Annular Dispersed Flow Research Articles

Application of the depth-averaged liquid film model to the annular flow regime in BWR fuel assemblies

The flow regime found in the downstream half of the BWR assembly can be described as an annular film flow with a central core of vapour with suspended droplets. This study presents the application of the fluid film model available in STAR-CCM+ to analyze the interplay between the vapour flow and the liquid film dynamics. The film model, based on depth-averaged equations, can be used to simulate film depth and velocity over complex geometries. Built on sound mathematical basis and applied to a wide variety of areas, the fluid film model, however, has not been widely used to study annular flow in nuclear fuel assemblies. Applying this model to a simplified geometry of a single rod with different gaps, the basic relationship between the vapour velocity, liquid film thickness and velocity distribution along the circumference of the rod is clarified. The minimum vapour velocity required to transport a liquid film along the surface of a fuel rod of a specific length against gravity was found using first-principles force balances. The model was then applied to a real, but simplified assembly geometry where the resulting flow pattern was primarily affected by the presence of part length rods. The minimum velocity required to transport a liquid film along the fuel rods in the assembly against gravity was found to be close to the range of velocities obtained using sub-channel codes. Strong circumferential variation in the wall shear across a short arc-length was observed for some fuel rods. The variation in wall shear also results in a corresponding variation in film velocity and film thickness and presumably has a strong impact on the local heat and mass-transfer characteristics. Cross flow from high resistance regions in the assembly towards lower resistance regions has a strong impact in terms of variations in the film thickness on the rods that are the furthest away from the low resistance paths.The modelling palette for the depth-averaged film model contains several features that can extend the validity of the modelling used in this study, such as the presence of droplets in the vapour core, deposition of droplets on to the film, heat transfer and boiling, and the influence of spacers in disrupting the liquid film. These features were not considered in this study and remain avenues for future work. The use of time-averaged turbulence modelling results in damping of unsteadiness of the vapour flow and perturbations in the film thickness. For better estimates of unsteady effects use of turbulence models such as the Large Eddy Simulation method is recommended. Current state-of-the-art in predicting dispersed annular flow using liquid film models is limited to single rods. The generalized depth-averaged film model can be applied to complex geometries including the effect of spacers.

Numerical model of heat and mass transfer and separation of the dispersed phase in high-speed dispersed-annular flows of gas and liquid

To solve scientific and technical problems, physical processes are considered and systems of equations are written numerical and approximate mathematical model of combined heat and mass transfer at contact cooling of gases and heating of water, as well as turbulent transfer of particles in an ascending dispersed annular flow of gas and liquid. The numerical model is based on a system of equations in particular derivatives in two-dimensional form with boundary conditions of the fourth kind. Approximate model built using a system of algebraic equations of the cell model of the flow structure for gas and liquid phases, where the main parameters are the number of cells of complete mixing, coefficients heat and mass transfer and turbulent transfer of particles. An example of solving a system of equations is shown cell model with calculation of gas and liquid temperature profiles, moisture and particle concentrations, and also the efficiency of the heat and mass transfer process, the efficiency of separation of the finely dispersed phase from the gas liquid film in cocurrent flow. Comparative characteristics of film devices are given. Noted introduction of scientific and technical developments in the purification of natural gas at production sites. Keywords: heat and mass transfer, gas cooling, particle separation, phase co-current flow, systems of transfer equations.

Численная модель тепломассообмена и сепарации дисперсной фазы в высокоскоростных дисперсно-кольцевых потоках газа и жидкости

To solve scientific and technical problems, physical processes are considered and systems of equations are written numerical and approximate mathematical model of combined heat and mass transfer at contact cooling of gases and heating of water, as well as turbulent transfer of particles in an ascending dispersed annular flow of gas and liquid. The numerical model is based on a system of equations in particular derivatives in two-dimensional form with boundary conditions of the fourth kind. Approximate model built using a system of algebraic equations of the cell model of the flow structure for gas and liquid phases, where the main parameters are the number of cells of complete mixing, coefficients heat and mass transfer and turbulent transfer of particles. An example of solving a system of equations is shown cell model with calculation of gas and liquid temperature profiles, moisture and particle concentrations, and also the efficiency of the heat and mass transfer process, the efficiency of separation of the finely dispersed phase from the gas liquid film in cocurrent flow. Comparative characteristics of film devices are given. Noted introduction of scientific and technical developments in the purification of natural gas at production sites.

