Direct Numerical Simulation of Complex Fuel Combustion with Detailed Chemistry: Physical Insight and Mean Reaction Rate Modeling (original) (raw)
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Energies
Flame propagation statistics for turbulent, statistically planar premixed flames obtained from 3D Direct Numerical Simulations using both simple and detailed chemistry have been evaluated and compared to each other. To achieve this, a new database has been established encompassing five different conditions on the turbulent combustion regime diagram, using nearly identical numerical methods and the same initial and boundary conditions. The discussion includes interdependencies of displacement speed and its individual components as well as surface density function (i.e., magnitude of the reaction progress variable) with tangential strain rate and curvature. For the analysis of detailed chemistry Direct Numerical Simulation data, three different definitions of reaction progress variable, based on and mass fractions will be used. While the displacement speed statistics remain qualitatively and to a large extent quantitatively similar for simple chemistry and detailed chemistry, there ar...
Modeling chemical flame structure and combustion dynamics in LES
Proceedings of the Combustion Institute, 2011
In turbulent premixed combustion, the instantaneous flame thickness is typically thinner that the grid size usually retained in Large Eddy Simulations (LES), requiring adapted models. Two alternatives to couple chemical databases with LES balance equations, the Thickened Flame (TFLES) and the Filtered Tabulated Chemistry (F-TACLES) models, are investigated here and compared in terms of chemical flame structure and dynamics. To avoid the uncertainties related to the modeling of sub-grid scale turbulence / flame interactions, this comparison is conducted in situations where the flame front is not wrinkled at sub-grid scale levels. The thinner quantity requiring an accurate discretization on the numerical grid mesh is the reaction rate of the thickened or filtered progress variable. The thermal flame structure is found to be considerably thicker in TFLES than when using F-TACLES. The simulation of a 2D unsteady Bunsen burner flame shows that the thermal thickness spreading strongly affects the flame dynamics giving a decisive advantage to F-TACLES compared to TFLES.
Combustion and Flame, 2013
In this work a new computational framework for the modeling of multi-dimensional laminar flames with detailed gas-phase kinetic mechanisms is presented. The proposed approach is based on the operatorsplitting technique, in order to exploit the best numerical methods available for the treatment of reacting, stiff processes. The main novelty is represented by the adoption of the open-source OpenFOAM Ò code to manage the spatial discretization of the governing equations on complex geometries. The resulting computational framework, called laminarSMOKE, is suitable both for steady-state and unsteady flows and for structured and unstructured meshes. In contrast to other existing codes, it is released as an open-source code and open to the contributions from the combustion community. The code was validated on several steady-state, coflow diffusion flames (fed with H 2 , CH 4 and C 2 H 4), widely studied in the literature, both experimentally and computationally. The numerical simulations showed a satisfactory agreement with the experimental data, demonstrating the feasibility and the accuracy of the suggested methodology. Then, the C 2 H 4 /CH 4 laminar coflow flames experimentally studied by Roesler et al. [J.F. Roesler et al., Combust. Flame 134 (2003) 249-260] were numerically simulated using a detailed kinetic mechanism (with 220speciesand220 species and 220speciesand6800 reactions), in order to investigate the effect of methane content on the formation of aromatic hydrocarbons. Model predictions were able to follow the synergistic effect of the addition of methane in ethylene combustion on the formation of benzene (and consequently PAH and soot).
Energies
In the present study, flame propagation statistics from turbulent statistically planar premixed flames obtained from simple and detailed chemistry, three-dimensional Direct Numerical Simulations, were evaluated and compared to each other. To this end, a new database was established encompassing five different conditions on the turbulent premixed combustion regime diagram, using nearly identical numerical methods and the same initial and boundary conditions. A detailed discussion of the advantages and limitations of both approaches is provided, including the difference in carbon footprint for establishing the database. It is shown that displacement speed statistics and their interrelation with curvature and tangential strain rate are in very good qualitative and reasonably good quantitative agreement between simple and detailed chemistry Direct Numerical Simulations. Hence, it is concluded that simple chemistry simulations should retain their importance for future combustion research...
