Exhaust flow diffuser for a steam turbine

TLDR: In this paper, an annular diffuser having its inlet located at the exit of a last row of blades of a steam turbine having initially very slowly increasing cross-sectional area with distance to accommodate the diffusion produced by the decaying wakes in the diffuser so as to prevent flow separation from diffuser walls and as a result to foster the diffusion process and to increase the efficiency of the steam turbine.

Abstract: An annular diffuser having its inlet located at the exit of a last row of blades of a steam turbine having initially very slowly increasing cross-sectional area with distance to accommodate the diffusion produced by the decaying wakes in the diffuser so as to prevent flow separation from diffuser walls and as a result to foster the diffusion process and to increase the efficiency of the steam turbine. The rate of increase of cross-sectional area, which is much smaller than that appropriate in diffusers having uniform incompressible flow at their inlets, allows wakes which form near the trailing edges of the last turbine blades to dissipate while avoiding flow separation. In the diffuser of this invention, whether it is one of fixed shape or one whose cross-sectional area can be changed by making use of an adjustable guide vane which surrounds at least a portion of the bearing cone, at a distance from inlet of one half of diffuser height at inlet, the cross-sectional area increase is smaller than 5.0% of the inlet cross-sectional area. This is equivalent to the corresponding two-dimensional straight-wall diffuser angle of, approximately, 2.9 degrees. For the diffuser whose cross-sectional area can be changed as required depending on its inlet flow conditions, the above limit applies for preferably most of the travel path of the adjustable guide vane but at least for the adjustable guide vane position closest to the turbine last blades. The length of the diffuser of this invention, in its preferred embodiment, measured along its mean line, is larger than or at least equal to 90% of the length of last turbine blades. The outer flow guide which defines the outer wall of the diffuser should have radius of curvature at its beginning larger than one half of the length of turbine last blades and should have a horizontal tangent there.