Study Of The Hydrodynamics Characteristics Of The Vertical Falling Film

To simulate the co-current gas-liquid two phase flow in the vertical falling filmtube，the two-dimensional numerical model is established using the VOF in theFLUENT solver. The model is used to explore waves, velocity, wall shear stress andheat transfer of falling film.The wave characteristics of falling film are different in the stream-wise direction.The film flow patterns consist of the droplet, laminar flow, laminar-turbulent flow andwave turbulence. The whole simulation domain of the vertical tube can be dividedinto three parts: the entry region, the developing region and the developed region,according to the distance from the liquid inlet. And the relationship between thesurface wave and film velocity and the wall shear stress are also investigated. It isfound that the wave trough of film thickness is synchronized with the peak of wallshear stress, and the peak of large amplitude wave is correspondent with the troughvalue of wall shear stress. A last the influences of surface tension and contact angle onfilm fluctuations are analyzed as well.The velocity profiles across the film are parabolic, and the velocity profiles in thesubstrate film are consistent with the Nusselt’s theory, maximum values of theprofiles in large waves are not the same as Nusselt’s prediction. The x, y velocityvector at the falling film surface in the three regions is investigated. The instantaneouswall shear stress at different liquid Reynolds number in the stream-wise direction iscompared. The simulated results of wall shear stress agree well with the Nusselt’sprediction. Based on the simulated results, new correlations of laminar and turbulentfalling film thickness are obtained and the predicted correlation is shown to agree wellwith popular experimental correlations.With constant wall temperature, the time variations of temperature in the film areanalyzed in the three regions. The surface temperature of the film becomes higherwith the increasing wall temperature. The fluctuations of surface temperature areopposite with the film wave. The dimensionless temperature profiles across the filmare analysis and it is found that the film is hardly affected by the wall temperaturewhen x/δ is beyond0.4, the heat transfer is also mainly in this range. The timevariations of heat transfer coefficient with different wall temperature are investigated in detail. When the inlet velocity distribution is introduced, the heat transfercoefficient is larger by8%in the entrance and the developing region, and15%in thedeveloped region.