•A new void fraction correlation for a wide range of gas–liquid two phase flow.•Proposed correlation is based on drift flux model and is flow pattern independent.•Drift flux parameters are modeled as ...a function of several two phase flow variables.•Proposed correlation verified against a comprehensive data bank of 8255 data points.•Proposed correlation gives better accuracy compared to other correlations.
The main objective of this study is to present new equations for a flow pattern independent drift flux model based void fraction correlation applicable to gas–liquid two phase flow covering a wide range of fluid combinations and pipe diameters. Two separate sets of equations are proposed for drift flux model parameters namely, the distribution parameter (Co) and the drift velocity (Ugm). These equations for Co and Ugm are defined as a function of several two phase flow variables and are shown to be in agreement with the two phase flow physics. The underlying data base used for the performance verification of the proposed correlation consists of experimentally measured 8255 data points collected from more than 60 sources that consists of air–water, argon–water, natural gas–water, air–kerosene, air–glycerin, argon–acetone, argon–ethanol, argon–alcohol, refrigerants (R11, R12, R22, R134a, R114, R410A, R290 and R1234yf), steam–water and air–oil fluid combinations. It is shown that the proposed correlation successfully predicts the void fraction with desired accuracy for hydraulic pipe diameters in a range of 0.5–305mm (circular, annular and rectangular pipe geometries), pipe orientations in a range of -90°⩽θ⩽90°, liquid viscosity in a range of 0.0001–0.6Pa-s, system pressure in a range of 0.1–18.1MPa and two phase Reynolds number in a range of 10 to 5×106. Moreover, the accuracy of the proposed correlation is also compared with some of the existing top performing correlations based on drift flux and separated flow models. Based on this comparison, it is found that the proposed correlation consistently gives better performance over the entire range of the void fraction (0<α<1) and is recommended to predict void fraction without any reference to flow regime maps.
•CFD-DEM formulation, including heat and mass transfer and long-range forces is described.•Implementation of CFD-DEM in simulation of different processes is discussed.•Different applications, ...including drying, coating, mixing combustion, gasification and etc. are discussed.
With increasing the computational resources, the number of publications about coupled computational fluid dynamics – discrete element method is in the rise in the recent years. This technique is very useful, especially in simulation of fluid-solid flows in process engineering. This paper provides an introduction to CFD-DEM modeling in process engineering systems, including heat and mass transfer and long range forces, and reviews the major researches in simulation of two-phase processes such as drying, coating, granulation, crystallization, chemical reactions (including combustion, gasification and pyrolysis) and mixing. Details of implementing unresolved CFD-DEM in these applications are explained in details and major assumptions and findings are discussed.
•The number of small bubbles in liquid plug/slug increases significantly with increasing jg.•The size of small bubbles decreases with increasing jg or increasing jf.•Increasing jg or L/D, or ...decreasing jf slightly increases the depth of the plug/slug bubble.•Increasing pipe size increases the contribution from large bubbles to total void fraction.•The large bubbles accelerate due to small bubble coalescence as flow develops, leading to α decreases.
The current work investigates the plug-to-slug transition in horizontal air–water two-phase flow in small (38.1 mm) and large (101.6 mm) diameter pipes. An extensive database is established to study the local interfacial structure in plug-to-slug transition flow. Detailed measurements across the flow area are performed for nine and six test conditions in small and large pipes, respectively, at three different axial locations downstream of the inlet using the local four-sensor conductivity probe. The effects of jg, jf, development length and pipe size are investigated. It is found that the number of small bubbles in the liquid plug/slug increases significantly in plug-to-slug transition with increasing jg, which are generated by the strong shear between the gas slug and liquid film. Due to the relative motion, these small bubbles either coalesce with the nose of the following plug/slug bubble, or slide between the plug/slug bubble and the wall, and then travel around the pipe circumference to reside beneath the large bubbles. This explains the large number of small bubbles observed at the top of the liquid film for the conditions at high gas flow rates. In the process of traveling downwards, some of the small bubbles coalesce with the slug bubbles. It is also found that increasing jg or jf decreases the size of the small bubbles. While shearing-off is believed to dominate as jg increases, turbulent-impact is enhanced as jf increases due to the increasing turbulence level in the liquid phase. Increasing jg, development length, or decreasing jf slightly increases the depths of the plug/slug bubbles; however, significant growth of plug/slug bubbles is observed in the axial direction. For the same condition, the contribution from large bubbles to total void fraction increases as pipe size increases, while the distribution of total void fraction is similar. The size of both small and large bubbles is found to be larger in the large diameter pipe. Due to the current bubble injection mechanism, small bubbles are generated at the inlet; they coalesce into large bubbles as the flow develops. The large bubble is found to accelerate as it grows along the axial direction, which can lead to a decreasing void fraction although pressure keeps decreasing.
