Flows In Large Diameter Pipes Research Articles

Two-group drift-flux model for dispersed gas-liquid flow in large-diameter pipes

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.

Soft measurement of oil–water two-phase flow using a multi-task sequence-based CapsNet

Flow parameters measurement facilitates the understanding of two-phase flow. Due to the changeable structures of the flow, the prediction of superficial velocity of oil–water two-phase flow in large diameter pipes is still a challenging problem. Therefore, we first conducted a vertical upward oil–water two-phase flow experiment in a 125 mm ID pipe, and obtained the response signals under different flow conditions by a vertical multi-electrode array (VMEA) conductance sensor. Then, new data pre-processing (1D to 2D) techniques and information fusion techniques (network channels) are employed. Moreover, the front-end structure of the network is optimized using a combination of attention block and residual structure, and the middle structure is optimized using inception block; on the other hand, the back-end structure of the original capsule network is innovatively changed so that it can handle both the flow pattern classification and superficial velocity prediction tasks. The dynamic routing algorithm has also been improved to speed up model training. Extensive experiments validate the effectiveness of the improved modules. Finally, we compare the proposed network with its variants and other competing networks. The better performance results show that our multi-task sequence-based CapsNet has great potential for dealing with high-dimensional, time-varying and nonlinear problems in multiphase flow.

Two-phase flow pattern identification in CAES systems with dimensional analysis coupled with support vector machine

Two-phase bubbly flow can lead to challenges during the charging period for hydraulically-compensated Compressed Air Energy Storage systems. In this study, upward two-phase flow systems have been investigated in large diameter vertical pipes to better understand when the transitions to problematic flow regimes occur. A comprehensive dimensional analysis was conducted, considering vertical pipe geometry and fluid thermophysical properties. Nine dimensionless parameters and three two-phase flow maps were created. The plot of liquid Reynolds number, ReL, over the Two-phase Reynolds number, Retp,made classification between the Bubbly, Slug, and Churn flow difficult for large diameter pipes. The plot of the two-phase Capillary number,Catp, over the liquid Reynolds number, ReL, showed large overlap of the data sets extracted from the literature. This may have been due to the weakness of the capillary number in defining the larger pipe flow behaviours. The last flow map based on the Slippage number, Sltp, versus the two-phase Froude number, Frtp, showed an exponential decay relationship between the Froude and Slippage parameter. When this was converted to the semi-log scale it provided a valuable classification map of two-phase flow. Finally, a Support Vector Machine (SVM) technique applied to the 2D plot of (Sltp- Frtp), gave a useful transition boundary to separate regimes in air-water upward flows in large diameter pipes.

Modified correction for predicting slip velocity of oil-water two-phase bubble flow in large-diameter pipes

Modified correction for predicting slip velocity of oil-water two-phase bubble flow in large-diameter pipes

Validation of interfacial area concentration model for simulating bubbly and cap/slug flow behaviors in large diameter pipes using LSTF, ATLAS, and horizontal pipe experiment data

This study discusses Interfacial Area Concentration (IAC) models developed for bubbly and cap/slug flow regimes in a large diameter pipe incorporating recently developed small bubble fraction correlations in various flow conditions. The main focus of this study was the robust simulation of the behaviors of bubbly and cap/slug flows in large diameter pipes observed in Large Scale Test Facility (LSTF), Advanced Thermal-hydraulic test Loop for Accident Simulation (ATLAS), and horizontal flow test facility. Since the IAC was one of the key parameters to achieve this, an IAC model for large diameter pipes was first derived based on the existing Hibiki-Ishii IAC model for bubbly flow, and the Shen-Hibiki IAC model for cap bubbly-to-churn flow. The volume fraction of group-1 bubbles, i.e., small spherical bubbles, in the IAC model, which have been developed based on the assumption that the group-1 bubbles exponentially decay in the slug flow regime, was refined. As a result, a more physical model of the group-1 bubble number density that remains constant in the cap/slug flow regime was imposed for large diameter pipes. The validity of the IAC model for large diameter pipes in predicting the interfacial drag force between gas and liquid phases in large pipes of multiple test facilities was then examined. The validation of the proposed IAC model was conducted through simulations of 1) the loss of Residual Heat Removal (RHR) during the mid-loop operation of the LSTF, 2) the 6-inch Small Break Loss of Coolant Accident (SBLOCA) of the ATLAS, and 3) the two-phase flow behaviors under a wide range of fluid conditions in the horizontal pipe test facility. Improved results were obtained with the proposed model as it indicated that there was good agreement between the calculated results and the experimental data throughout the whole transient.

