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Based on the assumption of gas-liquid stratified flow pattern in inclined gas wells, considering the influence of wettability and surface tension on the circumferential distribution of liquid film along the wellbore wall, the influence of the change of the gas-liquid interface configuration on the potential energy, kinetic energy and surface free energy of the two-phase system per unit length of the tube is investigated, and a new model for calculating the gas-liquid distribution at critical conditions is developed by using the principle of minimum energy. Considering the influence of the inclination angle, the calculation model of interfacial friction factor is established, and finally closed the governing equations. The interface shape is more vulnerable to wettability and surface tension at a low liquid holdup, resulting in a curved interface configuration. The interface is more curved when the smaller is the pipe diameter, or the smaller the liquid holdup, or the smaller the deviation angle, or the greater gas velocity, or the greater the gas density. The critical liquid-carrying velocity increases nonlinearly and then decreases with the increase of inclination angle. The inclination corresponding to the maximum critical liquid-carrying velocity increases with the increase of the diameter of the wellbore, and it is also affected by the fluid properties of the gas phase and liquid phase. The mean relative errors for critical liquid-carrying velocity and critical pressure gradient are 1.19% and 3.02%, respectively, and the misclassification rate is 2.38% in the field trial, implying the new model can provide a valid judgement on the liquid loading in inclined gas wells.

The objective of this study was to reduce the uncertainties of correlations for flow characteristics in vertical pipes under flooding at the top end. The void fraction α, pressure gradient dP/dz, and countercurrent flow limitation (CCFL) were previously measured with diameter D = 40 mm and working fluid of air and water. The wall friction and interfacial friction factors (fw and fi) were obtained based on the annular flow model, and CCFL and fw were evaluated in detail. Hence, attention was turned to detailed evaluations of α and fi. Liquid film thickness δ and interfacial friction factor fi for smooth film (SF) due to flooding at the top end were obtained using the previously derived fw correlation and existing dP/dz data with D = 20 to 50.8 mm and pressure P = 0.1 to 4.1 MPa, and empirical correlations for δ and fi were derived. The δ term was well expressed by a function of the liquid Reynolds number ReL, and the uncertainty of the δ correlation was ±0.0062 for α = 0.87 to 0.98. fi was expressed by a function of δ/L (where L is the Laplace length) or the Kutateladze parameter KG*, the dimensionless diameter D* (=D/L), and the density ratio of the gas and liquid phases ρG/ρL. The applicability of the derived correlations to conditions of D = 300 mm and P = 7 MPa was evaluated, and the fi correlation was modified based on fi values computed with the δ correlation. The drift-flux parameters for SF were also considered. [ABSTRACT FROM AUTHOR]

[Display omitted] • The PCB liquid film sensor can accurately measure the thickness of liquid film. • A new correlation for predicting liquid film thickness is proposed. • The interfacial friction factor tends to be a constant at high gas velocity. • A new correlation for predicting interfacial friction factor is proposed. The interfacial shear stress and liquid film thickness are very important to establish the basic model of annular flow in practical applications. In this paper, the liquid film thickness of annular flow in a narrow vertical rectangular channel with a cross-sectional area of 70 mm × 2 mm was measured based on the PCB liquid film sensor. According to the experimental results, the average liquid film thickness, base film thickness and interfacial friction factor were calculated and analyzed. The experimental results showed that the liquid film thickness was affected by the gas phase velocity and the liquid phase velocity, but the effect of the gas phase velocity on the liquid film thickness decreased gradually with increasing gas phase velocity. Moreover, the interfacial friction factor was obvious affected by the liquid phase velocity at low gas phase velocity, but this influence gradually decreased with increasing gas phase velocity. Existing correlations for above parameters were summarized and evaluated with the current data, and there was a large deviation between the predicted results and the experimental results. Therefore, a new correlation for above parameters was proposed, respectively. [ABSTRACT FROM AUTHOR]

