Steam-water Flow Research Articles

Computational modeling of bubble coalescence in a high-pressure steam-water flow

A numerical analysis of the coalescence of two co-axial bubbles in an upward subcooled flow boiling system was performed. The study was motivated by the need to better control the cooling of a nuclear reactor core, hence the work was carried out at high system pressure and velocity. The modeling was performed with volume-of-fluid interface tracking method coupled with adaptive mesh refinement while the turbulence characteristics was simulated using large eddy simulation. A microlayer model was applied to capture the unresolved thermal sub-grid scale in the bubble microlayer in addition to performing a near-wall treatment. This numerical model was compared with experimental data available in open literature. The dynamics and contact moment of two co-axial leading and trailing bubbles along the heated wall and far-field regions were then investigated at various thermal–hydraulic conditions.

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.

Modeling of inertial multi-phase flows through high permeability porous media: Friction closure laws

During a severe accident in a nuclear reactor, the core may be fragmented in a debris bed made of millimetric particles. The main safety procedure consists in injecting water into the core leading to a steam-water flow through a hot porous medium. To assess the coolability of debris bed, there is a need for an accurate two-phase flow model including closure laws for the pressure drop. In this article, a new model for calculating pressure losses in two-phase, incompressible, Newtonian fluid flows through homogeneous porous media is proposed. It has been obtained following recent developments in theoretical averaging of momentum equations in porous media. The pressure drops in the momentum equations are determined by eight terms corresponding to the viscous and inertial friction in liquid and gas phases, and interfacial friction between the phases. Analytical correlations with the void fraction have been formulated for each term using an original experimental database containing measurements of pressure drops, average velocities and void fractions from the IRSN CALIDE experiment. The new model has then been validated against the experimental data for various liquid and gas Reynolds numbers up to several hundreds. Finally, it has been compared to the models, usually used in the “severe accident” codes, which are based on a generalization of the Ergun law for multi-phase flows. The results show that the new model gives a better prediction both for the pressure drop and for the void fraction.

CFD prediction of droplet deposition in steam-water annular flow with flow obstacle effects

Recent model development on dryout prediction relies on the annular flow modeling, with three fields of gas, droplets and liquid film accounted for. Therefore one unified computational fluid dynamics (CFD) model for annular flow based on Lagrangian particle tracking approach was developed for dryout applications. On the other hand, it is well acknowledged that dryout performance could be improved if flow obstacles are placed in a flow channel. Therefore, to study and predict the dryout, the governing phenomena of droplet deposition and entrainment in annular flow with and without obstacles need to be investigated. The current work tested the CFD model against experimental data from a steam-water flow experiment. Both data with and without obstacles were employed to test the model capability on deposition calculation. The calculated deposition results without obstacles agree reasonably well with both the experimental data and the existing empirical correlations. In case of the flow with obstacles, the calculations also show reasonably good agreement with the experimental data. The current work laid a basis for further work on annular flow model development with dryout capabilities.

Effect of inverse load on critical heat flux of steam-water two-phase flow in a tube

Effect of inverse load on critical heat flux of steam-water two-phase flow in a tube

A Generalized turbulent dispersion model for bubbly flow numerical simulation in NEPTUNE_CFD

