Variation Of Void Fraction Research Articles

K-1440 上昇急拡大円管における気液2相流の流動構造(S16-4 キャビテーションおよび気液系流れのダイナミクス(4))(S16 キャビテーションおよび気液系流れのダイナミクス)

The variations of void fraction, bubble diameter and bubble velocity are measured using an electrical resistivity probe when two-phase flow goes through the region of sudden area expansion in the vertical pipe (from 0.05m i.d. to 0.1m i.d.). Air bubbles in the size of 4 to 10mm are made by bubble generator and injected at the position of z=-4m. (z axis is along the vertical pipe and z=0m is taken as the position of sudden area expansion.) In this study two different radial void distributions in the pipe are considerd. One is called as sliding flow regime, which occurs when the bubble diameters are less than 5mm. The other is termed as coring flow regime, which takes place when the bubble diameters are greater than 5mm. At z=0.4m the void fraction has a similar distribution to that in the region before the area expansion. But for just after expansion, average bubble diameter increases by 20%. This shows that a part of bubbles just after expansion merge together, and then break up. The observation using CCD camera can also confirm the merging and breakup of bubbles

614 強制対流過渡気液二相流の研究(GS-6 二相流)

This paper shows the experimental and analytical study of forced convective transient airwater two-phase flow. In experiment, the flow rates of water and air were independently at the entrance, the transient bubbly two-phase flow was realized in vertical channel. Then, measurement the transient variation of pressure, differential pressure and void fraction at measuring position. Concerning the measure of void fraction , use the character of air which is not conductive. Moreover , supply the data for the analysis.

CFD Modeling of Mass-Transfer Processes in Randomly Packed Distillation Columns

The volume-averaged equations for velocity and concentration fields have been used to simulate the hydrodynamics and mass-transfer processes in randomly packed distillation columns. This approach is regarded as a second-generation computational fluid dynamics (CFD) based model, and a significant departure from the traditional one-dimensional, first-generation models. The model has ability to capture radial and axial variations in flow and mass-transfer conditions. The spatial variation of void fraction has been included to take into account the effect of bed structures. The simulation results have been compared with experimental data reported by Fractionation Research, Inc. (FRI) which performed their tests in a 1.22-m-diameter column with a packed bed height of 3.66 m. For validation, we have used data obtained with 15.9-, 25.4-, and 50.8-mm metal Pall rings at various operating conditions. Good agreement between CFD predictions and published experimental data has been obtained. This is regarded as an en...

Thermalhydraulic Investigation Using a Test Facility Simulating 200 MW Integral Heating Reactor

Since the middle of 1980's, a research program on heating reactor has been carried out at Institute of Nuclear Energy Technology (INET) of Tsinghua University in China, the heating reactor is a light water cooled, integral type natural circulation reactor with low system pressure and low core exit quality, it has the passive safety features and other advanced characteristics of the next generation reactors. Because of a relatively long riser and low system pressure, a small quality perturbation at the exit of the core will lead to a relatively large variation of void fraction in the riser, so will the driving force. All of the above features made it very different from the typical PWRs and BWRs. In order to guarantee the safe operation of the heating reactor, a full scale test loop HRTL-200, simulating HR-200 heating reactors in main geometry and operation parameters, was constructed and operated under the broad test conditions covering the 200 MW heating reactor operation conditions at INET. The HRTL-200 test facility, some of steady-state and two-phase flow instability test results from the test facility and the consideration how to choose the optimal design parameters for the heating reactors are shown in this paper. The range of experimentally studied parameters are: the system pressure is from 1.0 to 4.0 MPa, the largest heat flux is about 50 W/cm2, and the quality at the exit of the test section is less than 6%.

Analysis of Coupled Nuclear/Thermal Instabilities of Two-Phase Flows in Parallel Boiling Channels

An analytical model is proposed to study the two-phase flow instabilities of in-phase and out-of-phase modes in parallel boiling channels. This model takes into consideration the interaction between channels with different void fraction variations by modeling the diffusion of neutrons. Linear stability analysis in the frequency domain is performed to obtain stability boundaries and flow oscillation modes. In the case of parallel two channels, two types of stability boundaries are found to exist on the stability map; one for the instability of the in-phase mode and the other for that of the out-of-phase mode respectively. For a certain range of the void-reactivity coefficient, the void-reactivity feedback loop is found to be unstable in a certain region on the stability map, which has an effect on the stability of the in-phase mode. The interaction between channels affects the instability of the out-of-phase mode. The results are also obtained for the case of three and four channels. It is found that the channels having stronger interaction between them are more likely to oscillate in-phase with each other, while those having weaker interaction are more likely to oscillate out-of-phase with each other.