A generic model for the prediction of critical gas velocity and pressure gradient in slightly inclined pipes (β ≤ 6°) is presented in this article. The gas–liquid configuration was determined based on the minimum energy principle and consideration of wettability and surface tension. A visualization experiment was conducted to obtain the critical gas velocity and the critical pressure gradient of a gas–liquid flow through the 40 and 60‐mm pipe diameter. The theoretical study shows that the configuration is close to a convex interface shape at the critical conditions, which is in accord with the experimental phenomenon. Experimental study shows interfacial waves are the main cause of increased interfacial friction factor and a linear functional relationship between the inclination angle and the flow correction factor f(β). The results demonstrate that the new model is capable of providing satisfactory prediction results for the critical gas velocity, pressure drop, and liquid holdup. [ABSTRACT FROM AUTHOR]

Accurate prediction of interfacial friction factor is critical for calculation of pressure drop and investigation of flow mechanism of vertical annular two-phase flows. Theoretical models of interfacial friction factor based on physical insight have been developed; however, these are inconvenient in engineering practice as too many parameters need to be measured. Although many researchers have proposed various empirical correlations to improve computation efficiency, there is no generally accepted simple formula. In this study, an efficient prediction model based on support vector regression machine (SVR) is proposed. Through sensitivity analysis, five factors are determined as the input parameters to train the SVR model, relative liquid film thickness, liquid Reynolds number, gas Reynolds number, liquid Froude number and gas Froude number. The interfacial friction factor is chosen as the output parameter to check the overall performance of the model. With the help of particle swarm algorithm, the optimization process is accelerated considerably, and the optimal model is obtained through iterations. Compared with other correlations, the optimal model shows the lowest average absolute error (AAE of 0.0004), lowest maximum absolute error (MAE of 0.006), lowest root mean square error (RMSE of 0.00076) and highest correlation factor (r of 0.995). The analysis using various data in the literature demonstrates its accuracy and stability in interfacial friction prediction. In summary, the proposed machine learning model is effective and can be applied to a wider range of conditions for vertical annular two-phase flows.

As part of its design certification effort, NuScale Power has completed a series of low mass flux [<1000 kg/(m2‧s)] critical heat flux (CHF) tests for a wide range of pressures at Stern Laboratories in Canada. Earlier studies have demonstrated that under annular flow conditions, disturbance waves with circulating vortices traverse the rod surface. The disturbance wave slides over and significantly influences energy transport in the co-current vapor-liquid sublayer at the heater interface. This paper describes the mechanisms leading to the onset of CHF in a vertical rod bundle experiencing annular mist flow conditions. The paper presents a new CHF model that implements a local disturbance wave velocity. A comparison of the model to the U1 CHF data set from Stern Laboratories shows excellent agreement over the full range of annular mist flows, pressures, and subcooled conditions for the specific spacer grids implemented in the study. [ABSTRACT FROM AUTHOR]

Proper selection and application of interfacial friction factor correlations has a significant impact on prediction of key flow characteristics in gas–liquid two-phase flows. In this study, experimental investigation of gas–liquid flow in a vertical pipeline with internal diameter of 0.060 m is presented. Air and oil (with viscosities ranging from 100–200 mPa s) were used as gas and liquid phases, respectively. Superficial velocities of air ranging from 22.37 to 59.06 m/s and oil ranging from 0.05 to 0.16 m/s were used as a test matrix during the experimental campaign. The influence of estimates obtained from nine interfacial friction factor models on the accuracy of predicting pressure gradient, film thickness and gas void fraction was investigated by utilising a two-fluid model. Results obtained indicate that at liquid viscosity of 100 mPa s, the interfacial friction factor correlation proposed by Belt et al. (2009) performed best for pressure gradient prediction while the Moeck (1970) correlation provided the best prediction of pressure gradient at the liquid viscosity of 200 mPa s. In general, these results indicate that the two-fluid model can accurately predict the flow characteristics for liquid viscosities used in this study when appropriate interfacial friction factor correlations are implemented.

