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Supercritical water, known for its abrupt changes in fluid properties near the pseudocritical region, has always posed a design challenge, leading to enhancement or deterioration in heat transfer. Additionally, the lack of complete understanding of the thermodynamic behavior of supercritical water near the pseudocritical zone, unlike in the subcritical zone, highlights the necessity of a systematic study. Hence, this paper focuses on characterizing the convective upward flow and heat transfer characteristics inside a vertical smooth annular square channel. The numerical analysis uses a steady-state, pressure-based, compressible, Newtonian solver with body force term and two-equation turbulence model to study the influence of variation in pressure, heat flux, and heat-to-mass flux ratios. The flow characteristics and heat transfer behavior of deteriorated, normal, and enhanced heat transfer modes are studied. The heat transfer deterioration is suppressed with the increase in pressure; however, it increases with the increase in inlet bulk temperature. The supercritical boiling number that determines the heat transfer mode is calculated and analyzed for the square annulus section. The evolution of the flow in different heat transfer modes differs based on the effect of buoyancy on the velocity field and the turbulence. The calculated Nusselt numbers match the existing Jackson and Hall correlation well with a maximum absolute error of 9.91%. [ABSTRACT FROM AUTHOR]

Interactions of heat transfer enhancement and fouling can occur due the change in nature of the surface, operating condition, or the fluid itself. Interactions arising relating to higher clean heat transfer coefficients, and higher wall shear stresses of the enhanced surfaces are discussed in this paper. On the lab-scale, enhancement was often achieved by static elements, such as wire wrapping on a heated rod in an annular flow. Lab-scale tests of necessity suffered from effects of relatively short duration of runs, and re-circulation of a batch of liquid with the danger of changing the fluid composition during experiment. Nevertheless, initial fouling rate results consistent with expected trends could be achieved for hydrocarbon fouling, and particulate fouling. There is a continued need to better represent and compare performance of plain and enhanced surfaces over prolonged operating periods in industrial heat exchangers, using commercial enhancement technologies. Example case studies are discussed which demand different methods of comparing performances. A daily averaged cost analysis is identified as a possible systematic approach on the evaluation of the economic benefit of the use of enhancement devices. This is illustrated for a case study example where tube insert is used as an enhancement option. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of heat transfer coefficient and dryout inception during flow boiling of R134a in a circular channel with an internal diameter of 500 μ m for horizontal, 45° inclined, and vertical upward flows. The microchannel was heated by imposing the wall temperature by flowing hot water counter-currently to the test fluid through an annular region containing the test section. Experiments were performed for mass velocities ranging from 350 to 600 kg/(s·m2), heat fluxes up to 46 kW/m2, saturation temperature of 30 °C, and vapor qualities from 0 to 1. The experimental data were parametrically analyzed, and the effects of the experimental parameters (heating method, mass velocity, heat flux, and channel orientation) were identified. Almost similar heat transfer behaviors were found under conditions of imposing wall temperature and heat flux through the Joule effect. The effect of the flow orientation on the heat transfer coefficient prior to the dryout inception was negligible. The heat transfer coefficient increases with increasing vapor quality, heat flux, and mass velocity. The critical heat flux exhibited similar values independently of the flow orientation. Moreover, the critical heat fluxes increases as the dryout inception vapor quality reduces, regardless of the flow orientation. [ABSTRACT FROM AUTHOR]

Arc discharge machining was an efficient method in machining difficult-to-cut materials. However, there was a serious overcutting phenomenon during machining, causing serious machining errors. In order to improve the machining accuracy, this paper presented a novel kind of arc discharge milling technology, which was based on the arc driving mechanism using the controllable non-uniform flow field around the tubular electrode. The control system changed the distribution of the non-uniform flow field in real time according to the machining path, driving the arc to extend in a controlled direction and reducing the repeated erosion of the specific machining surface. Compared with the annular flow field, the non-uniform flow field could reduce the overcutting by 52.15%, while obtaining similar relative electrode wear ratio (REWR) and material removal rate (MRR). The effects of non-uniform flow field, peak current of pulse generator, and pulse width on machining accuracy, REWR, and MRR were investigated. The result shows that the non-uniform flow field can significantly improve the accuracy of arc discharge machining. [ABSTRACT FROM AUTHOR]

To fully realize the advantages of the two-fluid model, accurate prediction of the interfacial area concentration (IAC) is indispensable. Since conventional flow regime-based IAC correlations are not capable of dynamically describing the evolution of interfacial structure, the interfacial area transport equation (IATE) was developed to close the two-fluid model. In the past 30 years, intensive efforts have been made to improve the prediction performance of IATE and extend the experimental database for the IATE benchmark. Recent efforts of the IATE development and benchmark conducted by the Thermal-hydraulics and Reactor Safety Laboratory at Purdue University are reviewed in this paper. This review covers (1) the development of IATE; (2) the experimental database for IATE modeling, including instrumentation development, local measurement data of adiabatic/diabatic two-phase flow, and annular flow characterization; and (3) implementation and evaluation of IATE in one-dimensional/three-dimensional scenarios. Significant progress has been achieved since 2009, and future works required to advance the modeling of IATE are also suggested. [ABSTRACT FROM AUTHOR]

