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Highlights • Full visualization in diabatic upward flow boiling conditions has been undertaken. • The heat transfer has been shown to be insensitive to mass flux. • The transition from bubbly to slug flow causes an inflection in the boiling curve. • The results are very well predicted by a modification to the classical Chen correlation. Abstract Forced convective boiling experiments with HFE7000 as the working fluid were conducted for upward flow in a vertical tube at 122 kPa (1.2 bar), inlet subcooling of 2 K and mass flux ranging between 50 and 150 kg/m2 s. The experimental facility consisted of a smooth 8 mm inside diameter optically transparent sapphire tube with overall length of 120 mm. A central 80 mm long section was coated with optically transparent and electrically conductive Indium Tin Oxide (ITO) functioning as a heater to facilitate boiling whilst allowing for visual observation of the two-phase flow. Tests were performed by fixing the flow rate and progressively increasing the heat flux. The results show that after the onset of nucleate boiling, the flow regime is bubbly and increasing the heat flux acts to increase the boiling heat transfer coefficient due to increased nucleation site density, bubble frequency and size. This results in a linear increase in the boiling curve. However, once slug flow is established, the slope of the boiling curve increases indicating that this flow regime augments the heat transfer positively. Overall, the boiling curves were insensitive to mass flux for the range tested. However, the inflection on the boiling curve, associated with flow regime transition from bubbly to slug, was observed to be sensitive to mass flux. The Chen convective boiling heat transfer correlation was compared with the measured data showing reasonable agreement. A modified Chen correlation has been proposed for low mass flux convective boiling which predicts the measured data more accurately. [ABSTRACT FROM AUTHOR]

Highlights • Two-phase flow of R134a–DMAC becomes more complicated under vibration state. • Bubble jolting and disintegration phenomena are observed under vibration state. • Stability of bubble absorption is affected by vibration parameters significantly. • The maximal reduction of BAH occurred when amplitude is 0.4 mm frequency is 25 Hz. • Pressure, temperature, concentration and flow rate affects the bubble absorption. Abstract Two-phase flow pattern evolvement characteristic of working pair R134a–DMAC as well as the relationship between the bubble absorption height and vibration parameters (amplitude and frequency) were studied experimentally. Results showed that the bubble behavior and two-phase flow patterns became more complicated and disturbed under the vibration state, besides the bubble jolting and disintegration phenomena were observed. Stability of bubble absorption was also influenced by the vibration parameters. The most significant enhancement effectiveness of system vibration to the bubble absorption of R134a–DMAC occurred when amplitude was 0.4 mm and frequency was 25 Hz within the testing conditions. Finally, absorption pressure and temperature, solution flow rate and solution concentration were testified to influence the bubble absorption height and flow patterns under the vibration conditions. [ABSTRACT FROM AUTHOR]

Abstract A 2D PEMFC model has been built to simulate the current density distribution and liquid water distribution in cathode. In this simulation, the air mass flow rate, relative humidity of the air and cell voltage are considered as the input parameters into the model, the diagram of current density and liquid water distribution in cathode are shown according to the simulation result and compared to each other. The result indicates that the liquid water distribution has a strong positive relation to the current density, where water drops diffuse mainly by the pressure difference (capillary force) in the porous media (the porous media coefficient εp is very low), where the shear force of the air can hardly drive the water drops due to the high friction force of the porous media. In the thin porous media (the porous media coefficient εp is relatively high), the purge effect of the air flow can be enhanced, decreasing the flooding areas in the cathode. Due to the purge effect of the air flow, the outlet of the cathode is the critical areas, where the flooding is most likely to happen. The steep trough at the current density distribution may mean the local flooding, which blocks the oxygen and weaken the chemical reaction. [ABSTRACT FROM AUTHOR]

During a loss of coolant accident, the formation of porous heat-generating debris bed causes concerns with the further progression of accident through inadequate removal of the decay heat. The sustainable heat removal or cooling of the porous debris bed using emergency coolant water poses limitations on the cooling mechanisms by imposing a maximum removable heat flux, also known as the dryout heat flux. Modeling debris bed cooling and the dryout heat flux requires the use of parameters that are non-linear functions of the vapor fraction in the two-phase flow. These models need high resolution vapor fraction data for validation, which is currently lacking in literature. This paper presents first of a kind high-resolution vapor fraction measurements in an inductively heated debris bed using a non-optical and non-intrusive neutron imaging technique. The neutron beam from TRIGA Mark II reactor at Kansas State University was used to image the packed bed test section and gather vapor fraction data. The vapor fraction measurements are presented for a range of heat fluxes up to the limiting dryout heat flux. Images showing the formation of a dryout zone for heat fluxes higher than the limiting dryout heat flux are also presented. The measured vapor fractions are compared with predictions from the classical 1D Lipinski model. The mean absolute deviation between the model predictions and the measured vapor fractions is found to be less than 10%, providing the first validation of the classical 1D Lipinski model. • High resolution vapor fraction measurements in an inductively heated debris bed using neutron imaging are presented. • The vapor fraction measurements for a range of heat fluxes show steady boiling and dryout beyond the dryout heat flux. • The vapor fraction measurements are used to validate the Lipinski 1D model for the first time. [ABSTRACT FROM AUTHOR]

