The separator has a cylindrical separator chamber and an impeller on the same axis and driven in rotation. The impeller has a core, an inlet for receiving an axial flow of fluid, an outlet for injecting the fluid into the separator chamber, and guide channels formed between the inlet and the outlet of the impeller at the periphery of the core. The core has a portion of diameter equal to or greater than the inner diameter of the separator chamber. In this portion, the guide channels extend longitudinally, and they then continue as far as the outlet from the impeller, which outlet is cylindrical in shape and of diameter equal to the inner diameter of the separator chamber.