In the present paper, C1 chemistry with 29 species and 140 reactions has been used to investigate the oxidation of stoichiometric methanol/oxygen mixture at constant energy and volume. Three fixed elemental constraints: elemental carbon, elemental oxygen and elemental hydrogen and from one to nine variable constraints: moles of fuel, total number of moles, moles of free oxygen, moles of free oxygen, moles of free valence, moles of fuel radical, moles of formaldehyde H2CO, moles of HCO, moles of CO and moles of CH3O were used. The four to twelve rate equations for the constraint potentials (LaGrange multipliers conjugate to the constraints) were integrated for a wide range of initial temperatures and pressures. As expected, the RCCE calculations gave correct equilibrium values in all cases. Only 8 constraints were required to give reasonable agreement with detailed calculations. Results of using 9 constraints showed compared very well to those of the detailed calculations at all conditions. For this system, ignition delay times and major species concentrations were within 0.5% to 5% of the values given by detailed calculations. Adding up to 12 constraints improved the accuracy of the minor species mole fractions at early times, but only had a little effect on the ignition delay times. RCCE calculations reduced the time required for input and output of data in 25% and 10% when using 8 and 9 constraints respectively. In addition, RCCE calculations gave valuable insight into the important reaction paths and rate-limiting reactions involved in methanol oxidation.

Comparison Between RCCE and Shock Tube Ignition Delay Times at Low Temperatures

2015 ◽  
Vol 137 (6) ◽  
Author(s):  
Ghassan Nicolas ◽  
Hameed Metghalchi
Keyword(s):  
Free Energy ◽  
Shock Tube ◽  
Ignition Delay ◽  
Low Temperatures ◽  
Reduction Technique ◽  
Experimental Measurements ◽  
Ignition Delay Times ◽  
Delay Times ◽  
Constrained Equilibrium ◽  
Rate Controlled

The rate-controlled constrained-equilibrium (RCCE) method is a reduction technique based on local maximization of entropy or minimization of a relevant free energy at any time during the nonequilibrium evolution of the system subject to a set of kinetic constraints. In this paper, RCCE has been used to predict ignition delay times of low temperatures methane/air mixtures in shock tube. A new thermodynamic model along with RCCE kinetics has been developed to model thermodynamic states of the mixture in the shock tube. Results are in excellent agreement with experimental measurements.

Constrained-Equilibrium Modeling of Methane Oxidation in Air

2014 ◽  
Vol 136 (3) ◽  
Author(s):  
Ghassan Nicolas ◽  
Mohammad Janbozorgi ◽  
Hameed Metghalchi
Keyword(s):  
Kinetic Model ◽  
Ignition Delay ◽  
Combustion Process ◽  
Rate Equations ◽  
Time Step ◽  
Ignition Delay Times ◽  
Wide Range ◽  
Delay Times ◽  
Constrained Equilibrium ◽  
Detailed Kinetic Model

Rate-controlled constrained-equilibrium method has been further developed to model methane/air combustion. A set of constraints has been identified to predict the nonequilibrium evolution of the combustion process. The set predicts the ignition delay times of the corresponding detailed kinetic model to within 10% of accuracy over a wide range of initial temperatures (900 K–1200 K), initial pressures (1 atm–50 atm) and equivalence ratios (0.6–1.2). It also predicts the experimental shock tube ignition delay times favorably well. Direct integration of the rate equations for the constraint potentials has been employed. Once the values of the potentials are obtained, the concentration of all species can be calculated. The underlying detailed kinetic model involves 352 reactions among 60 H/O/N/C1-2 species, hence 60 rate equations, while the RCCE calculations involve 16 total constraints, thus 16 total rate equations. Nonetheless, the constrained-equilibrium concentrations of all 60 species are calculated at any time step subject to the 16 constraints.

Rate-Controlled Constrained-Equilibrium Calculations of Ethanol-Oxygen Mixture

2006 ◽  
Author(s):  
Mohammad Janbozorgi ◽  
Yue Gao ◽  
Hameed Metghalchi ◽  
James C. Keck
Keyword(s):  
Chemical Reactions ◽  
Initial Pressure ◽  
Kinetic Scheme ◽  
Rate Equations ◽  
Oxygen Mixture ◽  
Free Valence ◽  
Free Oxygen ◽  
Constant Volume Chamber ◽  
Constrained Equilibrium ◽  
Rate Controlled

The Rate-Controlled Constrained-Equilibrium method (RCCE) is a powerful technique for simplifying the treatment of chemical reactions in complex systems. The method is based on the assumption that slow chemical reactions impose constraints on the allowed composition of such systems. Since the number of constraints can be very much smaller than the number of species, the number of rate equations to be integrated can be considerably reduced. In the present work, a kinetic scheme with 55 species and 366 reactions has been used to investigate stoichiometric Ethanol-oxygen mixture in a constant energy constant volume chamber. The state of the system was determined by three fixed elemental constraints: elemental carbon, elemental oxygen and elemental hydrogen and six variable constraints: moles of fuel, moles of fuel radicals, total number of moles, moles of free valence, moles of free oxygen, moles of OH+O+H. The 9 rate equations for the constraint potentials (LaGrange multipliers associated with the constraints) were integrated over a range of 1400 K-1700 K for initial temperature and 1atm-10atm for initial pressure. The RCCE calculations were in good agreement with detailed calculations and were faster than detailed calculations, which required integration of 55 species rate equations.

