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The objective of the current study is to better understand the interfacial structure and its development in an air–water planar bubble jet, as well as to provide a unique benchmark data set for a 3D thermal-hydraulic analysis code. Both flow visualization and local measurements were performed in three characteristic flow conditions at four elevations along a test section with a cross section of 200 mm in width and 10 mm in gap. A high-speed digital video camera was applied in the flow visualization study to capture the flow structures and bubble interaction phenomena, while a miniaturized four-sensor conductivity probe was used to acquire the time-averaged local void fraction, interfacial velocity, and bubble number frequency. Also, the interfacial area concentration and the averaged bubble Sauter mean diameter were obtained from the local measurements. The lateral bubble transport and bubble interaction mechanisms were clearly demonstrated in the acquired data.

This paper presents the modeling of bubble interaction mechanisms in the two-group interfacial area transport equation (IATE) for confined gas–liquid two-phase flow. The transport equation is applicable to bubbly, cap-turbulent, and churn-turbulent flow regimes. In the two-group IATE, bubbles are categorized into two groups: spherical/distorted bubbles as Group 1 and cap/slug/churn-turbulent bubbles as Group 2. Thus, two sets of equations are used to describe the generation and destruction rates of bubble number density, void fraction, and interfacial area concentration for the two groups of bubbles due to bubble expansion and compression, coalescence and disintegration, and phase change. Five major bubble interaction mechanisms are identified for the gas–liquid two-phase flow of interest, and are analytically modeled as the source/sink terms for the transport equation in the confined flow. These models include both intra-group and inter-group bubble interactions.

The bubble interaction mechanisms have been analytically modeled in the first paper of this series to provide mechanistic constitutive relations for the two-group interfacial area transport equation (IATE), which was proposed to dynamically solve the interfacial area concentration in the two-fluid model. This paper presents the evaluation approach and results of the two-group IATE based on available experimental data obtained in confined upward flow, namely, 11 data sets in or near bubbly flow and 13 sets in cap-turbulent and churn-turbulent flows. The two-group IATE is evaluated in steady-state, one-dimensional (1D) form. To account for the inter-group bubble transport, the void fraction transport equation for Group-2 bubbles is also used to predict the void fraction for Group-2 bubbles. Agreement between the data and the model predictions is reasonably good and the average relative difference for the total interfacial area concentration between the 24 data sets and predictions is within 7%. The model evaluation demonstrates the capability of the two-group IATE focused on the current confined flow to predict the interfacial area concentration over a wide range of flow regimes.

The objective of the present work is to study and model the interfacial structure development of air–water two-phase flow in a confined flow passage. Experiments of a total of 13 flow conditions in cap-turbulent and churn-turbulent flow regimes are carried out in a vertical air–water upward two-phase flow experimental loop with a test section of 200 mm in width and 10 mm in gap. Miniaturized four-sensor conductivity probes are used to measure local two-phase parameters at three different elevations for each flow condition. Bubble characteristics captured by the probes are categorized into two groups in view of the two-group interfacial area transport equation, i.e., spherical/distorted bubbles as Group 1 and cap/churn-turbulent bubbles as Group 2. The acquired local parameters are time-averaged void fraction, interfacial velocity, bubble number frequency, interfacial area concentration, and bubble Sauter mean diameter for each group of bubbles. Also, the line-averaged and area-averaged data are presented and discussed in detail. The comparisons of these parameters at different elevations demonstrate the development of interfacial structures along the flow direction due to bubble interactions and the hydrodynamic effects. Furthermore, these data can serve as one part of the experimental data for investigation of the interfacial area transport in a confined two-phase flow.

The interfacial area transport equation applicable to the bubbly flow is presented. The model is evaluated against data acquired by a state-of-the-art miniaturized double-sensor conductivity probe in an adiabatic air–water co-current vertical test loop under atmospheric pressure condition. In general, a good agreement, within the measurement error of ±10%, is observed for a wide range in the bubbly flow regime. The evaluation of the individual particle interaction mechanisms demonstrates the active interactions between the bubbles and highlights the mechanisms playing the dominant role in interfacial area transport. The analysis employing the drift flux model is also performed for the data acquired. Under the given flow conditions, the distribution parameter of 1.076 yields the best fit to the data.

