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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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INSTRUMENTATION DESIGN CONSIDERATIONS<br />

This section presents a brief overview <strong>of</strong> the design <strong>of</strong> instrumentation<br />

sensors utilized in gas turbine testing. Volumes <strong>of</strong> information are available<br />

to the test engineer to support sensor selection and design.<br />

Instrumentation Requirements<br />

Selection <strong>of</strong> proper instrumentation is one <strong>of</strong> the most important aspects in<br />

turbomachinery component testing. Of equal importance are the design and<br />

integration <strong>of</strong> the sensors into the test rig. Obviously, one would desire<br />

instrumentation that does not influence or disturb the flow while recording<br />

data. Several test facilities are developing and using nonintrusive measurement<br />

techniques such as laser doppler velocimetry for turbomachinery<br />

testing. However, most turbomachine development test programs still use<br />

pressure and temperature sensors installed on probes and rakes to measure<br />

flow-field performance properties in the test stage.<br />

Instrumentation sensors should be designed to minimize flow<br />

disturbances. Local stream blockage due to probes and rakes should be<br />

minimized by aerodynamically shaping the rake assemblies and probe stems<br />

along with the sensing end <strong>of</strong> the probe assembly. A small frontal area with<br />

a low drag coefficient is desired.<br />

One must also consider the mechanical design <strong>of</strong> the sensing element.<br />

The assembly must be mechanically stiff so that it will not ‘‘flap in the<br />

breeze’’ or fatigue. The probe should be designed such that its natural<br />

frequency does not coincide with frequencies inherent on the test rig. Often,<br />

strain gauges and even miniature accelerometers are installed on the probe<br />

stems to monitor the mechanical health <strong>of</strong> the probe. Probe breakage during<br />

testing can have catastrophic consequences on high-speed turbomachine<br />

blading, flow ducting, and downstream instrumentation.<br />

Rake and probe assemblies are typically installed in the test rig<br />

through openings in the outer flow path. Easy instrumentation accessibility<br />

for inspection, cleaning, and recalibration during the test activity is<br />

important. Individual probes must be able to pass through small openings.<br />

Normally this will be a small circular hole, <strong>of</strong>ten not more than a quarter<br />

inch in diameter. The hole must be large enough to allow the probe stem to<br />

move freely but not so large that it becomes a major leak source during the<br />

test.<br />

Another important consideration in the design and installation <strong>of</strong><br />

sensing elements is its ability to be washed and cleaned during the course <strong>of</strong><br />

the test. Under normal circumstances the sensing elements are clean when<br />

flow checked and calibrated. However, during testing a considerable amount<br />

Copyright © 2003 Marcel Dekker, Inc.

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