Variation Of Void Fraction Research Articles

Development and Sultan Test Result of ITER Conductor Samples of Korea

As a way to improve performance of ITER TF conductors, two types of cable-in-conduit conductors were developed in Korea with variations of conduit thickness resulting in the different void fraction of the conductors. The estimated void fractions of the conductors are 31% and 33%. Here we report the details of the TF conductor development and the performance test result of them carried out in SULTAN. Regarding the conductor development, the internal-Sn-processed strand characteristics, strand cabling, twist pitch and characteristics of the conduits for the conductors are presented. For an extended understanding of the conductor design and performance, the SULTAN test results are presented and the effect of the void fraction variations is discussed based on the results.

An acoustic resonator for determining the void fraction of bubbly mercury flows

An acoustic resonator for measuring free-gas void fraction of a helium-mercury mixture is investigated. We employ a vertical, stainless steel cylindrical waveguide with a 5.08-cm i.d., a 1.27-cm wall thickness, a 40-cm length, and pressure-release boundary conditions at both ends. A bubble injection flow loop produces 2-phase mixtures of varying void fraction that flows upwards through the tube, spills over, and recirculates. The resonator is driven from the top by a 2.54-cm diameter circular piston affixed to an electrodynamic shaker. A hydrophone mounted 1 cm above the tube bottom is used to measure the frequency response of the system. Sound speed is inferred by assuming a linear dependence of axial mode number on mode frequency, and void fraction is calculated assuming a mixture sound speed for a bubble population with maximum sized much smaller than the resonant sizes in the modal frequency range (Wood's limit). The system was validated using non-bubbly water and water-air mixtures of different void fractions. Void fraction measurements for Helium-Mercury mixtures will be presented. [Supported by the ORNL Spallation Neutron Source, which is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U.S. Department of Energy.]

Hydrodynamic drag of a flow of steam-water mixture in a pebble bed

Analysis is performed of the effect of the basic process parameters on the pressure loss under conditions of two-phase flow in a pebble bed. Relations are suggested, which generalize the available data of experiments involving steam-water flows in pebble beds (the ranges of variation of pressure from 0.9 to 15.6 MPa, of mass velocity in the pebble bed from 180 to 770 kg/(m2 s), and of steam quality from zero to 0.48), as well as the data for porous structures of sintered spherical bronze particles of small diameter (0.16–0.63 mm) at relatively low pressures (0.2–0.59 MPa) and low mass velocities (10 to 27 kg/(m2 s)), and in a wide range of variation of void fraction (0–1.0).

Void Fraction Variations in a Fractal-Like Branching Microchannel Network

Based on predictions of lower pressure drop penalties in fractal-like branching channels compared to parallel channels, an experimental investigation of two-phase void fraction variations was performed. The flow network, mimicking flow networks found in nature, was designed with a self-similar bifurcating channel configuration and etched 150 μ m into a 38.1 mm diameter silicon disk. A Pyrex® cover was anodically bonded to the silicon disk to allow for flow visualization. The length and width scale ratios between channels on either side of a bifurcation are fixed. The channel widths range in size from 100 μ m to 400 μm over a total channel length of approximately 17 mm. Experimental results of flow boiling are presented for a heater energy input power of 66 W and an inlet water flow rate of 45 g/min at a fixed inlet fluid temperature of 88°C. High-speed, high-resolution imaging was used to visualize the flow and quantify void fraction values in several channels within a branching structure. Both time-averaged and instantaneous two-dimensional void fraction data are presented, showing a correlation between channels at the same bifurcation level and between channels at different bifurcation levels.

Hot Gas Filtration and Heat Exchange in a Packed Bed Using Lapilli as a Granular Medium

A study was conducted on gas filtration and heat exchange in a fixed-bed heat exchanger−filter (FHEF) where we have removed fine dust particles from gases and exchanged heat with the granular medium (Lapilli). The influence of a number of variables was examined including gas velocities, gas temperatures, concentration of dust, height of the FHEF, dust and granular mean size, and pressure drop. The collection efficiency was found to decrease with increasing temperature, and the pressure drop increases when the size of the granular medium decreases. A numerical model for gas filtration and heat exchange using the finite element method was developed. It predicts the two-dimensional transient response of both solid and fluid phases as well as the variation of the specific deposit and void fraction of the FHEF with time. The numerical model incorporates variation in void fraction, specific surface area of the medium, and transport coefficient because of the combined processes of filtration and heat exchange.