Studies of critical heat fluxes in small diameter channels

The paper presents the results of experimental studies of critical heat flows in vertical small diameter channels, when the coolant moves from bottom to top, which were carried out in the Obninsk branch of MEPhI in the 1970s of the last century but have not become widespread due to the lack of demand for their practical use. Nowadays, the interest in such works is manifested, first of all, in the development of compact plants and devices, particularly in nuclear power engineering. As a coolant, water, Freon-12 and 96% ethyl alcohol were used. High velocities of underheated liquid at high heat fluxes on the channel wall lead to the so-called “fast crisis” of heat transfer. In this case, the magnitude of the heat flux depends mainly on the parameters of the coolant flow in the wall zone rather than the flow core. The “slow crisis” is mainly observed at high vapor concentrations, relatively low mass flow rates and in an annular-dispersed flow. The value of the critical heat flow in this case depends mainly on the flow parameters in the core, which are probably close to the average coolant flow parameters. The conditions in the near-wall region are also largely determined by the flow in the core. High heat transfer coefficients in a flow moving at high speed usually result in a much smaller and slower rise in the wall temperature. Sometimes a DNB heat flux can occur bypassing the boiling process. In the core of a VVER-type reactor operating in its nominal mode, surface boiling is present in a number of fuel rods. Probably, surface boiling will also be present in transportable and small-size nuclear power plants. Therefore, an important task is to conduct relevant research in this area.

Pressure drop in two phase flow of condensing air-steam mixture inside PHE channels formed by plates with corrugations of different geometries

The pressure drop in the two-phase condensing flow of air-steam mixture inside channels of plate heat exchanger (PHE) with different geometries of corrugations is studied based on experiments and one-dimensional mathematical modelling. The experiments were made with five samples of the PHE channel. In three of them plates with corrugations inclination angles 30, 45 and 60° at the same height of corrugations 5 mm. The other two plates corrugations height was 7.5 and 10 mm at the same pitch to height ratio and inclination angle of 60°. The correlation of pressure drop data for all experimental samples by average process parameters is not able to give acceptable accuracy. The correlation for local pressure gradients in two-phase condensing flow is identified using a developed one-dimensional mathematical model. The model of separated flows of phases is employed for channel zones close to air-steam mixture entrance. Further on channel length with an increase of liquid phase quantities, its combination with the dispersed annular flow structure model is used. The proposed equations can be included in the mathematical model when designing PHE and optimising the geometrical form of corrugations on its plates for steam condensation processes from an air-steam mixture.

Updating of the models for critical heat flux calculation in annular-dispersed flow

This study presents updating of two existing physical models of dryout in annular-dispersed flow regime according to the updated data of the experimental studies. Relations for critical heat flux at high mass vapour qualities, in two ranges of mass flow rates were considered. The calculation results were compared with the critical heat flux look-up tables at full range of mass flow rates. The comparing showed satisfactory agreement of results in the ranges of regime parameters 0.6 ⩽ x ⩽ 0.9; 50 ⩽ ρω ⩽ 8000 kg m-2 s-1; 0.1 ⩽ p ⩽ 20 MPa.

Experimental study of droplet detachment from liquid film surface by a co-current flow inside the nozzle stagnation chamber

The paper presents the results of an experimental study on the interaction of near-wall liquid film with the co-current gas flow in the convergent channel. Conditions for transition (co-current gas flow velocity and liquid flow rate) of near-wall liquid film from annular flow mode to annular-dispersed flow accompanied by a detachment of droplets from the interface are elucidated.

Evolution of Views on the Wavy Structure of a Liquid Film in Annular Dispersed Gas-Liquid Flow

The wavy structure of a liquid film sheared by high-velocity gas flow with droplet entrainment from the film surface has been investigated. The paper presents an analysis of methods for measuring the local fluid film thickness and a review of classical concepts of the wave hydrodynamics of annular dispersed flow obtained in early studies using rather crude methods. A systematic description of the structure, evolution, and interaction of waves of various types is given based on an analysis of high-resolution spatio-temporal records of film thickness obtained by laser-induced fluorescence. The results are compared with the results of recent experiments, and the necessity of revising some classical concepts is shown. Directions for further research are suggested.

