Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

238 Fluid Mechanics, Thermodynamics of Turbomachinery
Vaned diffusers
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FIG. 7.15. Element of flow path in radial diffuser.
In the vaned diffuser the vanes are used to remove the swirl of the fluid at a higher
rate than is possible by a simple increase in radius, thereby reducing the length of flow
path and diameter. The vaned diffuser is advantageous where small size is important.
There is a clearance between the impeller and vane leading edges amounting to about
0.04D2 for pumps and between 0.1D2 to 0.2D2 for compressors. This space constitutes
a vaneless diffuser and its functions are (i) to reduce the circumferential pressure gradient
at the impeller tip, (ii) to smooth out velocity variations between the impeller tip
and vanes, and (iii) to reduce the Mach number (for compressors) at entry to the vanes.
The flow follows an approximately logarithmic spiral path to the vanes after which
it is constrained by the diffuser channels. For rapid diffusion the axis of the channel is
straight and tangential to the spiral as shown. The passages are generally designed on
the basis of simple channel theory with an equivalent angle of divergence of between
8deg and 12deg to control separation. (See remarks in Chapter 2 on straight-walled
diffuser efficiency.)
In many applications of the centrifugal compressor, size is important and the outside
diameter must be minimised. With a vaned diffuser the channel length can be crucial
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