Abstract The Dynamic Thickened Flame (DTF) model has been widely used for Large Eddy Simulations (LES) of reacting flows over the past decades. This numerical approach was developed for simple flammable mixtures, assuming a constant amount of inert gas, if present. As a result, it is not possible to accurately compute premixed flames with local variations of inert species mass fraction using the standard DTF-LES model. The present paper addresses this limitation, and its purpose is twofold:(1) to propose and validate the extension of the DTF-LES model on a 3D LES deflagration of a hydrogen/air mixture at sub-atmospheric pressure, with a pressurized transverse injection of N 2 (P jet/P 0= 20) and,(2) to provide a comprehensive analysis of the key physical mechanisms responsible for the strong Flame Acceleration (FA). First, LES adequately replicates the experiment used as reference and shows that the extension of the DTF-LES model introduced in this work is necessary for such scenarios. Then, three mechanisms are identified as the main contributors to FA. At initial instants, the flame is sufficiently far from the jet exit to propagate unperturbed and, thus, it follows the laminar finger flame theory. Then, the flame-induced piston effect pushes fresh gases, as well as dilution gases, resulting in a delayed effect of the supersonic jet on the FA. This is further illustrated by simulating a premixed jet instead of N 2, showing no differences in absolute flame speed during this phase. In the next phase, hydrodynamically-induced flow contraction in the jet impingement zone is responsible for the sudden FA observed in the simulation. Finally, flame-turbulence interaction leads to a sudden rise in flame surface area, revealing a transition towards the turbulent combustion regime and, only then, dilution effects start to lower the flame propagation speed when compared to a fully premixed jet.