Spatio-temporal evolution of void fraction in liquid–solid inverse fluidized bed

Experiments were conducted to investigate the unsteady state void fraction characteristics of inverse fluidized bed (IFB) subjected to step change in liquid velocity. The resulting temporal and spatial evolution of void fraction in IFB is studied for the first time in the literature. The experimental profiles could be explained by the existing theories. The particle bed model proposed by Foscolo and Gibilaro [1987. Fluid dynamic stability of fluidized suspensions: the particle bed model. Chemical Engineering Science 42(6), 1489–1500.] is used to predict the experimental spatio-temporal variation of void fraction, and the time history of bed surface and voidage interface. In general, a good agreement is obtained between the experimental results and model prediction. The hyteresis phenomenon occurring during unsteady state conditions is also explained.

Single rod experiments on transient void behavior during low-pressure reactivity-initiated accidents in light water reactors

A series of experiments for fast transient void behaviors during cold shutdown reactivity-initiated accidents in light water reactors has been performed with a single simulated fuel rod at the atmospheric pressure. A fast-response impedance technique was applied to the void fraction measurement. It was confirmed that, based on the comparison with X-ray technique, the impedance technique was capable of measuring void fraction within an allowable range of accuracy for fast transient two-phase flows. Experimental data was accumulated concerning the onset of net vapor generation, local void fraction, water temperature and pressure. The observed tendency on the onset of net vapor generation was qualitatively agreed with the Saha and Zuber model. However, the model overestimated local water subcooling at the onset of net vapor generation. Inlet water subcooling largely influenced the variation of local void fraction. It was observed that larger inlet water subcooling resulted in repeated growth and collapse of voids to suppress a continuous increase in void fraction.

Study of periodically excited bubbly jets by PIV and double optical sensors

Interactions between large coherent structures and bubbles in two-phase flow can be systematically observed in a periodically excited bubbly jet. Controlled excitation at fixed frequency causes large eddy structures to develop at regular intervals. Thus, interactions between large vortices and bubbles can be studied with PIV and double optical sensors (DOS) using phase-averaging techniques. A number of results on the time and space dependence of velocities and void fractions are presented revealing physical interactions between the liquid flow field and bubble movement as well as feedbacks from bubble agglomeration on the development of flow structures. A clear indication of bubble trapping inside the vortex ring is the generation of a bubble ring that travels with the same velocity as the vortex ring. The DOS results indicate clustering of the bubbles in coherent vortex structures, with a periodic variation of void fraction during the excitation period.

Application of dual-plane ERT system and cross-correlation technique to measure gas–liquid flows in vertical upward pipe

The progress of process tomography provides a new method for two-phase flow measurement. The dual-plane electrical resistance tomography (ERT) is combined with the correlation measurement technique to carry out the two-phase flow measurement in which the continuous phase is conductive liquid. The method of the estimation of void fraction and the disperse phase velocity by extracting the eigenvalue of the dual-plane ERT boundary measured data is presented. This method is applied to the transient flow-rate measurement of the air–water two-phase flow in vertical pipe. The information of disperse phase void fraction and distribution variation with time change can be considered adequately, and the estimated value of disperse phase void fraction and velocity can be gained fairly accurately in this method, which provides the data for the calculation of the transient flow-rate. The experiment results indicate that this kind of measurement method is valid when the distance between the upstream and downstream measured cross section is short enough.

Two-phase flow pattern maps for vertical upward gas–liquid flow in mini-gap channels

An experimental visualization investigation is carried out to study the characteristics of flow boiling in a narrow channel represented by flow patterns, local void fraction variations and the net vapor generation (NVG) point. The experiment is conducted in a narrow rectangular channel with the size of 1 mm × 30 mm × 650 mm, which is heated by two sides using transparent ITO (Indium Tin Oxides) film for better visualization effect. In this study, it is found that the flow pattern transition boundary of heated vapor-water flow is consistent with several existing flow pattern transition lines for air-water flow experimental data and flow pattern transition criteria in narrow rectangular channel. Based on the experimental data of the void fraction and the thermodynamic equilibrium quality at the NVG point, assessments of the accuracy of existing models and correlations are performed. Finally, new empirical correlations for more accurate predictions for the void fraction and the NVG point are proposed based on the experimental results. The predictions using the present correlations agree well with the experimentally measured values with a mean absolute error (MAE) of 46.07% for the void fraction and 36.19% for the NVG point.

