Effects of liquid viscosity on inception of disturbance waves and droplets in gas-liquid annular two-phase flow

In order to clarify the effects of liquid viscosity on inception of disturbance waves and droplets, structure of gas-liquid interface and flow rates of droplets entrained with gas were measured simultaneously at five kinds of kinematic viscosities, 1.0, 3.2, 9.9, 30 and 70 mm2/s. Time-spatial traces of liquid film thickness measured by five holdup probes were presented and it was found that the inception of disturbance waves occurred at Rel= 200 or at the non-dimensional liquid film thickness of tfm+=6.5. Furthermore, entrainment ratios were measured and it was clarified that droplets occurred before the inception of disturbance waves with increasing liquid kinematic viscosity and the critical condition of the droplet inception was jl=0.02-0.03 m/s. The obtained results were compared with the published criteria on the inception of droplets and the previous correlations were found unsatisfactory. Considering the balance between shear force and surface tention acting on the wave crests, a new criterion of droplet inception was also proposed.