Thermal stratification and self-pressurization in a propellant storage tank due to heat leakage from the wall are key issues of space fluid management. Under low-gravity conditions, the gas/liquid two-phase flow in a tank is complicated owing to the irregular interface morphology caused by the capillary effect. To clarify the heat and mass transfer process, the gas/liquid two-phase flow with the capillary effect accounted at the interface is systematically investigated by taking into account the volume of fluid (VOF) method for two-phase capturing and the Lee model for phase change. Spatial-temporal evolutions of thermal and pressure distributions and mass transfer rates at the interface in an axisymmetric scaling capsule tank of ethanol are studied depending on various gravity levels, liquid filling ratios, and boundary heat fluxes. The results show that the overall temperature, pressure, and thermal distributions inside the tank are significantly affected by the gravity level, liquid filling ratio, and boundary heat flux, while the pressure distributions are quite similar under different conditions. The influence of gravity levels mainly originated from various interface configurations due to the capillary effect. Therefore, the capillary effect plays an important role in the heat and mass transfer process in low-gravity environments. [ABSTRACT FROM AUTHOR]

Gas- Liquid flows are by far the most important type of multiphase flow. This can be attributed to the wide range of industrial applications that the gas-liquid flow is discerned in. Popular examples of Gas-liquid flows are oil-gas mixtures, evaporators, boilers, condensers, refrigeration and cryogenics. The measurement of the liquid film thickness in two phase flows is prominent in various heat and mass transfer applications such as in boilers. To determine the thin film thickness is the aim of this study. A glass tube of diameter 4.7 mm is used for conducting the experiment and a laser pointer is used to obtain an image pattern on the screen. Using the principles of Optics, a method has been proposed to determine the thin film thickness and also to characterize the different types of flow. The thin film thickness obtained in the proposed method is validated using Image Processing.

The present paper describes a two-phase flow model used to study backward pumping of the low pressure fluid in the sealing interface of a mechanical seal. The Reynolds equation for a homogeneous mixture is solved as well as a transport equation used to determine the ratio of both fluids at each point. Several examples are presented to show that the second function of a seal (prevent the contamination of the process fluid by the operating environment) is not always insured because of possible reverse flows. The behavior of the seal can be greatly affected when it is partially lubricated by the low pressure fluid.

Steady state flow boiling experiments were carried out on several heated tubes with outer diameter 9.14 mm at outlet pressures 120, 200 and 300 kPa, inlet temperatures 64, 78 and 91 °C and approximately 400, 500, 600 and 800 kg/(m2·s) mass flux entering the vertically aligned test annulus until critical heat flux (CHF) was reached. The tubes were made of Inconel 625 with a length of 400 mm. The Inconel tubes were tested in three different modifications as smooth, abraded with 150 grit sandpaper and bead blasted. Multiple experiments were repeated on the same specimen to investigate the effect of surface characteristic changes (i.e., wettability, roughness and oxide layer morphology) on the occurrence of CHF. Despite the changes in initial wettability, the CHF dependency was not clearly observed, however, the changes in roughness led to an increase in CHF. The total number of 115 experimental runs were collected and the results were also compared with other literature experimental data.

Two-phase flow is encountered in various engineering areas, including the pharmaceutical, chemical, and food industries, desalination facilities, and thermal energy storage systems. Cost-effective and non-invasive monitoring of the solid volume fraction, which governs the thermos-physical properties of two-phase medium, is important for flow assurance. The flow loop having an inner diameter of 21.5 mm and length of about 12.2 m was equipped with square-edged orifice and slash plate pump. Tetrafluroethane (R134a) hydrate slurry of the specified solid volume fraction could be formed within the flow loop by removing an appropriate amount of water, and simultaneously injecting the pertinent amount of R134a while chilled at 275 K. The uncertainty in the thus-obtained solid volume fraction was smaller than 9%, with the largest contribution originating from the uncertain hydration number. The near power-law relationship between the orifice pressure loss coefficient and Metzner–Reed Reynolds number was recognized. However, the nonlinear nature of the Reynolds number with respect to the solid volume fraction inevitably makes the solution procedure iterative. The short span pressure differences across the orifice were regressed to yield empirical correlation, with which the solid volume fraction of R134a slurry could be determined from the measured pressure drop across the orifice and the flow rate. The uncertainty was less than 12% of the thus determined solid volume fraction.

In order to improve the inadequacy of the current research on oil droplet size distribution in aero-engine bearing chamber, the influence of oil droplet size distribution with the oil droplets coalescence and breakup is analyzed by using the computational fluid dynamics-population balance model (CFD-PBM). The Euler–Euler equation and population balance equation are solved in Fluent software. The distribution of the gas phase velocity field and the volume fraction of different oil droplet diameter at different time are obtained in the bearing chamber. Then, the influence of different initial oil droplet diameter, air, and oil mass flow on oil droplet size distribution is discussed. The result of numerical analysis is compared with the experiment in the literature to verify the feasibility and validity. The main results provide the following conclusions. At the initial stage, the coalescence of oil droplets plays a dominant role. Then, the breakup of larger diameter oil droplet appears. Finally, the oil droplet size distribution tends to be stable. The coalescence and breakup of oil droplet increases with the initial diameter of oil droplet and the air mass flow increasing, and the oil droplet size distribution changes significantly. With the oil mass flow increasing, the coalescence and breakup of oil droplet has little change and the variation of oil droplet size distribution is not obvious.

