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• A robust drift-flux model for upward high-viscosity two-phase flows was proposed. • The rigorous constitutive equations of the drift velocities were formulated based on drag law. • The dependence of drift-flux parameters on viscosities and flow regimes was identified. • The new theoretically-supported model achieved a superior predictive performance. The drift-flux model is of practical importance in predicting the void fraction of upward two-phase flows with high-viscosity liquid phase. This study aims to develop an accurate and robust drift-flux correlation for each type of upward high-viscosity two-phase flow to overcome the unsatisfactory predictive performance of the existing drift-flux correlations. First, an extensive literature review was conducted which collected more than 500 experimental void fractions of upward high-viscosity two-phase flows from 11 sources. The viscosity numbers for the collected database ranged from 0.0213 to 25.6. The existing drift-flux correlations of high-viscosity two-phase flows were critically reviewed. A comparison between the collected database and the existing drift-flux correlations indicated that none of the existing drift-flux correlations could predict the entire collected database with acceptable accuracy. The failure of the void fraction prediction using the existing drift-flux correlations drove the development of new drift-flux correlations based on the state-of-the-art understanding of the flow behaviors for upward high-viscosity two-phase flows. The rigorous constitutive equations of the drift velocities were formulated based on the drag law. The dependence of the distribution parameter on the physical properties and flow conditions was systematically analyzed. Finally, the new drift-flux correlation was developed for upward high-viscosity two-phase flows. This correlation was well-validated as demonstrating superior predictive performance to the existing correlations; for example, 92.2% of the collected void fractions were predicted within ± 20 % error without any systematic bias. The correlations are considered to have great practical potential in improving the analysis of high-viscosity two-phase flows. [ABSTRACT FROM AUTHOR]

• New one-group drift-flux model (DFM) was developed for dispersed flows in annuli. • New two-group DFM was developed for dispersed flows in annuli. • Developed one-group and two-group DFMs were validated by extensive data. • Two-group DFM can be used for calculating two-group interfacial area concentration. • Two-group DFM can calculate two-group gas velocity using mixture momentum equation. Interfacial area concentration (IAC) plays a vital role in mass, momentum, and heat transfers between gas and liquid phases in two-phase flows. The IAC of group-one bubbles, including spherical and distorted bubbles, differs from the IAC of group-two bubbles, including cap, slug and churn-turbulent bubbles. The two-group interfacial area transport equation (IATE) can be used as a mechanistic model to obtain the IAC of each group. The equation demands the two-group gas velocity fields, leading to the need to solve an extra momentum equation, which is complicated. The two-group drift-flux model can prevent this complexity by providing two-group gas velocities without introducing the extra momentum equation. Despite the importance of gas-liquid flows in annuli, the drift-flux model for gas-liquid flows is not well-developed like it is for pipe flows. The main objective of this study is to develop a two-group drift-flux model for dispersed vertical flows in annuli. The two-group drift-flux model provides the flow characteristics of group-one and group-two bubbles, such as void fractions and velocities, which strongly affect the prediction of the available interfacial area concentration of the group-one and group-two bubbles for heat and mass transfer. Hence, after comprehensively investigating the available experimental data, this study first improves a one-group drift-flux model for dispersed gas and low-viscosity liquid upward flows in annuli and proposes a new two-group drift-flux model. Then, the models are evaluated using the respective group experimental data. The consistency between one-group and two-group drift-flux models is also investigated. This consistency helps to reduce the two gas momentum equations to one gas mixture momentum equation. The results of the comparison between model predictions and experimental data are satisfactory. [ABSTRACT FROM AUTHOR]

• New method was developed to obtain subchannel average two-phase flow parameters. • New flow parameter mapping data was obtained for 8 × 8 rod bundle. • New two-group drift-flux model for 8 × 8 rod bundle was developed. • New two-group drift-flux model is useful for predicting interfacial area concentration. • Newly obtained data is useful for subchannel analysis code validation. Interfacial transfer terms in gas-liquid two-phase flows are formulated as the product of interfacial area concentration (IAC) and flux. The interfacial area transport equation (IATE) is essential for obtaining IAC in transient and developing two-phase flows. Two-group IATE was developed to account for the difference in the interfacial drag force between two groups of bubbles, where spherical and distorted bubbles are categorized as group one, while cap, slug, and churn turbulent bubbles are categorized as group two. The rigorous two-group approach requires two-group gas momentum equations, resulting in one additional momentum equation in the two-fluid model. The mixture gas momentum equation is considered to avoid the additional momentum equation. The two-group drift-flux model is necessary to calculate the two-group gas velocity from the mixture velocity. The present study develops an approximation methodology to acquire subchannel average and rod bundle average two-group two-phase flow parameters based on the validated power law assumption and a limited number of local data. The two-group model is developed for the distribution parameter and drift velocity for dispersed two-phase flows in rod bundles. The total performance of the newly developed two-group drift-flux model is evaluated by two-phase flow data in rod bundles. The results show that the developed two-group drift-flux model could well predict two-group gas velocities in rod bundles. This model is useful for subchannel thermal hydraulic analysis codes as a complement to the conventional one-group drift-flux model. [ABSTRACT FROM AUTHOR]

• Methodology to estimate subchannel average flow parameters was developed. • Subchannel average two-phase flow parameters were obtained in an 8 × 8 rod bundle. • Operating parameter effects on the flow characteristics were found. • The distribution parameter model for the rod bundle was validated independently. • The drift velocity model for the rod bundle was validated independently. Two-phase flows in rod bundles or tube bundles appear in heat transfer systems. The bundle-average (or one-dimensional) void fraction is a critical parameter in overall system design, performance evaluation, and safety assessment. Accurate bundle-average void fraction prediction requires a precise interfacial momentum transfer term formulated using the distribution parameter and drift velocity, two essential drift-flux parameters in the drift-flux model. The currently utilized modeling approach is based on the distribution parameter and drift velocity determined together through drift-flux plots. This results in possible compensation errors between the distribution parameter and drift velocity in the model validation process. The independent validation of the constitutive equations for the distribution parameter and drift velocity has been challenging due to experimental difficulty in measuring detailed two-phase flow structures in rod bundles. The present study proposes an approximation methodology to obtain subchannel average two-phase flow parameters using local two-phase flow data measured at two local points: the subchannel center and minimum gap center in a rod bundle. The distributions of subchannel average two-phase flow parameters are invaluable in benchmarking subchannel analysis codes. Comprehensive subchannel average void fraction and gas velocity mappings are given to discuss the effects of gas and liquid velocities, pressure, spacer grid, and developing length on the two-phase flow characteristics in the rod bundle. The bundle-average distribution parameter values calculated by subchannel average two-phase flow parameters are used for independent validation of the constitutive equations of the distribution parameter. The bundle-average drift velocity values back-calculated using the distribution parameter values are also used for independent validation of the constitutive equation of the drift velocity. [ABSTRACT FROM AUTHOR]

