Detailed Chemical Scheme Research Articles

Development and validation of a predictive combustion model for hydrogen-fuelled internal combustion engines

Internal combustion engines (ICEs) fuelled with hydrogen can play a major role in the short-term future transportation sector since they abate all criteria pollutants at engine-out reducing tailpipe CO2 emissions to near-zero levels. However, optimizing hydrogen ICEs is a challenging task that can be addressed through the development of a robust simulation tool capable to predict the H2 combustion process. In this study, a previously developed two-zone combustion model has been updated considering different laminar flame speed computations, both based on a detailed chemistry scheme: a polynomial correlation function and a tabulated approach. The predictive capabilities of the combustion model have been validated against experimental data coming from a 0.5L PFI single-cylinder engine under several operating conditions. The tabulated approach for laminar flame speed definition proved to be the best solution, leading to a combustion duration average error lower than 3 deg over a dataset containing more than 45 different operating conditions.

Infinitely Fast Heterogeneous Catalysis Model for Premixed Hydrogen Flame-Wall Interaction

In most computations of reacting flows, walls are considered as inert. This approach can produce non-physical results that lead to very large values of heat release at the wall for hydrogen Flame-Wall Interaction (FWI). This paper confirms this unexpected behavior at walls for H2 flames. It also explores the possibility that the problem does not come from a deficiency of the chemical scheme at low temperatures but from the wall boundary condition: due to the large amount of radical species produced by H2 flames, the “inert” assumption may not be valid anymore. The purpose of this study is twofold: (1) to propose a simplified model named Infinitely Fast Heterogeneous Catalysis (IFHC) that mimics surface reactions and (2) to test it on the canonical Head-On Quenching (HOQ) configuration. A large set of parameters is investigated: a skeletal (San Diego) and a detailed (Burke) chemical schemes are tested, the grid-dependence and the influence of operating conditions (ϕ, Tw and P) are assessed. Results show that the IFHC model can inhibit the chemical pathways that are responsible for the heat generation peak observed on the wall in hydrogen FWI with inert surfaces. The IFHC model also allows computations to converge when the mesh is refined, a property that cannot be reached for inert wall treatments. Using IFHC, neither the predicted quenching distance nor the flame structure are altered. However, the maximum wall heat flux is significantly decreased when compared to inert walls. Over the range of operating conditions studied, the maximum wall heat flux is reached at high pressure, low wall-temperature and equivalence ratios close to the maximum of reactivity of H2 flames (ϕ≈1.7). This study shows that, for H2 FWI, walls should be at least modeled with simple surface reactions – such as IFHC – and that assuming inert walls is not possible.Novelty and significance statementThe work presented in this paper is new, original and of interest at the present time as many groups simulate near-walls H2/Air flames. The IFHC model is relevant for all premixed hydrogen combustion close to walls and is, thus, relevant for most realistic premixed hydrogen combustion scenarios. Besides being conservative in terms of mass, momentum, energy and atom balance, IFHC allows to reach mesh convergence during quenching contrary to simplistic inert wall treatments. It also allows to describe walls at an affordable computational cost compared to a full set of heterogeneous reactions. This work also opens the discussion with experimentalists since, to the best of the authors’ knowledge, the dimensional maximum wall heat fluxes are not yet available in the literature and are, thus, provided here for future comparison.

