Design of low-reaction steam turbine blades

Abstract

Optimizing the aerodynamic design of turbine blades is a compromise between a large number of issues. These can be grouped into three areas:

(a) aerodynamic compromises; e.g. increasing the pitch—chord ratio improves profile loss but worsens secondary loss,

(b) mechanical constraints; e.g. the pitch-chord ratio affects the strength of a profile, which for a given unsteady stress level determines the width and hence strongly influences the secondary loss,

(c) costs; e.g. increasing the number of stages improves performance but also increases the cost of the turbine. It can also affect rotor stability and even the size of the turbine hall.

Some of the issues are difficult to quantify and may vary from day to day. For example, the marginal manufacturing cost of a given design will depend on the load on particular machine tools. Therefore the approach of a manufacturer evolves from experience. However, many other issues can be addressed systematically to achieve near optimum designs. This paper explores the aerodynamic design of low-reaction steam turbine blades and describes the technical arguments that lead to design decisions. Where the decision depends on cost and mechanical constraints these are also explored. A typical low-reaction stage is shown schematically in Fig. 1. The paper will concentrate on the design of short and intermediate height blades typically used in HP and IP cylinders and in the early stages of LP cylinders. In practice, long blades typically used in the later stages of LP cylinders are fairly similar for both ‘reaction’ and ‘impulse’ design manufacturers.