• A new two-group drift-flux model (DFM) was developed for large-diameter pipes. • Group-one drift velocity was proposed by considering group-two effects. • Group-one and group-two distribution parameters were determined. • The one-group DFM could be reproduced by the two-group DFM. • The model was applicable to adiabatic two-phase flows. Interfacial heat and mass transfer are prevalent in industrial processes. The interfacial transfer rate can be obtained by the product of their fluxes and interfacial area concentration (IAC) calculated by the interfacial area transport equation (IATE). Bubbles show different behavior according to their sizes. Hence, bubbles are classified into two groups. Consequently, two-group IATE is required causing to use of two gas momentum equations leading to more complexity. The present study suggests a new reliable two-group drift-flux modeling to reduce the two gas momentum equations to one gas mixture momentum equation for gas-liquid flow in large-diameter pipes. The model is developed based on the drift-flux model concept and experimental data. Group-one and group-two distribution parameters and drift velocities are validated through experimental data. The results show that the proposed two-group drift-flux model can support the concept of drift velocity from the bubbly to beyond the bubbly flow and consistency between the one-group and two-group drift-flux models. Moreover, steam-water data are used to validate the applicability of the model in steam-water flows condition. The developed two-group drift-flux model is indispensable for reducing the two gas momentum equations to one gas mixture momentum equation when two-group IATE is implemented into thermal-hydraulic codes to improve the prediction accuracy of IAC. [ABSTRACT FROM AUTHOR]

• A drift-flux correlation for a large annulus is developed. • A threshold length scale between medium and large annuli is proposed. • Applicability of drift-flux correlation to an eccentric annulus is confirmed. • Distribution parameter in churn flow is independent of channel size. In view of the practical importance of predicting void fraction in two-phase flow, a correlation based on the drift-flux model is often utilized in many industrial fields. To use the drift-flux type correlation, two drift-flux parameters such as distribution parameter and void fraction weighted mean drift velocity should be modeled accurately. Since the drift-flux parameters are susceptible to channel size, they are modeled differently depending on whether the channel size is characterized as small (or mini), medium or large. Although drift-flux type correlations with modeled distribution parameter and drift velocity for medium size channels with various geometries have been well-developed, limited drift-flux correlations are available for large size channels. Two-phase flow characteristics in a large size channel are quite different from those in a medium size channel at low flow conditions. Large slug bubbles observed in a medium size channel do not form in a large size channel due to the presence of surface instability. As large slug bubbles disintegrate into smaller cap bubbles, the drift velocity is increased. Complex two-phase flow characteristics in a large size channel also affect the distribution parameter. This study successfully models the distribution parameter and drift velocity for two-phase flow in a vertical concentric annulus. The drift-flux correlation with modeled distribution parameter and drift velocity is validated by existing data taken in large size annuli. The applicability of the drift-flux correlation to two-phase flow in a vertical eccentric annulus is also examined by existing data. [ABSTRACT FROM AUTHOR]

Abstract The study of two-phase flow in rectangular channels is of great importance due to their use in marine reactors, chemical reactors, test reactors, and neutron spallation sources. Due to the practical importance of the drift-flux correlation for two-phase flow analysis in industrial systems, distribution parameter and drift velocity have been studied for vertical upward two-phase flow in a large size rectangular channel. Due to the presence of liquid recirculation in the bubbly flow regime in large size rectangular channels, the distribution parameter, drift velocity, and flow regime transition criteria are different from those found in small or medium size rectangular channels. The use of hydraulic equivalent diameter as the characteristic length scale in channels with significant asymmetry and its effect on interfacial area concentration prediction are also discussed. Under these considerations a new drift-flux correlation has been developed for upward two-phase flow in large size rectangular channels. Comparison of the newly developed correlation with several datasets shows reasonable agreement. [ABSTRACT FROM AUTHOR]

Highlights • Constitutive equations for a vertical rod bundle are reviewed. • Dependence of distribution parameter on flow conditions is discussed. • 1D momentum equation by considering void fraction distribution is discussed. • Effect of channel size on interfacial area concentration is discussed. Abstract In view of the quality assurance of two-phase flow simulations, CSAU (Code Scalability, Applicability, and Uncertainty) methodology and code V & V (Verification and Validation) have been proposed. The estimation of simulation uncertainty is indispensable in using best-estimate computational codes. A key of successful two-phase flow simulations is to use the state-of-the-art constitutive equations to close the mathematical system used in two-phase flow analyses. The advanced constitutive equations should be developed based on “physics” behind phenomena and should consider scaling parameters which enable their application beyond test conditions used for a code validation. Two-phase flow simulations in a rod bundle is important in various industrial apparatuses such as heat exchangers and nuclear reactors. Constitutive equations for two-phase flows in a vertical rod bundles have been advanced in recent five years. In view of this, this paper provides a comprehensive review of most advanced constitutive equations for two-phase flow analyses in a vertical rod bundle. The constitutive equations of two-phase flow parameters reviewed in this paper are flow regime map, void fraction, void fraction covariance and relative velocity covariance, interfacial area concentration and wall friction. In addition, an exact formulation of one-dimensional momentum equation in two-fluid model considering void fraction distribution is discussed. [ABSTRACT FROM AUTHOR]

Utilization of one-dimensional system analysis code such as TRACE, RELAP5, TRAC-BF1 to evaluate gas-liquid two-phase flow behaviors in nuclear power plants is crucial for the plant-level safety assessment. In the two-fluid model, interfacial momentum transfer between two phases is expressed under interfacial drag term in the momentum equation. For the rigorous and accurate expression of interfacial drag term and drift flux parameter, covariance due to the area averaging of void fraction distribution must be considered. In the present paper, an effect of the covariance on void fraction prediction in pipe flow was numerically assessed by implementing Hibiki and Ozaki's model into the interfacial drag term in the one-dimensional two-fluid model. For the low flow rate with high void fraction conditions, it was found that the inclusion of covariance model slightly underestimated void fraction value than that calculated by the drift-flux model. This underestimation comes from the momentum source term in the two-fluid model, which was derived under the assumption of uniform void fraction distribution. Therefore, in this paper, momentum source term was rederived with consideration of void fraction covariance and a complete set of momentum equation and constitutive formulations for the one-dimensional two-fluid model is presented. [ABSTRACT FROM AUTHOR]

