Virtual nozzle phenomenon caused by separation bubble during CO₂ capture

The current work establishes and validates a CFD model to depict the mass and heat transfer of CO₂ in non-equilibrium environment and the virtual nozzle phenomenon caused by separation bubble. The influence of virtual nozzle effect on the mass and heat transfer are discussed, and the factors affecting the virtual nozzle effect are analyzed. The results show that the rapid expansion in the Laval nozzle causes the system to be in a non-equilibrium state, resulting in the spontaneous condensation of CO₂. The peak nucleation rate is as high as 3.5 × 10²¹m⁻³s⁻¹, and the axial nucleation range is about 0.01 m. The separation bubble in the nozzle reduces the flow area of main stream and trigger the virtual nozzle phenomenon. The interaction between the fluid in the virtual nozzle area and in the mainstream area triggers the exchange of momentum and energy, leading to an increase in static temperature and droplet evaporation in the virtual nozzle area. Increasing backpressure or wall roughness weakens the virtual nozzle effect. When the backpressure increases by 0.6 MPa, the length of virtual nozzle decreases from 16.66 mm to 1.28 mm; When the wall roughness increases by 0.02 mm, the length of virtual nozzle decreases from 10.25 mm to 7.91 mm.