•This provides a systematic, comprehensive review of two-phase instabilities.•Relevance to both macro- and micro-channels is carefully addressed.•Significant mechanistic differences among ...instabilities are addressed.•Confusion among studies over dominant instabilities is assessed.
Study of two-phase flow instabilities began in the late 1920′s, and in the nearly 100 years since, significant progress has been made in both experimental and theoretical understanding of them. Despite these advances, many key deficiencies remain, solution of which will provide appreciable value for system designers looking to leverage phase change heat transfer technologies in a safe and repeatable manner. The present review provides a systematic overview of all key two-phase instabilities focusing on the fundamental mechanisms leading to their occurrence. Emphasis is placed on how these mechanisms may change depending on whether flow may be classified as macro- or micro-channel, particularly relevant due to the modern proliferation of parallel micro-channel heat sinks. Extensive literature surveys are performed for each instability type, and strengths and weaknesses of existing literature assessed. Focus is placed on providing recommendations for future work based on the status of current literature. Important takeaways include the significant mechanistic differences for Density Wave Oscillations and Parallel Channel Instability between macro- and micro-channels, the need for better understanding of the role of parallel micro-channels on external pressure curves (impacting Ledinegg instability and Pressure Drop Oscillations), and the influence of size and position of compressible volume on Pressure Drop Oscillations.
This paper aims to provide a machine learning framework to simulate two-phase flow in porous media. The proposed algorithm is based on Physics-informed neural networks (PINN). A novel residual-based ...adaptive PINN is developed and compared with the residual-based adaptive refinement (RAR) method and with PINN with fixed collocation points. The proposed algorithm is expected to have great potential to be applied to different fields where adaptivity is needed. In this paper, we focus on the two-phase flow in porous media problem. We provide two numerical examples to show the effectiveness of the new algorithm. It is found that adaptivity is essential to capture moving flow fronts. We show how the results obtained through this approach are more accurate than using RAR method or PINN with fixed collocation points, while having a comparable computational cost.
•A basic framework to simulate two-phase flow in porous media using PINN is suggested.•A novel residual-based adaptive PINN is developed for accurate interface predictions.•Distinct training/collocation points are obtained for different terms of the loss.
•Increasing pipe size shifts bubbly-to-plug/slug transition to higher superficial liquid velocities.•Increasing pipe size has no effect on the frictional pressure loss analysis.•Increasing pipe size ...promotes bubbles to be more concentrated near top wall in bubbly flow.•Increasing pipe size has insignificant effect on the drift-flux analysis but needs further study.
This paper performs experimental studies to investigate the effects of pipe size on horizontal air–water two-phase flow in a wide range of flow configurations. Some of the major characteristic two-phase flow phenomena of interest include flow regime transitions, two-phase frictional pressure drop, local interfacial structures, and drift-flux analysis. Based on the experimental results obtained in two pipe sizes, the flow regime maps and the correlations for two-phase frictional pressure drop employed in conventional nuclear reactor system analysis codes (TRACE and RELAP) are evaluated. It is found that the flow regime maps in the codes incorrectly predict plug and slug flows as bubbly flow. Effects of pipe size on flow regime transitions are observed, while these effects are not reflected on the maps employed in the codes. The frictional pressure drop analysis suggests that the closure relations used in both TRACE and RELAP can be improved for horizontal two-phase flow. The local time-averaged two-phase flow parameters acquired by a four-sensor conductivity probe including void fraction, interfacial area concentration, bubble velocity, and bubble Sauter-mean diameter provide quantitative descriptions for the interfacial structures in different pipe sizes. It is found that increasing the pipe size tends to cause bubbles to be more concentrated near the top wall in bubbly flow. As a result, the bubble layer occupies a smaller portion of the flow area as pipe diameter increases. Based on the experimental database, drift-flux analysis using both the 〈〈vg〉〉 vs. 〈 j〉 and 〈α〉 vs. 〈β〉 methods is performed. The effect of pipe size on drift-flux analysis is found to be negligible. The global slip is found to be less than one, indicating that the gas phase moves slower than the liquid phase in horizontal bubbly flow. This analysis also provides a predictive method to estimate the void-weighted bubble velocity and void fraction, which can be predicted with an average percent difference of ±3.3% and ±3.1%, respectively.
•Heat transfer correlations and database for horizontal flows were reviewed extensively.•None of existing correlations predicted all horizontal flow databases satisfactorily.•The dependence of heat ...transfer coefficient on pressure drop was derived analytically.•A semi-theoretical heat transfer correlation for horizontal flow was developed.
