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Handbook of Turbomachinery Second Edition Revised - Ventech!

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still being sufficiently close to measure representative inlet pressures.<br />

Multiple inlet rakes are typically spaced uniformly around the duct<br />

circumference. Typically, three to four rakes are installed for the inlet<br />

measurement, as illustrated in Fig. 6. Provisions must be made should there<br />

be obstacles such as structural members, struts, and cross shafting in the<br />

inlet flow path. Inlet rakes should not be installed near or directly behind<br />

these objects.<br />

Test-cell ambient conditions can be used as inlet flow conditions if the<br />

test rig includes a well-designed and fabricated inlet bellmouth that is<br />

coupled to the test stage with a ‘‘short,’’ clean, axisymmetric duct. A test rig<br />

configured in this manner does not require inlet rake instrumentation. The<br />

assumption is that there are no significant sources <strong>of</strong> pressure loss through<br />

the bellmouth and ducting into the test rig and that the bellmouth inlet flow<br />

is uniform. Uniform inlet flow can be assumed if the bellmouth lip is located<br />

at least one diameter above the test cell floor and one diameter away from<br />

any obstructions in front <strong>of</strong> the bellmouth. In this case the ambient pressure<br />

is the test rig inlet pressure, with the ambient temperature being the inlet<br />

total temperature.<br />

Stage discharge total pressure radial immersion rakes are normally<br />

located as close as possible to the test stage exit plane. Using the trailing<br />

blade-row chord as a measure, the rakes are normally located less than a<br />

chord length downstream <strong>of</strong> the trailing edge. The actual placement <strong>of</strong> the<br />

rakes in the test rig may depend on many factors, including accessibility and<br />

structure/struts in the flow path.<br />

Figure 6 Typical inlet rake orientation.<br />

Copyright © 2003 Marcel Dekker, Inc.

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