Handbook of Turbomachinery Second Edition Revised - Ventech!

There is a good reason for this situation. It is difficult to determine by
tests when the physical area for the flow is just right. Gas will always fill the
space available for it, and the performance of diffusers is best when the flow
through them is on the verge of separating. A value of 0.98 has been used for
the value of B in the first stage and 0.96 for subsequent stages. This is
admittedly crude, and it suggests a fertile area for future research.
A practical review of the various blockages and losses in axial flow
compressors is presented in Koch and Smith [17]. This reference also offers a
good insight into some of the important items a compressor designer should
consider.
CENTRIFUGAL AND MIXED-FLOW COMPRESSORS
Attention must be given to all three directions of flow in the usual
configuration of these compressors (e.g., those shown in Figs. 3and 4). No
experiments have been devised that provide credible correlations about how
blade shapes affect the deviation angles, the optimum incidence angles, or
the losses. Consequently, the initial concepts for the shapes of both rotors
and stators are individually derived for each application.
Specific Speed
A quantity known as specific speed provides a correlation between
compressor efficiency and the energy that can efficiently add to the gas
flow by the compressor. Specific speed is defined by
Ns ¼ðN ffiffiffiffi p
0:75
QÞ=ðDHÞ
ð21Þ
A relation between specific speed and the best attainable compression
efficiency, derived from tests of a number of centrifugal impellers, is shown
by the data points of Fig. 16. (Be aware of the definitions for both Z* and
Q.)
P1 1 P2
Z*
c ¼ DT
T
pffiffiffiffiffiffiffiffiffi
N
Ns ¼
QAV DH :75
g 1
g
QAv ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
p
QINQOUT
The solid line of this figure represents a similar and widely used correlation
Copyright © 2003 Marcel Dekker, Inc.