The NEPTUNE_CFD code, based upon an Eulerian multi-fluid model, is developed within the framework of the NEPTUNE project, financially supported by EDF (Electricité de France), CEA (Commissariat à l’Energie Atomique et aux Energies Alternatives), IRSN (Institut de Radioprotection et de Sûreté Nucléaire) and AREVA-NP. NEPTUNE_CFD is mainly focused on Nuclear Safety applications involving two-phase water-steam flows, like two-phase Pressurized Shock (PTS) and Departure from Nucleate Boiling (DNB).Many of these applications involve bubbly flows, particularly, for application to flows in PWR fuel assemblies, including studies related to DNB.Considering a very usual model for interfacial forces acting on bubbles, including drag, virtual mass and lift forces, the turbulent dispersion force is often added to moderate the lift effect in orthogonal directions to the main flow and get the right dispersion shape.This paper presents a formal derivation of this force, considering on the one hand, the fluctuating part of drag and virtual mass, and on the other hand, Turbulent Pressure derivation obtained by comparison between Lagrangian and Eulerian description of bubbles motion. An extension of the Tchen’s theory is used to express the turbulent kinetic energy of bubbles and the two-fluid turbulent covariance tensor in terms of liquid turbulent velocities and time scale. The model obtained by this way, called Generalized Turbulent Dispersion Model (GTD), does not require any user parameter.The model is validated against Liu & Bankoff air-water experiment, Arizona State University (ASU) experiment, DEBORA experiment and Texas A&M University (TAMU) boiling flow experiments.

Approaches to modelling a solar field for direct generation of industrial steam

A thermal hydraulic study of water-steam flow in the absorber tubes of small-sized parabolic trough collectors for industrial process heat applications, and an analysis of the effects of inlet temperature and pressure are presented, considering the impact of different two-phase flow models. It was found that the impact is greatest at low inlet pressures and mass flow rates. The results show the potential of concentrating solar power for supplying a certain percentage of the Mexican industrial heat demand.

Steam-water flow instability in geothermal wells

The phenomenon of steam-water flow instability in geothermal well based on Newton’s second law is considered. The factor causing instability is the reduction of gravitational force that counteracts with an increase in flow rate, resulting in a decrease of mixture density with increasing flow rate. External pressure can effectively influence the stability in case of sufficiently rapid response to changes of flow rate. The specific of the development of instability in the geothermal well does not assume a sufficiently rapid reaction of bottom-hole pressure. In other words, bottom-hole pressure reaction cannot ensure flow stability. Reaction of wellhead pressure can be a stabilizing factor. The example of a typical well of Pauzhetka geothermal field shows, that inversion of output curves is associated with the stabilizing effect of the additional hydraulic resistance at the outlet. It is noted, that wells have extended sections with internal instability, which may be the source of oscillation of flow parameters.

Estimation of the Drop Size in Dispersed Flow

The formulas for calculating the characteristic drop size for the mean Sauter diameter have been compared. The question on various forms of the size distribution of drops has been considered. To substantiate the applicability of the compared formulas for calculating the thermohydrodynamics in the circuits of nuclear power plants, experimental data on the wall temperature in a dispersed flow have been used. It has been shown that the Sauter diameter values calculated using the wall temperature in the supercritical region are in good agreement with sparse direct measurements of the drop size in steam–water flows. The drop sizes calculated using the tested formulas obtained for two-component gas–liquid flows or for single-component flows of coolants (various kinds of freons) and liquefied nitrogen turned out to be much lower. It has been shown that it is necessary to recalculate the numerical coefficients in the considered formulas in using them for steam–water flows.

Modeling of annular two-phase flow using a unified CFD approach

A mechanistic model of annular flow with evaporating liquid film has been developed using computational fluid dynamics (CFD). The model is employing a separate solver with two-dimensional conservation equations to predict propagation of a thin boiling liquid film on solid walls. The liquid film model is coupled to a solver of three-dimensional conservation equations describing the gas core, which is assumed to contain a saturated mixture of vapor and liquid droplets. Both the Eulerian–Eulerian and the Eulerian–Lagrangian approach are used to describe the droplet and vapor motion in the gas core. All the major interaction phenomena between the liquid film and the gas core flow have been accounted for, including the liquid film evaporation as well as the droplet deposition and entrainment.The resultant unified framework for annular flow has been applied to the steam-water flow with conditions typical for a Boiling Water Reactor (BWR). The simulation results for the liquid film flow rate show good agreement with the experimental data, with the potential to predict the dryout occurrence based on criteria of critical film thickness or critical film flow rate.

