Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

Dq (deg)
240
160
80
Centrifugal Pumps, Fans and Compressors 239
a 2 = 80∞
1.2 1.6 2.0
FIG. 7.16. Flow path data for parallel-walled radial diffuser (incompressible flow).
when considering the final size of the compressor. Clements and Artt (1988) considered
this and performed a series of experiments aimed at determining the optimum diffuser
channel length to width ratio, L/W. They found that, on the compressor they
tested, increasing L/W beyond 3.7 did not produce any improvement in the performance,
the pressure gradient at that point having reached zero. Another significant result
found by them was that the pressure gradient in the diffuser channel when L/W > 2.13
was no greater than that which could be obtained in a vaneless diffuser. Hence, removing
completely that portion of the diffuser after this point would yield the same pressure
recovery as with the full diffuser.
The number of diffuser vanes can also have a direct bearing on the efficiency and
surge margin of the compressor. It is now widely accepted that surge occurs at higher
flow rates when vaned diffusers are used than when a simple vaneless diffuser design
is adopted. Came and Herbert (1980) quoted an example where a reduction of the
number of diffuser vanes from 29 to 13 caused a significant improvement in the surge
margin. Generally, it is accepted that it is better to have fewer diffuser vanes than
impeller vanes in order to achieve a wide range of surge-free flow.
With several adjacent diffuser passages sharing the gas from one impeller passage,
the uneven velocity distribution from that passage results in alternate diffuser passages
being either starved or choked. This is an unstable situation leading to flow reversal in
the passages and to surge of the compressor. When the number of diffuser passages is
less than the number of impeller passages a more uniform total flow results.
r 3/r 2
70∞
60∞