Stratified Steam-water Flow Research Articles

Measurement of water temperature and velocity fields by applying thermography on two-phase flow through horizontal rectangular channel

The infrared thermography and optical-flow based method were proposed for the measurement of water temperature and velocity fields in stratified steam-water flow through horizontal channel. Generally, this method is applicable for any turbulent flow with temperature gradient. Spinel polycrystalline (MgAl2O4) material was chosen for infrared optical window, as, besides excellent optical properties, it has exceptional durability in harsh environment. Different mounting types and dimensions were tested in order to investigate Spinel application possibilities and limitations. A significant influence of window specific thermal radiation on measurement results (>20% inaccuracy) was observed during experiments. The best identified configuration ensures good temperature measurement accuracy (<=2%). Graphical analysis was applied to infrared images, and good results of turbulence structure identification were achieved using combined local-global optical flow technique.

Numerical modelling of a direct contact condensation experiment using the AIAD framework

The Lithuanian Energy Institute (LEI) test case deals with direct contact condensation (DCC) in the two-phase stratified steam-water flow. The main goal of CFD simulations of these experiments is to compute new models of heat and mass transport from saturated vapour to liquid over a free surface and the temperature profiles across the liquid flow in a channel. Condensation occurs mainly on free surfaces for instance at PTS scenarios. The knowledge of the accurate coolant temperature is important for nuclear safety assessment.Three different direct contact condensation models for the heat transfer within the AIAD framework at the free surface were formulated and tested. The AIAD model describes a consistent set of model correlations for the interfacial area density, the drag, the non-resolved disturbances of a free surface and the turbulence damping the interface. The calculated surface temperature profiles agree well with the experiment. Further model development should focus on “CFD grade” experimental data and direct numerical simulations.

Numerical simulation of the direct contact condensation phenomena for PTS-related in single and combined-effect thermal hydraulic test facilities using TransAT CMFD code

The use of CFD for the industrial studies related to PTS, including DCC is already possible; improvements of the two-phase modeling capabilities have to be undertaken to qualify the codes for the simulation of such flows. The DCC in horizontally stratified flow regime constitutes very considerable challenge exercises for a computational fluid dynamics (CFD) simulation of the thermal hydraulics PTS phenomenon because the interplay between turbulence and interfacial heat and mass transfer problem. The main purpose of our study is to investigate numerically the DCC in horizontally stratified steam water flow in a 2D and 3D channel using TransAT CMFD code. The new methodology known as Large-Eddy & Interface (LEIS) have been implemented for treatment of turbulence combined with interface tracking ITM (level set approach). Among of the so-called ‘coarse-grained’ ITM's models, the modified original surface divergence has been chosen as well as the treatment of the turbulence by URANS and VLES. This contribution addressed on the validation of interfacial phase-change heat transfer and turbulence models with special correction of the damping function at the free surface for single and combined-effect thermal hydraulic studies for LIM and KAERI & KAIST test facilities. The LIES methodology was found to apply successfully to predict the condensing steam flow rate in the all cases of the LIM test case involving a Smooth to Wavy turbulent, concurrent stratified steam-water flow in a 2D channel. The CMFD TransAT code predicting capability is analyzed, comparing the liquid temperature and to much the velocity profile of KAERI & KAIST Countercurrent Stratified Flow (CCSF) combined-effect in 2D and 3D geometry to one predicted by analytical Biberg model. The vertical profiles of CFD local water velocity calculations have been corrected using AKN free surface damping model where the agreement with KAERI & KAIST data is sufficiently good. The KAERI & KAIST test case is actually more challenging than all low subcooling cases (COSI, TOPFLOW) where the application of the ASD model (and the other interfacial DCC models) for high subcooling (ΔTsub∼70.0°K) is still questionable. A modified ASD model should be implemented, to taking into account the effect of the degree of the water subcooling through the introduction of the Jakob number.

A New modelling strategy for phase-change heat transfer in turbulent interfacial two-phase flow

The paper presents a modelling strategy for phase-change heat transfer in turbulent interfacial two-phase flow. The computational framework is based on interface tracking ITM (level set approach), combined with large-scale prediction of turbulence, a new methodology known as Large-Eddy & Interface Simulation (LEIS), where super-grid scale turbulence and interfaces are directly solved, whereas the sub-scale parts are modelled. Because steady-state flow conditions are difficult to attain, recourse is made of the Very Large-Eddy Simulation (V-LES) instead of LES, where the flow-dependent cut-off filter is larger and independent from the grid. The computational approach is completed by a DNS-based interfacial phase-change heat transfer model built within the Surface Divergence (SD) theory. The original SD model is found to return better results when modified to account for scale separation, i.e. to segregate low-Re from high-Re number flow portions in the same flow. The model was first validated for an experiment involving a smooth to wavy turbulent, stratified steam-water flow in a 2D channel ( Lim et al., 1984, Condensation measurement of horizontal concurrent steam-water flow, ASME J. Heat Transfer 106, 425–432.), revealing that the original SD model performs better for high interfacial shear rates. This screening phase also demonstrated that the most critical issue is the accurate prediction of the interfacial shear using ITM. The model was then applied successfully to predict condensing steam in the event of emergency core cooling in a Pressurized Water Reactor (PWR), where water is injected into the cold leg during a postulated loss-of-coolant-accident. The simulation results agree fairly well with the COSI (short for COndensation at Safety Injections) data ( Janicot and Bestion, 1993, Condensation modelling for ECC injection, Nucl. Eng. Des. 145, 37–45).

Numerical approach for cocurrent stratified steam water flow in a horizontal configuration

The present study deals with the one-dimensional numerical approach in predicting the local flow properties for cocurrent stratified steam-water flow in a horizontal configuration. The turbulence-centered model, developed principally for gas absorption, has been modified and introduced for the condensation heat transfer coefficient using interfacial parameters, such as turbulent velocity and length scales. The calculated condensation rates, pressures and mean water layer thicknesses are in good agreement with Lim’s experimental data obtained from cocurrent stratified steam flow on a fairly thick layer of water. In addition, the approach was applied to the case of relatively thin liquid films, and the results were compared with Linehan’s experimental data. The comparison indicates that the one-dimensional numerical approach with the present condensation heat transfer correlation developed from the thick liquid data can be applicable to the prediction of the flow properties for thin liquid films.

Flooding and hysteresis effects in nearly-horizontal countercurrent stratified steam-water flow

Experimental data are presented on flooding transitions, together with a countercurrent-flow regime map for nearly-horizontal ( θ = 4.0) stratified steam-water flow. The influence of condensation on the initiation and basic mechanisms of flooding is explored. The mechanism of hysteresis effects owing to condensation, which are observed during the partial delivery stage in a steam/subcooled water flow, are also investigated.

Parametric effects on the onset of flooding in flat-plate geometries

Flooding of countercurrent stratified steam-water flow in flat-plate geometries is investigated. Both local and global effects of condensation on the onset of flooding are discussed, based on the results of top and bottom flooding tests. The influence of the channel depth and inclination angle on the flooding velocity is also explored. An envelope model, which considers flooding to be the limiting condition for countercurrent flow, is presented and compared with extensive data at various inclinations. Quite good agreement is shown at the nearly horizontal and moderately steep inclinations, but the onset of flooding in nearly vertical flow is expedited by significant droplet entrainment, and thus takes place earlier than the theory predicts.