Gamma Densitometer Research Articles

Development of an accelerator-based neutron source to prototype Mo-99 production, part I: A liquid LBE windowless target

Molybdenum-99 (Mo-99)’s decay product, technetium-99 (Tc-99 m), is one of the most critical isotopes for medical diagnostics. To provide U.S. domestic supply of Mo-99 without using high-enriched uranium (HEU), a subcritical uranium target assembly (UTA) is irradiated by an accelerator-based neutron source to create Mo-99 through fission. This study discusses the development of the accelerator-based neutron source. The high-energy electrons from the accelerator irradiate a liquid lead-bismuth eutectic (LBE) target to produce neutrons. Part I of this work focuses on numerical and experimental analysis towards the development of a liquid LBE windowless target. Unlike the existing windowless targets in literature, the current design creates a vertical free surface for a beam to irradiate. First, a hydrodynamic analysis of the LBE windowless target is performed. Simplified analytical calculations are assisted by 2D computational fluid dynamics (CFD) simulations to design the target, with the focus on eliminating recirculation zones and avoiding cavitation. With the optimized geometry, the experimental study is performed to investigate the flow hydrodynamics using liquid LBE. The experiments (1) compare pressure drop in the system to correlation predictions; (2) visualize the free surface liquid LBE flow from the beam view; (3) validate the LBE flow profile using temperature sensitive paint from the side view; and (4) validate the liquid LBE film thickness using gamma densitometer measurements. Second, the power handling capability of the designed windowless target is investigated. The divider plate in the current design is susceptible to overheating due to the thin LBE film in front. As LBE erosion and corrosion is likely to occur at an LBE velocity of 2.0 m/s and temperature above 500 °C, a power limit of 10 kW of beam power was established to prevent this corrosion from occurring, which is calculated by a Nusselt number correlation. The divider plate surface temperature at 10 kW agrees well with the 3D CFD simulation results. Part I demonstrates the fundamental physics in liquid LBE windowless target design and associated testing. A companion paper, Part II will demonstrate how to couple this windowless target into the Mo-99 production system, including an accelerator system operating under an ultra-high vacuum and the UTA cooled by water at room temperature.

Design and construction of an LSTM-powered high sampling rate dual-beam gamma densitometer for real-time measurement of the two-phase flow void fraction

In this work, an intelligent gamma densitometer gauge for non-intrusive real-time measurement of the two-phase flow void fraction has been designed and constructed. It has been implemented in a horizontal pipe over a wide range of gas and liquid superficial velocities. Moreover, a Long Short-Term Memory (LSTM) neural network has been used for analyzing the data sequence from a dedicated high sampling rate dual-beam gamma densitometer. This approach considerably reduced the complexity of the data processing and regression operations while providing a high capability to identify and compensate for the errors caused by intermittent, and inhomogeneous flow regimes in a real-time mode. A comprehensive sensitivity analysis has been performed to adjust the network parameters for the most accurate void fraction prediction in all studied flow regimes including slug, plug, wavy, stratified, bubbly, and semi-annular. The experimental evaluations revealed that considering a sampling time of a few seconds, the optimized model approximated the actual data with an R-squared of 0.98, and almost all measurements were carried out with the maximum absolute error of 10%. The high accuracy and relatively fast response of the proposed method make it a reliable technique for developing a high-performance real-time multiphase flow meter.

Development of an intelligent dual-beam gamma densitometer for real-time recognition of two-phase flow regime in horizontal pipe

This work includes the hardware and software development of a high sampling rate dual-beam gamma densitometer for real-time two-phase flow monitoring. The designed densitometer consists of two fast SiPM-based gamma detectors and a custom embedded system for digital signal processing and running flow regime recognition neural network. An exclusive LSTM deep neural network has been developed to classify flow regimes into six classes including stratified, annular, bubbly, wavy, slug, and plug. Therefore, the proposed design is superior to the traditional approaches by (i), unifying signal processing and flow identification units into a single cost-effective embedded system; (ii), reducing the need for time-consuming signal preprocessing steps from conventional to feed-forward neural networks, and (iii), real-time identification of the flow due to the high refresh rate of the data processing chain. The optimal neural network model has been obtained through comprehensive sensitivity analyses to have the best interpolation and generalization. The experimental evaluations showed that the optimized model recognizes the flow regime based on 6 s of gamma data sequence with an accuracy of ∼92%–97% depending on the flow regime in the pipeline. Furthermore, the real-time experiment proved the successful performance of the densitometer to trace the flow regime across the two-phase flow map.