Heat transfer crisis and fluid distribution in bundles of rods in the area of dispersed-annular flow mode

Heat transfer crisis and fluid distribution in bundles of rods in the area of dispersed-annular flow mode

Experimental study of hydrodynamics and heat transfer in two-phase natural circulation loop with reference passive cooling systems of nuclear power plants

In spite of long year practical use of natural circulation systems and not less long period of study working characteristics of these systems the problem of reliable predicting calculations of natural circulation loops (NCL) with boiling coolant at low reduced pressures is not likely can be considered as being solved. At the same time the interest to solution of this problem arises today, because low pressure natural circulation loops are considered as main type of post accident passive cooling systems for nuclear power plants. The main difficulties for calculations are concerned with determining friction pressure losses, which are strongly depended on two-phase flow pattern. At low reduced pressures large difference between liquid and vapour (gas) specific volumes predetermines the strong change of flow pattern even at small change of mass flow quality. An attempts to calculate local two-phase flow parameters (void fraction, circulation velocity) in the wide range of mass flow qualities without considering the change of flow patterns often lead to large deviations of calculated values from the results of experimental measurements. Another specific feature of two-phase low pressure NCL is hydrodynamic flow instability with circulation velocity pulsations of high amplitude and the occurrence of reverse flows. This also presents problems in developing calculation method. In present paper a method for calculating a low-pressure NCL has been developed, in which local two-phase flow parameters (void fraction, the phases velocities, pressure) are calculated using a modified homogeneous model with taking into account the distribution factor and the phases slip and a model of an annular-dispersed flow with considering droplets entrainment and deposition. A modified homogeneous model was applied for description quasihomogeneous flow regimes. At high values of void fraction annular-dispersed flow model was used. Recommendations for change from one model to another in practical calculations have been formulated and verified. The proposed calculation method has been verified by the comparison the calculated thermo-hydraulic characteristics of laboratory natural circulation loop with the experimental data. The experiments have been carried out for boiling in the loop of three different liquids – water, ethanol and perfluorohexane (FC-72) at atmospheric pressure. The comparison of calculated and experimental results showed their good agreement.

Experimental and Calculated Investigation of a Natural Circulation Loop’s Thermal-Hydraulic Characteristics

The results from experimental investigation into hydrodynamics and heat transfer in a two-phase natural circulation loop (NCL) under atmospheric pressure are presented. The experiments were carried out for liquids having essentially different properties: water, ethanol, and perfluorohexane C6F14 (the product trademark is FC-72). The circulation velocity in the NCL is not known in advance but is a complex function of the specified parameters (heat flux and liquid temperature at the heated section inlet) and of the two-phase flow internal characteristics. The liquid temperatures at the heated section inlet, the wall temperature over the section height, and also the circulation velocity were measured in the experiments at a specified heat flux; in addition, the two-phase flow at the loop riser leg outlet was filmed on video. The experiments and analysis have shown that flow hydrodynamic instability (circulation velocity pulsations) is really unavoidable in a two-phase NCL. Hydrodynamic instability with a high circulation velocity amplitude and with the occurrence of backward flows is typical for regimes involving significant liquid subcooling values at the heated section inlet and for NCLs containing an extended part with single-phase convection. This instability, which is characteristic for experiments with water, is due to the displacement of the boiling incipience section over the section height; the instability also persists at small subcooling values but with a low pulsation amplitude. Under the developed saturated liquid nucleate boiling conditions (at high heat flux values), the circulation velocity and wall temperature pulsations have small amplitudes, and the flow can be regarded as stable. In the experiments with perfluorohexane, the smallest wall temperature and circulation velocity pulsations were pointed out, which is attributed to a relatively high value of reduced pressure. In the experiments with ethanol, instability occurs in the developed nucleate boiling region (q > 35 kW/m2); this instability is caused by periodically alternating two-phase flow structure (regime). A procedure for calculating a low-pressure NCL is developed, in which the two-phase flow’s local parameters (void fraction, phase velocities, and pressure) are calculated according to a modified homogeneous model (taking into account the phase distribution factor and phases slip) and a dispersed-annular flow model taking into account the droplet entrainment and deposition phenomena. A comparison of the NCL calculation results with the experimental data obtained for three different liquids has shown that they are in good agreement with each other.

Horizontal Air-Water Two-Phase Flow Measurement Using an Electrical Impedance Probe and a Venturi Flow Meter

Two-phase flow rate measurement is extremely important in many industrial sectors (nuclear, chemical, oil and gas, etc.). Nevertheless, the instrumentation required to perform these measurements is still very limited despite the huge research efforts during the last years. In the frame of the development of instrumentation for the two-phase flow rate measurement, the main aim of this study is the characterization of a spool piece consisting of a Venturi Flow Meter, and two Electric Capacitance Probes mounted in a horizontal test section, where an air-water two phase flow occurs. The probes response is analyzed versus superficial velocities of liquid and gas, flow patterns and void fraction. The test section consists of a nominal ID 80 mm horizontal transparent pipe equipped with a Venturi Flow Meter, a new capacitance probe with concave electrodes and a SIET designed electrical capacitance probe with ten long linear electrodes. In this experimental campaign, the air flow rate is in the range 0.048-0.119 kg/s (superficial velocity from 8 to 19 m/s) and the range of water flow rate is 0.0040-0.0535 kg/s (superficial velocity from 0.0006 m/s to 0.0103 m/s). These ranges of superficial velocities correspond to wavy stratified and annular-dispersed flow patterns along the test section.