While most critical heat flux (CHF) studies focus on relatively straightforward geometries, actual equipment applications are complex. This paper summarizes literature which provides insights into the effects of selected inlet conditions on CHF and the occurrence of upstream CHF. Vertical smooth bore tube CHF tests found that the concept of "local conditions hypothesis", though not universally defensible, provides an adequate basis for applying CHF data to design. Upstream CHF indications have been found to be associated with a minimum in the CHF versus quality curve which has been attributed to the transition from intermediate flow regimes to annular flow regimes. Design implications are discussed including: 1) for vertical smooth bore tubes, upstream CHF is generally not observed except at very high masses where specific data are required - at lower mass fluxes, special design procedures are needed where nonuiform circumferential heating is present, 2) for inclined smooth bore tubes, design specific data are needed as upstream CHF observations are more prevalent, 3) for vertical and inclined multi-lead ribbed bore tubes (common in the power industry), minimum mass fluxes are specified to avoid upstream CHF, and 4) coiled and serpentine tubes are discussed. [ABSTRACT FROM AUTHOR]

This document is a special issue dedicated to Professor John Richard Thome on his 70th birthday. It contains nine articles written by his former colleagues and friends from various countries, covering topics such as green hydrogen energy systems, two-phase flows, and heat transfer. Professor Thome is a renowned expert in the field and has made significant contributions through his research, teaching, and engineering applications. He has published numerous papers and authored several books. The document also provides information about Professor Thome's background, career, and achievements. [Extracted from the article]

As part of its design certification effort, NuScale Power has completed a series of low mass flux [<1000 kg/(m2‧s)] critical heat flux (CHF) tests for a wide range of pressures at Stern Laboratories in Canada. Earlier studies have demonstrated that under annular flow conditions, disturbance waves with circulating vortices traverse the rod surface. The disturbance wave slides over and significantly influences energy transport in the co-current vapor-liquid sublayer at the heater interface. This paper describes the mechanisms leading to the onset of CHF in a vertical rod bundle experiencing annular mist flow conditions. The paper presents a new CHF model that implements a local disturbance wave velocity. A comparison of the model to the U1 CHF data set from Stern Laboratories shows excellent agreement over the full range of annular mist flows, pressures, and subcooled conditions for the specific spacer grids implemented in the study. [ABSTRACT FROM AUTHOR]

The technology of heavy oil transportation by core-annular flow (CAF) is attracting more and more attention. In this paper, the flow of oil-water annular flow in a 19 mm horizontal pipe in the presence of a CAF generator is studied by using the Volume of Fluent (VOF) model in Fluent 14.5. A detailed study of oil-water flow in the CAF generator device and effects of variation of gap thickness is analyzed, and the pressure distribution of CAF after passing through the CAF generator is discussed. The phase distribution obtained by numerical calculation agrees well with the experimental results. The influence of spiral blade on oil-water annular flow in CAF generator device is also analyzed. The results provide a suitable reference for the formation of CAF. [ABSTRACT FROM AUTHOR]

Magnetostrophic dynamos are studied in an annular core, adapting the seminal work of Taylor [The magnetohydrodynamics of a rotating fluid and the Earth's dynamo problem. Proc. R. Soc. London A. 1963, 274, 274] for a fluid-filled core. The model consists of an inviscid fluid core and a concentric solid inner core. The fluid is supposed to obey the Boussinesq equations of motion and is driven into motion by a flow-forcing function consisting of the buoyancy force of an adverse radial temperature gradient, opposed by the Lorentz force of the self-sustained magnetic field. Coriolis forces act but inertial and viscous forces are ignored. Taylor (1963) showed how such a "magnetostrophic dynamo" can be found when there is no solid inner core, but his ideas have to be non-trivially generalised when an inner core is present. That is undertaken in this paper. In the 1993, CUP book, "Theory of Solar and Planetary Dynamos", Hollerbach and Proctor gave examples in which the zonal flow created by a specified flow-forcing function may be singular on the "tangent cylinder", an imaginary cylinder tangential to the inner core and parallel to the polar axis. It is shown here how this singularity is related to the flow-forcing function, and how discontinuities of other components of the fluid velocity on the tangent cylinder are determined by that function. In appendix A, an identity is established between the leading terms in the Fourier expansion of two of the cylindrical components of an arbitrary vector field. In appendix B, eight examples are given relevant to annular dynamos. In appendix C, equatorial symmetry is considered. [ABSTRACT FROM AUTHOR]

To find the influence of the suction chamber and throat inlet profiles on the cavitation performance of an annular jet pump (AJP), a numerical method using a combination of the realizable k–ϵ turbulence model and the Schnerr–Sauer cavitation model was applied. This paper comprehensively studied the cavitation performance of AJPs with a polygonal line profile (POL) and a streamlined profile (STR). The results showed that the pump efficiency (η) can be increasedby using the STR and the largest increase was 1.4%. Since the throat is smoothly connected with the suction chamber in the AJP with STR, flow separation and local low pressure did not appear. The critical flow rate ratio (qc) of the AJP with STR was higher than that of the AJP with POL, implying that the former had a wider normal working range. During critical working, the cavitation region in the AJP with STR was narrower than that of the AJP with POL. Importantly, the region of cavitation inception and development in the AJP with STR was closer to the downstream. Hence, it was concluded that the cavitation in the AJP with STR had less influence on the inner flow field. [ABSTRACT FROM AUTHOR]

