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AbstractVoid fraction and relative velocity between phases are parameters of key importance when analyzing two-phase flow systems. The drift flux model is a simple, yet robust, method of predicting these parameters. Detailed drift flux analyses have focused on vertical flows, with recent work investigating horizontal flows. There is comparatively little work performed involving inclined two-phase flows—and fewer specifically for inclined bubbly flows. Depending on the orientation, the buoyancy force may not act parallel to the flow direction and can induce asymmetry and bubble clustering, which may affect relative velocity and void fraction. The current work seeks to characterize the effects of inclination on void fraction and relative velocity in bubbly two-phase flows. This is done by performing one-dimensional drift flux analyses and by comparing closure relations proposed in the literature. Experiments are performed focusing on bubbly flows in an inclinable test facility with an inner diameter of 25.4 mm. Time-averaged two-phase flow parameters are measured using four-sensor conductivity probes, and these measurements are benchmarked to ensure reliability. It is found that as the inclination angle is reduced from vertical, the relative velocity changes rapidly, yielding a slip ratio of less than unity for angles investigated below 80°. This corresponds to changes in the distribution parameter and drift velocity. The effects of angle are also apparent in changes in the area-averaged void fraction. These findings challenge previously developed correlations, which are generally unable to predict the experimental void fraction and have suggested slip greater than unity for inclined-upward bubbly flows. [ABSTRACT FROM AUTHOR]

A stable upwind finite-difference method for unsteady gas-liquid two-phase flows is proposed and applied to shock tube flows. The artificial dissipation terms in the flux difference splitting upwinding scheme are derived using a preconditioned matrix to enhance the stability and convergence of the numerical calculation of mixed compressible and incompressible flows with arbitrary void fractions. A homogeneous gas-liquid two-phase flow model is used. A stable four-stage Runge-Kutta method and the flux difference splitting upwind scheme combined with a third-order MUSCL TVD scheme are employed. Using the proposed method, we compute gas-liquid mixture shock tube problems and compare their results with the exact solution to check the reliability of the proposed method. Shock and expansion wave propagations through the gasliquid two-phase media are observed in detail. The effect of the preconditioned artificial dissipation on the numerical stability and convergence rate are investigated. We confirm that the proposed method is stable and effective for computations of unsteady two-phase complex flows with arbitrary Mach numbers. [ABSTRACT FROM AUTHOR]

A model is proposed for liquid film profile prediction in gas-liquid two-phase flow, which is able to provide the film thickness along the circumferential direction and the pressure gradient in the flow direction. A two-fluid model is used to calculate both gas and liquid phases' flow characteristics. The secondary flow occurring in the gas phase is taken into account and a sailing boat mechanism is introduced. Moreover, energy conservation is applied for obtaining the liquid film thickness distribution along the circumference. Liquid film thickness distribution is calculated accordingly for different cases; its values are compared with other models and available experimental data. As a result, the newly proposed model is tested and good performances are demonstrated. The liquid film thickness distribution in small pipes and inclined pipes is also studied, and regime transition is revealed by liquid film profile evolution. The observed inflection point demonstrates that the liquid film thickness decreases steeply along the circumference, when the circle angle ranges between 30° and 50° for gas-liquid stratified flow with small superficial velocities. [ABSTRACT FROM AUTHOR]

This study conducted various explosion experiments under different porous obstacle conditions of blockage ratio (BR) and void fraction (Φ f). The impact of porous obstacle on flame propagation velocity (v) and peak overpressure (P op) were analyzed. The results indicate that an increase in the BR enhances explosion pressure near the blockage area for both porous and non-porous obstacles. However, it also increases the downstream pressure decline gradient. An increase in the Φ f weakens the hindrance of porous obstacle on the flame propagation, allowing more flame to pass through the internal void channels of obstacles, thus delaying the downstream pressure decay. Compared to non-porous obstacles, gas explosions disturbed by porous obstacles with both high BR (BR＞75%) and high Φ f (Φ f ＞0.55) exhibit a lower downstream pressure decline gradient and a longer positive pressure duration. As a result, the explosion can produce a broader range of destruction. • The impact of porous and non-porous obstacles on flames was compared. • The propagation stability of flame gradually reduces with an increasing BR. • Increasing BR results in the forward shift of the maximum P op point. • Increasing V f tends to delay downstream explosion pressure decline. • Porous obstacles can amplify the destructive range of explosion. [ABSTRACT FROM AUTHOR]

Copyright of Lubrication Engineering (0254-0150) is the property of Editorial Office of LUBRICATION ENGINEERING and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Chemical Engineering (China) / Huaxue Gongcheng is the property of Hualu Engineering Science & Technology Co Ltd. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Void drift between subchannels in a rod bundle is a crucial phenomenon affecting the calculation accuracy of thermal-hydraulic parameters in SMRs. It holds significant importance in enhancing the precision of safety analysis for SMRs. Existing research on experiment and model of void drift between rod bundle subchannels is relatively rare, and the accuracy of model calculations requires improvement. In this study, experiments on gas-liquid two-phase non-equilibrium flow were conducted to measure the redistribution of two-phase flow induced by void drift in a 1 × 2 rod bundle. The experiment results indicated that in bubby flow regime with void fraction less than 0.3, the void diffusion coefficient showed little variation with changes in void fraction. However, in slug flow and annular flow regimes with void fraction exceeding 0.3, the void diffusion coefficient significantly increased with an increase in void fraction. Furthermore, a new void drift model was developed and validated based on a subchannel code. The overall predicted uncertainty for the outlet void fraction in the rod bundle benchmark was less than 13%.

