The design of the radial exhaust hood of a low pressure (LP) steam turbine has a strong impact on the overall performance of the LP 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 diffuser design 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 a method for the design optimization of the steam guide of a steam turbine for industrial power generation and mechanical drive of centrifugal compressors. This development is in the frame of a continuous effort in GE Oil and Gas to develop more efficient steam turbines. An existing baseline exhaust and steam guide design is first analyzed together with the last LP turbine stage with a frozen rotor full 3D Computational Fluid Dynamics (CFD) calculation. The numerical prediction is compared to available steam test turbine data. The new exhaust box and a first attempt new steam guide design are then first analyzed by a CFD computation. The diffuser inlet boundary conditions are extracted from this simulation and used for improving the design of the steam guide. The maximization of the pressure recovery is achieved by means of a numerical optimization method that uses a metamodel assisted differential evolution algorithm in combination with a 3D 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. In the design phase it is mandatory to achieve accurate results in terms of performance differences in a reasonable time. The pressure recovery coefficient is therefore computed through the 3D CFD solver excluding the last stage, 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.

Internal volutes have a constant outer radius, slightly larger than the diffuser exit radius, and the circumferential increase of the cross section is accommodated by a decrease of the inner radius. They allow the design of compact radial compressors and hence are very attractive for turbochargers and high-pressure pipeline compressors, where small housing diameters have a favorable impact on weight and cost. Internal volutes, however, have higher losses and lower pressure rise than external ones, in which the center of the cross sections is located at a larger radius than the diffuser exit. This paper focuses on the improvement of the internal volute performance by taking into account the interaction between the diffuser and the volute. Two alternative configurations with enhanced aerodynamic performance are presented. The first one features a novel, nonaxisymmetric diffuser̸internal volute combination. It demonstrates an increased pressure ratio and lower loss over most of the operating range at all rotational speeds compared with a symmetric diffuser̸internal volute combination. The circumferential pressure distortion at off design operation is slightly larger than in the original configuration with a concentric vaneless diffuser. Alternatively, a parallel-walled diffuser with low-solidity vanes and an internal volute allows a reduction of the unsteady load on the impeller and an improved performance, approaching that of a vaneless concentric diffuser with a large external volute.

Internal volutes have a constant outer radius, slightly larger than the diffuser exit radius, and the circumferential increase of the cross section is accommodated by a decrease of the inner radius. They allow the design of compact radial compressors and hence are very attractive for turbochargers and high-pressure pipeline compressors where small housing diameters have a favorable impact on weight and cost. Internal volutes, however, have higher losses and lower pressure rise than external ones in which the center of the cross sections is located at a larger radius than the diffuser exit. This paper focuses on the improvement of the internal volute performance by taking into account the interaction between the diffuser and the volute. Two alternative configurations with enhanced aerodynamic performance are presented. A first one features a novel, non-axi-symmetric diffuser/internal volute combination. It demonstrates an increased pressure ratio and lower loss over most of the operating range at all rotational speeds. The circumferential pressure distortion at off design operation is slightly larger than in the original configuration with a concentric vaneless diffuser. Alternatively, a parallel-walled Low-Solidity Diffuser (LSD) with an internal volute allows a reduction of the unsteady load on the impeller and an improved performance close to the one of a vaneless concentric diffuser with a large external volute.

This paper reports on an experimental and numerical study at low Reynolds number in order to evaluate the influence of the Coriolis forces on the flow in radial rotating channels. Operating conditions correspond to the flow in radial impellers for micro gasturbine applications. A comparison of detailed flow measurements with CFD results indicates that Navier Stokes solvers with standard k-ω and SST turbulence models predict the flow surprisingly well and that no extra corrections for Coriolis forces are required at these operating conditions

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.