Calculation of critical heat transfer in horizontal evaporator pipes in cooling systems of high-rise buildings

An exact calculation of the heat exchange of evaporative surfaces is possible only if the physical processes of hydrodynamics of two-phase flows are considered in detail. Especially this task is relevant for the design of refrigeration supply systems for high-rise buildings, where powerful refrigeration equipment and branched networks of refrigerants are used. On the basis of experimental studies and developed mathematical model of asymmetric dispersed-annular flow of steam-water flow in horizontal steam-generating pipes, a calculation formula has been obtained for determining the boundaries of the zone of improved heat transfer and the critical value of the heat flux density. A new theoretical approach to the solution of the problem of the flow structure of a two-phase flow is proposed. The applied method of dissipative characteristics of a two-phase flow in pipes and the principle of a minimum rate of entropy increase in stabilized flows made it possible to obtain formulas that directly reflect the influence of the viscous characteristics of the gas and liquid media on their distribution in the flow. The study showed a significant effect of gravitational forces on the nature of the phase distribution in the cross section of the evaporative tubes. At a mass velocity of a two-phase flow less than 700 kg / m2s, the volume content of the liquid phase near the upper outer generating lines of the tube is almost an order of magnitude lower than the lower one. The calculation of the heat transfer crisis in horizontal evaporative tubes is obtained. The calculated dependence is in good agreement with the experimental data of the author and a number of foreign researchers. The formula generalizes the experimental data for pipes with the diameter of 6-40 mm in the pressure of 2-7 MPa.

Prediction of friction pressure drop for low pressure two-phase flows on the basis of approximate analytical models

Wide use of natural circulation loops operating at low redused pressures generates the real need to develop reliable methods for predicting flow regimes and friction pressure drop for two-phase flows in this region of parameters. Although water–air flows at close-to-atmospheric pressures are the most widely studied subject in the field of two-phase hydrodynamics, the problem of reliably calculating friction pressure drop can hardly be regarded to have been fully solved. The specific volumes of liquid differ very much from those of steam (gas) under such conditions, due to which even a small change in flow quality may cause the flow pattern to alter very significantly. Frequently made attempts to use some or another universal approach to calculating friction pressure drop in a wide range of steam quality values do not seem to be justified and yield predicted values that are poorly consistent with experimentally measured data. The article analyzes the existing methods used to calculate friction pressure drop for two-phase flows at low pressures by comparing their results with the experimentally obtained data. The advisability of elaborating calculation procedures for determining the friction pressure drop and void fraction for two-phase flows taking their pattern (flow regime) into account is demonstrated. It is shown that, for flows characterized by low reduced pressures, satisfactory results are obtained from using a homogeneous model for quasi-homogeneous flows, whereas satisfactory results are obtained from using an annular flow model for flows characterized by high values of void fraction. Recommendations for making a shift from one model to another in carrying out engineering calculations are formulated and tested. By using the modified annular flow model, it is possible to obtain reliable predictions for not only the pressure gradient but also for the liquid film thickness; the consideration of droplet entrainment and deposition phenomena allows reasonable corrections to be introduced into calculations. To the best of the authors' knowledge, it is for the first time that the entrainment of droplets from the film surface is taken into consideration in the dispersed–annular flow model.

Quantitative analysis of transverse non-uniformity of liquid film at the initial stage of annular-dispersed flow

The process of formation of disturbance waves in annular gas-liquid flow was studied using three-dimensional configuration of brightness-based laser-induced fluorescence technique in two ducts of different geometry in the proximity of the inlet. The process was found to consist of three stages: formation of initial two-dimensional high-frequency waves; fragmentation into localised chaotic 3D-waves and formation of large-scale quasi-2D disturbance waves. A method of quantitative characterisation of the degree of transverse coherence of film surface was developed and used to estimate the coordinates of the borders of the three stages. All the stages occur closer to the inlet at higher superficial gas velocities and lower liquid flow rates. It was found that the shape of the duct does not exert significant influence of the distance of existence of two-dimensional initial waves, whilst the disturbance waves are formed closer to the inlet in the circular duct.

