Upward Air-water Two-phase Flow Research Articles

Axial Development of Flow Regime in Adiabatic Upward Two-Phase Flow in a Vertical Annulus

This study has investigated the axial development of flow regime of adiabatic upward air-water two-phase flow in a vertical annulus. The inner and outer diameters of the annulus are 19.1 mm and 38.1 mm, respectively. The hydraulic diameter of the flow channel, DH, is 19.0 mm and the total length is 4.37 m. The flow regime map includes 72 flow conditions within a range of 0.01 m/s<⟨jg⟩<30 m/s and 0.2 m/s<⟨jf⟩<3.5 m/s, where ⟨jg⟩ and ⟨jf⟩ are, respectively, superficial gas and liquid velocities. The flow regime has been classified into four categories: bubbly, cap-slug, churn, and annular flows. In order to study the axial development of flow regime, area-averaged void fraction measurements have been performed using impedance void meters at three axial positions corresponding to z/DH=52, 149, and 230 simultaneously, where z represents the axial position. The flow regime indicator has been chosen to be statistical parameters from the probability distribution function of the area-averaged void fraction signals from the impedance meters, and self-organized neural networks have been used as the mapping system. This information has been used to analyze the axial development of flow regime as well as to check the predictions given by the existing flow regime transition models. The axial development of flow regime is quantified using the superficial gas velocity and void fraction values where the flow regime transition takes place. The predictions of the models are compared for each flow regime transition. In the current test conditions, the axial development of flow regime occurs in the bubbly to cap-slug (low superficial liquid velocities) and cap-slug to churn (high superficial liquid velocities) flow regime transition zones.

Void Fraction Measurement and Signal Analysis from Multiple-Electrode Impedance Sensors

Void fraction sensors are important instruments not only for monitoring two-phase flow, but for furnishing an important parameter for obtaining flow map pattern and two-phase flow heat transfer coefficient as well. This work presents the experimental results obtained with the analysis of two axially spaced multiple-electrode impedance sensors tested in an upward air-water two-phase flow in a vertical tube for void fraction measurements. An electronic circuit was developed for signal generation and post-treatment of each sensor signal. By phase shifting the electrodes supplying the signal, it was possible to establish a rotating electric field sweeping across the test section. The fundamental principle of using a multiple-electrode configuration is based on reducing signal sensitivity to the non-uniform cross-section void fraction distribution problem. Static calibration curves were obtained for both sensors, and dynamic signal analyses for bubbly, slug, and turbulent churn flows were carried out. Flow parameters such as Taylor bubble velocity and length were obtained by using cross-correlation techniques. As an application of the void fraction tested, vertical flow pattern identification could be established by using the probability density function technique for void fractions ranging from 0% to nearly 70%.

Interfacial area transport of vertical upward air–water two-phase flow in an annulus channel

An experimental study on the interfacial area transport (IAT) of vertical, upward, air–water two-phase flows in an annulus channel has been conducted. The inner and outer diameters of the annular channel were 19.1 mm and 38.1 mm, respectively. Nineteen inlet flow conditions were selected, which cover bubbly, cap–slug, and churn–turbulent flows. The local flow parameters, such as void fraction, interfacial area concentration (IAC), and bubble interface velocity, were measured at nine radial positions for the three axial locations ( z/ D H = 52, 149 and 230). The radial and axial evolutions of local flow structure were interpreted in terms of bubble coalescence and breakup. The measured data can be used for the development of the bubble coalescence/breakup models for the IAT model and some closure models for computational fluid dynamics.

A study on the characteristics of upward air–water two-phase flow in a large diameter pipe

