Void Fraction Prediction Research Articles

A population balance cavitation model

We propose a novel polydisperse cavitation model based on population balances, which seeks to provide the bubble size distribution in addition to the void fraction, enabling natural prediction of transitions from large bubbles to small cavities in regions of massive condensation, or from small bubble to large cavities in regions of strong evaporation. The mass transfer rate is modeled from the asymptotic solution of the Rayleigh-Plesset equation with modifications for the polydisperse approach. Existing breakup and coalescence models, which do not include effects of mass change, are adapted and new models are developed to account for evaporation and condensation. In addition, homogenous and heterogeneous nucleation models are incorporated to treat the activation and size distribution of nuclei. The model is evaluated against data for the Delft twisted foil, showing satisfactory results for the pressure distribution, forces, and shedding frequency. Though no experimental data for the bubble size distribution is available, bubble sizes in different regions of the flow predicted by the model indicate the right trends observed in videos and photographs of the experiments. Refinements to the model based on experiments are necessary, especially to evaluate the ability to properly predict void fraction and bubble size distribution.

Numerical simulation and experimental validation of cavitating flow in a multi-hole diesel fuel injector

This study assesses the predictive capability of the ZGB (Zwart-Gerber-Belamri) cavitation model with the RANS (Reynolds Averaged Navier-Stokes), the realizable k-epsilon turbulence model, and compressibility of gas/liquid models for cavitation simulation in a multi-hole fuel injector at different cavitation numbers (CN) for diesel and biodiesel fuels. The prediction results were assessed quantitatively by comparison of predicted velocity profiles with those of measured LDV (Laser Doppler Velocimetry) data. Subsequently, predictions were assessed qualitatively by visual comparison of the predicted void fraction with experimental CCD (Charged Couple Device) recorded images. Both comparisons showed that the model could predict fluid behavior in such a condition with a high level of confidence. Additionally, flow field analysis of numerical results showed the formation of vortices in the injector sac volume. The analysis showed two main types of vortex structures formed. The first kind appeared connecting two adjacent holes and is known as “hole-to-hole” connecting vortices. The second type structure appeared as double “counter-rotating” vortices emerging from the needle wall and entering the injector hole facing it. The use of RANS proved to save significant computational cost and time in predicting the cavitating flow with good accuracy.

Three-component multi-fluid modeling of pseudo-cavitation phenomenon in diesel injector nozzles

Cavitation in fuel injectors occurs in the nozzle region where local pressure drops below the fuel saturation pressure. The pressure drop might simultaneously induce the formation of gas bubbles such as nitrogen dissolved in the fuel, also known as pseudo-cavitation. A new cavitation model has been developed that accounts for the nitrogen bubbles separation from the fuel stream by accounting for the solubility changes of nitrogen with the pressure drop. A multi-fluid model integrated with the volume-of-fluid interface tracking approach has been developed to capture the interface between the liquid fuel, fuel vapor and the de-gassed nitrogen. Differentiating between cavitation and pseudo-cavitation is a very challenging task experimentally. The new model allows distinguishing between the volume fraction occupied by fuel vapor and de-gassed nitrogen. Comparing the predicted void fraction along the nozzle with available experimental data demonstrates that this model significantly improves the predictions of size/location of the cavitation compared with single-fluid mixture models and the existing multi-fluid simulation results. For the standard fuel case, the bulk of bubble formation is correlated with the de-gassing and these bubbles are observed along the nozzle with higher concentration downstream of the flow. However, for the de-gassed fuel case, vapor cavitation bubbles are focused near the walls with higher concentrations near the entrance. The transient behavior of void formation shows that the local pressure at the nozzle entrance near the walls drops to below the saturation pressure initially and forms the vapor bubbles. As the pressure stabilizes at a pressure higher than the saturation pressure, the de-gassing phenomenon takes over vapor cavitation, leading to void formation throughout the nozzle attributed to the non-condensable gases. The sensitivity of the void fraction predictions to model parameters indicates that controlling the ratio of evaporation to condensation rate is essential for accurate prediction of steady state void fraction.

