## 1 Introduction  
Few engineering and medical applications need evacuated cavities during certain manufacturing and testing processes. These evacuated cavities may contain a variety of gases and liquids. So, the continuous removal of these undesired fluids and maintenance of the required evacuated cavities at a specific pressure becomes mandatory for these applications. The most used devices for venting these gases are categorized as a steam trap, liquid ring, vacuum pumps, manual valve, automatic valve and steam ejector. All these devices consume a considerable amount of energy to accomplish their duties. The use of an inverted U-tube is an innovative method for entraining such gases, which may lead to the consumption of less energy [1]. When water flows up in an inverted U-tube, it produces a low-pressure region at its upper part. This produces a low-pressure region, which can be exploited to draw gases from the required cavity by connecting the top of the inverted U-tube to the cavity through a side pipe. In the present paper, two-phase flow (water and gas) and three components (water liquid, air and water vapor) are considered in the proposed CFD computations. One of the most important pieces of equipment working under vacuum is the steam condenser of the steam power plant. The main function of the steam condenser, besides steam condensation, is to preserve the generated operational vacuum by evacuating the gases in the cycle. Currently, steam power plants share the largest percentage of electricity generation worldwide [2,3]. As commonly known in a condenser, the volume fractions of air, vapor, and water change rapidly, and this complicated behavior has a significant impact on plant efficiency. Water vapor can be taken out of the condenser cavity through condensation and it is pumped back to the water feeding system of the plant boiler. The non-condensable gas is mainly air that has leaked into the plant turbine and condenser, which are working below the atmospheric pressure or it comprises air and non-condensable gases released from the plant working fluid. Furthermore, these non-condensable gases may also be formed by water decomposition. For efficient plant operation, these gases are extracted out of the condenser cavity to avoid increasing the condenser operating pressure. Raising the condenser operating pressure diminishes the steam turbine output power. Additionally, these gases severely reduce the heat transfer rates in the condenser by the covering wrap of its tubes. Furthermore, the oxygen content raises the tube corrosiveness in the condenser, which lowers the condenser's operational life. So, these gases should be evacuated continuously for better operation and longer life of the steam condenser [4, 5, 6]. Few experiments consider the two-phase flow of steam condensation with the presence of gas in vertical tubes [7, 8, 9]. The main finding of these experiments was that raising both the gas mass fraction and the inlet properties reduced the heat transfer rate. Additionally, the local heat transfer was measured in the case of steam only and steam mixed with helium and air. The obtained temperature profiles for the considered cases provided a reasonable way of validating the proposed numerical model. The CFD computations of condensation in the existence of gases encountered some implementing difficulties but the resulted CFD flow profiles helped to investigate the physical mechanisms due to condensation. The effect of non-condensing gases on the steam condenser performance is presented theoretically by [PERSON] et al. [10]. In their study, the steam eject pump system is modeled by monitoring the gas extraction. Optimizing the steam ejector pump geometry was recommended for enhancing plant efficiency [10]. [PERSON] et al. [11] made a comparative study for the power consumptions in the case of using the ejector and electric vacuum pump. They presented an economic analysis and operating costs guidelines for the appropriate selection of the condenser evacuating system in the steam power plant.