Experimental investigation of upward vertical two-phase high-velocity flows in large-diameter pipes

An experimental investigation is carried out to analyze the effect of the diameter and high fluid velocities on two-phase flow in vertical large-diameter pipes. This study generated and gathered experimental data for diameters ranging from 97 to 297 mm. The working fluids for the experimental database are water and air, for superficial liquid velocities ranging from 0.2 to 4.7 m/s; and superficial gas velocities ranging from 0.1 to 30 m/s.From the experimental observations of flow regimes for diameters larger than 97 mm, cap-bubble flow is observed for conditions which slug flow would be expected in smaller diameter pipes. Based on experimental results obtained in this work and available in the literature, it is observed that pressure gradient only varies around 50% for changes of one order of magnitude on the pipe diameter (from 19 to 297 mm), for superficial gas velocities lower than the minimum point in the pressure gradient curve. The measurements of liquid holdup and pressure gradient are compared to a numerical model based on the drift-flux approach, which shows good agreement with the experimental observations.

Experimental study on vertical downward air-water two-phase flow in a large diameter pipe

Downward two-phase flows in large diameter pipes are important in various industrial applications, especially for the safety analysis in nuclear reactors. To address the issue that few data of downward flow in large diameter pipes is available for model evaluation, experiments of air–water downward flow in a pipe with inner diameter of 203.2 mm have been performed. Area-averaged void fraction and pressure measurement, as well as flow visualization, have been conducted at several axial locations. The flow conditions for superficial gas velocity range from 0.05 m/s to 3.00 m/s and for superficial liquid velocity range from 0.1 m/s to 1.5 m/s, which cover cap-bubbly flow, churn-turbulent flow and annular/falling film flow. The flow structure at several axial locations and the transition from churn-turbulent flow to annular/falling film flow have been discussed. Current available drift-flux models developed for downward flow in regular pipes as well as for upward flow in large pipes are evaluated using newly collected data. For churn-turbulent flow, the data indicates a larger drift velocity than the model prediction. Corresponding drift-flux constitutive equations are suggested which can reduce the prediction error from 34.37% to 11.79%.

Flow structure and flow regime transitions of downward two-phase flow in large diameter pipes

Downward two-phase flow in large diameter pipes appears in numerous industrial applications and nuclear reactor accidents. In this study, adiabatic air–water two-phase flow experiments in a 203.2 mm diameter pipe have been conducted to investigate flow regimes and their transitions in downward and horizontal flow. Three flow regimes (cap-bubbly, churn-turbulent and annular flow) were recognized in downward flow, as well three flow regimes (stratified, plug and pseudo-slug flow) were observed in horizontal section. Evolution of void fraction and flow structure along the loop under different flow conditions has been discussed. The Probability Density Function (PDF) and Cumulative Probability Density Function (CPDF) of area-averaged void fraction signals were utilized as the indicators for self-organized neural network (SONN) method to identify horizontal and vertical downward flow regimes, respectively. The downward flow regime maps for 203.2 mm diameter pipes have been proposed and compared with that for different diameter pipes. The results show that the flow regime maps agree well with that of 101.6 mm, but don’t agree well with that of smaller diameter pipes (25.4 mm and 50.8 mm). It is found that the transition between churn-turbulent and annular flow occurs at a certain superficial liquid velocity regardless of superficial gas velocity. A set of new transition criteria have been developed for downward flow regime transitions in large diameter pipes, and validated by the experimental data of 203.2 mm and 101.6 mm diameter pipes. Compared with existing models, these criteria provide more accurate predictions for downward flow regime transitions in large diameter pipes.