A Comparative Study of Closure Relations for CFD Modelling of Bubbly Flow in a Vertical Pipe

Commercial code CFX was used to examine the performance of a two-fluid model to predict the details of upward isothermal bubbly flow of air and water in a vertical pipe. The model equations are volume-averaged Navier-Stokes equations that require closure models for interfacial forces and bubble-induced turbulence effects. Two-equation SST and k-epsilon RANS turbulence models were also used. A parametric study of closure models included both standard options in CFX and previously published novel closure models that were implemented with user-defined functions. The CFD simulations were compared with two cases from the MTLoop experiments by Lucas et al. at the Helmholtz-Zentrum Dresden Rossendorf: one with wall-peak void fraction profile (MT039), and another with a core-peak void fraction profile (MT118). The effect of changing the drag force closures was not significant for the set examined. Poor predictions were found when the lift force and wall lubrication models were incompatible in magnitude. There was no significant effect of changing the liquid phase turbulence model. Changing the bubble-induced turbulence models, however, had a significant impact on the radial void fraction profile. The novel wall force from Lubchenko et al. at the Massachusetts Institute of Technology significantly improved the prediction of the near wall void fraction in the wall peak profile.

Evaluation of flow pattern recognition and void fraction measurement in two phase flow independent of oil pipeline’s scale layer thickness

The main objective of the present research is to combine the effect of scale thickness on the flow pattern and characteristics of two-phase flow that is used in oil industry. In this regard, an intelligent nondestructive technique based on combination of gamma radiation attenuation and artificial intelligence is proposed to determine the type of flow pattern and gas volume percentage in two phase flow independent of petroleum pipeline’s scale layer thickness. The proposed system includes a dual energy gamma source, composed of Barium-133 and Cesium-137 radioisotopes, and two sodium iodide detectors for recording the transmitted and scattered photons. Support Vector Machine was implemented for regime identification and Multi-Layer Perceptron with Levenberg Marquardt algorithm was utilized for void fraction prediction. Total count in the scattering detector and counts under photo peaks of Barium-133 and Cesium-137 were assigned as the inputs of networks. The results show the ability of presented system to identify the annular regime and measure the void fraction independent of petroleum pipeline’s scale layer thickness.

Distribution parameter and drift velocity for upward gas-liquid metal two-phase flow

In view of the needs in the developments of the fast-neutron nuclear reactor, the accelerator driven nuclear reactor system, the fusion reactor and so on, this study has conducted an extensive literature survey on the past experimental and theoretical studies of the upwardly-moving gas-liquid metal two-phase flows in the vertical flow channels with various channel geometries. The experimental data of the upward gas-liquid metal two-phase flows taken in the N2-Hg (nitrogen and mercury), N2-Pb/Bi (nitrogen and lead/bismuth eutectic alloy), N2-Ga (nitrogen and gallium) and N2-Na/K(nitrogen and sodium/potassium eutectic alloy) mixtures in the vertical circular, annular and rectangular flow channels and seven available drift-flux type correlations used for the void fraction predictions in the gas-liquid metal two-phase flows have been collected. The performances of these seven drift-flux type correlations have been checked against the collected experimental data, and no available drift-flux type correlation is able to predict well the void fraction of the gas-liquid metal two-phase flows with low gas-to-liquid density ratios. So, this study has analyzed the effects of the two-phase density ratio, the flow channel size and shape, the local two-phase flow conditions and the wettability between the liquid metal and the flow channel wall surface on the two-phase flow behaviors and has proposed general mathematical expressions for the distribution parameter and the drift velocity. By fitting the experimental data, a new constitutive correlation for the distribution parameter and the drift velocity consisting of two flow-regime-independent correlation sets, which are, respectively, for the gas-liquid metal two-phase flows with the low and high wettability between the liquid metal and the flow channel wall surface, has been developed. The newly-developed drift-flux type correlation has been evaluated with the collected experimental data of the gas-liquid metal two-phase flows, and their mean relative error is 0.109. The predicted mean relative errors of the low and high wettability drift-flux type correlation sets are, respectively, 0.132 and 0.0782 for the gas-liquid metal two-phase flows with the low and high wettability between the liquid metal and the flow channel wall surface.

