Numerical modeling of non-condensable gases transport in two-phase flow

Abstract

The presence of non-condensable gases at the gas-liquid interface significantly decreases the heat and mass transfer, and disturbs the fluid flow. Thus, modeling the transport of non-condensable gases in two-phase flow is of great importance in nuclear reactor safety. Non-condensable gases transport in two-phase flow is considered as a multi-component flow, where the non-condensable gases are assumed to be homogeneously mixed with steam in the gas phase. Modeling of non-condensable gases transport has been already done by Computational Fluid Dynamics, system thermal-hydraulics codes, and numerical simulations. This work is dedicated to the Apros thermal-hydraulics system code. Some stability and convergence issues can show up when modeling non-condensable gases in the the current version of the code Apros 6; for example, when the gas phase consists of a high mass fraction of non-condensable gas with a very low mass fraction of steam. In the present work, a new procedure is proposed to integrate the non-condensable gases transport equation solution in the two-fluid six equations solution algorithm to develop a new thermal-hydraulic solver. A new pressure equation is derived based on considering the non-condensable gas transport equation solution from the beginning of the numerical algorithm formulation. This is expected to improve the stability and convergence by making the non-condensable gas transport equation solution more tightly coupled with the pressure equation. The oscillating U-tube numerical benchmark test case was simulated to verify and validate the new implementation of the new pressure equation.

The results of the oscillating U-tube show good agreement with the analytical solution when using a dense nodalization. The second test performed was the simulation of experiments on condensation in the presence of non-condensable gases in the separate test facility CONAN. Experiments were simulated with a constant heat transfer coefficient and a heat transfer coefficient, obtained from the Uchida correlation. The simulation results were quantitatively reasonable. Moreover, the results showed qualitatively the same behavior as the experimental data. In addition, a hypothetical case with a varying velocity behavior was also simulated (with a constant heat transfer coefficient).