A stable upwind finite-difference method for unsteady gas-liquid two-phase flows is proposed and applied to shock tube flows. The artificial dissipation terms in the flux difference splitting upwinding scheme are derived using a preconditioned matrix to enhance the stability and convergence of the numerical calculation of mixed compressible and incompressible flows with arbitrary void fractions. A homogeneous gas-liquid two-phase flow model is used. A stable four-stage Runge-Kutta method and the flux difference splitting upwind scheme combined with a third-order MUSCL TVD scheme are employed. Using the proposed method, we compute gas-liquid mixture shock tube problems and compare their results with the exact solution to check the reliability of the proposed method. Shock and expansion wave propagations through the gas-liquid two-phase media are observed in detail. The effect of the preconditioned artificial dissipation on the numerical stability and convergence rate are investigated. We confirm that the proposed method is stable and effective for computations of unsteady two-phase complex flows with arbitrary Mach numbers.

The presence of gas in transmission fluid can disrupt the flow continuity, induce cavitation, and affect the transmission characteristics of the system. In this work, a gas void fraction model of gas–liquid two-phase flow in a transmission tube is established by taking ISO 4113 test oil, air, and vapor to accurately predict the occurrence, development, and end process of the cavitation zone as well as the transient change in gas void fraction. This model is based on the conservative homogeneous flow model, considering the temperature change caused by transmission fluid compression, and cavitation effects including air cavitation, vapor cavitation, and pseudo-cavitation. In this model, the pressure term is connected by the state equation of the gas–liquid mixture and can be applied to the closed hydrodynamic equations. The results show that in the pseudo-cavitation zone, the air void fraction decreases rapidly with pressure increasing, while in the transition zone from pseudo-cavitation to air cavitation, the air void fraction grows extremely faster and then increases slowly with decreasing pressure. However, in the vapor cavitation zone, the vapor void fraction rises slowly, grows rapidly, and then decreases, which is consistent with the explanation that rarefaction waves induce cavitation and compression waves reduce cavitation. [ABSTRACT FROM AUTHOR]

Air can have an adverse effect on the performance of an aero-engine lubrication system. A numerical analysis was conducted to explore the influence of inlet void fraction and pipe layout on the characteristics of oil-gas two-phase flow in a 90° elbow. The pipes were arranged horizontally and vertically with inlet void fractions of 0.05-0.15. The laws governing flow velocity, void fraction, and pressure along the pipe were determined separately. The results revealed the formation of large-scale vortices with high gas volume fractions inside both types of elbows, which exacerbate oil-gas separation and cause additional head loss. The maximum pressure drop was observed at approximately one pipe diameter downstream of the elbow outlet, which initially increases with the inlet void fraction and then gradually stabilizes. Asymmetric secondary flow vortices in the horizontal elbow were found to enhance oil-gas separation and accelerate lubricating oil to greater extent than in a vertical elbow under the same conditions. Consequently, the maximum pressure drop caused by flowing through the horizontal elbow is higher than that in the vertical elbow. [ABSTRACT FROM AUTHOR]

Conventional conductivity methods for measuring the void fraction in gas-liquid multiphase systems are typically affected by accuracy problems due to the presence of fluid flow and salinity. This study presents a novel approach for determining the void fraction based on a reciprocating dynamic conductivity probe used to measure the liquid film thickness under forced annular-flow conditions. The measurement system comprises a cyclone, a conductivity probe, a probe reciprocating device, and a data acquisition and processing system. This method ensures that the flow pattern is adjusted to a forced annular flow, thereby minimizing the influence of complex and variable gas-liquid flow patterns on the measurement results; Moreover, it determines the liquid film thickness solely according to circuit connectivity rather than specific conductivity values, thereby mitigating the impact of salinity. The reliability of the measurement system is demonstrated through laboratory experiments. The experimental results indicate that, in a range of gas phase superficial velocities 5–20 m/s and liquid phase superficial velocities 0.079–0.48 m/s, the maximum measurement deviation for the void fraction is 4.23%. [ABSTRACT FROM AUTHOR]

Two phase flow commonly occur in nature and in designed systems. The recent micro reaction-based technologies employed in laboratory on chip systems, micro propulsion devices, drug delivery systems require multi-phase fluid mixing in confined spaces of the order of micron size. Mixing index of the highest order is expected to be achieved in small channels. Limited experimental studies are available for real time measurement of void fraction and liquid film thickness which play a vital role during mixing of multiphase flow in mini/micro channels. In the present work an attempt has been made to reconstruct a two-dimensional Taylor bubble in real time. The cross-sectional void and liquid film thickness are deduced from the signal generated using a LASER-Photo resistor and the online reconstruction includes measurement of the interface velocity. A numerical model is developed to understand interaction of signals with two phase regimes and to estimate average liquid film thickness based on the experimental current signal. [ABSTRACT FROM AUTHOR]

