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Showing papers in "Combustion Theory and Modelling in 2013"


Journal ArticleDOI
TL;DR: In this article, the effect of sub-grid turbulent velocity fluctuation on the wrinkling factor of a premixed combustion has been investigated using large eddy simulation (LES) on the ORACLES burner.
Abstract: In the application of Large Eddy Simulation (LES) to premixed combustion, the unknown filtered chemical source term can be modelled by the generalised flame surface density (FSD) using algebraic models for the wrinkling factor Ξ. The present study compares the behaviour of the various models by first examining the effect of sub-grid turbulent velocity fluctuation on Ξ through a one-dimensional analysis and by the LES of the ORACLES burner (Nguyen, Bruel, and Reichstadt, Flow, Turbulence and Combustion Vol. 82 [2009], pp. 155–183) and the Volvo Rig (Sjunnesson, Nelsson, and Max, Laser Anemometry, Vol. 3 [1991], pp. 83–90; Sjunnesson, Henrikson, and Lofstrom, AIAA Journal, Vol. 28 [1992], pp. AIAA–92–3650). Several sensitivity studies on parameters such as the turbulent viscosity and the grid resolution are also carried out. A statistically 1-D analysis of turbulent flame propagation reveals that counter gradient transport of the progress variable needs to be accounted for to obtain a realistic flame thickn...

80 citations


Journal ArticleDOI
TL;DR: In this paper, a simplified mathematical model including negative ions is proposed based on a kinetic mechanism featuring 39 ionic reactions and 5 charged species, which is evaluated by comparison of a monodimensional premixed flame with the available experimental data.
Abstract: The application of an external electric field is known to improve flame stability significantly. Until now, few studies have proposed modelling approaches for combustion in the presence of an externally applied voltage. In these numerical studies, the negative ions are overlooked, and only the displacement of positive ions and electrons under the effect of a direct electric field was examined. In the present paper, a simplified mathematical model including negative ions is proposed based on a kinetic mechanism featuring 39 ionic reactions and 5 charged species. This mechanism is first evaluated by comparison of a monodimensional premixed flame with the available experimental data. Then it is used to analyse the stabilisation mechanism of a diffusion lifted flame in the presence of direct or alternating electric fields. It was concluded that the role of negative ions is crucial, and they are not to be neglected. Moreover, the simulations have shown that the magnitude of the flame stabilisation improvement ...

64 citations


Journal ArticleDOI
TL;DR: In this paper, three syngas combustion models have been calibrated using laminar flame speed measurements from high pressure experiments, and the resulting uncertainty in the parameters is propagated forward into the simulation of LAMR flame speeds.
Abstract: Syngas chemistry modelling is an integral step toward the development of safe and efficient syngas combustors. Although substantial effort has been undertaken to improve the modelling of syngas combustion, models nevertheless fail in regimes important to gas turbine combustors, such as low temperature and high pressure. In order to investigate the capabilities of syngas models, a Bayesian framework for the quantification of uncertainties has been used. This framework, given a set of experimental data, allows for the calibration of model parameters, determination of uncertainty in those parameters, propagation of that uncertainty into simulations, as well as determination of model evidence from a set of candidate syngas models. Here, three syngas combustion models have been calibrated using laminar flame speed measurements from high pressure experiments. After calibration the resulting uncertainty in the parameters is propagated forward into the simulation of laminar flame speeds. The model evidence is the...

58 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the use of dynamic adaptive chemistry (DAC) for efficient chemistry calculations in turbulent flame simulations, which is achieved through the directed relation graph (DRG) method, invoked for each computational fluid dynamics cell/particle to obtain a small skeletal mechanism that is valid for the local thermochemical condition.
Abstract: The use of large chemical mechanisms in flame simulations is computationally expensive due to the large number of chemical species and the wide range of chemical time scales involved. This study investigates the use of dynamic adaptive chemistry (DAC) for efficient chemistry calculations in turbulent flame simulations. DAC is achieved through the directed relation graph (DRG) method, which is invoked for each computational fluid dynamics cell/particle to obtain a small skeletal mechanism that is valid for the local thermochemical condition. Consequently, during reaction fractional steps, one needs to solve a smaller set of ordinary differential equations governing chemical kinetics. Test calculations are performed in a partially-stirred reactor (PaSR) involving both methane/air premixed and non-premixed combustion with chemistry described by the 53-species GRI-Mech 3.0 mechanism and the 129-species USC-Mech II mechanism augmented with recently updated NO x pathways, respectively. Results show that, in the...

