The aerodynamic design of an axial-flow compressor stage involves definition of the rotor and stator velocity triangles and blade geometry that will produce desired stage performance characteristics. When appropriate, this may include the design of inlet and exit guide vanes as well. In the context used in this book, there are subtle but important differences between stage design and compressor design. Stage design will involve the selection of basic dimensionless performance parameters and the design of appropriate blade rows to produce them. No specific reference is made to the precise application of the stage in an actual compressor. Without knowledge of the specific working fluid, Mach number levels, matching with adjacent stages, etc., stage design is a rather idealized process. It is not possible to establish precise end-wall contours and compute the losses in this context. The purpose of this chapter is to introduce basic dimensionless performance parameters and to discuss their application to the design process. Representative applications of the stage designs will be used to indicate some of the consequences of the choices made. Chapter 11 extends these basic design concepts to the design of blade rows for complete axial-flow compressors with specific operating conditions and working fluids.
The stage-design procedures described in this chapter offer far more than abstract educational value. They certainly can be used to design stages for actual applications within a compressor, although the procedures in chapter 11 are more efficient. They often play a key role in the design of industrial axial-flow compressors. Each industrial axial-flow compressor usually has a unique design. Development and manufacturing costs become primary considerations when there are no duplicate machines to share them. It is fairly common practice to employ a standard repeating stage design for these compressors. Blade stagger angles are commonly adjusted to accommodate the specific applications. Scaling of the blades with corresponding modification of the number of blades per row may be used to satisfy mechanical strength requirements. Since the precise application is not predefined and unique designs for all stages are not acceptable, the design of a standard repeating stage will precisely follow the process described in this chapter. The specific axial-flow compressor design will involve application of this standard stage design with appropriate sizing of the annulus and various minor adjustments to accommodate the application. An example of this use of stage design procedures is included at the end of this chapter.