Highlights • Finite volume GFM implementation of interfacial forces in PISO is presented. • GFM produces a sharp and accurate pressure jump compared to the traditional CSF. • Sharp material property implementation shows a reduction in shear stress diffusion. Abstract In this work, we implement interfacial forces (surface tension, hydrostatic and viscous forces) by enlisting the finite volume discretization of GFM (Ghost Fluid Method) using A-CLSVOF (Algebraic Coupled Level-Set/Volume-of-Fluid) method for the mass conservative, and smooth interface description. The widely used PISO momentum solution to resolve the pressure–velocity coupling is presented along with the present GFM discretization and its placement within the PISO loop. The pressure jump at the interface due to the interfacial forces is made sharp via direct calculation of the modified pressure matrix coefficients corresponding to targeted interfacial cells, and as a source term for the jump value itself. The Level-Set field is enlisted for curvature computation in A-CLSVOF and for the interpolation and weighting of the relative contribution of the capillary force in adjacent for the matrix coefficients in the FV framework. To assess the A-CLSVOF/GFM performance, four canonical cases were studied. In the case of a static droplet in suspension, A-CLSVOF/GFM produces a sharp and accurate pressure jump compared to the traditional CSF implementation of A-CLSVOF. The interaction of viscous and capillary forces is proven to be accurate and consistent with theoretical results for the classical capillary wave. For the linear two-layer shear flow, GFM sharp treatment of the viscosity captured the velocity gradient across the interface and removed the diffusion of the viscous stresses caused by the discontinuous material properties. Finally, the combination of all GFM improvements proposed in this study are compared to experimental findings of terminal velocity for a gaseous bubble rising in a viscous fluid. GFM outperforms CSF with errors of 4.6% and 14.0% respectively. [ABSTRACT FROM AUTHOR]

Abstract A moving mesh approach is employed to simulate two-phase flow with phase change. The mathematical model is based on the Arbitrary Lagrangian–Eulerian (ALE) description of the axisymmetric Navier–Stokes equations and energy conservation. These equations are discretized by the Finite Element Method (FEM) on a triangular unstructured mesh in which the phase boundary is represented by a set of interconnected nodes and segments that are part of the computational mesh. Here, phase change and surface tension are implemented as source terms, using the one fluid approach. The method is shown to provide an accurate description of the interfacial forces, heat and mass transfer between phases. Several different verifications are presented where the results are compared to analytical and semi-analytical solutions. [ABSTRACT FROM AUTHOR]

The purpose of this work was the study of the formation process of Newtonian drop in a continuous non-Newtonian fluid. This process was numerically studied by entering liquid into a submerged orifice in a cylindrical vessel. The simulations were carried out using SOLA-VOF method. In this code, the complete motion equations were predicted two dimensions and using finite difference method. In addition, power law model was used to simulate a non-Newtonian fluid. In this research, the effects of orifice diameter and Newtonian fluid flow rate were studied on the formation of the drop, size and its formation time. [ABSTRACT FROM AUTHOR]

Abstract In this study, ferrofluid droplet behavior in nonmagnetic viscous fluid under the uniform magnetic field is studied numerically by developing a robust two-dimensional hybrid lattice-Boltzmann/Finite-volume method. The lattice Boltzmann equation (LBE) with the magnetic force term is solved to update the flow field while the magnetic induction equation is solved using the finite volume method to calculate the magnetic field. To validate the flow field solution two test cases have been considered: the falling droplet and Laplace law. In order to validate the magnetic field, deformation of static drop under magnetic field is checked. The comparison of results between present study and previous researches shows a good agreement. The effects of the magnetic Bond number, susceptibility and magnetic field direction on deformation of the falling droplet are also investigated. The results show that the horizontal magnetic field accelerates the falling process of ferrofluid droplet while the vertical magnetic field slows down it by increasing the magnetic Bond number and susceptibility. Also, the increase in the magnetic Bond number and susceptibility enhances the magnitudes of magnetic force along the magnetic field. [ABSTRACT FROM AUTHOR]

In this paper, an experimental study is performed on the mixing of superheated gas and subcooled flowing liquid and the subsequent flow condensation process in a vertical visual flow channel using R123 as the working fluid. The flow patterns of the two phase flow in the vertical channel are verified and the flow condensation lengths of the gaseous phase under different operating conditions are obtained. Test results indicate that under the operating conditions in this experiment, the gaseous phase forms a cavity or a film at the gas orifices, and then forms a dispersive bubbly flow in the downstream region. The flow condensation length of the gaseous phase through a single orifice increases approximately linearly with the gas flow rate over a range of 0.35–0.76 m, and it is reduced to a range of 0.27–0.56 m by using multiple gas injection orifices. A computational fluid dynamics (CFD) model is established to simulate the two phase flow and interfacial heat and mass transfer in the channel. The simulation capability and prediction accuracy of the CFD model are validated by comparing numerical results of phase distributions as well as the flow condensation lengths with test data, and satisfying agreements are obtained. This work could be beneficial to the understanding and handling of interphase heat and mass transfer problems in in-tube two phase flows. [ABSTRACT FROM AUTHOR]

