Conditional Moment Closure for Turbulent Premixed Flames

The conditional moment closure (CMC) method has been successfully applied to various non-premixed combustion systems in the past, but its application to premixed flames is not fully tested and validated. The main difficulty is associated with the modeling of conditional scalar dissipation rate of the conditioning scalar, the progress variable. A simple algebraic model for the conditional dissipation rate is validated using DNS results of a V-flame. This model along with the standard k- turbulence modeling is used in computations of stoichiometric pilot stabilized Bunsen flames using the RANS-CMC method. A first-order closure is used for the conditional mean reaction rate. The computed non reacting and reacting scalars are in reasonable agreement with the experimental measurements and are consistent with earlier computations using flamelets and transported PDF methods. Sensitivity to chemical kinetic mechanism is also assessed. The results suggest that the CMC may be applied across the regimes of premixed combustion

Assessment of an Equivalent Reaction Networks Approach for Premixed Combustion

The pollutants produced by the burning of fossil fuels have a severe impact on the environment and on mankind. Computational fluid dynamics (CFD) is a powerful tool, which is widely used to predict the emission of these pollutants from industrial combustion systems. Nevertheless, to predict these emissions the chemical reaction must be represented by a detailed mechanism, which includes pollutant formation pathways. Thus, using a complex mechanism, especially in 3D simulations with a realistic geometry is prohibitively expensive computationally. In this article, the equivalent reaction networks (ERN) method is used in conjunction with a Reynolds-averaged Navier Stokes (RANS) approach to reduce the cost of these computations. For this purpose, a pilot stabilized stoichiometric methane-air flame is chosen with a specific interest in species with slow time scales, such as CO and NOx. The Favre averaged CFD results are then compared to previously-reported experimental measurements and earlier computations using conditional moment closure (CMC) at five axial locations within the flame. Despite the simplicity of the ERN method in contrast with other more complex combustion models, the comparison of the CFD results with the experimental measurements for the prediction of CO are extremely encouraging

Computations of turbulent lean premixed combustion using conditional moment closure

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-e{open} 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 with the experimental measurements and are consistent with the transported PDF computations. When the compounded effects of shear-turbulence and flame are strong, second order closures may be required for the CMC. © 2013 Copyright Taylor and Francis Group, LLC

Conditional Moment Closure for Turbulent Premixed Flames

The conditional moment closure (CMC) method has been successfully applied to various non-premixed combustion systems in the past, but its application to premixed flames is not fully tested and validated. The main difficulty is associated with the modeling of conditional scalar dissipation rate of the conditioning scalar, the progress variable. A simple algebraic model for the conditional dissipation rate is validated using DNS results of a V-flame. This model along with the standard k- turbulence modeling is used in computations of stoichiometric pilot stabilized Bunsen flames using the RANS-CMC method. A first-order closure is used for the conditional mean reaction rate. The computed non reacting and reacting scalars are in reasonable agreement with the experimental measurements and are consistent with earlier computations using flamelets and transported PDF methods. Sensitivity to chemical kinetic mechanism is also assessed. The results suggest that the CMC may be applied across the regimes of premixed combustion

Numerical Modelling of Two-Phase Flow in a Gas Separator Using the Eulerian–Lagrangian Flow Model

Gravity-driven separators are broadly used in various engineering applications to remove particulate matters from gaseous fluids to meet legislation demands. This study represents a detailed numerical investigation of a two-phase cyclone separator using the Eulerian–Lagrangian gas flow method. The turbulence is modelled using the Reynolds stress model (RSM). The technique has successfully predicted the typical trends and variations seen in such gas separators with an average error of approximately 5.5%. Also, the computed results show a realistic agreement with the experimental measurements

A Comparative Study of Conditional Moment Closure Modelling for Ignition of iso-octane and n-heptane in Thermally Stratified Mixtures

This paper presents a comparative study of the premixed conditional moment closure (CMC) model for modelling ignition of thermally stratified mixtures under homogeneous charge compression ignition (HCCI) conditions. For this purpose, the CMC model is applied to two sets of direct numerical simulations (DNSs) modelling ignition of lean n-heptane/air and iso-octane/air mixtures with various levels of thermal stratification. The results show excellent agreement for all n-heptane cases with thermal stratification of 15-60 K. However, an advanced ignition is predicted by the CMC model for the iso-octane case with thermal stratification of 60 K in comparison with the DNS data. Inspection of homogeneous ignition delay demonstrates that the ignition delay time fluctuations are much higher in the iso-octane cases compared with the n-heptane cases having same level of temperature inhomogeneities. This is because of the differing ignition responses to temperature between these two fuels. The observed discrepancies in the iso-octane case with (Formula presented.) K are due to the dominance of deflagration mode of combustion resulting in large conditional fluctuations, which occurs in the iso-octane case and not the n-heptane case because the temperature dependence of ignition delay is stronger for iso-octane. To further investigate the reasons for the observed discrepancies, a transport equation for the conditional variance is derived for premixed combustion. Assessment of the conditional variance equation using the DNS data shows that correlations between dissipation and conditional fluctuation and correlations between reaction and conditional fluctuations are the dominant sources of conditional fluctuations.close0