51 citations


Journal ArticleDOI
TL;DR: In this article, a computational study is performed on a series of piloted, lean, premixed turbulent jet flames using the Sydney Piloted Premixed Jet Burner (PPJB), and with jet velocities of 50, 100, 150 and 200 m/s are denoted PM150, PM1100, PM1150 and PM1200, respectively.
Abstract: A computational study is performed on a series of four piloted, lean, premixed turbulent jet flames. These flames use the Sydney Piloted Premixed Jet Burner (PPJB), and with jet velocities of 50, 100, 150 and 200 m/s are denoted PM150, PM1100, PM1150 and PM1200, respectively. Calculations are performed using the RANSPDF and LESPDF methodologies, with different treatments of molecular diffusion, with detailed chemistry and flamelet-based chemistry modelling, and using different imposed boundary conditions. The sensitivities of the calculations to these different aspects of the modelling are compared and discussed. Comparisons are made to experimental data and to previously-performed calculations. It is found that, given suitable boundary conditions and treatment of molecular diffusion, excellent agreement between the calculations and experimental measurements of the mean and variance fields can be achieved for PM150 and PM1100. The application of a recently developed implementation of molecular diffusion r...

48 citations


Journal ArticleDOI
TL;DR: In this article, direct numerical simulations of a stratified flow in a homogeneous compression charge ignition (HCCI) engine were performed to investigate the exhaust gas recirculation and temperature/mixture stratification effects on the autoignition of synthetic dimethyl ether (DME) in the negative temperature combustion region.
Abstract: Direct numerical simulations (DNSs) of a stratified flow in a homogeneous compression charge ignition (HCCI) engine are performed to investigate the exhaust gas recirculation (EGR) and temperature/mixture stratification effects on the autoignition of synthetic dimethyl ether (DME) in the negative temperature combustion region. Detailed chemistry for a DME/air mixture is employed and solved by a hybrid multi-time scale (HMTS) algorithm to reduce the computational cost. The effect of to mimic the EGR effect on autoignition are studied. The results show that adding enhances autoignition by rapid OH radical pool formation (34–46% reduction in ignition delay time) and changes the ignition heat release rates at different ignition stages. Sensitivity analysis is performed and the important reactions pathways affecting the autoignition are specified. The DNS results show that the scales introduced by thermal and mixture stratifications have a strong effect after the low temperature chemistry (LTC) ignition especi...

35 citations


Journal ArticleDOI
TL;DR: In this article, the authors used large eddy simulation (LES) to study swirling reacting flows by comparison with experimental observations and found that LES captures all salient features of an unconfined flame including velocity and temperature distributions.
Abstract: This paper utilises large eddy simulation (LES) to study swirling reacting flows by comparison with experimental observations. The purpose is to provide further insights in engineering designs, as well as to improve modelling. A reduced-scale swirl burner has been developed for the experiments. Comparison of particle image velocimetry (PIV) measurements with LES results using finite rate chemistry shows that LES captures all the salient features of an unconfined flame including velocity and temperature distributions. However, when the flame is confined within a cylindrical combustor, the simulated flame shape is initially not consistent with experimental observation. Investigations show that the discrepancy is caused by the often practised assumption of adiabatic wall temperature. With the use of an assumed wall temperature distribution guided by laboratory observation, results of LES are consistent with experiments. Although the latter LES approach requires more computational resources, the improvement i...

35 citations


Journal ArticleDOI
TL;DR: In this paper, the external flow of an atmospheric pressure burner-stabilised methane-oxygen-argon flame near an axisymmetric conical sampling probe with an orifice at the cone tip is simulated by solving the full set of unsteady Navier-Stokes equations using a recently developed original algorithm.
Abstract: A study is made of the external flow of an atmospheric-pressure burner-stabilised methane-oxygen-argon flame near an axisymmetric conical sampling probe with an orifice at the centre of the cone tip. The flow is simulated by solving the full set of unsteady Navier-Stokes equations using a recently developed original algorithm. Heat release due to chemical reactions is approximately described by a source term in the energy equation which provides a given temperature distribution in the unperturbed isobaric flame. On the probe orifice surface, the axial gas velocity is assumed to be equal to the local velocity of sound. A qualitative understanding of the nature and magnitude of the perturbing effects of the probe on the flame is obtained by comparing the steady flow near the probe and the unperturbed flame. Probe-induced distortions in the flame mixture composition are simulated by calculating concentration distributions of some species (CH4, CO2, H2O, and O2) in the flow field from the diffusion equation f...