This work presents and analyses the results of experimental activities aimed at a preliminary characterization of foamy flows for pipeline dewatering, in order to assess whether the addition of surfactants may effectively reduce the liquid holdup in horizontal pipelines. Static tests were run to compare the foam cycle (generation and decay) for three commercial surfactants and to choose the most suitable one. Dynamic tests with the selected product were performed in a 20 m long, 60 mm i.d. Plexiglas® pipe, where a 0.3% wt. solution of surfactant in tap water was pumped after mixing with an air flow at nearly atmospheric pressure and temperature. Superficial velocities ranged between 0.03 m/s and 0.05 m/s for water and between 1.5 m/s and 11.5 m/s for air, which would determine stratified/stratified wavy flows in the case of pure water-air flow, i.e. the benchmark case. Due to the presence of the surfactant, foam formed in the mixing section, which implied a significant change in the flow patterns that were photographically recorded and classified into three main types: plug, stratified wavy and stratified with foam entrainment, as far as the air superficial velocity was increased at constant water superficial velocity. The associated pressure drop, linearly distributed along the pipeline, resulted greater than the benchmark value in all the operating conditions, with a dramatic increase (even more than 100%) for plug flows. On the other hand, the percentage relative difference was found to lower with increasing the air superficial velocity, apart for stratified wavy flows where it seemed to keep constant at about 3.3%. Finally, a theoretical model for stratified flows was used to relate the pressure drop to the void fraction in order to get at least an approximate indication of the liquid load reduction due to the surfactant addition, which ranged between 6% and 39%. [ABSTRACT FROM AUTHOR]

A special sampler is proposed to extract a fraction of fluid from the main stream of gas-liquid two-phase flow. Four rectangular slots (2mm × 110 mm) were arranged along the circumference of the pipe wall, and a moving plate is used to adjust the fraction of taken off. Experiments were conducted in an air-water two-phase flow loop and the flow patterns tested were: wavy, slug and annular flow. Experimental results show that, on contrary to the phase split behavior of conventional horizontal T-Junction, the liquid preferentially enters the sample slots. Phase split is affected by the gas and liquid superficial velocities, slot length ratio ( L s /L m ) and pressure difference ratio between the sample slot and main slot (Δ P 3 / Δ P 2 ). In case of L s /L e equaling to 0.046, the fraction of liquid taken off can even exceed 0.4 due to much higher pressure difference of the sample slots compared with that of the main slots. [ABSTRACT FROM AUTHOR]

Two phase flow between two fractures commonly exists around fractured wells. Due to the fracture network, the flow velocity is extremely variable in different regions and it can be described by the temporal scale analysis. The flow velocity also changes with time during the change in water saturation, but these changes cannot be captured by the conventional temporal scale analysis. The temporal scales are obtained based on a principal equation, so it is difficult to accurately estimate the formation properties and to determine the exploitation scheme. In this study, the two phase flow model between two fractures has been proposed for the temporal scale description. The impact of reservoir heterogeneity on the temporal scales is considered in the model. Due to the tortuous flow channel shape between the fractures, the flow pressure affects the fluid flow afterwards. This is explained by the time lag effect in the model. The temporal scales are obtained by using finite element method and Laplace transformation at all time points with these considerations. Moreover, the well is soaked intermittently. The soaking duration is chosen based on temporal scales, and then discussed in terms of efficiency and recovery factor. The results showed that the water saturation profile position is reflected by the temporal scales, the flow resistance and the flow velocity are represented by their zero status response contributions, while the initial flow pressure is described by their zero input contribution. The highest zero status response contribution is different for different properties. Moreover, the time lag effect promotes the initial pressure fluctuation, and also upscales the fluid flow in tortuous channels. Finally, the soaking duration is an approximation of the time required to reduce the fracture flow pressure. [ABSTRACT FROM AUTHOR]

Debris flows, lahars, avalanches, landslides, and other geophysical mass flows can contain material in the order of O(106-1010)m³ or more. These flows commonly consist of a mixture of soil and rocks with a significant quantity of interstitial fluid. They can be tens of meters deep, and their runouts can extend many kilometers. The complicated rheology of such a mixture challenges every constitutive model that can reasonably be applied: The range of length and timescales involved in such mass flows challenge the computational capabilities of existing models. This paper extends recent efforts to develop a depth averaged "thin layer" model for geophysical mass flows that contain a mixture of solid material and fluid. Concepts from the engineering community are integrated with phenomenological findings in geoscience, resulting in a theory that accounts for the principal solid and fluid forces as well as interactions between the phases, across a wide range of solid volume fractions. A principal contribution here is to present drag and phase interaction terms that conform with the literature in geosciences. The Titan2F program predicts the evolution of the volumetric concentration of solids and dynamic pressure. The theory is validated with data from one-dimensional dam break solutions and with data from artificial channel experiments. [ABSTRACT FROM AUTHOR]

This paper is concerned with the development and implementation of an adaptive solution algorithm for the optimal control of a time-discrete Cahn-Hilliard-Navier-Stokes system with variable densities. The free energy density associated with the Cahn-Hilliard system incorporates the double-obstacle potential which yields an optimal control problem for a family of coupled systems in each time instant of a variational inequality of fourth order and the Navier-Stokes equation. A dual-weighted residual approach for goal-oriented adaptive finite elements is presented which is based on the concept of C-stationarity. The overall error representation depends on primal residuals weighted by approximate dual quantities and vice versa as well as various complementarity mismatch errors. Details of the numerical realization of the adaptive concept and a report on the numerical tests are given. [ABSTRACT FROM AUTHOR]

This paper presents the design and original fabrication of a fully transparent microfluidic reactor with integrated electrical sensors over the microchannel. The transparent chip design allows for microscale photochemistry, and permits direct, real-time visual and electrical observation. The microchip uses optically transparent indium tin oxide (ITO) electrodes as sensing elements for in-channel phase tracking. High-speed videography was used to validate the electrical measurement data. The device permits visual observation of microchannel bubble flow while simultaneously acquiring electrical data. The aim of this work is to develop and optically verify electrical data acquisition and signal analysis methodologies, which in future applications may be used for inexpensive non-destructive monitoring of two-phase microchannel flows, in systems where detailed visual observation may be infeasible. [ABSTRACT FROM AUTHOR]