SAE International Journal of Engines, 2019
Addition of hydrogen to hydrocarbons in premixed turbulent combustion is of technological interest due to their increased reactivity, flame stability and extended lean extinction limits. However, such flames are a challenge to reaction modelling, especially as the strong preferential diffusion effects modify the physical processes, which are of importance even for highly turbulent high-pressure conditions. In the present work, RANS modelling is carried out to investigate pressure and hydrogen content on methane/hydrogen/air flames. For this purpose, four different subclosures, used in conjunction with an algebraic reaction model, are compared with two independent sets of experimental data: 1. Orleans data consists of pressures up to 9 bar, with addition of hydrogen content up to 20% in hydrogen/methane mixture, for moderate turbulence intensities. 2. The Paul Scherrer Institute data includes same fuels with higher volume proportion of hydrogen (40%), at much higher turbulent intensities at 5 bar. The first model Model I is based solely on the increased reactivity of the hydrogen/methane mixture under laminar conditions. It shows that the increase of unstretched laminar burning velocity (S L0) is not sufficient to describe the increased reactivity in turbulent situations. This non-corroboration proves the importance of preferential diffusion effects in highly turbulent flames. Models II and III are formulated based on the localized increase in S L0 , local burning velocity which is a strong function of local curvature and flow strain. Model II over predicts the reactivity for higher pressures. Model III accurately predicts for nearly all studied flame conditions. Model IV is based on the leading point concept that the leading part of the turbulent flame brush is more important than rear part of premixed flame with the Lewis number less than unity. This model in its present formulation under predicts the average reaction rate compared with experiments.
Impact of detailed chemistry and transport models on turbulent combustion simulations
Progress in Energy and Combustion Science, 2004
More and more publications can be found in recent years where detailed models are employed to describe the chemical and molecular transport processes controlling flame structure. Up to a recent past, such studies were restricted to simple zero-or one-dimensional laminar computations, like ignition in a fully premixed mode, freely propagating laminar premixed flames or counter-flow flames. Since such models are now often used to investigate turbulent flames in multi-dimensional computations, we feel it is useful to review the literature on this subject and give a synthesis of the obtained results. To be more specific, we consider only in this review publications where (1) chemical processes are modeled with a multi-step reaction scheme, taking at least an intermediate species into account; or (2) molecular diffusion processes of the individual species are represented by a more elaborate model than assuming unity Lewis numbers; and (3) the retained configuration leads to unsteady strain-rate and curvature (or stretch-rate) variations in the reaction zone. Over 200 recent publications have been found to respect these criteria. Summarizing the results, one can say that there appears to be a growing need for simulations relying on detailed models for chemistry and transport processes, probably due to the fact that restrictions concerning pollutant emissions motivate a request for more accurate, quantitative results. Progress must still be accomplished concerning the identification of chemical pathways, the accurate determination of rate constants, and the development of reliable but efficient chemistry reduction techniques. The impact of the retained molecular diffusion model is higher than expected at the beginning of this study. Even for turbulent configurations, the global impact of these models can be comparable to switching between two different detailed chemical schemes. Concerning local flame structure, the transport models play an essential role, in particular for high flame curvatures and far from stoichiometry. As a whole, the need for matching the accuracy level of the chosen chemical and transport models is emphasized, since describing a physical phenomenon in great detail while, at the same time, representing another phenomenon of comparable importance with a very rough model, prevents really quantitative (and even perhaps qualitative) predictions. Specific difficulties concerning validation are also identified. q
Flow, Turbulence and Combustion
