A major goal of combustion research is to develop accurate, tractable, predictive models for the phenomena occurring in combustion devices, which predominantly involve turbulent flows. With the focus on gas-phase, non-premixed flames, recent progress is reviewed, and the significant remaining challenges facing models of turbulent combustion are examined. The principal challenges are posed by the small scales, the many chemical species involved in hydrocarbon combustion, and the coupled processes of reaction and molecular diffusion in a turbulent flow field. These challenges, and how different modeling approaches face them, are examined from the viewpoint of low-dimensional manifolds in the high-dimensional space of chemical species. Most current approaches to modeling turbulent combustion can be categorized as flamelet-like or PDF-like. The former assume or imply that the compositions occurring in turbulent combustion lie on very-low-dimensional manifolds, and that the coupling between turbulent mixing and reaction can be parameterized by at most one or two variables. PDF-like models do not restrict compositions in this way, and they have proved successful in describing more challenging combustion regimes in which there is significant local extinction, or in which the turbulence significantly disrupts flamelet structures. Advances in diagnostics, the design of experiments, computational resources, and direct numerical simulations are all contributing to the continuing development of more accurate and general models of turbulent combustion.

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Small scales, many species and the manifold challenges of turbulent combustion

Large Eddy Simulation of n-dodecane spray flames using Flamelet Generated Manifolds

A modified piloted burner for stabilizing turbulent flames of inhomogeneous mixtures

Tabulated chemistry is a popular technique to account for detailed chemical effects with an affordable computational cost in gaseous combustion systems. However its performances for spray combustion have not completely been identified. The present article discusses the chemical structure modeling of spray flames using tabulated chemistry methods under the hypothesis that the chemical subspace accessed by a two-phase reactive flow can be mapped by a collection of gaseous flamelets. It is shown that tabulated chemistry methods based either on pure premixed flamelets or on pure non-premixed flamelets fail to capture the structure of spray combustion. The reason is the complexity of the chemical structure of spray flames which exhibits both premixed-like and non-premixed-like reaction zones. To overcome this issue, a new multi-regime flamelet combustion model (called Partially-Premixed Flamelet Tabulation 2PFT) is presented in this paper. Information from premixed, partially-premixed and diffusion flames are stored in a 3-D look-up table parametrized as a function of the progress variable Y c , describing the progress of the reaction, the mixture fraction Y z , denoting the local equivalence ratio, and the scalar dissipation χ ∗ , which identifies the combustion regime. The performances of the 2PFT method are evaluated on counterflow laminar spray flames for different injection conditions of droplet diameter, liquid volume fraction and velocity. The 2PFT tabulation method better describes the chemical structure of spray flames compared to the classical techniques based on single archetypal flamelets. These results also confirm that the chemical structure of laminar spray flame can be modeled by a multi-regime flamelet combustion model based on gaseous flamelets.

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A tabulated chemistry method for spray combustion

Advances and challenges in modeling high-speed turbulent combustion in propulsion systems

Capabilities and limitations of multi-regime flamelet combustion models

Scalar dissipation rates and subfilter scalar variances are important modeling parameters in large eddy simulations (LES) of reacting flows. Currently available models capture the general behavior of these parameters, but these models do not always perform with the degree of accuracy that is needed for predictive LES. Here, two direct numerical simulations (DNS) are used to analyze LES dissipation rate and variance models, and to propose a new model for the dissipation rate that is based on a transport equation. The first DNS that is considered is a non-premixed auto-igniting C2H4 jet flame simulation originally performed by Yoo et al. [Proc. Combust. Inst. 33, 1619–1627 (2011)]10.1016/j.proci.2010.06.147. A LES of this case is run using algebraic models for the dissipation rate and subfilter variance. It is shown that the algebraic models fail to adequately reproduce the DNS results. This motivates the introduction of a transport equation model for the LES dissipation rate. Closure of the equation is add...

/pdf/modeling-scalar-dissipation-and-scalar-variance-in-large-53vw7m70ki.pdf

Modeling scalar dissipation and scalar variance in large eddy simulation: Algebraic and transport equation closures

Combustion models often appear in forms that are customized for specific applications. This process of custo- mization produces modeling approaches that tend to be very different from one another in terms of cost, accuracy, and ap- plicability. For example, many combustion models have been developed to describe either the asymptotic premixed or the asymptotic non-premixed combustion limit. These idealized regimes are chosen as the basis for modeling approaches be- cause the associated combustion physics are understood sufficiently well to be cast in a framework that accounts for how turbulence and chemistry interact. Partially premixed regimes and regimes that involve transitions between premixed and non-premixed behavior, however, are not very well understood. Consequently, most readily available modeling ap- proaches do not account for these mixed regimes in a very careful fashion. This presents a particular challenge to further model development, since these partially premixed and transition processes are very important in realistic combustion de- vices. In this review, the particular challenges associated with modeling partially premixed combustion in LES will be discussed and the applicability of common LES combustion models to partially premixed processes will be assessed.

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Large-Eddy Simulation for Combustion Systems: Modeling Approaches for Partially Premixed Flows

Turbulent flame brush thickness is a characteristic measure of the transition zone between the burned and unburned states of a premixed flame. It may be interpreted as the square root of flame position variance, and must be accurately described if the flow properties and heat release characteristics of a premixed flame are to be captured. In this work the transport equation for the flame brush thickness is presented and new modeling approaches to the unclosed production and dissipation terms are suggested. To validate and test these approaches, a series of direct numerical simulations (DNS) of turbulent level set propagation are performed. These simulations provide the data needed to set the constants in the RANS-type production and dissipation closure models. The resulting transport equation for the flame brush thickness accurately describes transient cases and thus offers an improved means of predicting flame properties in non-equilibrium regions, e.g., near burner nozzles or flame stabilizers.

E. Knudsen

Papers

Capabilities and limitations of multi-regime flamelet combustion models

Modeling scalar dissipation and scalar variance in large eddy simulation: Algebraic and transport equation closures

Large-Eddy Simulation for Combustion Systems: Modeling Approaches for Partially Premixed Flows

Modeling flame brush thickness in premixed turbulent combustion

Study of Combustion Instabilities Imposed by Inlet Velocity Disturbance in Combustor Using LES