An Analysis of the Merits of CFD for the Performance Prediction of a Low Pressure Steam Turbine Radial Diffuser

Abstract

The performance of the last stage and the diffuser of a low pressure steam turbine module are inherently linked. Design alterations in one part must take into consideration the subsequent effects imposed upon the other. Through a linked design system, including both the last stage and the exhaust, the last stage performance can be predicted more accurately and hence further performance improvements are possible. The importance of a valid prediction for the effects of the diffuser upon last stage power and efficiency become even more important when considering retrofit applications. In a retrofit project, a new flow path including modernised blades are installed in an existing older steam turbine casing. In these cases, the exhaust geometry does not lend itself to allowing the last stage to perform to its originally predicted level, but an optimization has to be established for a diffuser design, that links the last stage blade to the exhaust. This paper presents the coupled CFD design system, used by Alstom Power, to highlight the importance of including the 3D exhaust geometry, coupled to the last stage, when conducting last stage performance calculations. The test case is of a typical retrofit application. A comparison with measured test data shows the significance of the fidelity of any CFD system claiming to predict last stage performance, especially when used during the optimisation process of the diffuser.

A Literature Review of Low Pressure Steam Turbine Exhaust Hood and Diffuser Studies

“…This has proven to be the one of the most robust with the methodology, integrated with state of the art CFD codes; still used by a leading industrial supplier over 15 years later [1,14]. The hood inlet is circumferentially divided into segments, shown in Fig.…”

“…Tindell's numerical analysis with representative inlet conditions showed the separation point was dependent upon the operating point of the turbine. Accurate prediction of the separation point is vital, as Beevers observed in 2010 [1], the earlier the separation the less pressure the diffuser can recover. Liu in 2003 [13] investigated the effect of inlet conditions, comparing uniform with distorted inflow, and found separation was only present with nonuniform distributions at inlet to the diffuser.…”

“…The exhaust hood of a steam turbine is a vital area of power plant design, as the performance of the exhaust hood strongly influences the efficiency of the low pressure (LP) turbine [1,2]. The exhaust diffuser decelerates the flow leaving the last stage blades (LSB), converting the source of leaving kinetic energy to a rise in static pressure; a process known as static pressure recovery.…”

The influence of condenser pressure variation and tip leakage on low pressure steam turbine exhaust hood flows

“…Details of the operation point are not published. The data for Beevers [20] is taken from an 10% scale test facility at Alstom Power operating at comparable Mach number to field data. As the published data does not include an x-axis scale, a zero point is assumed for comparision.…”

“…Second order discretization was applied and the turbulence was modelled with the standard k-ε turbulence model with standard wall functions. This turbulence model was selected as it is widely adopted by other researchers [22,20,12]. Ris et al (2009) showed a small difference in the calculated pressure recovery (approximately 5%) with varying turbulence models [23].…”