An adiabatic upward co-current air–water two-phase flow in a vertical large diameter pipe (inner diameter, D: 0.2 m, ratio of pipe length to diameter, L/ D: 60.5) was experimentally investigated under various inlet conditions. Flow regimes were visually observed, carefully analyzed and classified into five, i.e. undisturbed bubbly, agitated bubbly, churn bubbly, churn slug and churn froth. Void fraction, bubble frequency, Sauter mean diameter, interfacial area concentration (IAC) and interfacial direction were measured with four-sensor optical probes. Both the measured void fraction and the measured IAC demonstrated radial core-peak distributions in most of the flow regimes and radial wall peak in the undisturbed bubbly flow only. The bubble frequency also showed a wall-peak radial distribution only when the bubbles were small in diameter and the flow was in the undisturbed bubbly flow. The Sauter mean diameter of bubbles did not change much in the radial direction in undisturbed bubbly, agitated bubbly and churn bubbly flows and showed a core-peak radial distribution in the churn slug flow due to the existence of certain amount of large and deformed bubbles in this flow regime. The measurements of interfacial direction showed that the main and the secondary bubbly flow could be displayed by the main flow peak and the secondary flow peak, respectively, in the probability density function (PDF) of the interfacial directional angle between the interfacial direction and the z-axis, η zi . The local average η zi at the bubble front or rear hemisphere ( η zi F and η zi R ) reflected the local bubble movement and was in direct connection with the flow regimes. Based on the analysis, the authors classified the flow regimes in the vertical large diameter pipe quantitatively by the cross-sectional area-averaged η zi at bubbly front hemisphere ( η zi F ¯ ). Bubbles in the undisturbed bubbly flow moved in a vertical way with some swerving motions and those in other flow regimes moved along the lateral secondary flow with an averaging net upward velocity.

Modélisation dynamique des structures d’interface via l’équation de transport de l’interface

The interfacial area transport equation dynamically models the two-phase flow regime transitions and predicts continuous change of the interfacial area concentration along the flow field. Hence, when employed in the numerical thermal-hydraulic system analysis codes, it eliminates artificial bifurcations stemming from the use of the static flow regime transition criteria. Accounting for the substantial differences in the transport phenomena of various sizes of bubbles, the two-group interfacial area transport equations have been developed. The group 1 equation describes the transport of small-dispersed bubbles that are either distorted or spherical in shapes, and the group 2 equation describes the transport of large cap, slug or churn-turbulent bubbles. The source and sink terms in the right-hand-side of the transport equations have been established by mechanistically modeling the creation and destruction of bubbles due to major bubble interaction mechanisms. In the present paper, the interfacial area transport equations currently available are reviewed to address the feasibility and reliability of the model along with extensive experimental results. These include the data from adiabatic upward air-water two-phase flow in round tubes of various sizes, from a rectangular duct, and from adiabatic co-current downward air-water two-phase flow in round pipes of two sizes.

Flow pattern, pressure drop and void fraction of two-phase gas–liquid flow in an inclined narrow annular channel

Two-phase flow pattern, pressure drop and void fraction in horizontal and inclined upward air–water two-phase flow in a mini-gap annular channel are experimentally studied. A concentric annular test section at the length of 880 mm with an outer diameter of 12.5 mm and inner diameter of 8 mm is used in the experiments. The flow phenomena, which are plug flow, slug flow, annular flow, annular/slug flow, bubbly/plug flow, bubbly/slug–plug flow, churn flow, dispersed bubbly flow and slug/bubbly flow, are observed and recorded by high-speed camera. A slug flow pattern is found only in the horizontal channel while slug/bubbly flow patterns are observed only in inclined channels. When the inclination angle is increased, the onset of transition from the plug flow region to the slug flow region (for the horizontal channel) and from the plug flow region to slug/bubbly flow region (for inclined channels) shift to a lower value of superficial air velocity. Small shifts are found for the transition line between the dispersed bubbly flow and the bubbly/plug flow, the bubbly/plug flow and the bubbly/slug–plug flow, and the bubbly/plug flow and the plug flow. The rest of the transition lines shift to a higher value of superficial air velocity. Considering the effect of flow pattern on the pressure drop in the horizontal tube at low liquid velocity, the occurrence of slug flow stops the rise of pressure drop for a short while, before rising again after the air velocity has increased. However, the pressure does not rise abruptly in the tubes with θ = 30° and 60° when the slug/bubbly flow occurs. At low gas and liquid velocity, the pressure drop increases, when the inclination angles changes from horizontal to 30° and 60°. Void fraction increases with increasing gas velocity and decreases with increasing liquid velocity. After increasing the inclination angle from horizontal to θ = 30° and 60°, the void fraction appears to be similar, with a decreasing trend when the inclination angle increases.