Experimental investigation of the development of surfactant stabilized oil-water pipe flow downstream of a choke valve

In wells and multiphase flowlines the presence of valves, pumps and other complex geometries will considerable contribute to dispersion of the phases. In combination with the droplet stabilizing effect of natural surfactants contained in the crude oil and other production chemicals, this may lead to a considerable development length of the flow. Such effects should be understood and considered by future simulation tools. In the presented work, the described scenario was experimentally tested under controlled conditions. The development of dispersed oil-water flow downstream of a valve was monitored. Surfactant was added to the mineral oil to obtain a suitable droplet stability. The near horizontal test section was 220 m long and had an inner diameter of 109 mm. The vertical phase fraction distributions were measured at six locations along the pipe with traversing gamma densitometers. The test section was further instrumented with nine pressure transducers, four optical sections, and three droplet sampling points to monitor flow development. The flow development showed a characteristic behavior of a rather sudden separation after a certain length with stable dispersed flow conditions. The degree of inlet mixing and thus initial droplet size governed by the pressure drop over the mixing valve was found to be crucial for the development length. Flow rate and initial water fraction were of minor importance. The separation behavior was coalescence controlled as it would be expected for a surfactant stabilized system. Still, even if static separation bench tests resulted in very long separation time scales, the in-flow separation happened much faster. This indicates the importance of dynamic flow conditions and associated effects such as shear and a certain mixing to be preferential for the separation. The measured pressure drop of the dispersed flow was more than 20% higher than separated flow with otherwise the same conditions. As the flow separates, the pressure drop decreases and, as expected, finally approaches the value for separated flow. The work presents a detailed data set as basis for model development.

Comparative Analysis of Riser Base and Flowline Gas Injection on Vertical Gas-Liquid Two-Phase Flow

Gas injection is a frequently used method for artificial lift and flow regime rectification in offshore production and transportation flowlines. The flow behaviour in such flowlines is complex and a better understanding of flow characteristics, such as flow patterns, void fraction/hold up distributions and pressure gradient is always required for efficient and optimal design of downstream handling facilities. Injection method and location have been shown to strongly affect downstream fluid behaviour that can have important implications for pumping and downstream facility design, especially if the development length between pipeline and downstream facility is less than L/D = 50 as reported by many investigators. In this article, we provide the results of an experimental investigation into the effects of the gas injection position on the characteristics of the downstream upwards vertical gas flow using a vertical riser with an internal diameter of 52 mm and a length of 10.5 m. A horizontal 40-m-long section connected at the bottom provides options for riser base or horizontal flow line injection of gas. The flowline gas injection is performed 40 m upstream of the riser base. A 16 by 16 capacitance wire mesh sensor and a gamma densitometer were used to measure the gas-liquid phase cross-sectional distribution at the riser top. A detailed analysis of the flow characteristics is carried out based on the measurements. These include flow regimes, cross-sectional liquid holdup distributions and peaking patterns as well as analysis of the time series data. Our findings show that flow behaviours differences due to different gas injection locations were persisting after a development length of 180D in the riser. More specifically, core-peaking liquid holdup occurred at the lower gas injection rates through the flowline, while wall-peaking holdup profiles were established at the same flow conditions for riser base injection. Wall peaking was associated with dispersed bubbly flows and hence non-pulsating as against core-peaking was associated with Taylor bubbles and slug flows. Furthermore, it was found that the riser base injection generally produced lower holdups. It was noted that the circumferential injector used at the riser base promoted high void fraction and hence low liquid holdups. Due to the bubbly flow structure, the slip velocity is smaller than for larger cap bubbles and hence the void fraction is higher. The measurements and observations presented in the paper provides valuable knowledge on riser base/flowline gas introduction that can directly feed into the design of downstream facilities such as storage tanks, slug catchers and separators.

