Time-averaged Void Fraction Research Articles

Void Fraction Measurement in Subcooled-Boiling Flow Using High-Frame-Rate Neutron Radiography

A high-frame-rate neutron radiography (NR) technique was applied to measure the void fraction distribution in forced-convective subcooled-boiling flow. The focus was experimental technique and error estimation of the high-frame-rate NR. The results of void fraction measurement in the boiling flow were described. Measurement errors on instantaneous and time-averaged void fractions were evaluated experimentally and analytically. Measurement errors were within 18 and 2% for instantaneous void fraction (measurement time is 0.89 ms), and time-averaged void fraction, respectively. The void fraction distribution of subcooled boiling was measured using atmospheric-pressure water in rectangular channels with channel width 30 mm, heated length 100 mm, channel gap 3 and 5 mm, inlet water subcooling from 10 to 30 K, and mass velocity ranging from 240 to 2000 kg/(m2·s). One side of the channel was heated homogeneously. Instantaneous void fraction and time-averaged void fraction distribution were measured parametrically. The effects of flow parameters on void fraction were investigated.

Local void fraction and velocity field description in horizontal bubbly flow

The internal phase distribution of concurrent, air–water bubbly flow in a 50.3-mm i.d. transparent horizontal pipeline has been experimentally investigated using the hot-film anemometry technique. Liquid and gas volumetric superficial velocities ranged from 3.8 to 5.0 m s −1 and 0.25 to 0.8 m s −1, respectively. The time-averaged local values of the void fraction, bubble-passing frequency, the mean liquid velocity, and the liquid turbulence fluctuations were measured. The experimental results indicate that the void fraction and bubble-passing frequency have local maxima near the upper pipe wall, and the profiles tend to flatten with increasing liquid flow rate. For the horizontal bubbly flow, the observed time-averaged local void fraction can reach 65%, whereas the bubble frequency may reach a value of 1400 s −1. The fact that the local peak void fraction in all cases never exceeds 65% indicates that the maximum bubble packing exists within the channel. It was found that increasing the gas flow rate at fixed liquid flow rate would increase the local void fraction and bubble-passing frequency. The axial liquid mean velocity showed a relatively uniform distribution except near the upper pipe wall, where a sharp reduction in velocity was noticed. The local mean liquid velocity and turbulence fluctuations increased with gas flow rate. An interesting feature of the liquid velocity distribution is that it tends to form a fully-developed turbulent pipe-flow profile at the lower part of the pipe irrespective of the liquid and gas superficial velocities. At very low local void fractions, the turbulent intensity consistently tended to be slightly lower than the single-phase. However, at high void fraction, i.e. high bubble population, introduction of gas strongly enhances the turbulence level as characterized by the turbulent velocity fluctuation and turbulent intensity. In general, it was concluded that the local turbulence intensity is mainly affected by the local void fraction distribution.

Measurement of Boiling Flow by High-Frame-Rate Neutron Radiography. 1st Report. Error Estimation and Void Fraction Measurement.

Void fraction measurement by high-frame-rate neutron radiography and the error estimation were conducted. Measurement error on instantaneous void fraction and time-averaged void fraction of forced-convective subcooled boiling was evaluated experimentally and analytically. Measurement error would be within 18% and 2% for instantaneous void fraction (measurement time is 0.89 ms), and time-averaged void fraction, respectively. Void fraction distribution of subcooled-boiling was measured using atmospheric-pressure water in rectangular channels with channel width 30 mm, heated length 100 mm, channel gap 3 mm and 5 mm, inlet water subcooling from 10 to 30 K and mass velocity ranging from 240 to 2000 kg/(m2·s) . One-side of the channel was heated homogeneously. Instantaneous void fraction and time-averaged void fraction distribution were measured par-

Investigation of large amplitude stratified waves in a CANDU-type 37 rod nuclear fuel channel by a real-time neutron radiography technique

A real-time neutron radiography (RTNR) system is used to determine two-phase flow parameters for a horizontal co-current two-phase flow channel with a cylindrical 37 rod bundle. Image processing techniques are applied to visualize the two-phase flow, and to determine flow regime, cross-sectional averaged void fraction, time averaged void fraction, and void distribution. The experimentally determined flow regime map disagrees with existing flow regime models developed for the cylindrical rod bundles. A new flow regime is observed and designated large amplitude stratified wavy (LASW) flow. The results show that the LASW flow regime may be due to a combination of undeveloped flow phenomena, boundary conditions, and circumferential cross flow occuring in the bundle. The rods in the bundle may act as a dampener to the vertical flow component and hinders the development of the wave into plug or slug flow by changing the momentum of the fluid in the circumferential direction. The effect of gas and liquid superficial velocities and axis position on the void fraction is observed. The existence of a cylindrical bundle in a circular flow channel is found to affect the flow regime, void fraction, void migration, and void fraction fluctuation and these effects are discussed in detail.