35 citations


Journal ArticleDOI
TL;DR: In this article, the development of fingering char patterns on the surface of porous thin materials has been investigated in the framework of reverse combustion, and the results of the formal asymptotic procedure are justified by qualitative multiscale numerical simulations at the microscopic and macroscopic levels.
Abstract: The development of fingering char patterns on the surface of porous thin materials has been investigated in the framework of reverse combustion. This macroscopic characteristic feature of combustible media has also been studied experimentally and through the use of phenomenological models. However, not much attention has been given to the behaviour of the emerging patterns based on characteristic material properties. Starting from a microscopic description of the combustion process, macroscopic models of reverse combustion that are derived by the application of the homogenisation technique are presented. Using proper scaling by means of a small scale parameter ϵ, the results of the formal asymptotic procedure are justified by qualitative multiscale numerical simulations at the microscopic and macroscopic levels. We consider two equilibrium models that are based on effective conductivity contrasts, in a simple adiabatic situation, to investigate the formation of unstable fingering patterns on the surface o...

33 citations


Journal ArticleDOI
TL;DR: In this article, an extended kinetic model involving the submechanism of nitric oxide formation in the presence of singlet oxygen O2(a 1Δg) or O 2(b 1Σg +) molecules in the methane-air mixture is presented.
Abstract: Mechanisms of homogeneous charge compression ignition (HCCI) combustion enhancement are investigated numerically when excited O2(a 1Δg) molecules are produced at different points in the compression stroke. The analysis is conducted with the use of an extended kinetic model involving the submechanism of nitric oxide formation in the presence of singlet oxygen O2(a 1Δg) or O2(b 1Σg +) molecules in the methane-air mixture. It is demonstrated that the abundance of excited O2(a 1Δg) molecules in the mixture even in a small amounts intensifies the ignition and combustion and allows one to control the ignition event in the HCCI engine. Such a method of energy supply in the HCCI engine is much more effective in advancement of combustion timing than mere heating of the mixture, because it leads to acceleration of the chain-branching mechanism. The excitation of O2 molecules to the a 1Δg electronic state makes it possible to organise the successful combustion in the cylinder at diminished initial temperature of the...

31 citations


Journal ArticleDOI
TL;DR: In this article, a turbulent premixed flame stabilised behind a bluff body burner is simulated and two regularisation methods are implemented to solve the flow field in either the reactive or non-reactive case.
Abstract: In the present work, simulations are performed in order to study a turbulent premixed flame stabilised behind a bluff body burner. Conditional Source-term Estimation (CSE) is the combustion model adopted for this purpose and it is coupled with a trajectory gene- rated low-dimensional manifold method for chemistry reduction: conditional-averaged chemical source terms are closed by conditional-averaged scalars, which are obtained by inverting an integral equation. Two regularisation methods are implemented. The optimal regularisation parameter is determined and a sensitivity analysis is performed related to the effect of the value of the regularisation parameter. In the present study, only small differences in the conditional and unconditional averages are noticed for values of the regularisation parameter located in the optimal range determined by the L-curve. Two-equation k–e and Shear Stress Transport (SST) turbulence models are used to solve the flow field in either the reactive or non-reactive case. Di...

