Experimental and numerical research on a bluff-body stabilised combustion process has a long history outlined in [1]. The role of bluff-body is to redirect the gas flowing along its walls so that a recirculation zone is created. It stabilises the flame and improves combustion efficiency. This research concentrates on a nitrogen-diluted hydrogen flame formed downstream a bluff-body with a cylindrical and star-shaped wall placed in an oxidiser duct. To intensify the mixing process an excitation is imposed on an oxidiser mass flow stream. The study utilises the Large Eddy Simulation (LES) method, providing detailed insights into the intricate physics of unsteady flow. A two-stage computational approach is employed, incorporating the ANSYS Fluent software for modelling the flow around the bluff bodies and generating time-varying boundary conditions at the combustion chamber inlet. The combustion process in the chamber is modelled using the in-house high-order SAILOR code [2]. The main objective of the research is to evaluate the impact of bluff-body shape and excitation parameters (amplitude, frequency) on the formation of large- and small-scale vortical structures and to analyse their influence on various flame parameters (flame shape, fuel consumption, temperature and species distribution). The findings reveal that, in the case of the cylindrical bluff body, the mixing process is driven by periodically generated large toroidal vortices resulting from Kelvin-Helmholtz instability. The excitation enhances the formation of these toroidal vortices, leading to an intense injection of oxidiser into the recirculation zone. On the contrary, the star-shaped bluff body induces azimuthal disturbances that disrupt large toroidal vortical structures. This results in the generation of small vortices fostering intense small-scale mixing. Consequently, this causes elongation of the flame and a more uniform temperature distribution. Its maximum level is decreased approximately 200 K that may have significant impact on the thermal reduction of NOx. [1] Shanbhogue, S. J., Husain, Lieuwen T., Lean blowoff of bluff body stabilized flames: Scaling and dynamics. Prog. En. Combust. Sci. Vol. 35, pp. 98-120, 2009. [2] Tyliszczak A., A high-order compact difference algorithm for half-staggered grids for laminar and turbulent incompressible flows. J. Comput. Phys. Vol. 276, pp. 438-467, 2014.