Countercurrent Flow Limitation Characteristics Research Articles

Experimental Study on Two-Phase Countercurrent Flow Limitation in Horizontal Circular Pipes

The two-phase countercurrent flow limitation (CCFL) in horizontal channels is important in relation to nuclear reactor safety. In this study, we aim to investigate the CCFL characteristics and the flow behaviors in horizontal circular pipes with small diameters. The effects of pipe diameter and the water head in the upper plenum on CCFL characteristics are experimentally studied. An image-processing technique and statistical treatments are implemented to analyze the horizontal countercurrent flow. The results show that the CCFL characteristics for the horizontal circular pipes with small diameters can be well correlated using the dimensionless parameters, which are based on adding fluid viscosity to the Wallis parameters. The CCFL characteristics are significantly affected by the pipe diameter and are slightly affected by the water head above the horizontal pipe. The gas–liquid interface fluctuates with certain periods, and flow pattern transitions happen in the horizontal air–water countercurrent flow. As the air flow rate increases, the occurrence location of the liquid slug appears to shift towards the water entrance. In addition, the further away from the water entrance, the lower the average of liquid holdup.

Experimental Investigations and Numerical Studies of Two-Phase Countercurrent Flow Limitation in a Pressurized Water Reactor: A Review

Gas–liquid two-phase countercurrent flow limitation (CCFL) phenomena widely exist in nuclear power plants. In particular, the gas–liquid countercurrent flow limitation phenomena in a pressurized water reactor (PWR) during a loss-of-coolant accident (LOCA) or a small-break loss-of-coolant accident (SBLOCA) play an important role in nuclear reactor safety research. Over several decades, a series of experimental investigations and numerical studies have been carried out to study the CCFL phenomena in a PWR. For the experimental investigations, numerous experiments have been conducted, and different CCFL mechanisms and CCFL characteristics have been obtained in various test facilities simulating different scenarios in a PWR. The CCFL phenomena are affected by many factors, such as geometrical characteristics, liquid flow rates, and fluid properties. For the numerical studies, more and more numerical models were presented and applied to the calculations of two-phase countercurrent flow over the past several decades. It is considered that the computational fluid dynamics (CFD) tools can simulate most of the two-phase flow configurations encountered in nuclear power plants. In this paper, the experimental investigations and the numerical studies on two-phase countercurrent flow limitation in a PWR are comprehensively reviewed. This review provides a further understanding of CCFL in a PWR and gives directions regarding future studies. It is found that relatively fewer investigations using steam–water under high system pressures are performed due to the limitation of the test facilities and test conditions. There are a number of numerical studies on countercurrent two-phase flow in a PWR hot leg geometry, but the simulations in other flow channels were relatively rare. In addition, almost all of the numerical simulations do not include heat and mass transfer. Thus, it is necessary to investigate the effects of heat and mass transfer experimentally and numerically. Furthermore, it is of significance to perform numerical simulations for countercurrent two-phase flow with a fine computational grid and suitable models to predict the formation of small waves and the details in two-phase flow.

An experimental study on the effect of liquid properties on the counter-current flow limitation (CCFL) during gas/liquid counter-current two-phase flow in a 1/30 scaled-down of Pressurized Water Reactor (PWR) hot leg geometry

To promote the safety of the nuclear reactors, numerous authors have carried out investigations of gas/liquid counter-current two-phase flow in order to understand the characteristics during the inception of flooding or CCFL. The present work covers an experimental and analytical study on the effect of liquid properties on CCFL characteristics in a complex geometry representing 1/30 scaled-down version of a German-Konvoi PWR hot leg. The obtained results reveal that the gas velocity to initiate the flooding monotonically decreases with the increase of the liquid velocity. At high liquid flow rates, it is noticed that with the increase of glycerol percentage, the gas flow rate needed to initiate the flooding significantly decrease due to either the increase in liquid viscosity or the decrease in corresponding surface tension. Here under the same flow condition, the flooding is initiated faster by the higher glycerol percentage. Moreover, instead of the well-known Wallis superficial velocity, the Kutateladze-type parameter or a combination of the Wallis parameter and liquid property numbers by Zapke & Kröger (1996) which were previously proposed by numerous authors to investigate the effects of liquid properties on CCFL characteristics, a modified non-dimensional Wallis parameter combined with the inverse viscosity number introduced by Ma et al. (2020) is found herein to more accurately correlate the present experimental data. Through a dimensional similarity analysis, a new empirical correlation reveals that the liquid Froude number is the greatest determinant of CCFL characteristics with respect to the effect of the liquid properties.

