The paper provides an explanation why new methods of reducing kinetic energy losses have to be developed for annular axial-radial diffusers with a sharp turn of the stream, used in exhaust hoods of condensing steam turbines. The results of numerical simulation of separated flow in an annular axial-radial diffuser with vortex cells intended for control of flow separation are presented. Numerical simulation is based on the solution of the Reynolds-averaged Navier–Stokes equations in a steady axisymmetric formulation. The efficiency of the vortex cell is estimated depending on its size, location on the surface of the diffuser near the flow separation zone and the intensity of the slit suction. The conclusion is that the vortex cell with a slit suction can be regarded as a promising method for controlling flow separation in an axial-radial diffuser with a sharp turn of the stream.