Pascale Domingo

Partially premixed flamelets in LES of nonpremixed turbulent combustion

Partially premixed flames are observed in nonpremixed turbulent combustion when fuel and oxidizer have mixed before burning. This combustion regime combines the properties of both premixed and diffusion flames. A procedure based on the resolved fields is proposed to associate premixed and diffusion flame descriptions in large eddy simulation. Using basic and well known subgrid modelling of premixed and diffusion flames, the proposed methodology is tested for flames lifted in a two-dimensional turbulent wake. Very recent experimental observations concerning the dynamics of the flow field at the turbulent flame base are reproduced.

Modelling of the effect of DC and AC electric fields on the stability of a lifted diffusion methane/air flame

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 depends, mainly, on the intensity and polarity of the applied voltage. If the applied voltage is alternating, its frequency is also found to influence the extent of the flame stabilisation improvement.

Self-similar behavior and chemistry tabulation of burnt-gas diluted premixed flamelets including heat-loss

In many combustion systems, the reactive gases feeding the reaction zones are diluted by burnt products, to favor flame stabilization, homogenize the temperature distribution and reduce pollutant emission. The objective of this paper is to discuss a premixed flamelet detailed chemistry tabulation strategy for vitiated and non-adiabatic combustion. Dilution by burnt products is parameterized here with two controlling quantities: the amplitude of the heat-loss in the burnt gases, for instance at walls, and the level of reactant vitiation. The chemical response of premixed flames to variations of these parameters is studied and it is shown that most chemical properties of burnt-gas diluted flames feature self-similar behavior, which can be used to dramatically downsize chemical tables based on canonical flamelets. The self-similar behavior of the flamelets is studied for both molecular diffusion and chemical source budgets in a progress variable composition space. It is found that two different scaling relations are needed to ensure self-similar behavior of both major and radical species.

Scalar sub-grid energy in large-eddy simulation of turbulent flames: mesh quality criterion

In Large-Eddy Simulation (LES), scalar fluctuations are decomposed into a resolved part and a complementary Sub-Grid Scale (SGS) part. Accordingly, it is usually assumed that the scalar energy contained in these two parts sum up, so that the time average of the scalar energy equals the time average of the resolved part of the scalar energy to which the time average of the SGS scalar variance is added. Conditions are discussed under which an additional residual term must be added to close this scalar energy budget. For this residual term to stay at a moderate level, the LES filter must be small enough compared to the integral length-scale of the scalar field, a condition that is verified from a canonical manufactured turbulent scalar solution. A mesh-quality criterion is derived from these observations and the minimum Reynolds number that a Direct Numerical Simulation (DNS) should feature for SGS scalar variance to be accurately studied from a priori filtering is obtained as a corollary.

Reliability of Large-Eddy Simulation of Nonpremixed Turbulent Flames: Scalar Dissipation Rate Modeling and 3D-Boundary Conditions

The intricate coupling between the numerical discretization of scalar field transport and the modeling of unresolved sub-grid scale fluctuations of chemical species is discussed. It is shown how the closures for the sub-grid scale scalar dissipation rate combine modeling of small scale diffusion with two error terms measuring the lack of accuracy in the transport of scalar field fluctuations energy. Then, the need of accounting for the three-dimensional character of turbulent flows at boundaries of computational domains is illustrated.