Flow characteristics in vertical circular pipes with the square top end under flooding conditions

Previously, we evaluated flow characteristics in a vertical pipe of diameter D = 20 mm under flooding conditions. Now, we have measured the void fraction α, pressure gradient dP/dz and countercurrent flow limitation (CCFL) characteristics in a vertical pipe of D = 40 mm with the square (i.e. sharp-edged without curvature) top end, and air and water to evaluate the film thickness δ, the wall friction factor fw and the interfacial friction factor fi. The comparison of flow characteristics between pipes of D = 20 and 40 mm showed that the CCFL characteristics and the transition from the smooth film (SF) to the rough film (RF) due to flooding near the bottom end were different. On the other hand, δ and fw for SF were expressed by the same correlation for D = 20 and 40 mm. We discussed the CCFL characteristics and fw in detail for SF; these items are important for assessment of α and fi from the dP/dz data in the absence of α data.

Experimental study on the effect of diameter on gas–liquid CCFL characteristics of horizontal circular pipes

The gas–liquid two-phase countercurrent flow in horizontal pipes is closely related to the safe operation of marine diesel engines with underwater exhausts, and nuclear power plants with pressurized water reactors. A visualization experiment on the effect of the diameter size on the counter current flow limitation (CCFL) characteristics of horizontal pipes was performed in the diameter range of 20–130 mm using air and water as the two phases. The results indicate that with a certain gas flow rate, the flow rate of the backflow liquid significantly increases with increasing diameter. Wallis models and those using the Froude–Ohnesorge numbers can correlate the effect of the change in diameter size when the diameters are greater than 100 mm, whereas they lose normalization capability with diameters less than 100 mm. From an analysis of the physical mechanism of formation of this diameter effect, a novel correlation model based on a new dimensionless group for each phase, consisting of the dimensionless inertia and dimensionless viscous force, was proposed for predicting the CCFL characteristics of small diameter pipes. Meanwhile, another correlation based on the Wallis model was also advanced for predicting the CCFL characteristics of large diameter pipes.

On CCFL at a PWR Hot-Leg Pipe Geometry and Comparison Between Results in COLLIDER 1/4th- and Kobe 1/15th-Scaled Facilities

Countercurrent flow limitation (CCFL) in a pressurized water reactor hot-leg pipe geometry with a 190-mm pipe diameter was investigated experimentally and numerically at the COLLIDER test facility of the Technical University Munich in the past 3 years. This paper summarizes the most important CCFL findings learned from the COLLIDER test facility and tries to explain the reasons for obtaining different descriptions, results, and conclusions at different CCFL experimental investigations. The factors that can affect CCFL experimental results are explained in detail including some scale effects. The necessary preconditions to compare two sets of data from different CCFL experimental investigations are discussed in detail. The difference among CCFL-related limits/curves is clarified taking data at the COLLIDER as an example. The limits included the limit of the transition from a supercritical into a subcritical flow (SSTL); the onset of CCFL limit (iCCFL) inside the hot-leg pipe; the onset of CCFL limit (eCCFL) at the entrance of the steam generator; the deflooding limit (CCFLd); the CCFL characteristics curve (CCFLch), which predicts the water delivery rate after the onset of iCCFL; and the onset of hysteresis limit. It will be shown that among these limits only SSTL, CCFLch, and eCCFL are original limits while the rest are derivatives of them. In particular, it will be shown that the iCCFL limit is a combination of the SSTL and CCFLch limits. The effect of scale upon the eCCFL’s mechanism (whether a water accumulation or droplet entrainment at the entrance to the steam generator) is clarified via a comparison to a 50-mm CCFL facility at Kobe University. This paper tests the scalability of interface distribution at quasi-stationary conditions (i.e., points along the CCFLch curve) via a comparison of time-averaged interface distributions obtained at similar inlet conditions ( at the COLLIDER 190-mm and Kobe 50-mm channels. The comparison will show that interface distributions (which are directly linked to the pressure drop and interfacial momentum transfer) cannot be scaled at the bend/riser/entrance region because of the influence of the channel diameter upon occurring CCFL mechanism. Meanwhile, the water level gradient can be similar at the horizontal part, but not the relative water depth.

