6 Preliminary Aerodynamic Design of Axial-Flow Turbine Stages

The aerodynamic design of an axial-flow turbine stage can be conveniently accomplished in a series of well-defined steps. First a preliminary design is accomplished to define the basic stage characteristics such as the stage flow coefficient, work coefficient, reaction and nozzle exit swirl vortex type. In this phase, standard efficiency chart estimates are used to approximate the stage performance. Flow profiles are computed using the simple normal equilibrium model to identify the fluid turning required in the nozzle and rotor to achieve the desired performance. When a preliminary design is configured, its geometry is exported to an input file for the performance analysis of chapter 5 for a more thorough evaluation of its performance characteristics. Some iteration will normally be necessary to obtain a preliminary design that yields satisfactory performance. Then the basic blade characteristics matching the fluid turning requirements are exported from the preliminary design system to the detailed airfoil design system of chapter 7 to design a series of constant-radius airfoil sections and stack them to define the nozzle and rotor blades. The detailed geometry of these blades is exported from the airfoil design system to update the input file for the performance analysis to accomplish a more precise performance analysis.