Three-dimensional investigation of liquid film structure at the initial area of annular-dispersed flow

Initial stage of downward flow of gas-sheared liquid film in a vertical rectangular duct was studied using brightness-based laser-induced fluorescence technique. Measurements were resolved along both longitudinal and transverse coordinates and time. The initial high-frequency waves which are formed at the inlet were found to be two-dimensional. These waves are promptly broken into localised horseshoe-shaped waves which merge downstream to form large-scale quasi-2D disturbance waves. Peculiarities of three-dimensional evolution of waves of different types were studied in a wide range of flow parameters.

Hydrodynamics of a natural circulation loop in a scaled-down steam drum-riser-downcomer assembly

Complex interactions of different phases, widely varying frictional characteristics of different flow regimes and the involvement of multiple scales of transport make the modelling of a two-phase natural circulation loop (NCL) exceedingly difficult. The knowledge base about the dependency of downcomer flow rate on riser-side flow patterns, particularly for systems with multiple parallel channels is barely developed, necessitating the need for detailed experimentation. The present study focuses on developing a scaled-down test facility relevant to the Advanced Heavy Water Reactor conceived in the atomic energy programme of India to study the hydrodynamics of the NCL using air and water as test fluids. An experimental facility with two risers, one downcomer and a phase-separating drum was fabricated. Conductivity probes and photographic techniques are used to characterize the two phase flow. Normalized voltage signals obtained from the amplified output of conductivity probes and their subsequent analysis through probability distribution function reveal the presence of different two-phase flow patterns in the riser tubes. With the increase in air supply per riser void fraction in the two-phase mixture increases and gradually flow patterns transform from bubbly to fully developed annular through slug, churn and dispersed annular flow regimes. Downcomer flow rate increases rapidly with air supply till a maximum and then starts decreasing due to enhanced frictional forces. However, the maximum value of downcomer water flow for any magnitude of total air flow can be obtained only for identical air supply through both risers. Any disparity in air flow through the risers has been found to cause as much as 2% reduction in downcomer flow rate owing to the interaction of varying flow patterns through the two riser tubes.

Liquid–Liquid Two-Phase Flow Patterns in a Serpentine Microchannel

Liquid–liquid two-phase flow patterns in a serpentine glass microchannel have been studied using a camera with very high shutter speed. The phase system is equilibrated water−succinic acid−n-butanol system. Observed flow patterns have been classified into seven different types: slug flow, slug and droplet flow, droplet flow, unstable annular flow, annular flow, annular dispersed flow, and fully dispersed flow. Two different ways of introducing the aqueous and organic phases into the microchannel have been studied. Flow regime maps are presented and discussed. Voronoi diagrams of the flow regime maps are also presented.

Diminution de la température de la paroi observée à proximité du flux de chaleur critique de type d'assèchement

An analysis of the wall temperature drop phenomenon before critical heat flux (CHF) takes place was carried out. The study of the heat transfer in the liquid film of the dispersed-annular flow permitted to conclude that this temperature drop is related to an important thinning of the liquid film when conditions approach to ones of the CHF. In this case, the heat is principally transmitted through the film by conduction. Thus, the power increase leads to a decrease of the thickness and of the thermal resistance of the film and, consequently, to a wall temperature drop.

In-pile Experiment in JMTR on the Radiation Induced Surface Activation (RISA) Effect on Flow-boiling Heat Transfer

In-pile flow-boiling experiments were performed to investigate the possible enhancement of heat transfer by the radiation induced surface activation (RISA) effect. The test section was a 2-mm diameter 100-mm long bore in a SUS-316L stainless steel block heated electrically. The test section, housed in an irradiation capsule, was inserted into one of the irradiation holes in the Japan Materials Testing Reactor (JMTR) of the Japan Atomic Energy Agency (JAEA). Quasi-steady state experiments were conducted before irradiation (out-of-pile and in-pile before reactor operation), during irradiation and after irradiation (in-pile), for the same boundary conditions using the same test section block. This approach allowed direct evaluation of the RISA effect through comparison of experimental data. Boiling curves were obtained up to the onset of dryout in an annular dispersed flow, for mass fluxes ranging from 180 to 630kg/(m2s) under a fixed pressure of 420 kPa. The critical heat flux obtained during and after irradiation indicated an about 17% increase, on average, from that before irradiation. Meanwhile, the wall superheat at subcritical heat fluxes generally became greater than that before irradiation.