Numerical investigation of severe slugging under conditions of a parabolic trough power plant with direct steam generation

The present study reveals a numerical investigation of severe slugging with the system code ATHLET. It is aimed to close knowledge gaps about the two-phase flow within the connection pipes of two adjacent collectors in a solar thermal power plant with direct steam generation. The underlying ATHLET model provides a one-dimensional 6 equation model with a mass, momentum and energy equation for each phase, respectively. This comprehensive model provides all features for the examination of water steam flows in this type of power plants. A validation of ATHLET for severe slugging conditions is performed and the obtained periods of severe slugging as well as the pressure amplitudes are in good agreement with experimental data. The probability of severe slugging is studied for numerous flow conditions and geometric conditions which are close to the operation conditions and the piping system at the DISS test facility at the Plataforma Solar de Almería, Spain. Basically, larger vertical and downwards inclined pipe sections increase the probability of severe slugging. Severe slugging can be strongly suppressed by a high pressure operation. In a pipe geometry that shows easily severe slugging a pressure of P⩾60bar prevents severe slugging. In the case of a specific pipe geometry of the DISS test facility, a pressure of P⩾30bar is sufficient to have a non-oscillating flow behaviour.

Poly-disperse simulation of condensing steam-water flow inside a large vertical pipe

The condensation of saturated steam bubbles in sub-cooled water inside a vertical pipe was studied by poly-disperse CFD simulations. Six test cases with varied pressure, liquid sub-cooling and diameter of the gas injection orifices were simulated. Baseline closures presented for non-drag forces in previous work were found to be reliable also in non-isothermal cases. The effect of bubble coalescence and breakup is over-weighting in the region close to steam injection in case of small orifice diameter. With the increase of orifice diameter, breakup becomes dominant in determining bubble size change. The effect of interphase heat transfer coefficient correlations was investigated. The widespread Ranz–Marshall correlation was found to under-estimate the condensation rate, especially at high pressure levels. In contrast, satisfying agreement with the experimental data was obtained by the Tomiyama correlation.

Biomass conversion to hydrogen-rich synthesis fuels using water steam plasma

In this study, an experimental plasma-chemical reactor equipped with an arc discharge water steam plasma torch was used for biomass conversion to hydrogen-rich synthesis fuels. Glycerol and crushed wood were used as biomass sources. The effects of different conversion parameters including the water steam flow rate, treated material flow rate, and plasma torch power were studied. The experimentally obtained results were compared with the model based on the thermodynamic equilibrium. Additionally, the quantification of the plasma conversion system in terms of energy efficiency and specific energy requirement was performed. It has been found that the synthesis gas can be effectively produced from the biomass using water steam plasma.

Numerical Investigation of the Flow Characteristics of Liquid Film in the Process of Falling Film Evaporation

In order to study the flow characteristics of fluid falling film evaporation in a vertical pipe and obtain the dynamic parameters of steam-water two-phase flow, a three dimensional numerical simulation on flow behavior of falling film evaporation was carried out to capture the free surface of falling liquid film with the volume of fluid (VOF) model. The steam-water two-phase flow dynamic characteristics including liquid film flow pattern, liquid film thickness, wall shear stress and velocity distribution of flow field were analyzed through changing feed flow velocities. The results show that under different inlet flow velocities, the average thickness of liquid film from simulation agrees with empirical value well. The film thickness decreases sharply in the entry section which is less than 100mm. The change of wall shear stress is similar to that of film thickness along flow direction, but the change of film thickness has hysteresis quality. Under the influence of gravity and wall shear stress, the accelerated motion and decelerated motion of liquid film alternate, and the flow velocity inside liquid film increases with the distance from liquid film to wall surface, and reaches the maximum at the steam-water interface.