[Display omitted] • The void fraction (41.31%) formed by large-particle size gravel was rapidly reduced. • High rate of gas flow in the pipeline made landfill gas diluted rapidly. • A larger void fraction (44.62%) led to formation of a smaller anaerobic zone. The main disposal method for municipal solid waste (MSW), including the growing worldwide volumes of kitchen waste, involves transport to landfills. Because kitchen waste is mainly composed of organic matter and has a high moisture content, large amounts of leachate and landfill gas are generated when it is sent to landfills. Therefore, rapid waste stabilization is essential. In this study, four semi-aerobic bioreactors (named NS, SS, MS, and LS) were established with void fractions of 33.76%, 39.84%, 44.62%, and 41.31%, respectively. The results showed that the void fractions of landfill directly affected the gas flow path. When the landfill void fraction was small (e.g., NS), most airflow traveled directly through the pipeline and minimal airflow entered the waste layer. When the landfill void fraction was large (e.g., MS), air easily entered the waste layer and some air flowed into the gas vent with the landfill gas. As the reaction proceeded, the void fraction gradually decreased due to gravity-induced sedimentation. During the water addition experiment, the voids were occupied by water, leading to formation of an anaerobic area. Among the four bioreactors, only MS had negligible formation of an anaerobic zone in the center. Methane (CH 4) generation was detected only at the connection between the gas vent and the leachate collection pipe. A larger void fraction led to formation of a smaller anaerobic zone. The ratio of air flowing in pipeline was lowest in MS. These results indicated that a large void fraction promotes the decomposition of organic matter. [ABSTRACT FROM AUTHOR]

The flow characteristics of void fraction and specific surface area are important to predict the mass transfer and reaction performances in gas–liquid microreactors. However, the models and trends for the two parameters in different flow patterns remain ununified and insufficient. Accordingly, this work first time studies the void fraction and specific surface area in a very wide flow pattern range (Taylor, short bubble, bubbly, and bubble swarm flow). The results show that at the same gas–liquid flow rates, the void fraction decreases with bubble size in Taylor and short bubble flow but is totally opposite in bubbly and bubble swarm flow. Importantly, the variation of specific surface area with bubble size is independent of the flow pattern and decreases with bubble size. Finally, based on bubble size, two general models for the two parameters are developed. This work provides a new direction for the unification of flow characteristics. [ABSTRACT FROM AUTHOR]

Air can have an adverse effect on the performance of an aero-engine lubrication system. A numerical analysis was conducted to explore the influence of inlet void fraction and pipe layout on the characteristics of oil-gas two-phase flow in a 90° elbow. The pipes were arranged horizontally and vertically with inlet void fractions of 0.05-0.15. The laws governing flow velocity, void fraction, and pressure along the pipe were determined separately. The results revealed the formation of large-scale vortices with high gas volume fractions inside both types of elbows, which exacerbate oil-gas separation and cause additional head loss. The maximum pressure drop was observed at approximately one pipe diameter downstream of the elbow outlet, which initially increases with the inlet void fraction and then gradually stabilizes. Asymmetric secondary flow vortices in the horizontal elbow were found to enhance oil-gas separation and accelerate lubricating oil to greater extent than in a vertical elbow under the same conditions. Consequently, the maximum pressure drop caused by flowing through the horizontal elbow is higher than that in the vertical elbow.

Subcooled flow boiling contains complex bubble behaviors and interactions. Accurate prediction and analysis of the subcooled flow boiling are of great significance for the safe operation and economy of the nuclear reactors. The bubble-layer-based model is a new theoretical model in predicting the flow boiling, which divides the flow filed into two regions in the radial direction, which are the bubble layer region and the core region. Bubble behaviors in each region, mass and energy exchanges between different regions, and variations of parameters in the axial direction are analyzed and reflected through a set of two-dimensional steady-state conservation equations of mass, momentum and energy. Therefore, this model can provide more detailed information of the flow field than current one-dimensional models. However, this bubble-layer-based model for the subcooled flow boiling has only been verified in conventional tubes. When the channel type and sizes change, the accuracy of the model is still uncertain. Considering that rectangular channels are widely encountered in multiphase flow equipment, this paper aims to further extend this model to the prediction of the subcooled flow boiling in rectangular channels. Firstly, several flow boiling experiments in both conventional rectangular channels and narrow rectangular channels were selected. The working conditions are 0.1-8 MPa for pressure, 211-1 700 kW/m2 for heat flux, 240-2 200 kg/(m2·s) for mass flux, 4.42×103-3.72×105 for liquid Reynold number and 0.83-1.76 for Prandtl number. Then, variations of void fraction obtained by this model were compared with these experimental data. The results show that this bubble-layer-based model shows high accuracy when it is applied to both conventional rectangular channels and narrow rectangular channels, which reflects that the channel size and shapes have limited impact on the accuracy of this model. In addition, to further verify the accuracy of this model, one of the existing models (Cai model) in predicting the void fraction in subcooled flow boiling was also selected for comparison. The results show that the bubble-layer-based model shows higher accuracy than Cai model, with the relative error of 32.7% and 52.6% respectively. Finally, this model was applied to the analysis of the flow boiling in one narrow rectangular channel. It describes the variations of several two-phase flow parameters, such as the void fraction, pressure, velocity and temperature. These results not only provide a better understanding of the mechanism of subcooled flow boiling, but also provide basic data for the thermal physical coupling analysis of the rectangular fuel elements in nuclear reactors.