Study on bubbly and cap-bubbly flow in a square channel using dual wire-mesh sensors

The experiments of co-current vertical upstream air-water two-phase flow in a 66 mm × 66 mm square channel were carried out. The superficial liquid velocity ranges from 0.765 m/s to 1.53 m/s and the superficial gas velocity ranges from 0.0215 m/s to 0.313 m/s, including bubbly flow and cap-bubbly flow. A dual wire-mesh sensors (WMSs) was used to measure the bubbles in the two-phase flow. The local superficial gas velocity from the dual WMSs was compared with the rotameters and most results were within the deviation of 5%. It can ensure the measurement accuracy of the WMSs. The experimental results include the void fraction projection, sectional averaged void fraction variation, time-averaged void fraction profile, gas velocity profile, and the bubble size distribution. Within the range of flow conditions of this paper, the void fraction was relatively uniformly distributed among the cross-section in the bubbly flow and core peak in the cap-bubbly. Both cross-correlation velocity and bubble velocity were used to calculate the gas velocity profiles. Since the bubble lateral displacement, the cross-correlation method trend to miss the correlation caused by small bubbles and get a higher velocity. Bubble velocity was recommended to be used for determining gas velocity profiles. The results of void fraction profiles, gas velocity profiles, and bubble size distributions can be used for the validation of two-phase models of CFD codes. The void fraction predictions of two drift-flux correlations were compared against the experimental data and both models provided reasonable prediction accuracy.

Comparison of 63 different void fraction correlations for different flow patterns, pipe inclinations, and liquid viscosities

Gas–liquid two-phase flow is commonly encountered in the oil industry, especially in transport pipelines. Thus, the correct prediction of operational parameters such as void fraction or pressure drop is necessary for the development of efficient processes and facility design. Nowadays, there are different studies focused on predicting the void fraction parameter with empirical correlations. This study was aimed at analyzing and comparing 63 different void fraction correlations in order to determine if there was a unique correlation capable of accurately predicting the void fraction for the different operational conditions of flow patterns, pipe inclinations, and liquid viscosity. For this reason, a database of 11,895 experimental points was used to compare the results against the different empirical correlations available in the literature and determine the best one using statistical analyses based on indicators, such as relative error, absolute average percent error, among others. It must be mentioned that the database was strictly filtered and divided depending on the flow pattern (i.e., slug, churn, bubbly, and annular), the different pipe inclinations (i.e., vertical, horizontal and inclined), and the liquid viscosities. The results showed that there was not a unique unified correlation to determine the void fraction accurately for the different operational conditions and fluid properties. However, the best correlations for each specific set of flow patterns, pipe inclinations, and intervals of liquid viscosities were determined according to the statistical indicators, the general assumptions of each correlation, and information reported in the literature. Moreover, it was possible to analyze and provide some recommendations for the patterns evaluated, taking into account that some specific cases require further study, as there was no correlation capable of providing reliable results for the prediction of void fraction.

An improved drift-flux correlation for gas-liquid two-phase flow in horizontal and vertical upward inclined wells

Drift-flux model has been diffusely applied to characterize two-phase flow in pipes and wellbores owing to its accuracy and simplicity. The constitutive correlations used in the drift-flux model specifying the relative motion between the phases have been extensively investigated leading to the development of multitudinous correlations. However, previous studies reveal that most of the correlations can either be proper for limited two-phase flow conditions or the applicability is extended by increasing the complexity of the correlation. This study attempts to propose an improved drift-flux correlation concerning the distribution coefficient and drift velocity applying for a wide range of fluid properties, pipe orientations, and flow patterns without complicating the correlation. In this study, the drift velocity is defined in terms of pipe diameter, inclination angle, and void fraction while the distribution coefficient is correlated as a function of mixture Reynolds number and a Yield-Power-Law correlation in terms of void fraction. A comprehensive experimental database consisted of 1865 data records collected from 13 distinct sources is used for correlation parameterization and performance estimation. The proposed correlation shows comparable or even better performance in comparison to the other three well-established drift-flux correlations for predicting void fraction. The proposed correlation has also demonstrated outstanding performance in case where high liquid viscosity is involved.

A hybrid multiphase flow model for the prediction of both low and high void fraction nucleate boiling regimes

The improvement of a hybrid (a combination of Volume-of-Fluid (VOF) and Eulerian model) multiphase flow solver for the numerical prediction of high- and low-void fraction flow boiling regimes is presented in this article. These flow regimes could simultaneously occur in core-catcher and in-vessel retention-external reactor cooling systems during a severe accident in nuclear power plant. To enhance the prediction of the low void fraction boiling regime, a turbulent rough wall function model is implemented in the hybrid model to reproduce the impact of coarseness induced by the existence of growing bubbles along the heating wall on the liquid velocity profile. With this wall function, a more accurate prediction of the radial velocity profile is achieved within the uncertainty of the velocity measurements. Moreover, an improved prediction of the radial void fraction is achieved using the model proposed by Lopez de Bertodano for turbulence dispersion force without compromising the prediction of the radial gas velocity profile and radial liquid temperature profile. Although the hybrid model shows potential in capturing the interface and dynamic behavior of large-scale bubbles (vapor slug) for high void fraction regime, the predicted wall superheat is higher than the measured values. This highlighted the need for the extension of the present wall boiling model to cover flow boiling involving sliding vapor slugs on the heated wall.

