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In order to understand clearly the transition characteristics of thermocapillary convection of low Prandtl number fluid in shallow annular pool with surface heat dissipation, some bifurcation routes to chaos of thermocapillary convection have been numerically investigated by using the finite volume method in this paper. The annular pool was filled with the low Prandtl ( Pr ) number fluid of Pr = 0.011. The range of Biot number is from 0 to 1. Results indicate that the flow bifurcation route of thermocapillary convection depends intensively on surface heat dissipation. When surface heat dissipation is smaller, with the increase of Marangoni number the flow bifurcation sequence of thermocapillary convection is two-dimensional axisymmetric steady flow → three-dimensional steady flow → coexisting hydrothermal waves and radial moving waves → radial moving waves → chaos. When surface heat dissipation is larger, the bifurcation sequence becomes simple as two-dimensional axisymmetric steady flow → hydrothermal waves → chaos. Furthermore, every bifurcation of thermocapillary convection is always accompanied by the variations of the wave number and the oscillatory frequency. Furthermore, the temperature fluctuation amplitude on the free surface increases gradually with the increase of Marangoni number. [ABSTRACT FROM AUTHOR]

In annular two-phase gas-liquid flow, the liquid film on the wall consists of relatively quiescent substrate regions which are traversed by large amplitude, high velocity waves known as disturbance waves. The turbulent disturbance wave regions have relatively high average heat transfer coefficients (low average wall temperatures) compared to the (probably laminar) substrate regions. Nevertheless, there is evidence that nucleate boiling (necessitating a higher wall temperature) occurs first in the wave regions. This paper explores the hypothesis that wall temperature fluctuations due to turbulence in the disturbance waves are of sufficient magnitude to give localized triggering of nucleation sites and hence nucleate boiling. This hypothesis was explored using Computational Fluid Dynamics (CFD). The turbulence was modelled using wall-resolved LES. The results lend weight to the hypothesis that the nucleate boiling observed in disturbance waves is due to transient local high temperatures induced by the turbulence. [ABSTRACT FROM AUTHOR]

• An annular monitoring device is used on an injection molding machine. • The change in material viscosity leads to a change in viscous dissipation. • Detection of material changes via temperature measurement is possible. • A new viscous dissipation approach is tested for viscosity identification. • Identification results are consistent with the data available in the literature. Recent studies emphasize the importance of monitoring equipment/strategy for energy efficiency and quality control of polymer processing. New devices and new methods are proposed for temperature and viscosity measurements on a polymer production line. In this paper, an annular measuring device is used on an injection molding machine to detect material changes through temperature measurement. When the material becomes more viscous, the melt temperature increases due to the viscous dissipation in the production line. An original inverse method based on the viscous dissipation phenomenon is proposed for viscosity identification using temperature measurements of the annular device. The identification process successfully distinguishes the difference in viscosity between two different polymers, and the identification results are in good agreement with other viscosity curves available from the literature. [ABSTRACT FROM AUTHOR]

Dual-layer pipe dual-gradient drilling technology is an emerging technology for solving the problem of the narrow safety density window in deepwater drilling. The unstable spacer fluid interface in this technology directly affects the dual-gradient pressure system in the annulus, causing changes in the drilling mud performance and affecting the control of bottom hole pressure and rock removal with drilling mud. Therefore, the key to the stable operation of dual-layer pipe dual-gradient drilling technology is to maintain the stability of the spacer fluid interface. Based on this, a seawater-spacer fluid-drilling mud annular flow model was established in this study, with a bottom hole pressure control step of 0.2 MPa, and the spacer fluid height after a single control was used as the evaluation index to study the influence of annular flow velocity, the spacer fluid properties, and the drill string rotation speed on the stability of the spacer fluid interface. The results show that in the determined conditions of the seawater and drilling mud system, the annular fluid flow rate and the physical parameters of the spacer fluid are the main factors affecting the stability of the spacer fluid interface. When the annular fluid flow rate increased within the range of 0.04~0.2 m/s, the liquidity index of the spacer fluid increased between 0.5 and 0.9, the consistency coefficient increased in the range of 0.6 to 1.4 P a ⋅ s n , and the stability of the spacer fluid interface decreased. However, the stability of the spacer fluid interface increased with the increase in its density in the range of 1100~1500 kg/m3. The results obtained in this study can provide a reference for selecting the operating parameters to ensure the stable operation of dual-gradient pressure systems. [ABSTRACT FROM AUTHOR]

The surface wettability is important in the change in the relative permeability of gas and water. Due to the heterogeneous property of coal, it has a mixed wetting state, which makes it difficult to predict the change in permeability. To investigate the influence of different wettabilities on two-phase flow, a total of three different rank coal samples were collected and were treated with different chemicals. The alteration of the coal's wettability, characteristics of gas–water flow, and relative permeability of the coal after the chemical treatments were analyzed. The research conclusions suggest that (1) the coal samples treated with SiO2 and H2O2 increased the hydrophilicity of the coal surface, while the coal samples treated with DTAB increased the hydrophobicity of the coal surface. Compared to SiO2, both H2O2 and DTAB can form a uniform wetting surface. (2) The wettability alteration mechanism among the three different chemical reagents is different. (3) All the chemicals can change the gas–water interface. The water migrates more easily through the cleats after H2O2 treatment, while it is more difficult for the water to migrate through cleats after the DTAB treatment. (4) There are two types of flow states of gas and water on different wetting surfaces. A slug flow is formed on a hydrophilic surface, while an annular flow is formed on a hydrophobic surface. (5) The crossover point and the residual water saturation of the relative permeability curves were influenced by the surface wettability. [ABSTRACT FROM AUTHOR]

An integrated flow model was developed to simulate the fluidization hydrodynamics in a new bubble‐driven gas–liquid–solid fluidized bed using the computational fluid dynamic (CFD) method. The results showed that axial solids holdup is affected by grid size, bubble diameter, and the interphase drag models used in the simulation. Good agreements with experimental data could be obtained by adopting the following parameters: 5 mm grid, 1.2 mm bubble diameter, the Tomiyama gas–liquid model, the Schiller–Naumann liquid–solid model, and the Gidaspow gas–solid model. At full fluidization state, an internal circulation of particles flowing upward near the wall and downward in the centre is observed, which is in the opposite direction compared with the traditional core‐annular flow structure in a gas–solid fluidized bed. The simulated results are very sensitive to bubble diameters. Using smaller bubble diameters would lead to excessive liquid bed expansions and more solid accumulated at the bottom due to a bigger gas–liquid drag force, while bigger bubble diameters would result in a higher solid bed height caused by a smaller gas–solid drag force. Considering the actual bubble distribution, population balance model (PBM) is employed to characterize the coalescence and break up of bubbles. The calculated bubble diameters grow up from 2–4 mm at the bottom to 5–10 mm at the upper section of the bed, which are comparable to those observed in experiments. The simulation results could provide valuable information for the design and optimization of this new type of fluidized system. [ABSTRACT FROM AUTHOR]

The surface morphology of X80 steel with hydrophilic underwater oleophobic characteristic is described greater comprehensively and quantitatively in this work by combining fractal dimension and multifractal. X80 steel with hydrophilic underwater oleophobic surface characteristics was constructed using a chemical etching method. Then, with the aid of three wettability parameters—contact angle, rolling angle, and adhesion work—this study investigated the relationship between the surface fractal dimension of X80 and the stability of the water ring in the core annular flow. The results showed that: (1) the fractal dimension of X80 steel specimens increased first and then decreased with the increase of reaction time. Besides that, the value of it was greater than 2, indicating that the surface had obvious fractal characteristics. The spectral difference, Δf(α), and the spectral width, Δα, supplemented the description of the X80 steel surface morphology, which was consistent with the scanning electron microscope results. (2) When the maximum fractal dimension was 2.0808, the minimum contact angle of distilled water on its surface was 50.2°, and the maximum contact angle of underwater oil droplets was 166.4°. The larger the fractal dimension of X80 steel with hydrophilic underwater oleophobic properties, the more hydrophilic and underwater oleophobic it is. This illustrated that there was a strong binding force between the water and the X80 steel pipe wall, and hence the quality and efficiency of the core annular flow was improved, which was more conducive to the promotion of this technology in the field of heavy oil transportation. [ABSTRACT FROM AUTHOR]