Journal ArticleDOI
TL;DR: In this article, a simple analysis of linear and spiral counterflow heat-recirculating combustors was conducted to identify the dimensionless parameters expected to quantify the performance of such devices.
Abstract: A simple analysis of linear and spiral counterflow heat-recirculating combustors was conducted to identify the dimensionless parameters expected to quantify the performance of such devices. A three-dimensional (3D) numerical model of spiral counterflow ‘Swiss roll’ combustors was then used to confirm and extend the applicability of the identified parameters. It was found that without property adjustment to maintain constant values of these parameters, at low Reynolds number (Re) smaller-scale combustors actually showed better performance (in terms of having lower lean extinction limits at the same Re) due to lower heat loss and internal wall-to-wall radiation effects, whereas at high Re, larger-scale combustors showed better performance due to longer residence time relative to chemical reaction time. By adjustment of property values, it was confirmed that four dimensionless parameters were sufficient to characterise combustor performance at all scales: Re, a heat loss coefficient (α), a Damkohler number (...

Journal ArticleDOI
TL;DR: In this article, a computational fluid dynamics model for high-temperature oxy-natural gas combustion is developed and exercised, and the model features detailed gas-phase chemistry and radiation treatments (a photon Monte Carlo method with line-by-line spectral resolution for gas and wall radiation) and a transported probability density function (PDF) method to account for turbulent fluctuations in composition and temperature.
Abstract: A computational fluid dynamics model for high-temperature oxy–natural gas combustion is developed and exercised. The model features detailed gas-phase chemistry and radiation treatments (a photon Monte Carlo method with line-by-line spectral resolution for gas and wall radiation – PMC/LBL) and a transported probability density function (PDF) method to account for turbulent fluctuations in composition and temperature. The model is first validated for a 0.8 MW oxy–natural gas furnace, and the level of agreement between model and experiment is found to be at least as good as any that has been published earlier. Next, simulations are performed with systematic model variations to provide insight into the roles of individual physical processes and their interplay in high-temperature oxy–fuel combustion. This includes variations in the chemical mechanism and the radiation model, and comparisons of results obtained with versus without the PDF method to isolate and quantify the effects of turbulence–chemistry inte...

Journal ArticleDOI
TL;DR: In this paper, the influence of countergradient transport on the speed of a statistically stationary, planar, 1D premixed flame propagating in frozen turbulence is studied theoretically and numerically by considering the normalised magnitude NB of the countergradient flux to be an input parameter.
Abstract: The influence of countergradient transport on the speed of a statistically stationary, planar, 1D premixed flame that propagates in frozen turbulence is studied theoretically and numerically by considering the normalised magnitude NB of the countergradient flux to be an input parameter. Spectra of admissible flame speeds are analytically determined and explicit travelling wave solutions are found for two algebraic relations widely used to close the mean rate of product creation. A problem of selecting the physically relevant solution that is approached for sufficiently steep initial conditions is addressed. It is argued that, if NB is larger than an analytically determined critical number NcrB, then the type of the physically relevant solution is drastically changed. If NB NcrB, the physically relevant solution is of pushed wave type, i.e. its speed is controlled by processes in the entire flame brush. Analytical expressions for the speed of the physically relevant solution as a function of NB and the density ratio are obtained. For NB > NcrB, the mean flame brush thickness and the spatial profile of the Favre-averaged combustion progress variable are also determined analytically. These results are validated by numerical simulations. Both analytical expressions and numerical data indicate that (i) both turbulent flame speed and thickness are decreased when NB is increased and (ii) the direction of total scalar flux (i.e. the sum of countergradient and gradient contributions) is strongly affected not only by NB, but also by the shape of the dependence of the mean rate of product creation on the mean combustion progress variable.

Journal ArticleDOI
TL;DR: The Rate-Controlled Constrained Equilibrium (RCCE) method is a thermodynamic based dimension reduction method which enables representation of chemistry involving n s species in terms of fewer n r constraints as mentioned in this paper.
Abstract: The Rate-Controlled Constrained-Equilibrium (RCCE) method is a thermodynamic based dimension reduction method which enables representation of chemistry involving n s species in terms of fewer n r constraints. Here we focus on the application of the RCCE method to Lagrangian particle probability density function based computations. In these computations, at every reaction fractional step, given the initial particle composition (represented using RCCE), we need to compute the reaction mapping, i.e. the particle composition at the end of the time step. In this work we study three different implementations of RCCE for computing this reaction mapping, and compare their relative accuracy and efficiency. These implementations include: (1) RCCE/TIFS (Trajectory In Full Space): this involves solving a system of n s rate-equations for all the species in the full composition space to obtain the reaction mapping. The other two implementations obtain the reaction mapping by solving a reduced system of n r rate-equatio...

