Subcooled Water Pool Research Articles

CFD simulation of steam condensation in a subcooled water pool

In present work interfacial dynamics of steam injection to a sub cooled stagnant water pool using a vertical blowdown pipe has been simulated using ANSYS FLUENT 14. Multiphase Volume of Fluid (VOF) model is used to track the interface and standard k-є model was adopted for modeling the turbulence. Effects of pool temperature and steam injection velocity are explored to understand the hydrodynamic and thermal characteristics of the pool. Also the effect of both these parameters on the rate of condensation is investigated. It has been observed that increase in the steam injection velocity will decreases the interfacial temperature which in turn enhances the rate of condensation.

On the fragmentation characteristics of melt jets quenched in water

Experiments were carried out to investigate the characteristics of jet breakup and debris formation after melt jets fall into a subcooled water pool, which may occur in industrial processes such as the interactions of molten corium with coolant during a severe accident of light water reactors. A high-speed visualization system developed previously at KTH was used to capture the jet fragmentation phenomenon. Molten metal (Woods metal or tin) and mixture of binary oxides (WO3-Bi2O3 or WO3-ZrO2) were employed separately as melt materials to address different breakup mechanisms (e.g., hydrodynamic vs. thermodynamic fragmentation) and material effect. Moreover, the parameters related to melt and water conditions, including superheat, jet diameter and velocity of melt as well as subcooling of water, were scrutinized for their influences on jet fragmentation characteristics. The experimental data were acquired on the melt jet fragmentation patterns, breakup length, droplet size spectrum, droplet breakup and solidification as well as debris morphology, which can be useful for validation of the codes used for the prediction of debris formation phenomena.

Simulation of melt jet breakup experiments by JASMINE with an empirical correlation for melt particle size distribution

ABSTRACTWe modified JASMINE code, a fuel–coolant interaction simulation code developed at Japan Atomic Energy Agency (JAEA), to extend the applicability for ex-vessel melt coolability assessment. The modification included addition of a melt particle size distribution model based on an empirical correlation and a simple non-local radiation heat transfer model, improvement in the treatment of melt particle generation, and re-agglomeration of settled particles. The modified code was tested by simulating melt jet breakup experiments, namely selected cases of ALPHA/GPM series with alumina–zirconia mixture and steel melt by JAEA, and FARO experiments with urania–zirconia mixture by Joint Research Center Ispra. Simulation results showed that the code reproduces the experimental results well for the cases with a deep subcooled water pool where the melt breaks up completely. On the other hand, significant underestimation of heat removal from the melt and overestimation of agglomeration of settled melt was encountered for conditions with a shallow or saturation temperature water pool. The melt agglomeration behavior in the simulation was sensitive to model parameters on the agglomeration criterion and heat transfer depending on conditions.

Steady State Vapor Bubble in Pool Boiling.

Boiling, a dynamic and multiscale process, has been studied for several decades; however, a comprehensive understanding of the process is still lacking. The bubble ebullition cycle, which occurs over millisecond time-span, makes it extremely challenging to study near-surface interfacial characteristics of a single bubble. Here, we create a steady-state vapor bubble that can remain stable for hours in a pool of sub-cooled water using a femtosecond laser source. The stability of the bubble allows us to measure the contact-angle and perform in-situ imaging of the contact-line region and the microlayer, on hydrophilic and hydrophobic surfaces and in both degassed and regular (with dissolved air) water. The early growth stage of vapor bubble in degassed water shows a completely wetted bubble base with the microlayer, and the bubble does not depart from the surface due to reduced liquid pressure in the microlayer. Using experimental data and numerical simulations, we obtain permissible range of maximum heat transfer coefficient possible in nucleate boiling and the width of the evaporating layer in the contact-line region. This technique of creating and measuring fundamental characteristics of a stable vapor bubble will facilitate rational design of nanostructures for boiling enhancement and advance thermal management in electronics.

Numerical simulation of direct contact condensation of subsonic steam injected in a water pool using VOF method and LES turbulence model

The direct-contact condensation (DCC) of subsonic steam injected into a subcooled water pool has been simulated using the Volume of Fluid (VOF) multiphase flow model and Large Eddy Simulation (LES) turbulent flow model of FLUENT. A thermal equilibrium model has been introduced to simulate the phenomenon of DCC. The condensation model is a two resistance model which takes care of the heat transfer process on both sides of the interface. The numerical results for the steam plume can be divided into four stages: the initial stage, the maximum stability stage, the oscillatory stage and the detachment stage. Fairly good qualitative agreement was obtained with the available experimental data for the plume shape and evolution characteristics of the steam plume. In addition, it was found that the centerline axial temperature increases with the decrease of the centerline axial velocity at the pipe exit and the pressure oscillation is mainly affected by the steam velocity, steam condensation and subcooled water hydrostatic pressure.