Application of CFD-PBM coupling model for analysis of gas-liquid distribution characteristics in centrifugal pump

The gas and liquid distributions in the pump are significant for understanding the pump performance under bubble flow inlet. An accurate numerical simulation method will contribute to obtain the gas and liquid distribution characteristics in the pump. In this study, the simulation method based on the CFD-PBM (CFD-Population Balance Model) coupling model was developed and validated by experiments. The simulation results predicted by the new numerical method agreed better with the experimental results than calculations based on Eulerian-Eulerian model. The coalescence and breakup of bubbles in the centrifugal pump should be considered, which is well handled by the CFD-PBM coupling model. The influences of inlet gas volume fraction and liquid phase flow rate on the void fraction and bubble size distribution in the centrifugal pump were investigated. The results show that more bubbles accumulated in the impeller than in the volute as inlet gas volume fraction increased to a certain value. Small gas pockets accumulated in the impeller and formed large gas masses around the impeller inlet leading to “Surging” of the pump. The percentages of bubbles with large diameter increased with the inlet gas volume fraction increasing, rather, the bubbles with small diameter decreased. In addition, the bubbles were more likely to break up into smaller bubbles and difficult to aggregate into bigger bubbles when the liquid flow rate increased. Several void fraction and bubble size prediction models were also compared for the present cases. This study provides an effective method to simulate the pump characteristics under gas and liquid conditions, which is helpful to investigate the gas-liquid performance of the pump.

Validation of the Interfacial Area Transport Equation Coupled with the Void Transport Equation for Prediction of Flashing Flows

The static correlations from RELAP5 and TRACE as well as the interfacial area transport equation (IATE) are benchmarked for flashing flow with selected cases from a recently published experimental data set. The RELAP5 correlation is able to predict the interfacial area concentration more accurately than the TRACE correlation. The one-group decoupled IATE, supplied with experimental void fraction, shows overprediction of interfacial area concentration, especially at low-pressure conditions. Additionally, the one-group IATE is solved simultaneously with the void transport equation where at low pressures, the accuracy of the predicted interfacial area concentration improves even with the void fraction being underpredicted. However, as the pressure increases, the improving accuracy of the predicted void fraction leads to an overprediction of the interfacial area concentration. The two-group IATE is also benchmarked, first using the interfacial mass generation model from RELAP5 and TRACE and then with a model derived through a mass-energy balance approach. The accuracy of the two-group IATE is observed to be sensitive to the choice of the heat transfer length scale and Nusselt number correlations.

Validation and Uncertainty Quantification for Two-Phase Natural Circulation Flows Using TRACE Code

The work presents validation of the TRAC/RELAP Advanced Computational Engine (TRACE) code for natural circulation two-phase flow in a vertical annulus. Natural circulation experiments were recently conducted for a vertical internally heated annulus at the Multiphase Thermo-Fluid Dynamics Laboratory at the University of Illinois. The experimental matrix consists of 107 experiments with system pressure in the range of 145 to 950 kPa and heat flux up to 275 kW/m2. Void fraction, gas velocity, and interfacial area concentration were measured in five axial locations along the test section with six measurements of bulk liquid temperature and pressure. To validate the capability of the TRACE code under natural circulation flow conditions, a complete model of the experimental facility was created and validated using forced convection and single-phase natural circulation data. Sensitivity and uncertainty quantification were performed. The sensitivity to important simulation parameters was studied using Sobol’s variance decomposition and the Morris screening method. The sensitivity of boundary conditions on void fraction measurement was investigated. The sensitivity study has shown significant differences in model sensitivity between different experimental conditions. With heat flux being the most influential parameter for high-pressure cases without flashing and pressure, temperature and heat flux have a combined strong effect in the case of low-pressure experiments when flashing occurs. Additionally, higher uncertainty in void fraction prediction was observed for experimental conditions at low pressure with flashing.

