The main challenge of turbulent combustion simulation is to resolve turbulent mixing together with the chemistry of combustion involving hundreds of molecular species, in a solution time that is compatible with engineering design. Steady diffusion flamelet-based turbulent combustion models have been used for nearly three decades. The computational efficiency of flamelet-based models has been the key to their widespread success in industrial applications. However, increasingly stringent emission requirements continuously push designers to incorporate more finite-rate chemistry effects for the engine simulations in a more comprehensive manner.
Due to expensive chemistry computations, the reduced-order turbulent-combustion modeling is still a popular modeling choice for engine design, particularly in the gas turbine industry. The concept of a Flamelet Generated Manifold (FGM) offers an advantage over the widely used steady diffusion flamelet models, as it does not make any assumption about the thin flame structure. It can therefore be used to model finite rate chemistry effects in a broad range of combustion applications. The additional advantages of the FGM is that it is computationally as efficient as the prevailing steady flamelet-based models. FGM however, can be used for a wider range of combustion conditions, for both premixed and non-premixed combustion regimes. ANSYS CFD provides a highly optimized multi-dimensional FGM model for accurate modeling of finite rate controlled phenomena like flame quenching, and the formation and oxidation of carbon monoxide inside combustors.
ANSYS CFD uses a primitive configuration to generate the manifold for FGM modeling. The manifold then represents the chemistry inside a three dimensional CFD simulation. The turbulence chemistry interaction for the entire manifold is pre-tabulated and requires no additional cost at the CFD run time. A three-dimensional visualization tool allows analysis of the shape of the manifolds for instantaneous as well as PDF averaged mixture state.
Multiple combustion regimes within combustor
Although FGM offers several advantages over a steady diffusion flamelet approach, it also inherits few approximations of low dimensional manifold modeling. One of such approximations of the FGM modeling is the selection of the primitive one-dimensional configuration for creation of manifold. The manifold can be created either based on a premix flame configuration or can be based on pure diffusion flame structure. These two manifolds can be different, particularly for minor species and intermediate radical concentrations.
A proper manifold creation is an integral part of an accurate prediction of combustion characteristics in combustors. The optimal choice of manifold creation is driven by the combustion regime in the combustor for which the model is to be used. If the combustion is predominantly in the non-premixed mode inside the combustor, then a diffusion flame configuration is the right choice for the manifold creation. In other extreme, a premix flame is the appropriate choice for manifold creation for devices predominantly operating in the premixed combustion mode. However, the combustors may exhibit different degree of premixed and diffusion flame characteristics in different regions. The combustion could be premixed dominated in one region of the combustor while it could be diffusion controlled in another section. For such cases, a single primitive configuration to generate the manifold may not be the optimal choice and can lead to compromise the accuracy.
Incorporating the multiple regimes using reduced order models is an area of active research. ANSYS CFD combustion modeling provides a hybrid solution strategy for modeling multiple combustion regimes in the framework of FGM modeling. The multi-regime FGM modeling approach embeds the premixed and diffusion flame based manifolds. The appropriate regime selection in CFD simulation is then automatically done based on the prevailing local conditions of reactants and products, determined by a flame index indicator. The CFD solution locally computes the normalized flame index to identify the combustion regime at each point, which allows automatic selection of the appropriate manifold during the simulation without any user specific tuning. A smooth transition of regimes is done to ensure a numerically robust solution. The multi regime methodology is a promising tool for a generalized multi-dimensional FGM model to accurately capture wide range of combustion physics in a computationally efficient manner.
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