Effect of Inlet Deflector Angle on the Oil–Water Separation Efficiency of Free Water Knock Out

This study quantitatively investigates how the inclination of the inlet deflector affects oil–water separation efficiency in a Free Water Knock Out (FWKO) vessel through Computational Fluid Dynamics-based multiphase flow analysis. Three different conditions were examined: Case 1 with a vertical wall, Case 2 inclined by 30° toward the inlet, and Case 3 inclined by 30° toward the outlet. The Volume of Fluid (VOF) method was employed to evaluate variations in internal flow distribution and separation performance. Separation efficiency was calculated as the time-averaged mass flow ratio of oil at the outlet during the statistically steady period, and a localized initial-filling analysis near the inlet deflector was conducted to investigate the early-stage flow behavior. The results show that Case 1 produces the best impingement and turbulent mixing, resulting in wide oil dispersion and a separation efficiency of 96.7%. In Case 2, the backward-wall impingement reduces the incoming kinetic energy, forming a thin wall film that alleviates interfacial disturbance and maintains stable stratification, thereby achieving the highest efficiency of 97.8%. In Case 3, the emulsion is rapidly directed downstream, leading to a shorter residence time and persistent mixing zones, which yield the lowest efficiency of 95.6%. The initial-filling analysis supports these trends: Case 2 develops a wide and uniform wall film, whereas Case 3 shows narrow and elongated spreading, consistent with the global separation results.