The steam generator tube rupture (SGTR) accident is one of the design basis accidents of lead-cooled fast reactor (LFR). In the event of such an accident, a subcooled water jet is generated in the lead pool of primary circuit, which forms a complicated distribution of the liquid metal-gas two-phase flow. The detection of the two-phase interface and migration of steam bubbles is an significant part of reactor safety analysis. Due to the opacity, corrosiveness, high temperature of liquid metal, and the current limitations of measurement methods involving liquid metal-gas two-phase flow, a diamond-like carbon (DLC) coated wire-mesh sensor (WMS) based on electrical principles was proposed in this paper. The sensor could achieve the image of the liquid metal-gas two-phase distribution inside the flow channel, while preventing the corrosion failure of the sensor. Based on the COMSOL software, a numerical model of the DLC coated WMS was established. Selecting the materials of two phases as liquid GaInSn alloy and air, the simulation of the electric field in the liquid metal environment containing a single bubble was performed, and an electric signal value matrix representing the liquid metal-gas two-phase was output. The signal matrix was further normalized through the linear relation to obtain the two-phase distribution image, which verified the application feasibility of the coated WMS in liquid metal. In addition, a prototype of the DLC coated WMS was manufactured, and the numerical simulation results were experimentally validated in order to demonstrate the rationality of the numerical model. Furthermore, the influence of different structural parameters on the measurement accuracy of coated WMS was studied in detail by changing the transverse interval, axial interval, electrode diameter, coat thickness, and the size of air bubble. The simulation results show that the electric field distribution in the liquid metal-gas two-phase environment differs significantly from that in conventional fluid medias such as water-gas due to the high relative permittivity of liquid metal. On the other hand, considering the resolution of the WMS and the phenomenon of electric signal crosstalk, the transverse interval of electrode should be selected between 2-3 mm, and the axial interval should be selected between 1.5-2 mm to achieve high measurement accuracy. The coat thickness and electrode diameter have little influence on the measurement accuracy, but to reduce the intrusion effect of the WMS, the electrode diameter should be selected between 0.4-0.5 mm and the coat thickness should not exceed half of the electrode radius. This study could guide the practical application of DLC coated WMS in the liquid metal-gas two-phase environment and provide a technical basis for the analysis of multi-phase distribution under SGTR accidents.

To address the issue of horizontal well production affected by the distribution of perforation density in the wellbore, a numerical model for simulating two-phase flow in a horizontal well is established under two perforation density distribution conditions (i.e. increasing the perforation density at inlet and outlet sections respectively). The simulation results are compared with experimental results to verify the reliability of the numerical simulation method. The behaviors of the total pressure drop, superficial velocity of air-water two-phase flow, void fraction, liquid film thickness, air production and liquid production that occur with various flow patterns are investigated under two perforation density distribution conditions based on the numerical model. The total pressure drop, superficial velocity of the mixture and void fraction increase with the air flow rate when the water flow rate is constant. The liquid film thickness decreases when the air flow rate increases. The liquid and air productions increase when the perforation density increases at the inlet section compared with increasing the perforation density at the outlet section of the perforated horizontal wellbore. It is noted that the air production increases with the air flow rate. Liquid production increases with the bubble flow and begins to decrease at the transition point of the slug-stratified flow, then increases through the stratified wave flow. The normalized liquid flux is higher when the perforation density increases at the inlet section, and increases with the radial air flow rate. [ABSTRACT FROM AUTHOR]

Copyright of Atomic Energy Science & Technology is the property of Editorial Board of Atomic Energy Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Void fraction is one of the dominant parameters of gas-water two-phase flow. Its accurate measurement plays an important role in achieving parameter control and reliable operation in industrial processes. This article proposes a more practical method for the measurement of void fraction in gas-water bubbly flow using a derived multi-eigenvalue sequence from a normalized electrical impedance tomography impedance matrix. The relations between eigenvalues and void fraction, bubble radius, number of bubbles are investigated by numerical simulations, which illustrates the superiority of using multi-eigenvalue rather than the largest eigenvalue for void fraction prediction. The nonlinear mapping between the multi-eigenvalue sequence and void fraction is established by applying the XGBoost model with a sliding window of time series. This proposed method is verified by static and dynamic experiments using a self-developed setup in our laboratory, generating stable gas-water bubbly flow with void fraction of less than 0.12. It is shown that the proposed method can predict void fraction with a relative deviation of 10%. Compared with the conventional method based on the largest eigenvalue, the proposed method efficiently improves the measurement accuracy of void fraction in gas-water bubbly flow and applicability in actual measurement of two-phase flow, which can be further extended to other flow regimes. [ABSTRACT FROM AUTHOR]

In this study, a novel three-dimensional DEM-CFD numerical method was used to simulate the void fraction, pressure drop, velocity, and local flow in the packed bed under the small column-to-particle-diameter (D/d) ratios. The pseudo-continuum model method is used in the interior of particles, and for the simulation of the fluid outside the particles, the models are established based on DEM to avoid the wall effect. The simulation results of void fraction and pressure drop show that the void fractions generated by the DEM method are consistent with the ones calculated from the empirical correlations; and the empirical correlations used for pressure drop in this work are basically consistent with the results of the pressure drop predicted by DEM-CFD method. The simulation results of velocity and local flow show that velocity distribution is related to the void fraction distribution; the radial velocity distribution depends mainly on the structure of the packed bed and is independent of the inlet velocity; and the velocity uniformities of fluid flow are improved with the increasing of D/d ratio. [ABSTRACT FROM AUTHOR]