Condensation induced water hammer: Numerical prediction

Contact of steam and subcooled water in a pipe or a pressurized vessel leads to intensive condensation accompanied by a pressure drop in the volume of condensing steam and an acceleration of the surrounding water mass towards the steam volume, which can result in a severe water hammer and plant damage. This phenomenon is known as the condensation induced water hammer (CIWH). A one-fluid model is developed for the prediction of pressure surges during CIWH. It is shown that the reliable prediction of pressure surges strongly depends on the calculation of the condensation rate, transient friction and the water column–steam interface tracking. Due to the lack of the CIWH condensation models, a new approach is derived. The one-fluid model predictions of pressure surges are compared with available measured data from a CIWH experimental facility and acceptable agreements are obtained. In addition, the ability of the developed model to simulate the water cannon event, which takes place during the steam drainage into the pool of subcooled water, is demonstrated. Experimentally observed considerable scattering of test data under the same conditions is related to the condensation rate and its dependence on the entrained droplets–steam interfacial area concentration in the vicinity of the water column head.

Ultrasonic enhancement of subcooled pool boiling of freely oscillated wires

New methods for cooling of microelectronic elements have been recently developed, including application of ultrasonic fields, which can enhance the heat transfer in two-phase cooling. Here we deal with ultrasonic enhancement of heat transfer from wires in subcooled water pool boiling at subcooling of 80K. The main purpose is to find the effect of wire diameter on the heat transfer coefficient. The wire heaters were used as a source of constant heat flux, and as a thermometers. The experiments were carried out using wires of different diameters: 20, 50, 90, 200 and 0.250μm, submerged in a bath of water. The frequency of the ultrasonic field was 40kHz and the intensity was 0.5W/cm2. The wire wall temperature was measured as a function of wire surface heat flux. The effect of acoustic field on heat transfer was measured by the change in the average surface temperature of the heater. When the ultrasonic field was applied, the wall temperature decreased depending on wire diameter and heat flux. Video images of vapor bubble dynamics within the sound field aided in the analysis, and are presented here. The highest heat transfer augmentation was registered for the heated wire of diameter 200μm during pool boiling in the ultrasound field.

Phase change heat transfer and bubble behavior observed on twisted wire heater geometries in microgravity

Phase change is an effective method of transferring heat, yet its application in microgravity thermal management systems requires greater understanding of bubble behavior. To further this knowledge base, a microgravity boiling experiment was performed (floating) onboard an aircraft flying in a parabolic trajectory to study the effect of surface geometry and heat flux on phase change heat transfer in a pool of subcooled water. A special emphasis was the investigation of heat transfer enhancement caused by modifying the surface geometry through the use of a twist of three wires and a twist of four wires. A new method for bubble behavior analysis was developed to quantify bubble growth characteristics, which allows a quantitative comparison of bubble dynamics between different data sets. It was found that the surface geometry of the three-wire twist enhanced heat transfer by reducing the heat flux needed for bubble incipience and the average wire temperature in microgravity. Simulation results indicated that increased local superheating in wire crevices may be responsible for the change of bubble behavior seen as the wire geometry configuration was varied. The convective heat transfer rate, in comparison to ground experiments, was lower for microgravity at low heating rates, and higher at high heating rates. This study provides insights into the role of surface geometry on superheating behavior and presents an initial version of a new bubble behavior analysis method. Further research on these topics could lead to new designs of heater surface geometries using phase change heat transfer in microgravity applications.

