Enhanced direct-contact condensation and entrainment in a three-nozzle eductor for thermal vacuum desalination: experimental and CFD study

Eductors offer a compact and low-maintenance alternative to conventional thermally driven vacuum systems in desalination, which typically rely on energy-intensive condensers and pumps. By employing direct-contact condensation (DCC), eductors can simultaneously generate a vacuum and entrain vapor, enabling simpler and more energy-efficient desalination modules. However, optimization of multi-nozzle geometries for two-phase flows remains limited. This study presents a combined experimental and numerical investigation of single- and three-nozzle eductors, focusing on the effects of inter-nozzle spacing ratio (D n /D) and nozzle splitting ratio (R s ) under two-phase steam–water operation. Three-dimensional ANSYS Fluent simulations were validated against experimental pressure and temperature data at five axial stations, showing RMSE within 2–3%. Results indicate that inter-nozzle spacing significantly affects entrainment and condensation. At D n /D = 0.29, the three-nozzle eductor achieved optimum performance, improving the entrainment ratio by 107–163% compared with the single-nozzle design. Flow visualizations confirmed enhanced vapor entrainment and intensified condensation within inter-nozzle gaps. Lower suction vapor temperature deepened the vacuum and extended the condensation region, while higher back pressure suppressed entrainment. The optimized three-nozzle eductor entrained up to 24 kg/h of vapor, corresponding to ~24 kg·m −2 ·h −1 VMD flux for a 1 m 2 membrane, with specific energy consumption reduced by ~2 kWh·m −3 relative to the single nozzle. These findings establish condensation-driven entrainment, rather than vacuum generation alone, as the dominant vapor entrainment mechanism and demonstrate that optimized multi-nozzle eductors can substantially enhance the performance and compactness of thermal vacuum desalination systems. • Combined experimental and CFD study of a two-phase three-nozzle eductor. • Optimum spacing and split increased entrainment by 107–163%. • Interacting jets intensified vapor–liquid mixing and condensation. • Condensation-driven entrainment sustained stronger vacuum generation. • Achieved ~24 kg m −2 h −1 flux with lower energy use than single nozzle.