Numerical study on two-phase expansion performance and quantitative analysis of wetness loss in cryogenic turbo-expander

Abstract

Cryogenic two-phase turbo-expander becomes a prevalent option in process design of high efficiency cryogenic liquefaction system, especially in modern air separation system and helium system. Computational code of turbine off-design performance prediction in overheated expansion and non-equilibrium condensation module are well established in our former works, and the mechanism of nucleation and droplet growth in spontaneous condensation is revealed. However, the analysis between single and two-phase expansion as well as wetness influence on turbine performance is not conducted yet. In this study, we investigate a turbo-expander performance analysis in a wide temperature range (Tin = 118 K ~ 98 K), and present the wetness loss assessment and quantitative calculation in two-phase expansion process. By referring to the previous off-design performance of overheated expansion, the two-phase expansion performance under different pressure ratio and rotational speed is obtained and compared. According to the continuous inlet temperature drop process, the expansion is categorized into three types and their zone boundaries are divided. The transitional feature of unexpected change is frequently figured out on the expansion zone boundary, and the reaction ratio and pressure ratio in diffuser during the continuous inlet temperature drop process are analyzed detailed, which shows the unique and significant role of wetness effect in two-phase expansion. Based on the experimental wetness loss assessment method established by Baumann in wet steam turbine, two numerical assessments by using overheated expansion and equilibrium condensation model as reference expansion are analyzed. To quantitatively analyze the wetness loss influence, the efficiency drop caused by wetness and the relative wetness loss are calculated by eliminating other component losses in turbine, and the relative wetness loss is fitted with outlet liquid mass fraction mathematically to assist future design of cryogenic two-phase turbo-expanders.