Investigation of flow-induced radial force and its influence on rotor dynamics for a cryogenic liquid turbine expander

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 flowinduced 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 flowinduced 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 flowinduced 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.