Void Fraction Measurement and Prediction of Two-Phase Boiling Flows in a Tubular Test Section

Abstract Void fraction is one of the most important parameters that affect two-phase flow heat transfer and pressure drop. In this paper, a commercial gamma densitometer and a high-speed X-ray radiography system developed at the University of Michigan (UM) are used to measure the void fraction in two-phase boiling flows, with water as the working fluid, in a tubular test section. The test section is made of Incoloy 800H/HT with a total length of 1.589 m, an inner diameter of 12.95 mm, and a wall thickness of 3.05 mm. These two instrumentation systems are installed on a traversing platform that travels along the vertical test section to perform measurements at multiple elevations. Subcooled flow boiling and natural convection boiling experiments are performed to measure the void fraction in the test section. Flow visualization images are obtained for bubbly and slug flows from the X-ray radiography system. The wall temperature of the test section is measured at 17 elevations by thermocouples. In addition to the experiments, a multiphase computational fluid dynamics (MCFD) model is developed using ansysfluent to simulate the subcooled flow boiling. The measured wall temperature and void fraction from the experiments are compared with the MCFD simulation results. The root-mean-square (RMS) relative deviations are 3.6% and 16.1% for the wall temperature and void fraction, respectively, between the experimental data and MCFD simulations.

Simulation of the two-phase flow across tube bundle

Experiments on two-phase flow across an in-line tube bundle are analyzed with the STEG code, which has been developed for modeling thermal-hydraulic processes in a horizontal steam generator (SG). An adiabatic, vertical two-phase flows of air-water across horizontal in-line, 5 x 20 rod bundles, with a pitch-to-diameter ratio P/D=1.3 are considered, the mass velocity is varied in the range 27 - 818 kg/m2s. The calculated values of void fraction in the tube bundle are compared with the experimental ones measured by a gamma densitometer. A reasonable agreement between the calculations and the experimental data is obtained.

Non-invasive measurement of oil-water two-phase flow in vertical pipe using ultrasonic Doppler sensor and gamma ray densitometer

Oil–water two-phase flow experiments were conducted in a vertical pipe to study the liquid–liquid flow measurement using a non-invasive ultrasound Doppler flow sensor and a gamma-ray densitometer. Tap water and a dyed mineral oil are used as test the fluids. A novel Doppler effect strategy is used to estimate the mixture flow velocity in a vertical pipe based on flow velocity measured by the ultrasound sensor and the shear flow velocity profile model. Drift-flux flow model was used in conjunction phase fractions to predict the superficial velocities of oil and water. The results indicate that the proposed method estimated oil-continuous flow and water-continuous flow have average relative errors of 5.2% and 4.5%; and superficial phase flow velocities of oil and of water have average relative errors of 4.5% and 5.9% respectively. These results demonstrate the potential for using ultrasonic Doppler sensor combined with gamma densitometer for oil–water measurement.

Dual-Isotope Spectrometric Gamma Densitometer for Diagnostics of Three-Phase Oil–Water–Gas Flows

The operation of a gamma densitometer (GD) of ID = 100 mm is considered; it is intended for the diagnostics of three-phase oil–water–gas flows using the separationless method, when the composition of components is determined by a spectrometric analysis of gamma irradiation with two different photon energies. The 241Am and 137Cs isotopes, installed in the same housing, are used as gamma sources, while a gamma detector is made on the basis of the BGO crystal of 40 mm in diameter and height. Information about the design, measuring electronics, and main characteristics of the GD is given. The method used for the temperature stabilization of the recorded radiation intensities for both isotopes within a range of ambient temperature from 10 to 35oC is presented.

Large-Scale Experiments on Slug-Length Evolution in Long Pipes

Summary We present a unique set of two- and three-phase slug-flow experiments conducted in a 766-m-long, 8-in. pipe at 45-bara pressure, using Exxsol™ D60 fluid (ExxonMobil Chemical, Houston, Texas, USA) as the oil phase and nitrogen as the gas phase. The first one-half of the pipe was horizontal, while the second one-half was inclined by 0.5°. A total of 10 narrow-beam gamma densitometers were mounted on the pipe to study flow evolution, and in particular slug-length development. The results show that the mean slug length initially increases with the distance from the inlet, but this increase slows down, and the mean slug length typically reaches a value between 20 and 50 diameters at the outlet. At low mixture velocities (<3 m/s), the slug-length distributions tend to be extremely wide, sometimes with standard deviations approaching 100%. The longest slugs that we observed were more than 250 pipe diameters (50 m). At higher mixture velocities (>3 m/s), the slug-length distributions are in general narrower. The effect of the water cut (WC) on the slug-length distribution is significant but complex, and it is difficult to establish any general trends regarding this relationship. Finally, it was observed that slug flow often requires a very long distance to develop. Specifically, in most of the slug-flow experiments, the flow regime 57 m downstream of the start of the horizontal section was not slug flow.

