Handbook of Turbomachinery Second Edition Revised - Ventech!

tests. One should take pains to provide compressors with uniform flow rates
and properties at the inlet during these tests.
If compressor specifications require it to operate with gradients in the
flow at the inlet, this ability can be verified by separate tests. Usually,
specified deviations from uniform flow can be established with the use of
overlapping screens upstream of the inlet. Under some conditions, however,
ingenuity is required to effect suitable tests.
Instrumentation
The object of overall tests is to determine how the rotating speed and flow
rate affect the pressure ratio, the temperature rise, and the position of the
surge line. A number of instruments are available for sensing and measuring
rotating speed, flow rate, pressures, temperatures, and velocities, as well as
variations of the last four items with time. Torque measurements are useful
for confirming the amount of energy added by the rotors: the effect of any
backflow over rotor blade tips is then properly identified as an increase in
entropy instead of a transfer of mechanical energy.
The location of the sensing elements deserves careful study. Figure 18
illustrates a typical problem. This figure shows how the total pressure, as
measured behind the last blade row of a compressor, can vary throughout
the annulus. This result was observed even though uniform flow was
presented to the compressor inlet. Measurements of total temperature often
exhibit similar trends. Experience shows that this type of distribution can
change with flow rate and speed. The section on energy degradation noted
how this flow is subsequently mixed as it moves downstream. The mass flow
rate, the power, and the momentum of the gas are preserved during mixing,
while the available total pressure is reduced. Many data points can be
required to obtain the information necessary to accurately evaluate the
available total pressure or the true total temperature.
Whenever possible, the data should be taken far enough downstream
so that the flow is sufficiently uniform that the effects of subsequent mixing
can be ignored. In this case the data at the measuring points can be weighted
either by the local mass flow rate or by volume flow rate. The result is
practically the same.
When gradients in flow are inevitable at the measuring station, some
method must be used to get meaningful averages. Some measure of the
statistical variance is also required. Previous discussions have demonstrated
that both radial and circumferential gradients in pressure may have been the
result of the design and inlet conditions. They are not random in a statistical
sense. The method of averaging must recognize this situation, and those
Copyright © 2003 Marcel Dekker, Inc.