1216 異なる表面状態の円柱発熱体における定常及び過渡プール沸騰CHFの写真観察(GS-6 二相流)

1216 異なる表面状態の円柱発熱体における定常及び過渡プール沸騰CHFの写真観察(GS-6 二相流)

Study on fragmentation behavior of liquid lead alloy droplet in water

Fragmentation behavior of molten lead alloys droplet in water was investigated experimentally by releasing liquid LBE (45w%Pb–55w%Bi) and lead droplets into a pool of subcooled water. The fragmentation occurred when the temperature of the interface between a molten droplet and water was higher than the spontaneous nucleation temperature of water and lower than the minimum film boiling temperature. With increasing the droplet temperatures, the peak pressure in fragmentation of LBE droplet increased from 5 to 8 kPa, and for lead, the value remained around 2 kPa. With increasing the water subcooling, the peak pressure in fragmentation remained constant at 5 kPa for LBE droplet and at 2 kPa for lead droplet. The lead alloy fragmentation process in water was numerically simulated by embedding a semi-empirical fragmentation model for droplet fragmentation rate into the computer code of two-phase flow: JASMINE code. The corresponding results, such as pressure history and fragmentation peak pressure, agreed well with the experimental results.

CFD ANALYSIS OF TURBULENT JET BEHAVIOR INDUCED BY A STEAM JET DISCHARGED THROUGH A VERTICAL UPWARD SINGLE HOLE IN A SUBCOOLED WATER POOL

Thermal mixing by steam jets in a pool is dominantly influenced by a turbulent water jet generated by the condensing steam jets, and the proper prediction of this turbulent jet behavior is critical for the pool mixing analysis. A turbulent jet flow induced by a steam jet discharged through a vertical upward single hole into a subcooled water pool was subjected to computational fluid dynamics (CFD) analysis. Based on the small-scale test data derived under a horizontal steam discharging condition, this analysis was performed to validate a CFD method of analysis previously developed for condensing jet-induced pool mixing phenomena. In previous validation work, the CFD results and the test data for a limited range of radial and axial directions were compared in terms of profiles of the turbulent jet velocity and temperature. Furthermore, the behavior of the turbulent jet induced by the steam jet through a horizontal single hole in a subcooled water pool failed to show the exact axisymmetric flow pattern with regards to an overall pool mixing, whereas the CFD analysis was done with an axisymmetric grid model. Therefore, another new small-scale test was conducted under a vertical upward steam discharging condition. The purpose of this test was to generate the velocity and temperature profiles of the turbulent jet by expanding the measurement ranges from the jet center to a location at about 5% of <TEX>$U_m$</TEX> and 10 cm to 30 cm from the exit of the discharge nozzle. The results of the new CFD analysis show that the recommended CFD model of the high turbulent intensity of 40% for the turbulent jet and the fine mesh grid model can accurately predict the test results within an error rate of about 10%. In this work, the turbulent jet model, which is used to simply predict the temperature and velocity profiles along the axial and radial directions by means of the empirical correlations and Tollmien's theory was improved on the basis of the new test data. The results validate the CFD model of analysis. Furthermore, the turbulent jet model developed in this study can be used to analyze pool thermal mixing when an ellipsoidal steam jet is discharged under a high steam mass flux in a subcooled water pool.

CFD analysis of a turbulent jet behavior induced by a steam jet discharge through a single hole in a subcooled water tank

A computational fluid dynamics (CFD) analysis for a turbulent jet flow induced by a steam jet discharged into a subcooled water pool was performed for 10 s of transients to investigate whether the currently available CFD codes can be suitably used as a tool to investigate the applicability of the existing semi-analytical correlations to a condensing jet-induced turbulent jet and to analyze the thermal-hydraulic behavior, such as global circulation and local hot spot, in a condensation pool for advanced light water reactors. As for the numerical experiment, a series of sensitivity calculations was conducted systematically to elucidate the major factors which can cause different analysis results by varying the mesh distributions, numerical models for a convection term and an eddy viscosity term. The effect of a difference in the velocity and the temperature distribution in a region between the sparger and the pool wall has not been observed in the afore-mentioned sensitivity calculations. The comparison of the CFD results with the test data shows that the CFD analysis does not accurately simulate the local phenomenon of a turbulent jet existing downstream of a steam jet. It was found that the value of the turbulent intensity at the inlet of the turbulent jet region is the most important factor because it can determine the boundary of a turbulent jet through a momentum diffusion process in a radial direction. The comparison of the CFD results with the test data shows that the CFD analysis can accurately simulate the local phenomenon of a turbulent jet existing downstream of a steam jet only when the CFD analysis reflects the physics of a turbulent jet.