Flow Regimes of Upward Gas-liquid Two-Phase Flow in a Vertical Pipe with an Axisymmetic Sudden Expansion

Experimental study was made on the multi-dimensional behavior of upward air-water two-phase flow through a vertical pipe with an axisymmetric sudden expansion, which is one of the typical multi-dimensional channel geometry. The aims of this study are to clarify the multidimensional behavior of bubbly flow or slug flow affected by sudden expansion channel geometry, and to accumulate the experimental data for two-phase flow analysis, which is applicable to predict the multi-dimensional behavior with appropriate accuracy. The observation using a high-speed video camera was performed to clarify the multi-dimensional dynamic flow behavior with the sudden expansion. It revealed the characteristic phenomena such as gas-slug breakup and bubble deformation and bubble size below and above the sudden expansion point. Furthermore the effect of the sudden expansion channel on the flow regime map can be also described by using these experimental result.

Two-phase phase distribution in a vertical large diameter pipe

In view of the practical importance of the application of two-phase flow in a large diameter pipe to various fields of engineering, the characteristics and phase distribution patterns of two-phase flow in a vertical large diameter pipe have been experimentally and theoretically studied for various flow conditions. The local measurements for the interfacial parameters (void fraction, Sauter mean diameter and pressure loss) in a vertical upward air–water two-phase flow in a pipe with 0.2 m in inner diameter and 24 m in height have been performed by using the optical probes and differential pressure transducers. The two-phase flow characteristics have been analyzed with experimental data, which shows that the phase distribution patterns in the vertical large diameter pipe can be divided into two basic patterns, namely, wall peak and core peak. With the application of the concept of skewness, the two-phase distribution patterns have been quantitatively distinguished by establishing a phase distribution pattern transition criterion. An empirical relation for the phase distribution transition from wall peak to core peak was fitted by using the phase distribution pattern transition criterion and the present experimental data and verified by other researchers’ experimental data. This study also showed that there existed the flow plugging phenomena in the low region of the test section at high superficial gas velocity conditions in the vertical large diameter pipe.

Image processing methods to obtain symmetrical distribution from projection image

Flow visualization and measurement of cross-sectional liquid distribution is very effective to clarify the effects of obstacles in a conduit on heat transfer and flow characteristics of gas–liquid two-phase flow. In this study, two methods to obtain cross-sectional distribution of void fraction are applied to vertical upward air–water two-phase flow. These methods need projection image only from one direction. Radial distributions of void fraction in a circular tube and a circular-tube annuli with a spacer were calculated by Abel transform based on the assumption of axial symmetry. On the other hand, cross-sectional distributions of void fraction in a circular tube with a wire coil whose conduit configuration rotates about the tube central axis periodically were measured by CT method based on the assumption that the relative distributions of liquid phase against the wire were kept along the flow direction.

Effects of the Opening Area Ratio of a Ring-Type Obstacle on Liquid Film Thickness in Upward Gas-Liquid Two-Phase Flow in a Vertical Tube

A ring-type obstacle, which simulated a spacer in a light water reactor or a flow obstruction supporting heat exchanger tube, was set in an upward air-water two-phase flow in a vertical tube to investigate the effects of the obstacle on liquid film thickness in relation to the drypatch generation. We measured simultaneously the time varying liquid film thickness at several axially different locations near the obstacle under the conditions of the superficial water velocity ranging from 0.06 m/s to 0.4 m/s and the superficial air velocity from 0.5 m/s to 36 m/s and discussed the effects of the opening area ratio of the obstacle on the liquid film thickness. The results are summarized as follows : (1) The minimum film thickness near the obstacle becomes thinner as the clearance between the obstacle and the inner surface of tube becomes smaller. (2) The obstacle makes the disturbance wave smooth and it makes the base film thick downstream of the obstacle when the clearance is small.

Effects of the Inner Diameter of a Ring-Type Obstacle on Liquid Film Thickness in Upward Gas-Liquid Two-Phase Flow in a Vertical Tube.

A ring-type obstacle, which simulated a spacer in a light water reactor or a flow obstruction supporting heat exchanger tube, was set in an upward air-water two-phase flow in a vertical tube to investigate the effects of the obstacle on liquid film thickness in relation to the drypatch generation. We measured the axial distributions of time varying liquid film thickness near the obstacle under the conditions of the superficial water velocity ranging from 0.06m/s to 0.4m/s and the superficial air velocity from 0.5m/s to 36m/s and discussed the effects of the inner diameter of the obstacle on the liquid film thickness. The results are summarized as follows : (1) The minimum film thickness and the mean film thickness near the obstacle become thinner as the inner diameter of the obstacle becomes smaller. (2), The maximum duration of the existence of thin film becomes larger and the occurrence frequency of thin film becomes higher as the inner diameter of the obstacle becomes smaller.

