An extension of the artificially thickened flame approach for premixed hydrogen flames with intrinsic instabilities

Despite the growing interest in hydrogen as a fuel, current combustion models are still incapable of considering the effects of intrinsic flame instabilities, such as increased flame surface area and local stratification. A significant challenge arises due to the substantial impact on the laminar flame consumption speed, complicating the transfer of established combustion models. This study proposes an extension of the artificially thickened flame (ATF) approach to encompass laminar self-wrinkling flames subject to intrinsic instabilities, serving as a foundation for advancing turbulent combustion models. Several 2D numerical simulations were conducted to analyze the interaction of ATF and thermodiffusive instabilities in unstable lean hydrogen flames. The assessment of characteristic length scales revealed a linear scaling relationship with the thickening factor , causing the domain size at which instabilities manifest to expand and modify the consumption speed of the thickened flame. To compensate for these deviations, a novel efficiency function is derived from the fractal characteristics of the flame front. Using the critical wavelength and a geometric length scale as inner and outer cut-offs, respectively, the improved ATF model was evaluated successfully in fully coupled simulations of the 2D planar flames. To facilitate the use of the new model, a practical on-the-fly procedure for estimating a characteristic geometric length scale required for the efficiency function is proposed and successfully employed.