Copyright of Coal Geology & Exploration is the property of Xian Research Institute of China Coal Research Institute and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper analyzes the developing adiabatic two-phase flow after an expansion valve in an air conditioning system. R245fa and POE 32 mixture is used as the working fluid. The developing two-phase flow patterns are visualized in an 8 mm straight tube after the expansion valve from 15 to 595 mm. To explore the effect of fluid properties on the developing two-phase flow, the experimental results from previous research by the same authors are compared with those of R245fa in this paper. An image analysis technique and a one-dimensional developing two-phase flow model proposed in a previous paper by the same authors (Yao & Hrnjak, 2023c) are applied to both R134a and R245fa. The flow patterns near the expansion valve exhibit some similarities when comparing the two refrigerants. However, a significant contrast emerges in the presence of a greater number of droplets suspended within the vapor phase in the case of R245fa flow. This disparity is primarily attributed to R245fa's higher vapor velocity. Furthermore, R245fa exhibits a shorter developing length than R134a. For fully developed flow, R245fa showcases a more pronounced turbulent behavior, and the transition to annular flow happens at a lower vapor quality in comparison to R134a. All of these distinctions can be adequately accounted for by the proposed model. The developed flow patterns of both refrigerants are compared with some existing flow pattern maps. At last, a new flow pattern map is proposed based on the observed flow patterns in this research. [ABSTRACT FROM AUTHOR]

Copyright of Atomic Energy Science & Technology is the property of Editorial Board of Atomic Energy Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Machine Tool & Hydraulics is the property of Guangzhou Mechanical Engineering Research Institute (GMERI) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Flow boiling in the rod bundle channel is an important phenomenon in nuclear reactors. The local inhomogeneous void fraction distribution in the flow channel will affect the flow and heat transfer in two phase flow. A 5 × 5 rod bundle test section is designed and utilized in the experiments and a conductivity probe is utilized to measure the local void fraction of two phase flow. A remote-control stepping driving assembly is designed to drive the void probe. The local void fraction characteristics at the outlet of a 5 × 5 rod bundle channel during subcooled boiling and saturated boiling are experimentally studied when the system pressure is 2∼3 MPa. The outlet subcooling ranges from 0 °C to 18 °C. The mass flux is 2000 kg/m2s and the heat flux ranges from 186.3 kW/m2 to 320.8 kW/m2.The effects of thermal parameters on local phase interfacial parameters in subcooled boiling and saturated boiling are obtained. The research results are important for revealing the two phase flow mechanisms in rod bundle channels.

Ciliary movement holds substantial importance in the realm of biomedical research. Disorders arising from impaired ciliary motion, such as primary ciliary dyskinesia and specific respiratory ailments, underscore the critical need to investigate both the normal functioning and dysfunction of cilia. Such research is fundamental to the development of effective treatments and therapies. This work explores the application of Levenberg–Marquardt backpropagation neural networks (LMBP-NNs) for simulating the flow of a non-Newtonian Jeffery fluid in peristaltic motion. Inspired by synchronous ciliary movement, microfluidic systems mimicking cilia functionality have been developed. These artificial cilia can generate controlled fluid flow patterns, enabling various applications such as drug delivery, mixing, and particle sorting in lab-on-a-chip devices. The present study focuses on measuring thermal and mass transfer in beating cilia during multiphase peristaltic pumping of Jeffery fluid in two- and three-dimensional channels using transverse magnetohydrodynamics in a porous medium. The governing equations for the non-Newtonian Jeffery fluid in both the fluid and dusty phases are formulated, and exact solutions are calculated for fluid phase velocity, particulate phase velocity, temperature, concentration, and pressure gradient. The physical behavior is investigated by graphing dimensionless parameters for velocity, temperature, and concentration profiles in both solid and liquid phases. The trapping phenomena is depicted by showing streamlines of various dimensionless characteristics such as porosity velocity and volume percentage. The LMBP-NNs approach is then used to train, test, and verify the neural network models using the reference datasets. The accuracy of the LMBP-NN is assessed using statistical data such as mean square error, curve fitting graphs, regression plots, and error histograms. The study also investigates flow model limits for momentum, temperature, and concentration profiles using visual representations. This study illustrates the computational capability of LMBP-NNs in modeling the peristaltic flow of Jeffery fluid and gives useful insights into fluid flow behavior in microfluidic devices containing artificial cilia. The findings help to better understand and optimize these systems for a variety of fluid manipulation and transportation applications.

Multi-scale voids including fractures and bed separations will appear and dynamically develop as the underground mining-induced disturbance increases. The void fraction, which is an index for characterizing the void scale, is an important parameter for understanding the heat and mass transfer (including methane flow, coal spontaneous combustion, underground water flow) in the fractured overburden disturbed by underground mining of coal seam. The stochastic caving tests were conducted to simulate the roof caving and overburden movement induced by coal seam mining with different inclination angle and excavation length. The expectations and variances of random variables about the overall and local void fractions showed that the void fractions gradually increase with increases in excavation lengths and obey a normal distribution. The distribution fields of the mean and stochastic void fractions indicated that the rich voids present an arch-type distribution in the disturbed overburden. The random distribution field of void fractions obtained from stochastic inversion presents a small range of fluctuation around the mean values of void fractions and is more realistic to reflect the randomness of separation and caving of rock strata induced by underground mining of coal seam. [ABSTRACT FROM AUTHOR]

