Design optimization of the helium turbines intended to operate close to the saturation curve.

The design of cryogenic turbines intended for the expansion of gas near saturation curve has its own specifics. As for helium at temperatures below ca 10 K, there is an interesting h-s diagram region significantly influenced by real gas behaviour. Due to substantial changes of compressibility coefficient along the expansion curves in alternative operational modes, the flow density deviations in decisive flow cross sections can cause important changes in internal parameters of the turbine stage. This is associated with a resulting negative impact on turbine isentropic efficiency, turbine mass flow and cooling power. The topic of design optimization when the turbine is to be operated in different operation modes was investigated by computational way and internal parameters of the turbine stage were analysed. The essential results of these analyses are presented here. The possibilities to eliminate the consequences of non-optimal choice of the design regime with respect to the properties of the reaction radial turbine stage and to the regulation potential of the turbines equipped with eddy current brake were also investigated. Some conclusions and recommendations useful for designers of the helium liquefiers/refrigerators are presented in general.