PIV measurements of turbulent jet and pool mixing produced by a steam jet discharge in a subcooled water pool

This experimental research is on the fluid-dynamic features produced by a steam injection into a subcooled water pool. The relevant phenomena could often be encountered in water cooled nuclear power plants. Two major topics, a turbulent jet and the internal circulation produced by a steam injection, were investigated separately using a particle image velocimetry (PIV) as a non-intrusive optical measurement technique. Physical domains of both experiments have a two-dimensional axi-symmetric geometry of which the boundary and initial conditions can be readily and well defined. The turbulent jet experiments with the upward discharging configuration provide the parametric values for quantitatively describing a turbulent jet such as the self-similar velocity profile, central velocity decay, spreading rate, etc. And in the internal circulation experiments with the downward discharging configuration, typical flow patterns in a whole pool region are measured in detail, which reveals both the local and macroscopic characteristics of the mixing behavior in a pool. This quantitative data on the condensing jet-induced mixing behavior in a pool could be utilized as benchmarking for a CFD simulation of relevant phenomena.

CFD simulation of steam jet-induced thermal mixing in subcooled water pool

This study examined the IRWST thermal mixing phenomena induced by a steam jet in a subcooled water pool. Due to the limitation of the current CFD code to simulate condensation, the steam condensation region model was developed to evaluate the thermal mixing phenomena. Within this region, all the steam was condensed into water, and the steam mass and energy inputs were treated as the source. This calculation was treated using single-phase CFD methods. The benchmark calculation for a thermal mixing experiment in the water tank was performed to develop an optimized 3D evaluation methodology of the thermal-hydraulic behavior in APR1400 IRWST. Steam discharge through the sparger and condensation phenomenon was modeled with the choking flow and thermal mixing model in the quenching tank using CFX11. Three types of thermal mixing experiments, local phenomena test, thermal mixing tests in cylindrical water pool and annulus water pool, were designed to provide data representative of the behavior of the prototype for CFD simulations of the thermal-hydraulic behavior in IRWST. A comparison of the calculated and experimentally measured temperature profiles showed some disagreement particularly around the sparger. The main reason for this disagreement was caused by the difference in the test and simulating conditions at the tank wall. However, moving away from the sparger, the trends of the temperature rise became similar to that in the experiment. Despite these problems, this model is the best way of evaluating the thermal mixing phenomena caused by a steam jet in a subcooled water pool.

Analytical tool development for coarse break-up of a molten jet in a deep water pool

A computer code JASMINE-pre was developed for the prediction of premixing conditions of fuel–coolant interactions and debris bed formation behavior relevant to severe accidents of light water reactors. In JASMINE-pre code, a melt model which consists of three components of sub-models for melt jet, melt particles and melt pool, is coupled with a two-phase flow model derived from ACE-3D code developed at JAERI. The melt jet and melt pool models are one-dimensional representations of a molten core stream falling into a water pool and a continuous melt body agglomerated on the bottom, respectively. The melt particles generated by the melt jet break-up are modeled based on a Lagrangian grouped particle concept. Additionally, a simplified model pmjet was developed which considers only steady state break-up of the melt jet, cooling and settlement of particles in a stationary water pool. The FARO corium quenching experiments with a saturation temperature water pool and a subcooled water pool were simulated with JASMINE-pre and pmjet. JASMINE-pre reproduced the pressurization and fragmentation behavior observed in the experiments with a reasonable accuracy. Also, the influences of model parameters on the pressurization and fragmentation were examined. The calculation results showed a quasi-steady state phase of melt jet break-up during which the amount of molten mass contained in the premixture was kept almost constant, and the steady state molten premixed masses evaluated by JASMINE-pre and pmjet agreed well.

THERMAL HYDRAULIC PHENOMENA IN A WATER POOL DUE TO AIR BUBBLE OSCILLATION

In both a boiling water reactor and an advanced pressurized water reactor such as the one currently under construction in Korea named APR1400, when a pressure relieving system is in operation, water, air and steam discharge successively into a sub-cooled water pool through spargers. Among the phenomena occurring during the discharging processes, the air bubble clouds with a low-frequency and high-amplitude oscillation may result in significant damage to the submerged structures if a resonance between the bubble clouds and structures occur. This study deals with a numerical prediction of the pressure field generated by the oscillation of air bubbles. The analysis was performed by using a commercial thermal hydraulic analysis code, FLUENT, version 4.5. The multiphase flows of water, air and steam were simulated by the VOF (Volume Of Fluid) model contained in the code. Unlike Kim et al. [1] , the LRR (Load Reduction Ring) of the sparger is artificially blocked for the investigation of LRR effects on the pressure field. It also includes the effects of air mass and inlet pressure in the piping on the pressure field.