A drift-flux correlation has been often used to predict void fraction of gas-liquid two-phase flow in a horizontal channel due to its simplicity and practicality. The drift-flux correlation includes two important drift-flux parameters, namely, the distribution parameter and void-fraction-weighted-mean drift velocity. In this study, an extensive literature survey for horizontal two-phase flow is conducted to establish void fraction database and to acquire existing drift-flux correlations. A total of 566 data is collected from 12 data sources and 4 flow-regime-dependent and 1 flow-regime-independent drift-flux correlations are identified. The predictive capability of the existing drift-flux correlations is assessed using the collected data. It is pointed out that the drift velocity determined by a regression analysis may include a significant error due to a compensation error between distribution parameter and drift velocity. In this study, a simple flow-regime-independent drift-flux correlation is developed. In the modeling approach, the void-fraction-weighted mean drift velocity is approximated to be 0 m/s, whereas the distribution parameter is given as a simple function of the ratio of non-dimensional superficial gas velocity to non-dimensional mixture volumetric flux. The newly developed correlation shows an excellent predictive capability of void fraction for horizontal two-phase flow. Mean absolute error (or bias), standard deviation (random error), mean relative deviation and mean absolute relative deviation of the correlation are 0.0487, 0.0985, 0.0758 and 0.206, respectively. The prediction accuracy of the correlation is similar to the correlation of Chexal et al. (1991), which was formulated based on the drift-flux parameters by means of many cascading constitutive relationships with numerous empirical parameters. [ABSTRACT FROM AUTHOR]

An experimental investigation of horizontal forced vibration effect on void fraction variation of subcooled boiling flow was carried out in this study. In order to simulate the fuel assembly subchannel of a boiling water reactor (BWR), an annular test section with inner and our diameters of 19.1 and 38.1 mm was utilized for subcooled boiling tests under atmospheric pressure. The annular test section was attached to an eccentric-cam vibrator, which was driven by a low-speed motor and can produce horizontal forced vibrations with frequency up to 20 Hz and maximum displacement of 22.2 mm. The inlet liquid velocity and subcooling were set as v f , in = 0.25–1.00 m/s and Δ T Sub = 5–20 °C. Different heat fluxes of q ″ = 0.058–0.193 MW/m 2 were loaded through the center heater rod, and the void fraction and fluid temperature were measured during the tests under stationary (no vibration) and vibration conditions. Test results show that in the subcooled boiling region, the void fraction and fluid temperature can vary under horizontal forced vibrations, and the variation trends were presented in N Zu - N Sub and v f , in -〈 α 〉 plots. These variations can be explained by the potential changes of thermal boundary layers (TBL) and the heat transfer enhancement under vibrations. In addition, no significant change of void fraction and fluid temperature was found in or near the saturated boiling conditions under vibrations. [ABSTRACT FROM AUTHOR]

Drift-flux parameters have been often used to formulate one-dimensional interfacial drag force in dispersed two-phase flow, which is one of key parameters to predict void fraction using one-dimensional thermal-hydraulic codes. This approach is called “Andersen approach”, which has been widely used in one-dimensional nuclear thermal-hydraulic system analysis codes such as TRACE, RELAP5 and TRAC-BF1. However, the current formulation of one-dimensional interfacial drag force ignores important void fraction covariance and relative velocity covariance when local interfacial drag force is converted to one-dimensional interfacial drag force. The impact of neglecting void fraction covariance and relative velocity covariance on one-dimensional interfacial drag force and relative velocity has been discussed in detail. In view of the importance of the drift-flux parameters, void fraction covariance and relative velocity covariance on one-dimensional formulation of the interfacial drag force, three constitutive equations have been developed for upward boiling two-phase flow in a vertical pipe. The validity of the modeled void fraction covariance and relative velocity covariance for subcooled and bulk boiling flow in a vertical pipe has been verified by boiling R12 data taken in a vertical pipe with the diameter of 19.2 mm under the pressure simulating prototypic nuclear reactor thermal-hydraulic conditions. The correlation of void fraction covariance agrees with the boiling flow data in the vertical pipe with the mean absolute error, standard deviation, mean relative deviation and mean absolute relative deviation being 0.828, 3.43, 10.3% and 33.5%, respectively. The correlation of relative velocity covariance agrees with the boiling flow data in the vertical pipe with the mean absolute error, standard deviation, mean relative deviation and mean absolute relative deviation being −0.00394, 0.0663, −0.184% and 5.11%, respectively. Due to the great importance of the void fraction covariance and relative velocity covariance on one-dimensional interfacial drag force formulation, it is highly recommended to include the void fraction covariance and relative velocity covariance in the one-dimensional formulation of the interfacial drag force used in nuclear thermal-hydraulic system analysis codes. [ABSTRACT FROM AUTHOR]

The void fraction prediction is critical in nuclear safety analyses. Available one-dimensional computational codes heavily depend on the two-fluid model. The interfacial drag force in the momentum equation of the two-fluid model is currently formulated using the drift-flux model. Thus, the drift-flux model is critical for advanced two-phase flow analysis computational codes. The drift-flux model is characterized by two essential parameters: the distribution parameter and the drift velocity. The distribution parameter expresses the covariance of the area-averaged product of a void fraction and mixture volumetric flux. Flow channel geometry is a significant factor affecting the distribution parameter. The drift velocity expresses the relative velocity between gas and liquid phases. A bubble shape regime affects the drift velocity. The bubble shape regime is susceptible to flow regime dependence on the bubble Reynolds number. Bubbles in a spherical or distorted particle regime (group-1 bubbles) dominate in bubbly flow at a low void fraction, whereas bubbles in a slug or cap bubble regime (group-2 bubbles) play a critical role in beyond-bubbly flow. Whether slug bubbles or cap bubbles appear depends on the channel size. Thus, flow channel size significantly affects the drift velocity. The drift velocity dynamically changes along the two-phase flow, transforming from bubbly to beyond-bubbly flows. The present study first formulates the evolution process of the drift velocity using the two-bubble-group approach. A two-group-based formulation is simplified to model the drift velocity behavior at the bubbly-to-beyond-bubbly flow transition. A scheme is proposed to explicitly calculate the distribution parameter and drift velocity using operating parameters. A drift-flux correlation with the new drift velocity model is evaluated by 750 data collected from 8 different sources. Tested fluid systems are nitrogen-water, air-water, and steam-water systems. Flow channel diameter ranges from 0.0508 to 0.305 m. Test section height (L)-to-channel diameter (D) ratio L / D ranges from 9.41 to 130. Superficial gas velocity is changed from 0.0100 to 11.2 m/s, whereas superficial liquid velocity is changed from 0 (pool condition) to 2.6 m/s. Operating pressure covers from 0.1 to 4.6 MPa. A systematic evaluation demonstrates the validity of the newly developed drift velocity model for a wide range of test conditions. This model is simple enough to be implemented into existing one-dimensional computational codes without any substantial code architecture change. • A new drift-flux correlation was developed for medium-to-large-size channels. • Drift velocity was modeled by the two-group bubble approach. • The applicable range covered pressures from 0.1 to 4.6 MPa. • The correlation was validated for channel sizes up to 0.305 m. • The correlation was applicable to adiabatic and boiling two-phase flows. [ABSTRACT FROM AUTHOR]