Journal ArticleDOI
TL;DR: In this article, the conditional moment closure (CMC) method is used to compute the conditional scalar dissipation rate of the conditioning scalar, the progress variable, and turbulence is modelled using the standard k-α model.
Abstract: Conditional Moment Closure (CMC) is a suitable method for predicting scalars such as carbon monoxide with slow chemical time scales in turbulent combustion. Although this method has been successfully applied to non-premixed combustion, its application to lean premixed combustion is rare. In this study the CMC method is used to compute piloted lean premixed combustion in a distributed combustion regime. The conditional scalar dissipation rate of the conditioning scalar, the progress variable, is closed using an algebraic model and turbulence is modelled using the standard k–ɛ model. The conditional mean reaction rate is closed using a first order CMC closure with the GRI-3.0 chemical mechanism to represent the chemical kinetics of methane oxidation. The PDF of the progress variable is obtained using a presumed shape with the Beta function. The computed results are compared with the experimental measurements and earlier computations using the transported PDF approach. The results show reasonable agreement w...

Journal ArticleDOI
TL;DR: A Memory Abstraction Layer (MAL) has been developed that handles requested LUT entries efficiently by splitting the database file into several smaller blocks and keeps the total memory usage at a minimum using thin allocation methods and compression to minimise filesystem operations.
Abstract: A large number of methods for simulating reactive flows exist, some of them, for example, directly use detailed chemical kinetics or use precomputed and tabulated flame solutions. Both approaches couple the research fields computational fluid dynamics and chemistry tightly together using either an online or offline approach to solve the chemistry domain. The offline approach usually involves a method of generating databases or so-called Lookup-Tables (LUTs). As these LUTs are extended to not only contain material properties but interactions between chemistry and turbulent flow, the number of parameters and thus dimensions increases. Given a reasonable discretisation, file sizes can increase drastically. The main goal of this work is to provide methods that handle large database files efficiently. A Memory Abstraction Layer (MAL) has been developed that handles requested LUT entries efficiently by splitting the database file into several smaller blocks. It keeps the total memory usage at a minimum using th...

Journal ArticleDOI
TL;DR: In this paper, an unsteady three-dimensional numerical model has been formulated, coded, and solved to study ignition and flame development over a composite solid fuel sample upon heating by a localised radiant beam in a buoyant atmosphere.
Abstract: An unsteady three-dimensional numerical model has been formulated, coded, and solved to study ignition and flame development over a composite solid fuel sample upon heating by a localised radiant beam in a buoyant atmosphere. The model consists of an unsteady gas phase and an unsteady solid phase. The gas phase formulation consists of full Navier-Stokes equations for the conservation of mass, momentum, energy, and species. A one-step, second-order overall Arrhenius reaction is adopted. Gas radiation is included by solving the radiation transfer equation. For the solid phase formulation, the energy (heat conduction) equation is employed to solve the transient solid temperature. A first-order in-depth solid pyrolysis relation between the solid fuel density and the local solid temperature is assumed. Numerical simulations provide time-and-space resolved details of the ignition transient and flame development and the existence of two types of ignition modes: one with reaction kernel initiated on the surface a...

Journal ArticleDOI
TL;DR: In this article, large eddy simulations were employed to investigate the effect of the inlet turbulence intensity on the H2/CH4 flame structure in a hot and diluted co-flow stream which emulates the (Moderate or Intense Low-oxygen Dilution) MILD combustion regime.
Abstract: Large eddy simulations (LES) are employed to investigate the effect of the inlet turbulence intensity on the H2/CH4 flame structure in a hot and diluted co-flow stream which emulates the (Moderate or Intense Low-oxygen Dilution) MILD combustion regime. In this regard, three fuel inlet turbulence intensity profiles with the values of 4%, 7% and 10% are superimposed on the annular mixing layer. The effects of these changes on the flame structure under the MILD condition are studied for two oxygen concentrations of 3% and 9% (by mass) in the oxidiser stream and three hot co-flow temperatures 1300, 1500 and 1750 K. The turbulence-chemistry interaction of the numerically unresolved scales is modelled using the (Partially Stirred Reactor) PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influences of the turbulence intensity on the flame structure under the MILD condition are studied by using the profile of temperature, CO and OH mass fractions in both physical and mixtur...