Three-stage modelling and parametric analysis of a downdraft biomass gasifier

Syngas production from biomass resources suggest considerable privileges to attain energy sustainability in future. Design and control strategies are essential to extend the technology of biomass gasifiers, which require reliable model developments. The overarching contribution of this study is to develop and evaluate a three-stage biomass gasifier model. The model consists of three successive sub-models for drying and pyrolysis, partial oxidation and char reduction reactors. The pyrolysis of raw material is simulated based on the ligno-cellulosic structure of the biomass by adopting comprehensive kinetic rate modelling approach. The partial oxidation sub-model is built by axis-symmetric 2D transport equations including detailed chemical scheme. Char reduction sub-model is extended based on axis-symmetric 2D DPM model accompanied by appropriate chemical kinetic scheme considering heterogeneous and homogeneous reactions. Accuracy of each sub-model is separately verified by comparison with available numerical and experimental data. Moreover, the correctness and predictability of the complete gasifier model is evaluated using two experimental reports. For both cases, investigations demonstrate high resolution agreement between the results of the developed model and available experimental measurements both thermally and chemically. Furthermore, a parametric analysis is conducted to investigate the gasifier performance against the variations of the main system operating parameters including the type of the feeding biomass and its moisture content, equivalence ratio and air initial temperature. Based on the results, higher volatiles mass fractions and lower char mass fraction have been produced from pyrolyzing of hardwood in comparison with beech wood. Also, Results reveal that with increase of the moisture content from 15% to 35%, syngas LHV and cold gas efficiency reduce by 1.9559 MJ . kg −1 and 25.78% respectively, while H 2 mole fraction at the gasifier outlet rises by 0.90%. On the contrary, growth of equivalence ratio from 2 to 10 leads to the drastic increase of syngas LHV by 5.6404 MJ . kg −1 , however, cold gas efficiency peaks to 82.75% at the equivalence ratio of 4. Besides, varying the inlet air temperature over a range of 500 K–1300 K causes 0.6938 MJ . kg −1 growth of syngas LHV as well as 9.43% rise of cold gas efficiency. • A conceptual three-stage model is developed to simulate the performance of a downdraft biomass gasifier. • The model includes three separate successive sub-models for pyrolysis, partial oxidation and char reduction. • Results of each sub-model and the whole three-stage model are experimentally validated. • Three-stage model is evaluated for two different lignocellulosic feedstocks. • The effects of the key operating parameters on the gasifier performance are analyzed.

Molecular level simulations of combustion processes using the DSMC method

A technique called the Direct Simulation Monte Carlo (DSMC) method is used for simulating laminar one-dimensional hydrogen-air flames with an aim to establish the feasibility of molecular-level simulations for combustion using the Quantum-Kinetic (QK) reaction model. In DSMC, simulation particles are employed which represent many molecules with similar properties. The DSMC method effectively solves the Boltzmann equation by decoupling the motion of molecules into two phases: a Move phase and a Collision phase. Chemistry is treated using the Quantum-Kinetic model in which the total energy exchange resulting from molecular collision determines the outcome of the chemical reactions. A major advantage of DSMC is that it avoids using continuum Arrhenius reaction rates or simplified gradient laws for the diffusivities of mass, momentum and heat. Instead, any such laws and their parameters emerge naturally from the DSMC results. Simulations of one-dimensional hydrogen-air flames using the DSMC method with a detailed chemical scheme show promising results for molecular-level simulations of combustion. An adjustment to the activation energy of a key reaction is made in order to achieve the correct flame speed. The species and temperature profiles from DSMC show reasonable agreement with those obtained from Direct Numerical Simulation (DNS) using a conventional continuum approach. Likewise, the molecular diffusivity of hydrogen and oxygen also show reasonable agreement with the DNS results, although differences in the results still exist, especially for oxygen diffusivity. These are expected to improve with further development in the DSMC method for combustion.

Capabilities and limitations of Large Eddy Simulation with perfectly stirred reactor assumption for engineering applications of unsteady, hydrogen combustion sequences

This work evaluates the capabilities of Large Eddy Simulation with perfectly stirred reactor (PSR) assumption and detailed chemistry to predict unsteady, premixed, hydrogen combustion sequences. The model was benchmarked with hydrogen-air experimental tests and with numerical data of flame acceleration in an obstructed channel. Results permit to identify major shortcomings that should be addressed with this approach and to assess the uncertainties linked to the use of different sub-models. Spatial resolution was found to be critical and limits the applications of this approach due to the increase of the computational costs with the flame surface. While the influence of the detailed kinetic chemical scheme used was low, the impact of the sub-grid turbulence model used was high. Results showed that simulations provided good agreement with the experimental data when a minimum spatial resolution of 1/8 of the laminar flame thickness was imposed. This threshold permits to simulate with good results the early stages of the combustion sequence (ignition and initial flame acceleration) but limits the model applications when the flame surface increases. In-situ Adaptive Tabulation (ISAT) was an effective strategy to overcome the limitations and partially reduce the computational cost when detailed chemistry models are used together with PSR-LES.