Characteristics of horizontal gas-liquid two-phase flow measurement in a medium-sized pipe using gamma densitometry

Two-phase flows are common occurrences in many industrial applications. The understanding of their characteristics in industrial piping systems is vital for the efficient design, optimization, and operation of industrial processes. Most of the previous experimental studies involving the use of gamma densitometers for holdup measurements in air-water mixtures are limited to smaller diameter pipes (generally regarded as those with < 50 mm in nominal diameter). Further, very few literature report experimental data obtained using gamma desitometers. This paper presents an application of a gamma densitometer in the measurement of two-phase flow characteristics in an intermediate diameter pipe (nominal diameter between 50 mm and 100 mm). Scaled air-water experiments were performed in a 17-m long, 0.0764-m internal diameter horizontal pipe. Liquid superficial velocity ranged between 0.1–0.4 m/s while gas superficial velocity ranged from 0.3 to 10.0 m/s. The measured parameters include liquid holdup, pressure gradient, flow pattern, and slug flow features. The flow patterns observed were stratified, stratified-wavy, plug, slug, and annular flows. Plug and slug flow patterns showed good agreement with established flow pattern maps. Furthermore, the slug translational velocity was observed to increase with increasing mixture velocity, as reported by previous authors, hence establishing the reliability of the instrumentation employed. The slug body length was also measured using the gamma densitometer and was found to be within the range 24–36D with a mean length of 30.6D.

X-ray measurements of thin liquid films in gas–liquid pipe flow

This paper presents a method for reconstructing the thickness profile of the thin liquid film being present in steady, fully developed gas–liquid annular pipe flow, taking advantage of nonintrusive X-ray measurements. Experiments have been performed with low liquid loading, annular flows, in a near horizontal pipe system (inclination +2.5°), using a high-density gas (SF6) and Exxsol D60 oil. Special care was taken to ensure the flows were fully developed, and statistically steady. The superficial gas velocity was maintained at a constant value of 10 m/s, while the superficial liquid velocity was varied between 0.5 mm/s and 42 mm/s. The X-ray measurements were supplemented by gamma ray densitometer data and isokinetic data. An explicit mathematical method is presented, whereby the shape and thickness of very thin liquid films can be reconstructed if X-ray images of the flow have been obtained from two independent directions: one with a vertical view of the pipe, and another with a horizontal side view. The method can be straightforwardly applied to cases where the film volume dominates the total volume of droplets suspended in the gas phase. This condition is generally satisfied for the experiments presented in this paper, as verified by isokinetic measurements.Mean liquid hold-up values determined by gamma densitometer are in reasonable agreement with the X-ray results. It is found that the mean film thickness at any position on the pipe wall increases with the superficial liquid velocity. While the film thickness is maximal at the pipe bottom in virtually all experimental cases, its profile exhibits multiple local maxima and minima, with the smallest thickness apparently 20°–30° away from at the pipe top. The presented film reconstruction method is highly sensitive; even at extremely low liquid levels, with a superficial liquid velocity less than 0.01% of the mixture velocity, the film and its thickness profile are clearly detected. An analytical fit for the film profile as a function of perimeter angle and superficial liquid velocity is given. The fit captures the main features of the profile as derived from X-ray measurements, for thin films with maximum thickness less than 5% of the pipe diameter.

Investigation of oil-water flow in concentric and fully eccentric annuli pipes

Experimental investigations are performed on co-current flow of oil (Exxsol D60) and water in concentric and fully eccentric annuli with the inner pipe located at bottom of the outer pipe. The annulus outer and inner pipe have an inside diameter of 99 mm and outside diameter of 50 mm, respectively. This yields a diameter ratio of K = 0.505. The flow conditions studied span mixture velocities and input water cuts in the range 0.50–1.75 m/s and 10–90%, respectively, at pipe inclinations of 0° and 4° upward. Flow regimes have been identified and maps constructed using instantaneous images of the flow from high-speed cameras (shadowgraph) and X-ray chordal holdup measurements along the vertical projection. Flow regimes in the concentric annulus exhibit a higher level of mixing than that observed in the fully eccentric configuration. The transition to dispersed flow occurs at lower mixture velocities in the concentric annulus. Measurements from broad-beam gamma densitometers reveal that the mean water holdup is higher in the fully eccentric annulus for a given mixture velocity and input water cut. The higher water accumulation in this annulus configuration can be attributed to a low velocity region in the narrow gap at the annulus pipe bottom. The frictional pressure gradient in the concentric annulus is higher as compared with the fully eccentric configuration. Peaks in the pressure gradient profile, for a constant mixture velocity, are observed at high water cuts (i.e. WC ≥ 50%) at the transition between dual continuous and dispersed flows. Pressure gradient data are compared with predictions using the homogeneous and two-fluid model. In general, the homogeneous model using a modified Brinkman (Brinkman, 1952)/Roscoe (Roscoe, 1952) dispersion viscosity model shows the best agreement with data in both concentric and fully eccentric annuli.

