A novel flamelet manifold parametrization approach for lean CH4–H2-air flames

Abstract

Hydrogen is a carbon-free energy carrier that can substantially support the decarbonization of the power generation and transportation sector in the near future. Blending H2 into natural gas represents a feasible option to continue using the current infrastructure, allowing a smooth transition to pure H2 combustion technologies. In the present study, a flamelet model is proposed to describe lean CH4–H2-air laminar Bunsen flames with inert gas as a coflow, simultaneously taking into account multiple complex physical phenomena such as differential diffusion, heat losses at the burner wall and mixing between the main flow and coflow. In most previous works based on manifold-based reduction methods, transport equations are solved for the progress variable, mixture fraction and enthalpy. In contrast, transport equations for several species and enthalpy are solved in the present study and species diffusivities are evaluated with a mixture-averaged diffusion model. The control variables of the manifold are then reconstructed with those transported variables. In order to consider the mixing between the main flow and the inert coflow, two different approaches based on a three-dimensional and a four-dimensional manifold, respectively, are formulated and assessed. Utilizing a “linear mixing” assumption, the three-dimensional manifold is parameterized by a progress variable, an approximate Bilger mixture fraction and enthalpy. The four-dimensional manifold relaxes this assumption and additionally includes the inert gas mass fraction as the fourth control variable. The accuracy of the two approaches is evaluated by comparison with detailed chemistry results and experimental measurements. Overall, results obtained with both approaches show good agreement with the reference data, both qualitatively and quantitatively, indicating that all the effects mentioned above are well captured. Slightly better agreement is achieved with the four-dimensional manifold, which shows superiority in the mixing layer between the main flow and the inert coflow.