Approximation of laminar flame characteristics on premixed ammonia/hydrogen/nitrogen/air mixtures at elevated temperatures and pressures

Ammonia shows a promising potential to be a carbon-free and sustainable fuel for gas turbines and internal combustion engines. The design of such complex combustion systems using computational reactive fluid dynamics in combination with detailed reaction mechanisms is a time-consuming challenge. Combustion models are a solution to reduce the computation time, but require data of fundamental laminar flame characteristics. In order to provide such data, in this present study correlations of laminar flame speed and flame thickness for ammonia/hydrogen/nitrogen/air mixtures based on a prior selected detailed reaction mechanism were developed. A validation study for three detailed chemical schemes was conducted using experimentally obtained laminar flame speed data from literature to determine one mechanism with the best predictive capabilities. A database with 222250 data entries of laminar flame speed and flame thickness was generated by performing numerical one-dimensional simulations using the selected mechanism. From that correlations were derived. The database as well as the derived correlations contain a broad range of initial conditions: air/fuel equivalence ratio: (0.5–1.7), ammonia: (0–100) mol%, hydrogen addition: (0–60) mol%, nitrogen addition: (0–20) mol%, fresh gas temperatures: (300–1100) K and pressures: (0.1–25) MPa. For the substitution of fuel within existing burners or engines which so far are operated with methane, it is of special interest, if a defined ammonia/hydrogen/nitrogen/air mixture could be used with similar properties. For that it is shown that such a mixture composition can be found which has a similar laminar flame speed than methane, depending on the given initial conditions.

BEATBOX v1.0: Background Error Analysis Testbed with Box Models

Abstract. The Background Error Analysis Testbed (BEATBOX) is a new data assimilation framework for box models. Based on the BOX Model eXtension (BOXMOX) to the Kinetic Pre-Processor (KPP), this framework allows users to conduct performance evaluations of data assimilation experiments, sensitivity analyses, and detailed chemical scheme diagnostics from an observation simulation system experiment (OSSE) point of view. The BEATBOX framework incorporates an observation simulator and a data assimilation system with the possibility of choosing ensemble, adjoint, or combined sensitivities. A user-friendly, Python-based interface allows for the tuning of many parameters for atmospheric chemistry and data assimilation research as well as for educational purposes, for example observation error, model covariances, ensemble size, perturbation distribution in the initial conditions, and so on. In this work, the testbed is described and two case studies are presented to illustrate the design of a typical OSSE experiment, data assimilation experiments, a sensitivity analysis, and a method for diagnosing model errors. BEATBOX is released as an open source tool for the atmospheric chemistry and data assimilation communities.

Auto-thermal reforming (ATR) of natural gas: An automated derivation of optimised reduced chemical schemes

A fully automated strategy is discussed to construct reduced chemistry suitable for the numerical simulation of stationary combustion systems of large dimension, such as auto-thermal reforming (ATR) of natural gas for syngas production. Because of computing limitations in terms of space and time resolution, three-dimensional simulations of an ATR unit cannot be addressed with detailed chemistry. A procedure is proposed to automatically derive optimised and reduced chemical schemes under specific ATR operating conditions. A stochastic model problem is first designed to probe the dynamical response of a detailed chemical scheme, over a large range of chemical compositions of the mixture. Reference composition space trajectories are built, featuring turbulent micro-mixing and reactions. Following these trajectories, the chemical response is analysed using directed relation graphs with error propagation combined with quasi-steady state hypothesis, to reduce the number of species and elementary reactions. Then, the time evolution of the model problem is coupled with a genetic algorithm, to optimise on the fly the chemical rates of the reduced kinetics, according to the reference composition space trajectories. The accuracy of the reduced scheme is monitored with a fitness function and the results are tested against the reference detailed chemistry.

