Numerical study on direct contact condensation of countercurrent subcooled water-steam flow in a steam-filled vertical rectangular narrow channel

A two-dimensional computational fluid dynamics (CFD) model was developed to investigate direct contact condensation (DCC) during counter-current flow of subcooled water and steam in a steam-filled vertical rectangular narrow channel. The model incorporates the Volume of Fluid (VOF) method, Lee phase-change model, Continuum Surface Force (CSF) model, and RNG k–ε turbulence model. The effects of subcooled-water and steam inlet velocities on flow evolution, pressure response, and counter-current flow limitation (CCFL) were systematically analyzed. The results show that the channel pressure exhibits pronounced periodic pulsations. Increasing the inlet velocities of both phases strengthens interfacial shear and flow disturbances, thereby intensifying interfacial instability, promoting condensation-induced water hammer (CIWH), and increasing the pressure oscillation intensity. The maximum pressure is mainly controlled by the subcooled-water inlet velocity and increases monotonically with it, while steam injection markedly amplifies the pressure peak compared with single-phase liquid injection. The average pressure increases with either inlet velocity, with the liquid velocity exerting a stronger effect. Flooding and liquid-bridge formation are observed during counter-current flow, triggering CCFL. As the subcooled-water-to-steam velocity ratio increases, CCFL weakens, whereas higher steam velocities lead to earlier liquid-bridge formation.