The invention concerns a device wherein the diffusion chamber (6) comprises, between its upstream end and its downstream end, in the direction of the fluid flow, at least an insert (12) formed by an annular ring having an external diameter identical to the internal diameter of the chamber, said annular ring being arranged perpendicular to the axis of the supply conduit and projecting towards the inside part of the chamber from the side wall(s) thereof, to form a lateral obstacle to the fluid flow, the solid surface of the insert (12) being sufficient to generate in the upstream part of the chamber a pressure higher than that of the downstream part.