Annular flow is one of the most important multiphase flow regimes primarily because of its presence in extensive industrial applications,such as steam generators, evaporators, condensers and boiling water reactors. There is a great need to have specific techniques capable of extracting characteristics of annular flow pattern via experiment and/or modelling, which is essential to understand specific annular flow phenomena. The intention of developing a mathematical decomposition method for flow regime recognition comes from the fact that each regime is characterised by unique dynamic behaviour. The direct approach with Proper Orthogonal Decomposition (POD) was previously introduced by Lumley, which required a very large amount of relevant data from experimental multiphase and process conditions is necessary. The presented approach is to apply POD on a large amount of tomograms for identification of annular flow pattern in a gas-liquid two phase flow, which also provides a new method to extend the current evaluation procedure of Electrical Impedance Tomography (EIT) for imaging of air-water pipeline flows.The paper demonstrates the capability of the proposed method with a study on annular flow patterns in vertical pipeline. [ABSTRACT FROM AUTHOR]

The mold filling behavior of gradual expansion structure in low pressure casting was studied by two phase flow model using the Volume of Fluid method, and was verified by water simulation with a Plexiglas mold. To get smooth mold filling process and provide a guide for the pressurizing speed design in the producing practice, the mathematical model with the pressurizing speed, expansion angle and height of the gradual expansion structure was established. For validation experiments, ZL205A alloy castings were produced under two different pressurizing speeds. Weibull probability plots were used to assess the fracture mechanisms under different pressurizing speeds. Mechanical properties of ZL205A alloy were applied to assess the entrainment of oxide film. The results show that the filling process of a gradual expansion structure in a low pressure casting can be divided into the spreading stage and filling stage by gate velocity. The gate velocity continues to increase in the gradual expansion structure, and increases with the increase of pressurizing speed or expansion angle. Under the effect of the falling fluid raised by the jet flow along the sidewall, the fluid velocity decreases in the jet zone from ingate to free surface. As such, oxide film entrainment does not occur when the gate velocity is greater than the critical velocity, and the gate velocity no longer reflects the real state of the free surface. The scatter of the mechanical properties is strongly affected by the entrainment of oxide films. [ABSTRACT FROM AUTHOR]

Electrolysis with polymer electrolyte membranes (PEMs) plays an increasingly important role in the development of inconsistent renewable energy technologies and seasonal storage. An effect that reduces the efficiency of PEM electrolysis is the mass transport limitation (MTL), which occurs at higher current densities and leads to a sudden increase in cell potentials. The oxygen generated on the anode side prevents the water from being supplied to the catalyst. Neutron-based imaging (neutron visualization techniques) make it possible to visualize mass transfer processes in the porous transport layer (PTL). When PTL materials are varied and operating modes used, it is possible to investigate the critical point at which the MTL is generated. This paper presents the results of neutron radiography measurements. In the course of the measurements, we have observed PEM electrolysis cells in operation, using different materials as anode PTL. The PTLs are metal plates made of sintered titanium particles, as well as titanium fiber. During the measurements, it was possible to visualize the water-gas distribution in the cell during operation in order to understand the mechanisms of mass transport. [ABSTRACT FROM AUTHOR]

An experimental investigation was carried out on the effects of high liquid viscosities on slug length in a 0.0762-m ID horizontal pipe using air-water and air-oil systems with nominal viscosities ranging from 1.0 to 5.5 Pa s. The measurements of slug length were carried out using two fast sampling gamma densitometers with a sampling frequency of 250 Hz. The results obtained show that liquid viscosity has a significant effect on slug length. An assessment of existing prediction models and correlations in the literature was carried out and statistical analysis against the present data revealed some discrepancies, which can be attributed to fluid properties in particular, low viscous oil data used in their derivation Hence, a new high viscous oil data presented here from which we derive a new slug length correlation was derived using dimensional analysis. The proposed correlation will improve prediction of slug length as well as provide a closure relationship for use in flow simulations involving heavy oil. This is important since most current fields produce highly viscous oil with some reaching 10 Pa s. [ABSTRACT FROM AUTHOR]

This work investigates boiling heat transfer for a small scale thermosyphon. Heat transfer results were analysed for three fluids; water, ethanol and HFE-7000. The influence of confinement and vapour mass flux on thermosyphon heat transfer was studied using a fully transparent two-phase closed thermosyphon. By considering the thermal performance of the thermosyphon together with the observed flow regimes, diabatic flow maps were developed. Of particular importance is the reduced pressure of the system which influences the size of the generated vapour bubbles in the evaporator. At low reduced pressures, bubble diameters may be large enough to span the tube diameter and the thermosyphon exhibits confined flow behaviour similar to that of micro-scale convective two-phase flows. Flows of this nature result in oscillatory behaviour and the boiling heat transfer is less effective. The phenomenon can be avoided by increasing the reduced pressure or tube diameter of the system. The most favourable flow regime in the context of thermosyphon heat transfer was found to be churn-type flow. It was found that slug/plug-type flow was not conducive to heat transfer where vapour production rates, and thus latent heat transfer, were relatively low. The flow regime maps relating phase superficial momentum flux the heat transfer coefficient may be used as an aid to the design of thermosyphons for specific applications. [ABSTRACT FROM AUTHOR]

