Unstructured Adaptive Mesh Technique for Gas-Liquid Two-Phase Flows

Abstract

In the design study of large-sized sodium-cooled fast reactors in Japan (JSFR), the suppression of gas entrainment (GE) phenomena at a free surface in the reactor vessel is very important to establish an economically superior design. However, the GE phenomena are highly non-linear and too difficult to be evaluated theoretically. Therefore, we are developing high-precision CFD method for gas-liquid two-phase flows to evaluate the GE phenomena accurately. To reproduce the GE phenomena by CFDs, there are three key issues, i.e. geometry dependency, interfacial dynamics and locality. Former two issues are already addressed by employing unstructured mesh schemes and a high-precision simulation method for gas-liquid two-phase flow based on the PLIC (Piecewise Linear Interface Calculation) method, respectively. In fact, the simulation results of the GE phenomena in a simple GE experiment showed good agreements with experimental data. Recently, therefore, we focus on the locality of the GE phenomena. In our previous study (presented in ICONE17), the two-dimensional unstructured adaptive mesh technique for single-phase flows was developed to address the third issue. The isotropic cell refinement method was employed and the connection cell method was proposed to eliminate the edge incompatibility. The verification/validation results showed that the developed unstructured adaptive mesh technique succeeded in providing a high-precision solution, even though a poor-quality distorted mesh at the initial state was employed. In this study, the unstructured adaptive mesh technique is extended to the numerical simulations of gas-liquid two-phase flows. The redistribution methods of two-phase flow variables are newly developed to satisfy the conservations of the variables, i.e. the volumes of gas and liquid phases, the location of interfaces and the momentum of each phase. This improved unstructured adaptive mesh technique for gas-liquid two-phase flows is validated by solving the well-known slotted disk revolution and dam-break problems. As a result, the unstructured adaptive mesh technique succeeds in maintaining the slotted-disk shape after one revolution and shows more than first order accuracy (grid convergence) in the slotted-disk revolution problem. In addition, thanks to the momentum-conservative formulation, the dam-break phenomenon is well simulated by the unstructured adaptive mesh technique. Especially, wave-breaking phenomena are simulated by refined cells near the gasliquid interface. It should be noted that these simulation results are obtained by using relatively small number of cells because of the efficient mesh adaptation by the unstructured adaptive mesh technique.