Automatic reduction and optimisation of chemistry for turbulent combustion modelling: Impact of the canonical problem

In a preliminary part, reduced chemical schemes from the literature on methane/air turbulent combustion are tested in one-dimensional premixed flames at various equivalence ratios. As discussed by Peters (1985), for the very same number of chemical species transported with the flow, a scheme introducing essential intermediate species through analytical relations derived from quasi-steady state and equilibrium assumptions, provides much better predictions than the global schemes limited to the transported species. Along these lines, a fully automated reduction procedure is discussed. The calibration of reduced chemistry usually relies on canonical problems, also computed with reference detailed chemical schemes. To cover at once a given range of chemical compositions, equivalence ratios and temperatures, the chemical properties of stochastic particles, submitted to micro-mixing and chemical reactions, are combined with the computation of deterministic one-dimensional composition-space trajectories, issued from an arbitrary number of inlet conditions. Along these trajectories, well-established methods to reduce the numbers of species and reactions are applied, cast in a fully automated manner. The rates of the resulting reduced scheme are then optimised, following a genetic algorithm, to match the detailed chemistry response. The discussed strategy is applied to methane/vitiated-air combustion. The accuracy of the resulting reduced scheme is evaluated in the simulation of freely propagating one-dimensional premixed flames, at various equivalence ratios, and also in strained diffusion flames in a one-dimensional counter-flowing jet configuration, up to the quenching point. The accurate reproduction of the flame speed response versus equivalence ratio requires adding premixed flames as target in the optimisation loop. Once the flame speed is captured, the quenching scalar dissipation rate of the diffusion flames is also well reproduced, as anticipated by Peters in a relation between the flame speed and the quenching scalar dissipation rate (Peters, 1991, 2000). Finally, the optimisation of the chemical rates is demonstrated under specific operating conditions, featuring four inlets with a large variety of flow compositions.

Experimental and numerical studies of pressure effects on syngas combustor emissions

Pressure effects on NOx and CO emissions of a model syngas combustor were experimentally and numerically studied. The validated numerical method was employed to analyze pressure effects on the combustor emissions such as different mixing levels and extra reactant addition. The model combustor, fueled with 10MJ/Nm3 coal-derived syngas, was installed in a pressurized test-rig, and emissions were measured within 0.1–0.35MPa. Based on the flow, temperature and species fields calculated by the CFD method, a chemical reactor network (CRN) model was established. With a detailed chemical scheme, the emissions calculated by the CRN model agreed well with experimental results. The model was then employed to calculate emissions within pressure range from 0.1 to 2.0MPa, with the adiabatic flame temperature in the primary zone varied from 1477 to 2317K. The calculated NOx and CO emissions generally showed exponential relationship with the operating pressure, except that the NOx emission decreased at higher pressure when the primary adiabatic flame temperature was lower than 1800K. Through the analyses of the NOx formation pathways, it was found that the higher pressure would generally enhance the N2O pathway, which dominated NOx formation at low temperature, but NOx emission concentration through N2O pathway decreased with pressure above 1.0MPa at low temperature, which could explain the NOx emission behavior. The CRN modeling method was applied to analyze the effects of pressure on emission behaviors under the different mixing modes and with the extra reactant such as H2O and NH3 in fuel. Premixed and steam dilution burning modes showed better performance on reducing emissions at elevated pressures. NH3 could obviously increase the emission levels, especially at low pressure. Comprehensive understanding of the relationship between the emissions and the operating pressure in full pressure range was obtained, which could be valuable for the predicting and analyzing the syngas combustor pollutant emissions.

Influence of H2O, CO2 and O2 addition on biomass gasification in entrained flow reactor conditions: Experiments and modelling

Biomass gasification in Entrained Flow Reactor (EFR) is both studied with experiments in a drop tube reactor and modelling with a 1-D model (GASPAR). Operating conditions are chosen thanks to results of a preliminary modelling of an industrial EFR. Influence of addition of steam (0.55g/gdb), carbon dioxide (0.87g/gdb) and oxygen (Equivalent Ratio: 0–0.61) is investigated between 800 and 1400°C with beech wood particles sieved between 315 and 450μm as feedstock. The model takes into account pyrolysis reaction, gas phase reaction with a detailed chemical scheme (176 species, 5988 reactions), char gasification by steam and CO2 and soot formation. H2O or CO2 addition has no influence on gasification product yields at 800 and 1000°C, while at 1200 and 1400°C the char gasification is significantly enhanced and soot formation is certainly inhibited by OH radical which reacts with soot precursors. The modification of output gas phase composition is mostly due to WGS reaction which reaches thermodynamic equilibrium from about 1200°C. As expected, O2 has a significant influence on gas and tar yields through combustion reactions. Char and soot yields decrease as ER increases. The GASPAR model allows a good prediction of gas and char and gives relevant evolution of soot and tar yields on the large majority of conditions studied.