Two-fluid model studies for high density two-phase liquid metal vertical flows

Liquid metal magnetohydrodynamic power converters (LMMHD PC) have been recently proposed for electrical power generation. These systems contain two-phase vertical flows consisting of high density liquid metals and suitable gas–vapor. Optimum design of LMMHD power plants require accurate modeling of two-phase flows in the riser. A two-fluid model has been developed for this purpose. One-dimensional, steady state two-fluid flow equations consisting of conservation of mass, momentum of each phase along with auxiliary relations have been solved numerically by the Runge–Kutta method. Interfacial drag force corresponding to multi-bubble, churn turbulent and slug flow based on Ishii et al. and Taitel classification has been used. Effect of variation of void fraction and phase velocities of the fluids across the cross section of the pipe has been studied based on Ishii et al. model by modifying relative velocity and incorporating appropriate coefficients in the conservation equations. Bubble size at the mixer orifice exit has been calculated using the equations of Kumar et al. In order to verify the accuracy of the model, a nitrogen–mercury experimental system has been set up. Void profiles have been measured using gamma-ray attenuation method. Void fraction, slip and pressure at different locations were determined for the mass fluxes varying from 0.125 to 2.302 kg/sm 2 for nitrogen and 5.52×10 3 to 12.26×10 3 kg/sm 2 for mercury. The predicted values have been compared with the experimental data. The void fraction values matched well with the experimental data within 10% and within 20% when cross-sectional effects were included. The over all pressure values were within 13% and 8%, respectively, while the slip values deviated within 25% and 27%, respectively. In general, the model matched better with experimental data when the cross-sectional effects were not included. This is due to the high density of the liquid metal and relatively larger pipe diameter.

Measurements of void fraction by an improved multi-channel conductance void meter

An improved multi-channel conductance void meter (CVM) was developed to measure a void fraction. Its measuring principle is based upon the differences in electrical conductance of a two-phase mixture due to the variation of void fraction around a sensor. The sensor is designed to be flush-mounted to the inner wall of the test section to avoid flow disturbances. The signal processor with three channels is specially designed so as to minimize inherent bias error due to the phase difference between channels. It is emphasized that the guard electrodes are electrically shielded in order not to affect the measurement of two-phase mixture conductance, but to ensure that the electrical fields are evenly distributed in the measuring volume. Void fraction is measured for bubbly and slug flow regimes in a vertical air–water flow and statistical signal processing techniques are applied to show that CVM has good dynamic resolution which is required to investigate the structural developments of bubbly flow and the propagation of void waves in a flow channel.

Hot gas filtration, using a moving bed heat exchanger-filter (MHEF)

A study was conducted on the use of a Moving Bed Heat Exchanger-Filter (MHEF) for removing fine dust particles from gases. The influence of a number of variables was examined, including gas velocities, solid velocities, gas temperatures and dust sizes. The collection efficiency was found to decrease with increasing temperature; the total collection efficiency decreases strongly when the solid velocity increases. A stable numerical model for filtration and heat exchange was developed that predicts the two dimensional transient response of both solid and fluid phases. The numerical model incorporates variation in void fraction, velocities and transport coefficient due to combined processes of filtration and heat exchange.

Slug flow regime identification from dynamic void fraction measurements in vertical air-water flows

An existing design of conductivity probe for the measurement of void fraction has been developed and tested. Calibration checks showed that the time averaged output of the instrument was close to linear over the whole range of void fraction, with a tendency to overestimate the true value. Computer modelling of the probe's response to different void distributions supported the calibration results. Two meters were used to examine the dynamic variation of void fraction in air water flows in a vertical 32 mm diameter tube. Flow patterns ranging from bubbly to annular were observed. Examination of the void fraction traces and their probability distribution functions enabled the identification of six flow regimes: discrete bubble flow, spherical cap bubble flow, stable slug flow, unstable slug flow, churn flow and annular flow. The locations of flow regime transition boundaries on a flow pattern map, deduced from this objective analysis, are in agreement with the data of other workers. The unstable slug flow regime corresponds to a region some workers have identified as churn flow, it provides explanations for some of the apparent anomalies concerning the location of the slug to churn flow boundary. Cross-correlation of the signals from the two probes has provided statistical data on the lengths of slugs and Taylor bubbles which suggested modifications to the previous theories enabling the entry to and exit from the unstable slug flow regime to be predicted.