Using a newly developed experimental setup, the features and advantages of an autothermal single-casing atmospheric subbituminous coal fluidized bed air-blown gasifier, combining a combustion and gasification section, and mixing the dispersed phase (inert material, char) and heat exchange between them through an annular transfer device, have been revealed. To increase the efficiency of the gasifier, an experimental-computational method was developed find the conditions for optimal operation, combining changing the annular flow's geometry and regulating the primary air for gasification. A simple and reliable multizone thermodynamic calculation model makes it possible to predict the composition of char and syngas in the gasification section with acceptable accuracy. This method confirmed that a two-section fluidized bed gasifier can provide efficient gasification of solid fuels and is suitable for use in small-scale cogeneration plants. Syngas with a heating value of 3.6–4.5 MJ/m3 and CGE of 38.2%–42.3% was obtained in the experimental setup without optimizing the primary air flow rate. With optimization, the indicators increased to the heating value of syngas of 5.20–5.34 MJ/m3 and CGE of 42.5%–50.0%. With heat regeneration of 0.8, CGE increases to 70%. [ABSTRACT FROM AUTHOR]

In this paper, the feasibility of using an X-ray tube instead of radioisotope sources for measuring volume fractions of gas, oil, and water in two typical flow regimes of three-phase flows, namely, annular and stratified, is evaluated. This study's proposed detection system is composed of an X-ray tube, a 1 inch × 1 inch NaI detector, and one Pyrex-glass pipe to model different volume fractions for two flow regimes, annular and stratified. Group method of data handling (GMDH), a powerful regression tool, was also implemented to analyze the obtained data. The obtained results in this work indicate that a simple system based on an X-ray tube and just one NaI detector could be a potential alternative to radioisotope-based systems for separate measurements of gas, oil, and water volume fractions in annular and stratified flow regimes of a three-phase flow. [ABSTRACT FROM AUTHOR]

This paper presents a series of experiments focused on the displacement of viscoplastic fluids by various Newtonian and non-Newtonian fluids from a long horizontal, eccentric annulus. The flow regimes range from high Reynolds number laminar regimes through to fully turbulent. These experiments represent the primary cementing operation in a horizontal well. The main objective of our experiments is to gain insight into the role of the flow regime in the fluid-fluid displacement flows of relevance to primary cementing. We study strongly eccentric annuli and displaced fluids with a significant yield stress, i.e., those scenarios where a mud channel is most likely to persist. For fully eccentric annuli, the displacements are uniformly poor, regardless of regime. This improves for an eccentricity of 0.7. However, at these large eccentricities that are typical of horizontal well cementing, the displacement is generally poor and involves a rapid "breakthrough" advance along the wide upper side of the annulus followed only by a much slower removal of the residual fluids. This dynamic renders contact time estimates meaningless. We conclude that some of the simple statements/preferences widely employed in industry do not necessarily apply for all design scenarios. Instead, a detailed study of the fluids involved and the specification of the operational constraints is needed to yield improved displacement quality. [ABSTRACT FROM AUTHOR]

• Flow boiling bubble dynamic experiment is carried out with transparent heating wall. • The oscillation after bubble coalescence is divided into violent and moderation region. • A model is proposed to predict the relative velocity at the moment of bubble collision. The behavior of bubble coalescence in subcooled boiling has a significant influence on the void fraction and heat transfer capacity. In this paper, a visual flow boiling experiment is carried out to investigate the phenomenon of bubble coalescence and the effect of sliding bubbles on coalescence. The phenomenon of bubble coalescence, the velocity of bubble and interface evolution characteristics after coalescence are analyzed in detail. The annular waves caused by the rupture of the thick liquid film and the severe deformation caused by bubble coalescence are observed during the experiment. The propagation of annular waves on bubble surfaces makes bubbles unstable and then easily breaks down to produce secondary bubbles. Unlike the secondary bubbles produced by air bubble coalescence, the critical diameter of initial average bubbles that produce secondary bubbles after bubble coalescence is about 0.87 mm. This difference is because the viscosity of the fluid at high temperature in boiling two-phase flow is different from that in adiabatic water-air two-phase flow. Besides, it is found that there is an oscillation adjustment zone after bubble coalescence. This stage is divided into the violent oscillation region and moderation region, and their duration is basically the same. The relative velocity of sliding bubbles during collision is obtained according to the velocity difference between bubbles and the growth rate of the sliding bubble. Also, the effect of sliding bubbles on bubble coalescence is analyzed in this paper. [ABSTRACT FROM AUTHOR]

The paper presents the results of a study concerned with the hydrodynamics of an annular downward multiphase flow of gas and two mutually non-mixing liquids through a vertical pipe with a diameter of 12.5 mm. The air, oil and water were used as working media in this study with changes in superficial velocities in the ranges of jg = 0.34–52.5 m/s for air, jo = 0.000165–0.75 m/s for oil, and jw = 0.02–2.5 m/s for water, respectively. The oil density and viscosity were varied within the ranges of ρo = 859–881 kg/m3 and ηo = 29–2190 mPas, respectively. The research involved the identification of multiphase flow patterns and determination of the void fraction of the individual phases. New flow patterns have been identified and described for the gravitational flow conditions of a two-phase water–oil liquid and a three-phase air–water–oil flow. New flow regime maps and equations for the calculation of air, oil and water void fractions have been developed. A good conformity between the calculated and measured values of void fraction were obtained. The map for the oil–water–air three-phase flow is valid for the following conditions: j3P = 0.35–53.4 m/s (velocity of three-phase mixture) and oil in liquid concentration βo* = 0.48–94% (oil in liquid concentration). In the case of a downward annular oil–water two-phase flow, this map is valid for liquid mixture velocity jl = 0.052–2.14 m/s and βo* = 0.48–94%. [ABSTRACT FROM AUTHOR]

Helically coiled tube (HCT) is widely used in nuclear engineering and other fields due to its effective compactness, excellent structure robustness, heat transfer efficiency and flow stability. As one of the most important gasliquid twophase flow patterns in HCT, the characteristic of annular flow has a great influence on the heat transfer performance of helical oncethrough steam generator. Especially, the circumferential distribution of liquid film thickness in annular flow directly determines the dryout onset location. Although the liquid film flow in straight tubes has been extensively studied, due to the limitation of measurement technology, the precise measurement data of liquid film thickness in HCT are still very rare. In this paper, based on the selfdeveloped noninvasive contact ringisland array sensor (RIAS), airwater twophase flow experiments were carried out to study the threedimensional spatialtemporal distribution of liquid film thickness in vertical HCT with different structures. According to the results, four typical liquid film flow regimes were defined based on the circumferential position of the thicker liquid film appearing in the tube wall. That is, the bottom distribution (BD) dominated by gravity force, the outside distribution (OD) dominated by liquid centrifugal force, the inside distribution (ID) dominated by gas centrifugal force and the insideoutside distribution (IOD) dominated by secondary flow. The effects of coil diameter and coil pitch of HCT as well as gas and liquid velocities on the distribution of liquid film flow regimes were detailed analyzed. It is implied that the coil diameter determines the intensity of centrifugal force and secondary flow. Generally, the smaller the coil diameter, the easier the circumferential distribution forms of OD, ID and IOD are to appear. Also, increasing the coil pitch weakens the effect of centrifugal force on the liquid film. Then a small coil pitch normally results in the circumferential distribution form of OD while it is ID for large coil pitch. It is also found that the IOD occurs only when both gas and liquid velocities are large enough. By taking the influence of HCT geometry and fluid parameters into account, a general submap of liquid film flow in HCT was proposed using the modified Dean number De* and the modified LockhartMartinelli parameter X*. The transition criteria between different liquid film flow regimes were also established and verified with experimental data in present work and previous references. From this work, the mechanism of curvatureinduced centrifugal force and secondary flow on the spatiotemporal evolution of liquid film in HCT is revealed.