Journal ArticleDOI
TL;DR: In this article, a turbulent combustion model, Conditional Source-term Estimation (CSE), is applied to a non-premixed turbulent jet methane flame, where the conditional chemical source terms are determined on the basis of first order closure and the conditional averaged species concentrations are obtained by inverting an integral equation.
Abstract: A turbulent combustion model, Conditional Source-term Estimation (CSE) is applied to a non-premixed turbulent jet methane flame. The conditional chemical source terms are determined on the basis of first order closure and the conditional averaged species concentrations are obtained by inverting an integral equation. The Tikhonov method is implemented for regularisation. Detailed chemistry is tabulated using the trajectory generated low-dimensional manifold method. Radiation due to the gaseous species is included. Reynolds Averaged Navier–Stokes calculations are performed using two different turbulence models. The objectives of the paper are (i) assessment of the impact of the main numerical parameters in CSE and (ii) comparison of the CSE numerical predictions with available experimental data and results from previous simulations for the selected flame. The number of CSE domains and the number of points in each CSE domain are shown to have a significant impact on the results if not selected appropriately....

Journal ArticleDOI
TL;DR: In this paper, the extinction and re-ignition processes observed experimentally in thin reaction zones of piloted turbulent non-premixed methane flames approaching blow-off are analysed using Large Eddy Simulation (LES) along with the Eulerian stochastic field method representing the unresolved sub-grid turbulence-chemistry interactions.
Abstract: Extinction and re-ignition processes observed experimentally in thin reaction zones of piloted turbulent non-premixed methane flames approaching blow-off are analysed using Large Eddy Simulation (LES) along with the Eulerian stochastic field method representing the unresolved sub-grid turbulence–chemistry interactions. Eight stochastic fields in conjunction with a reduced chemical mechanism involving 19 species are employed to perform simulations of the Sydney flames L, B and M, which exhibit increasing levels of extinction. The agreement of the flame statistics of the velocities, mixture fraction and selected reactive species were found to be encouraging and highlight the ability of the method to capture quantitatively the effects of increasing jet velocity in this series. In a subsequent analysis of the flame structure using the LES simulation data, the strong three-dimensionality of the flame was emphasised. Quantitative comparisons with recent measurements using high-speed Planar Laser-Induced Fluores...

Journal ArticleDOI
TL;DR: In this article, a premixed stagnation flame stabilised by a wall is analyzed theoretically considering thermally sensitive intermediate kinetics, and the effects of heat conduction and radical quenching on the wall surface are examined.
Abstract: The premixed stagnation flame stabilised by a wall is analysed theoretically considering thermally sensitive intermediate kinetics. We consider the limit case of infinitely large activation energy of the chain-branching reaction, in which the radical is produced infinitely fast once the cross-over temperature is reached. Under the assumptions of potential flow field and constant density, the correlation for flame position and stretch rate of the premixed stagnation flame is derived. Based on this correlation, the effects of heat conduction and radical quenching on the wall surface are examined. The wall temperature is shown to have great impact on flame bifurcation and extinction, especially when the flame is close to the wall. Different flame structures are observed for near-wall normal flame, weak flame, and critically quenched flame. The fuel and radical Lewis numbers are found to have opposite effects on the extinction stretch rate. Moreover, it is also demonstrated that only when the flame is close t...

