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Handbook of Turbomachinery Second Edition Revised - Ventech!

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The circumferential placement <strong>of</strong> the exit rakes must be carefully<br />

selected. The flow field immediately downstream <strong>of</strong> a stator vane blade row<br />

is characterized by periodic blade wakes that mix out while propagating<br />

downstream. If the measurement rakes are placed far enough downstream,<br />

the wakes are essentially mixed and the flow is relatively uniform. Should<br />

this be the case, the circumferential placement <strong>of</strong> the rakes is not critical.<br />

For most test applications, the rakes are placed relatively close to the<br />

stage exit. Here the flow is nonuniform because <strong>of</strong> the wakes generated by<br />

the test stage blading. Should all the sensing elements be located in wakefree<br />

flow-field regions, the integrated pressures will be high. Likewise, should<br />

all the sensors be located in the wake, the integrated pressures will be low. In<br />

order to get a more representative pressure average, the positions <strong>of</strong> the<br />

individual rakes are indexed equally across the blade passage so that the<br />

different wake flow properties are equally measured.<br />

An example <strong>of</strong> how this might be done is illustrated in Fig. 7. Here we<br />

have arbitrarily selected a stage exit plane that includes 11 stator blades<br />

spaced 32.78 apart and 4 individual immersion rakes to measure the total<br />

pressure. As seen, the four rakes have been located approximately 908 apart.<br />

Rake positioning is accomplished by indexing each rake a specific<br />

circumferential distance from a fixed reference point such as a stator blade<br />

Figure 7 Exit rake placement.<br />

Copyright © 2003 Marcel Dekker, Inc.

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