As the last stage of a moisture separator, the drain device is an important part of the secondary separator which can affect the separation efficiency. The emphasis of this paper is placed on studying the flow structure and predicting the free liquid surface height in the drain device. Four different turbulence models including k - ε , k - ω , Shear Stress Transport (SST) and Scale-Adaptive Simulation (SAS-SST) were used to simulate the drainage process with air-water mixture (cold state) under ambient temperature and atmosphere pressure. These models were validated by the experimental data obtained in our previous study. Simulation results showed that the free liquid surface height predicted by the SAS-SST model is in best agreement with the experimental results under cold state. Therefore it was further used to perform the simulation for steam-water mixture (hot state) under the standard working condition (138.1 m 3 /h, 7 MPa and 285.8 °C). The maximum free liquid surface height is simulated to be 269.3 mm for the hot state by the SAS-SST model which is 4.5% higher than that of the cold state. However, it could also meet the design object. [ABSTRACT FROM AUTHOR]

In the current study, two-phase flow of water and air over a stepped spillway is probed in the form of a two-dimensional incompressible viscous flow. A novel numerical approach is used for the numerical simulation which is a combination of two models: volume of fluid (VOF) which uses an interface tracking algorithm for the simulation of the two-phase flow and two-fluid model which is based on time and space averaged equations and cannot track the interface explicitly. The most important issue in the introduced approach is to couple the two basic methods and select a proper criterion for status change between two basic methods. The latter criterion is based on an approximation from local distribution of the interface at each cell. In the hybrid method. In order to investigate the aeration effect in the stepped spillway, the air suction is generated by designing some holes at the upper edge of the steps and considering atmosphere pressure for these areas. The obtained results divulge the amount of dispersion is low at the beginning part of the step and also the hybrid model take more advantages from VOF, while in the lower steps where the flow disperses two-fluid model has hegemony. The results are compared in the form of pressure contours and streamlines as well as volume fraction counters. The comparison shows that the results of the proposed method is closer to the experimental results with respect to each of the basic model. [ABSTRACT FROM AUTHOR]

Previous work has described the use of Coriolis mass flow metering for multiphase flows, with applications in the oil and gas industry. This paper describes the development and testing of a Coriolis two-phase flow metering solution for nitrogen/synthetic oil mixtures with viscosities ranging from 50 cSt to 500 cSt, and with Gas Volume Fractions (GVFs) ranging from 0% to 90%. Formal trials took place at the UK National Flow Laboratory. A single synthetic oil was used; its viscosity was adjusted by controlling temperature. The results show that Coriolis metering is well suited to high viscosity oil/gas mixtures: in the formalized trials, mass flow errors for gas were mostly within 5% and for liquid were mostly within 2.5%. Achieving this accuracy up to 90% GVF represents an advance on previous work, which has been limited to around 50% GVF for low viscosity oils in two-phase and three-phase mixtures. [ABSTRACT FROM AUTHOR]

Droplet microfluidic systems have attracted a large amount of research due to their numerous applications in biomedical micro-devices and drug discovery/delivery platforms. One of the most important problems in such systems is to investigate deformation, coalescence, and breakup of droplets within the channel. The present study demonstrates numerical simulation of a falling droplet subject to gravitational force in a channel with embedded rectangular obstacles. The lattice Boltzmann method incorporated using He–Chen–Zhang method for two phase flow is employed. Two rectangular obstacles with inverse aspect ratios are introduced to investigate the mechanism of breakup and deformation of the droplet. The influence of gravity magnitude, viscosity and surface tension on the deformation rate of droplet for two different aspect ratios of the obstacle is studied. It is observed that increasing the gravity force, decreasing the viscosity or surface tension increase droplet deformation rate resulting in more stretched filaments and so breakup occurs in a shorter time. [ABSTRACT FROM AUTHOR]

In this work, coupled Cahn–Hilliard phase field and Navier–Stokes equations were solved using finite element method to address the effects of micro-fracture and its characterizations on water-oil displacements in a heterogeneous porous medium. Sensitivity studies at a wide range of viscosity ratios (M) and capillary numbers (Ca), and the resultant log Ca–log M stability phase diagram, revealed that in both media, with/without fracture, the three regimes of viscous fingering, capillary fingering and stable displacement similarly occur. However, presence of the fracture caused water channeling phenomenon which resulted in reduction of the number of active fingers and hence the final oil recovery factor. At high Ca (especially in the stable regime, with log Ca ≥ −2.5 and log M ≥ 0), recovery factor for the fractured medium was relatively identical with the non-fractured one. At log M ≥ 0, the fracture was fully swept, but flow instabilities were observed inside the fracture at lower M values, especially for log Ca > −4.6. In the case of the fractured medium at log Ca = −4.6 and log M = 0 (capillary dominant flow), it is observed that the primary breakthrough took place by a finger progressed through the matrix, not those channeled through the fracture. Geometrical properties of the fracture, including length, aperture and orientation, highly affected both displacement profile and efficiency. The fracture length inversely influenced the oil recovery factor. It was observed that there is a critical fracture width (almost half of the medium average pore diameter) at which the recovery factor of the medium during displacement is minimum, compared to the media having thinner and thicker fractures. Minor channeling effect in the media with thinner fracture and larger fracture swept volume as well as high fracture/matrix cross flow in the media with thicker fracture were detected as the main cause of this non-monotonic behavior. In the models with thick fractures (with the thickness higher than the average pore diameter), considerable trapped oil volumes were observed inside the fracture at low M values. The fracture orientation had the most impressive effect on oil recovery compared to the other studied parameters; where the oil recovery factor incremented more than 20% as the fracture rotated 90° from flow direction. Due to the dominant effect of the channeling phenomenon, the change in the medium wettability from slightly oil-wet to slightly water-wet, did not considerably affect the displacement profile in the fractured medium. However, oil recovery factor increased as the medium became more water-wet. The fracture area was fully swept by the injected water in the oil-wet and neutral-wet media. However, flow instabilities were observed inside the fracture of the water-wet medium due to counter-current imbibition between fracture/matrix. Micro-scale mechanisms of pore doublet effect, interface coalesce, snap-off and reverse movements were captured during the studied unstable displacements. [ABSTRACT FROM AUTHOR]

