Effect of Orifice Geometry and Air-Inlet Position on Air–Water Entrainment: A CFD Study

Orifice-induced air entrainment plays an important role in hydraulic aeration, mixing systems, and energy dissipation devices. However, the combined influence of orifice geometry and air-inlet location on air–water entrainment characteristics remains insufficiently understood. Therefore, this study presents a combined experimental and computational investigation of air–water discharge through circular, square, and triangular sharp-edged orifices with different opening dimensions and air-inlet configurations. Approximately 120 numerical and experimental cases are examined under a wide range of Reynolds numbers. A three-dimensional CFD model is developed in ANSYS Fluent using the Volume of Fluid (VOF) multiphase approach coupled with the realizable k–ε turbulence model. The numerical predictions are validated against experimental measurements, showing good agreement with most deviations remaining within a few percent. The results show that reducing the orifice opening enhances the normalized discharge ratio R = (Qa Qw ) by increasing jet acceleration, pressure reduction, and suction near the vena contracta. Considering the circular orifice as baseline design, triangular orifices achieved the highest entrainment performance, with normalized ratios R∗ = 1.239–2.189, corresponding to improvements of 23.9–118.9% over the baseline circular case. Square orifices showed moderate enhancement, with R∗ = 0.814–1.322 and a maximum improvement of 32.2%. Forward shifting the air inlet in circular orifices produced R∗ = 0.520–1.099, indicating that inlet relocation can improve entrainment by up to 9.9%.