This paper presents an algorithm for accurately solving the full two-dimensional governing equations, along with the interface conditions that govern laminar/laminar annular/stratified internal condensing flows. The simulation approach - which can be generalized to adiabatic and evaporating flows, a 3-D level-set technique, and so on - uses a sharp-interface model, separate liquid and vapor domain computational solutions with interface conditions embedded as boundary conditions, and a moving grid technique to locate the dynamic wavy interface (in amplitude and phase) by amethod of characteristicssolution of the interface tracking equation. The moving grid is spatially fixed for a defined number of instants, but changes when the current marker instant advances in time. [ABSTRACT FROM PUBLISHER]

Flow boiling heat transfer in microchannels is used today in many diverse applications. The previous studies addressing the effect of channel size, heat flux, vapor quality, and mass flux on heat transfer during flow boiling are reviewed in the present paper. The relationship between flow characteristics and flow boiling heat transfer was studied experimentally for refrigerant R-C318 at moderate reduced pressures where the contribution of nucleate boiling is decisive. Flow boiling mechanisms were identified using an annular microchannel with transparent outer wall for successive visualization of boiling. The considerable suppression of nucleate boiling heat transfer was observed at transition to annular flow and explained by formation of a liquid flow with thin film and dry spots. A general equation for prediction of two-phase flow boiling heat transfer inside the circular, annular, and rectangular microchannels is proposed and verified using the experimental data. This equation accounts for the nucleate boiling suppression, forced convection, and thin film evaporative heat transfer in the form that allows to distinguish more clearly the contribution of each mechanism of heat transfer under the conditions, when it is predominant. A new approach for prediction of transition to the annular flow is proposed and verified, using the experimental data. [ABSTRACT FROM AUTHOR]

An experimental campaign is conducted to investigate the pressure drop during the restart of a heavy oil-water core-annular flow (CAF) from a stratified configuration in a polyvinyl chloride (PVC) horizontal pipe. The evolution characteristic of restart pressure drop along time is explored, and the effects of various factors including oil holdup (0.26-0.76), oil viscosity (1.0553-3.02 Pa·s), standstill period (0.5 & 1.0 h) and water cleaning superficial velocity (0.25-1.01 m/s) on maximum restart pressure drop are emphatically investigated. The results demonstrate that the restart process can be divided into decay and steady two stages. The maximum restart pressure drop generally increases along with the increase of oil holdup, oil viscosity, standstill period and water cleaning superficial velocity. Moreover, of all the measured variables, oil holdup and water cleaning superficial velocity have a significant influence on maximum restart pressure drop. The results obtained can provide a theoretical reference and practical guidance for the development of appropriate restart schemes for on-site shutdown pipelines. [ABSTRACT FROM AUTHOR]

The effective determination of pressure losses depends on accurate knowledge of the drilling fluid rheology. As the fluid circulates deeper around the wellbore, its rheological behavior undergoes significant alterations due to the variations in downhole conditions encountered. The present study investigates the effects of the rheological properties of Yield-power law fluid at various pressures and temperatures on annular pressure losses and velocity profiles. Simulations were performed using Computational Fluid Dynamics to examine the fluid flow in turbulent and laminar regimes. Comparison between numerical, experimental and slot approximation model results showed a good agreement. Results indicated that pressure losses have reduced in both regimes with increasing temperature, at a constant pressure. However the pressure has the opposite effect at a constant temperature. For a drilling fluid flow velocity of 1 m/s, the elevation of temperature from 25 °C to 90 °C, decreases the pressure drop gradient by (31% to 48%) at low and high- pressure conditions respectively. Whereas, the influence of increasing pressure on pressure losses is more apparent at 25 °C. Earlier transition from laminar to turbulent is observed with temperature rise. Therefore, the temperature effect on pressure losses in the turbulent region; is shown for different Generalized Reynolds numbers. [ABSTRACT FROM AUTHOR]

This paper focuses on the unified modeling suite for annular flow that the authors have developed and continue to develop. The annular flow suite currently includes models to predict the void fraction, the entrained liquid fraction, and the wall shear stress and pressure gradient, and a turbulence model for momentum and heat transport inside the annular liquid film. The turbulence model, in particular, allows prediction of the local average liquid film thicknesses and the local heat transfer coefficients during convective evaporation and condensation. The benefit of a unified modeling suite is that all the included prediction methods are consistently formulated and are proven to work well together, and provide a platform for continued advancement based on the other models in the suite. First the unified suite of methods is presented, illustrating in particular the most recent updates. Then results for the pressure drop and the heat transfer coefficient during convective evaporation and condensation are presented and discussed, covering both water and refrigerants flowing through circular tubes and noncircular multi-microchannel configurations for microelectronics cooling. [ABSTRACT FROM PUBLISHER]

Phase distribution within an annular channel is desired to know in cooling systems. Electrical resistance tomography (ERT) is a promising technique to achieve this task in a noninvasive and visualized manner. However, high nonlinearity and ill condition of ERT restrict its spatial resolution. The internal structure that forms the annular channel with the pipe wall makes the image reconstruction of phase distribution different from the circular cross section that ERT usually applied to. In this paper, a hybrid solver coupling the finite-element method and boundary-element method is proposed to solve the forward problem more efficiently. This algorithm is based on an elaborately designed finite-element forward solution, which speeds up the image reconstruction by eliminating the unknowns inside the known internal structure using boundary element method. The Levenberg–Marquardt (L-M) method is used to achieve a fast convergence and a high numerical accuracy and then to reconstruct the phase distribution within the annular channel. The linear and nonlinear forms of the L-M methods are compared in the cases of changing the internal structure conductivity, in order to understand the influence of the internal structure. [ABSTRACT FROM PUBLISHER]

The article focuses on a procedure that helps to predict the transition and choking flow conditions in circular conduit bends. Configuration for the experiment with Froude numbers are presented. Also included are the scale effects, equations for transition and choking conditions and several diagrams which shows the annular flow conditions.