The external flow field over wave interface is reached by the aid of conformal transformation thought, and velocity boundary layer of inner flow field is deduced also, which gives fundamental description of shear stress over wave interface. The viscous drag coefficient is induced with considering the characteristics of interface wave, such as wave height, wave length and ratio between wave lengths of both ascending and descending semi periods, which significantly impact the velocity gradient over the wave interface that accounts for the differences in distribution of local shear stress. The influence of fluid flow is studied also, which gives support that the earlier separating point of the fluid flow over wave interface the stronger depression to the turbulent perturbation is made, and a reduced drag force arrived. There are otherness and commonness between fluid flow over wave interface and interface appearing in air-water wave stratified flow, and a new model is proposed to intrinsically construct the interfacial friction factor encountered in wave stratified flow. The models for viscous drag coefficient and interfacial friction factor are tested, different models are studied, predicted values are compared with experimental data and that computed by solving there-dimension unsteady Navier-Stokes equations, which gives proof that the values predicted by newly proposed models well fit the real values. [ABSTRACT FROM AUTHOR]

Highlights • A new correlation for interfacial friction factor of vertical upward adiabatic annular flow in pipes has been proposed. • The new correlation is based on a group of non-dimensional numbers including We f , We g ″ and N μ f , which are easy to obtain. • From data comparison, the new correlation has a mean absolute percentage error of 15% for all data listed in four databases. Abstract Interfacial shear stress in annular two-phase flow is essential both for detailed modeling of two-fluid model and calculation of pressure gradient. Most of existing correlations on interfacial friction factor are based on Wallis's (1969) correlation, which considers the interfacial friction factor as a function of film thickness. As film thickness is a parameter not easy to obtain, it needs either high accuracy instrument to measure from experiment or to be calculated from available correlations. The calculated film thickness from available correlations introduces additional uncertainties to interfacial friction factor. Because of this, a new correlation of interfacial friction factor based on a group of non-dimensional numbers which are easy to obtain has been proposed. From data comparison, the new correlation has a mean absolute percentage error of 15% for all data listed in four databases. [ABSTRACT FROM AUTHOR]

Abstract The increasing use of commercial flow simulators in the oil and gas industry provides the impetus to accurately model multiphase flows in pipelines. The current work investigates stratified oil-water flows in horizontal pipelines by experimentally measuring interfacial deformations, correlating them to interfacial friction. The derived interfacial model is then implemented into the two-fluid model to predict pressure losses in pipelines. An experimental campaign was undertaken using process oil (ρ = 845 k g / m 3 , μ = 0.03 P a. s) and water as the test fluids in acrylic pipes of 40 mm diameter, with objectives of capturing interfacial deformations via image processing techniques, while also measuring pressure gradients in the pipe. The interfacial deformations were seen to increase with increasing flow velocities, plateauing with wave amplitudes of 3 mm. Further tests with identical fluids, conducted on larger diameter pipes (54.8 mm and 108.4 mm) also showed similar trend. Wave aspect ratios were correlated with a modified Froude number, linking the interfacial deformations to input flow variables. An empirical equation, utilizing the amplitude of interfacial deformations as roughness heights is derived for the interfacial friction factor. Incorporating the derived equation into the one dimensional two-fluid model, pressure losses were calculated and compared against experimental pressure drop measurements conducted in current work, as well as with those in literature. The performance of this model is also compared with existing prediction models and is found to provide improved accuracies across wide range of viscosity ratios and pipe diameters. Highlights • Experimentally captured and quantified liquid-liquid interface deformations using image processing tools. • Pressure drop measurements in liquid-liquid stratified and stratified-wavy flow regimes. • Interfacial friction factor model is presented to incorporate interfacial wave amplitudes as roughness parameters. • Pressure prediction model developed and tested against experimental data and data from literature. [ABSTRACT FROM AUTHOR]