An acoustic model of NIST’s long-wavelength acoustic flowmeter for gas flow in large-diameter pipes

NIST is investigating long-wavelength acoustic flowmeter (LWAF) technology to accurately measure gas flow in large-diameter pipes, such as smokestacks used by coal-burning power plants. To aid in the data analysis and development of the method, we constructed a lumped-element acoustic model of the LWAF based on existing theory for sound propagation in circular ducts, modified to include flow. The model calculates the ratio of the acoustic pressure amplitudes and phase differences between two locations in a partial standing wave downstream of a continuous sound source up to the duct’s cut-on frequency. We used the numerical calculations of the reflection coefficient by Munt [J. Sound Vibration 142, 413–436 (1990)] to model the radiation impedance as a function of flow speed. In the absence of flow, the model was used to calibrate the positions of several microphones in the LWAF. In the presence of flow, the model predicts qualitatively the measured amplitude ratios and phase differences as a function of flow rate. Quantitative comparison is limited by the uncertainty of the radiation impedance and its flow dependence. This limitation prompts us to investigate ways to either measure the radiation impedance or eliminate it by using multiple coherent sound sources.

Interfacial area concentration in gas–liquid bubbly to churn flow regimes in large diameter pipes

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.

A correlation for interfacial area concentration in high void fraction flows in large diameter channels

Two phase flows exist as a part of many industrial processes, including chemical processes, nuclear reactor systems, and heat exchangers. In all of these applications the interfacial area concentration is an important parameter for evaluating the interactions between the phases, including drag forces, heat transfer or chemical reaction rates. Many models for interfacial area concentration exist for dispersed bubbly flows; however this type of flow only exists at relatively low void fractions. Very few correlations exist for the prediction of cap-turbulent, slug, or churn-turbulent flows. In this paper a new correlation for predicting the interfacial area concentration beyond bubbly flows in large diameter pipes is derived using a two-bubble-group method (spherical and distorted bubbles as Group-1 bubbles and cap and churn-turbulent bubbles as Group-2 bubbles) and the two-group interfacial area transport equation. The derivation assumes steady state and fully developed flow, and is based on interfacial area transport source and sink terms for large diameter pipes developed by Smith et al., 2012a. Int. J. Heat Fluid Flow 33, 156–167. The resulting equations can be used to predict the void fraction for each group of bubbles and the Sauter mean diameter for each group of bubbles in addition to the total interfacial area concentration. The model is then benchmarked based on the data collected by Schlegel et al., 2012. Exp. Therm. Fluid Sci. 41, 12–22; Schlegel et al., 2014. Int. J. Heat Fluid Flow 47, 42–56. It is found that the correlation predicts the data for Sauter mean diameter of Group 1 bubbles with RMS error of 23.3% and bias of +1.83%. For Group 2 bubbles the RMS error is 24.0% and the bias is +5.35%. This indicates that the correlation somewhat over-predicts the bubble sizes. In spite of this the prediction error remains reasonable compared to the accuracy of previous correlations, and given that the experimental uncertainty can be as high as 15% for some flow conditions. The RMS error and bias in the total interfacial area concentration are 22.6% and −4.29%, respectively. This is consistent with the over-prediction of the Sauter mean diameters, but again is reasonable considering the experimental uncertainty and the prediction error of previous correlations. The model is also able to predict the trends found in the experimental data with varied liquid and gas velocities, representing a large improvement over previous modeling efforts. An expanded database of accurate interfacial area concentration measurements at higher pressures would allow further improvement of the model benchmark and expansion of the range of applicability of the model.

Evaluation of interfacial area transport equation in vertical bubbly two-phase flow in large diameter pipes

The introduction of interfacial area transport equation (IATE) has greatly improved the overall performance of two-fluid model due to the fact that the IATE can predict the interfacial area concentration (IAC) more accurately. However, models in the source and sink terms of IATE that are developed for predicting bubble coalescence and breakup are mainly based on small diameter pipe flows. Since the two-phase flow in a large diameter pipe is characterized by its multi-dimensional nature, the IATE should be confirmed before being applied to large pipe flows. From this point of view, local measurements in air–water bubbly flow systems in a large pipe with 101.6mm diameter were performed by using four-sensor optical probes. The radial profiles of the void fraction, IAC, bubble Sauter mean diameter and interfacial velocity were obtained at two axial locations of z/D=2 and 29. The simplified one-dimensional, steady-state, adiabatic one-group IATE with eight sets of bubble coalescence and breakup models was evaluated against the present experimental data and that from the literatures. The evaluation results showed that the Sun et al. (2004a) model is the best one for large diameter pipes. This model can well reflect the interfacial transfer mechanisms and can provide a reasonable prediction in general. Besides, the models of Smith et al. (2012a) and Ishii and Kim (2001) are also recommended due to their relative good performance in the prediction of IAC. Further work should be undertaken to develop a new bubble coalescence and breakup model with extra consideration of the multi-dimensional flow behavior in large pipes, if necessary.

