The design of the radial exhaust hood of a low pressure (LP) steam turbine has a strong impact on the overall performance of the steam turbine. A higher pressure recovery of the diffuser will lead to a substantial higher power output of the turbine. One of the most critical aspects in the design of such devices is the steam guide, which guides the flow near the shroud from axial to radial direction and has a high impact on the pressure recovery.
This paper presents an extension of a design activity previously reported for the design optimization of the steam guide of a steam turbine for industrial power generation and mechanical drive of centrifugal compressors. Whereas this previous work only focused at peak efficiency, this paper will look into the off-design aspect. Peak performance, as usually used as design criteria, will now be replaced by proper off-design criteria guaranteeing a high performance level at both design and off-design conditions. On the basis of these considerations a multi-objective optimization of the steam guide has been performed keeping the exhaust outer casing unchanged.
The maximization of the objective functions is achieved by means of a numerical optimization method that uses a metamodel assisted differential evolution algorithm in combination with a CFD solver. The profile of the steam guide is parameterized by a Bezier curve. This allows for a wide variety of shapes respecting the manufacturability constraints of the design. The pressure recovery coefficient is computed over a wide operating range through several RANS computations including the last stage but introducing a mixing plane between the rotating blade and the diffuser inlet to reduce the computational burden. Steam tables are used for the accurate prediction of the steam properties.
Finally, the optimized design is analyzed by a frozen rotor computation to validate the approach. Also off-design characteristics of the optimized diffuser are shown.