To improve the operating stability of a cryogenic liquid turbine expander, the influence of flow behavior on rotor radial force and critical speed is investigated numerically. At both design and off-design conditions, unsteady flow simulation is conducted in the entire expander environment where the physical model is constituted by a volute, nozzle, impeller and diffuser duct. The asymmetric flow characteristics are captured at both design and off-design conditions and they are responsible for the significant radial force on rotor. The radial component of flow-induced resultant force is calculated with direct integration approach and it is significant due to apparent asymmetric impeller flow. The influence of such radial component of flow-induced resultant force on the rotor critical speed is further investigated, where the flow-induced radial force is considered as an equivalent mass and superposed on the impeller mass integrity to obtain a resultant mass of impeller, which is used in the prediction of rotor critical speed.

To predict rotor critical speed, a finite element method is developed and incorporated into a FORTRAN code, and it is validated and then used to predict the rotor critical speed with and without consideration of the radial component of flow-induced resultant force respectively. The following is described: at both design condition and off-design conditions, the predicted critical speeds with consideration of flow-induced radial force are significantly below that without flow-induced radial force. The influence of impeller flow behavior on rotor dynamics of the turbine expander is not negligible.

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