A visualization experimental investigation on the condensation heat transfer of refrigerant R410a in a rectangular micro-channel with hydraulic diameter of 0.67 mm is conducted in this paper. Experiments are conducted with saturation temperatures of 36–41 °C, and the refrigerant mass fluxes is in the range of 109–1042 kg/(m 2 s) over the entire range of vapor qualities. One-dimension annular flow model is established to study the condensation heat transfer characteristics inside micro-channel. The condensate film thickness, dimensionless condensation liquid flow velocity, local heat transfer coefficient and average heat transfer coefficient are obtained to analyze the condensation heat transfer mechanism in micro-channel. The predicted value of heat transfer coefficient has great agreement with experimental data in this study. Visualization experimental results agree with condensation flow patterns map in micro-channel proposed by other researcher. Annular flow, wavy-annular flow, slug flow and bubble flow are observed along the flow direction in micro-channel under the low mass flux. Saturation pressure, mass flux, sub-cooled and vapor quality of R410a are considered. The results show that the heat transfer coefficient increases with mass fluxes and vapor quality remarkably. Experiment results are 17% higher than other research results, especially in high mass flux and vapor quality. In addition, the significant influence of sub-cooled is proposed and discussed in micro-channel condensation study. The lower sub-cooled will bring higher heat transfer coefficient. [ABSTRACT FROM AUTHOR]

The present paper investigates void fraction and pressure drop characteristics of vertical air-water downflow through millichannels (0.83 ≤ Eotvos Number ≤ 20.6). Experiments have revealed the hydrodynamics to be a function of tube diameter and phase velocities with the functional form being different for different flow patterns. Accordingly, separate mechanistic models have been proposed to predict the aforementioned parameters for bubbly, slug, falling film and annular flow distributions. The proposed analysis have been validated with experimental results of the present study and those reported in literature. [ABSTRACT FROM AUTHOR]

The paper presents a numerical prediction of reacting flow field and temperature distribution of turbulent diffusion flame in two air blast nozzle swirling flow namely co and counter with different swirl intensities. This quantity is 0.46 for the central flow and ± 1.0 for the annular flow. Current study was focus on the rotation effect of the secondary flow. The calculated results were validated on real time through laser anemometry for both cases. The reasons behind the choice of this issue is that it is realistic in the industry but with a larger scale and our possession of the experimental values to validate the simulation. The use of two flows produced less NOx than a single flow due to its high capacity which homogenizes the temperature in the burner. The obtained results show that LES had successfully predicted the recirculation zones (CTRZ and CRZ) and the PVC with good precision. [ABSTRACT FROM AUTHOR]

Two-phase gas–liquid annular flows are encountered in ventless aero-engine oil system pipes. The droplet size in the flow has an important impact on the performance of downstream equipment as breathers and de-aerators. However, literature studies present semi-empirical models that are not in the range of operating conditions of the oil system. To investigate the effect of the use of lubrication oil on the droplet sizes, this paper presents experimental results of annular flow with oil flow rates from 160 to 640 l/h and air flow rates from 60 to 120 Nm 3 / h . Comparison of the Sauter–Mean Diameter predicted by existing correlation show an error of minimum 30% compared to experimental values for higher oil flow rates, which are the most important in oil systems. To address this issue, correlations were adapted to fit experimental results. With the new set of parameters, the Sauter-Mean Diameter is estimated with an error of maximum 18% for higher oil flow rates. Results analysis illustrate that the main difference between existing and new correlations could be due do the surface tension and viscosity of lubrication oil, which are very different from water at low temperature. The results are also consistent with the transition between bag and ligament break-up droplet generation mechanism at a flow rate of 80 Nm 3 / h . [ABSTRACT FROM AUTHOR]

Measuring the strong local variation in heat transfer coefficients in multi-microchannel evaporators is related to the inverse heat conduction problem (IHCP). As the local flow heat transfer coefficients change greatly in magnitude from single-phase liquid at the entrance to a peak in slug flow and then to a minimum at the transition to the onset of annular flow and finally a new substantial rise up to the outlet, a significant heat spreading occurs due to the heat transfer process itself, and this has to be accounted for when processing the data. Until now, IHCP has not been introduced in the experimental study of heat transfer in such evaporators when reducing local experimental data. In this paper, a new method for processing experimental local heat transfer data by solving the 3D IHCP is proposed. This method is then applied and validated using two sets of single- and two-phase flow experimental data obtained with infrared (IR) camera temperature measurements. The 14 400 raw pixel temperatures per image from the IR camera are first pre-processed by a filtering technique to remove the noise and then to smooth the data, where the IR camera has undergone a prior inhouse pixel by pixel insitu temperature calibration. Three filtering techniques (Wiener filter, spline smooth, and polynomial surface fitting) are compared. The polynomial surface fitting technique was shown to be more suitable for the current type of data set. Then the 3D IHCP is solved based on a finite volume method using the TDMA (Tridiagonal Matrix Algorithm) solver with a combination of Newton-Raphson iteration and a local energy balance method. Furthermore, the present 3D TDMA method (named as 3D TDMA) is compared with three other post-processing methods currently used in the literature, among which the present one is found to be more accurate for reducing the local heat transfer data in multi-microchannel evaporators. [ABSTRACT FROM AUTHOR]

This paper presents an improved model for gas-liquid two-phase flow in the churn and annular flow regimes for small- and large-diameters in vertical and near-vertical pipes. Many investigators consider churn flow to be the least understood flow regime in upward gas-liquid flows in vertical pipes. Nevertheless, this flow regime occurs commonly in several applications in the oil and gas industry, such as: gas-lift operations, production of gas-condensate wells, and two-phase flows with high gas-liquid-ratio in general. Additionally, the accuracy of two-phase flow models for large pipe diameters, irrespective of flow regimes, is still questionable. Only a limited number of studies can be found in the open literature that validates two-phase flow models for pipe diameters larger than 0.203 m (8 in.). The model developed in this study proposes a modification to an approach originally proposed in previous studies for pipe diameters smaller than 0.0508 m (2 in.). These modifications have proven to improve accuracy in the prediction of pressure gradient and liquid holdup for small and large pipe diameters under churn and annular flow conditions. The absolute average error in the prediction of experimental pressure gradient decreases from 155% (for the original model) to 36% (for the modified version). This study validates the proposed model with field and laboratory experimental data from several different studies from the literature, in terms of pressure-gradient and liquid holdup results, for pipe diameters ranging from 0.0318 to 0.279 m (1.25–11 in.), pressures from 1 to 613 bara (14.6–8900 psia), and fluids types such as air-water and oil-natural gas systems. This study also compares its results with other widely accepted models in the commercial packages. When compared to field and laboratory data, the simulation results show that this new model has an overall better performance compared to other models widely used in the oil and gas industry. [ABSTRACT FROM AUTHOR]

This paper describes new measurements on the characteristics of waves in churn and annular flow. The measurements were for air-water flow in an 11 mm diameter vertical tube, a diameter close to the equivalent diameter of nuclear reactor channels. These measurements extend the parameter space of previous publications into the low and gas and liquid mass flow rates important for nuclear reactor fault studies. Detailed measurements of the flow characteristics are reported, including the identification of a new regime we designate ‘pre-annular flow’, in which the liquid film between the waves appears to be stationary. The results of these measurements at low mass fluxes are compared with predictions in this low mass flux region using the principal literature correlations, all of which were actually obtained from elsewhere in the parameter space. Reassuringly, the broad conclusion is that despite this, these correlations obtained from elsewhere are safe to continue to be used in this low mass flux region. A new interpretation of the criteria for onset of entrainment was also obtained from the study. [ABSTRACT FROM AUTHOR]

An experimental study has been carried out to investigate the effects of periodic forced flow oscillations on gas-liquid two-phase flows using a 42 m long, 0.05 m ID vertical pipe system. From the knowledge of the authors, there is no experimental work available in the literature on the forced flow oscillations for churn flow regime. Therefore, the main objective of this paper is to experimentally characterize churn and annular flows in a long vertical tube. The time variation of liquid holdup, pressure drop and pressure gradient were analyzed under two superficial liquid velocities (0.017 and 0.3 m/s), and superficial gas velocities oscillations ranging from 4 m/s to 21 m/s. The impact of flow oscillations in two-phase flow regime transitions is also investigated. The observations of flow regimes under oscillatory conditions showed significant differences compared to the ones expected in steady-state flow under the same conditions. Under oscillatory conditions, the local liquid holdup and the amplitude of its waves decreased with the distance from the inlet. For the experimental runs with oscillatory gas flow and low superficial liquid velocities (U ls = 0.017 m/s), the liquid holdup at the bottom of the tube decreased while the gas flow rate was lowered, whereas in steady-state conditions the liquid holdup should increase while reducing superficial gas velocities. This process is described in this study by the formation of large waves in the bottom of the pipe as a consequence of the flow oscillations. [ABSTRACT FROM AUTHOR]