Laboratory Measurements of Void Fraction and Turbulence in the Bore Region of Surf Zone Waves

Laboratory measurements of the instantaneous free surface, horizontal velocity, and void fraction fluctuations were made simultaneously for three cases of regular waves breaking on a plane slope. The data were reduced by ensemble averaging to quantify the temporal variation of the turbulence intensity and void fraction above trough level in the aeration region of the breaking waves. The cross-shore location of the measurements was restricted to the transition region marked by a rapid decrease in wave height. The study showed that the maximum ensemble-averaged void fractions were between 15 and 20% and that the temporal variation of the normalized void fraction above the still water level could be modeled by linear growth followed by exponential decay. The temporal variation of void fraction above the still water normalized by the wave period and average void fraction appears to be self-similar.

A Gas-Liquid Flowmeter Combining Vertical and Horizontal Pressure Fluctuations Induced by Local Void Fraction Variations

Two AC signals generated by two sensors mounted in two elbows at the ends of vertical and horizontal branches of a metering pipe are simultaneously recorded and analyzed. The AC signals are mainly due to fluctuations in stagnation-pressure caused by the local oscillations of local void fraction and to transport velocity of the gas-liquid flow. Features extracted from stochastic interpretation of the two signals are strongly related to gas and liquid flowrates. Laboratory and field testing of the new meter demonstrated that, for the same gas and liquid flowrates, probability distribution functions (PDF) determined from statistical analysis of “vertical” and “horizontal” AC signals are unique. For broad ranges of gas-liquid ratios, features extracted from the PDF are linearly related to gas and liquid flowrates.

2116 2x3 ロッドバンドル流路におけるボイドドリフト(二流体モデルによる流量再配分計算)

The purpose of this study is to describe analytically two-phase void drift phenomena in a 2×3 rod bundle channel simulating BWR fuel rod bundle. For that, a subchannel analysis code, which was developed by the present authors and based on a two-fluid model, was used, and prediction of gas and liquid flow redistribution due to void drift between subchannels in the 2×3 rod bundle channel was conducted. In the code, a void settling model for evaluating the void drift effect was incorporated with usual conservation equations of mass and momentum. From a comparison, it was found that the subchannel analysis code can predict well the data on subchannel flow and void fraction variations in the 2×3 rod channel if an appropriate correlation was employed to evaluate wall and interfacial friction forces.

CFD Simulation and Experimental Study of Liquid Dispersion in Randomly Packed Metal Pall Rings

The radial dispersion of liquid phase in a large scale packed column (0.6 m diameter) filled with 25.4 mm metal Pall rings was studied in terms of liquid radial concentration profiles using a tracer technique. The effects of operating condition and liquid distributor design on liquid dispersion at different packed bed heights were investigated. It was found that the effect of gas flow rate is most significant, especially above the loading point. The extent of radial mixing in the liquid phase was found to increase with the gas flow rate. To simulate the liquid dispersion in a random packed column, a set of volume averaged equations for the hydrodynamics and mass transport has been solved with the commercial Computational Fluid Dynamics (CFD) software, CFX4.2. The empirical correlations for the two-phase flow pressure drop, inter-phase interaction, and mass dispersion have been incorporated into the volume averaged equations to provide the closure relations. The spatial variation of void fraction in packed beds due to inhomogeneity of the bed, particularly near the wall has also been included in the models. This approach can be used to predict the flow and concentration development in both radial and axial directions taking into account different radial and axial dispersion. The simulation results agree well with the experimental data. Capturing these features is an important first step in building a Computational Fluid Dynamics based mass transfer model to study distillation and absorption in packed columns.

A numerical investigation of unsteady bubbly cavitating nozzle flows

The effects of unsteady bubble dynamics on cavitating flow through a converging-diverging nozzle are investigated numerically. A continuum model that couples the Rayleigh–Plesset equation with the continuity and momentum equations is used to formulate unsteady, quasi-one-dimensional partial differential equations. Flow regimes studied include those where steady-state solutions exist, and those where steady-state solutions diverge at the so-called flashing instability. These latter flows consist of unsteady bubbly shock waves traveling downstream in the diverging section of the nozzle. An approximate analytical expression is developed to predict the critical backpressure for choked flow. The results agree with previous barotropic models for those flows where bubble dynamics are not important, but show that in many instances the neglect of bubble dynamics cannot be justified. Finally the computations show reasonable agreement with an experiment that measures the spatial variation of pressure, velocity and void fraction for steady shockfree flows, and good agreement with an experiment that measures the throat pressure and shock position for flows with bubbly shocks. In the model, damping of the bubble radial motion is restricted to a simple “effective” viscosity, but many features of the flow are shown to be independent of the specific damping mechanism.