• Effects of liquid viscosity on interfacial and wall friction factors of swirling annular flows were investigated. • An interfacial swirl number was obtained in each liquid phase with different viscosity. • Friction factors were evaluated by correlations with the interfacial swirl number and liquid viscosity. Experiments of gas–liquid swirling annular flows in a vertical pipe of diameter D = 30 mm were carried out to obtain interfacial and wall friction factors, f i and f w , based on a three-fluid model. The liquid phase was two kinds of glycerol-water solutions with the kinematic viscosity ν L of two or four times larger than that of water. Experimental data were obtained in two axial sections with short and long axial distances from the swirler, which corresponds to swirling and non-swirling annular flow regimes, respectively. Both the azimuthal and axial components of interfacial velocity decayed with increasing ν L. The interfacial swirl number s i , which is the ratio of the azimuthal component to the axial component of interfacial velocity, was not affected by ν L for z *(= z / D) ≤ 3.8, where z is the axial coordinate in the test section. The mean interfacial swirl number s i ∗ , which is the arithmetic mean of s i in z * ≤ 3.8, was expressed in terms of dimensionless numbers based on the gas and liquid volumetric fluxes without ν L. The interfacial friction factor decreased with increasing ν L in swirling flows. The functional form of the Colebrook equation had a prospect of correlating f i in both swirling and non-swirling annular flows. The wall friction factor ratio of f w in the swirling annular flow regime to that in the non-swirling annular flow regime decreased as ν L increased for s i ∗ > 0.33. The wall friction factor ratio was well correlated in terms of s i ∗ and ν L. [ABSTRACT FROM AUTHOR]

A mechanistic model for void fraction prediction with improved interfacial friction factor in nearly horizontal tubes has been proposed in connection with the development of a condensation model package for the passive auxiliary feedwater system of the Korean Advanced Power Reactor Plus. The model is based on two-phase momentum balance equations to cover various types of fluids, flow conditions, and inclination angles of the flow channel in a separated flow. The void fraction is calculated without any discontinuity at flow regime transitions by considering continuous changes of the interfacial geometric characteristics and interfacial friction factors across three typical separated flows, namely stratified–smooth, stratified–wavy, and annular flows. An evaluation of the proposed model against available experimental data covering various types of fluids and flow regimes showed a satisfactory agreement.

Abstract: Correlations for the interface roughness are proposed for conditions without or with disturbance waves for vertical annular flow. The decisive role of interface roughness in the interfacial friction factor is specified through a comparison of ten correlations for the interfacial friction factor with an experimental database. The effect of the disturbance wave on the interface roughness is analyzed to figure out the influence parameters, which are the superficial gas and liquid Reynolds numbers for conditions without disturbance waves and gas core and liquid film Reynolds numbers for conditions with disturbance waves. The proposed correlations improve the prediction precision of the interfacial friction factor, especially in smooth and transitional regimes. [ABSTRACT FROM AUTHOR]

Experiments on air–water two-phase swirling annular flows in a vertical pipe of 40 mm diameter were carried out at atmospheric pressure and room temperature to investigate interfacial and wall friction factors, f i and f w . The friction factors were evaluated using measured pressure drops and void fractions. Measurements of liquid film thickness and flow observation were also conducted. The ranges of the gas and liquid volume fluxes, J G and J L , were 12.5 ≤  J G  ≤ 20.0 m/s and 0.03 ≤   J L  ≤ 0.11 m/s, respectively. The main conclusions obtained are as follows: (1) f i and f w in swirling annular flows are several times larger than those in non-swirling flows, (2) f i is well correlated in terms of the liquid volume fraction and the gas Reynolds number, Re G , (3) Re G and the liquid Reynolds number, Re L , are required for correlating f w , and (4) the liquid film thicknesses in two-phase swirling flows in a one-fifth model of a BWR separator are well predicted using the two-fluid model and the correlations of f i and f w developed based on the experimental data. [ABSTRACT FROM AUTHOR]