A linear temporal stability of confined swirling annular liquid layers involving heat and mass transfer at the gas–liquid interface is presented in this paper. A normal-mode stability analysis that includes the effect of both swirling and heat transfer is performed. The flow in a gas-center coaxial swirl injector gives rise to the flow configuration explored in this work. The effects of various non-dimensional parameters on the instability of the flow are discussed. The heat transfer at the interface has been characterized by introducing a heat flux ratio between the conduction heat flux and the evaporation heat flux. The heat and mass transfer at the interface are found to destabilize the flow. Increasing confinement destabilizes the flow, which is opposite to the condition without heat and mass transfer. [ABSTRACT FROM AUTHOR]

• Flow boiling heat transfer of water in a coil was experimentally investigated. • Severe inhomogeneity of circumferential wall temperature existed. • The flow pattern of two-phase boiling flow in the coil was divided into four regions. • Influencing factors on CWTI and heat transfer coefficient were explored and analysed. • A new FBHT correlation in the HCT with better accuracy was proposed. Helically coiled tubes (HCTs) have been widely used in chemical industry and nuclear engineering because of its compact structure and excellent heat transfer performance. This paper designed and built an experimental platform to study the flow boiling heat transfer (FBHT) of subcritical water with high temperature and high pressure in an HCT. The results showed that the wall temperature on the inner side is higher and it gradually decreases when moving to the outer side along the circumferential direction. The variation law of circumferential wall temperature inhomogeneity (CWTI) under different pressures, mass velocities, heat fluxes and vapour qualities was obtained. The distribution of circumferential wall temperature varies greatly in the single-phase region while it varies slightly and is relatively uniform in the two-phase boiling region. As the vapour quality increases, the CWTI decreases first, and maintains at a low value when the vapour quality is about 0.0 ∼ 0.9. When the vapour quality reaches about 0.9, the CWTI increases rapidly. When the vapour quality is less than 0.5, the heat flux and mass velocity have little effect on the CWTI. When the vapour quality is larger than 0.5, reducing the heat flux or increasing the mass velocity can effectively reduce the CWTI. In addition, the flow pattern of two-phase boiling flow in the HCT is divided into four regions, namely bubble flow, slug flow, annular flow and mist flow, and the transition vapour quality x t1 = 0.2, x t2 = 0.5 and x t3 = 0.93 are respectively selected according to the sudden abrupt change of the wall temperature or the differential pressure. Finally, the existing FBHT correlations are collected and evaluated and the calculation accuracy needs to be improved for more accurate calculation. Therefore, a new FBHT correlation with better accuracy is proposed for calculating the heat transfer coefficient of two-phase flow boiling in the HCT. [ABSTRACT FROM AUTHOR]

The present paper discusses and analyzes experiments on the dynamics of hanging cantilever tubular beams conveying fluid downwards and subjected to partially-confined reverse external axial flow. A bench-top-size facility, consisting of a reservoir filled with water, a hanging flexible pipe conveying fluid downwards and a shorter outer rigid tube surrounding the pipe at its upper portion containing an upwards flow, was utilized. A non-contacting optical approach was employed to obtain the displacement time-series of pipe motions from which, using qualitative and quantitative nonlinear dynamics tools, the nature of the motions was explored. The oscillatory motions observed were found to be unsteady with both periodic and chaotic content. The influence of some system parameters on the dynamics, namely, external flow confinement, pipe slenderness, pipe material, placement of a constraint at the external annular flow inlet/outlet, and eccentric positioning of the outer rigid tube relative to the central pipe, was examined. It was found that varying the system parameters in some cases gives rise to quite interesting qualitative or quantitative changes in this Fluid–Structure Interaction (FSI) system. [ABSTRACT FROM AUTHOR]

• Pre-separation is observed in the swirler. • The flow patterns inside the vane-type separator are investigated. • The reason for the formation of swirling chain flow is analyzed. • The effect of the first light phase outlet on the flow behavior is studied. • Multi-stage vane-type separator performs better than single stage separator. The axial separator for gas-liquid separation receives amounts of attention due to its compact geometry. Since the inlet flow pattern does a significant impact on the separation performance of the centrifugal separator, it is necessary to figure out the flow behavior inside the separator and give some guidance to the practical application. In this paper, a vane-type separator was used to investigate the flow behaviors inside the separator at different inlet flow patterns. Pre-separation is found in the swirler, which is caused by the curving flow channel. Swirling chain flow, swirling intermittent flow and swirling annular flow inside the separation chamber are observed in the experimental range. Besides, the reason for the formation of swirling chain flow is explained. The phenomenon of bubble accumulation can be eliminated by opening the first light phase outlet. Then the effect of first light phase outlet was studied further. Additionally, a multi-stage vane-type separator was tested to study the flow behavior in the two swirl chambers. From the experimental investigation, it is suggested that the vane-type separator would better be applied in high liquid velocity and low gas void fraction condition. [Display omitted] [ABSTRACT FROM AUTHOR]

• The swirling flow patterns are 3-D reconstructed in a horizontal pipe. • Detailed information on the liquid film circumferential distribution is given. • The PDFs, CPDFs, time-trace of the liquid film thickness are analyzed. • The periodicity and frequency of the interfacial wave are analyzed by PSD and ACF. A comprehensive understanding of swirling flow liquid film characteristics is key to swirling flow applications. Previous studies lost sight of the swirling liquid film characteristics in horizontal tubes. In this paper, the liquid film characteristics of swirling flow in a horizontal tube with a diameter of 32 mm were investigated using water and air as process fluids. Three inlet flow patterns, including stratified flow, annular flow, and slug flow, are covered. A swirler, with a pitch of 40 mm giving a total of 5 pitches, was utilized to induce swirling flow, which has the advantages of uncomplicated structure, convenient installation and excellent rectification effect. The experimental data was collected via a pair of 16 × 16 conductivity wire mesh sensors (WMS). The results indicate that, in all three inlet flow patterns, the swirling flow liquid film at the pipe top is slightly thicker than that at the pipe bottom, breaking the gravitational effect. The circumferential and temporal distributions of the swirling liquid film thickness become more uniform as gas and liquid velocities increase. With increasing gas velocities, the peaks of power spectral density (PSDs) decrease, i.e., the interfacial wave perturbation is weakened. For inlet slug flow, the swirling flow interface wave disturbance is the largest and exhibits strong periodicity. The period decreases as gas velocities increase. These findings have enhanced the understanding of swirling flow, which provide new insights and scientific guidance for research related to pressure drop and heat and mass transfer in swirling flow. [ABSTRACT FROM AUTHOR]

• Ring island array sensor for liquid film thickness measurement is developed. • Circumferential distribution of liquid film in helically coil tube is obtained. • Sub-flow regime map of annular flow in helically coil tube is proposed. • Prediction model of flow regime transition boundaries is established. The distribution of liquid film thickness in helically coiled tubes (HCTs) directly affects the heat transfer performance of the annular flow region. However, due to the limitation of measurement technology, accurate measurement data of liquid film thickness in HCTs is very rare. In this paper, a non-invasive contact liquid film thickness sensor (ring island array sensor) is successfully developed to obtain the refined three-dimensional spatial-temporal distribution data of liquid film thickness in HCTs. According to the results, four typical liquid film flow regimes are defined based on the circumferential position of the thicker liquid film appearing in the tube wall, i.e., the bottom distribution (BD) dominated by liquid phase gravity force, the outside distribution (OD) dominated by liquid phase centrifugal force, the inside distribution (ID) dominated by gas centrifugal force and the inside-outside distribution (IOD) dominated by the secondary flow. These circumferential distributions of liquid film thickness are mainly affected by the HCT structure (e.g., coil diameter and coil pitch) and superficial gas-liquid velocities. Based on the modified Dean number De * and the modified Lockhart-Martinelli parameter X *, which consider the influence of HCT structures, fluid properties and operating parameters, a general sub-flow regime map of annular flow is proposed. The prediction models of transition boundaries of different liquid film flow regimes are also established and verified with present and available data in the literature. [ABSTRACT FROM AUTHOR]