F112 強制対流気液二相流の微細流動構造

Microscopic structure of forced-convective air-water two-phase flow was experimentally investigated. In the experiment, setting the flowrates of water and air at several values, bubbly and slug flows were realized in a vertical circular tube of 19.7mm I.D. The holdup sensor was used to measure the time variation of instantaneous cross-sectional area-averaged void fraction about 1m downstream from the two-phase mixer. The void data were analyzed by means of the discrete Fourier transformation (DFT). The analytical results showed that the change from bubbly to slug flows are rather continuous. It was considered that the bubble clusters formed in the transition regime between bubbly and slug flows play important role in the present experiments.

Analysis of liquid outflow from single-outlet vessels

AbstractThe paper presents experimental and theoretical investigation of the outflow of liquids from single-outlet vessels incorporating a vertical downcomer. In such vessels the liquid leaving through the outlet of the downcomer is replaced by another fluid, which enters the vessel-downcomer system through the same opening. Experimental results of the variation of the void fraction and pressure in the vessel with elapsed time are presented. It is shown that the flow is generally oscillatory, and the conditions required for the establishment of the fully developed oscillatory flow are reported. A theoretical model of the fully developed oscillatory flow is developed. The model describes well the frequency of the oscillatory flow and the variation of the gas void fraction in the vessel with elapsed time. The model somewhat overpredicts the amplitude of the pressure oscillations. Thermodynamic aspects of this flow are also considered.

Analysis of liquid outflow from single-outlet vessels

The paper presents experimental and theoretical investigation of the outflow of liquids from single-outlet vessels incorporating a vertical downcomer. In such vessels the liquid leaving through the outlet of the downcomer is replaced by another fluid, which enters the vessel-downcomer system through the same opening. Experimental results of the variation of the void fraction and pressure in the vessel with elapsed time are presented. It is shown that the flow is generally oscillatory, and the conditions required for the establishment of the fully developed oscillatory flow are reported. A theoretical model of the fully developed oscillatory flow is developed. The model describes well the frequency of the oscillatory flow and the variation of the gas void fraction in the vessel with elapsed time. The model somewhat overpredicts the amplitude of the pressure oscillations. Thermodynamic aspects of this flow are also considered.

The Behaviour of Gas-Liquid and Vapour-Liquid Upward Bubbly Flows Passing Through a Vertical Venturi

A comparison between the flow of air-water and steam-water mixtures through a venturi has established key differences between the two systems. The variations of local void fraction of two-phase flows through a converging-diverging venturi have been studied using gamma ray attenuation. The independent variables are the liquid and gas flow rates, the temperature and the axial position in the flow direction. The difference between the two systems is found to be most evident at, and downstream of the throat. In steam-water, void fraction increases substantially at, and beyond the throat as a result of the low local pressure at the throat. On the other hand, with air-water the void fraction at the throat decreases at low temperature because of local changes in the slip ratio of the two phases. The effect of temperature on flow through a venturi is found to be more significant than it is in a straight pipe. This phenomenon becomes more important as the system approaches its boiling point.

Acoustic propagation through surface ship wakes

To predict acoustic propagation through a ship wake, an understanding is required of the physical properties (void fraction, spatial and temporal variation of the bubble structure, oceanic ambient dynamics, for example) of the wake, as well as an acoustic propagation model capable of handling the changes in these properties. The wakes of large surface ships contain high concentrations of air bubbles of various sizes and nonuniform distributions, which give the wake decidedly inhomogeneous acoustic properties. The extant literature contains little quantitative information. Limited material has been collected and combined with a notional hypothesis regarding the random structure to produce a physical model for some of the relevant physical properties. The model has been used as a basis for propagation calculations for this complex situation. Two methods have been employed for those calculations: range-dependent, Gaussian beam ray tracing and a 2-D parabolic equation technique, the latter having previously proven useful for turbulent atmospheric conditions. The acoustic model predictions provide a clear indication of the complexity of the problem and motivation for development of a 3-D propagation model. [Work supported by ONR Code 321 (ARL Project).]