Correlation of Interfacial Friction Coefficients for Predicting Countercurrent Flow Limitation at a Sharp-Edged Lower End of Vertical Pipes

One-region (1-R) sensitivity computations with the annular-flow model were carried out for countercurrent flow limitation (CCFL) at a sharp-edged lower end in vertical pipes to generalize the prediction method for CCFL there (i.e., predicting effects of diameters and fluid properties on CCFL characteristics). In our previous study, we selected a correlation of interfacial friction coefficients, fi, with a function of average void fraction which gave a good prediction of the trend for air–water CCFL data, and we modified it to get good agreement with steam–water CCFL data under atmospheric pressure conditions, but it failed to predict CCFL reasonably at high pressure conditions. We recently found a Russian report on CCFL data at high pressure conditions, by which we improved the fi correlation using the dimensionless diameter and the viscosity ratio or density ratio of gas and liquid phases to get good agreement with CCFL data at high pressures. The improved fi correlation with the viscosity ratio and the improved fi correlation with the density ratio gave similar computed results, but the number of adjustment functions was one for the density ratio and two for the viscosity ratio (i.e., minimum value of two functions).

Semi-empirical correlation for counter-current flow limitation at the upper or lower end of sharp-edged vertical pipes

A semi-empirical correlation for CCFL (counter-current flow limitation) in vertical pipes was derived from one-dimensional momentum equations. Available correlations such as the empirical correlations proposed by Wallis (1969) and Zapke and Kröger (1996) can be deduced from the derived fundamental functional form of the semi-empirical correlation in limiting flow conditions. Comparisons between the semi-empirical correlation with available experimental data of CCFL taking place at the sharp-edged lower end of a vertical pipe showed that the correlation is applicable for various fluid properties and pipe diameters. The fundamental functional form of the correlation was also transformed so as to express the characteristics of CCFL occurring at the sharp-edged upper end of a vertical pipe. The present fundamental functional form of CCFL correlation is useful not only to express CCFL data but also to understand how relevant parameters play their roles in the CCFL characteristics.

Image-processing of time-averaged interface distributions representing CCFL characteristics in a large scale model of a PWR hot-leg pipe geometry

Countercurrent Flow Limitation (CCFL) was experimentally investigated in a 1/3.9 downscaled COLLIDER facility with a 190mm pipe’s diameter using air/water at 1 atmospheric pressure. Previous investigations provided knowledge over the onset of CCFL mechanisms. In current article, CCFL characteristics at the COLLIDER facility are measured and discussed along with time-averaged distributions of the air/water interface for a selected matrix of liquid/gas velocities. The article demonstrates the time-averaged interface as a useful method to identify CCFL characteristics at quasi-stationary flow conditions eliminating variations that appears in single images, and showing essential comparative flow features such as: the degree of restriction at the bend, the extension and the intensity of the two-phase mixing zones, and the average water level within the horizontal part and the steam generator. Consequently, making it possible to compare interface distributions obtained at different investigations. The distributions are also beneficial for CFD validations of CCFL as the instant chaotic gas/liquid interface is impossible to reproduce in CFD simulations. The current study shows that final CCFL characteristics curve (and the corresponding CCFL correlation) depends upon the covered measuring range of water delivery. It also shows that a hydraulic diameter should be sufficiently larger than 50mm in order to obtain CCFL characteristics comparable to the 1:1 scale data (namely the UPTF data). Finally, the study shows that the change of the flow condition inside the hot-leg is not only related to the water and air inlet velocities, but is also dependent upon the existent interface distribution within the hot-leg, and that several CCFL cases of identical inlet flow conditions can exist with different interface distribution and pressure difference. The last result is of a special importance to the investigation of this phenomenon during SBLOCA accidents, since the entire phenomenon is driven by pressure difference between the steam generator and reactor vessel, as well as by gravity. This result show also that CCFL characteristics cannot be investigated using 1D codes, as the interface distribution within the hot-leg during a SBLOCA accident will depend upon flow history or previous interface distribution. Current investigations support the effort to provide more knowledge over CCFL in order to extrapolate results obtained in downscaled models into the 1:1 scale.