Two-component gas-liquid two-phase slug flow in horizontal pipes occurs frequently in many industrial applications. The flow and heat transfer characteristics are complicated due to the intermittent flow structures. In view of the importance of predicting the heat transfer characteristics of the slug flow, this present study aims at developing a semi-theoretical heat transfer correlation for two-component two-phase slug flow based on the concept of Reynolds and Chilton-Colburn analogies. Firstly, an extensive literature review on existing databases and correlations of heat transfer coefficient for two-component two-phase slug flow was conducted. More than 500 experimental data and 8 heat transfer correlations were collected. The comparison between collected database and correlations indicated that none of the correlations could estimate the whole database satisfactorily. The relationship between heat transfer and pressure drop was investigated theoretically and an improved semi-theoretical heat transfer correlation was developed based on Reynolds and Chilton-Colburn analogies and the collected experimental results from 16 sources. The comparison analysis demonstrated that the newly-developed correlation achieved an excellent predictive capability with a wide range of test conditions. The newly developed correlation demonstrated that 91.5% of the data was predicted within ±30% error with the mean absolute relative deviation of 14.0%. In addition, the extension of the new correlation to other horizontal two-phase flow regimes was discussed. The newly-developed semi-theoretical correlation would be useful for predicting heat transfer coefficient of two-component two-phase flow in horizontal pipes.
The dynamic characteristics of shale gas wells are complexly affected by the gas–water two-phase flow. Based on the special flow mechanism of gas–water two-phase flow in shale gas reservoir, this ...paper establishes a mathematical model for gas–water two-phase flow in shale gas multi-stage fractured horizontal wells, introduces the eigenvalue method and orthogonal transformation, and obtains the analytical solution of the two-phase flow model. The gas–water two-phase flow rules and main influence factors of shale gas wells were identified, further combined with the flowback test characteristics and data of the shale gas wells in southern Sichuan, the characteristic parameters for the evaluation of the gas well flowback effect were determined, and an index system was established for the evaluation of shale gas well flowback. The analysis result shows that the shale gas well flowback effect has a good relationship with its production capacity, which is mainly reflected in the flowback characteristic parameters such as gas breakthrough time, gas breakthrough flowback rate, 30 d flowback rate, and maximum production flowback rate. The shale gas wells with lower flowback factors have a better production capacity than those with higher flowback factors. The flowback evaluation index system can accurately forecast the shale gas well production capacity in its initial stage, and furthermore offer guidance to promptly ascertaining the block development potential and formulating the development schemes.
•A comprehensive 3D multiphase anisotropic model of PEMFC is developed.•Effects of surface tension, wall adhesion and gravity in channel are included.•Anisotropy of GDL and liquid saturation jump are ...taken into account in this model.•Contact angle at GDL/channel interface can affect the performance of PEMFC.•Adding baffles in channel can prevent PEMFC from concentration loss effectively.
A comprehensive 3D (three-dimensional) multiphase model of PEMFC (proton exchange membrane fuel cell) is developed, in which the gas and liquid two-phase flow in channel and porous electrodes are investigated in detail. In the simulation of gas and liquid two-phase flow in channels, the effect of surface tension, wall adhesion and gravity is taken into account, including the influence of pressure difference between the inlet and outlet on inlet reactant gas concentration; while in porous electrodes, the anisotropy of GDL (gas diffusion layer) and liquid saturation jump at the interface of two different porous layers (e.g. GDL and MPL (micro-porous layer)) are also considered in this model. It is found that the amount of liquid water in channels increases with the increment of current density. In addition, increasing the contact angle at GDL/channel interface is found to be able to improve the performance of PEMFC by facilitating the water removal process in channels. Moreover, it can be concluded that adding baffles in cathode channel not only increases the oxygen concentration in porous electrodes but also facilitates the water removal process, both of which prevent PEMFC from concentration loss effectively.
A hybrid Eulerian-Lagrangian solver RYrhoCentralFoam is developed based on OpenFOAM® to simulate detonative combustion in two-phase gas-liquid mixtures. For Eulerian gas phase, RYrhoCentralFoam ...enjoys second order of accuracy in time and space discretizations and is based on finite volume method on polyhedral cells. The following developments are made based on the standard compressible flow solver rhoCentralFoam in OpenFOAM®: (1) multi-component species transport, (2) detailed fuel chemistry for gas phase combustion, and (3) Lagrangian solver for gas-droplet two-phase flows and sub-models for liquid droplets. To extensively verify and validate the developments and implementations of the solver and models, a series of benchmark cases are studied, including non-reacting multi-component gaseous flows, purely gaseous detonations, and two-phase gas-droplet mixtures. The results show that the RYrhoCentralFoam solver can accurately predict the flow discontinuities (e.g. shock wave and expansion wave), molecular diffusion, auto-ignition and shock-induced ignition. Also, the RYrhoCentralFoam solver can accurately simulate gaseous detonation propagation for different fuels (e.g. hydrogen and methane), about propagation speed, detonation frontal structures and cell size. Sub-models related to the droplet phase are verified and/or validated against analytical and experimental data. It is also found that the RYrhoCentralFoam solver is able to capture the main quantities and features of the gas-droplet two-phase detonations, including detonation propagation speed, interphase interactions and detonation frontal structures. As our future work, RYrhoCentralFoam solver can also be extended for simulating two-phase detonations in dense droplet sprays.
PDF