Hydrodynamic characteristics and capillary-assisted heat transfer enhancement of non-condensing/condensing two-phase flows
Interfacial characteristics of two-phase flows were studied through visualization experiments and numerical simulation using computational fluid dynamics (CFD) based on the volume-of-fluid (VOF)-continuum surface force (CSF) method. An experimentally-validated analytical method was also presented for the geometrical correction of the optically-distorted objects in cylindrical tubes that is applicable to geometrical measurements (e.g., liquid-gas interfaces, solid particles, gas bubbles, void fraction) inside the tubes. The numerically-simulated two-phase flows agreed favorably with the visually-observed flows. The simulation of two-phase flows under reduced gravities indicated the important contribution of gravity on hydrodynamics of intermediate scale two-phase flows such as void fraction, pressure drop, slip ratio, and bubble velocity; the pressure drop of horizontal plug/bubble flows and Taylor bubble velocity of vertical slug flow is minimum around normal gravity. The computational model is then extended to account for convective and condensation heat transfer. The numerical results for a vertical slug flow show that a porous-tube-insert (PTI) promotes the internal liquid circulations in both axial and radial directions resulting in an enhanced convective heat transfer up to five times of that in bare tube. In addition, the PTI enhances the flow condensation heat transfer up to three times mainly due to the enforced ultra-thin liquid film near the tube wall and increased area for thin-film condensation.