Numerical study on modeling of liquid film flow under countercurrent flow limitation in volume of fluid method

Countercurrent flow limitation (CCFL) in a heat transfer tube at a steam generator (SG) of pressurized water reactor (PWR) is one of the important issues on the core cooling under a loss of coolant accident (LOCA). In order to improve the prediction accuracy of the CCFL characteristics in numerical simulations using the volume of fluid (VOF) method with less computational cost, a thin liquid film flow in a countercurrent flow is modeled independently and is coupled with the VOF method. The CCFL characteristics is evaluated analytically in condition of a maximizing down-flow rate as a function of a void fraction or a liquid film thickness considering a critical thickness. Then, we have carried out numerical simulations of a countercurrent flow in a vertical tube so as to investigate the CCFL characteristics and compare them with the previous experimental results. As a result, it has been concluded that the effect of liquid film entrainment by upward gas flux will cause the difference in the experiments.

Experimental Investigation of Air-Water CCFL in the Pressurizer Surge Line of AP1000

Countercurrent flow limitation (CCFL) may occur under certain flow conditions in the surge line, restricting the draining of water from the pressurizer and thus affecting the coolant inventory and water level in the reactor pressure vessel (RPV). The complexity of the AP1000 pressurizer surge line structure makes predicting CCFL fairly difficult, and there are still not enough CCFL studies on this complex structure. Based on an extensive literature survey, the authors of this paper are particularly aware of the need for improved CCFL models for the pressurizer surge line of AP1000. To investigate the CCFL phenomenon in the surge line assembly fixture of AP1000, a whole-visual test model of the surge line is designed with a scaling ratio of 1:4, and a test loop is established to carry out visualization experiments with air-water countercurrent flow (CCF). The whole-visual test section made of acrylic material is composed of a pressurizer simulator, a surge line tube, a hot leg T-type tube, and an RPV simulator. The air-water CCF experiments are conducted at atmospheric pressure and room temperature with the pressurizer simulator water level varying from 150 to 900 mm. The visual CCF experimental processes and CCFL phenomena are filmed by a high-speed camera and analyzed in detail. The pressure drops at different CCFL locations are measured and evaluated to explore the relationships between the CCFL characteristics and flow patterns in the surge line. The development process of the CCFL is defined as the CCFL region, which can be divided into different regions according of the changes in water mass flow and CCF flow behavior. The CCFL data are analyzed and compared using the air and water superficial velocities to study the effects of hysteresis and water level. Small discrepancies are found between the data of different water levels, reflecting the small but not-negligible influence of the upper tank water level. Empirical models for the CCFL in the surge line assembly fixture are explored preliminarily using Kutateladze-type correlation and Froude-Ohnesorge correlation. Deficiencies still exist in the present semiempirical models, inspiring a more in-depth study on the empirical models for CCFL in the surge line assembly fixture that considers the complex two-phase flow behaviors in the upper tank and near the joint between the upper tank and surge line tube. The present CCFL data are compared broadly and in detail with groups of CCFL data of similar former experiments to demonstrate the applicability of the present air-water CCFL data to the development of a CCFL prediction model for the prototype large-diameter surge line assembly fixture of the AP600/AP1000. We will perform much more experimental and theoretical work to study the detailed mechanism of these special phenomena and to develop a more applicable CCFL model for the geometry and conditions of the prototype large-diameter surge line assembly fixture.

Prediction Method of Countercurrent Flow Limitation in a Pressurizer Surge Line and Its Evaluation for a 1/10-Scale Model

The method for predicting countercurrent flow limitation (CCFL) and its uncertainty in an actual pressurizer surge line of a pressurized water reactor (PWR) using 1/10-scale air–water experimental data, one-dimensional (1D) computations, and three-dimensional (3D) numerical simulations was proposed. As one step of the prediction method, 3D numerical simulations were carried out for countercurrent air–water flows in a 1/10-scale model of the pressurizer surge line to evaluate capability of the 3D simulation method and decide uncertainty of CCFL characteristics evaluated for the 1/10-scale model. The model consisted of a vertical pipe, a vertical elbow, and a slightly inclined pipe with elbows. In the actual 1/10-scale experiment, air supplied into the lower tank flowed upward to the upper tank and water supplied into the upper tank gravitationally flowed downward to the lower tank through the pressurizer surge line. In the 3D simulation, however, water was supplied from the wall surface of the vertical pipe to avoid effects of flooding at the upper end (the 3D simulation largely underestimated falling water flow rates at the upper end). Then, the flow pattern in the slightly inclined pipe was successfully reproduced, and the simulated CCFL values for the inclination angle of θ=0.6 deg (slope of 1/100) agreed well with the experimental CCFL data. The uncertainty among air–water experiments, 1D computations, and 3D simulations for the 1/10-scale model was dC=±0.015 for the CCFL constant of C=0.50. The effects of θ (θ=0,1.0 deg) on CCFL characteristics were simulated and discussed.