CFD modelling of stratified/atomization gas–liquid flow in large diameter pipes

A CFD model of stratified flow with atomization has been developed to study the transport of water droplets in gas–liquid multiphase systems in 38in. diameter pipes. Entrained droplets may deposit onto the wall surface, forming a thin draining film associated with an effective interfacial roughness. Secondary flows related to large gas flow rates may enhance the transport of droplets in the top region of the pipe. Gas–liquid surface tension also appears as an important parameter; it is shown to affect the entrainment rate and subsequently, the droplet deposition rate. This work involving water droplets extends the initial study performed with oil droplets entrainment in the gas phase, validated for 8in. pipes, and scaled-up on larger diameter pipes.

Modified distribution parameter for churn-turbulent flows in large diameter channels

Two phase flows in large diameter channels are important in a wide range of industrial applications, but especially in analysis of nuclear reactor safety for the prediction of BWR behavior and safety analysis in PWRs. To remedy an inability of current drift-flux models to accurately predict the void fraction in churn-turbulent flows in large diameter pipes, extensive experiments have been performed in pipes with diameters of 0.152m, 0.203m and 0.304m to collect area-averaged void fraction data using electrical impedance void meters. The standard deviation and skewness of the impedance meter signal have been used to characterize the flow regime and confirm previous flow regime transition results. By treating churn-turbulent flow as a transition between cap-bubbly dispersed flow and annular separated flow and using a linear ramp, the distribution parameter has been modified for churn-turbulent flow. The modified distribution parameter has been evaluated through comparison of the void fraction predicted by the drift-flux model and the measured void fraction.

Mechanistic modeling of interfacial area transport in large diameter pipes

Flow in large pipes is important in a wide variety of applications. In the nuclear industry in particular, understanding of flow in large diameter pipes is essential in predicting the behavior of reactor systems. This is especially true of natural circulation Boiling Water Reactor (BWR) designs, where a large-diameter chimney above the core provides the gravity head to drive circulation of the coolant through the reactor. The behavior of such reactors during transients and during normal operation will be predicted using advanced thermal-hydraulics analysis codes utilizing the two-fluid model. Essential to accurate two-fluid model calculations is reliable and accurate computation of the interfacial transfer terms. These interfacial transfer terms can be expressed as the product of one term describing the potential driving the transfer and a second term describing the available surface area for transfer, or interfacial area concentration. Currently, the interfacial area is predicted using flow regime-dependent semi-empirical correlations; however the interfacial area concentration is best computed through the use of the one-dimensional interfacial area transport equation (IATE). To be useful in practical applications the IATE requires mechanistic models for the change in interfacial area concentration due to interactions between bubbles. Therefore using previously collected data and proper scaling methods, current models for bubble interactions for the IATE are briefly discussed and new models applicable to large pipes are developed and compared with the existing data.

Two-phase flow structure in large diameter pipes

Flow in large pipes is important in a wide variety of applications. In the nuclear industry in particular, understanding of flow in large diameter pipes is essential in predicting the behavior of reactor systems. This is especially true of natural circulation Boiling Water Reactor (BWR) designs, where a large-diameter chimney above the core provides the gravity head to drive circulation of the coolant through the reactor. The behavior of such reactors during transients and during normal operation will be predicted using advanced thermal–hydraulics analysis codes utilizing the two-fluid model. Essential to accurate two-fluid model calculations is reliable and accurate computation of the interfacial transfer terms. These interfacial transfer terms can be expressed as the product of one term describing the potential driving the transfer and a second term describing the available surface area for transfer, or interfacial area concentration. Currently, the interfacial area is predicted using flow regime dependent empirical correlations; however the interfacial area concentration is best computed through the use of the one-dimensional interfacial area transport equation (IATE). To facilitate the development of IATE source and sink term models in large-diameter pipes a fundamental understanding of the structure of the two-phase flow is essential. This understanding is improved through measurement of the local void fraction, interfacial area concentration and gas velocity profiles in pipes with diameters of 0.102m and 0.152m under a wide variety of flow conditions. Additionally, flow regime identification has been performed to evaluate the existing flow regime transition criteria for large pipes. This has provided a more extensive database for the development and evaluation of IATE source and sink models. The data shows the expected trends with some distortion in the transition region between cap-bubbly and churn-turbulent flow. The flow regime map for the 0.102m and 0.152m diameter test sections agree with the existing flow regime transition criteria. It may be necessary to perform further experiments in larger pipes and at higher gas flow rates to expand the range of conditions for which models can be developed and tested.

