14 Quasi-Three-Dimensional Blade Passage Flow Field Analysis

A quasi-three-dimension flow analysis employs two-dimensional flow analyses in the hub-to-shroud and blade-to-blade surfaces to approximate the three-dimensional flow in a blade passage. The fundamental concept is generally credited to Wu [26] and was introduced previously in chapter 3, section 3.3. The specific analysis described in this chapter is widely used for centrifugal compressor design and analysis [1]. It is also employed for axial-flow compressor design and analysis [2], but in a more limited role. Its primary role for turbine design and analysis is to support the detailed aerodynamic design of radial-inflow turbine impellers as described in chapter 11, section 11.9. It has also been used effectively to design nozzle blades for radial-inflow turbines. But the detailed nozzle blade design procedure presented in chapter 11 has largely replaced it for that application.

The present analysis achieves exceptional computational speed and reliability largely due to its use of the linearized blade-to-blade flow analysis described in chapter 13, section 13.4. But that also imposes some limitations on the method that are particularly significant for turbines. Its limitation to subsonic or low transonic Mach number levels excludes a number of turbine applications. As noted in chapter 13, its accuracy is compromised when it is applied to the rather thick airfoils often used for turbines. It certainly could be extended for more general use on turbines by substituting a more general blade-to-blade flow analysis such as the time-marching method of chapter 13. But that would substantially increase the computation time required and significantly reduce its reliability. It is very doubtful that this quasi-three-dimensional flow analysis would remain an attractive design tool if that were done. Indeed, it would lose most of its advantages over commercially available viscous computational fluid dynamic (CFD) codes while offering a less general solution.