Linear Stability Analysis on Instabilities of In-phase and Out-of-phase Modes in Boiling Water Reactors

An analytical model is proposed to study the two-phase flow instabilities of in-phase and out-of-phase modes in boiling water reactors. This model, developed for parallel boiling channels with the void-reactivity feedback loop, takes into consideration the interaction between channels with different void fraction variations. Linear stability analysis in the frequency domain is performed to reveal the mechanisms of the instability. Stability boundaries for the system are obtained on the stability map. In the case of parallel two channels, two types of stability boundaries are found to exist on the stability map; one for the instability of the in-phase mode and the other for that of the out-of-phase mode respectively. It is found that the interaction between channels affects the instability of the out-of-phase mode. For a certain range of the void-reactivity coefficient, the void-reactivity feedback loop is found to be unstable in a certain region on the stability map, which has an effect on the stability of the in-phase mode. Froude number is important to the stability of the system, especially to the instability of the in-phase mode.

Linear Stability Analysis on Instabilities of In-phase and Out-of-phase Modes in Boiling Water Reactors.

An analytical model is proposed to study the two-phase flow instabilities of in-phase and out-of-phase modes in boiling water reactors. This model, developed for parallel boiling channels with the void-reactivity feedback loop, takes into consideration the interaction between channels with different void fraction variations. Linear stability analysis in the frequency domain is performed to reveal the mechanisms of the instability. Stability boundaries for the system are obtained on the stability map. In the case of parallel two channels, two types of stability boundaries are found to exist on the stability map; one for the instability of the in-phase mode and the other for that of the out-of-phase mode respectively. It is found that the interaction between channels affects the instability of the out-of-phase mode. For a certain range of the void-reactivity coefficient, the void-reactivity feedback loop is found to be unstable in a certain region on the stability map, which has an effect on the stability of the in-phase mode. Froude number is important to the stability of the system, especially to the instability of the in-phase mode.

Bubble to slug flow transition in vertical upward two-phase flow of cryogenic fluids

An experimental study on the dispersed bubble to slug flow transition in a cryogenic gas-liquid two-phase flow is reported in this work. The transition is detected based on an analysis of the probability distribution function of pulse widths produced by a multivibrator in response to the temporal variation of local void fraction in the flow. An analytical model is also presented to explain the experimental results. The model is based on a bubble population balance including both breakage and coalescence in an unsteady turbulent two-phase mixture. It predicts the transition as an effect of the occurrence of certain bubbles above the critical size which have a very high tendency to coalesce with other bubbles.

Experimental study of the two-phase flow dynamics in nucleate and film pool boiling

An experimental study is conducted to measure the transient characteristics of the local boiling process close to the heated wall. Nucleate and film boiling two-phase parameters were measured for power oscillations in a small horizontal heater immersed in stagnant liquid. A method for measuring fast temporal variations of void fraction and the interfacial impact rate is presented. Ensemble averages of the data have been carried out to obtain time evolution of the void fraction and the interfacial impact rate at the probe tip. R-113, R-11 and water have been used as working fluids, finding important differences between the behavior of the freons and water at the boiling crises. The critical heat flux in freons is associated with a sudden transition in the impact rate and the void fraction, which supports the theory of hydrodynamic instability. On the other hand, the experiments in water suggest the existence of two transitions: a hydrodynamic transition from bubbling regime to “mushroom” regime followed by a liquid film dryout type of CHF, in agreement with the theory of Haramura & Katto.

Wave form of short shock wave reflected from bubble layers in water

Experimental and theoretical investigation of an influence of the local layer parameters, presence of rigid boundary on the bubble dynamics under the action of strong pressure pulse as well as the formation of reflected signal are reported. It is shown that duration of reflected pulse, its amplitude and presence of an oscillations behing it depend not only on the amplitude and duration of incident pulse, but on the spatial gradient of void fraction and distribution of the bubble radii within the layer. This phenomenon is described within the framework of the model, taking into account a specific variation of void fraction inside the layer as well as the specific bubble dynamics in it.

Characteristics of gas-liquid two-phase flow in a capillary tube

Gas-liquid two-phase phenomena in capillary tubes were investigated with special attention on the flow patterns, the time varying void fraction and pressure loss. The directions of flow were vertical upward, horizontal and vertical downward. Pipe inner diameters used were 1 mm, 2.4 mm and 4.9 mm. As a result it is made clear that due to the strong effects of surface tension the flow pattern is not severely affected by the direction of flow, the smaller the pipe inner diameter, the easier the formation of liquid slug and the pressure loss in a unit length takes much larger than the estimated value by the Chisholm equation.