Nanoscale two-phase flow is important during shale gas production. In this work, molecular dynamics simulations are employed to investigate fluid flow regimes of water and methane under different pressure gradients inside the nanoscale slit pores. The illite mineral is selected to model the clay pores based on the Small-Angle X-ray Diffraction (SAXD) experiment. The different flow regimes of gas-water two-phase are visualised and the structures are analysed. As the driving pressure gradient increases, it is observed that the two-phase flow in the slit undergoes transitions from intermittent flow to a transition stage between intermittent flow and annular flow, and finally fully converts to annular flow. The slip conditions during the two-phase flow on the hydrophilic illite surface are investigated. We found that the negative slip length stays around 0.25 nm within the range of practical production pressure gradients, which can be used for accurate production prediction. This work provides detailed insights into the nanoscale water and gas two-flow inside the clay nanopores during shale gas exploration. [ABSTRACT FROM AUTHOR]

There are numerous correlations available for predicting frictional pressure drop of condensing flow in horizontal tubes. It is necessary to evaluate and analyze the prediction accuracy and applicability of existing correlations. In order to explore the versatility of frictional pressure drop correlations for condensation flow with different working fluids, four existing models are investigated: phase-separation conversion model, full-phase flow model, two-phase multiplier flow model, and homogeneous flow model. Combined with five different working fluids (R170, R12, R600a, R22, R1234yf), 402 groups of experimental data on frictional pressure drop for condensation flow in horizontal circular tubes are considered with respect to flow pattern. The prediction results based on each correlation are compared and analyzed. Mean absolute relative deviation (MARD) and mean relative deviation (MRD) are selected as indicators to evaluate each correlation. The results showed that the versatility of the existing correlations needs improvement. The prediction accuracy of frictional pressure drop for condensation flow can be improved by selecting different correlations for different flow patterns. The phase-separation conversion model has a good prediction effect and a wide application scope in annular flow. The maximum MARD is 15.85%, and the maximum MRD is 8.72%. 93.08% of the data predicted deviation is less than 30%. This study will provide some constructive instructions to predict frictional pressure drop of condensation flow in horizontal circular tubes for designing heat exchangers. [ABSTRACT FROM AUTHOR]

Swirling gas flow is an important topic of research that helps in the design of rockets, atomizers,and gas turbine combustors. In the present work, swirl flow inside annular geometries with constant and varying cross-sectional areas are examined using computational fluid dynamics (CFD). Effects of changing the flow and geometric parameters on the swirl behaviour were studied. Reynolds number was found to increase the swirl number in straight and diverging cross-sectional geometries while no significant effect of Reynolds number was observed in converging cross-sectional geometry. Radius ratio, defined as the ratio of the inner to the outer radius of the annular geometry, was found to have a significant effect on the swirl number. Decreasing the radius ratio, in straight and diverging annular geometry decreases the swirl number, however, an opposite trend was observed in the case of converging annular geometry due to significant increase in axial velocity as a result of reduced cross-sectional area. Increasing the swirler vane angle increased the swirl number. At higher vane angles of 60° and 70°, a recirculation zone is developed near the exit of the swirler. Using small cone angles was found to lower the swirl decay rate in converging and diverging nozzles. [ABSTRACT FROM AUTHOR]

A numerical simulation approach is substantiated and verified for predicting the Taylor-Couette flow with an impermeable outer stationary cylinder and porous rotating inner one. An imposed throughflow is considered, supplied via one end of the annular gap and leaving the domain through another gap end (retentate) and from inside of the rotating cylinder (permeate). The flow is typical for dynamic filtration by rotating cylindrical filters. In contrast to known publications, the rotating porous cylinder is included into the computational domain and the liquid flow inside of it is fully resolved. The influence of the permeate-retentate ratio and permeability of the rotating inner cylinder onto the centrifugal stability boundary and supercritical flow details is discussed. It is found that filtration velocity becomes distributed not uniformly along the porous cylinder when its hydraulic resistance is less than a definite value. After some critical threshold, the flow structure drastically changes, and spiral vortices appears, which crosses the porous rotating cylinder back and forth several times, providing multiple flow recirculation through the porous cylinder. The results obtained create the basis for the development of dynamic rotational filters for various applications. [ABSTRACT FROM AUTHOR]

The algorithm for the accurate and relevant numerical solution technique given in part I of this article is used to obtain the results for shear-driven annular condensing flows in horizontal channels—with or without transverse gravity. The unsteady wave simulation capability is used to implement a unique non-linear stability analysis. The steady and unsteady simulations’ results for millimeter scale (hydraulic diameter 4–8 mm), modest mass-flux (5–120 kg/m2/s), and refrigerant vapors (FC-72, R113, etc.) are used to mark the approximate location beyond which the annular regime typically transitions to a non-annular regime. These are used to develop correlations for local heat transfer coefficients and the approximate length that marks the transition from annular to non-annular regimes. [ABSTRACT FROM PUBLISHER]