The paper presents the results of an experimental study of flow and heat transfer in an impinging annular jet. Using the PIV-system the distribution of average and pulsation velocities was measured; and heat fluxes and their pulsations were detected using miniature heat flux sensors. The measurement results have been compared at identical mass flow rate of air with similar data for a round jet with a diameter equal to the outer diameter of the annular jet. It is shown that under these conditions of comparison the values of velocity and turbulent pulsations in the annular jet are significantly higher than the same values in the round jet. The heat transfer intensity of the impinging annular jet is also higher than that of the round jet, and the degree of heat transfer enhancement depends on the annular gap size and distance from the nozzle to the obstacle. [ABSTRACT FROM AUTHOR]

In this paper, the heat transfer rate and the pressure drop of metal porous media inserts are analyzed in the single phase. The testing range of the Reynolds number is between 3654 and 14617. Tap water is used as the working fluid. The effect of particle size and particle shape on the heat transfer performance is studied. The results show that an optimal value existed among the testing samples for dendritic particle. The best heat transfer rate is resulted by the sample D-125 instead of by the sample D-25 with the small particle size, which is approximately 2.2–3.0 times as that of the smooth tube. This is mainly because the velocity at the inlet of inserts increases greatly and the fluid mixing intensity at the annular is also high. Two factors work together to bring about a best heat transfer coefficient. The friction factor increases with the decrease in the particle size. The best Performance Evaluation Criterion (PEC) value is resulted by sample D-125, which is about 1.26–1.71. The particle shape also has an impact on the heat transfer rate. The heat transfer coefficient of the spherical particle is higher than that of dendritic particle. However, the higher heat transfer rate is paid at the cost of high flow resistance. This high flow resistance finally deteriorates the heat transfer performance. As a result, the best heat transfer performance is achieved by the dendrictic particle, which brings a high heat transfer rate as well as a low flow resistance. Therefore, the PEC values have to be taken into consideration in the design of heat exchanger. [ABSTRACT FROM AUTHOR]

A new concept of simulation modeling of dynamic systems is developed. The basic notions and terms of the concept are outlined, as well as the principles of creating a simulation model of a physical process. The proposed concept is applied to the problem of modeling the process of developing a volatile oil formation operated by a rod pump in the pump–drill hole–formation system. The developed computer simulation model, taking into account the presence of the annular space and the fluid flow between the annular space and the lifting pipe, is used to study the operation process. Full filling of the rod pump cylinder does not always coincide with the maximum flow rate of the drill hole. The coincidence of the maxima of filling the cylinder and pump delivery is a condition for the optimal mode. The optimal mode can only be achieved by switching to the periodic mode of operation of the pump. It is established that in order to switch to a periodic mode, a condition is necessary when, at maximum flow, the maximum filling of the pump cylinder cannot be achieved. [ABSTRACT FROM AUTHOR]

A thorough understanding and accurate calculation of the annulus pressure losses are paramount to drilling design and construction. However, due to the influence of the drillstring rotation and the abrupt dimensional variation at the joint, it is difficult to calculate the pressure loss by theoretical analysis in the annulus at the drillstring joints. Considering the geometric nonlinearity of drillstring joints and the unsteady flow of annular fluid caused by pipe rotation, based on the turbulence model k − ε , the flow field characteristics demonstrate the annulus pressure losses at the drillstring joint are studied by numerical simulation. Simulation results indicate that drillstring rotation speed, annular gap, and eccentricity greatly influence flow field and pressure losses at drillstring joints. The annular pressure loss would increase with the decrease of the annular gap and eccentricity but decrease with the increase of the drillstring rotation rate. The investigation method and results would help guide the design of drilling hydraulic parameters. [ABSTRACT FROM AUTHOR]

The Air-Core-Liquid-Ring (ACLR) atomizer is an innovative internal-mixing pneumatic atomization technique, suitable for energy-efficient spray drying of highly viscous liquid feeds, with high solid contents. However, pneumatic atomizers such as the ACLR can suffer from unstable internal flow conditions, which may lead to a wide variation in the droplet diameter obtained. Therefore, the internal flow conditions of an ACLR-atomizer needs to be properly studied and comprehended. With that in mind, a computational fluid dynamic (CFD) model was implemented and tested with experimental data collected for different air pressures and liquid feed viscosities. The model used can predict average lamella thickness with a relative error of less than 10%, when compared to experimental results, although some degree of artificial dampening of the flow instabilities occurs at high viscosities and low pressures. These instabilities have to be investigated in more detail from both the numerical side, by further refining the CFD model to capture the moment-to-moment behavior of the flow, as well as on the experimental side, by studying the instability development at higher recording speeds. [ABSTRACT FROM AUTHOR]

This paper presents the results of analyses of the impact of heat transfer conditions on the hydrodynamics of downward co-current annular flow in vertical tubes of very viscous liquid and gas. The research was conducted within the range of gas velocities of 0–30.0 m/s and liquid velocities of 0.001–0.254 m/s, while the viscosity was in the unprecedented range of 0.046–3.5 Pas. The research demonstrates that the volume and nature of the liquid waves with various amplitudes and frequencies arising on the surface of the film are relative to the flow rate and viscosity of the gas phase. At the same time, we found that, under the condition of liquid cooling, an increase in viscosity resulted in the formation of a smooth interface whereas, under the conditions where the liquid is heated at the end of the channel section, a greater number of capillary waves were formed. This research resulted in the development of new dependencies which take into account the influence of selected thermal and flow parameters (including mass fraction) on the values of volumes specific to very viscous liquid film flows. These dependencies improve the accuracy of calculation by 8–10% and are fully applicable to the description of the performance of an apparatus with a hydraulically generated liquid film. [ABSTRACT FROM AUTHOR]

• Liquid film thickness profiles in gas–liquid annular flow can be reconstructed from stereoscopic X-ray data. • Using X-ray data, the film thickness can be obtained at hundreds of points around the pipe perimeter. • The film thickness detection limit is approximately 30 µm with the presented setup. • The method is applicable to 2-phase or 3-phase systems, to horizontal or inclined pipes, and to steady or dynamic systems. • Experiments have been performed with high-density SF6 gas and Exxsol D60 oil. This paper presents a method for reconstructing the thickness profile of the thin liquid film being present in steady, fully developed gas–liquid annular pipe flow, taking advantage of nonintrusive X-ray measurements. Experiments have been performed with low liquid loading, annular flows, in a near horizontal pipe system (inclination +2.5°), using a high-density gas (SF 6) and Exxsol D60 oil. Special care was taken to ensure the flows were fully developed, and statistically steady. The superficial gas velocity was maintained at a constant value of 10 m/s, while the superficial liquid velocity was varied between 0.5 mm/s and 42 mm/s. The X-ray measurements were supplemented by gamma ray densitometer data and isokinetic data. An explicit mathematical method is presented, whereby the shape and thickness of very thin liquid films can be reconstructed if X-ray images of the flow have been obtained from two independent directions: one with a vertical view of the pipe, and another with a horizontal side view. The method can be straightforwardly applied to cases where the film volume dominates the total volume of droplets suspended in the gas phase. This condition is generally satisfied for the experiments presented in this paper, as verified by isokinetic measurements. Mean liquid hold-up values determined by gamma densitometer are in reasonable agreement with the X-ray results. It is found that the mean film thickness at any position on the pipe wall increases with the superficial liquid velocity. While the film thickness is maximal at the pipe bottom in virtually all experimental cases, its profile exhibits multiple local maxima and minima, with the smallest thickness apparently 20°–30° away from at the pipe top. The presented film reconstruction method is highly sensitive; even at extremely low liquid levels, with a superficial liquid velocity less than 0.01% of the mixture velocity, the film and its thickness profile are clearly detected. An analytical fit for the film profile as a function of perimeter angle and superficial liquid velocity is given. The fit captures the main features of the profile as derived from X-ray measurements, for thin films with maximum thickness less than 5% of the pipe diameter. [ABSTRACT FROM AUTHOR]