The interfacial structure of the two-phase flows is of great importance in view of theoretical modeling and practical applications. In the present study, the focus is made on obtaining detailed local two-phase flow parameters in the air–water bubbly flow in an adiabatic rectangular vertical duct using the double-sensor conductivity probe. The characteristic wall peak is observed in the profiles of the interfacial area concentration and the void fraction. The development of the interfacial area concentration along the axial direction of the flow is studied in view of the interfacial area transport and bubble interactions. The experimental data is compared with the drift flux model with C0=1.35.

The breakdown of the liquid film at the wall in annular gas-liquid flow may lead to the formation of a stable dry patch. For the case of heat transfer surfaces this causes a hot spot. Dry patch stability depends on a balance of body and surface forces. In the present study the film is driven by the interfacial shear force and the gravity force is negligible. Hartley and Murgatroyd proposed a model for dry patches of shear driven films based on a balance of surface tension and inertia but the film contact angle had to be adjusted to an unrealistic value to fit the model to experimental data. Murgatroyd later proposed an additional force because the wall and the interfacial shear stresses on the film are unbalanced near the dry patch. The magnitude of the net shear force on the film is determined by a characteristic length, λ, over which this imbalance occurs. However, Murgatroyd did not validate the model with a mathematical solution for the distribution of the shear stresses but determined λ empirically to fit the experimental data. A new computational fluid dynamics (CFD) solution of the flow field in the film around the dry patch has been obtained. The CFD results confirm Murgatroyd’s hypothesis, although the details are more complex. In addition new experimental data for adiabatic upward annular air-water and air-ethylene glycol flows provide further validation for Murgatroyd’s model.

Two-fluid model predictions of film dryout in annular flow are limited by the uncertainties in the entrainment rate of droplets from the liquid film. The main cause of these uncertainties is the lack of separate effects experimental data in the range of the operating conditions in industrial applications. Air–water and Freon-113 entrainment rate data have been obtained in 10 mm tubes using the double film extraction technique. These experiments have been scaled to approach high-pressure steam–water flow conditions. The effects of surface tension and density ratio, missing from most previous data sets, have been systematically tested. The entrainment rate mechanism is scaled by the Kelvin–Helmholtz instability at the film surface. Based on this analysis and the new data, a new correlation is proposed that is valid for low-viscosity fluids in small ducts in the ripple-annular regime.

High pressure steam–water annular flow was simulated with scaled air–water and Freon-113 experiments. A unique Freon-113 loop was built to obtain low surface tension and high vapor density data on entrainment fraction. The results were compared with two correlations available in the open literature. The Ishii and Mishima correlation was capable to collapse the data for air–water, Freon-113 and steam–water experiments satisfactorily. However the correlation needs to be adjusted for high Weber numbers of the gas phase. The data was limited to the ripple-annular regime because the film extraction technique is not reliable when the film becomes frothy.

Two-fluid model predictions of film dryout in annular flow are limited by the uncertainties in the constitutive relations for the entrainment rate of droplets from the liquid film. The main cause o...

In the two-fluid model, interfacial concentration is one of the important parameters. The objective of this study is to develop an interfacial area equation with the source and sink terms being properly modeled. For bubble coalescence, the random collisions between bubbles due to turbulence, and the wake entrainment process due to the relative motions of the bubbles, were included. For bubble breakup, the impact of turbulent eddies is considered. Compared with measured axial distributions of the interfacial area concentration under various flow conditions, the adjustable parameters in the source/sink terms were obtained for the simplified one-dimensional transport equation.

