563 / 2019-01-15 09:44:13

Synergy analysis of flow characteristics and loss mechanism in a radial-inflow turbine with shrouded impeller

Radial-inflow turbine, Synergy, Labyrinth seal, Flow resistance, Loss mechanisms

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The radial-inflow turbine with shrouded impeller is commonly employed to avoid the leakage flow in the internal flow field of rotor, especially in the small mass flow rate condition. Nevertheless, the shrouded impeller whose sealing structure would be designed as the labyrinth seal typically could lead to another two losses: work reduction at the impeller inlet and the mixing loss with the passage vortex system at the impeller outlet. Moreover, it’s difficult to measure the aerodynamic parameters by common probes at such narrow gap between the impeller and the labyrinth seal. The lack of experimental data makes it difficult to interpret these losses. In summary, a lack of clarity in the causal relationship between these losses and their drivers still prevents a systematic approach to the design of the sealing structure.
To address these questions, a numerical model of radial turbine coupled with labyrinth seals has been established. The effect of the dimensionless seal clearance, which ranges from 0.43% to 2.98%, and rotating speed, which ranges from 28000 rpm to 48000 rpm at off-designed conditions, are investigated. The physical quantity synergy is innovatively employed to explain the flow characteristics and loss mechanism based on the current numerical data. The understanding gained shows that the synergy angle presents a larger increasing tendency in rotor than that in seal when rotating speed rises, which means the flow resistance is more sensitive to the raise of rotating speed in rotor. The blade loading reducing at the leading edge of rotor blade is due to the leakage flow into the seal, where the surface synergy angle is less than 90°. The zones where the cavity vortex in seal cavities and the passage vortex in rotor passages locate has larger entropy generation rate obviously. These regions move towards the shroud endwall when rotating speed rises, and the synergy angle shows an effective way to indicate the loss cores. Overall, the synergy angle is more effective in characterizing the flow resistance and loss distribution in the turbine.