This paper reports the results of a study concerned with air−water and air−oil two–phase flow pattern analysis in the channels with open–cell metal foams. The research was conducted in a horizontal channel with an internal diameter of 0.02 m and length of 2.61 m. The analysis applied three foams with pore density equal to 20, 30 and 40 PPI (pore per inch) with porosity, typical for industrial applications, changing in the range of 92%–94%. Plug flow, slug flow, stratified flow and annular flow were observed over the ranges of gas and liquid superficial velocities of 0.031–8.840 m/s and 0.006–0.119 m/s, respectively. Churn flow, which has not yet been observed in the flow through the open–cell foams, was also recorded. The type of flow patterns is primarily affected by the hydrodynamic characteristics of the flow, including fluid properties, but not by the geometric parameters of foams. Flow patterns in the channels packed with metal foams occur in different conditions from the ones recorded for empty channels so gas−liquid flow maps developed for empty channels cannot be used to predict analyzed flows. A new gas−liquid flow pattern map for a channel packed with metal foams with the porosity of 0.92–0.94 was developed. The map is valid for liquids with a density equal to or lower than the density of water and a viscosity several times greater than that of water. [ABSTRACT FROM AUTHOR]

• Novel two-phase flow regime identification technique is presented. • Analysis of local temperature data allows predominant flow regimes to be inferred. • Flow patterns for condensing flows of steam are presented. • Annular flow nearest tube inlet progresses to stratified flow at tube exit. Two-phase flows underpin some of our most ubiquitous technologies, ranging from micro-scale liquid-liquid cooling of electronics to macro-scale liquid-vapour boiling and condensation in thermal power plants. Establishing the morphology of a two-phase flow, under a prescribed set of conditions, is considered particularly important in the design stage. As the pressure loss and heat transfer characteristics of a two-phase flow are intimately linked to the fluidic arrangement, knowledge of the prevailing flow topology enhances understanding, and can lead to the development of flow-specific correlations and/or models. This paper presents a novel experimental measurement technique for identifying the predominant two-phase flow regime in a circular tube. Specifically, the investigation presented in this paper focuses on condensing flows of steam, at typical Rankine cycle cooling conditions. However, it is proposed that the experimental arrangement and methodology can be applied to any two-phase flow scenario. The approach presented herein employs a temperature measurement platform - composed from localised instrumentation - to measure the temperature drop, associated with the presence of a liquid phase, at any point in the tube. Through analysis and interpretation of local temperature difference measurements around the inside tube circumference, and along the tube length, the predominant flow regime can be identified. In this study, measurements were taken from a 25 mm internal diameter round tube, with steam flow rates in the range of 0.42–0.94 g · s - 1. The flow regime was seen to transition from an annular-type profile nearest the tube inlet to a stratified-wavy topology towards the tube exit in all instances. [ABSTRACT FROM AUTHOR]

As a clean, low-carbon, and green hydraulic transportation technology, wheeled capsule pipeline hydraulic transportation is a transportation mode conducive to the sustainable development of the social economy. Based on the method of a physical model experiment and hydraulic theory, the flow velocity characteristics in the pipeline when the wheeled capsule with a length–diameter ratio of 2.5 and 2.14, respectively, was transported in the straight pipe section with an inner diameter of 100 mm were studied in this paper. The results show that in the process of transporting materials, the flow velocity distribution of the cross section near the upstream and downstream section of the capsule was basically the same, and the axial velocity was smaller in the middle of the pipe and larger near the inner wall of the pipe. The radial velocity distribution was more thinly spread near the pipe wall and denser near the center of the pipe. The circumferential flow velocity was distributed in the vicinity of the support body of the wheeled capsule. For any annular gap section around the wheeled capsule, the radial velocity of annular gap flow was very small, and the average radial velocity of annular gap flow was about 1/30 of the average axial velocity of annular gap flow and about 0.7 of the average circumferential velocity of annular gap flow. The axial, circumferential, and radial flow velocities on the same radius measuring ring changed with the polar axis in a wave pattern of alternating peaks and troughs. These results can provide the theoretical basis for optimizing structural parameters of the wheeled capsule. [ABSTRACT FROM AUTHOR]

In this paper, a nonlinear theoretical model is developed for the dynamics of a flexible cantilevered pipe that is simultaneously subjected to internal and partially-confined external axial flows. The pipe under consideration discharges fluid downwards, which accumulates in a relatively large tank, and then flows upwards through a generally shorter annular region surrounding the pipe. Thus, the internal and external flows are interdependent and in opposite directions. A practical application of this system may be found in solution mining processes for brine production, and in the subsequent usage of the salt-mined caverns for hydrocarbon storage. The equation of motion is derived using the extended Hamilton's principle to third-order accuracy with a separate derivation of the fluid-related forces associated with the internal and external flows. The equation is discretized using Galerkin's scheme and solved via the pseudo-arclength continuation method and a direct time integration technique. Two pipes of different dimensions and materials are considered in this study; the stability of these pipes is investigated with increasing flow velocity. Also, the influence of varying the length and tightness of the annular region on the dynamical behaviour of the pipes is explored theoretically. The predictions of the proposed model are compared to experimental observations from the literature for systems with the same parameters as those considered in this paper, as well as to predictions of an earlier linear theory. The results obtained are in excellent qualitative and good quantitative agreement with the experimental observations. Furthermore, this model predicts the frequencies of oscillation more accurately than linear theory. • The dynamics of a pipe subjected to internal and external axial flows is examined. • The pipe loses stability via second-mode flutter at high enough flow velocity. • The amplitude and frequency of oscillation increase with increasing flow velocity. • Increasing the length or tightness of the annulus generally destabilizes the system. • For sufficiently long annuli, decreasing the tightness leads to first-mode flutter. [ABSTRACT FROM AUTHOR]

• New ultrasonic method is developed for disturbance wave measurement. • Experiments under pressures are conducted in horizontal wavy and annular flow. • Experimental results are compared and analyzed with existing predicted correlation. • A new wave velocity correlation applied to different system pressures is proposed. Gas-liquid interface shows obvious disturbance characteristics in horizontal wavy and annular flow. The study of the characteristic parameters of interfacial disturbance wave is significant for understanding the gas-liquid two-phase flow. In this paper, using the ultrasonic echo resonance main frequency (UERMF) method, the non-contact measurement experiments are carried out to measure the liquid film thickness of gas-liquid wavy and annular flow in a 50 mm diameter horizontal pipe. Then the disturbance wave velocity is obtained by cross-correlation method under 220 different experimental conditions for pressures ranging from 0.1 MPa to 0.7 MPa, the superficial velocity of gas and liquid ranging from 10 m/s to 35 m/s and 0.006 m/s to 0.6 m/s, respectively. The experimental results are compared with existing predictive wave velocity correlations such as Marmottant correlation, Schubring correlation, etc. As a result, the existing correlations have certain limitations on reflecting the disturbance wave variation in both wavy and annular flows, and are also not applicable to high pressures. Considering all these factors, a new modified disturbance wave velocity correlation is proposed which satisfies horizontal wavy, annular flow and high pressure system. Besides, the new correlation is compared with all the experimental data in this paper and existing correlations in the references. The mean absolute error (MAE) is less than 15%, which verifies the extrapolation and applicability of the new interfacial disturbance wave velocity correlation. [ABSTRACT FROM AUTHOR]

The void fraction and interfacial area concentration are important phase interfacial parameters in two-phase flow. Accurate acquisition of void fraction and interfacial area concentration under churn flow and annular flow in a narrow rectangular channel is the key to construct and improve the two-fluid model. In this paper, a visualization experiment was conducted to study the flow characteristics of gas-liquid two-phase flow in a narrow rectangular channel with a cross-section of 65 mm×2 mm. The range of gas-phase superficial velocity is 1.9 m/s and liquid-phase superficial velocity is 0.1-1.5 m/s. Flow pattern includes churn flow and annular flow. Based on the idea of gas-phase splitting calculation, an analytical calculation method to obtain the void fraction and interfacial area concentration under the churn flow and annular flow was proposed using high-speed camera. And the calculation method for the local parameters in the direction of the wide edge of the narrow rectangular channel was also proposed. For the two-phase flow video under each experimental condition, picture extraction and data processing were performed at 0.1 s time interval, and the duration of data processing is 60 s for each condition. The relative deviation between the calculated void fraction of the empirical formular and experiment value in the narrow channel is within 10%. The correlation coefficient K between interfacial area concentration and void fraction was proposed. As the void fraction increases, the value of K gradually converges to 1 from greater than 1. This result is consistent with the theoretical analysis and verifies the rationality of interfacial area concentration calculation method. To verify the accuracy of the local parameter calculation method, the experimental images were divided into 10 equal parts along the broad side of the narrow rectangular channel. The relative deviation between the average values of the local void fraction and interfacial area concentration and the results calculated without partitioning is within 5%. The calculation method can be helpful for the study of phase interfacial parameters in narrow rectangular channels under complex flow patterns.

