Abstract
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 CH4,H2O and O2 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 are pronounced differences for its individual contributions which to a large extent depend on the definition of reaction progress variable as well as on the chosen isosurface level. It is concluded that, while detailed chemistry simulations provide more detailed information about the flame structure, the choice of the reaction progress variable definition and the choice of the resulting isosurface give rise to considerable uncertainty in the interpretation of displacement speed statistics, sometimes even showing opposing trends. Simple chemistry simulations are shown to provide (a) the global flame propagation statistics which are qualitatively similar to the corresponding results from detailed chemistry simulations, (b) remove the uncertainties with respect to the choice of reaction progress variable, and (c) are more straightforward to compare with theoretical analysis or model assumptions that are mostly based on simple chemistry assumptions.
Highlights
The use of fossil resources for the supply of primary energy leads to increasing concerns about emissions and their environmental, and health impact [1]
For simple chemistry (SC), the behaviour is in good agreement with the detailed chemistry (DC) results and detailed explanations are provided for the change of correlation strength with the value of reaction progress variable [49], while the present analysis focuses on the differences between SC and DC and on the differences resulting from different definitions of reaction progress variable
Direct Numerical Simulations (DNS) have been compared to results obtained from detailed chemistry and transport simulations using a new database consisting of five simple chemistry (SC) and detailed chemistry and transport (DC) simulations of statistically planar turbulent premixed, stoichiometric methane-air flames for a range of different Damköhler, Karlovitz and turbulent Reynolds number values
Summary
The use of fossil resources for the supply of primary energy leads to increasing concerns about emissions and their environmental, and health impact [1]. Energies 2021, 14, 5695 requires the definition of a reaction progress variable together with the identification of the isolevel of reaction progress variable It has been shown in the first part of this work [23] 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 DNS. Different species participating in the chemical reaction are characterised by different individual Lewis numbers and the global Lewis number is known to influence flame propagation statistics Such analysis has been pioneered by Rutland and Trouvé [27] for generic planar flame simulations showing a pronounced Lewis number dependent correlation between surface curvature and the local flame speed. This will be followed by a detailed discussion of the results and conclusions will be drawn
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