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Handbook of Turbomachinery Second Edition Revised - Ventech!

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simply arithmetically averaging the several measurements in the measurement<br />

plane.<br />

Individual pressure probes inserted into the flow path can also be used<br />

in performance testing. These sensors are usually used in flow-path regions<br />

where it is impractical to install rakes, where flow-path blockage must be<br />

minimized, or at locations in the flow path where isolated measurements are<br />

required.<br />

As an alternative to radial immersion rakes, two-dimensional<br />

traversing actuators, usually controlled by preprogrammed microprocessors,<br />

have been developed to survey the flow path with single-element<br />

sensors. The probe is traversed both radially and tangentially to preselected<br />

positions. Once the probe is in place, data are recorded before the probe<br />

moves to the next position. Measurement locations can be tailored to<br />

maximize measurements in areas <strong>of</strong> interest (such as a wake behind a strut<br />

or blade) while minimizing measurements in areas <strong>of</strong> uniform flow.<br />

Advantages <strong>of</strong> this type <strong>of</strong> measurement system include minimizing flowfield<br />

blockage, minimizing instrumentation-induced flow disturbances, and<br />

eliminating probe-to-probe measurement variations.<br />

Disadvantages include the amount <strong>of</strong> time required to complete a twodimensional<br />

survey. Each measurement requires time for the probe to be<br />

positioned and data to be recorded. In addition, a large amount <strong>of</strong> data can<br />

be acquired that requires complicated data reduction and analysis schemes.<br />

The use <strong>of</strong> two-dimensional traversing mechanisms can also be complicated<br />

by available real estate on the test rig. In some cases, special test hardware<br />

must be designed and integrated onto the test rig to accommodate these<br />

traversing systems.<br />

Static Pressure Measurements<br />

Static pressures are normally measured with flush-mounted taps located on<br />

outer and inner flow paths. Static pressures coupled with measured total<br />

pressures can be used to calculate local flow velocities. In order to do this,<br />

static pressure taps are normally located in the same measurement plane as<br />

the total pressure sensing elements as illustrated in Fig. 9. Typically, static<br />

pressure taps are equally spaced around the circumference on both the inner<br />

and outer flow paths. As shown in Fig. 9, the static taps are located<br />

equidistant between each <strong>of</strong> the rakes. This is done so that the rake<br />

installations will have minimum impact on the static pressure measurements.<br />

Pitot-static pressure probes can be used to measure both total and<br />

static pressures at the same point in the flow field. A typical Pitot-static<br />

probe configuration is seen in Fig. 10. Most Pitot-static probe applications<br />

Copyright © 2003 Marcel Dekker, Inc.

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