Journal ArticleDOI
TL;DR: In this paper, the authors examined the flow field inside a premixed flat-flame reactor during the formation of carbon (soot) and iron oxide (from Fe(CO)5) nanoparticles, and how it affects the measurements of nanoparticle size distribution.
Abstract: Premixed low-pressure flat-flame reactors can be used to investigate the synthesis of nanoparticles. The present work examines the flow field inside such a reactor during the formation of carbon (soot) and iron oxide (from Fe(CO)5) nanoparticles, and how it affects the measurements of nanoparticle size distribution. The symmetry of the flow and the impact of buoyancy were analysed by three-dimensional simulations and the nanoparticle size distribution was obtained by particle mass spectrometry (PMS) via molecular beam sampling at different distances from the burner. The PMS measurements showed a striking, sudden increase in particle size at a critical distance from the burner, which could be explained by the flow field predicted in the simulations. The simulation results illustrate different fluid mechanical phenomena which have caused this sudden rise in the measured particle growth. Up to the critical distance, buoyancy does not affect the flow, and an (almost) linear growth is observed in the PMS experiments. Downstream of this critical distance, buoyancy deflects the hot gas stream and leads to an asymmetric flow field with strong recirculation. These recirculation zones increase the particle residence time, inducing very large particle sizes as measured by PMS. This deviation from the assumed symmetric, one-dimensional flow field prevents the correct interpretation of the PMS results. To overcome this problem, modifications to the reactor were investigated; their suitability to reduce the flow asymmetry was analysed. Furthermore, ‘safe’ operating conditions were identified for which accurate measurements are feasible in premixed low-pressure flat-flame reactors that are transferrable to other experiments in this type of reactor. The present work supports experimentalists to find the best setup and operating conditions for their purpose.

Journal ArticleDOI
TL;DR: In this paper, the impact of turbulence on the autoignition of a diluted hydrogen jet in a hot co-flow of air is studied numerically, and the LES combustion model used is successfully validated against experimental measurements and 3D DNS.
Abstract: The impact of turbulence on the autoignition of a diluted hydrogen jet in a hot co-flow of air is studied numerically. The LES combustion model used is successfully validated against experimental measurements and 3D DNS. Parametric studies are then carried out by separately varying turbulent intensity and integral length scale in the co-flow, while keeping all other boundary conditions unchanged. It is found that the impact of turbulence on the location of autoignition is non-trivial. For weak to mild turbulence, with a turbulent time scale larger than the minimum ignition delay time, autoignition is facilitated by increased turbulence. This is due to enhanced mixing between fuel and air, creating larger most reactive mixture fraction regions. On the other hand, for turbulent time scales smaller than the ignition delay time, the increased scalar dissipation rate dominates over the effect of increased most reactive mixture fraction regions, which leads to a rise in the autoignition length. Turbulence–chemi...

Journal ArticleDOI
TL;DR: Curran et al. as discussed by the authors studied the combustion chemistry of the first stage ignition and chemistry/flow interactions for DME with a mathematical analysis of two systems: a plug flow reactor study is used to reduce the reaction chemistry systematically.
Abstract: The combustion chemistry of the first stage ignition and chemistry/flow interactions are studied for dimethyl ether (DME) with a mathematical analysis of two systems: a plug flow reactor study is used to reduce the reaction chemistry systematically. A skeletal reaction mechanism for the low temperature chemistry of DME until the onset of ignition is derived on the basis of the detailed DME mechanism of the Lawrence Livermore National Laboratory – see Curran, Fischer and Dryer, Int. J. Chem. Kinetics, Vol. 32 (2000). It is shown that reasonably good results for ignition delay times can be reached using a simple system of three ordinary differential equations and that the resulting analytical solution depends only on two reaction rates and the initial fuel concentration. The stepwise reduction of the system based on assumptions yields an understanding on why these reactions are so important. Furthermore, the validation of the assumptions yields insight into the influence of the fuel and the oxygen concentra...

Journal ArticleDOI
TL;DR: In this paper, a numerical model incorporating ambipolar diffusion and plasma kinetics has been developed to predict gas temperature, species, and ion and electron concentrations in laminar premixed flames without applied electric fields.
Abstract: Flame stabilisation and extinction in a number of different flows can be affected by application of electric fields. Electrons and ions are present in flames, and because of charge separation, weak electric fields can also be generated even when there is no externally applied electric field. In this work, a numerical model incorporating ambipolar diffusion and plasma kinetics has been developed to predict gas temperature, species, and ion and electron concentrations in laminar premixed flames without applied electric fields. This goal has been achieved by combining the existing CHEMKIN-based PREMIX code with a recently developed methodology for the solution of electron temperature and transport properties that uses a plasma kinetics model and a Boltzmann equation solver. A chemical reaction set has been compiled from seven sources and includes chemiionisation, ion-molecule, and dissociative–recombination reactions. The numerical results from the modified PREMIX code (such as peak number densities of posit...