• Optimized empirical correlations for liquid film thickness and interfacial friction factor in vertical gas-liquid annular flow have been presented. • The liquid film thickness is provided as a novel five-variable correlation. • For the thickness of the liquid film, the experimental values and suggested correlations deviate by a maximum of 3.15%. • For the interfacial friction coefficient, the experimental values and suggested correlations deviate by a maximum of 4.56%. The annular flow regime is one of the most prevalent two-phase flow regimes, occurring throughout a broad range of gas and liquid flow rates. This flow pattern is distinguished by a thin layer of liquid around the walls and a core of quickly flowing gas in the middle. Using image recording and processing methods, the generation of an annular flow pattern in a counter-current two-phase flow in a vertical transparent pipe was explored in the current experimental study. In this regard, the range of air (bottom to top) and water (top to bottom) velocities for annular flow was 20.94–3.66 m/s and 0.3–1.06 m/s, respectively. In this paper, the liquid film thickness is provided as a novel five-variable correlation function of gas phase superficial velocity, liquid phase velocity, pipe diameter, Weber number, and Froude number, with an R2 value of 0.98. In addition, the interfacial friction factor (liquid and gas phases) is provided as a five-variable correlation based on gas flow Reynolds number, liquid flow Reynolds number, pipe diameter, liquid film thickness, and Froude number with coefficient of determination of 0.97. The sensitivity analysis of the acquired experimental findings reveals that the liquid film thickness and the interfacial friction factor are most sensitive to changes in the superficial velocity of the gas phase and the Reynolds number of the liquid phase, respectively. Also, for the thickness of the liquid film and the interfacial friction coefficient, the experimental values and suggested correlations deviate by a maximum of 3.15 percent and 4.56 percent, respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

One-region computations with the annular flow model were done for counter-current flow limitation (CCFL) at the sharp-edged lower end of vertical pipes to evaluate effects of diameters and fluid properties on CCFL characteristics. CCFL characteristics computed with several correlations for interfacial friction factors were compared with CCFL data and the correlation proposed by Bharathan et al. (which is a function of void fraction) was selected. The adjustment factors were obtained to give good agreement between CCFL characteristics computed with the correlation by Bharathan et al. and CCFL data and were correlated with the viscosity ratio of gas and liquid phases and the dimensionless diameter. By using the correlation for interfacial friction factors by Bharathan et al. modified with the viscosity ratio of gas and liquid phases and the dimensionless diameter, effects of diameters and fluid properties on CCFL characteristics were computed. [ABSTRACT FROM AUTHOR]

Foam lift is one of the most cost effective methodologies for unloading gas wells. The surfactants are either injected intermittently or continuously to lift the liquid to the surface. By reducing the gravitational gradient and increasing the frictional gradient, the critical velocity at which liquid loading occurs is shifted to lower gas velocities. Currently, we do not have a methodology to predict the critical velocity (at the transition boundary of annular and intermittent flow) and the pressure drop under foam flow conditions. To address this, we measured several foam flow characteristics in both small scale and large scale facilities. Small scale facility involved measurement of foam carryover capacity as a function of time and surfactant concentration. Large scale facility involved measurement of liquid holdup, pressure drop, fraction of gas trapped in foam and foam holdup in 40-ft 2-in. and 4-in. tubing. We developed closure relationships for liquid hold up, foam holdup, fraction of gas trapped in the foam and interfacial friction factor by combining the small scale data with the data collected in the large scale experiments. These closure relationships are applicable to four different surfactants tested. A new transition criterion was developed and successfully used to predict onset of liquid loading under foam flow. Using a force balance over the gas core in annular flow, we developed a new procedure to calculate the pressure drop under foam flow conditions. We compared our model results with actual measurements in the large scale facility. Our model was reasonably able to predict the pressure drop within ±30%. The reason for such a large variance is that the small scale facility was not able to capture all the characteristics of the foam which were observed in the large scale facility. It is very difficult to reproduce the foam characteristics exactly in two different experiments. This is discussed further in this paper. The procedure developed is the only one currently available to calculate the pressure drop under the foam flow conditions using the small scale data. It is superior to conventional annular flow pressure drop prediction models which are currently available in the literature. [ABSTRACT FROM AUTHOR]

Interfacial friction is one of the key variables for predicting annular two-phase flow behaviours in vertical pipes. In order to develop an improved correlation for interfacial friction factor in downward co-current annular flow, the pressure gradient, film thickness and film velocity data were generated from experiments carried out on Cranfield University’s Serpent Rig, an air/water two-phase vertical flow loop of 101.6 mm internal diameter. The air and water superficial velocity ranges used are 1.42–28.87 and 0.1–1.0 m/s respectively. These correspond to Reynolds number values of 8400–187,000 and 11,000–113,000 respectively. The correlation takes into account the effect of pipe diameter by using the interfacial shear data together with dimensionless liquid film thicknesses related to different pipe sizes ranging from 10 to 101.6 mm, including those from published sources by numerous investigators. It is shown that the predictions of this new correlation outperform those from previously reported studies. [ABSTRACT FROM AUTHOR]