The main aim of this study is to experimentally investigate two-phase flow patterns during flow boiling of R245fa in a micro channel array. The array consists of 10 rectangular micro channels connected in parallel, each having a hydraulic diameter of 1 mm. Base area of the channels is 14.5 x 180 mm. Micro channels without and with 0.5 mm wide inlet restrictions are examined to determine their influence on the flow. Four two-phase flow patterns are observed using a high-speed camera: bubbly, slug, churn-annular and annular flows. Two former are grouped together as intermittent flow, and two latter as annular flow. In the paper a special focus is put on the determination of the transition vapor quality corresponding to the change from intermittent to annular flow as well as how it is influenced by heat flux, mass flux and saturation temperature. Tested heat flux ranges from 30 to 50 kW/m 2 , mass flux from 400 to 1000 kg/m 2 s, examined saturation temperatures are 49, 63 and 82 ° C and inlet subcooling varies between 13 and 19.4 K. It was found that transition vapor quality increases at lower mass flux, higher heat flux or saturation temperature. Coupling data on two-phase flow patterns with the data from our previous study on heat transfer (Halon et al., 2022) shows that flow patterns do not determine the dominant heat transfer mechanism. Intermittent flow occurs both in forced convection and nucleate boiling dominated zones, and annular flow, typically associated with the domination of forced convection, accompanies nucleate boiling dominated heat exchange. Micro channels with restrictions exhibit similar behavior, but transition vapor qualities shift to higher values. Comparison with four recognized models for transition vapor quality shows that it is best predicted by the model of Costa-Patry and Thome (2013) with M A P E of 47.9% calculated for both studied geometries. • Two-phase flow patterns during flow boiling in micro channels are investigated. • Two micro channel geometries are tested: without and with inlet restrictions. • The focus is on transition from intermittent to annular flows. • Transition shows dependence on heat flux, mass flux and saturation temperature. • The link between flow patterns and the dominant heat transfer mechanism is analyzed. [ABSTRACT FROM AUTHOR]

The gas–solid particle two-phase flow in a fluidized bed shows complex physics. The multi-scale algorithm composed of the coupled gas-kinetic scheme (GKS) for the gas phase and unified gas-kinetic wave-particle method (UGKWP) for the solid particle phase is developed for three-dimensional fluidized bed study. For the solid-particle phase, different from the widely-used Eulerian and Lagrangian approaches, the UGKWP unifies the wave (dense particle region) and discrete particle (dilute particle region) formulation seamlessly according to a continuous variation of particle cell's Knudsen number (Kn). The GKS-UGKWP for the coupled gas-particle evolution system can automatically become an Eulerian–Eulerian (EE) method in the high particle collision regime and Eulerian–Lagrangian (EL) formulation in the collisionless particle regime. In the transition regime, the UGKWP can achieve a smooth transition between the Eulerian and Lagrangian limiting formulation. More importantly, the weights of mass distributions from analytical wave and discrete particle are related to the local Kn by (1 − exp (− 1 / Kn)) for wave and exp (− 1 / Kn) for discrete particle. The UGKWP provides an optimal modeling in capturing the particle phase in difference regimes with the full consideration of physical accuracy and numerical efficiency. In the numerical simulation, the UGKWP does not need any prior division of dilute/dense regions, which makes it suitable for the fluidized bed problem, where the dilute/transition/dense regions instantaneously coexist and are dynamically interconvertible. In this paper, based on the GKS-UGKWP formulation two lab-scale fluidization cases, i.e., one turbulent fluidized bed and one circulating fluidized bed, are simulated in 3D and the simulation results are compared with the experimental measurements. The typical heterogeneous flow features of the fluidized bed are well captured and the statistics are in good agreement with experiment data. • A multi-scale GKS-UGKWP algorithm is constructed for gas-particle two phase flow. • GKS-UGKWP connects Eulerian–Eulerian and Eulerian–Lagrangian methods smoothly. • This method is validated in turbulent fluidized bed simulations. • The core-annular flow structures in circulating fluidized bed are predicted. [ABSTRACT FROM AUTHOR]

The alternating appearance of elongated bubbles and liquid slugs of slug flow in the pipe causes severe pressure fluctuation. As a result, measuring the flow rate of the slug flow with the throttling unit based differential pressure method is difficult. This paper investigates a new swirler-based flow measurement method in slug flow. The swirler converts the slug flow into a swirling annular flow, and the differential pressure method is used to measure the flow rate. The influences of gas and liquid flow rates on the differential pressure ΔP X across the swirler as well as its downstream axial differential pressure ΔP Z are investigated. ΔP X 0.5 increases linearly as the liquid mass flow rate increases, and the slope of the curve increases as the gas mass flow rate increases. The influence of gas mass flow rate on ΔP X 0.5 is comparable to that of liquid mass flow rate on ΔP X 0.5. ΔP Z 0.5 increases linearly with increasing gas/liquid mass flow rate, and the slope of the curve of ΔP Z 0.5 with m l differs slightly from the slope of the curve in single-phase water conditions. Based on the research presented above, new empirical correlations of mass flow rate based on ΔP X and ΔP Z are established respectively. The superficial liquid velocity ranges from 0.6 to 2 m per second, while the superficial gas velocity ranges from 2 to 6 m per second. If the gas mass flow rate and ΔP X are known, the relative error of liquid mass flow is less than 3%. The relative error of the gas mass flow rate is less than 10% if the liquid mass flow rate and ΔP X are given. The calculation accuracy of the flow measurement model using ΔP X is better than the calculation accuracy of the flow measurement model using ΔP Z. • A new flow measurement method based on swirler in slug flow is studied. • Correlation of gas-liquid flow rates based on differential pressure is established. • The calculation accuracy of differential pressure signals is evaluated. [ABSTRACT FROM AUTHOR]

The annular flume is an ideal hydrodynamic test device for studying river sediment, and it has been widely used in recent years to study the movement patterns of sediment and other particulate matter. Annular flumes have made outstanding contributions to research in fields related to sediment transport and the diffusion and migration of pollutants. The existence of circumfluence structures in annular flumes leads to complex and variable flow structures. To obtain a more stable and controllable water flow structure, a sophisticated three-dimensional mathematical model based on the Fluent software was established to study the development law of water flow structure in the flume by changing the size of the annular flume speed ratio. The results show the following: (1) The overall trend of the simulation results basically matched with the measured results; the average relative error was 3.54% and the Nash efficiency coefficient was 0.9934, close to 1. The model calculation data were highly credible. (2) The axial flow velocity of the water tank gradually showed a "U"-shape distribution with the increase in the speed ratio. (3) When the speed ratio was R ≤ 0.17 (where the speed ratio R refers to the ratio of annular groove to shear ring speed), there was only one vortex in the tank; when the speed ratio was R > 0.17, there were multiple vortices in the tank, and the flow pattern was more complicated. (4) When the rotational speed ratio R = 0.28, the secondary flow intensity of the annular flume reached the lowest point, which was only 39.28% of the secondary flow intensity of the conventional annular flume. (5) It was determined that the annular flume water flow structure was most stable and controllable when the rotational speed ratio R = 0.24. The results of the study can provide a further theoretical basis for research on sediment dynamics and its related fields conducted by applying an annular flume. [ABSTRACT FROM AUTHOR]

