Numerical study of cavitating flow in two-phase lng expander

LNG expander is developed and used as a replacement of a J-T valve in liquefaction process of natural gas to reduce significantly the energy consumption in the LNG plant. Similar to conventional hydraulic turbines, the unexpected cavitation also occurs in the LNG expander. In the present study, cavitating flow in two-phase LNG expander is investigated. With the justified Rayleigh-Plesset cavitation model, cavitating flow characteristics is investigated for the LNG expander in the entire stage environment including an annular bend, nozzle ring, and radial inflow impeller. On the basis of cavitating flow analysis, a coaxial rotating exducer is developed and fitted downstream to the impeller, so as to reduce the cavitation in impeller and subsequently prevent impeller damage. The following are demonstrated: (1) without exducer, significant cavitating flow is encountered at the impeller trailing edge and also in half streamline-wise region, and they are resulted from the viscous dissipation and flow separation; (3) with exducer, the impeller cavitation has diminished entirely but it has occurred in the successive exducer; (3) a use of exducer enhances the energy conversion capability of the rotors, but reduces the overall temperature drop and efficiency of the expander; (4) the design optimization of exducer is required to suppress the exducer cavitation, which also needs to be incorporated with the impeller design to achieve a better match between rotor/stator, so as to maximize the design optimization benefits.