Pressurized metered dose inhalers (pMDI) are one of the most common devices to deliver therapeutic treatment to patients with asthma or COPD. The atomization mechanism responsible for droplet production is poorly understood because of the short length and timescales involved, making experimental investigations to characterize fluid flow structures and spray formation processes difficult. This article reports the findings of new high-speed and high-resolution imaging and temperature measurements seeking to improve the fundamental understanding of the atomization mechanism of the pMDI. An annular flow regime of gaseous core in liquid annulus was observed in the orifice. Shock diamonds within the flow, viewed via Schlieren imaging, confirmed experimentally for the first time the choked nature of the orifice exit flow. Propellant formulation flow in the orifice was found to be superheated to a degree that significant flashing and evaporation at the orifice exit is likely to take place. This information points to a hybrid mechanism of predominantly aerodynamic atomization with significant liquid flashing upon exit from the orifice producing a near instantaneous formation of respirable droplets. Large irrespirable droplets are produced by two mechanisms; breakup of liquid slugs ejected from the orifice, due to unsteady transient fluid structures in the actuator sump and stripping of ligaments from a liquid pool around the orifice exit, which subsequently break up into large droplets in the periphery of the plume. This new understanding will aid development of next generation, high efficiency pMDIs, particularly those employing low global warming potential propellants such as HFA152a or HFO1234ze(E). Copyright © 2022 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Evaluation analysis on the approaches for predicting two-phase pressure drop of refrigerant-lubricant oil mixture in compressor suction line is presented in this study. A total of 274 pressure drop data for refrigerant-oil mixture in suction line were adopted to develop a novel consolidated database. These data were collected from the authors' measurement and four published literatures. The working fluids included R410A, R134a and R32 with their relevant lubricant oils and the database covered the inner diameter, local quality, liquid and vapor Reynolds numbers with the ranges of 7.1–18.5 mm, 0.927–0.994, 0.32–19.1 and 2.59–20.4 × 104, respectively. Two analytical models namely annular flow model (AFM) and double-circle model (DCM) and four representative pressure drop correlations were introduced to conduct an assessment. The comparison result indicated that the analytical models tend to over predict the experimental data of annular flow pattern, while the DCM underestimates the data under stratified-wavy flow conditions. Among the four correlations, Hu correlation provided relatively satisfactory prediction and based on its format, an improved correlation was proposed. The proposed correlation incorporated the surface tension effect and it displayed the best agreement with the database, which yielded mean relative error of 18.16% and captured 85.40% of data within ±30% deviations. [ABSTRACT FROM AUTHOR]

Heat transfer results for subcooled and saturated inverted annular film boiling (IAFB) obtained from a 7×7 rod bundle during transient reflood are presented in this paper. The test section consists of heater rods of 9.5-mm diameter and 12.6-mm pitch arranged in a square array. Flooding rates considered are 0.076 and 0.152 m/s, pressure varied from 138 to 414 kPa, and inlet subcooling up to 83 K. Evaluation of the data includes estimation of the local void fraction and Nusselt number during IAFB as well as in the inverted slug film boiling (ISFB) regime, which occurs when the inverted annular liquid column disintegrates. Experimental heat transfer results are compared with several film boiling models, and a new correlation for the Nusselt number is proposed for the IAFB and ISFB regimes. Predicted Nusselt numbers using the new correlation deviate from the experimental data by an average error of 15% and root-mean-square error of ~30%. [ABSTRACT FROM AUTHOR]

Experiments of flow regime visualization and identification of air–water two-phase flow were conducted in a horizontal helically coiled rectangular channel. The test superficial liquid and gas velocities are 0.09–2 m/s and 0.18–16 m/s, respectively. Flow regimes were observed with a high-speed video camera and the corresponding local and average void fractions were measured with an electric conductivity probe method and with a quick-close valve method, respectively. Four main flow regimes including unsteady pulsating flow, bubbly flow, intermittent flow, and annular flow were observed. The bubbly flow identification criteria depend on more than 90% bubbles whose chord length was smaller than the channel equivalent diameter. The annular flow identification criteria is the local void fraction in the gas core larger than 0.97. Then the flow regimes and their transition mechanisms are analyzed. Furthermore, new transition criteria among these flow regimes have been proposed. The results show that the critical transition average void fraction from bubbly to intermittent flow is 0.23. A complete air–water flow regime map has been developed for the horizontal helically coiled rectangular channel and the map predicts the observed flow regimes well. [ABSTRACT FROM AUTHOR]

An annular water-film lubricates an oil-core by functioning as a barrier between the core and the pipe wall in the water-lubricated flow of non-conventional viscous oil. A dependable model for appraising the frictional energy losses in such a core annular flow system is necessary to ensure its widespread implementation in the industry. In the current study, the modeling was conducted using an artificial neural network (ANN) based on 223 data sets. Seven input variables applied in the current ANN model are pipe diameter, average velocity, fluid properties, and water fraction. The optimum architecture was identified as a feed-forward neural network with backpropagation technique involving two hidden layers, each of which was consisted of 20 neurons or nodes. Comparative statistical analysis demonstrated promising accuracy of the current model, the coefficient of determination was 0.992, and the root mean square error was 0.111. In addition to validating the model, the relative significance of the input parameters was evaluated with a sensitivity analysis. [ABSTRACT FROM AUTHOR]