In order to investigate the potential seismic vibrations effect on two-phase flow in an annular channel, experimental tests with air-water two-phase flow under horizontal vibrations were carried out. A low-speed eccentric-cam vibration module capable of operating at motor speed of 45–1200 rpm ( f  = 0.75–20 Hz) was attached to an annular channel, which was scaled down from a prototypic BWR fuel sub-channel with inner and outer diameters of 19.1 mm and 38.1 mm, respectively. The two-phase flow was operated in the ranges of 〈 j f 〉 = 0.25–1.00 m/s and 〈 j g 〉 = 0.03–1.46 m/s with 27 flow conditions, and the vibration amplitudes controlled by cam eccentricity ( E ) were designed for the range of 0.8–22.2 mm. Ring-type impedance void meters were utilized to detect the area-averaged time-averaged void fraction under stationary and vibration conditions. A systematic experimental database was built and analyzed with effective maps in terms of flow conditions (〈 j g 〉-〈 j f 〉) and vibration conditions ( E - f and f - a ), and the potential effects were expressed by regions on the maps. In the 〈 j g 〉-〈 j f 〉 maps, the void fraction was found to potentially decrease under vibrations in bubbly flow regime and relatively lower liquid flow conditions, which may be explained by the increase of distribution parameter. Whereas and the void fraction may increase at the region closed to bubbly-to-slug transition boundary under vibrations, which may be explained by the changes of drift velocity due to flow regime change from bubbly to slug flows. No significant change in void fraction was found in slug flow regime under the present test conditions. [ABSTRACT FROM AUTHOR]

A steam generator thermal-hydraulic code based on homogeneous flow model has been useful based on its numerical stability and simpler formulation. One of key parameters for a steam generator thermal-hydraulic analysis is void fraction which determines two-phase mixture density and affects two-phase mixture velocity. These parameters are important for a heat transfer tube vibration analysis. A void fraction-quality correlation is very important to accurately convert the quality into the void fraction. The void fraction-quality correlation should preferably be applicable to parallel and cross flows in rod or tube bundles since two-phase flow in the steam generator encounters flow configuration change from the parallel flow along the tube bundle in the riser section of the steam generator to the cross flow in the U-bend section of the steam generator. A set of correlations depending on flow configuration such as parallel and cross flows, rod or tube array pattern and mass flux is developed based on legacy Smith correlation. The correlation agrees with the parallel and cross flow data with the mean absolute error (or bias) of 0.117% and the standard deviation (random error) of 2.26% and with the mean absolute error (or bias) of 0.760% and the standard deviation (random error) of 6.21%, respectively. The correlations are further simplified to a single correlation applicable for parallel and cross flow in rod or tube bundles. The Smith correlation with a modified constant entrainment parameter e being 0.5 is recommended for predicting void fraction in the steam generators. The Smith correlation with e = 0.5 is expected to be applicable for parallel and cross flows with various rod or tube array patterns including normal square, parallel triangular and normal triangular arrays. [ABSTRACT FROM AUTHOR]

In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube heat exchangers, a drift-flux correlation has been developed to meet the demand on the study of two-phase flow gas and liquid velocities, two-phase pressure drop, heat transfer, flow patterns and flow induced vibrations in the shell side. Two critical parameters such as distribution parameter and drift velocity have been modeled. The distribution parameter is obtained by constant asymptotic values and taking into account the differences in channel geometry. The drift velocity is modelled depending on the density ratio and the non-dimensional viscosity number. The relationship between the channel averaged and gap mass velocity has been discussed in order to obtain the superficial gas and liquid velocities in the drift-flux correlation. The newly developed drift-flux correlation agrees well with cross-flow experimental databases of air-water, R-11 and R-113 in parallel triangular, normal square and normal triangular arrays with the mean absolute error of 1.06% and the standard deviation of 4.47%. In comparison with other existing correlations, the newly developed drift-flux correlation is superior to other studies due to the improved accuracy. In order to extend the applicability of the newly developed drift-flux correlation to void fraction of unity, an interpolation scheme has been developed. The newly developed drift-flux correlation is able to calculate the void fraction of cross-flow over a full range with different sub-channel configurations in shell-tube type heat exchangers. [ABSTRACT FROM AUTHOR]

• Experiments to investigate the flow features in a vertical narrow rectangular channel are performed. • A database of two-group bubble local parameters in the two-phase flow is presented. • Abrupt and gradual axial two-group bubble evolutions at flow regime transition happen. • The two-group interfacial area transport equation (IATE) is evaluated using the narrow rectangular channel data. • The IATE evaluation results show that the cap-bubbly and slug flow prediction needs improvement. This study has experimentally investigated flow characteristics of two group bubbles in upward two-phase flows in a vertical narrow rectangular channel with 0.993 mm in gap, 40 mm in width, 2000 mm in height, and 1.94 mm in equivalent hydraulic diameter, D h. The local measurements are performed at seven axial positions (z / D h =52, 104, 155, 207, 415, 622, 829 (z : the axial distance from the inlet) using a high-speed camera. The database of the locally evolving parameters has been established by processing the images from the high-speed camera measurements. The measured parameters include the void fraction, interfacial area concentration (IAC), Sauter mean diameter and number density of group-1 and -2 bubbles, and gas-phase velocity of all bubbles. A total of 14 flow conditions are set at four different superficial liquid velocities ranging from 0.214 m/s to 2.08 m/s and different superficial gas velocities ranging from 0.0755 m/s to 1.70 m/s at the inlet. The observed flow regimes cover the bubbly, cap-bubbly, and slug flows. The measured void fraction ranges from 3.92 % to 42.6 %. The axially developing features of the two-phase flows are analyzed for the three flow regimes. Abrupt and gradual axial two-group bubble evolutions with intensive bubble coalescence at the flow regime transition are, respectively, observed at low and high superficial liquid velocity conditions. The newly-obtained experimental database has been used to evaluate the existing two-group interfacial area transport equation (IATE) and its corresponding source and sink models for the two-phase flow in the vertical narrow rectangular channel. Group-1 bubble IAC mean relative error of 21.1% and group-2 bubble IAC mean relative error of 33.4% have been obtained in the evaluations. The existing two-group IATE cannot satisfactorily predict the intensive bubble coalescence and the intragroup transport of the two-group bubbles at the transition from bubbly to slug flow in the vertical narrow rectangular channel. Therefore, it is necessary to improve the existing models of the intergroup and intragroup bubble interactions in the existing two-group IATE in the future. [ABSTRACT FROM AUTHOR]