Packed bed reactors play a crucial role in various industrial applications. This paper utilizes the Discrete Element Method (DEM), an efficient numerical technique for simulating the behavior of packed beds of particles as discrete phases. The focus is on generating densely packed particle beds. To ensure the model accuracy, specific DEM parameters were studied, including sub-step and rolling resistance. The analysis of the packed bed model extended to a detailed exploration of void fraction distribution along radial and vertical directions, considering the impact of wall interactions. Three different samples, spanning particle sizes from 0.3 mm to 6 mm, were used. Results indicated that the number of sub-steps significantly influences void fraction precision, a key criterion for comparing simulations with experimental results. Additionally, the study found that both loosely and densely packed beds of particles could be accurately represented by incorporating appropriate values for rolling friction. This value serves as an indicator of both inter-particle friction and friction between particles and the walls. An optimal rolling friction coefficient has been thereby suggested for the precise representation for the densely packed bed of spherical char particles. [ABSTRACT FROM AUTHOR]

In severe accidents in light-water reactors, the core melt, which falls to the bottom of the containment vessel, is expected to be cooled in the retained water. The particulate debris is generated after the molten material falls from the pressure vessel. For example, the cooling characteristics remain to be further elucidated in a system where particulate debris and structure material coexist and the gas-liquid two-phase flow inside particulate debris. This study investigated air–water two-phase flow inside a particulate bed and near a structure wall using a high-speed camera with a refractive index matching the transparent bed with purified water and a wire-mesh sensor (WMS). Particles with diameters of ø 3, 5, and 10 mm were used and subjected to air–water two-phase flow tests with a superficial gas velocity of 4.0–2.0×103 mm/s and superficial liquid velocity of 0.5–75.3 mm/s. Visualization results of bubble behavior near the wall and inside the bed showed the advection of bubbles from within the bed to the vicinity of the structure wall. This finding explains the higher void fraction measured in the vicinity of the structure wall using the WMS. Analysis of the void fraction distribution based on the theoretical models indicated that the higher void fraction near the structure wall observed in the case with larger particle size may be attributed to the slip velocity between the gas and liquid phases in the particulate bed. The developed techniques and insights gained in this study contribute to a detailed understanding of the thermal-hydraulic phenomena that accompany two-phase flows in particulate debris.

The primary focus of this research is to create a precise, yet straightforward drift-flux model tailored for water-gas flow in variously angled pipes. This model is pivotal in estimating the volumetric flow rate of each phase in two-phase pipe flow scenarios. The proposed correlation is developed using twenty-two different (4367 data points) sources with a wide range of input data with different flow patterns. The drift-flux model created in this study underwent validation through the use of experimental data previously documented in the literature. Results indicate this model's stability and practicality, achieving 94.7 % accuracy within a margin of error of ±20 % for the chosen datasets. Furthermore, when compared against several drift-flux models, this newly proposed model consistently outperformed them across a liquid holdup range of 0.01–1. A comparative analysis with the state-of-the-art Unified Model underscores the accuracy of the proposed drift-flux model, highlighting its performance across diverse inclination angles and flow regimes. The proposed model demonstrates excellent predictive accuracy, particularly in vertical flow scenarios. Extending the scope of the study, further research could focus on the application of the developed drift-flux model to other fluid combinations and flow conditions, as well as its integration with mechanistic modeling and computational fluid dynamics simulations to provide more comprehensive insights into the behavior of two-phase flow in pipes with different inclination angles, enhancing the model predictive ability and operational range.

Nitrogen pressurization is widely used in nuclear reactor (such as passive nitrogen-type accumulators and nitrogen regulator) because of passive mechanism. However, nitrogen pressurization brings new challenges. Loss of coolant accident (LOCA) can lead to rapid reduction of pressure in primary circuit of pressurized water reactor (PWR), putting dissolved nitrogen in the water into a supersaturated state, which contributes to nitrogen release. In order to prevent the accumulation of released nitrogen, which may act as a threat to reactor safety, the migration and release characteristics of nitrogen were studied. In this paper, a nitrogen wall release model was established, the physical significance of the release source term was explained and the specific process of nitrogen release was revealed. In the case of LOCA in primary circuit of a PWR, the pressure in the primary circuit decreases and the susaturated dissolved nitrogen degas from the liquid and form nitrogen bubbles on nucleate sites. The susaturated dissolved nitrogen continuously degas at the gas-liquid interface of the bubble, leading to the sustained growth of nitrogen bubble. After reaching departure diameter, the buoyancy force on the nitrogen bubble is greater than surface tension and it escapes from the wall finally. In this paper, the mathematical modeling of nitrogen dissolution equilibrium, dissolved nitrogen convection and diffusion, the susaturated dissolved nitrogen wall release and nitrogen bubbles upward floating was carried out. Combined with the one-dimensional lumped parameter method, the mass conservation of the control volume was analyzed and the one-dimensional nitrogen mass conservation equation was deduced. To solve the one-dimensional nitrogen mass conservation equation, a one-dimensional lumped parameter nitrogen migration solution program was written based on MATLAB. The program was verified by the experimental data of small break loss of coolant accident (SBLOCA). The trends of dissolved nitrogen concentration and void fraction under pressure relief condition during SBLOCA were simulated. The results suggest that susaturated nitrogen degas on the wall in the form of bubbles. The bubble grows at a rate proportional to t1/2 and also proportional to (C-Ci)12. The influence of the initial nucleation radius on the growth period of the bubble is negligible and the bubble rises uniformly in the main stream after departure. Under pressure relief condition during LOCA, the nitrogen wall release effect is significant and the diffusion process of dissolved nitrogen is weak. Controlled by nitrogen wall release, the average concentration of dissolved nitrogen decreased by 19.5% and the void fraction became 4.33 times of the original, which indicates that wall release has significant effect on circuit, potentially affecting reactor safety. The simulation results are in good agreement with experimental data of SBLOCA, proving the reliability of wall release model and program.