An experimental investigation of single-phase and two-phase (evaporation and condensation) heat transfer and pressure drop characteristics is performed for five micro-fin tubes with various geometries using refrigerant R22 as the working fluid. The mass flux is in the range of 130–550 kg/m2s. Evaporation tests are conducted at 6 °C saturation temperature, 0.1 inlet quality and 0.9 outlet quality, while condensation tests are carried out at 45 °C saturation temperature, 0.8 inlet quality and 0.1 outlet quality. The results suggest that the micro-fin tubes single-phase heat transfer coefficients increased by 46.3%–63.6% compared to smooth tubes. Tube geometries that have the best evaporation or condensation heat transfer performance are identified. A new evaporation heat transfer correlation that takes the micro-fin effects and heat transfer mechanisms into consideration is developed, which predicts the experimental data within 15% error compared with the previously reported correlations which show prediction errors beyond 30%. The new model is applied to intermittent and annular flow patterns in micro-fin tubes. As for condensation, the Choi et al. correlation well predicts the pressure drop within 20% error, and the Yu and Koyama correlation provides the best heat transfer predictions within 25% error. [ABSTRACT FROM AUTHOR]

In the present study, flow boiling in a single minichannel with a longitudinal step on its each corner is studied experimentally. Bubble/flow dynamics, and thus, underlying physical phenomenon is investigated through high-speed flow visualization experiments. The results are also compared with those of the conventional single rectangular minichannel (plane channel) in terms of heat transfer and total pressure drop. Experiments are performed for different values of the mass flux and wall heat flux at a constant inlet temperature. It is obtained that the stepped channel improves heat transfer performance up to 13.2% compared to the plane channel. During the visualization studies, five flow patterns are observed: bubbly flow, slug flow, churn flow, annular flow, and inverted annular flow. [ABSTRACT FROM AUTHOR]

The flow condensation heat transfer performances of R1234ze(E) and R32 in a horizontal single circular minitube with the inner diameter of 2 mm are investigated experimentally. Tests are carried out at saturation temperatures of 35, 40, and 45 °C, mass fluxes ranging from 100 to 400 kg/m2s, and heat fluxes ranging from 4 to 28 kW/m2. Flow patterns of R32 are observed by using a visualization approach. Plug, slug, annular-wavy, and annular flow patterns are presented in this research. The effects of the operating parameters (mass flux, heat flux, saturation temperature, and vapor quality) on the heat transfer characteristics of R1234ze(E) and R32 are discussed. A database involving R1234ze(E), R1234yf, R32, and R134a is built to make further analysis. Several typical correlation models are used to predict our measured data and the collected database. The Baird et al. model and the Müller-Heck model can well predict the heat transfer coefficients and frictional pressure drops, respectively. The flow pattern boundary between intermittent flow and annular wavy flow is depicted by three dimensionless numbers: the Lockhart-Martinelli parameter, the Bond number, and the Reynolds number. [ABSTRACT FROM AUTHOR]

Understanding the impact mechanism of the oil properties and the bend geometric parameters on oil–water core annular flow in bends is of great importance for the design and operation of bend pipelines. To address this, the oil–water core annular flow through U-bends is analyzed numerically, by using the volume of fluid technique together with the continuum surface stress method. A good match is observed between the numerically simulated phase contours and those taken from reported experiments. The influence of the inclination angle, the inlet diameter ratio, oil properties, and the pipe materials on the hydrodynamics is investigated together with the fouling characteristics of core annular flows. Through the numerical simulations, the flow in a range of operating conditions has been determined, together with the influence of geometric parameters and pipe material. [ABSTRACT FROM AUTHOR]

Experimental research is performed on two-phase flow boiling heat transfer in micro-channels. FC-72 is used as the working fluid. In order to analyze the heat transfer mechanism during two-phase flow boiling, the dimensionless parameters, e.g., boiling number and convection number, are used, and the effect of these parameters on the heat transfer can be confirmed during flow boiling in the micro-channel. In addition, the transition criterion from bubbly/slug flow to annular flow is proposed from the modified Weber number. Based on the boiling heat transfer mechanism obtained from the experiments, a new correlation is proposed to predict the heat transfer coefficient. The new correlation predicts well the experimental results within a mean absolute error of 5.2%. [ABSTRACT FROM AUTHOR]

Stable surfactant-free Al2O3/deionized (DI) water nanofluids are prepared by a two-step process and are stabilized using an ultrasonic homogenizer. The thermal conductivity enhancement measured by a transient hot wire technique demonstrated a nonlinear relationship with increase in volume fraction of dispersed nanoparticles and attains a maximum enhancement of 15% for 1 vol% of Al2O3loading in deionized water at 70°C. The stabilized Al2O3/DI water nanofluids were employed as the working fluid in a screen mesh wick heat pipe placed horizontally. The straight heat pipe configuration is altered for more practicality in use, with crimped edges, extended conduction lengths, and minute surface depressions. The heat pipe is tested at various levels of heat inputs and concentrations of Al2O3nanoparticles. The evaporator section is heated by circulating water through a heating chamber, and the condenser section is cooled under free convection. The experimental results show an optimum reduction of 22% in the thermal resistance value using 1 vol% of Al2O3/DI nanofluids as compared to DI water at low heat input of 12 W. The stabilized operation of the heat pipe is observed at high heat input of 73 W and at low concentration of 0.005 vol% Al2O3/DI water nanofluids. The findings emphasize potential for nanofluids as future heat pipe fluids. [ABSTRACT FROM PUBLISHER]