Two-fluid model predictions of film dryout in annular flow, leading to nuclear reactor fuel failure, are limited by the uncertainties in the constitutive relations for the entrainment rate of droplets from the liquid film. The main cause of these uncertainties is the lack of separate-effects experimental data in the range of the operating conditions in nuclear power reactors. An air–water experiment has been performed to measure the entrainment rate in a small pipe. The current data extend the available database in the literature to higher gas and liquid flows and also to higher pressures. The measurements were made with the film extraction technique. A mechanistic model was obtained based on Kelvin–Helmholtz' instability theory. The dimensionless model includes the Weber number of the gas and the liquid film Reynolds number. Kataoka and Ishii's correlation (Kataoka, I., Ishii, M., 1982. NUREG/CR-2885, ANL-82-44) is modified based on this model and the new data. The new correlation collapses the present air–water data and Cousins and Hewitt's data (Cousins, L.B., Hewitt, G.F., 1968. UKAEA Report AERE-R5657) The effects of pressure and surface tension were considered in the derivation so it may be applied for boiling water reactor operating conditions.

New experimental data are obtained for pressure drop and entrainment for annular upflow in a vertical pipe. The 9.5-mm pipe has a hydraulic diameter similar to the subchannels in the fuel assemblies of water-cooled reactors. The test section has a length-to-diameter ratio of 440 to ensure fully developed annular flow. The pressure covers the range from 140 to 660 kPa. Therefore, the density ratio is varied by a factor of -4. This allows the investigation of the effect of pressure on the interfacial shear models. Gas superficial velocities between 25 and 126 m/s are tested, extending the range of previous data to higher gas velocities. The data are compared with well-known models for interfacial shear that represent the state of the art. Good results are obtained with the models by Wallis, and Henstock and Hanratty. When the model by Asali, Hanratty, and Andreussi is modified for the effect of pressure, the agreement is also good, and the data collapse with very little scatter. There is a close relationship between these models and mixing length theory such that the models may be viewed as correlations for the surface roughness. This points toward a more fundamental approach in terms of the interfacial structure.

The objective of the present work is to study and model the interfacial structure development of air-water two-phase flow in a confined test section. Experiments of a total of 9 flow conditions in cap-turbulent and churn-turbulent flow regimes are carried out in a vertical air-water upward two-phase flow experimental loop with a test section of 200-mm in width and 10-mm in gap. Miniaturized four-sensor conductivity probes are used to measure local two-phase parameters at three different elevations for each flow condition. The bubbles captured by the probes are categorized into two groups in view of the two-group interfacial area transport equation, i.e., spherical/distorted bubbles as Group 1 and cap/churn-turbulent bubbles as Group 2. The acquired parameters are time-averaged local void fraction, interfacial velocity, bubble number frequency, interfacial area concentration, and bubble Sauter mean diameter for both groups of bubbles. Also, the line-averaged and area-averaged data are presented and discussed. The comparisons of these parameters at different elevations demonstrate the development of interfacial structure along the flow direction due to bubble interactions.

The breakdown of the liquid film at the wall in annular gas-liquid flow may lead to the formation of a stable dry patch. For the case of heat transfer surfaces this causes a hot spot, The dry patch is a partial area on the solid surface that is non-wetted due to a local disturbance of the flow and is sustained by surface tension. Dry patch stability is dependent on a balance of body and surface forces. In the present study the interfacial shear force drives the film and the gravity force is negligible. A new computational fluid dynamics (CFD) solution of the flow field in the film around the dry patch has been obtained. The CFD results confirm Murgatroyd's shear force model (1965), although the details are more complex. Furthermore, there is agreement between the CFD solution and the experimental value of the characteristic length scale, L, for the shear force. In addition new experimental data have been taken for adiabatic upward annular air-water and air-ethylene glycol flows at room temperature in a 9.5 mm diameter tube. They provide validation of Murgatroyd's model over a wider range of the film's Reynolds number than previous data.

Air/water and vapor/freon were utilized to scale and simulate annular two-phase flow for high pressure steam/water conditions. A unique vapor/liquid Freon loop was built to obtain the high pressure data. The results were compared with two correlations available in the open literature. The Ishii and Mishima dimensionless group was able to scale the data remarkably well even for vapor/liquid Freon. However, the correlation needs to be adjusted for high Weber numbers of the gas phase.