Experimental analysis of steam condensation in vertical tube with small diameter

Thermal design of tubular heat exchanger based on condensation heat of water steam requires knowledge of condensation heat transfer in the each tube. This paper is just aimed on experimental analysis of steam condensation in vertical copper tube in length of 1285mm with 2.0mm inner diameter and 0.5mm wall thickness. Experimental measurement is performed in 12 steps with variable inlet temperature and mass flow rate of water steam. The heat transfer coefficient on the inner surface of tube in condensation zone is calculated by Thermal resistance method and Wilson plot method. The correlation quality of results obtained from both methods is 98.8%. The results are compared with other experimental studies and also correlated with five chosen equations for prediction of condensation heat transfer coefficient. The correlation quality of results obtained from this experimental analysis and four tested equations is over 96.6%, only theoretically determined Nusselt equation undervalues condensation heat transfer coefficient as is known. The Nusselt equation does not take to account waves on condensate surface. These waves on condensate surface are caused by flow of water steam in tube and the wave’s effect is approximately 20.5% in this presented case.

Behaviour and Stability of the Two-Fluid Model for Fine-Scale Simulations of Bubbly Flow in Nuclear Reactors

Abstract In the present work, we formulate a simplistic two-fluid model for bubbly steam-water flow existing between fuel pins in nuclear fuel assemblies. Numerical simulations are performed in periodic 2D domains of varying sizes. The appearance of a non-uniform volume fraction field in the form of meso-scales is investigated and shown to be varying with the bubble loading and the domain size, as well as with the numerical algorithm employed. These findings highlight the difficulties involved in interpreting the occurrence of instabilities in two-fluid simulations of gas-liquid flows, where physical and unphysical instabilities are prone to be confounded. The results obtained in this work therefore contribute to a rigorous foundation in on-going efforts to derive a consistent meso-scale formulation of the traditional two-fluid model for multiphase flows in nuclear reactors.

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.

Investigation of air–water two phase flow through a venturi

Study of two phase flow characteristics is of prime importance in various modern engineering fields including nuclear. In a 700MWe Indian Pressurized Heavy Water Reactor (PHWR), limited boiling is allowed in the channels to extract more power. In this pressure tube type of reactor, the channel power computation requires the measured channel flow and outlet quality. Though the channel flow-rate can be measured at the inlet feeder to the reactor core, the channel outlet quality needs to be derived from the measured two phase pressure drop across a venturi located at the outlet feeder of the same channel. The measured pressure drop shall be correlated to the mixture density of two phase steam–water flow in the channel. In order to study the pressure drop characteristics of the specific venturi under two phase conditions, an experimental study was performed with air–water two phase flow, with same the venturi channel system as that in the prototype reactor. A series of tests were carried out by varying the air/water flow rates, in a multiphase loop. The pressure drop across the venturi was measured using a DPT (Differential Pressure Transmitter). The measured data was compared with the correlations available in literature.

Steam-water flow in geothermal wells

A mathematical model was developed for calculation of steam-water flow in a geothermal well for the feeding interval. The model assumes a variable mass flow rate over the channel length. The basis for this model are the flow continuity equation, momentum and energy conservation equations, taken with account for variable mass flow. The model was implemented as a computer code suitable for calculation of flow parameters upstream (downward the top level of the feeding zone). Then this model was applied for wells in the Mutnovskii geothermal field, this revealed a geyser-type mechanism of flow instabilities with the pressure oscillation period about ten minutes. The remedy for these oscillations was offered.

РАСЧЕТ ТЕЧЕНИЙ В ПАРОВОДЯНЫХ ГЕОТЕРМАЛЬНЫХ СКВАЖИНАХ ПО МАТЕМАТИЧЕСКИМ МОДЕЛЯМ WELL

РАСЧЕТ ТЕЧЕНИЙ В ПАРОВОДЯНЫХ ГЕОТЕРМАЛЬНЫХ СКВАЖИНАХ ПО МАТЕМАТИЧЕСКИМ МОДЕЛЯМ WELL