The flooding phenomenon has an important influence on the safety of equipment, especially the nuclear power plant. A new semi-empirical correlation for flooding was proposed based on the experimental investigation of the physical characteristics of the flooding in our previous work. The model was derived from the momentum equations in the annular flow model. The liquid volume fraction, the wall and interfacial friction have been taken into account. The interfacial friction factor is emphatically discussed in this study since the interfacial friction has a significant influence on flooding. A modified model of interfacial friction factor was presented containing the Bond number since the Bond number can remove the influence of the geometry. The predicting error was basically within 20% compared with the existing experimental data obtained in different geometries and sizes. • A modified interfacial friction model for countercurrent flow was proposed based on the experimental results. • The interfacial and wall friction, liquid volume fraction have been taken into account in the new flooding correlation. • Bond number was introduced into the flooding correlation to represent the geometry effect. [ABSTRACT FROM AUTHOR]

• Detailed experimental characterization of flow boiling in a millimetric tube in vertical upward and downward flows configurations. • Characterization of the flow patterns, void fraction, wall and interfacial shear stresses and wall heat transfer. • Modelling of the wall shear stress based on the contribution of both the convection and nucleate boiling. • Modelling of the interfacial shear stress, based on the characterization of the interfacial waves. Upward and downward vapor-liquid vertical flows inside a vertical 6 mm sapphire tube was investigated using HFE-7000 as working fluid. The goal was to investigate the effect of flow direction and wall heating on two-phase parameters such as flow pattern, wall shear stress (τ w), interfacial shear stress (τ i) and interfacial wave structures. The mass flux, heat flux and vapor quality ranges were 50 ≤ G ≤ 400 k g / m 2. s , 0 ≤ q ≤ 3 W / c m 2 and 0 ≤ x ≤ 0.7 respectively. Bubbly, slug, churn, annular flow patterns were observed in upward and downward flows. A falling film regime occurred in downward flow at low mass fluxes. Void fractions were higher in downward flow than in upward flow due to gravity effect, and were in good agreement with drift flux models of the literature. The wall shear stress increased with the wall heat flux due to the bubble nucleation at the wall. Following a similar approach to Kim and Mudawar (2013b), a correlation for the wall shear stress taking into account the forced convection and the bubble nucleation was derived and provided a good estimation of the experimental data within ± 20 %. From image processing of the high-speed visualizations, velocities (U w) and frequencies (F w) of the disturbance waves in annular flow were measured. The interfacial shear stress was found to directly depend of the product U w × F w and a prediction of the interfacial friction factor was proposed in flow boiling for both upward and downward flows. [ABSTRACT FROM AUTHOR]

Highlights: [•] New data were obtained on the effect of physical properties of the two phases. [•] Semi-theoretical transitions are suggested for the stratified flow subregimes. [•] An improved model is suggested for pressure drop and holdup in horizontal stratified flow. [•] The model is applicable to slightly downward stratified flow. [Copyright &y& Elsevier]

Accurate prediction of interfacial friction factor is critical for calculation of pressure drop and investigation of flow mechanism of vertical annular two-phase flows. Theoretical models of interfacial friction factor based on physical insight have been developed; however, these are inconvenient in engineering practice as too many parameters need to be measured. Although many researchers have proposed various empirical correlations to improve computation efficiency, there is no generally accepted simple formula. In this study, an efficient prediction model based on support vector regression machine (SVR) is proposed. Through sensitivity analysis, five factors are determined as the input parameters to train the SVR model, relative liquid film thickness, liquid Reynolds number, gas Reynolds number, liquid Froude number and gas Froude number. The interfacial friction factor is chosen as the output parameter to check the overall performance of the model. With the help of particle swarm algorithm, the optimization process is accelerated considerably, and the optimal model is obtained through iterations. Compared with other correlations, the optimal model shows the lowest average absolute error (AAE of 0.0004), lowest maximum absolute error (MAE of 0.006), lowest root mean square error (RMSE of 0.00076) and highest correlation factor (r of 0.995). The analysis using various data in the literature demonstrates its accuracy and stability in interfacial friction prediction. In summary, the proposed machine learning model is effective and can be applied to a wider range of conditions for vertical annular two-phase flows. [ABSTRACT FROM AUTHOR]