Slug Translational Velocity for Highly Viscous Oil and Gas Flows in Horizontal Pipes

Slug translational velocity, described as the velocity of slug units, is the summation of the maximum mixture velocity in the slug body and the drift velocity. Existing prediction models in literature were developed based on observation from low viscosity liquids, neglecting the effects of fluid properties (i.e., viscosity). However, slug translational velocity is expected to be affected by the fluid viscosity. Here, we investigate the influence of high liquid viscosity on slug translational velocity in a horizontal pipeline of 76.2-mm internal diameter. Air and mineral oil with viscosities within the range of 1.0–5.5 Pa·s were used in this investigation. Measurement was by means of a pair of gamma densitometer with fast sampling frequencies (up to 250 Hz). The results obtained show that slug translational velocity increases with increase in liquid viscosity. Existing slug translational velocity prediction models in literature were assessed based on the present high viscosity data for which statistical analysis revealed discrepancies. In view of this, a new empirical correlation for the calculation of slug translational velocity in highly viscous two-phase flow is proposed. A comparison study and validation of the new correlation showed an improved prediction performance.

Holdup and frequency characteristics of slug flow in concentric and fully eccentric annuli pipes

Slug liquid holdup and frequency for gas-liquid flows in horizontal and upward low-inclination annuli are investigated. The annulus consists of an outer pipe of 99 mm inside diameter and an inner pipe of 50 mm outside diameter. Two annulus configurations are studied, namely, concentric and fully eccentric with the inner pipe at the bottom of the outer pipe. The test fluids are water and Exxsol D60 as the liquid phases and sulphur hexafluoride (SF6) as the gas phase. The slug characteristics are obtained from time-traces of the cross-sectional average liquid holdup from broad-beam gamma densitometers. Results show that the eccentricity of the annulus has limited effect on the slug liquid holdup and slug frequency. For a given superficial liquid velocity, the slug frequency has a parabolic profile as function of the mixture velocity with a minimum value at UM ≈ 3 m/s. Once UM is greater than 3 m/s, the slug frequency sharply increases. Experimental results are compared with predictions calculated using existing correlations and models developed for full pipe systems in the literature. These models fail to capture the slug characteristics in annulus configurations. One of the existing models for predictions of slug liquid holdup has been modified in this paper to account for the effect of the annulus geometry. In addition, the paper documents a new correlation development for slug frequency using the Taylor bubble (or translational velocity) and the slug liquid holdup in an annulus geometry. The proposed models, for both slug liquid holdup and frequency, perform better than the existing models in literature when tested against experimentally obtained annulus flow data.

Operation features of a narrowing device in separationless three-phase flow-meter

On the basis of experimental data, operation of the conical narrowing device, which is a part of the separationless three-phase flow-meter “oil-water-gas” of ID = 100 mm, is analyzed. The component composition is determined by a spectrometric dual energy gamma densitometer in this flow-meter. With the involvement of the study of model horizontal single-phase, two-phase and three-phase flows, operation features of the narrowing device are revealed and an approach to the assessment of the main parameters of the flow meter is justified. Experimental data were obtained for more than 500 combinations of component compositions of single-phase, two-phase and three-phase flows. Most of the obtained relative errors in determining the flow rates of liquid components are within the limits recommended with Russian State Standard − GOST R 8.615-2005.

