Numerical Study of Cavitating Behavior in a Cryogenic Liquid Turbine

Abstract

The cryogenic liquid turbine is used in the internal compression air-separation unit to replace the Joule-Thomson valve for the purpose of energy-saving. Evaporation of the liquefied air must be reduced to a minimum to increase gas production of the air-separation unit and simultaneously prevent cavitation damage to the turbine structure during throttling process. In the present study, cavitation behavior in the liquid turbine is investigated and some influential factors are identified. A numerical flow model is established in a turbine stage environment, which includes an asymmetrical volute, variable geometry nozzle, impeller, and diffuser. The simulation is conducted by using the ANSYS-CFX, where an iterative model is incorporated to update the liquefied gas properties at local temperature, and the Rayleigh-Plesset model is used for describing cavitation behavior. The cavitation model is validated first by experimental data of a hydrofoil in liquid nitrogen and its suitability for cryogenic fluid flow has been justified. Both steady and unsteady simulations are conducted respectively and the results are compared. At the design condition, the vapor fraction in the impeller produced by the unsteady simulation is similar to that obtained by the steady simulation. At off-design condition, the turbine flow behavior is far from the steady assumption. The influence of the diffuser duct cone angle on cavitation behavior is also investigated, which demonstrate that geometry modification of diffuser duct is effective to suppress the cavitation flow.