Modelling the future distribution of ammonium nitrate concentrations in The Netherlands for 2020: The sensitivity to meteorological parameters

We present a parameterization developed to simulate Ammonium particle (NH4+) concentrations in the Operational Priority Substances (OPS) source-receptor model, without the necessity of using a detailed chemical scheme. By using the ratios of the main pre-cursor gases SO2, NO2 and NH3, and utilising calculations performed using a chemical box-model, we show that the parameterization can simulate annual mean NH4+ concentration fields to within ∼15% of measured values at locations throughout the Netherlands. Performing simulations for different decades, we find a strong correlation of simulated NH4+ distributions for both past (1993–1995) and present (2009–2012) time periods. Although the total concentration of NH4+ has decreased over the period, we find that the fraction of NH4+ transported into the Netherlands has increased from around 40% in the past to 50% for present-day. This is due to the variable efficiency of mitigation practises across economic sectors. Performing simulations for the year 2020 using associated emission estimates, we show that there are generally decreases of ∼8–25% compared to present day concentrations. By altering the meteorological fields applied in the future simulations, we show that a significant uncertainty of between ∼50 and 100% exists on this estimated NH4+ distribution as a result of variability in the temperature dependent emission terms and relative humidity. Therefore, any projections of future NH4+ distributions should be performed using well chosen meteorological fields representing recent meteorological situations.

Numerical study of the behavior of methane-hydrogen/air pre-mixed flame in a micro reactor equipped with catalytic segmented bluff body

In this work, combustion characteristics of premixed methane-hydrogen/air in a micro reactor equipped with a catalytic bluff body is investigated numerically. In this regard, the detailed chemistry schemes for gas phase (homogeneous) and the catalyst surface (heterogeneous) are used. The applied catalytic bluff body is coated with a thin layer of platinum (Pt) on its surface. Also, the lean reactive mixture is entered to the reactor with equivalence ratio 0.9. The results of this study showed that the use of catalytic bluff body in the center of a micro reactor can significantly increase the flame stability, especially at high velocities. Moreover, it is found that a catalytic bluff body with several cavities on its surface and also high thermal conductivity improves the flame stability more than a catalytic bluff body without cavities and low thermal conductivity. Finally, it is maintained that the most advantage of using the catalytic bluff body is its easy manufacturing process as compared to the catalytic wall. This matter seems to be more prevalent when we want to create several cavities with various sizes on the bluff-body.

Radical chemistry at night: comparisons between observed and modelled HO<sub>x</sub>, NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> during the RONOCO project

Abstract. The RONOCO (ROle of Nighttime chemistry in controlling the Oxidising Capacity of the AtmOsphere) aircraft campaign during July 2010 and January 2011 made observations of OH, HO2, NO3, N2O5 and a number of supporting measurements at night over the UK, and reflects the first simultaneous airborne measurements of these species. We compare the observed concentrations of these short-lived species with those calculated by a box model constrained by the concentrations of the longer lived species using a detailed chemical scheme. OH concentrations were below the limit of detection, consistent with model predictions. The model systematically underpredicts HO2 by ~200% and overpredicts NO3 and N2O5 by around 80 and 50%, respectively. Cycling between NO3 and N2O5 is fast and thus we define the NO3x (NO3x=NO3+N2O5) family. Production of NO3x is overwhelmingly dominated by the reaction of NO2 with O3, whereas its loss is dominated by aerosol uptake of N2O5, with NO3+VOCs (volatile organic compounds) and NO3+RO2 playing smaller roles. The production of HOx and ROx radicals is mainly due to the reaction of NO3 with VOCs. The loss of these radicals occurs through a combination of HO2+RO2 reactions, heterogeneous processes and production of HNO3 from OH+NO2, with radical propagation primarily achieved through reactions of NO3 with peroxy radicals. Thus NO3 at night plays a similar role to both OH and NO during the day in that it both initiates ROx radical production and acts to propagate the tropospheric oxidation chain. Model sensitivity to the N2O5 aerosol uptake coefficient (γN2O5) is discussed and we find that a value of γN2O5=0.05 improves model simulations for NO3 and N2O5, but that these improvements are at the expense of model success for HO2. Improvements to model simulations for HO2, NO3 and N2O5 can be realised simultaneously on inclusion of additional unsaturated volatile organic compounds, however the nature of these compounds is extremely uncertain.