On modeling fluidelastic instability in tube arrays subjected to two-phase cross-flow

An experimental program was undertaken in order to improve our understanding of the mechanism of fluidelastic instability in two-phase flows and to provide practical design guidance to avoid vibration problems in heat exchangers. Different ways of defining the traditional parameters, such as void fraction, damping, velocity, and 2-phase fluid density, were examined. It was found that the damping in quiescent fluid combined with the gamma densitometer measured void fraction and the interfacial velocity, collapses the available data well and provides the expected trend of two-phase flow stability data over the void fraction range from liquid to gas flows. The resulting stability maps represent a significant improvement over existing maps for predicting fluidelastic instability of tube bundles in two-phase flows. This result also tends to confirm the hypothesis that the basic mechanism of fluidelastic instability is the same for single and two-phase flows, providing that the parameters are properly defined.

Two-phase gas-liquid flow in concentric and fully eccentric annuli. Part I: Flow patterns, holdup, slip ratio and pressure gradient

Horizontal and upward low-inclination gas-water and gas-oil flows are investigated using an annulus pipe configuration in a high-pressure system (∼400 kPa). The annulus test section, or the so-called pipe-in-pipe configuration, consists of an outer pipe of 99 mm inside diameter and an inner pipe of 50 mm outside diameter. Two different vertical positions of the inner pipe with respect to outer pipe have been tested; namely concentric configuration where both pipes have the same centreline and fully eccentric where the inner pipe is placed at the bottom wall of outer pipe. The flow is studied using high-speed photography, differential pressure transducers, and broad-beam gamma densitometers to characterise the flow as function of the liquid phase properties, annulus eccentricity, and pipe inclination. Flow regime maps reveal that slug flow appears to be the most common flow feature across the conditions studied in this paper. Flow in the eccentric annulus shows a more consistent behaviour (i.e. well-defined flow structures axially and in time) than that observed in the concentric annulus. Gas-oil flows show more stable phase fraction behaviour across the cross-section compared to that observed in gas-water flows in which the interface shows significant activity promoting the formation of droplets and ligaments that break from the liquid film. This is especially the case with the concentric annulus configuration. The oil phase regardless of the geometrical configuration tends to fully wet the pipe (made of PVC) as the gas velocity increases, creating a continuous film along the pipe walls. Conversely, the thin film in gas-water flow is not continuous. Discontinuity in the thin film increases with an increase in effective pipe roughness. Given the same inlet flow conditions, the concentric annulus configuration causes larger pressure drop along the pipe than that obtained in the eccentric.

Investigation of the effect of the electrode distance on the impedance void meter performance in the two-phase flow measurement

Experimental study on the effect of electrode distance on the performance of impedance void meter is presented. The study includes multiple approaches. In the numerical simulation approach, based on the mathematical similarity between electric and temperature fields, the electric fields with different electrode distances are simulated using ANSYS Fluent to generate temperature fields. In the experimental study, an adiabatic air-water experiment is performed to compare two impedance meters with different electrode distances. The test conditions cover the flow regimes from bubbly flow to annular flow. Gamma densitometer and multi-single sensor conductivity probe are utilized for the void fraction comparison. The effect of the electrode distance on the flow regime identification using the artificial neural network is investigated, the signals of the different impedance meters are applied as the inputs for the artificial neural network. It is found that impedance meter with a properly large electrode distance has better performance in void fraction measurement, especially at high superficial velocity.

Bubble velocity in horizontal and low-inclination upward slug flow in concentric and fully eccentric annuli

The Taylor bubble velocity for gas–liquid flows, which is of great importance in multiphase flow models, has been thoroughly studied for a wide range of conditions in full pipe flows. The applicability of models developed for these full pipe systems to flows in annuli has not been fully verified as very little data are available. This work presents experimental data on concentric and fully eccentric horizontal and 4° upward annulus for gas–liquid flows at high-pressure (400 kPa, absolute). The test fluids are water and Exxsol D60 as the liquid phases and sulphur hexafluoride (SF6) as the gas phase. The test section consists of a 45 m long PVC pipe with an annulus pipe diameter ratio of K = 0.505 and an inside diameter of the outer pipe of 99 mm. Gamma densitometer sensors have been used to measure the instantaneous cross-sectional average holdup at different locations along the test section. Results show that the bubble velocity follows a linear trend, similar to that observed in full pipe systems, with a critical Froude number at FrM,C ≈ 3.3. For Froude numbers lower than the critical value, the bubble velocity is well predicted by models developed for full pipe using the hydraulic diameter. For higher Froude numbers, a new correlation has been developed based on the experimental observations with excellent agreement for all cases studied.