Constrained-Equilibrium Modeling of Methane Oxidation in Air

2013 ◽  
Author(s):  
Ghassan Nicolas ◽  
Mohammad Janbozorgi ◽  
Hameed Metghalchi
Keyword(s):  
Combustion Process ◽  
Rate Equations ◽  
Experimental Measurements ◽  
Combustion Modeling ◽  
Ignition Delay Times ◽  
Wide Range ◽  
Constrained Equilibrium ◽  
Detailed Kinetic Model ◽  
Rate Controlled ◽  
Oxidation In Air

The Rate-Controlled Constrained-Equilibrium (RCCE) has been further developed and applied to model methane/air combustion process. The RCCE method is based on local maximization of entropy or minimization of a relevant free energy at any time during the non-equilibrium evolution of the system subject to a set of constraints. The constraints are imposed by slow rate-limiting reactions. Direct integration of the rate equations for the constraint potentials has been employed. Once the values of the potentials are obtained, the concentration of all species can be calculated. A set of constraints has been developed for methane/air mixtures in the method of Rate-Controlled Constrained-Equilibrium (RCCE). The model predicts the ignition delay times, which have been compared to those predicted by detailed kinetic model (DKM) and with shock tube experimental measurements. The DKM includes 60 H/O/C1–2/N species and 352 reactions. The RCCE model using 16 constraints has been applied for combustion modeling in a wide range of initial temperatures (900–1200 K), pressures (1–50 atmospheres) and fuel-air equivalence ratio (0.6–1.2). The predicted results of using RCCE are within 5% of those of DKM model and are in excellent agreement with experimental measurements in shock tubes.

Development of the Rate-Controlled Constrained-Equilibrium Method for Modeling of Ethanol Combustion

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
Ghassan Nicolas ◽  
Hameed Metghalchi
Keyword(s):  
Ignition Delay ◽  
Stable Equilibrium ◽  
Combustion Process ◽  
Detailed Chemical Kinetics ◽  
Equilibrium Method ◽  
Constrained Equilibrium ◽  
Ethanol Combustion ◽  
Rate Controlled ◽  
Detailed Chemical

The rate-controlled constrained-equilibrium method (RCCE) has been further developed to model the combustion process of ethanol air mixtures. The RCCE is a reduction technique based on local maximization of entropy or minimization of a relevant free energy at any time during the nonequilibrium evolution of the system subject to a set of kinetic constraints. An important part of RCCE calculation is determination of a set of constraints that can guide the nonequilibrium mixture to the final stable equilibrium state. In this study, 16 constraints have been developed to model the nonequilibrium ethanol combustion process. The method requires solution of 16 differential equations for the corresponding constraint potentials. Ignition delay calculations of ethanol oxidizer mixtures using RCCE have been compared to those of detailed chemical kinetics using 37 species and 235 reactions. Agreement between the two models is very good. In addition, ignition delay of C2H5OH/O2/Ar mixtures using RCCE has been compared with the experimental measurements in the shock tube and excellent agreement has been reached validating the RCCE calculation.

Modeling of Combustion of Mono-Carbon Fuels Using the Rate-Controlled Constrained-Equilibrium Method

2004 ◽  
Author(s):  
Sergio Ugarte ◽  
Mohamad Metghalchi ◽  
James C. Keck
Keyword(s):  
Combustion Process ◽  
Chemical Species ◽  
Second Law Of Thermodynamics ◽  
Equilibrium System ◽  
Equilibrium Method ◽  
Ignition Delay Times ◽  
Constrained Equilibrium ◽  
Rate Controlled ◽  
Induction Times ◽  
Non Equilibrium

Modeling of a non-equilibrium combustion process involves the solution of large systems of differential equations with as many equations as species present during the process. The process of chemical reaction and combustion is complicated since it may be governed by hundreds, sometimes thousands of microscopic rate processes. Integration of these equations simultaneously becomes more difficult with the complexity of the combustible. In order to reduce the size of these systems of equations, the Rate-Controlled Constrained-Equilibrium method (RCCE) has been proposed to model non-equilibrium combustion processes. This method is based on the Second Law of Thermodynamics, assuming that the evolution of a complex system can be described by a small number of rate-controlling reactions which impose slowly changing constraints on all allowed states of the system, therefore a non-equilibrium system will relax to its final equilibrium state through a sequence of rate controlled constrained equilibrium states. Oxidation induction times and concentration of species during a combustion process are found in a less complicated way with this method, as equations for constraints rather than for species determine the composition and evolution of the system. The time evolution of the system can be reduced since the number of constraints is much smaller than the number of species presents, so the number of equations to solve. The RCCE method has been applied to the stoichiometric combustion of mono-carbon fuels using 29 chemical species and 139 chemical reactions at different sets of pressure and temperature, ranging from 1 atm to 100 atm, and from 900 K to 1600 K respectively. Results of using 8, 9, 10 and 11 constraints compared very well to those of the detailed calculations at all conditions for the cases of formaldehyde (H2CO), methanol (CH3OH) and methane (CH4). For these systems, ignition delay times and major species concentrations were within 5% of the values given by detailed calculations, and computational saving times up to 50% have been met.

