Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 18 Typical test rig schematic.
ducting. Screens and honeycomb flow straightening sections are used to
remove swirl and total pressure distortion, hence ensuring a uniform inlet
flow into the test rig. The plenum is also a low-velocity region in which
pressure and temperature can accurately be measured. The flow transitions
from the plenum to the test rig with a bellmouth.
Fan and compressor stage operating points are determined by
matching the stage pressure rise to the pressure loss of the test rig discharge
system. The downstream pressure losses are modulated with a variable-flow
resistance generator such as a throttle valve or variable solidity screens
placed over the discharge duct. As the resistance increases, the system
pressure drop increases, forcing the compressor stage to match, and
consequently to operate at a higher pressure ratio.
The variation in downstream system loss with flow rate can be seen by
plotting the pressure loss characteristics on a performance map as shown in
Fig. 19. The pressure loss varies as the square of downstream flow velocity
and consequently the square of flow rate. Different valve settings or screen
solidities result in different loss coefficients, hence defining different loss
characteristics. It is important to realize that the pressure characteristic that
represents the ‘‘open’’ throttle valve or the ‘‘no-screen’’ configuration is the
lowest pressure limit at which the test rig will operate. Should the
performance map require lower pressure ratios at high flows, the test rig
discharge ducting must be tailored to reduce pressure losses.
Constant-speed performance characteristics seen on performance
maps are generated by setting the desired test rig speed with minimum
downstream resistance and recording the performance test data. The test rig
must have the capability to hold fixed speeds very closely, especially for fan
Copyright © 2003 Marcel Dekker, Inc.