Design and Analysis of Expansion Wheel for Low Temperature Helium Turbine

Abstract

Design and development on a kW class indigenous liquid helium plant is in progress in Institute For Plasma Research. The Helium Refrigerator/Liquefier (HRL) needs a turbine as expansion machine to produce cooling effect which is further used for the production of liquid helium. Turboexpander constitutes many parts among which Nozzle, Expansion wheel, diffuser and shaft are taken in consideration and designed at primary stage. Design is carried out using mean line analysis, which considers the flow of fluid in one dimension. This assumption reduces bulky mathematical calculations. The results obtained through this analysis are later validates with the results obtained through CFD analysis results, which considers the three dimensional fluid flow. CFD analysis was carried considering both ideal and real behavior of gas. Cryogenic turboexpander are smaller in size and runs at very high speed about few kHz, which turns into special design consideration such as proper balancing, even a small imbalance in rotating part can cause failure of machine. Because of very small size of expansion wheel nozzle are designed for fixed angle of incidence. Expansion wheel is designed considering backward swept blades. Complex Shape of blade profile, which varies in three dimensions, is modeled in bladegen module, available in Ansys. Diffuser is designed to recover maximum energy of outgoing flow, placed after expansion wheel. Design of shaft is done considering natural frequency, for different diameter and thickness of collar. Latter on work is carried forward for structural analysis of expansion wheel and shaft, using FEA in Ansys. Structural analysis is carried to check whether a maximum deflection, maximum principle stress, maximum shear stress are within limit or not, and it is found that all are within limit and are acceptable. Work is further extended for off nominal design of turboexpander to predict performance of turbine if it does not runs at optimum designed speed. Off nominal performance is tested by varying different running speed, inlet temperature, inlet pressure and outlet pressure.