Numerical study of condensation heat transfer performance and liquid film distribution characteristics in small-scale helium liquefiers

At present, the liquefaction rate of small-scale helium liquefiers using 4 K cryocoolers is generally low. Helium condensation, the most critical heat transfer process in the system, is seldomly studied, and the design of condenser lacks accurate theoretical guidance. In this study, a CFD model for the film condensation of helium on the surfaces of condensers in small-scale helium liquefiers was established and verified. Compared with cryogenic fluids such as nitrogen, in the condensation of helium, the temperature difference and thermal resistance between the gas-liquid interface and saturated vapor cannot be ignored under small condensation temperature differences. On different surfaces of a cylindrical condenser, the thickness of liquid film on the vertical surface is lower than those on the horizontal surfaces, and its heat transfer coefficient is 1–3 times than those of horizontal surfaces. When the cooling capacity increases from 1 W to 2 W at 4.21 K, with the increase of the condensation temperature difference and thermal resistance brought by liquid film, the average heat transfer coefficient of condenser decreases from 530 W/(m2 K) to 446 W/(m2 K), the condensation efficiency decreases by 11.7 %. This study aims to provide theoretical guidance for the optimal design of the helium condensers in small-scale helium liquefiers.