Deep sea compression systems must work under strenuous conditions with either gas in liquid or liquid in gas mixtures, mostly inhomogeneous. Off-design operation affects the mechanical system's overall efficiency and reliability, with penalties in leakage and rotordynamic performance of secondary flow components, namely, seals. This article introduces a test rig to characterize the leakage and dynamic force coefficients of a short-length annular seal (L/D = 0.36, clearance = 0.127 mm) operating under various flow regimes ranging from pure gas, to bubbly (liquid in gas), to foamy (gas in liquid), to pure liquid. The test rig includes of rotating journal and a softy supported cartridge that make a clearance annular seal that is supplied with a liquid/gas mixture. Flowmeters record the fluid's passage, and with manual control of the streams, the mixture has a known liquid (or gas) volume fraction at the seal inlet plane. Two orthogonally mounted electromagnetic shakers excite the cartridge with periodic (single-frequency) forces spanning a wide frequency range. Eddy current sensors and accelerometers record the seal cartridge motions and a frequency domain parameter identification method delivers the seal dynamic force coefficients. For tests with a pressure supply/pressure discharge ratio = 3.0 and 3.5 and a nonrotating journal, the article reports the flow rate for an ISO VG10 oil in air mixture with liquid volume fraction (LVF) at the inlet plane increasing from pure gas to pure liquid. Wet seal stiffness and mass and damping force coefficients follow for a seal operating with a pressure supply/pressure discharge ratio = 2.0 and operating with air (only) and also with an oil-in-air mixture with inlet LVF = 2% and 4%. The experimental results, the first reported, reveal that a small amount of liquid increases the damping coefficients of the wet seal 10-fold (or more). Predictions from a computational bulk flow model also demonstrate that the seal damping coefficient varies greatly with small contents of liquid in the oil/gas mixture, although agreement with the experimental force coefficients is not compelling due to the likely inhomogeneity of the mixture flowing though the seal. [ABSTRACT FROM PUBLISHER]

This article examines the concentric and eccentric annular flow of non-Newtonian fluids. A new effective diameter definition that accounts for flow geometry and fluid rheology is presented for pressure loss prediction. Satisfactory agreements were obtained between this new effective diameter definition in combination with laminar Fanning friction factor correlations and extensive flow data in the literature. In addition, an explicit friction factor correlation as a function of the generalized Reynolds number, diameter ratio, and relative roughness is presented for a fully eccentric annular drag-reducing turbulent flow. A good agreement was obtained between this correlation and the gathered flow data. The correlations reported provide an effective means of determining friction pressures of non-Newtonian fluids in eccentric annuli. [ABSTRACT FROM PUBLISHER]

In this review, various aspects of geological carbon sequestration are discussed in relation to the principles of two-phase flow in porous media. Literature reports on geological sequestration of CO2show that the aquifer storage capacity, sealing integrity of the caprock, and the in situ processes, for example, the displacement of brine by supercritical CO2(scCO2), convection–diffusion–dissolution processes involving scCO2and brine, geochemical reactions, and mineral precipitation depend on the fluid–fluid–rock characteristics as well as the prevailing subsurface conditions. Considering the complexity of the interrelationships among various processes, experimental investigations and network of mathematical functions are required for the ideal choice of geological site with predictable fluid–fluid–rock behaviors that enhance effective monitoring. From a thorough appraisal of the existing publications, recommendations are made for improvement in the existing simulators to fully couple the entire processes involved in the sequestration operations and in situ mechanisms which include injection rate and pressure, brine displacement, simultaneous flow of free and buoyant phases of CO2, various trapping mechanisms, convection–diffusion–dissolution processes, scCO2–brine–rock reactions, precipitation of the rock minerals, and the consequences on the hydraulic and hydrogeological properties in the course of time as well as the quantity of injected CO2. Suggestion is made for the inclusion of leakage parameters on site-specific basis to quantify the risks posed by the prevailing fluid–fluid–rock characteristics as well as their immediate and future tendencies. Calls are also made for thorough investigations of factors that cause nonuniqueness of the two-phase flow behavior with suggestions for the use of appropriate experimental techniques. The review comprehensively synthesizes the available knowledge in the geological carbon sequestration in a logical sequence. [ABSTRACT FROM PUBLISHER]

The influence of oxygen enrichment of the air co-jet on the combustion of a laminar methane jet diffusion flame is studied experimentally by the measurement of the heat flux transferred to an assembly of heat exchangers with oxygen concentration ranging from 21% to 41% and firing rates ranging from 0.5 to 1.2 kW. It is observed that, as expected, heat flux increases with oxygen concentration; less methane is necessary to produce the same heating effect as oxygen index is increased. The radiant fraction is found to increase with the oxygen concentration. Deducing the convection part, it is shown that convection is highly dependent on the flame regime with a lesser rate of increase when the flame is reattached to the burner. Evolution of the convective coefficient may be used as a detection of the transition from a lifted flame to a flame attached to the burner. [ABSTRACT FROM PUBLISHER]