Water and glycerine solutions were used to produce nominal liquid viscosities of 1, 50, 200 and 550 mPa s. These liquids flowed concurrently with air to allow measurements of momentum flux, void fraction and pressure distributions to be made at mass fluxes consistent with an exit “homogeneous” Mach number of 0.4. The flow pattern in the pipe was, in the main, annular up-flow. The data are shown to be consistent with the annular flow model. However, entrained liquid fractions were estimated at conditions where they were unexpected and the interfacial friction factors were significantly different to those implied from low viscosity fluid correlations. A new correlation for the interfacial friction factor is proposed for the highly viscous dataset. Choking tests were carried out with air–liquid flows with liquid viscosities of 1 and 50 mPa s. These data showed that the annular model approach extends to cover these flow conditions. More traditional methods are shown not to represent the measured void fraction, frictional pressure drop or choking mass flow rates. [Copyright &y& Elsevier]

Proper selection and application of interfacial friction factor correlations has a significant impact on prediction of key flow characteristics in gas–liquid two-phase flows. In this study, experimental investigation of gas–liquid flow in a vertical pipeline with internal diameter of 0.060 m is presented. Air and oil (with viscosities ranging from 100–200 mPa s) were used as gas and liquid phases, respectively. Superficial velocities of air ranging from 22.37 to 59.06 m/s and oil ranging from 0.05 to 0.16 m/s were used as a test matrix during the experimental campaign. The influence of estimates obtained from nine interfacial friction factor models on the accuracy of predicting pressure gradient, film thickness and gas void fraction was investigated by utilising a two-fluid model. Results obtained indicate that at liquid viscosity of 100 mPa s, the interfacial friction factor correlation proposed by Belt et al. (2009) performed best for pressure gradient prediction while the Moeck (1970) correlation provided the best prediction of pressure gradient at the liquid viscosity of 200 mPa s. In general, these results indicate that the two-fluid model can accurately predict the flow characteristics for liquid viscosities used in this study when appropriate interfacial friction factor correlations are implemented. [ABSTRACT FROM AUTHOR]

• The interfacial friction factor in the IAFB regime is computed from experimental data. • Key mechanisms for interfacial friction factor in smooth and wavy IAFB regions differ. • Nine interfacial friction factor correlations are assessed and recommendations made. The interfacial friction factor is an important closure model in the two-fluid model for the inverted annular film boiling (IAFB) regime. Almost all the interfacial friction factor correlations used in IAFB modeling were obtained based on experimental data from annular two-phase flows. This paper first reviews the interfacial friction factor models/correlations used in IAFB modeling. A laminar vapor film model is then derived for the smooth IAFB region based on a laminar flow assumption. Experimental data obtained from subcooled and low-quality film boiling experiments in the literature are reduced to calculate the interfacial friction factor by solving one-dimensional two-fluid momentum equations for the liquid and vapor phases. The trend of the interfacial friction factor indicates that the transition from the smooth to wavy IAFB regions occurs at a void fraction range of 0.2 to 0.4. Comparisons between the laminar vapor film model and corresponding experimental data and nine correlations in the literature indicate that the laminar vapor film model appears to provide the lower bound of the interfacial friction factor in the smooth IAFB region. Two interfacial friction factor correlations from the literature are recommended for the smooth and wavy IAFB regions based on the analysis. It can be concluded that the interfacial friction factor in the smooth IAFB region is dependent on the gas Reynolds number and vapor film thickness, while in the wavy IAFB region, it is primarily dependent on the amplitude of the interfacial waves. [ABSTRACT FROM AUTHOR]