Inclusion of Preferential Diffusion in Simulations of Premixed Combustion of Hydrogen/Methane Mixtures with Flamelet Generated Manifolds

Abstract

In this paper we study the possibility to account for preferential diffusion effects in lean turbulent premixed flames in numerical predictions with reduced chemistry. We studied the situation when hydrogen is added to methane at levels of 20% and 40% by volume in the fuel, at lean combustion (φ = 0.7) with air. The base case of pure methane was used as a reference. In this case preferential diffusion effects are negligible. First the sensitivity of the mass burning rate to flame stretch was investigated, in one dimensional computations with detailed chemistry, to set reference values. Then the framework of the Flamelet Generated Manifolds (FGM) was used to construct an adequate chemical method to take preferential diffusion into account, without the need for using detailed chemistry. To that end a generalization of the method was presented in which five controlling variables are required. For this system, proper transport equations and effective Lewis numbers where derived. In practice not all five variables are necessary to include and as a first step we limited the amount in the numerical tests in this study to two controlling variables. The method was then tested in configurations in which there was an interaction of coherent vortices and turbulence with flames. It was demonstrated that a minimum of two controlling variables is needed to account for the changed mass burning rate as function of stretch and curvature. It was shown that one-dimensional FGM as well as one-step Arrhenius kinetics can not describe this relation.

A computational study of preferential diffusion and scalar transport in nonpremixed hydrogen-air flames

“…where ρD YZ is the diffusion coefficient for preferential diffusion calculated by the FGM technique. The diffusivity D YZ is related with the non-unity Lewis number, which has been derived by Swart et al [42]. It is worth noting that the species diffusion contribution in Eq.…”

Development of a novel flamelet-based model to include preferential diffusion effects in autoignition of CH4/H2 flames

“…16 and 17 separately on the predicted autoignition time scales. For this purpose, we define a sensitivity parameter S τ ig that is computed as follows:…”

“…Successful reduction techniques to accommodate preferential diffusion are mainly based on flamelets [12], such as FGM (Flamelet Generated Manifolds) [13], FPV (Flamelet Progress Variable) [14] and REDIM (ReactionDiffusion Manifolds) [15]. Inclusion of preferential diffusion in FGM has been studied by van Oijen et al [13] and later on by de Swart et al [16] in the context of premixed flames. It has been shown that two controlling variables are needed to account for local variations in equivalence ratio and mass burning rate.…”

A Priori Analysis of a Compressible Flamelet Model using RANS Data for a Dual-Mode Scramjet Combustor

“…[18][19][20][21][22] However, for a compressible flow, the static pressure cannot be assumed constant and may vary by an order of magnitude or more. The effect of p on the chemical kinetics and composition must be considered within the FPV model.…”