A phenomenological model of the thermal hydraulics of convective boiling during the quenching of hot rod bundles. Part III: Model assessment using Winfrith steady-state, post-CHF, void-fraction and heat-transfer measurements and Berkeley transient-reflood test data

Alter completion and assessment of the thermal-hydraulic model developed in companion papers, we performed further independent assessment calculations using recently available Winfrith steady-state, post-CHF, void-fraction and heat-transfer data, and Berkeley transient-reflood test data. This paper discusses the results of those calculations. The thermal-hydraulic model in the Transient Reactor Analysis Code, Modification 2 (TRAC/PF1-MOD2, or simply TRAC), was used to predict the axial variation of void fraction as measured in Winfrith post-CHF tests. The predictions for reflood calculations were reasonable. The model correctly predicted the trends in void fraction due to the effect of pressure and power, with the effect of pressure being more apparent than that of heat flux. The effects of pressure, test-section power, and flow rate on the axial variation of tube wall temperature are predicted reasonably well for a large range of operating parameters. The predicted precursory cooling rates in Berkeley transient-reflood tests were in reasonably good agreement with the measured data. For high-test section input powers, oscillations associated with the void-fraction predictions observed. The predicted quench-front velocities (rewetting velocities) and their variations along the test tube were also found to be in reasonably good agreement with measured data.

Prediction of pressure drops in forced convection subcooled boiling water flows

A semi-empirical model is described for the prediction of the pressure drop in subcooled convective boiling of water flows in tubes subject to uniform high heat fluxes. The flow in the single-phaseliquid (SPL) regime, the partially-developed boiling (PDB) regime and the fully-developed boiling (FDB) regime is modeled including the effects of nonequilibrium quality and void fraction and property variations along the tube. Improved models for the PDB and FDB regimes and extensive comparisons with experimental data are described. Pressure drop predictions of the model are compared with over 100 experimental runs and found to agree within approximately ± 20%.

Heat Transfer With Vaporization of a Liquid by Direct Contact in Another Immiscible Liquid: Experimental and Numerical Study

An experimental setup was designed to study direct-contact evaporators using a liquid dispersed in another immiscible liquid. The study was carried out on an n-pentane–water system to determine the influence of different parameters on these systems, and consequently to construct a model for this type of evaporator. An optical probe was used to measure the local void fraction. At different column abscissas along a selected diameter, the local void fraction variations were determined. The shape of the curves can be attributed to the different processes occurring in the spray column. A one-dimensional heat transfer model in the spray column was established. Simplifying assumptions were used to establish and resolve the set of equations governing heat transfer and two-phase flow. The vaporization process induces a volumetric expansion of the two-phase mixture. A theoretical model was used, in which the coalescence between the spherical fluid particles is taken into account. Different coalescence laws dependent on particle density were introduced into the theoretical model and then tested. The numerical results are discussed and compared with the experimental data obtained for the n-pentane–water system.

In situ ellipsometric study of amorphous silicon/amorphous silicon-carbon interfaces

An ellipsometric study of the growth of hydrogenated amorphous silicon/hydrogenated amorphous silicon-carbon interfaces and multilayer structures is presented. Kinetic ellipsometry is used at a fixed wavelength to study the growth of the materials at the interface between the a-Si:H and a-Si1−xCx:H layers as a function of the carbon content. It is observed that for samples with carbon content below 36%, the a-SiC:H grows uniformly on top of the a-Si:H. However, as the carbon content is increased beyond this point, the growth of the alloy material becomes nonuniform. Various growth models were used to fit the experimental data and the best fits were obtained using a multilayer model of varying void fraction to describe the inhomogeneous growth. Multilayers were grown using a-Si:H and a-SiC:H. It was observed that for low carbon concentrations (x<0.36), the successive layers of the multilayer structure are reproducible and the growth remains homogeneous for all the layers.

The dependence of axial dispersion on non-uniform flows in beds of uniform packing

Axial dispersion in packed beds in analyzed using both the dispersed plug flow model and a nonplug flow model which has been previously used successfully to treat dispersion in laminar and turbulent flow in tubes. The velocity profiles required for predicting dispersion with the nonplug flow model was based on a modified Navier—Stokes equation to account for the resistance of the packing to flow. This term allows radial variations in void fraction and predicts their effect on the velocity profile. Numerical solution of the flow model allows the prediction of dispersion coefficients for flow in packed beds. Experimental results obtained in this study compare favorably with predicted values and with dispersion data available in the literature.