Numerical study on effects of wettability on performances of CO2 ejector

Abstract

Inner wall surface wettability could affect ejector performances, however, no-slip wall boundary conditions were mostly assumed in CO₂ ejector simulation studies. In this study, we introduced velocity slip boundary condition into the 3D heterogeneous mixture model for a two-phase CO₂ ejector, by using different apparent contact angle at the inner wall of mixing chamber, motive nozzle or diffuser. Numerical simulation results show that the inner wall wettability of mixing chamber and diffuser affects flow and mass transfer in a CO₂ ejector significantly, while the inner wall wettability of motive nozzle affects ejector performances very slightly. As apparent contact angle increases from 5° to 175°, resulting in the change of slip condition from negative slip to no-slip then to positive slip, velocity in mixing chamber increases, pressure in mixing chamber decreases, expansion angle after motive nozzle outlet decreases, thus backflow vortices become smaller then disappear. This leads to an increase in suction mass flow rate, and entrainment ratio can be enhanced over 15 %. Correlations between entrainment ratio and apparent contact angle at the wall of different parts in ejector were developed. As the wall wettability changes from hydrophilic to superhydrophobic, the variation trends of ejector performances are similar under different operating conditions with motive nozzle inlet in the supercritical, near-critical, and subcritical regions. This study is helpful to understand flow mechanisms inside the CO₂ ejector with different wall wettability, and provides a new approach to improve the performances of a two-phase CO₂ ejector by changing wall surface wettability.