Wet friction clutch is the key functional component of the high-speed helicopter variable-speed transmission system, which is used to change the power transmission path. In the engagement process of wet friction clutch, the driving/driven disc will produce drag torque under the shearing of lubricating oil, which reduces the transmission efficiency. This unnecessary drag torque reduces efficiency and increases clutch temperature. The temperature increase promotes the wear of gears and bearings and the aging deformation of friction plates, which leads to local wear and reduces the service life of the clutch. From the principle of wet friction clutch, the oil groove structure is directly related to the drag torque and the temperature rise of friction disc. It is very important for the long-distance flight and service life of high-speed helicopters to obtain the groove structure with low drag torque and low temperature rise. In order to solve this problem, taking the wet friction clutch of a high-speed helicopter as the research object, based on the radial and annular compound groove, the thermal-fluid-solid coupling simulation model of the wet friction clutch is established to obtained the flow characteristics and temperature field distribution of the lubricating oil in the friction disc oil groove, and to analyze the influence law of the oil groove structure parameters on the drag torque and temperature field. In order to improve the transmission efficiency and the service life of friction disc, Taguchi experiment and non-dominated neighborhood immune algorithm were used to optimize the structural parameters of the oil grooves. The comparison results show that the optimized structural can effectively reduce the drag torque and the temperature rise. This work can provide a theoretical reference for the structure design of a wet friction clutch. [ABSTRACT FROM AUTHOR]

The nanosecond pulsed discharge in a gas–liquid mixture of a Venturi tube with the injection of oxygen was numerically and experimentally investigated. It was found that the density of the gas–liquid mixture close to the electrode at the water inlet is significantly lower than that at the water outlet. The lowest void fraction close to the water outlet is higher than 50%. When the oxygen flow rises from 0.5 SLPM to 3 SLPM, the flow mode changes from the bubble flow to the annular flow. If the applied voltage is kept at 45 kV in amplitude, the probability of the breakdown increases with the water flow and it reaches 100% as the water flow rises to 9.5 L/min. Since the pulsed breakdown voltage changes from shot to shot under the same experimental conditions, the distribution of 200 breakdown voltages was measured and the median in the distribution of the probability density, defined as U50, was determined. The curve of the U50 as a function of the oxygen flow takes the shape of "V," similar to that of the Paschen curve obtained from the gas breakdown. An abnormal polarity effect in the breakdown voltage was observed, and the reason was given. By combining the results from the experiment with the numerical simulation, the spatial distribution of the reduced field at the time of breakdown was determined. The oxygen flows not higher than 0.5 SLPM are the better choice for getting a reduced field higher than 500 Td. [ABSTRACT FROM AUTHOR]

To guarantee the transporting efficiency of microdevices associated with fluid transportation, mixing, or separation and to promote the heat transfer performance of heat exchangers in microelectronics, the hydrodynamic behaviors at unsteady and steady states, as well as the thermal characteristics at the steady state in a pressure-driven electrokinetic slip flow of power-law fluid in a microannulus are studied. To present a more reliable prediction, the slip phenomenon at walls and nonlinear rheology of liquid are incorporated. The modified Cauchy momentum equation applicable to all time scales and energy equations, are analytically solved in the limiting case of a Newtonian fluid and numerically solved for power-law fluids. The transient velocity profile, time evolution of flow rate, temperature profile, and heat transfer rate are computed at different flow behavior indices, electrokinetic width, slip lengths, and Brinkman numbers, thereby, the coupling effect of nonlinear rheology, slip hydrodynamics, and annular geometry on flow and thermal behaviors is explored. The unsteady flow takes a longer time to achieve the steady state for shear thinning fluids or greater slip lengths. The flow behavior index and slip length play a significant role in the flow rate and heat transfer performance. The relevant discussion can serve as a theoretical guide for the operation and thermal management of annular geometry-related flow actuation systems. [ABSTRACT FROM AUTHOR]

Purposes: To explore the value of Microflow Imaging (MFI) in renal solid tumors. Methods: A total of 195 patients with 199 lesions pathologically confirmed masses were retrospectively analyzed. The 199 masses were divided into the tumor ≤ 4 cm group (n = 104) and tumor > 4 cm group (n = 95). The diagnostic efficacy of Color Doppler Flow Imaging (CDFI), Power Doppler Imaging (PDI) and MFI in renal tumors sizes were compared by determining the Adler grade, vascular morphology and peripheral blood flow. Results: Among 199 tumors, 161 lesions were malignant and 38 lesions were benign. MFI in malignant tumor ≤ 4 cm demonstrated statistically significant differences in Adler grade and vascular morphology as compared to CDFI and PDI (p < 0.05). In malignant tumor > 4 cm group, MFI showed significant difference in vascular morphology compared with CDFI (p < 0.05). MFI showed a significant difference in the peripheral annular blood flow of malignant tumors when compared to CDFI and PDI (p < 0.05). In addition, the malignant tumors of the two sizes by MFI in peripheral annular blood flow detection showed significant difference (p < 0.05). The area under the curve of ROC by MFI in the tumor ≤ 4 cm was 0.771, which was higher than CDFI and PDI (p < 0.05), but no obvious difference among the tumor > 4 cm (p > 0.05). Conclusion: MFI provides a new method for the differential diagnosis of small renal carcinoma. Based on the convenience and non-radiation of MFI, we can choose MFI as an imaging diagnostic tool for patients who need long-term active surveillance (AS) follow-up. [ABSTRACT FROM AUTHOR]

In natural gas wells, liquid loading is a severe problem threatening production safety. Published studies have verified that liquid loading is closely related to film reversal in gas–liquid annular flow, but the evolution of the film reversal is still unclear. This article reports on experiments conducted to reveal the film reversal evolution characteristics. Experiments were conducted in a 50-mm-diameter vertical pipe with superficial gas velocities ranging from 5.66 to 22.64 m/s and superficial liquid velocities ranging from 0.014 to 0.170 m/s. A camera and electrical resistance tomography were used to obtain qualitative and quantitative results, and an error analysis verified the experiments' repeatability and reliability. The evolution process is divided objectively into three stages to clarify the film reversal: no film reversal (No-FR), the onset of film reversal (Onset-FR), and complete film reversal (Complete-FR). The three stages occur successively with decreasing gas velocity. The characteristics of the individual stages are elaborated, including the interfacial structures, morphological features, and motion trajectories. The void fractions are analyzed in both the time and frequency domains, where the statistical parameters of the probability density function, average value, and standard deviation are presented. The results show that the Onset-FR stage exhibits characteristics similar to both the No-FR and Complete-FR stages, indicating that it plays an intermediate role in the gradual evolutionary process. The current experimental results also achieve excellent agreement with published datasets and correlations. [ABSTRACT FROM AUTHOR]

This work is devoted to the numerical study of particle dispersion prediction of gas–solid coaxial jet with a larger concentration difference. In pulverized coal gasifier, the pulverized coal is very dense at the nozzle outlet, and its volume fraction can reach 0.3. Then, it is dispersed rapidly under the action of high‐speed annular gas, and the particle volume fraction in the space is less than 0.002. In order to better predict the motion characteristics of a gas–solid jet with a high concentration gradient, the applicability of the three models is compared. The results show that the dense discrete phase model (DDPM) and Eulerian two‐fluid model (TFM) models considering particles collision can accurately predict the dense jet flow at low annular gas velocity. The DDPM and discrete phase model (DPM) show a good simulation for the dispersion characteristics of particles at high annular velocities where the particles' collision could be ignored. Therefore, DDPM has better adaptability for coaxial jet with large concentration gradient. The DDPM was used to predict the particle velocity and concentration for different annular gas velocities and different particle mass loads. It is found that particle flow is contracted first and then dispersed gradually under the action of airflow. The particle dispersion range increases with the increase of solid loading rate, and the corresponding radial distribution of particle velocity is greater. [ABSTRACT FROM AUTHOR]