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Improving Understanding of Turbine Shroud Losses
Posted Tue June 04, 2002 @02:26PM
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Application by Paul Roach, ALSTOM Power UK Ltd.

A world-leading supplier to the energy market, ALSTOM offers its customers a comprehensive range of power generation solutions from turnkey power plants to gas-, steam- and hydro-turbines, generators and boilers. Gas turbine design and manufacture has been undertaken at Alstom's Lincoln site since the late 1940ís, with engine ratings up to the 13MW Cyclone introduced in 1997.

To help maximize powerplant efficiency, the Company uses state-of-the-art CFD methods to design compressor and turbine blading. In addition to in-house software, ALSTOM has also been making use of CFX-TASCflow in the last few years. To date, its main use in the Turbine Department has been to examine secondary and parasitic gas flows, such as can occur in blade cooling passages.

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In a recent study, we have used CFX-TASCflow to examine leakage flows in shrouded turbine blade passages. We have modeled the gas flow through the complete stage, including the shroud, disc cavities and associated leakage gaps. The operating conditions for the modeled stage were a stagnation pressure ratio of 4.0, with exit Mach numbers from both blade rows of about 1.1. Using CFX-TASCflow, we could appreciate the full impact of the shroud leakages, and understand how their domination of the main flow features immediately downstream of the stage means that they cannot be neglected. Flow patterns through the shroud itself were also complex and highly three-dimensional. Again CFX-TASCflow revealed significant effects, with the small tangential gaps running the full length of the shroud potentially causing very strong temperature gradients in the shroud metal.

Comparison of the results with available experimental data for the performance characteristics showed that the inclusion of shroud and other leakages is essential for accurate prediction of the flow field. CFX-TASCflow is helping us to understand these complex shroud cavity flows, and improve future turbine designs.

flow pathlines

flow pathlines
Highly three-dimensional and complex flow distribution through the axial gaps upstream and downstream of the shroud, with a gap running the full length of the shroud. Surface contours are of static temperature, the yellow streaklines originate in the inlet plane of the upstream axial gap, the blue streaklines in the tangential gap.

flow pathlines

flow pathlines
3-D views of the mainstream flows, without shroud leakage. These show contours of entropy on the blade surfaces and in the wakes, with streaklines introduced at the stator/rotor interface very near the hub wall. Clearly visible are the locations of the strong shock waves on the suction surfaces, and the rotor wakes downstream. Secondary flows are evidenced by the larger "blobs" of entropy in the wakes, also highlighted by the streaklines. This shows that the shroud leakage clearly dominates the flow features immediately downstream of the stage.

Variation of Mach number
Numerical and experimental radial variation of Mach number showing the importance of leakage flows.

Predicted isentropic efficiency
When all of the shroud leakages are included, predicted stage isentropic efficiencies are in close agreement with experiments.

The author gratefully acknowledges the financial support from the EU Framework 5 Programme (ENK5-CT2000-00065).

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