• Void fraction data for the tight-lattice rod bundle at various operation conditions were collected. • The dependency of drift-flux parameters on physical properties and bundle geometries were assessed. • The existing drift-flux correlations developed for the reactor safety analysis for rod bundles were reviewed. In view of the energy system design involving two-phase flow in various industrial apparatus, the void fraction is one of the most critical parameters, which characterizes gas fraction in a two-phase mixture. Accurate prediction of a void fraction is indispensable to estimate two-phase mixture levels and represent the flow behaviors in a given flow channel. A drift-flux type correlation has been utilized in existing one-dimensional thermal-hydraulic system analysis codes to calculate various two-phase flow parameters in a rod bundle. Due to the demand for higher performance of equipment, a tight-lattice configuration of the rod array has been given increased attention. The tight-lattice rod-bundle geometry has a much smaller rod diameter-to-rod pitch length than a typical rod-bundle utilized in equipment, such as compact heat exchangers, advanced steam generators, and next-generation nuclear reactors. As a result, it is expected that the flow characteristics may differ from conventional rod-bundle geometries. Therefore, developing the reliable void fraction prediction model for accurately simulating the two-phase flow behavior in a tight-lattice rod-bundle will be essential from the safe, economical, and robust design aspect. The present study extensively reviews existing drift-flux correlations applicable to rod-bundle geometries. Their performances were assessed against available experimental data collected from test facilities equipped with tight-lattice configuration, and applicability towards tight-lattice bundle geometry was considered. [ABSTRACT FROM AUTHOR]

This study performed a survey on existing correlations for interfacial area concentration (IAC) prediction and collected an IAC experimental database of two-phase flows taken under various flow conditions in large diameter pipes. Although some of these existing correlations were developed by partly using the IAC databases taken in the low-void-fraction two-phase flows in large diameter pipes, no correlation can satisfactorily predict the IAC in the two-phase flows changing from bubbly, cap bubbly to churn flow in the collected database of large diameter pipes. So this study presented a systematic way to predict the IAC for the bubbly-to-churn flows in large diameter pipes by categorizing bubbles into two groups (group 1: spherical or distorted bubble, group 2: cap bubble). A correlation was developed to predict the group 1 void fraction by using the void fraction for all bubble. The group 1 bubble IAC and bubble diameter were modeled by using the key parameters such as group 1 void fraction and bubble Reynolds number based on the analysis of Hibiki and Ishii (2001, 2002) using one-dimensional bubble number density and interfacial area transport equations. The correlations of IAC and bubble diameter for group 2 cap bubbles were developed by taking into account the characteristics of the representative bubbles among the group 2 bubbles and the comparison between a newly-derived drift velocity correlation for large diameter pipes and the existing drift velocity correlation of Kataoka and Ishii (1987) for large diameter pipes. The predictions from the newly-developed two-group IAC correlation were compared with the collected experimental data in gas–liquid bubbly to churn flow regimes in large diameter pipes and their mean absolute relative deviations were obtained to be 28.1%, 54.4% and 29.6% for group 1, group 2 and all bubbles respectively. [ABSTRACT FROM AUTHOR]

In view of an important role of a one-dimensional drift-flux correlation in nuclear thermal-hydraulic system analysis codes, several drift-flux correlations such as Lellouche–Zolotar, Chexal–Lellouche, TRAC-BF1 and Ozaki correlations have been reevaluated by rod bundle test data taken in FRIGG and NUPEC test facilities. The mean absolute error of void fraction representing a correlation bias of the Lellouche–Zolotar, Chexal–Lellouche, TRAC-BF1 and Ozaki correlations are, respectively, −1.0, 0.5, −6.3 and −3.3% for the FRIGG test data and 2.0, 2.3, −0.4 and −0.7% for the NUPEC test data. The effects of unheated rods, axial and radial power distributions, large unheated center rod and geometry of a shroud or casing on void fraction are identified. The presence of unheated rods with similar size of other heated rods tends to increase a distribution parameter in a drift-flux correlation, whereas the presence of a large unheated center rod tends to decrease the distribution parameter. The axial and radial power distributions do not have significant effect on void fraction within the tested axial and radial power distribution range. The Ozaki correlation is recommended for predicting void fraction in a BWR core but it is suggested to reduce the distribution parameter in the Ozaki correlation if a large unheated center rod exists in the core. It is indicated that drift-flux correlations developed based on bounded rod bundle test facility data may overestimate the distribution parameter for a PWR core. [ABSTRACT FROM AUTHOR]

• The channel size effect on drift-flux parameters is detailed. • Critical size such as 30 times Laplace length does not apply to non-circular channels. • The bundle distribution parameter depends on bundle casing geometry and rod array. • The channel size effect is more pronounced at low pressure and low flow conditions. • The channel size has less impact for beyond-bubbly flows. Gas-liquid two-phase flows are utilized in various heat and mass transfer processes that appear in numerous industrial apparatus. Typical examples are chemical reactors, heat exchangers, nuclear reactors, deaerators, condensers, etc. The numerical simulation of gas-liquid two-phase flows, which is crucial for efficient, safe, and optimized apparatus design, requires an accurate model developed based on the physics of the two-phase fluid dynamics. The two-fluid model is considered the most accurate two-phase conservation equations used in computer simulation codes to predict the thermal-hydraulic behavior of two-phase flows. The drift-flux parameters, such as the distribution parameter and drift velocity in the drift-flux model, are utilized in formulating the interfacial drag force in the two-fluid model-based simulation codes. The drift-flux model is an insightful model considering the difference between gas and liquid velocities. The effect of phase and velocity distributions on the void fraction is considered through the two drift-flux parameters. The distribution parameter and drift velocity are critical parameters in the two-phase flow formulation through the two-fluid model, which are the backbone of thermal-hydraulic analysis codes. The constitutive equations for the distribution parameter and drift velocity developed for medium-size channels reach a mature level but do not apply to industrial-size channels or large-size channels. The complicated two-phase flow dynamics in large-size channels affect the distribution parameter and drift velocity significantly. The distribution parameter in a large-size channel increases at low pressure and low liquid flow conditions due to induced secondary flow in the large-size channel. The drift velocity also increases due to cap bubbles formed by the surface instability of large bubbles. Multi-dimensional two-phase flow dynamics observed in large-size channels complicate the distribution parameter and drift velocity modeling. Thus, the drift-flux modeling in large-size channels is a much more subtle task than that in medium-size channels. This fact has driven the research to establish the drift-flux type correlations by modeling the distribution parameter and drift velocity in large-size channels. The current paper aims to provide state-of-the-art knowledge of the current status of the recent development of the drift-flux type correlations in large-size channels. The discussed items cover the formulation of the one-dimensional drift-flux model, typical drift-flux correlations developed for medium-size channels, distribution parameters for subcooled and saturated boiling flows, unique two-phase flow dynamics in large-size channels, critical size at the boundary between medium and large-size channels, and existing drift-flux correlations for large-size channels. The flow channel geometries discussed in the current paper are circular, annulus, rectangular, square, vertical rod bundle, and horizontal tube bundle. The future research direction is also discussed. [ABSTRACT FROM AUTHOR]