The Rate-Controlled Constrained-Equilibrium Combustion Modeling of n-Pentane/Oxygen/Diluent Mixtures

2019 ◽  
Vol 141 (8) ◽  
Author(s):  
Linghao Du ◽  
Guangying Yu ◽  
Ziyu Wang ◽  
Hameed Metghalchi
Keyword(s):  
Experimental Data ◽  
Hydrocarbon Fuel ◽  
Chemical Mechanism ◽  
Pure Oxygen ◽  
Oxygen Mixture ◽  
Ignition Delay Times ◽  
Constraint Set ◽  
Constrained Equilibrium ◽  
Rate Controlled ◽  
Reacting Systems

Rate-controlled constrained equilibrium (RCCE) is a reduction technique used to describe the time evolution of complex chemical reacting systems. This method is based on the assumption that a nonequilibrium system can reach its final equilibrium state by a series of RCCE states determined by maximizing entropy or minimizing relevant free energy. Those constraints are imposed by some small number of slow reactions. Much research has been done on this method and many RCCE models of C1−C4 hydrocarbon fuel combustion have been established by the previous researchers. Those models show good performance compared with the result of detailed kinetic model (DKM). In this study, RCCE method is further developed to model normal pentane (n-C5H12) combustion with least number of constraints. The chemical mechanism for DKM contains 133 species and 922 reactions. Two sets of constraints were found during the study: (1) 16 constraints for the normal pentane and pure oxygen mixture and (2) 14 constraints for the mixture of normal pentane and oxygen with argon as diluent. Results of the first constraint set were compared with result of DKM and results of the second constraint set were compared with those of DKM and experimental data by calculating their ignition delay times. Comparisons showed that the first set of constraints had relatively good accuracy and the second set of constraints agreed very well with the experimental data.

Ignition Characteristics of a Fischer-Tropsch Synthetic Jet Fuel

2008 ◽  
Author(s):  
P. Gokulakrishnan ◽  
M. S. Klassen ◽  
R. J. Roby
Keyword(s):  
Kinetic Model ◽  
Ignition Delay ◽  
Flow Reactor ◽  
Kinetic Mechanism ◽  
Jet Fuel ◽  
Synthetic Jet ◽  
Jet Fuels ◽  
Ignition Delay Times ◽  
Delay Times ◽  
Ignition Characteristics

Ignition delay times of a “real” synthetic jet fuel (S8) were measured using an atmospheric pressure flow reactor facility. Experiments were performed between 900 K and 1200 K at equivalence ratios from 0.5 to 1.5. Ignition delay time measurements were also performed with JP8 fuel for comparison. Liquid fuel was prevaporized to gaseous form in a preheated nitrogen environment before mixing with air in the premixing section, located at the entrance to the test section of the flow reactor. The experimental data show shorter ignition delay times for S8 fuel than for JP8 due to the absence of aromatic components in S8 fuel. However, the ignition delay time measurements indicate higher overall activation energy for S8 fuel than for JP8. A detailed surrogate kinetic model for S8 was developed by validating against the ignition delay times obtained in the present work. The chemical composition of S8 used in the experiments consisted of 99.7 vol% paraffins of which approximately 80 vol% was iso-paraffins and 20% n-paraffins. The detailed kinetic mechanism developed in the current work included n-decane and iso-octane as the surrogate components to model ignition characteristics of synthetic jet fuels. The detailed surrogate kinetic model has approximately 700 species and 2000 reactions. This kinetic mechanism represents a five-component surrogate mixture to model generic kerosene-type jets fuels, namely, n-decane (for n-paraffins), iso-octane (for iso-paraffins), n-propylcyclohexane (for naphthenes), n-propylbenzene (for aromatics) and decene (for olefins). The sensitivity of iso-paraffins on jet fuel ignition delay times was investigated using the detailed kinetic model. The amount of iso-paraffins present in the jet fuel has little effect on the ignition delay times in the high temperature oxidation regime. However, the presence of iso-paraffins in synthetic jet fuels can increase the ignition delay times by two orders of magnitude in the negative temperature (NTC) region between 700 K and 900 K, typical gas turbine conditions. This feature can have a favorable impact on preventing flashback caused by the premature autoignition of liquid fuels in lean premixed prevaporized (LPP) combustion systems.

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