A Thickened flame model extension for the simulation of lean hydrogen-air explosions in confined environments

This paper investigates the coupling between wall confinement and flame front instabilities during lean H2-air deflagrations in tubes. Flame-Resolved Simulations (FRS) show that confinement significantly affects flame behavior: (1) in narrow tubes, confinement effects dominate over flame instabilities and flame acceleration is driven dominantly by the finger flame mechanism, (2) while in wider tubes, instabilities have more space to develop, thereby enhancing their contribution to flame acceleration. In a large-scale modeling perspective, the paper delves into ways to reproduce the complex interaction between confinement and flame front instabilities using coarser meshes. Strong limitations of the Thickened Flame (TF) model, a classical approach for the Large Eddy Simulations (LES) for reactive flows, are first highlighted. The inherent inability of the TF approach to reproduce the specificities of lean H2-air combustion is solved by employing the Thermo-Diffusive-Stretched- Thickened Flame (TD-S-TF) model initially developed in Hok et al. (2024) and extending it to account for confinement effects: the model incorporates a time-dependent efficiency function mimicking the effects of subgrid thermo-diffusive instabilities on flame acceleration, and saturated to account for the limited instability growth in confined spaces. Although such saturation is only demonstrated for the simple tube configuration, this strategy solves issues encountered with the TF model, thereby paving the way for accurate confined H2-air explosions simulations.