The presence of gas in transmission fluid can disrupt the flow continuity, induce cavitation, and affect the transmission characteristics of the system. In this work, a gas void fraction model of gas–liquid two-phase flow in a transmission tube is established by taking ISO 4113 test oil, air, and vapor to accurately predict the occurrence, development, and end process of the cavitation zone as well as the transient change in gas void fraction. This model is based on the conservative homogeneous flow model, considering the temperature change caused by transmission fluid compression, and cavitation effects including air cavitation, vapor cavitation, and pseudo-cavitation. In this model, the pressure term is connected by the state equation of the gas–liquid mixture and can be applied to the closed hydrodynamic equations. The results show that in the pseudo-cavitation zone, the air void fraction decreases rapidly with pressure increasing, while in the transition zone from pseudo-cavitation to air cavitation, the air void fraction grows extremely faster and then increases slowly with decreasing pressure. However, in the vapor cavitation zone, the vapor void fraction rises slowly, grows rapidly, and then decreases, which is consistent with the explanation that rarefaction waves induce cavitation and compression waves reduce cavitation.

Drift Flux model is widely used in literature to predict void fraction in two-phase gas-liquid flow. Drift flux model has been used for all flow regimes. The distribution parameter implemented in the model is very crucial for the accuracy of the model. A new distribution parameter was developed in this paper as a function of two dimensionless parameters and flow regime (slug or plug). The new model showed a superior predicted void fraction accuracy over all available models in literature. In this paper, the influence of the flow regimes was implemented in the formulation of the drift flux model distribution parameter for the first time in literature. The drift velocity was found to be negligible in the horizontal configuration. The proposed model was validated using unbiased data from literature from different sources and for a wide range of liquid viscosity from water up to high viscosity oil (600 cP) and pipe diameter from 19 mm up to 152 mm. The mean relative absolute error of the proposed model using all data bank is around 16% while the least error model available in literature is around 19%. Moreover, the most recent models of Rassame and Hibiki (2018) and Kong et al. (2018b) give 33% and 50%, respectively.

In an open-pool type research reactor with a downward forced flow in the core, pipes can be under sub-atmospheric pressure because of the large pressure drop at the reactor core in the atmospheric pool. Sub-atmospheric pressure can result in air inflow into the pipe from the pressure difference between the atmosphere and the inside of the pipe, which in a postulated pipe break scenario can lead to the breakdown of the cooling pump. In this study, a plant-scale experiment was conducted to study air inflow in large piping systems by considering the actual operational conditions of an advanced research reactor. The air inflow rate was measured, and the entrained air was visualized to investigate the behavior of air inflow and flow regime depending on the pipe break size. In addition, the developed drift-flux model for a large vertical pipe with a diameter of 600 mm was compared with other correlations. The flow regime transition in a large vertical pipe under downward flow was also studied using the newly developed drift-flux model. Consequently, the characteristics of two-phase flow in a large vertical pipe were found to differ from those in small vertical pipes where liquid recirculation was not dominant.

This paper applies an image analysis technique to quantify the two-phase flow videos after an expansion valve. This method first finds the liquid-vapor interface and then estimates the void fraction and velocity of each phase. This paper also proposes a one-dimensional developing two-phase flow model based on the force-momentum balance. The void fraction of the developing flow predicted by the model shows a similar trend to the quantified experiment data from image analysis: void fraction will decrease and then stabilize as flow develops along the tube. In essence, the change in void fraction and flow velocity is a result of unbalanced force in liquid and vapor phase. This model is also capable of estimating the distance to achieve fully developed flow after an expansion valve. The ability to predict two-phase flow behavior is not only important to the theoretical study of two-phase flow, but also critical to two-phase flow system design to optimize heat transfer and pressure drop. [ABSTRACT FROM AUTHOR]

Large vertical pipes are key structures connecting subsea wells to offshore platforms. However, existing studies mainly focus on small vertical pipes. In a vertical acrylic pipe with 80 mm inner diameter and 11 m height, a high-speed camera was used to visually research the influences of pipe diameters, liquid properties and inlet effect on air-water co-flow characteristic. Different flow regime maps of vertical pipes (diameters are in the range of 50–189 mm) were compared and the critical gas velocity of the transition boundary from bubble to slug flow tended to increase with the increase of diameters at D ≥ 80 mm. Drift-flux models were established in different flow regimes and liquid properties have a significant effect on drift coefficients of bubble flow and slug flow (void fraction α ≤ 0.4). The influence of inlet turbulent effect on the gas-liquid interface distribution gradually weakened and disappeared from the pipe base to 85D, where the flow was fully developed. Slug frequency has a trend of increase first and then decrease with the gas Weber numbers increasing at low liquid superficial velocities (JL ≤ 0.31 m/s). And on the basis of this law, a new slug frequency correlation was proposed. It was found that there was an exponential relationship between the ratio of lengths of Taylor bubble to slug and the void fraction. [ABSTRACT FROM AUTHOR]

