Page 91 - Fluid Mechanics and Thermodynamics of Turbomachinery

72 Fluid Mechanics, Thermodynamics of Turbomachinery
tends to produce thick boundary layers and eventual flow separation. Lieblein states
the general hypothesis that in the region of minimum loss, the wake thickness
and consequently the magnitude of the loss in total pressure, is proportional to
the diffusion in velocity on the suction-surface of the blade in that region. The
hypothesis is based on the consideration that the boundary layer on the suction-
surface of conventional compressor blades contributes the largest share of the blade
wake. Therefore, the suction-surface velocity distribution becomes the main factor
in determining the total pressure loss.
Figure 3.15 shows a typical velocity distribution derived from surface pressure
measurements on a compressor cascade blade in the region of minimum loss. The
diffusion in velocity may be expressed as the ratio of maximum suction-surface

FIG. 3.15. Compressor cascade blade surface velocity distribution.

FIG. 3.16. Model variation in velocity in a plane normal to axial direction.