Two-phase flow characteristics and leakage in the shaft seal of steam screw expanders

Twin-screw expander is an efficient technology for steam pipeline pressure recovery applications. It operates at lower pressures than turbines and exhibits good performance with droplets. Leakage through the suction end face directly reduces the expander's volumetric efficiency, highlighting the significance of research on shaft seals. This study confirms that the Wet Steam Model is more accurate than the Ideal and Real Gas Models in reflecting the phase change phenomena and flow characteristics of steam after throttling. The effects of four key operating parameters were further investigated on the leakage and flow characteristics of the shaft seal, including suction pressure, superheat degree, rotating speed and liquid mass fraction. The results indicate that, within the specified pressure range, droplet condensation occurs after the final throttling when the superheat ranges from 0 to 30 K. Furthermore, leakage decreases by 1.5 % to 1.7 % with every increment of 10 K in superheat. Higher rotating speeds alter the steam flow path due to wall drag, thereby reducing leakage. An optimal liquid mass fraction of 0.1 was found for two-phase operating conditions, resulting in a 3.6 % reduction in leakage and a 20 K decrease in outlet temperature.