• Two-group interfacial area concentration models are implemented into TRAC-BF1 code. • Large and small interfacial area concentration models are developed for sensitivity analyses. • Sensitivity analyses of interfacial area effect on code predictions are conducted. • Sensitivity analyses include steady-state and transient flows and flow regime transition. • Interfacial area concentration has a minor effect on void fraction prediction in adiabatic flow. In one-dimensional two-fluid model-based codes, an interfacial drag force appears in a momentum conservation equation. The interfacial drag force is an essential parameter in predicting a void fraction. The accurate modeling of the interfacial drag force is indispensable for evaluating thermal–hydraulic characteristics in a nuclear reactor core. The interfacial drag force is formulated as the product of an 'overall' drag coefficient and the square of the relative velocity between gas and liquid phases. The 'overall' drag coefficient is expressed by the product of a drag coefficient, interfacial area concentration, and density of continuous phase. The rigorous model of the drag coefficient depending on a bubble shape regime has been established. The modeling of the interfacial area concentration depending on a flow regime is one of the weakest links in thermal–hydraulic analysis. The interfacial area transport equation was proposed to predict the dynamic change of the interfacial area concentration but has not reached a level accurate enough to predict the interfacial area concentration. Due to its incomplete development situation, an alternative way to predict the interfacial area concentration through a semi-theoretical interfacial area correlation has been proposed. This study aims to elucidate the effect of the interfacial area concentration on void fraction prediction in a one-dimensional thermal–hydraulic analysis. A one-dimensional two-fluid model-based code, such as TRAC-BF1, has been modified by implementing two existing constitutive equations of the interfacial area concentration into the code. This study also introduces large and small interfacial area concentration models of the interfacial area concentrations. The large and small interfacial area concentration models are designed to intentionally provide hypothetical large and small interfacial area concentrations within physically possible ranges. A total of four interfacial area concentration models is tested under adiabatic two-phase flow conditions. Code calculations with the four different models under steady-state and transient-state conditions and flow regime transitions have identified that the effect of the interfacial area concentration on the void fraction is insignificant for the adiabatic two-phase flows. The findings obtained in this study suggest that a simple interfacial area correlation is sufficient in modeling the interfacial drag force for adiabatic two-phase flows. However, robust and accurate modeling of the interfacial area concentration is still indispensable for two-phase flows with phase change because the interfacial heat transfer term in an energy conservation equation includes the interfacial area concentration. [ABSTRACT FROM AUTHOR]

• A two-phase flow structure in large-sized square channels (LSCs) was discussed. • Drift-flux analysis was performed with bubbly flow data in LSCs. • Correlations of void fraction and interfacial area in LSCs were recommended. • Frictional pressure drop calculation procedure in LSCs was recommended. • A criterion of developing length was recommended. Economic Simplified Boiling Water Reactor (ESBWR) concept has been proposed to improve the reactor safety feature. The core flow in ESBWR is driven by natural circulation induced by the difference between two-phase flow density in a partitioned chimney and single-phase flow density in a downcomer. The accurate prediction of the void fraction in the chimney is the key to calculate the two-phase flow density. The geometry of the partitioned chimney is a square channel with 0.61 m in the current ESBWR design, which is considered a large-sized square channel. Extensive research has been conducted for two-phase flows in large-sized circular channels, but the research for two-phase flows in large-sized square channels is very limited. In view of this, the current paper summarized the current understanding of the two-phase flow behavior in large-sized square channels and reviewed existing data collected in large-sized square channels. The two-phase flow parameters considered in the assessment were critical channel size at the boundary between medium and large-sized channels, developing length, void fraction, interfacial area concentration, flow regime transition, and frictional pressure drop. The assessment clearly indicated that experimental study in large-sized channels was very limited. Most datasets were collected in atmospheric air–water bubbly flows. Thus, the constitutive equations of two-phase flows were not fully evaluated. Some analytical studies for critical channel size and distribution parameter were introduced to give insight into the geometrical effect on the two-phase flow parameters. This paper strongly recommended collecting fundamental data. [ABSTRACT FROM AUTHOR]

• Void fraction data and correlations for the tight-lattice rod bundle were collected. • The existing drift-flux correlations for rod bundles were reviewed. • The existing drift-flux correlations failed to predict the tight-lattice rod bundle database. • The dependence of distribution parameters on physical properties was analyzed. • A robust drift-flux correlation for the tight-lattice rod bundle was developed. The concept of a resource-renewal boiling water reactor (RBWR) that uses spent nuclear fuel as a nuclear fuel has been proposed to reduce the period required for the radiotoxicity of spent nuclear fuel decaying to the same level as natural uranium ore from about 100,000 years to about 300 years. The RBWR core is designed based on the tight-lattice core concept to reduce the ratio of a moderator to fuel. Since the void fraction is one of the most critical design parameters for the RBWR, this study developed the drift-flux correlation applicable to two-phase flow in the tight-lattice rod bundle. The prediction error of the newly developed drift-flux correlation is ±11.7 %. The validated rod bundle geometrical conditions are the number of rods from 4 to 37, the rod spacing from 1.0 to 2.0 mm, the rod diameter from 9.0 to 13.7 mm, the mixture volumetric flux from 0.11 to 211 m/s, and the pressure from 0.1 to 7.2 MPa. The comparison in the distribution parameter between the tight-lattice rod bundle and the conventional BWR rod bundle indicates that the distribution parameter for the tight-lattice rod bundle is larger than that for the conventional BWR rod bundle. The increased distribution parameter in the tight-lattice core may be due to the small rod spacing and the triangular array of the rods. Both of the small rod spacing and the triangular array of the rods tend to increase the distribution parameter. [ABSTRACT FROM AUTHOR]

Abstract: The design and safety analysis for miniature heat exchangers, the cooling system of high performance microelectronics, research nuclear reactors, fusion reactors and the cooling system of the spallation neutron source targets requires the knowledge of the gas–liquid two-phase flow in a narrow rectangular channel. In this study, flow measurements of vertical upward air–water flows in a narrow rectangular channel with the gap of 0.993mm and the width of 40.0mm were performed at seven axial locations by using the imaging processing technique. The local frictional pressure loss gradients were also measured by a differential pressure cell. In the experiment, the superficial liquid velocity and the void fraction ranged from 0.214m/s to 2.08m/s and from 3.92% to 42.6%, respectively. The developing two-phase flow was characterized by the significant axial changes of the local flow parameters due to the bubble coalescence and breakup in the tested flow conditions. The existing two-phase frictional multiplier correlations such as and were verified to give a good prediction for the measured two-phase frictional multiplier. The predictions of the drift-flux model with the rectangular channel distribution parameter correlation of and several existing drift velocity correlations of and agreed well with the measured void fractions and gas velocities. The interfacial area concentration (IAC) model of was modified by taking the channel width as the system length scale and the modified IAC model could predict the IAC and Sauter mean diameter acceptably. [Copyright &y& Elsevier]

