Abstract
A simple method for modelling steam turbines using a nozzle analogy was recently published. The method describes both stator and rotor components of a turbine with an appropriate nozzle definition, and makes use of the traditional velocity triangles to determine work output. What makes the method convenient is that it only requires a reasonable estimate of the effective nozzle throat area per blade row, as well as properly chosen loss coefficients. A complete turbine train with multiple stages can easily be solved using a simple 1-D pressure correction method. The method was demonstrated to match detailed inter-stage turbine data quite accurately.
This paper elaborates on the application of the method. It presents a more thorough procedure for establishing the nozzle throat areas using a turbine lay-out drawing, as well as consideration of possible reaction ratio variation between successive stages. Furthermore, the loss coefficients are determined using the methods described by Traupel and Zehner. The paper presents these in a compressed and digitized format for easy implementation in a software code.
A case study is presented to show how the model can quantify improvements to blade profiles. Another case study models a complete boiler feed pump turbine connected to the feedwater pumps and includes the partial arc governing stage and cold re-heat supply. This specific model demonstrated the method’s ability to accurately model a turbine at low load and high back pressure conditions.
Turbine system modelers can apply the method presented here to their own equipment and obtain trustworthy results without the need for detailed profile geometry or complex CFD analyses. The method is however limited to subsonic flow between blade rows, hence some modern transonic last stage blade designs might not be adequately represented.