Experimental and numerical study of premixed hydrogen/air flame propagating in a combustion chamber

An experimental and numerical study of dynamics of premixed hydrogen/air flame in a closed explosion vessel is described. High-speed shlieren cinematography and pressure recording are used to elucidate the dynamics of the combustion process in the experiment. A dynamically thickened flame model associated with a detailed reaction mechanism is employed in the numerical simulation to examine the flame-flow interaction and effect of wall friction on the flame dynamics. The shlieren photographs show that the flame develops into a distorted tulip shape after a well-pronounced classical tulip front has been formed. The experimental results reveal that the distorted tulip flame disappears with the primary tulip cusp and the distortions merging into each other, and then a classical tulip is repeated. The combustion dynamics is reasonably reproduced in the numerical simulations, including the variations in flame shape and position, pressure build-up and periodically oscillating behavior. It is found that both the tulip and distorted tulip flames can be created in the simulation with free-slip boundary condition at the walls of the vessel and behave in a manner quite close to that in the experiments. This means that the wall friction could be unimportant for the tulip and distorted tulip formation although the boundary layer formed along the sidewalls has an influence to a certain extent on the flame behavior near the sidewalls. The distorted tulip flame is also observed to be produced in the absence of vortex flow in the numerical simulations. The TF model with a detailed chemical scheme is reliable for investigating the dynamics of distorted tulip flame propagation and its underlying mechanism.

Multi-scale modelling of pulsed nanosecond dielectric barrier plasma discharges in plane-to-plane geometry

An integrated theoretical and numerical framework is developed to study the dynamics of energy coupling, gas heating and generation of active species by repetitively pulsed nanosecond dielectric barrier discharges (NS DBDs) in air. The work represents one of the first attempts to simulate, in a self-consistent manner, multiple (more than 100) nanosecond pulses. Detailed information is obtained about the electric-field transients during each voltage pulse, and accumulation of plasma generated species and gas heating over ms timescales. The plasma is modelled using a two-temperature, detailed chemistry scheme, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. The analysis is conducted with pressures and pulsing frequency in the range 40–100 Torr and 1–105 Hz, respectively. The input electrical energy is directly proportional to the number density, and remains fairly constant on a per molecule basis from pulse to pulse. Repetitive pulsing results in uniform production of atomic oxygen in the discharge volume via electron-impact dissociation during voltage pulses, and through quenching of excited nitrogen molecules in the afterglow. The ion Joule effect causes rapid gas heating of ∼40 K/pulse in the cathode sheath and generates weak acoustic waves. Conductive heat loss to the walls during the time interval between voltage pulses prevents overheating of the cathode layer and development of ionization instabilities. A uniform ‘hat-shaped’ temperature profile develops in the discharge volume after multiple pulses, due to chemical heat release from quenching of excited species. This finding may explain experimentally observed volumetric ignition (as opposed to hot-spot ignition) in fuel–air mixtures subject to NS DBD.

An optimization-based approach to detailed chemistry tabulation: Automated progress variable definition

Detailed chemistry tabulations built from canonical combustion problems, such as premixed or diffusion flamelets, have been the subject of multiple studies. Chemical look-up table construction strategies may rely on the definition of progress variables, which are usually obtained from combination of reactants and final products mass fractions. Finding a progress variable such that all relevant species can be retrieved from knowledge of it is not always straightforward; especially for fuel mixtures composed of more than a single hydrocarbon, or for simpler fuels but under conditions where the global reaction progress is not continuous, as in the case of cool-flame ignition. To overcome these difficulties, automated methods are discussed for defining progress by variables in which all species of a chemical scheme, even minor ones, are involved. This is done by formulating constraints applied to progress variable definition: they should evolve in a monotonic manner from fresh to burnt gases and the species derivative in progress variable space should stay moderate for tabulation accuracy. This set of constraints is discretized along chemical trajectories observed in laminar canonical flames, to be formulated in terms of an ensemble of inequalities, which are solved using optimization tools. The outcome is a set of weighting coefficients to be applied to every species of the detailed chemical scheme, in order to construct the progress -variable space. The methods are successfully applied to methane and kerosene premixed flamelets and to n-heptane self-ignition, under conditions with cool-flame effects.