水平管での気液対向流制限に対する一次元計算の予測精度

Countercurrent flow limitation (CCFL) may occur in a hot leg (consisting of an inclined pipe, a vertical 50-deg elbow, and a horizontal pipe) and a pressurizer surge line (consisting of a vertical pipe, a vertical 90-deg elbow, and a slightly inclined pipe with elbows) under postulated accident conditions in a pressurized water reactor (PWR). In our previous study, we developed a one-dimensional (1D) computation method to predict CCFL characteristics in an inclined pipe with elbows, to generalize the prediction method for CCFL in piping systems. In this study, we did 1D computations for CCFL experiments in horizontal pipes (diameters of D = 0.03-0.65 m and ratios of the length to the diameter of L/D = 4.5-63) to evaluate prediction accuracy of the 1D computation method. The differences of the CCFL constant C (value of JG*1/2 at JL*1/2 = 0 where JG* and JL* are the Wallis parameters) between measured and computed values were within dC = ±0.055 for C = 0.4-0.7.

Countercurrent Flow Limitation in Slightly Inclined Pipes With Elbows

One-dimensional (1D) sensitivity computations were carried out for air–water countercurrent flows in a 1/15-scale model of the hot leg and a 1/10-scale model of the pressurizer surge line in a pressurized water reactor (PWR) to generalize the prediction method for countercurrent flow limitation (CCFL) characteristics in slightly inclined pipes with elbows. In the 1D model, the wall friction coefficient fwG of single-phase gas flows was used. The interfacial drag coefficient of fi=0.03, an appropriate adjustment factor of NwL=6 for the wall friction coefficient fwL of single-phase liquid flows (NwG=1 for fwG of single-phase gas flows), and an appropriate adjustment factor of Nde=6 for the pressure loss coefficient ζe of elbows in single-phase flows were determined to give good agreement between the computed and measured CCFL characteristics. The adjusted factors were used to compute and then discuss effects of the inclination angle and diameter on CCFL characteristics.

Condensation experiments for counter-current flow limitation in an inverted U-tube

In this study, we measured counter-current flow limitation (CCFL) characteristics in an inverted U-tube (18.4 mm diameter and 1.0 m straight-part length) simulating steam generator (SG) U-tubes under conditions of steam condensation at pressures of 0.1–0.14 MPa. Differential pressure ΔP between the top of the inverted U-tube and the lower tank was measured, and the flow patterns wave estimated by comparing the waveforms of ΔP with those in air–water experiments. As a result, we classified the flow patterns under CCFL conditions into CCFL-P, CCFL-L and CCFL-T. The falling water flow rate under CCFL conditions slightly increased as the pressure increased and the cooling water temperature decreased (subcooling of cooling water increased). In the case of CCFL-L, CCFL characteristics in the inverted U-tube were between those in air–water and saturated steam–water experiments at 0.1 MPa. Furthermore, we derived a Wallis type CCFL correlation and its uncertainty from CCFL data, including previously measured data, i.e., J*1/2G + 0.88JL*1/2 = 0.76 ± 0.05.

Effects of fluid properties on CCFL characteristics at a vertical pipe lower end

The purpose of this study is to derive a counter-current flow limitation (CCFL) correlation and evaluate its uncertainty for steam generator (SG) U-tubes in a pressurized water reactor (PWR). Experiments were conducted to evaluate effects of the liquid viscosity on CCFL characteristics using air–40 wt% or air–60 wt% glycerol water solution and saturated steam–water at atmospheric pressure with vertical pipes simulating the lower part of the SG U-tubes. The steam–water experiments confirmed that CCFL characteristics could be expressed in terms of the Wallis parameters (JG* and JL*) for the pipe diameters of D = 14, 20, and 27 mm. A CCFL correlation was derived using the ratio μG/μL of the viscosities of the gas and liquid phases, μG and μL, as a correction term representing effects of fluid properties, where JG*1/2(μG/μL)−0.07 was expressed by a cubic function of JL*1/2(μG/μL)0.1. In the correlation, the constant C indicating the value of JG*1/2(μG/μL)−0.07 at JL* = 0 was (1.04 ± 0.05), and this uncertainty of ±0.05 would cover most of the previous experimental data including the ROSA-IV/LSTF data at 1, 3, and 7 MPa.

