Design optimisation of axial flow steam turbines

Abstract

This study at Cranfield University was initiated by my sponsoring company’s desire to improve its Steam Turbine design capability. In particular it was thought that emerging Computational Fluid Dynamics software might provide an alternative and improved approach to existing turbomachinery design techniques in use. My course of study has enabled me to understand the fundamentals of the numerical approaches and methods employed by the commercially available software codes. The brief was to review and if appropriate select and implement a CFD solution into Peter Brotherhood Ltd. The current range of steam turbines have power outputs ranging from 500 kW to 30 MW. These machines are primarily Rateau impulse turbines operating under a pressure compounding arrangement. More recent developments particularly on condensing machines incorporating twisted and tapered blades have led to partial reaction stages being used. Most of the machines produced today are impulse combined with partial reaction. After a review of software vendors AEA TASCflow software was purchased and was used throughout this study. This work concentrates on the technical design o f steam turbine nozzles and blading. It proposes alternative blade and nozzle geometry along with new methods of construction and manufacture. It is recognised however that in order to evaluate nozzle performance it is necessary to consider the downstream blade and thus the performance of the complete turbine stage. Throughout comparisons are made with existing fortran PITCH software described in chapter 1. A literature survey investigated many approaches and factors that can improve turbine efficiency and power output. A selection of these more applicable to the smaller power output designs of turbine produced by Peter Brotherhood Ltd have been investigated to evaluate their merits. These are outlined in chapters 2, 3 and 4. Results from these studies indicate that new complex geometry nozzles when matched to improved blading with improved flow incidence angles, correct axial spacing and casing shroud flaring can lead to stage power increases of over 15%. CFD has provided a much improved insight into the three dimensional aspects and flow phenomena. The introduction of CFD has provided a boost to the design capability and it is used regularly with confidence as a development tool within turbomachinery research.