Modelling of stratified two phase flows using an interfacial area density model

Abstract

Stratified two-phase flow regimes can occur in chemical plants, nuclear reactors and oil pipelines. A relevant problem is the development of wavy stratified flows which can lead to slug generation. The slug flow regime is characterized by an acceleration of the gaseous phase and by the transition of fast liquid slugs, which carry a significant amount of liquid with high kinetic energy. It is potentially hazardous to the structure of the system due to the strong oscillating pressure levels formed behind the liquid slugs as well as the mechanical momentum of the slugs. Because these flow conditions cannot be predicted with the required accuracy and spatial resolution by the one-dimensional system codes, the stratified flows are increasingly modelled with computational fluid dynamics (CFD) codes. In CFD, closure models are required that must be validated. The recent improvements of the multiphase flow modelling in the ANSYS CFX code make it now possible to simulate these mechanisms in detail. In order to validate existing multiphase flow models and to further develop these, measurement data with a high-resolution in time and also in space are needed. For the experimental investigation of co-current air/water flows, the HAWAC (Horizontal Air/Water Channel) was built. The channel allows in particular the study of air/water slug flow under atmospheric pressure. Parallel to the experiments, CFD calculations were carried out. The two-fluid model was applied with a special turbulence damping procedure at the free surface. An Algebraic Interfacial Area Density (AIAD) model on the basis of the implemented mixture model was introduced, which allows the detection of the morphological form of the two phase flow and the corresponding switching via a blending function of each correlation from one object pair to another. As a result this model can distinguish between bubbles, droplets and the free surface. The behaviour of slug generation and propagation at the experimental setup was qualitatively reproduced by the simulation, while local deviations require a continuation of the work. The creation of small instabilities due to pressure surge or an increase of interfacial momentum should be analysed in the future. Furthermore, experiments with pressure and velocity measurements are planned and will allow quantitative comparisons, also at other superficial velocities.

Simulação de escoamento bifásico aplicando a técnica da fluidodinâmica computacional

“…O modelo desenvolvido no trabalho trata-se de um tubo horizontal com escoamento bifásico líquido-gás [3]. A configuração do tubo retangular contém duas entradas, onde ambos os fluidos são injetados de modo concorrentes sendo separados por uma lâmina de 1 mm de espessura.…”

“…Na literatura vários modelos interfaciais têm sido explorados, porém a grande maioria, embora sejam sensíveis a transferências de movimento próximo à interface, ainda não são capazes de descrever de forma completa todas as informações morfológicas, como a formação de bolhas e gotas. No entanto, recentemente, um modelo proposto por Hohne e Vallee (2009) [3] mostrou-se capaz na descrição dessas características morfológicas, esse novo modelo denominado Algebriac interfacial Area Density (AIAD) tem como base a definição de um novo coeficiente de arrasto oriundo de um conjunto de funções e correlações.…”

CFD Study of Gas Holdup and Frictional Pressure Drop of Vertical Riser Inside IC Reactor

“…It must be noted that the cost in terms of computational effort is very high for capturing the phase interface. The multiple flow regimes (MFR) model was developed to simulate multi-scale multiphase flow combining the advantages for Eulerian-Eulerian model and VOF model [23][24][25][26][27]. The model has been verified in typical two-phase flow, such as dam break process, stratified flow process, and liquid drop oscillation process [28].…”

Two‐fluid model with interface sharpening

“…An example of such approach can be found in the work of Minato [24] 577 and Nagayoshi [25], who solved the transport equation with a stable and an accurate high-resolution technique. Similar type of interface tracking is implemented in the computer code CFX [26] and has been used for modeling of horizontally stratified pipe flows by Hohne [4,27,28], Frank [29], Wintterle [30], Scheuerer [6] and Bartosiewicz [31]. The simulations of horizontal stratified flow with the two-fluid model and anisotropic drag force (wall-like in direction parallel to the free surface and bubble-like in direction perpendicular to the free surface) were performed by Coste [32].…”

“…One of the first attempts to simulate mixed flows was presented byČerne [13] who coupled the volume of fluid method (with geometrical interface reconstruction) with the two-fluid model. Experiment in a horizontal channel with transition from horizontal stratified to slug flow was simulated with the two-fluid model implemented in CFX by Hohne [4,27,28]; however, large interface was not recognized. A recent study by Herrmann [34] should be mentioned, who coupled Eulerian interface tracking and Lagrangian point particle by removing/adding small structures (droplets) from the Eulerian field into the Lagrangian frame.…”