Abstract: In order to develop the interfacial area transport equation for the interfacial transfer terms in the two-fluid model, accurate data sets on axial development of local parameters such as void fraction, interfacial area concentration, interfacial gas velocity and Sauter mean diameter are indispensable to verify the modeled source and sink terms in the interfacial area transport equation. From this point of view, local measurements of both group 1 spherical/distorted bubbles and group 2 cap/slug bubbles in vertical upward air–water two-phase flow in a large diameter pipe with 200mm in inner diameter and 26m in height were performed at three axial locations of z/D =41.5, 82.8 and 113 as well as 11 radial locations from r/R =0–0.95 by using four-sensor probe method. Here, z, r, D and R are the axial distance from the inlet, radial distance from the pipe center, pipe diameter and pipe radius, respectively. The liquid flow rate and the void fraction ranged from 0.0505m/s to 0.312m/s and from 1.98% to 32.6%, respectively in the present experiment. The flow condition covered extensive region of bubbly flow, cap turbulent flow as well as their transition. The extensive analysis on the radial profiles of local flow parameters and their axial developments demonstrate the development of interfacial structures along the flow direction due to the bubble coalescence and breakup and the gas expansion. The significant decrease in void faction and interfacial area concentration and the increase in Sauter mean diameter and interfacial velocity were observed when the gradual flow regime transition occurred. Finally, the net change in the interfacial area concentration due to the bubble coalescence and breakup was quantitatively investigated in the present paper to reflect the true transfer mechanisms in observed two-phase flows. [Copyright &y& Elsevier]

Abstract: The drift-flux model is one of the most significant models for the prediction of two-phase flows being used today, with applications in many fields of engineering. Even more advanced models such as the two-fluid model require the use of drift-flux based models as constitutive relations. For this reason, it is necessary that accurate models for the drift-flux parameters exist for various geometries and fluid systems. One of the main weaknesses in a comprehensive set of drift-flux models is a lack of well-developed models for large diameter pipes, especially at higher void fractions. Thus one major step towards defining a comprehensive set of drift-flux models is to verify models in large pipes. To this end, a review of previous experiments has yielded a database of void fraction information in large pipes. Additionally, experiments have been performed in tests sections with diameters of 0.15 m and 0.20 m with liquid velocities up to 1 m/s and void fractions up to 0.85 under two pressure conditions. A comprehensive set of drift-flux models has been recommended based on the new data and the previously existing data. [Copyright &y& Elsevier]

Abstract: In view of the practical interest of the drift-flux model for two-phase flow analysis, the distribution parameter and drift velocity constitutive equations have been obtained for subcooled boiling flow in a sub-channel of rod bundle geometry. The constitutive equation of the distribution parameter for subcooled boiling flow in a sub-channel is obtained from the bubble-layer thickness model. In this derivation an existing constitutive equation for subcooled boiling flow in a round pipe is modified by taking account of the difference in the flow channel geometry between the sub-channel and round pipe. The constitutive equation of the drift velocity is proposed based on an existing correlation and considering the rod wall and sub-channel geometry effects. The prediction accuracy of the newly developed correlations has been checked against experimental data in a 3×3 rod bundle sub-channel, obtaining better predicting errors than the existing correlations most used in literature. [Copyright &y& Elsevier]

Local measurements of flow parameters were performed for vertical upward subcooled boiling flows in an internally heated annulus. The annulus channel consisted of an inner heater rod with a diameter of 19.1 mm and an outer round pipe with an inner diameter of 38.1 mm, and the hydraulic equivalent diameter was 19.1 mm. The double-sensor conductivity probe method was used for measuring local void fraction, interfacial area concentration, and interfacial velocity. A total of 11 data were acquired consisting of four inlet liquid velocities, 0.500, 0.664, 0.987 and 1.22 m/s and two inlet liquid temperatures, 95.0 and 98.0 °C. The constitutive equations for distribution parameter and drift velocity in the drift-flux model, and the semi-theoretical correlation for Sauter mean diameter, namely, interfacial area concentration, which were proposed previously, were validated by local flow parameters obtained in the experiment. [Copyright &y& Elsevier]

In view of the great importance to two-fluid model, structure of downward bubbly flows in vertical pipes has been discussed intensively based on available data sets of local flow parameters including extensive air–water data sets recently measured by the authors. In this study, an approximate radial phase distribution pattern map has been proposed based on available data sets, and radial profiles of local flow parameters such as void fraction, interfacial area concentration, interfacial velocity, and bubble Sauter mean diameter have been discussed in detail. The one-dimensional drift-flux model for a downward two-phase flow and the correlation of the interfacial area concentration have been compared with the downward flow data. The correlations applicable to the predictions of one-dimensional void fraction and interfacial area concentration for a downward bubbly flow have been recommended by the comparison. [Copyright &y& Elsevier]

In view of the practical importance of the drift-flux model for two-phase flow analysis in general and in the analysis of nuclear-reactor transients and accidents in particular, the kinematic constitutive equation for the drift velocity has been studied for various two-phase flow regimes. The constitutive equations that specify the relative motion between phases in the drift-flux model has been derived by taking into account the interfacial geometry, the body-force field, the shear stresses, the interfacial momentum transfer and the wall friction, since these macroscopic effects govern the relative velocity between phases. A comparison of the models with existing experimental data shows a satisfactory agreement. [Copyright &y& Elsevier]

In view of the practical importance of the drift-flux model for two-phase flow analysis in general and in the analysis of nuclear-reactor transients and accidents in particular, the distribution parameter and the drift velocity have been studied for downward two-phase flows. The constitutive equation that specifies the distribution parameter in the downward flow has been derived by taking into account the effect of the downward mixture volumetric flux on the phase distribution. It was assumed that the constitutive equation for the drift velocity developed by Ishii for a vertical upward churn-turbulent flow determined the drift velocity for the downward flow over all of flow regimes. To evaluate the drift-flux model with newly developed constitutive equations, area-averaged void fraction measurement has been extensively performed by employing an impedance void meter for an adiabatic vertical co-current downward air–water two-phase flow in 25.4-mm and 50.8-mm inner diameter round tubes. The newly developed drift-flux model has been validated by 462 data sets obtained in the present study and literatures under various experimental conditions. These data sets cover extensive experimental conditions such as flow system (air–water and steam–water), channel diameter (16–102.3 mm), pressure (0.1–1.5 MPa), and mixture volumetric flux (−0.45 to −24.6 m/s). An excellent agreement has been obtained between the newly developed drift-flux model and the data within an average relative deviation of ±15.4%. [Copyright &y& Elsevier]

