Simulation of Saturated Vapor Condensation from a Downflow on the Surface of a Horizontal Pipe by the VOF Method

Abstract

Various literary sources present the results of experiments that were carried out in order to investigate the process of condensation on a horizontal cylinder of a moving steam of freon R-113. These results demonstrate a qualitative disagreement with the trends following from the available theoretical dependencies. The authors of these experimental data indicated some possible reasons for this difference, but a detailed verification of the above assumptions is difficult due to the difficulties in obtaining information about the local characteristics of the condensation process. In this work, the VOF (Volume of Fluid) method is used to simulate the experimental modes of R-113 freon condensation on the surface of a horizontal cylinder from a downward flow moving at a velocity of up to 6 m/s at a pressure close to atmospheric. The Lee model was used to simulate interfacial mass transfer. The selection of its constant was carried out using the algorithm proposed earlier by the authors of this work. Data on changes in the local characteristics of heat transfer during condensation from a moving vapor flow, obtained using the VOF method, are presented. The calculation results are in good agreement with the “unusual” experimental data and confirm the experimentally recorded anomalous (compared to the existing theoretical dependences) increase in the heat-transfer coefficient with an increase in the oncoming flow velocity. It is shown that one of the reasons for the increase in the heat-transfer coefficient is the interaction of the falling condensate film with the vortex structures formed behind the streamlined cylinder. At a certain velocity of the oncoming flow, the falling condensate film is periodically “flooded,” which, in turn, leads to a significant intensification of heat transfer near the lower generatrix of the cylinder. This mechanism is not taken into account in the existing models since, as a rule, it is assumed in them that, after flow separation, the film flows down only due to the action of gravitational forces. A criterion dependence is proposed for determining the boundary of “anomalous” (compared to the theoretical value) heat-transfer intensification.