Void Fraction Measurement of Oil-Gas-Water Three-Phase Flow Using Mutually Perpendicular Ultrasonic Sensor.

The complex flow structure and interfacial effect in oil–gas–water three-phase flow have made the void fraction measurement a challenging problem. This paper reports on the void fraction measurement of oil–gas–water three-phase flow using a mutually perpendicular ultrasonic sensor (MPUS). Two pairs of ultrasonic probes are installed on the same pipe section to measure the void fraction. With the aid of the finite element method, we first optimize the emission frequency and geometry parameters of MPUS through examining its sensitivity field distribution. Afterward, the oil–gas–water three-phase flow experiment was carried out in a vertical upward pipe with a diameter of 20 mm to investigate the responses of MPUS. Then, the void fraction prediction models associated with flow patterns (bubble flow, slug flow, and churn flow) were established. Compared to the quick closing valves, MPUS obtained a favorable accuracy for void fraction measurement with absolute average percentage error equaling 8.983%, which indicates that MPUS can satisfactorily measure the void fraction of oil–gas–water three-phase flow.

Improved drift-flux correlation to enhance the prediction of void fraction in nuclear reactor fuel bundles at low flow and elevated pressure conditions

ABSTRACTThe EPRI (1991) and the Clark et al. (2014) drift-flux correlations implemented in RELAP5/MOD3 code were benchmarked against the low-pressure void fraction data collected by Clark et al. (2014) at Purdue University and the high-pressure data collected by Anklam and Miller (1982) at ORNL. The Clark et al. correlation (2014) was found to perform best at low pressures of Clark et al. tests (2014), but the accuracy was deteriorated at higher pressures of Anklam and Miller tests (1982). To account for this effect, a pressure scaling factor has been added to the distribution parameter correlation to reduce the distribution parameter peaking at high pressures. This factor is dependent on the density ratio and it eliminates the distribution parameter enhancement effect for pressures over 0.5 MPa. It was confirmed that the new correlation with the correction improved the void fraction prediction performance at a higher pressure of Anklam and Miller tests (1982). The new correlation provides a transition between the Clark et al. correlation (2014) at low pressures and the correlation of Ozaki et al. (2013) and Ozaki and Hibiki (2015) at high pressures, which is excellent both in physical characteristics and scalability of two-phase flow in rod bundles.

An assessment of gas void fraction prediction models in highly viscous liquid and gas two-phase vertical flows

Gas void fraction plays a significant role in determination of several multiphase flow parameters. Good insight of its behaviour coupled with accurate prediction is imperative for design of efficient equipment which has the potential to translate to higher production rates in the petroleum industry. Against the background of the prevalence of higher viscous and imminent application of highly viscous liquids in the petroleum industry, air-water and air-low viscous liquid mixtures dominate gas void fraction research in vertical pipes. In this work, gas-liquid (μl=100−7000mPas) mixtures are used to investigate the behaviour of gas void fraction in vertical pipes. The influence of superficial phase velocities and liquid viscosity are observed. Further, a combined database consisting of experimental and the reported data of Schmidt et al. (2008) is employed to evaluate the predictions of 100 existing correlations. The results indicate that the Hibiki and Ishii (2003) and Bestion (1990) correlations are the overall best and second-best performing correlations. In the absence good performing correlations for churn and annular flows, two correlations each, based on drift flux and slip ratio, are developed respectively. Predictions from these correlations show good agreement with the database and comparable performance with the overall best correlations.

Drift-flux correlation for upward two-phase flow in inclined pipes

In view of the practical importance of the drift-flux model to predict void fraction for gas-liquid flow in various engineering fields including chemical engineering, this study aims at developing a drift-flux correlation with a wide range of applicability for upward two-phase flows in inclined pipes. Firstly, over 3000 experimental data of void fraction were collected from 14 sources. Then, the correlations of distribution parameter and void-fraction-weighted-mean drift velocity were established. The comparison between the collected database and correlations indicated none of the existing correlations could predict the whole database satisfactorily. Therefore, the dependence of drift-flux parameters on inclination angles was comprehensively analysed, and a new drift-flux correlation was developed. The newly-developed drift-flux correlation demonstrated superior performance to existing correlations. More than 95% of the predicted void fraction were predicted within ±20% error. In summary, the newly-developed drift-flux correlation is useful to predict void fraction in various engineering fields including chemical engineering.