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The wall friction term in a one-dimensional model vanishes under the stagnant liquid conditions (i.e. the liquid volume flux JL = 0). Hence void fraction αG(ΔP) in rod bundles have been evaluated using measured pressure gradients dP/dz. However, effects of the wall friction term on αG(ΔP) have not been quantitatively evaluated. In this study, we therefore carried out air-water experiments using a 3x3 rod bundle under the conditions of JL = 0, and measured dP/dz and αG,exp with the closing valve method. We obtained the dimensionless wall friction force Fw* (= αG(ΔP)-αG,exp) from the measured dP/dz and αG,exp. The Fw* values were expressed by using the gas volume flux JG, because the liquid flow field was induced by the gas flow (i.e. JG). The difference between αG,exp and αG(ΔP) was reduced by half after αG(ΔP) was corrected using the obtained correlation for Fw*. The Fw* correlation was applied to existing αG(ΔP) data for 4x4 and 8x8 bundles, and effects of Fw* on the αG(ΔP) values were confirmed to be small in a large rod bundle due to the small wall area per unit fluid volume. [ABSTRACT FROM AUTHOR]

In cooling towers and condensers, accurate measurement of the void fraction in gas-liquid two-phase flow can help optimize heat transfer efficiency. By monitoring the void fraction, operational parameters can be adjusted to maximize heat exchange effectiveness. As such, the development of a technique for measuring void fraction in high-temperature and opaque containers is necessary. This paper presents a method for measuring the void fraction of the section during direct contact heat transfer using electromagnetic tomography. In order to enhance the magnetic field, a certain amount of nanofluid is introduced into the direct contact evaporator. With the aid of a self-developed EMT system, the gas and liquid phases are differentiated, ultimately yielding the section void fraction value. The final results are qualitatively consistent with three classical models for void fraction calculation. This is the first report on measuring cross-sectional void fraction in direct contact heat transfer processes using the EMT system. The findings demonstrate the feasibility of the measurement method based on EMT and provide a reference for research on high-precision and automated measurement techniques of void fraction in gas-liquid two-phase flow. [Display omitted] • The EMT technique is suitable for flow pattern detection of direct contact heat transfer processes. • The void fraction rate of EMT data has the highest correlation with the drift flow model trend. • The void fraction measured by EMT method has a consistent trend with the results calculated by three classic models. [ABSTRACT FROM AUTHOR]

The two-dimensional instability of a horizontal layer of boiling liquid with a finite height was experimentally studied. In this layer, "vapor columns" rose at the corners of a square rectangular grid, and the symmetry of "vapor column" location on the heating surface was considered. The model adopts an approach to the boiling crisis from the side of both developed nucleate boiling and transitional boiling (the Zuber problem). When dealing with developed nucleate boiling, the layer of boiling liquid is considered in calculations as an isotropic homogeneous system (foam). It is shown how the conditions on the heating surface (capillary-porous coating) affect the external hydrodynamics of the liquid layer and, ultimately, the value of the critical heat flux. The calculation ratio obtained by approaching the boiling crisis from the side of developed nucleate boiling takes into account the dependence of the critical heat flux on the void fraction of the boiling liquid layer. A new solution to the boiling crisis problem is proposed when approaching the crisis from the side of transitional boiling (the Zuber problem). This new solution eliminates some shortcomings of the classical problem (in particular, the void fraction of the layer corresponds to the experiments). [ABSTRACT FROM AUTHOR]

As a special flow state of the downer, the downer moving bed has been applied in many fields. With the rapid development of the downer moving bed, experimental studies on the flow and heat transfer characteristics in the bed are urgently needed to promote the further development of it. Therefore, a complete set of downer moving bed experimental equipment is completed in this paper. Key parameters of flow characteristics such as void fraction and particle flow velocity are indirectly calculated through direct measurement of pressure and other data. The experimental results show that the factors affecting the solid circulation flow rate include bed inventory and inlet air velocity. Due to the high packing density of the particles, the void fraction of the downer moving bed has a small relative change. The particle flow velocity increases significantly with the increase of the inlet air velocity. Under the joint effect of particle velocity and void fraction, the variation of heat transfer coefficient along the height of the bed presents a single peak curve. And the heat transfer performance of the downer moving bed is obviously better than that of the downer fluidized bed at the same solid circulating flow rate. The differences between different tube bank arrangements and the single tube are compared. It is concluded that in practical applications, the requirements for heat transfer and heat transfer coefficient should be considered together and the best tube bank arrangement should be selected. [ABSTRACT FROM AUTHOR]

• We analyzed the non-uniform flow in the cross-corrugated passage of the shell-and-plate heat exchanger. • We summarize three typical gas and liquid flow paths under different working conditions. • We simulated three flow patterns and established the relationship between mean void fraction and flow patterns. • We proposed a correlation of gas-liquid two-phase flow in shell-and-plate heat exchanger. The flow path of the fluid in the passage of the shell-and-plate heat exchanger shows the characteristics of first dispersing and then gathering, which significantly affects the efficiency of the heat exchanger. Compared with the traditional rectangular plate heat exchanger (PHE), an in-depth understanding of the flow characteristics in SPHE is insufficient. The properties of the flows between the adjacent plates were numerically studied in the present study. The single-phase fluid passes rapidly between the staggered contact points, and there is noticeable pressure fluctuation at the entrance and exit. In addition, the flow diffusion and convergence cause noticeable differences in local velocity and pressure variation. For two-phase flows, three flow patterns are defined, and a subregional discussion on the flow pattern in the passage and their pressure gradient and void fraction are also provided. The void fraction is also introduced to determine the flow patterns, i.e., the dominant flow pattern is bubbly flow when α < 0.4 , 0.4 ≤ α ≤ 0.7 is for slug flow, and the primary type is film flow when α > 0.7. By analyzing the pressure drop, an empirical equation of pressure drop prediction is proposed. [ABSTRACT FROM AUTHOR]