A three-dimensional compressible Direct Numerical Simulation (DNS) analysis has been carried out for head-on quenching of a statistically planar stoichiometric methaneair flame by an isothermal inert wall. A multi-step chemical mechanism for methane-air combustion is used for the purpose of detailed chemistry DNS. For head-on quenching of stoichiometric methane-air flames, the mass fractions of major reactant species such as methane and oxygen tend to vanish at the wall during flame quenching. The absence of OH at the wall gives rise to accumulation of carbon monoxide during flame quenching because CO cannot be oxidised anymore. Furthermore, it has been found that low-temperature reactions give rise to accumulation of HO 2 and H 2 O 2 at the wall during flame quenching. Moreover, these low temperature reactions are responsible for non-zero heat release rate at the wall during flame-wall interaction. In order to perform an in-depth comparison between simple and detailed chemistry DNS results, a corresponding simulation has been carried out for the same turbulence parameters for a representative single-step Arrhenius type irreversible chemical mechanism. In the corresponding simple chemistry simulation, heat release rate vanishes once the flame reaches a threshold distance from the wall. The distributions of reaction progress variable c and non-dimensional temperature T are found to be identical to each other away from the wall for the simple chemistry simulation but this equality does not hold during head-on quenching. The inequality between c (defined based on CH 4 mass fraction) and T holds both away from and close to the wall for the detailed chemistry simulation but it becomes particularly prominent in the near-wall region. The temporal evolutions of wall heat flux and wall Peclet number (i.e. normalised wall-normal distance of T = 0.9 isosurface) for both simple and detailed chemistry laminar and turbulent cases have been found to be qualitatively similar. However, small differences have been observed in the numerical values of the maximum normalised wall heat flux magnitude ( max ) L and the minimum Peclet number (P e min ) L obtained from simple and detailed chemistry based laminar head-on quenching calculations. Detailed explanations have been provided for the observed differences in behaviours of ( max ) L and (P e min ) L . The usual Flame Surface Density (FSD) and scalar dissipation rate (SDR) based reaction rate closures do not adequately predict the mean reaction rate of reaction progress variable in the near-wall region for both simple and detailed chemistry simulations. It has been found that recently proposed FSD and SDR based reaction rate closures based on a-priori DNS analysis of simple chemistry data perform satisfactorily also for the detailed chemistry case both away from and close to the wall without any adjustment to the model parameters.
Proceedings of the Combustion Institute, 2002
Direct numerical simulations (DNS) are ideally suited to investigate in detail turbulent reacting flows in simple geometries. When considering such problems as pollutant emission or stability limits, detailed models must generally be employed to describe the chemical processes with sufficient accuracy. Due to the huge cost of such simulations, they have been mostly restricted to two-dimensional configurations up to now, leading to unsolved questions concerning the generality of the obtained results. We have recently developed a three-dimensional DNS code leading to reasonable computing times, thanks to the low-Machnumber approximation and to a new chemistry reduction technique. This code is used here to investigate the evolution of premixed methane/air flame kernels placed in a homogeneous isotropic turbulence field. This situation typifies the initial flame development after spark ignition in a gas turbine or an internal combustion engine. Laminar reference computations are carried out in one and two dimensions and are compared with turbulent results obtained in two and three dimensions. Evolution of flame surface area, stretch rate, and flame front curvature are in particular presented and show considerable differences between two-dimensional and three-dimensional computations. The interest of repeating the computations to increase the statistical validity of the results is demonstrated in two dimensions, but is not sufficient to explain the discrepancy obtained with the three-dimensional computation. Further three-dimensional simulations are nevertheless needed to quantify more precisely the observed changes (slower increase of the equivalent radius, higher stretch rate, and curvature shifted toward positive values).
Thermal Science, 2020
The characteristics of partially-premixed flames is investigated by simulating a series of triple flames with different variations of chemical equivalent ratio. A two-dimensional Reaction-Diffusion Manifold (REDIM) chemistry tabulation method is employed in the simulation and the results are compared with the methane-air 19-step chemical reaction mechanism. The performance of these two mechanisms is then assessed by using direct numerical simulations coupled with GRI3.0 detailed mechanism. It is shown that both 2D REDIM table and 19-step simplified mechanism can describe the temperature and main products accurately, however, for some minor intermediary products, predictions from 2D REDIM table is observed to be better than 19-step simplified mechanism. Compared with the 19-step mechanism, 2D REDIM table only needs to solve only the transport equations for CO2 and N2 species, which greatly simplifies the solution process of chemical reaction and provides a reliable solution for the n...