In view of the practical importance of the drift–flux model for two-phase-flow analysis in general and in the analysis of nuclear-reactor transients and accidents in particular, the distribution parameter and the drift velocity have been studied for vertical upward two-phase flow in a large diameter pipe. One of the important flow characteristics in a large diameter pipe is a liquid recirculation induced at low mixture volumetric flux. Since the liquid recirculation may affect the liquid velocity profile and promote the formation of cap or slug bubbles, the distribution parameter and the drift velocity in a large diameter pipe can be quite different from those in a small diameter pipe where the liquid recirculation may not be significant. A flow regime at a test section inlet may also affect the liquid recirculation pattern, resulting in the inlet-flow-regime dependent distribution parameter and drift velocity. Based on the above detailed discussions, two types of inlet-flow-regime dependent drift–flux correlations have been developed for two-phase flow in a large diameter pipe at low mixture volumetric flux. A comparison of the newly developed correlations with various data at low mixture volumetric flux shows a satisfactory agreement. As the drift–flux correlations in a large diameter pipe at high mixture volumetric flux, the drift–flux correlations developed by Kataoka–Ishii, and Ishii have been recommended for cap bubbly flow, and churn and annular flows, respectively, based on the comparisons of the correlations with existing experimental data. [Copyright &y& Elsevier]

Local measurements of flow parameters were performed for vertical upward bubbly flows in an annulus. The annulus channel consisted of an inner rod with a diameter of 19.1 mm and an outer round tube with an inner diameter of 38.1 mm, and the hydraulic equivalent diameter was 19.1 mm. Double-sensor conductivity probe was used for measuring void fraction, interfacial area concentration, and interfacial velocity, and laser Doppler anemometer was utilized for measuring liquid velocity and turbulence intensity. A total of 20 data sets for void fraction, interfacial area concentration, and interfacial velocity were acquired consisting of five void fractions, about 0.050, 0.10, 0.15, 0.20, and 0.25, and four superficial liquid velocities, 0.272, 0.516, 1.03, and 2.08 m/s. A total of eight data sets for liquid velocity and turbulence intensity were acquired consisting of two void fractions, about 0.050, and 0.10, and four superficial liquid velocities, 0.272, 0.516, 1.03, and 2.08 m/s. The constitutive equations for distribution parameter and drift velocity in the drift-flux model, and the semi-theoretical correlation for Sauter mean diameter namely interfacial area concentration, which were proposed previously, were validated by local flow parameters obtained in the experiment using the annulus. [Copyright &y& Elsevier]

In relation to the development of the interfacial area transport equation, axial developments of local void fraction, interfacial area concentration, and interfacial velocity of vertical upward bubbly flows in an annulus with the hydraulic equivalent diameter of 19.1 mm were measured by the double-sensor conductivity probe. A total of 20 data were acquired consisting of five void fractions, about 0.050, 0.10, 0.15, 0.20, and 0.25, and four superficial liquid velocities, 0.272, 0.516, 1.03, and 2.08 m/s. The obtained data will be used for the development of reliable constitutive relations, which reflect the true transfer mechanisms in subcooled boiling flow systems. [Copyright &y& Elsevier]

• Downward two-phase flows were focused in relation to nuclear safety analyses. • Void fraction data and correlations of vertical downward two-phase flows were reviewed. • The predictive performance of existing correlations was evaluated with the collected databases. • A new drift-flux correlation was developed for vertical downward two-phase flows. • The new correlation achieved superior predictive performance to existing correlations. The two-fluid model has been adopted as a platform of nuclear thermal–hydraulic system analysis code because it can treat the mechanical and thermal non-equilibrium between phases through the interfacial transfer terms. Precise modeling of the area-averaged interfacial drag force in the interfacial momentum transfer term is essential in predicting void fraction accurately. The drift-flux parameters, such as the distribution parameter and drift velocity, play an essential role in formulating the area-averaged interfacial drag force. This study aims at developing a drift-flux correlation with a wide application range for vertical downward two-phase flows. First, over 1200 experimental void fraction data of vertical downward two-phase flows were collected from 13 sources. Then, the existing correlations of the distribution parameter and drift velocity were reviewed and examined. Finally, a new drift-flux correlation was developed based on a large amount of experimental data and state-of-the-art knowledge of two-phase flow behaviors. The comparison between the experimental and calculated void fractions by the newly-developed correlation demonstrated that the new correlation could achieve superior performance to the existing correlations. More than 93% of the predicted void fractions were predicted within ± 20% error with the mean relative deviation and mean absolute relative deviation of 0.609% and 9.93%, respectively. [ABSTRACT FROM AUTHOR]

For the modeling of the interfacial structure characteristics, an experiment of vertical adiabatic air-water flow has been conducted under an atmosphere pressure condition. The experimental facility is composed of 5 × 5 rods arranged on a square pitch in a square casing with 9.5 mm outside rod diameter and 12.6 mm pitch, simulating a prototypic rod bundle in nuclear reactors. The miniaturized four-sensor conductivity probe has been developed to allow for the experimental measurement in the small flow channel. Extensive data are acquired for one-dimensional flow parameters including axial development of void fraction, interfacial area concentration and gas velocity. The effect of a prototypic spacer grids with mixing vanes on the flow structure has been discussed at various flow conditions based on the experimental data. Two competing effects, “swirling effect” and “bubble breaker effect”, are identified. The effect vanishes out within a short distance from the space grid. A drift-flux correlation is developed for predicting void fraction of adiabatic bubbly two-phase flow in a vertical rod bundle. Existing interfacial area correlations are tested and Hibiki-Ishii correlation (2002b) is recommended as most accurate correlation to predict the interfacial area concentration. It is expected that the newly obtained data in the 5 × 5 rod bundle is useful for developing the interfacial area transport equation and benchmarking a computational code. [ABSTRACT FROM AUTHOR]

In engineering fields such as power generation systems (nuclear and thermal power plants), chemical processing, oil industry and so on, large-diameter channels have been extensively used to increase the mass, momentum and heat transport capability of the working fluid. Compared with small-diameter pipes, two-phase flow in the large-diameter channels shows more complicated flow characteristics. Much larger cap bubbles can exist and the interfacial instability prevents the large cap bubbles from forming large stable Taylor bubbles. So, the flow regimes and the radial void fraction profiles are different and the relative velocities between the two phases are significantly increased compared to those in small-diameter pipes. This paper reviews the recent progress in the research on two-phase flows in large-diameter channels. Recent progress on the state-of-the-art tool of four-sensor probe is explained and the necessary two-group bubbles can be classified through the measured bubble diameter, instead of the present method using bubble chord length, in 3-dimensional two-phase flow. The databases on the flows in large-diameter channels are presented and their typical multi-dimensional characteristics are discussed in detail. The most updated constitutive equations covering flow regime transition criteria, drift-flux correlations, interfacial area concentration (IAC) correlations and one- and two-group interfacial area transport equation(s) (IATE(s)) are summarized and their merits and drawbacks are analyzed. The important assumption that the area-averaged interfacial velocity weighted by IAC is equal to the area-averaged gas velocity weighted by void fraction in the 1D IATE has been confirmed by the present newly-obtained experimental data. The 1D numerical simulations of multi-dimensional two-phase flows in large-diameter channel are reviewed. Finally, the future research directions are suggested. [ABSTRACT FROM AUTHOR]