11 Multistage Axial-Flow Compressor Aerodynamic Design

The basic concepts introduced in chapter 10 to design an ideal axial-flow compressor stage are easily extended to the design of a complete axial-flow compressor. When the specific working fluid, rotation speed, mass flow rate and inlet thermodynamic conditions are known, the design process can include the influence of total pressure loss and Mach number levels. This is possible as long as the design is restricted to blade operating conditions reasonably close to the design incidence angles, i*, such that the blades operate in the low-loss incidence angle range where loss coefficients are reasonably constant. This is normal practice at the compressor's design point, i.e., for the mass flow rate and speed condition at which the stages are to be well matched throughout the compressor. An axial-flow compressor design system of this type is unlikely to produce all of the aerodynamic and structural design features desired and all of the off-design operating characteristics required. But it can produce an excellent preliminary configuration to be fine-tuned using the performance analysis of Chapter 9.

The same procedures can be used to design individual stages or blade rows intended for specific applications. For example, it might be used to design replacements for blade rows that are too badly damaged to be reverse-engineered. When this type of design system is available, the benefits of the ideal stage design system of Chapter 10 may seem questionable. Indeed, if sufficient care is taken, this more rigorous design approach may be used in place of the ideal stage design system to generate a standard industrial compressor stage design. It is necessary to avoid imposing restrictions that may preclude using the standard stage for intended applications different from the design conditions used. The procedures defined in this chapter size the compressor annulus to match the specific operating conditions. This imposes a specific stream surface pattern and limits the span of the blades designed to whatever is required to conserve mass. The influence of end-wall boundary layers is also significantly affected by the specific design conditions. If this system is applied at the lowest Mach number level expected and at a higher volume flow rate than is expected to be required, a stage design may be sufficient for general use as a standard stage. But that typically reduces the design procedure to a more complicated version of the methods described in Chapter 10, effectively nullifying the benefits of the more rigorous models used. Indeed, the design methods of Chapter 10 are often useful for exploring alternative stage performance parameters before attempting a design using the methods of this chapter.