The Direct Contact Condensation of Steam in a Pool at Low Mass Flux

The direct contact condensation of steam at low mass flux (in the chugging region) and the pressure oscillation induced by steam condensation were investigated and analyzed. During the experiment, the steam was discharged into a square subcooled water pool in the horizontal direction. The main experimental parameters were the steam mass flux in the range of 10–80 kg/m2·s and the subcooled water temperature in the range of 30–80°C. Also, two different nozzles (5/8, 6/8 inch) were used. In the chugging region, the high pressure pulses were generated with relatively low frequency. The frequency was little affected by the subcooled water temperature, but increased with increasing steam mass flux. A critical value of steam mass flux was found where the pressure pulse generation rate increased suddenly. This critical value for the 5/8 inch nozzle was found to be lower than that for the 6/8 inch nozzle. The pressure pulse was observed to be generated on the occurrence of sudden condensation of the steam bubble which was detached from the nozzle exit by the necking phenomenon.

Fuel coolant interaction experiments in TROI using a UO 2/ZrO 2 mixture

This paper discusses the results of steam explosion experiments using reactor material carried out under “Test for Real cOrium Interaction with water (TROI)” program. About 4–9 kg of corium melt jet is delivered into a sub-cooled water pool at atmospheric pressure. Spontaneous steam explosions are observed in four tests among six tests. The dynamic pressure, dynamic load, and morphology of debris clearly indicate the cases with steam explosion. The initial conditions and results of the experiments are discussed.

ICONE11-36212 NUMERICAL STUDY ON THE CHARACTERISTICS OF AIR BUBBLE OSCILLATION IN WATER

In both a boiling water reactor and an advanced type of pressurized water reactor under construction in Korea named APR1400,when a pressure relieving system is in operation, water, air and steam discharge successively into a sub-cooled water pool through spargers. Among the phenomena occurring during the discharging processes, the air bubble clouds with a low-frequency and high-amplitude oscillation may result in significant damage to the submerged structures if the resonance between the bubble clouds and structures occur. The phenomena involved are so complicated that most predictions of frequency and pressure loads have resorted to experimental work and computational approach has been precluded. This paper deals with a numerical simulation on the behavior of air bubble discharging into a sub-cooled water pool through a sparger without LRR (Load Reduction Ring) by using a commercial CFD code, FLUENT. The simulation includes both the transient flows of water, air and steam in the pipe and the oscillation of the air bubble subsequently formed in the pool. The VOF (Volume Of Fluid) model is selected to simulate the multi-flows of water, air and steam. It precludes a modeling of steam condensation in the pool since the resultant loads of steam condensation are relatively lower than those from the air bubble oscillation. The simulation shows that one large air bubble which is formed at the sparger head oscillates and moves upward to the water surface subsequently. Comparison of the cases with and without LRR (Note that the authors had subsequently. Comparison of the cases with and without LRR (Note that the authors had previously performed a simulation on the behavior of air bubble with LRR.) shows that the maximum dynamic pressures at the wall without LRR are much larger than those with LRR, which implies that LRR has a great impact on reducing the dynamic pressures. The frequency of the case with LRR is slightly lower due to the larger size of air bubble cloud formed only at the sparger head. The pressure and temperature distributions obtained in the computation seem to be reasonable from a physical viewpoint. It seems that the change of initial air mass in the computation range does not have an impact on the dynamic pressures at the wall. Whereas, the frequency increases as the air mass decreases, which is consistent with the well-known trend that the frequency of a large air bubble is inversely proportional to the air bubble radius. The dynamic pressures and the frequency at the wall increase as the inlet boundary pressure increases, which is consistent with the previous investigations.

Insight into steam explosions with corium melts in KROTOS

This paper describes two KROTOS tests in which ∼4 kg of corium melt was injected into a ∼1-m deep subcooled water pool. These tests were performed in a test vessel that allowed direct visual observations of melt injection and mixing conditions. Visual observations showed that the corium jet penetrated deep into the water while maintaining its shape. The tests did not produce spontaneous explosions. However, the second test with an external trigger produced an explosion with a relatively low efficiency of 0.15%. This is different from the behaviour of alumina melts, which exhibit one order of magnitude higher explosion efficiencies. This paper describes possible mechanisms that could have contributed to the reduced efficiency with corium melt.