This paper presents a Computational Fluid Dynamic (CFD) simulation for dry air/water-liquid and two-phase flow mixing in a vertical inverted U-tube using the mixture multiphase and turbulence models. This study is to investigate the flow behaviors and underlying some physical mechanisms encountered in dry air/water-liquid flow in the inverted U-tube. Water flows through the inverted U-tube while the dry air is entrained using the side-tube installed after the water flow downward. The inverted U-tube is tested at water mass flow rates of 2,4,6 and 8 kg/s, air mass flow rates, 0.000614–0.02292 kg/s, with dry air volume fractions 0.2–0.9. The obtained results are compared with the experimental data for model validation and the present CFD model is able to give an acceptable agreement. Also, the results show that, at water mass flow rate of 2 kg/s, there are vortices and turbulent intensity disturbances are noticed at the inverted U-tube higher part, which refers to an air entrainment occurrence from the side-tube. Theses disturbances starts to be stabilized at air mass flow rate around 0.00736 kg/s and air volume fraction, αa = 0.75. This means, if the air mass flow rate increases above this limit, the air entrainment may be blocked. On the other side, at water mass flow rate of 4 kg/s, there are little noticed disturbances until air mass flow rate of 0.00368 kg/s and αa = 0.43 and thereafter stabilized. After this point for water mass flow rate of 4 kg/s, increasing air mass flow rate may block the water flow and the whole inverted U-tube system possible stop flowing. Therefore, this study is able to estimate the required operational conditions and mass ratios for stable air entrainment process. Beyond these operational conditions, air entrainment may be blocked and the whole system discontinues its normal induced gravitational flow. In addition, this study proves that the inverted U-tube is able to generate a vacuum pressure up to 53.382 kPa based on the present geometrical configuration. This generated low-pressure by the inverted U-tube can be used for engineering applications which are working under vacuum and need continuous evacuating form the dry air and non-condensable gases. Furthermore, these findings motivate the utilizing of inverted U-tube for the air evacuation purposes for less power consuming in power plants.