This study explored flow patterns and void fraction prediction models for co-current upward liquid-gas two-phase flow in narrow annular spaces. Experiments on two-phase flow in both narrow and wide annular conduits were performed in a 4-inch (0.102 m) outside-diameter test section, with 3.5-inch (0.089 m) and 2-inch (0.051 m) inner pipes as representations of different annular geometries. Flow patterns and liquid holdup were recorded while flowing air and water upward through the test sections. The experimental results showed that the two-phase flow pattern maps in narrow annulus and wide annulus are with different major regime transition lines. A new drift-flux model for water-air two-phase flow in narrow annuli was developed using experimental data collected in this study. The new model was compared against previous models that were developed for wide annuli, and the results showed a better match in void fraction predictions for two-phase flow in narrow annuli with experimental data. The understanding of two-phase flow in narrow annuli is vital for real-time prediction of void fraction and annular pressure loss in wells with gas influx during casing/liner/slimhole drilling operations. An enhanced prediction can ensure delivery of safer wells with lower non-productive time and associated trouble encountered during well control operations. [ABSTRACT FROM AUTHOR]

Seals are considered one of the most important flow elements in turbomachinery applications. The most traditional and widely known seal is the labyrinth seal but in recent years other types like the brush or carbon seals were introduced since they considerably reduce the sealing air consumption. When seals are used for sealing of aero engine bearing chambers they are subjected to high "bombardment" through oil particles which are present in the bearing chamber. These particles mainly result from the bearings as a consequence of the high rotational speeds. Particularly when carbon or brush seals are used, problems with carbon formation (coking) may arise when oil gets trapped in the very tight gap of these seals. In order to prevent oil migration into the turbomachinery, particularly when the pressure difference over a seal is small or even negligible, significant improvement can be achieved through the introduction of so called windback seals. This seal has a row of static helical teeth (thread) and below this thread a scalloped or smooth shaft section is rotating. Depending on the application, a windback seal can be used alone or as a combination with another seal (carbon, brush or labyrinth seal). A CFD analysis carried out with ANSYS CFX version 11 is presented in this paper with the aim to investigate this seal type. The simulations were performed by assuming a two-phase flow of air and oil in the bearing compartment. Design parameters like seal clearance, thread size, scallop width, were investigated at different operating conditions. [ABSTRACT FROM AUTHOR]

A numerical methodology based on dual grid dual level set function is presented for simulating 2D multiphase flows through complex geometries on a non-body fitted Cartesian computational grid. A Cut Cell based immersed boundary method (IBM) is used to sharply resolve complex geometries. The cut cells near the immersed boundary are identified using a fictitious level set function. The governing equations are discretized using the finite volume method (FVM) on a m × n staggered grid. The dynamics of fluid-fluid interface is tracked by an other level set function defined on dual grid (2m × 2n). The use of dual grid arrangement for fictitious level set function results in an accurate calculation of diffusion and advection fluxes at the cut cell near the immersed boundary. A module by module validation of the developed numerical methodology is carried out with several test cases. Two single phase flow problems corresponds to Poiseuille flow and flow through a convergent-divergent section while the two multiphase problems selected are the standard Young’s Laplace law test – semicircular fluid rod on a horizontal surface – and the bubble rise phenomenon in an inclined channel. The obtained results show an excellent agreement with those derived analytically or taken from literature. Furthermore, the performance study carried out on the present model shows that the method is between first and second order accurate. [ABSTRACT FROM AUTHOR]

Studies of the rock matrix acidizing for enhanced recovery of oil or gas have entirely focused on the fully water saturated conditions. In fact, matrix acidizing can be conducted in low-water-cut oil-production wells without pre-flushing of water or in high water production by injecting gas or oil ahead of the acid injection. These conditions yield a multiphase system, where the dynamics of acid transport and reactions can be altered by the presence of an immiscible phase. Against this backdrop, we present an investigation of the impact of initial saturation of an immiscible phase in the damaged zone on the efficiency of wormhole generation and growth in acidizing operations. We present a dimensionless two-phase reactive transport modelling tailored for studying the processes associated with rock acidizing. For a case study of acid injection into calcite with random porosity and permeability distribution, we show that an initial two-phase condition has positive feedback on the generation of wormholes. The results, however, indicate that the relative magnitude of reduction in the amount of pore volume of injected acid to produce effective wormholes depends on the mobility ratio, so that a higher mobility ratio facilitates a faster wormhole generation process. Under the conditions of modelling study presented, we demonstrate that in addition to the commonly used pair of Péclet-Damköhler regime identification, the mobility ratio of the displacing/displaced fluids, the relative permeability and phase condition need be accounted for in the analysis, if there are two-phase flow conditions across the target region of acidizing. [ABSTRACT FROM AUTHOR]

