Partial admission is applied wildly to regulate the power in turbomachinery. The prominent unsteady characteristic has a great effect on the aerodynamic performance and unit safety. The RNG k-ε turbulence model, dual time step method, sliding interface model and structural hexahedron mesh were adopted and the 3-D unsteady viscous model for the full annulus of the control stage of 300MW steam turbine with admission room and reinforced rib was constructed. The flow phenomena and exciting force were investigated detailed in multiple working conditions. The time-averaged unsteady wheel efficiency is over 3% lower than that from steady analysis with two different degrees of admission. The rotor blades experience large unsteady axial and tangential forces while passing the blocked channel. Spectrum analysis was performed and 50.0Hz is seen as the typical low frequencies. The largest amplitude of unsteady forces under the condition of three valves opened (ξ = 0.70) is at the first multiple rotational frequency due to the circumferential nonuniformities of flow domain, while the largest amplitude under the condition of two valves opened (ξ = 0.47) is at the second multiple rotational frequency due to the circumferential periodicities of flow domain. These provide essential parameters for strength and vibration analysis of blade. The pressure and static entropy plots show that the circumferential distributions are nonuniform and the nonuniformities decrease while moving towards the downstream sections in the two conditions. The entropy is significantly large, which means that the mixing and windage losses are huge resulting in the decrease of the blade power. The pressure drop moves towards the downsteam blade surface while rotor passing into the blocked channel. The pressure is recovering while rotor passing out the blocked channel.

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