Global Reaction Mechanism for Ethylene Flames with Preferential Diffusion

Two compact global mechanisms for ethylene (C2H4) diffusion flame combustion have been tailored to include important reaction steps for acetylene and benzene production. One mechanism (G11) contains 11 species with 10 reaction steps including acetylene (C2H2), and the other mechanism (G12) contains 12 species with a total of 11 reactions steps to include also the formation of benzene (C6H6). The reaction steps have been carefully selected to minimize the mechanism size for the use in large-scale computational fluid dynamics (CFD) simulations. Hence, the reaction constants have been optimized for the correct prediction of important radical concentrations. Particular focus has been on the mechanisms ability to reproduce important preferential diffusion effects and on the formation of H and C2H2 due to its importance to soot formation. The two global chemical models have been validated for a transient 1-dimensional diffusion flame configuration and show very good agreement with various detailed chemical schemes. The mechanisms are found to be nonstiff reducing typical computing time for a transient flamelet calculation (F. Mauss, 1998) from a few hours (171 species mechanism) to only a few minutes (G11).

A linear CO chemistry parameterization in a chemistry-transport model: evaluation and application to data assimilation

Abstract. This paper presents an evaluation of a new linear parameterization valid for the troposphere and the stratosphere, based on a first order approximation of the carbon monoxide (CO) continuity equation. This linear scheme (hereinafter noted LINCO) has been implemented in the 3-D Chemical Transport Model (CTM) MOCAGE (MOdèle de Chimie Atmospherique Grande Echelle). First, a one and a half years of LINCO simulation has been compared to output obtained from a detailed chemical scheme output. The mean differences between both schemes are about ±25 ppbv (part per billion by volume) or 15% in the troposphere and ±10 ppbv or 100% in the stratosphere. Second, LINCO has been compared to diverse observations from satellite instruments covering the troposphere (Measurements Of Pollution In The Troposphere: MOPITT) and the stratosphere (Microwave Limb Sounder: MLS) and also from aircraft (Measurements of ozone and water vapour by Airbus in-service aircraft: MOZAIC programme) mostly flying in the upper troposphere and lower stratosphere (UTLS). In the troposphere, the LINCO seasonal variations as well as the vertical and horizontal distributions are quite close to MOPITT CO observations. However, a bias of ~−40 ppbv is observed at 700 Pa between LINCO and MOPITT. In the stratosphere, MLS and LINCO present similar large-scale patterns, except over the poles where the CO concentration is underestimated by the model. In the UTLS, LINCO presents small biases less than 2% compared to independent MOZAIC profiles. Third, we assimilated MOPITT CO using a variational 3D-FGAT (First Guess at Appropriate Time) method in conjunction with MOCAGE for a long run of one and a half years. The data assimilation greatly improves the vertical CO distribution in the troposphere from 700 to 350 hPa compared to independent MOZAIC profiles. At 146 hPa, the assimilated CO distribution is also improved compared to MLS observations by reducing the bias up to a factor of 2 in the tropics. This study confirms that the linear scheme is able to simulate reasonably well the CO distribution in the troposphere and in the lower stratosphere. Therefore, the low computing cost of the linear scheme opens new perspectives to make free runs and CO data assimilation runs at high resolution and over periods of several years.

The autoignition in air of some binary fuel mixtures containing hydrogen

A predictive model for the autoignition and combustion of fuel–air mixtures employing detailed full chemical schemes was used to examine the autoignition and combustion characteristics in air of hydrogen in the presence of a range of common fuels. These included the gaseous fuels: methane, carbon monoxide and the higher hydrocarbon fuel n-heptane. A wide range of relative concentrations of the fuel components in the binary mixtures with hydrogen for different values of initial mixture temperature and pressure were considered under constant volume adiabatic conditions. It is shown that the presence of hydrogen in turn with these fuels can bring about complex changes to the autoignition behaviour of the fuel mixtures that show hydrogen may behave as an accelerant or retardant depending on the fuel, initial temperature, pressure and equivalence ratio considered.