Handbook of Turbomachinery Second Edition Revised - Ventech!

the compressor system, as well as the pressure ratio being developed. The
slope at the surge point is practically zero for large volumes or high pressure
ratios.
Observe that a positive slope of a constant-speed curve requires a cell
of separated flow to be either developing or growing within a blade row.
Chapter 13 of NASA SP [1] contains a discussion of some of the factors
influencing the slope. However, experience has not yet produced a method
of precisely anticipating the surge point of a given design. At this time, it is
prudent to assume that surge will occur whenever the incidence angle of any
part of any blade row starts to exceed the angle for stall. If part of a blade
row is already stalled, however, the compressor will probably be free from
surging as long as none of the blade rows is either beginning to stall or
experiencing the growth of an established stall zone. This discussion
admittedly lacks the positive directions that one desires, but this is the
current state of the art.
Centrifugal and Mixed-Flow Compressors
Energy Addition and Losses
The very nature of the shape of these compressors precludes the existence of
anything similar to the two-dimensional cascade data that are available for
axial-flow compressor design. The latter four techniques discussed for axialflow
compressors are equally applicable to these units.
The third method of the previous section, corrections for streamline
curvature, is used when calculating facilities are limited. The slip factor is
estimated as previously noted to determine the energy added to the flow.
The effect of off-design incidence angles is approximated by resolving the
relative inlet velocity into two components—one parallel to the convex
surface of a blade at the inlet and the other perpendicular to it. The kinetic
energy of the second component is presumed to be lost. The losses thus
calculated are added to that estimated from the curve of efficiency versus
specific speed. Although this concept is not precise, it does account for the
trends generally observed in both rotors and stators. The compressor map is
prepared by using the calculated increase in enthalpy and the losses to
compute the pressure ratio and efficiency of each stage. The overall
performance is then readily found.
UNSTEADY FLOW
The importance of and interest in this subject are evidenced by Reference
[39]. There are several sources of unsteady flow in compressors. Notice first
Copyright © 2003 Marcel Dekker, Inc.