This article describes an investigation of flame shapes and flow configurations in a premixed, swirl-stabilized dump combustor. High swirl, annular nozzle flows of this nature enable a variety of different flame configurations and heat release distributions with their associated flow fields. These differences are significant, since each of these configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle lifetime, and liner heating. These different configurations arise because multiple flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. We present results from high-speed luminosity imaging, particle image velocimetry (PIV), and OH-planar laser induced fluorescence (PLIF) to illustrate time-averaged and instantaneous flame shapes and flow fields associated with the different configuration “families.” Selected cases are compared with large eddy simulations (LES). Particular emphasis is given to the distinctly different flame and flow topologies that exist in these flows, and their sensitivity to geometric (such as centerbody size and shape, combustor diameter, exhaust contraction) and operational (e.g., bulkhead temperature, preheat temperature, fuel/air ratio) parameters. We particularly emphasize the importance of the centerbody shape, and its associated impact on the structure of the central recirculating flow, as differentiating between two different families of flame shapes. [ABSTRACT FROM AUTHOR]

In gas–liquid two-phase flow, the prediction of two-phase density and hence the hydrostatic pressure drop relies on the void fraction and is sensitive to the error in prediction of void fraction. The objectives of this study are to analyze dependence of two-phase density on void fraction and to examine slip ratio and drift flux model-based correlations for their performance in prediction of void fraction and two-phase densities for the two extremes of two-phase flow conditions, that is, bubbly and annular flow or, alternatively, the low and high region of the void fraction. It is shown that the drift flux model-based correlations perform better than the slip ratio model-based correlations in prediction of void fraction and hence the two-phase mixture density. Another objective of this study is to verify performance of different two-phase dynamic viscosity models in prediction of two-phase frictional pressure drop. Fourteen two-phase dynamic viscosity models are assessed for their performance against 616 data points consisting of 10 different pipe diameters in annular flow regime. It is found that none of these two-phase dynamic viscosity models are able to predict the frictional pressure drop in annular flow regime for a range of pipe diameters. The correlations that are successful for small pipe diameters fail for large pipe diameters and vice versa. [ABSTRACT FROM AUTHOR]

There is wide disagreement between experimental investigations and correlations for heat transfer during condensation in micro-channels. The major problem is the fact that the vapor-side resistance is usually appreciably smaller than that on the coolant side so that methods where only the overall resistance is measured, and the vapor-side heat transfer coefficient obtained by subtraction of resistances, are prone to large uncertainty. A few more recent correlations, based mainly on data for R134a, are in fair agreement when predictions for R134a and for the same conditions are compared. The fact that wide discrepancies are found when the correlations are used for fluids with widely different properties indicates that some or all of the correlations do not capture all of the essential mechanisms. That closely similar predictions are found when the correlations are applied to R134a indicates that the datasets used in the different studies were in essential agreement. Similar comments apply for pressure drop. The special case of annular laminar condensate flow permits wholly theoretical solution without recourse to empirical input. For this mode of condensation and for specified fluid, channel geometry, flow parameters and tube wall temperatures, local heat transfer-coefficient, and local pressure gradient can be calculated as well as local quality and void fraction. The theory is outlined in the article, and recent developments are discussed. Comparisons with the correlations for heat transfer and pressure gradient are given. For the heat transfer coefficient, the results of the annular flow theory are in surprisingly good agreement with the correlations when applied to R134a. For ammonia, the theoretical results lie between the widely spread values obtained from the correlations. Results for pressure gradient given by the annular laminar flow model are generally lower than those given by the correlations. [ABSTRACT FROM AUTHOR]

Gaseous detonation propagating in an annular cylinder was studied for hydrogen/oxygen/nitrogen mixtures numerically and experimentally. In experiments performed in an annular combustor, pressure gauges were used to measure the pressure and propagation velocity, and smoked foil measurement was performed to record the cellular structure. Meanwhile, based on the two-dimensional Euler equations with detailed finite-rate chemistry, the numerical calculations were performed by a fifth-order weighted essentially non-oscillatory (WENO) scheme and a semi-implicit Runge–Kutta method. The results show that the detonation is strengthened near the outer concave wall and weakened near the inner convex wall, so that the detonation front is continuously realigning itself to the local channel axis, to maintain the detonation steadily propagates with a planar front. In addition, the local explosion occurs on the inner wall periodically, which protects the detonation from quenches. The cellular size near the outer concave wall is smaller than that near the inner convex wall. [ABSTRACT FROM AUTHOR]

An exact solution for calculating the frictional pressure losses of yield-power law (YPL) fluids flowing through concentric annulus is proposed. A solution methodology is presented for determining the friction factor for laminar and nonlaminar flow regimes. The performance of the proposed model is compared to widely used models as well as the experimental results of 10 different mud samples obtained from the literature. The results showed that the proposed model could estimate the frictional pressure losses with an error of less than 10% in most cases for both laminar and nonlaminar flow regimes, more accurately than the widely used models available in the literature. [ABSTRACT FROM AUTHOR]