CFD calculations of flow pattern maps and LES of multiphase flows

Abstract

Boiling crisis and flows occurring in a steam generator or a heat exchanger remain a major limiting phenomenon for the analysis of operation and safety of both nuclear reactors and conventional thermal power systems. Firstly, the choice is made to investigate a hybrid modeling of the flow, considering the gas phase as two separated fields, each one being modeled with different closure laws. In so doing, the small and spherical bubbles are modeled through a dispersed approach within the two-fluid model, and the distorted or large bubbles are simulated with an interface locating method. This kind of approach is requiring a set of mass transfer terms between the continuous and the dispersed fields of the same physicochemical phase. The main outcome is the simulation of three field cases with a complete set of coupling terms between the two gas fields. Firstly, validation results are presented in the Castillejos test case where air is injected at the bottom of a cylindrical bubble column. Next, METERO experiments are simulated with this multifield approach. METERO is an experimental rig dedicated to the study of turbulent mixing of air and water in horizontal flows. The different regimes encountered in the METERO experiments, i.e. stratified flows, slug flows, plug flows, stratified dispersed bubbly flows and dispersed bubbly flows are simulated and presented in the paper. Finally, MAXI2 experiment (liquid/vapour flow in oblique tubes bundle geometry) is simulated: the three-field approach gives a reasonable agreement with experimental data whereas the standard two-field approach (dedicated to dispersed bubbly flow) fails. The important topic of turbulence modelling for two-phase flows is also addressed in the paper. The key point is that standard LES models may fails in the simulation of large interfaces. But the ADM model could constitute an interesting alternative in further studies.

The emptying of a bottle as a test case for assessing interfacial momentum exchange models for Euler–Euler simulations of multi-scale gas-liquid flows

“…The LBM ( Denèfle, 2013;Denèfle et al, 2015;Mimouni et al, 2017 ) is the most sophisticated of the models considered in the present study. Here, distinct dispersed ( dg ) and continuous ( cg ) gas phases are considered, making the LBM a 3 −field model involving separate cl → cg and cl → dg momentum exchange closures.…”

“…An important consequence of this difference is that the drag force on uncontaminated bubbles grows linearly rather than quadratically with the rise velocity ( Batchelor, 1967 ). In NEPTUNE_CFD, this state of affairs led Fleau (2017) (see also Mimouni et al, 2017 ) to assume that the interfacial momentum transfer toward a cg entity involves a characteristic length scale l g and is proportional to μ l l g (u cl − u cg ) , so that the momentum exchange rate per unit volume may be written in the form…”

“…Last, an exchange procedure combining numerical requirements and basic physical principles has to be designed to allow a LI to break up into smaller bubbles, and such bubbles to coalesce and generate a LI. Such an approach has been implemented both in the aforementioned GENTOP formulation ( Montoya et al, 2015 ), and in the NEP-TUNE_CFD code where it is termed the Large Bubble Model (LBM, Denèfle et al, 2015;Mimouni et al, 2017 ). Preliminary assessment of this approach in canonical configurations, such as the Kelvin-Helmholtz and Rayleigh-Taylor instabilities, was reported by Fleau et al (2015Fleau et al ( , 2016 .…”

Dispersed Two-Phase Flow Modelling for Nuclear Safety in the NEPTUNE_CFD Code

“…As a first step towards this new approach, we have simplified the general concept by considering the liquid phase as continuous in our simulations. The main results obtained with NEPTUNE CFD are presented in [61][62][63][64].…”

LES of turbulent non-isothermal two-phase flows within a multifield approach

“…The criteria to switch from the large interfaces treated with the LBMo to the dispersed field and the opposite are not given in this paper since we will focus on the large interfaces. Readers are invited to refer to Mimouni et al [2017] for details. However, industrial studies require the ability of dealing with both turbulence and phase change effects.…”