Analytical Approach to Steam Turbine Heat Transfer in a Combined Cycle Power Plant

Abstract

Combined cycle units have become very popular in recent years as a source of power generation. Such units have a gas turbine as the topping cycle and a steam turbine as the bottoming cycle and can reach combined cycle efficiencies as high as 60%. The exhaust from the gas turbine is passed through a heat exchanger in which steam is generated for the steam turbine. This combined arrangement makes it less polluting as well. An important element of a combined cycle power plant is the steam turbine, which is the subject of this paper. Improvements to the design of advanced steam turbines require an improved understanding of the heat transfer within the various components of the unit. Physics-based ANSYS models for typical GE high pressure and intermediate pressure units have been developed. Components such as the rotor, diaphragm, and shells have been analyzed. The boundary conditions were derived from full-load, steady state flow analyses, steam turbine performance code outputs and computational fluid dynamics (CFD) analyses to develop normalized (non-dimensional) local flow conditions, with the normalizing parameters based on key cycle parameters. These normalized local flow conditions and cycle parameters were then used to define local transient boundary temperatures and heat transfer coefficients for input to the thermal ANSYS models. Transient analyses of components were performed. The results were compared with temperature measurements taken during the complete cycle of an operational steam turbine to validate and improve the methodology, and were applied to structural models of the components to predict their thermal growth and the net impact on the clearance between the rotor and diaphragms and other secondary flow paths in the steam turbine, including the packing seals. This paper will focus on the thermal modeling of a typical steam turbine. It will discuss the process used (summarized above) and the basic equations employed in the analyses. Results will be compared with shell temperature measurements obtained during the start up of a steam turbine in the field. Implications of the thermal results on power systems operation will be discussed. Plans for future improvements will be presented.