Handbook of Turbomachinery Second Edition Revised - Ventech!

include not only the pressure change across the turbomachine blading of
interest but also the pressure losses due to flow-path ducting from the inlet
pressure measurement plane to the stage of interest and from the blading
exit plane to the discharge measurement station.
A careful accounting of both stage pressure ratio characteristics and
flow-path total pressure losses must be made. Often flow-path duct losses
are ‘‘doubled-dipped’’ because the analyst fails to recognize that flow-path
duct losses are included in the stage performance map because of the way
the test rig was instrumented. Likewise, losses may be missed should the
analyst not include duct losses that were thought to be included, but were
not, in the stage performance map pressure characteristics.
Radial immersion rakes, similar to the rake configuration seen in Fig.
5, are used to measure turbomachine stage inlet and exit pressures. Total
pressures are measured with impact or kiel sensors mounted on rakes that
are immersed into the flow path. The individual impact sensor tube axes
should be aligned parallel to the incoming flow direction. A choice must be
made by the engineer to select the number of rakes such that the flow field is
adequately defined but the flow path is not blocked nor significantly altered
by the installed rakes.
A generally accepted practice is to install inlet rakes at least a duct
diameter upstream of the test stage. This separation distance minimizes flow
and mechanical interactions due to the rake’s wake on the test blading, while
Figure 5 Radial immersion rake.
Copyright © 2003 Marcel Dekker, Inc.