On bubble forces in turbulent channel flows from direct numerical simulations

The prediction of void fraction, which relies on interfacial force models, is a major issue in the context of boiling. The two-fluid model requires the modelling of the momentum transfer between phases. When bubbles are small (particle hypothesis), the momentum transfer is related to interfacial forces acting on bubbles. However, the splitting of these forces into drag, lift, added mass, etc., is not straightforward from the local point of view, where only the total interfacial force is defined as an integral of the constraint over the interface. For large-size bubbles, the particle hypothesis can be questioned. The momentum transfer can then be connected to the forces acting on a fluid element of the vapour phase. Based on the local and averaged formulations of the Navier–Stokes equations, a new balance equation for forces enables us to define lift, drag, added-mass and dispersion forces acting on a fluid element of the vapour phase. This equation gives a local definition for all the forces responsible for spatial distribution of bubbles and reflects the meaning usually assigned to the interfacial forces in the particle approach. Through this means, the link between the local formulation and physical phenomena is established and a new way of modelling the lift force is proposed. Furthermore, a new laminar dispersion force which relies on surface tension and pressure effects is introduced. The analysis of the budget equation on our direct numerical simulation database brings into light the large influence of this laminar dispersion force in the migration process. Different well-known physical behaviours can be modelled via this new force: the horizontal clustering of spherical bubbles in laminar flows and the oscillating trajectories of deformable bubbles.

Surrogate modeling of advanced computer simulations using deep Gaussian processes

The continuous advancements in computer power and computational modeling through high-fidelity and multiphysics simulations add more challenges on assessing their predictive capability. In this work, metamodeling or surrogate modeling through deep Gaussian processes (DeepGP) is performed to construct surrogates of advanced computer simulations drawn from the nuclear engineering area. This work is centered around three major ideas: (1) surrogate modeling through deep Gaussian processes (DeepGP), (2) simulation assessment through surrogate-based uncertainty quantification (UQ) methodologies, and (3) drawing conclusions regarding the underlying uncertainty of the four simulations considered in this paper. First, DeepGP models are trained, optimized, and validated to yield variety of features: (1) achieving high accuracy (small error metrics) on the validation set, (2) automatically capturing the surrogate model uncertainty (i.e. interpolation errors), (3) fitting multiple outputs with different scales simultaneously, (4) handling high dimensional input spaces, and (5) learning from small data amounts. Second, the validated DeepGP surrogates are utilized to efficiently perform UQ tasks such as uncertainty propagation (through Monte Carlo sampling), parameter screening (through Morris screening), and variance decomposition (through Sobol Indices) to investigate the selected simulations. Third, the thermal-hydraulics (fluid flow) results demonstrate the importance of inlet temperature uncertainty in void fraction predictions. For the reactor physics application (fuel depletion/consumption), DeepGP accurately captures the uncertainty in criticality calculations, which is about 0.6% (i.e. a considerable value for this application). For the application of kinetic parameters (nuclear data), DeepGP successfully explains 95% or more of the variance in all 12 outputs. Finally, DeepGP-based UQ analysis of the fuel performance application (materials science) shows the importance of the clad surface temperature, fuel porosity, and linear heat rate in explaining the variance of the maximum fuel centerline and surface temperatures.

Void fractions in a rod bundle geometry at high pressure–part Ⅰ: Experimental study

An investigation of void fraction measurement and prediction in a triangular-array rod bundle geometry was carried out under high pressure conditions (P = 5–9 MPa and G = 100–350 kg·m−2·s−1). The aims of this work were to expand the existing database and assess and modify the predictive models. The present paper is the Part I of this study, where the experimental work was presented to measure local, chordal and area-average void fractions respectively by a comprehensive measuring system composed by optical probes and gamma-ray densitometry. Non-uniform distribution of void fraction was investigated by local-to-local and local-to-average comparisons of void fractions. The results indicated that non-uniform distribution was not only related to the geometry, but also affected by the transition of flow regimes in rod bundle. The effects of pressure and mass flux on void fractions were also analyzed in detail in consideration of variations of fluid properties and drift-flux parameters. Additionally, both void fraction and interface frequency were compared with the flow regime map in TRACE code. The variation of interface frequency indicated that the droplet entrainment would occur in annular flow at high mass flux. In the Part Ⅱ of this study, an assessment of existing models for void fraction prediction in rod bundles was carried out according to the present data and a newly-developed drift-flux model was proposed.