Water blocking due to the capillary end effect near the wellbore vicinity creates significant formation damage and decreases well productivity for oil and gas. The alteration of the rock wettability by a chemical well treatment is an effective way to reduce water blockage and enhance oil and gas production. Presently, several surfactants and nanofluids are used in the industry for contact angle alteration. After the treatment, the porous medium in the well vicinity (or along the core) has stepwise contact angle. The previous paper (Naik et al., 2015) develops analytical models for incompressible steady-state two-phase linear and axi-symmetric flows, accounting for the piecewise constant contact angle and contact-angle-dependent capillary pressure and relative permeability. The current paper adds the effect of compressibility, which is particularly important for high rate gas wells and wells in unconventional and tight-sand reservoirs. The obtained semi-analytical model has been validated by comparison with a set of laboratory coreflood tests. The modelling reveals a complex interplay between the competing effects of compressibility, viscous and capillary forces, yielding non-monotonic dependencies of well productivity on the wettability and the optimal contact angle. The model closely matches the productivity index data for two production wells from a field case before and after wettability treatment using surfactant. [ABSTRACT FROM AUTHOR]

In this paper, the three dimensional ventilated cavitating flow in the steady condition around a projectile model is simulated using CFD method combined with a sst k-ω turbulence model and volume-of-fluid technique, With the aid of CFD software ANSYS CFX. The numerical model is validated using comparisons between numerical predictions and existing experimental data and fairly good agreement is revealed. The numerical results show that with increasing the ventilation gas rate at constant Froude number, the cavity length gradually increases to a critical value and then remains fixed upon further increase in gas ventilation rate. Also, it has been observed that rear portion of larger cavity moves upwards due to gravitational effect. With increasing the ventilation gas rate, the gas leakage mechanism at rear portion of ventilated supercavity changes from the re-entrant jet closure mode to twin vortex closure mode. The variation of ventilation gas rate versus cavity length is a function of Froude number and the critical ventilation gas rate increases linearly with Froude number. [ABSTRACT FROM AUTHOR]

• Local two-phase flow data is collected for bubbly flows at different orientations. • Decreasing angle causes relative velocity to transition from positive to negative. • Reducing liquid flow rate induces bubble coalescence, mostly at low angles. • Changes in bubble size are from the interface between bubble and liquid layer. The present work seeks to develop a detailed experimental database and characterize the local effects of inclination on horizontal and upward bubbly air–water two-phase flows. Experimental conditions are selected to investigate bubbly flows at a variety of orientations while reducing the influence of flow regime transition. Detailed time-averaged data is collected using state-of-the-art four-sensor conductivity probes for measurements across the flow area for five flow conditions at five inclination orientations in a 25.4 mm inner diameter pipe. Using the newly established database, effects of inclination on bubbly flows are analyzed, as are changes in liquid and gas flow rate at different orientations. The observations include (1) bubble migration and agglomeration at the top pipe wall as orientation changes from vertical to horizontal; (2) reducing the angle from vertical reduces the local gas velocity, and correspondingly reduces the global slip from greater than unity at 90 ° to less than unity for 60 ° and lower; (3) changes in the bubble size distribution are primarily at the interface between the bubble cluster and the liquid region of the pipe, not coalescence within the bubble cluster. Physical mechanisms for these observations are proposed, and the present trends at different orientations support previous experiments in horizontal two-phase flows. [ABSTRACT FROM AUTHOR]

This paper presents the pressure drop prediction of BWR fuel assembly without experiment. To evaluate the core performance, pressure drop of fuel assembly is one of the key design parameters. The shape of fuel assembly is complicated and the spacer has a strong effect on pressure drop. Therefore, to obtain the pressure drop of fuel assembly, the experiments are needed. However, the experiments need a lot of time and money. In the authors' previous study, the friction coefficient and spacer loss coefficient under single phase flow condition were evaluated using CFD analysis and total pressure drop under two phase flow condition were predicted using these coefficients and verified by the data of the tight lattice bundle. The purpose of this study is to predict the pressure drop of BWR fuel assembly without experiments. The predicted pressure drop by the previous study's method was compared with the pressure drop data for full-scale high-burnup 8x8 fuel assembly covering the operating BWR conditions. The following conclusions are obtained; (1)The average and standard deviations of prediction accuracy (calculation/experiment) were 1.046 and 0.023, (2)The prediction accuracy of this method is almost same as that of the current BWR design method.

In air/oil lubrication systems, the flow parameters, e.g., flow pattern, liquid film thickness, and air/oil flow rate, are of great importance to the transportation efficiency. In most cases, the on-going two-phase flow is annular flow with the oil moving along the tube wall and the air travelling at high speed in the center. This usually results in the formation of a thin oil film, the thickness of which is a key parameter determining the efficiency of the lubrication system. As the oil film thickness of the on-going air/oil flow varies dynamically, there is actually no applicable method for a non-intrusive test. In this paper, the use of electrical capacitance tomography (ECT) to investigate the air/oil flow has been studied. Capacitance measurements are made from an externally mounted electrode array in a non-invasive and non-intrusive manner. Both average and distributed oil film thicknesses can be calculated from the reconstructed ECT images. Simulation and experimental results show that the ECT technique can provide satisfactory results of online oil film thickness estimation