Journal ArticleDOI
TL;DR: In this paper, a Burke-Schumann formulation for diffusion flames between a fuel and oxidiser with Lewis numbers of unity was developed, subject to addition to the fuel and/or oxidiser stream of a different reactant for which the Lewis number differs from unity.
Abstract: A Burke–Schumann (flame-sheet) formulation is developed for diffusion flames between a fuel and oxidiser with Lewis numbers of unity, subject to addition to the fuel and/or oxidiser stream of a different reactant for which the Lewis number differs from unity. This formulation is applied to laminar counterflow diffusion-flame experiments, reported here, in which hydrogen was added to either methane–nitrogen mixtures or oxygen–nitrogen mixtures at normal atmospheric pressure, with both feed streams at normal room temperature. Experimental conditions were adjusted to fix selected values of the stoichiometric mixture fraction and the adiabatic flame temperature, and the strain rate was increased gradually, maintaining the momentum balance of the two streams, until extinction occurred. At the selected sets of values, the strain rate at extinction was measured as a function of the hydrogen concentration in the fuel or oxidiser stream. The ratio of the fraction of the oxidiser flux that consumes hydrogen to the ...

Journal ArticleDOI
Xinyan Wang1, Hui Xie1, Liyan Xie1, Lianfang Zhang1, Le Li1, Tao Chen1, Hua Zhao1 
TL;DR: In this article, a SI-CAI hybrid combustion model (HCM) has been constructed on the basis of the 3-Zones Extended Coherent Flame Model (ECFM3Z).
Abstract: SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In this study, a SI-CAI hybrid combustion model (HCM) has been constructed on the basis of the 3-Zones Extended Coherent Flame Model (ECFM3Z). An ignition model is included to initiate the ECFM3Z calculation and induce the flame propagation. In order to precisely depict the subsequent auto-ignition process of the unburned fuel and air mixture independently after the initiation of flame propagation, the tabulated chemistry concept is adopted to describe the auto-ignition chemistry. The methodology for extracting tabulated parameters from the chemical kinetics calculations is developed so that both cool flame reactions and main auto-ignition combustion can be well captured under a wider range of thermodynamic conditions. The SI-CAI hybrid combusti...

Journal ArticleDOI
TL;DR: In this paper, a numerical study has been carried out to gain physical insight into the phenomena of opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment.
Abstract: In this work a numerical study has been carried out to gain physical insight into the phenomena of opposed flow flame spread over an array of thin solid fuel sheets in a microgravity environment. The two-dimensional (2D) simulations show that the flame spread rates for the multiple-fuel configuration are higher than those for the flame spreading over a single fuel sheet. This is due to reduced radiation losses from the flame and increased heat feedback to the solid fuel. The flame spread rate exhibits a non-monotonic variation with decrease in the interspace distance between the fuel sheets. Higher radiation heat feedback primarily as gas/flame radiation was found to be responsible for the increase in the flame spread rate with the reduction of the interspace distance. It was noted that as the interspace distance between the fuel sheets was reduced below a certain value, no steady solution could be obtained. However, at very small interspace distances, steady state spread rates were obtained. Here, due to...

Journal ArticleDOI
TL;DR: In this article, the analysis of dilution effects on the opposed-jet H2/CO syngas diffusion flames was performed using OPPDIF coupled with narrowband radiation calculation, and the results showed that the maximum flame temperature decreases the most by CO2 dilution, followed by H2O and N2.
Abstract: This paper reported the analysis of dilution effects on the opposed-jet H2/CO syngas diffusion flames. A computational model, OPPDIF coupled with narrowband radiation calculation, was used to study one-dimensional counterflow syngas diffusion flames with fuel side dilution from CO2, H2O and N2. To distinguish the contributing effects from inert, thermal/diffusion, chemical, and radiation effects, five artificial and chemically inert species XH2, XCO, XCO2, XH2O and XN2 with the same physical properties as their counterparts were assumed. By comparing the realistic and hypothetical flames, the individual dilution effects on the syngas flames were revealed. Results show, for equal-molar syngas (H2/CO = 1) at strain rate of 10 s−1, the maximum flame temperature decreases the most by CO2 dilution, followed by H2O and N2. The inert effect, which reduces the chemical reaction rates by behaving as the inert part of mixtures, drops flame temperature the most. The thermal/diffusion effect of N2 and the chemical ef...