EFFECT OF IMPELLER BLADE SHAPE ON PERFORMANCE

The following types of blade shapes are used for impellers of centrifugal compressors:

1. Backward-curved blades (β₂ < 90°)
2. Radial blades (β₂ = 90°)
3. Forward-curved blades (β₂ > 90°).

The characteristics of these blades are shown in Fig. 14.8. Centrifugal effects on the curved blades create a bending moment and produce increased stresses which limit the maximum speed at which the impeller can run.

Slightly backward-curved impeller blades (i.e., β₂ < 90°) give optimum efficiency because the degree of reaction increases with decreasing blade tip angles. With increase in degree of reaction, the part of kinetic energy that is transferred into pressure energy within the impeller is also increased. By selecting very small tip blade angles β₂, the length of impeller channel increases and consequently the friction loss increases considerably, thereby reducing the efficiency. The optimum blade angle is about 45°.

Figure 14.8 Characteristics of blades

Radial blades are used for aircraft centrifugal compressors because they can be manufactured easily, have the lowest centrifugal stresses, are free from bending stresses, and have equal energy conversion in impeller and diffuser, giving higher pressure ratio and efficiency.