Flow Characteristics of Centrifugal Gas-Liquid Separator (Investigation with Air-Water Two-Phase Flow Experiment)

Air water two phase flow experiment was conducted to examine the basic flow characteristics of a centrifugal gas liquid separator. Vertical transparent test section, which is 4 m in height, was used to imitate the scale of a BWR separator. Flow rate conditions of gas and liquid were fixed at 0.1 m3/s and 0.033 m3/s, respectively. Radial distributions of two-phase flow characteristics, such as void fraction, gas velocity and bubble chord length, were measured by traversing dual optical void probes in the test section, horizontally. The flow in the standpipe reached to quasi-developed state within the height to diameter aspect ratio H/D=10, which in turn can mean the maximum value for an ideal height design of a standpipe. The liquid film in the barrel showed a maximum thickness at 0.5 to 1 m in height from the swirler exit, which was a common result for three different standpipe length conditions, qualitatively and quantitatively. The empirical database obtained in this study would contribute practically to the validation of numerical analyses for an actual separator in a plant, and would also be academically useful for further investigations of two-phase flow in large-diameter pipes.

New Interfacial Drag Force Model Including Effect of Bubble Wake, (II)

In the previous study, we proposed a new interfacial drag force model based on experimental data of steam-water bubbly flow in a large-diameter pipe. This is because our experimental results had suggested that effect of bubble wake should be included in the interfacial drag force model, although it had not been taken into account in the existing models. A preliminary method for including the effect of bubble wake was hence developed and used in the new model. A new bubble size prediction method was also adopted in the model. In the present study, after improving the measuring equipment and signal processing procedure, another series of experiments was carried out. Using the new experimental data, the methods of predicting bubble size and effect of bubble wake were slightly modified. To test the validity of the new model, predicted results were compared with available experimental data sets of steam-water bubbly two-phase flow in large-diameter pipes. One-dimensional and two-dimensional two-fluid models were used for the calculation. Comparisons showed that the new model is in good agreement with the experimental data, whereas the model which does not take into account the effect of bubble wake overestimated the void fractions.

New Interfacial Drag Force Model Including Effect of Bubble Wake, (I) Model Development for Steam-Water Bubbly Flow in Large-Diameter Pipes

Several experimental results show that bubbles can easily be captured in the wake formed by leading bubbles when multiple bubbles are rising in a liquid. It is suggested from this experimental result that the effect of bubble wake should be included in the constitutive relationships representing the interfacial drag force. In the present study, steam-water bubbly flow experiments were performed to develop a new interfacial drag force model including the effect of bubble wake. Since the validity of the existing constitutive equations have been tested mainly for two-phase flow in small-diameter pipes, our study focused on two-phase flow in a large-diameter pipe. Using a one-dimensional two-fluid model, the applicability of the new interfacial drag force model to our experimental conditions was investigated. As a result, it was shown that the present model markedly improves the accuracy of the predicted results. It was therefore demonstrated that the present bubble wake model is effective at least for the conditions which were used for model development. Its applicability to different conditions will be discussed in a subsequent study.

New Interfacial Drag Force Model Including Effect of Bubble Wake, (I) Model Development for Steam-Water Bubbly Flow in Large-Diameter Pipes.

In the previous study, we proposed a new interfacial drag force model based on experimental data of steam-water bubbly flow in a large-diameter pipe. This is because our experimental results had suggested that effect of bubble wake should be included in the interfacial drag force model, although it had not been taken into account in the existing models. A preliminary method for including the effect of bubble wake was hence developed and used in the new model. A new bubble size prediction method was also adopted in the model. In the present study, after improving the measuring equipment and signal processing procedure, another series of experiments was carried out. Using the new experimental data, the methods of predicting bubble size and effect of bubble wake were slightly modified. To test the validity of the new model, predicted results were compared with available experimental data sets of steam-water bubbly two-phase flow in large-diameter pipes. One-dimensional and two-dimensional two-fluid models we...