text: The present invention relates generally to turbine engines and more particularly to methods and apparatus for improving performance of turbine engines that are axially coupled together.
  At least some known power generating systems include at least two turbines coupled axially together via a coupling. More specifically, the turbines are connected such that their rotor shafts rotatably coupled and such that fluid flow exiting a final stage of an upstream turbine enters the first stage of a downstream turbine through a cavity defined between the turbines.
  The cavity formed between the turbines may facilitate undesirable energy losses between the turbines. For example, because the rotating shaft and coupling are exposed to the flow path, as the shaft is rotated, fluid may become entrained and become ejected into flow path in a condition known as windage loss. In addition, undesirable flow separation losses may occur as the fluid contacts the coupling enroute to the downstream turbine. In addition, if an exit annulus of the upstream turbine has a different height or diameter than the entrance annulus of the downstream turbine, additional energy losses may occur as the fluid flow is channeled through the coupling.
  Some known power generation systems supply additional steam to the coupling region. Additional steam is admitted as required by the thermodynamic cycle so as not to affect the coupling losses. However, the introduction of such steam may cause an undesirable disturbance to the fluid flowing through the coupling.
  As such, other known power generation systems include a generally cylindrical coupling cover which overlies the rotating shaft and coupling and has an axis that is generally coincident with the axis of rotation of the turbines. Although the coupling cover facilitates mitigating losses associated with the rotating shaft and coupling, the additional cover also produces energy losses itself, and does not address recovering energy from the flowpath. Additionally, the coupling cover does not provide a means for retrofitting previously commissioned turbines.