Gamma radiation densitometry for studying the dynamics of fluidized beds

In this paper γ-radiation techniques are used to study the behaviour of a 40 cm fluidized bed, consisting of 1.9 mm spherical polystyrene particles. The particles are fluidized at high velocity, i.e. 50% of their terminal velocity, the bed being in the turbulent fluidization regime. Two parallel source–detector pairs are used to study the time-averaged void fraction profile. Instead of a direct inversion using the Abel-transform, a least-squares based algorithm with basis functions is proposed. The dynamics are investigated by inspection of the probability density functions of the recorded time series. It is shown, that close to the static bed height the bed can be almost packed to almost completely transparent. The bed is seen to move violently: large voids and particle clusters travel through the column. Using generalized cross-correlation techniques both upward and downward velocities are found.

Generation of Void in a Molten-Metal Bath by Gas Injection from Below

An experimental apparatus for investigating the injection of nitrogen gas and water into the base of a steel tank containing molten tin has been developed. A first set of experiments based on only gas injection has been used to develop a diagnostic technique using continuous high-energy X rays and digital imaging to observe the mixing process and to measure local and average void fractions in the test section as a function of time and space. This unique application of real-time, high-energy, X-ray imaging has been used to generate two-dimensional mappings of the chordal-average void fraction with spatial resolution corresponding to a 0.43-mm2 cross-sectional area perpendicular to the X-ray path and time resolutions of <5 ms. Void fraction measurements with superficial gas injection velocities from 0.07 to 0.14 m/s into a 0.08-m-deep pool of 683 K molten tin indicate that the time and spatial average integral void fraction at these gas injection rates is relatively constant, in the range from 0.26 to 0.31. Similar injections into pools of 0.14- and 0.15-m depths have also exhibited relatively constant average integral void fractions in the range from 0.18 to 0.26. These values are in good agreement with past integral experimental measurements in mercury, Wood’s metal, and molten steel.

Three-dimensional visualization of void fraction distribution in steady two-phase flow by thermal neutron radiography

Three-dimensional void fraction distributions of a steady air–water two-phase flow in a 4×4 rod-bundle with circular ferrule type spacers were measured by neutron radiography using a CT method. The high flux thermal neutron radiography system at JRR-3M in JAERI was used. Two-phase flow was visualized with a SIT tube camera and time-averaged one-dimensional cross sectional averaged void fraction distributions were calculated. Visualization with high spatial resolution up to 0.18 mm was carried out by using a cooled CCD camera. Projections in 250 directions were obtained and were reconstructed by a filtered back projection method after using some image processing techniques. Animations were made to show the three-dimensional distributions. One-dimensional and three-dimensional void fraction distributions of the steady state two-phase flow in the rod bundle near the spacer were clearly visualized.

Real-time cross-sectional averaged void fraction measurements in vertical annulus gas-liquid two-phase flow by neutron radiography and X-ray tomography techniques

Abstract A Real-Time Neutron Radiography (RTNR) system and a high speed X-ray Computed Tomography (X-CT) system are used to determine the flow regime, the instantaneous cross-sectional averaged void fraction, and the time averaged void fraction in a vertical annulus flow channel. A standard optical video system is also used to observe the flow regime. The annulus flow channel is operated as a bubble column and measurements are obtained for gas flow rates form 0.0 to 30.0 1/min. The flow regimes observed by all three measurement systems via image analysis shows that the results agree well with each other. Both the RTNR and the X-CT systems show that the time averaged and cross-sectional averaged void fraction increases with increasing superficial gas velocity. Time and cross-sectional averaged void fractions determined by the RTNR system agree within 4% with those determined by the high speed X-ray CT technique.

Large bubbles attached to spargers in downwards two-phase flow

Suitably formed gas injection geometries permit optimizing the volume of the bubbles created in oil production pipes without recourse to having to change the flow rates of gas. Unfortunately, little research attention has been dedicated to such a study, which may affect the gas lift method of EOR. To address this knowledge gap and to gain more understanding about the coalescence and entrainment processes of the stationary Taylor bubble, an experimental investigation in a bubble drop-down facility using an air–water system was carried out using advanced instrumentation, wire mesh sensor (WMS) and high-speed video photography. The experiments were performed in a 67 mm internal diameter pipe on a stationary Taylor bubble in a downward liquid flow using three different gas injection geometries. Taylor bubble length, the gas and liquid flow rates approaching the stationary bubble were varied. Also, the wake length below, the stationary bubble was measured at different conditions, of gas and liquid superficial velocities and comparisons were made with existing data. A high-speed camera was used to obtain video images of the flow to validate the measurements taken on the wake lengths. Also, the WMS was placed at two different positions, below the gas injection point to obtain time and cross-sectionally resolved information about the spatial distribution of the flowing phases. This information was used to produce time-averaged void fraction, bubble size distribution and contour plots of the two-phase flow structure. Probability density function (PDF) of void fraction of the wake section of the stationary bubbles showed that the flows are in the bubbly flow region whereas the PDF of void fraction for the entire slug unit depicted a typical twin-peaked slug flow. The results of the measurement of Taylor bubble lengths and the corresponding wake of the bubble lengths showed a good match against current published works. The results revealed that the bubble wake length depends on both the nature of the fluid and the pipe diameter, while on the contrary, the bubble length mainly depends on the gas superficial velocity.

