CFD validation of new Nu-Re correlations for the condensation of large steam bubbles directly injected into a flow in a vertical pipe and effects of bubbles forces upon momentum transfer

摘要

The condensation of large steam bubbles directly injected into a 100 mm diameter vertical pipe with flowing low subcooled water was experimentally investigated at the SCUBA test facility of the Technical University Munich (TUM). The condensation process was observed by using high-speed camera recording and a special image processing tool was developed to extract bubbles' parameters and calculate bubbles' Reynolds and condensation Nusselt number. The experimental investigations showed the significant influence of the bubbles' surface structure and of the initial gas turbulence upon the condensation process of steam bubbles. This effect was quantitatively demonstrated via a higher dependency of the experimentally measured bubbles' Nusselt number upon the bubbles' Reynolds number. The experimentally developed Nu-Re correlations are implemented into a 3D CFD model (using ANSYS-CFX v16.0), and the calculations' results using different Nu-Re correlations are compared. A special treatment of experimental data was used to extract an initial steam fraction distribution to apply to the model. Other required initial parameters such as the average value of the initial bubbles' equivalent diameter (D) over bar (B,eq,0,) the bubbles' initial rising velocity (V) over bar (B,0), the water's velocity distribution V-L(r), and the saturation temperature T-s were also extracted from experiments. Additionally, the treatment provided an experimental 2D steam fraction distribution that is used for the validation of the simulation results. Steady state simulations were carried out using an Euler-Euler two-fluid approach with the Two-Resistance heat transfer model with zero resistance on the steam side and different Nu-Re correlations on the water's side. Simulations were carried out for two experimental cases with two types of steam injection: vertical and horizontal. The simulations show a better agreement when our experimentally developed Nu-Re correlations are applied. It also shows the inadequacy of previously developed correlations for the case of larger steam bubbles which are directly injected into the liquid flow. The effect of the drag force, the added mass force, and the lift force upon the simulation results was investigated and discussed in detail. The discussion shows the need for further investigation of the large steam bubbles' dynamics. The current results are of significance for the simulation of steam bubble condensation that occur as a result of direct steam injection (rather than during a subcooled boiling) in nuclear safety equipment. (C) 2017 Elsevier Ltd. All rights reserved.