Effects of Shape and Size on Countercurrent Flow Limitation in Flow Channels Simulating a PWR Hot Leg

A numerical study is presented to examine the effects on countercurrent flow limitation (CCFL) of the shape and size of hot leg models with a rectangular cross section. The CCFL was described in terms of Wallis parameters using the channel height H as the characteristic length. Numerical simulations, using the computational fluid dynamics software code FLUENT 6.3.26, were done for the air-water CCFL experiments carried out previously at Helmholtz-Zentrum Dresden-Rossendorf in a 1/3-scale hot leg model with a rectangular channel (H×W = 0.25×0.05 m2), and the results were compared with the air-water CCFL data obtained at Kobe University in a 1/5-scale hot leg model with rectangular cross section (H×W = 0.15×0.01 m2) and the results of simulations. It was found that both the height-to-width ratio and the size of the cross section affected the CCFL characteristics in the Wallis diagram. Comparison of CCFL characteristics in rectangular channels with those in circular channels showed that the hydraulic diameter Dh was a major cross-section geometry term influencing the CCFL characteristics. CCFL constants of the Wallis correlation were ~0.61 on average for the range 0.05 m ≤ Dh ≤ 0.75 m but became small for Dh ≤ 0.0254 m, and these tendencies were well reproduced by the numerical simulations.

Air-Water Tests on Counter-current Flow Limitation at Lower End of Vertical Pipes Simulating Lower Part of Steam Generator U-tube

Air-water tests on counter-current flow limitation (CCFL) were carried out using vertical pipes with the diameter of D=20 mm and a sharp lower edge which simulated the lower part of U-tubes in a steam generator. Flow patterns were observed to identify the location of CCFL occurrence, and effects of the pipe length L and the water level in the upper tank h on CCFL were measured under the conditions of L=100-700 mm and h=100-600 mm. As a result, CCFL appeared at the upper end under the conditions of L/D=5 and h/D=5, but at the lower end for L/D≧15 and h/D=5～30. In the case of the CCFL occurrence at the lower end, L and h did not affect CCFL characteristics. Using the Wallis parameters, JG* and JL*, a CCFL correlation for the sharp lower edge was derived. The CCFL constant, which is the value of JG*1/2 at JL*1/2=0, was C=0.73±0.02.

蒸気発生器Ｕ字管下部を模擬した鉛直管下端での気液対向流制限の数値シミュレーション

Numerical simulations were done for counter-current flow limitation (CCFL) at the lower end of a vertical pipe simulating lower part of steam generator U-tubes by using the volume of fluid method (VOF) implemented in the CFD software FLUENT6.3.26. The simulated CCFL characteristics agreed well with air-water experimental data but flooding in simulations appeared at the upper end of the vertical pipe. To avoid flooding at the upper end, water was supplied through the pipe wall simulating condensation on the inner surface and flooding at the lower end was successfully simulated. However, computations by the standard k-ε turbulence model became unstable for pressures lower than 1.0 MPa and significantly underestimated falling water flow rates. On the other hand, computations by the laminar flow model were stable even for low pressures and significantly overestimated falling water flow rates. Computations by the k-ω SST turbulence model were unstable for pressures lower than 1.0 MPa but gave good agreement of a CCFL value with a steam-water experimental value at 1.0 MPa.

VOF Simulations of Countercurrent Gas-Liquid Flow in a PWR Hot Leg

In order to evaluate flow patterns and CCFL (countercurrent flow limitation) characteristics in a PWR hot leg under reflux condensation, numerical simulations have been done using a two-fluid model and a VOF (volume of fluid) method implemented in the CFD software, FLUENT6.3.26. The two-fluid model gave good agreement with CCFL data under low pressure conditions but did not give good results under high pressure steam-water conditions. On the other hand, the VOF method gave good agreement with CCFL data for tests with a rectangular channel but did not give good results for calculations in a circular channel. Therefore, in this paper, the computational grid and schemes were improved in the VOF method, numerical simulations were done for steam-water flows at 1.5 MPa under PWR full-scale conditions with the diameter of 0.75 m, and the calculated results were compared with the UPTF data at 1.5 MPa. As a result, the calculated flow pattern was found to be similar to the flow pattern observed in small-scale air-water tests, and the calculated CCFL characteristics agreed well with the UPTF data at 1.5 MPa except in the region of a large steam volumetric flux.