A model for correcting for the effect of fluctuating voids in radiometric diagnosis of two-phase-flow

A simple plane model is proposed for correcting the effect of void fluctuations on void-fraction determination in two-phase-flow systems employing the modified (one-shot) photon-attenuation method. Through a series of benchmark tests, an experimental observation is that if the static void assumption is used, the experimental result will consistently exceed the actual time-averaged void fraction. The accuracy of the void-fraction prediction can be much improved by the correction procedure established in this work.

Isothermal Air-Water Two-Phase Up-and Downward flows in a Vertical Capillary Tube 1st Report, Flow Pattern and Void Fraction.

Knowledge of two-phase flow in a capillary tube is important to understand flow phenomena in narrow passages, for example, in a compact heat exchanger. The present authors have previously reported the characteristics of flow parameters such as pressure drop, and void fraction as well as flow pattern in an air-water two-phase flow in a horizontal capillary tube. In the present paper, we discuss the effect of flow direction on void fraction and flow patterns in two-phase flow in a vertical capillary tube. Two correlations relating bubble velocity with total volume flux and time-averaged void fraction with gas volume flow quality were proposed.

Determination of two-phase interfacial areas by an ultrasonic technique

Two modes of single beam ultrasonic techniques, the pulse-echo and the transmission mode, are developed to measure instantaneous liquid levels, time averaged void fraction and interfacial area in a gas-liquid two phase flow system. The method is non-intrusive and can be easily used in metallic pipes. Flow regime observation by current technique is also discussed.

Stratified gas-liquid two-phase electrohydrodynamics in horizontal pipe flow

Stratified gas-liquid two-phase electrohydrodynamics in pipe flow has been studied experimentally and numerically. Experimental studies were conducted using 1.27 and 1.9*10/sup -2/ m-inner diameter horizontal tubes with air-water two-phase flow for the range of gas surface velocity of 10+or-/sup 2/-10 m/s, liquid surface velocity of 10/sup -2/-2*10/sup -1/ m/s, and applied voltage of 0-20 kV. Experimental results are analysed by an area-averaged two-fluid one-dimensional model. The results show that the effect of the applied electric field is significantly influenced by the flow regime transition boundaries between stratified smooth-to-wavy and stratified wavy-to-intermittent flow. However, the time-averaged void fraction was not observed to be significantly influenced by applied electric fields.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A capacitance sensor for two-phase void fraction measurement and flow pattern identification

A capacitance sensor has been developed for measuring time-averaged void fractions in two-phase pipe flow. The electrode configuration consists of a helical cross capacitor in which is placed a dielectric tube. To render edge effects negligible guard electrodes are used. For annular and dispersed flow configurations theoretical calibration curves are derived by solving the Laplace equation. The thickness of the wall of the dielectric tube appeared to be a very important design parameter. For annular and stratified flow experimental curves are presented. For many practical flow situations the time-averaged void fraction can be determined with an absolute accuracy of ±0.03. Due to the relatively long axial extension local void fraction measurements are not possible and for mixed flow patterns, such as intermittent flow with frothy slugs, the accuracy is less. The time-varying output signal is useful for flow pattern identification. The sensor is easy to machine and only a standard capacitance meter is needed.

Void signal analysis and gas-liquid two-phase flow regime determination by a statistical pattern recognition method.

A statistical pattern recognition method was applied to the analysis of the signals of a cross-sectional mean void fraction for discriminating gas-liquid two-phase flow regimes. The analysis and discrimination were carried out based on six key flow patterns: bubble, cap-bubble, plug, froth (FI and FII), and annular flow. For each flow condition, 100 void signals with a recording dimension of 1 second were used and transferred to discrete data, the sampling frequency of which was selected as 100 Hz by comparison with correct recognition rates obtained from different frequencies. The magnitude of the time-averaged void fraction was partly employed supplementary to the pattern recognition method. The boundaries between the six flow regimes were determined corresponding to a correct recognition rate of 80% and were drawn on a superficial gas-liquid velocities diagram. These flow boundaries were also compared with those available in the literature.

Preliminary study on application of X-ray CT scanner to measurement of void fractions in steady state two-phase flows.

Preliminary studies are made in order to apply an X-ray CT scanner to the measurement of cross-sectional distributions of void fractions in steady state two-phase flows. Using air-water flows in a horizontal U-shaped tube, results of CT scanner measurements are compared with those of the volumetric measurements with quick-closing valves and of hotfilm anemometer measurements. Studied are cross-sectionally averaged void fractions and void fraction profiles. The comparisons give good agreements. The effect of density fluctuations on the reconstruction of void fraction distributions in two-phase flows is also discussed. Consideration of the statistical fluctuation in the projection data for a stationary object gives the sufficient conditions required for the evaluation of the cross-sectional distribution of the scanning time-averaged void fraction in steady state two-phase flows.