低温透平膨胀机内非平衡自发凝结两相流动的数值研究

In view of 3-D two-phase flow with spontaneous condensation in cryogenic turbo-expanders, a mathematical model of non-equilibrium condensation is employed to predict the condensation nucleation and droplet growth in gas repaid expansion, where the nucleation model corrected for non-isothermal effect and Gyarmathy droplet growth model are considered. Following the analysis for simulation results, the effects of inlet superheat degree and rotation on the spontaneous condensation in impeller flow passage are investigated, and the distributions of droplet number density, droplet diameter and wetness are obtained. A higher inlet superheat degree results in that repaid nucleation occurs in guiding section and consequently the process develops slowly, thus the gas remains in a higher supercooled state and the nucleation region is enlarged. Because of the interaction of condensation and secondary flow in impeller flow passage, the adverse temperature gradient causes an additional thermodynamic loss. Moreover, the strength of adverse pressure temperature gradient promotes the separation of boundary layer and further causes additional flow loss. For radial and mixed-flow turbines, the small droplets very likely gather and form bigger ones in the secondary flow region, wall of suction side and wake flow region, which leads to secondary nucleation, secondary droplets on a large-scale and the further additional mechanical loss.