An experimental investigation was performed to study the details of flow in the tip clearance gap of a linear turbine blade cascade. The cascade was designed and built to be geometrically similar to the earlier VPI&SU cascade; however, the new cascade also had a tip gap (2.1 percent of blade height) and two endwall boundary layer bleeds upstream of the blade row. The boundary layer bleeds were designed to reduce secondary flow other than the tip gap leakage flow in the cascade, and they performed well. The cascade flow had an exit Reynolds number based on the axial chord of 4.5 x 10⁵.
Static pressure measurements were made on the blades and on the endwall with particular attention given to the tip gap. Also, flow visualizations on the endwall and on the suction surface of the middle blade were performed.
From the pressure measurements, a minimum static pressure coefficient of -6.85 (based on the freestream velocity head) was obtained along the bottom of the blade, near the tip gap inlet. Avena contracta was evident, also in the tip gap entrance region, and a contraction coefficient of 0.61 was calculated from measured data. Mixing occurred after the vena contracta with the static pressure across the tip gap exit being fairly uniform. The flow visualizations showed a separation and reattachment on the endwall under the blade and a tip gap leakage vortex in the passage.
Models of the tip gap flow, based on potential flow theory and potential flow theory with mixing were discussed and developed. Potential flow theory accurately models the unloading along the pressure surface of the blade, and the endwall static pressure distribution of the tip gap, up to the vena contracta. It also predicts a contraction coefficient of 0.61.
The combined potential flow and mixing model accounts for the pressure rise in the tip gap due to mixing. It predicts a minimum static pressure coefficient under the blade of -6.81, which agrees well with measured data. / M.S.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/101324 |
| Date | January 1986 |
| Creators | Tilton, James S. |
| Contributors | Mechanical Engineering |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | xii, 101 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 15256060 |
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