The flow characteristics of an upward gas‐liquid two‐phase flow in a vertical tube with a wire coil: Part 1. Experimental results of flow pattern, void fraction, and pressure drop

AbstractExperiments were carried out to investigate the flow pattern, average void fraction, and pressure drop of an upward air‐water two‐phase flow in vertical tubes of 25‐mm inside diameter with wire coils of varying wire diameter, pitch, and number of coils in cross section. Five kinds of flow patterns—bubble, slug, churn, semiannular, and annular flow—were defined based on the observation of flow behavior in the experiments. At higher water flowrates, the bubble‐to‐slug transition occurred at lower air flowrates in tubes with wire coils than in smooth tubes. The average void fraction was found by using the drift flux model. Further, the experimental results of the friction pressure drop were compared with the Lockhart‐Martinelli correlation. As a result, a correlation with the constant C in Chisholm's equation was obtained as a function of the wire coil pitch‐to‐diameter ratio. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 639–651, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10066

Effects of the Length of a Ring-Type Obstacle on Liquid Film Thickness in Upward Gas-Liquid Two-Phase Flow within a Vertical Tube.

A ring-type obstacle, which simulated a spacer in Light Water Reactor or a flow obstruction supporting heat exchanger tube, was set in an upward air-water two-phase flow within a vertical tube to investigate the effects of the obstacle on liquid film thickness in relation to the drypatch generation. We measured the axial distributions of time varying liquid film thickness near the obstacle in the range of the superficial water velocity of 0.06-1.6 m/s and the superficial air velocity of 0.5-36 m/s and discussed the effects of the length of the obstacle on the liquid film thickness. The results are summarized as follows: (1) The minimum film thickness and the mean film thickness near the obstacle become thinner as the length of the obstacle becomes shorter. (2) The length of the obstacle has stronger effects on both the minimum and the mean film thickness near the obstacle as the superficial water velocity becomes lower. (3) The generation of drypatch is possible under the flow conditions examined in the present experiment if heat is applied.

Co-current air–water two-phase flow patterns in vertical triangular microchannels

Characteristics of co-current upward air–water two-phase flow patterns in vertical equilateral triangular channels with hydraulic diameters of 2.886, 1.443 and 0.866 mm were investigated experimentally. Flow regimes were identified by both visual observations using a high-speed motion analyzer and dynamic pressure-drop measurements. The experimental results show that the typical flow patterns encountered in the conventional, large-sized vertical circular tubes, such as dispersed bubbly flow, slug flow, churn flow and annular flow, were also observed in the channels having larger hydraulic diameters ( d h=2.886 and 1.443 mm). However, for the smallest channel ( d h=0.866 mm), dispersed bubbly flow pattern, characterized by randomly dispersed bubbles in continuous liquid phase, was not found, although the other typical flow patterns remained in the channel. Moreover, the experiments reveal that, for the channel of d h=0.866 mm, a so-called capillary bubbly flow pattern, characterized by a single train of bubbles, essentially ellipsoidal in shape and spanning almost the entire cross-section of the channel, existed at low gas flow rates. It is further found that in the slug flow regime, slug-bubbles were substantially elongated. Finally, flow regime transition boundaries for the triangular microchannels were compared with relevant flow regime transition models and correlations as well as the existing experimental data for small round tubes and square channels.

Flow Characteristics of Upward Gas-Liquid Two-Phase Flow in a Vertical Tube with a Wire Coil. 1st Report. Experimental Results of Flow Pattern, Void Fraction and Pressure Drop.

Experiments were carried out to investigate the flow pattern, average void fraction, and pressure drop of upward air-water two-phase flow in vertical tubes of 25 mm inside diameter with wire coils fo varying wire diameter, pitch and number of coil. Five kinds of flow patterns, i.e., bubble, slug, churn, semi-annular and annular flow, were defined based on the observation of flow behaviors in the experiments. At higher water flow rate, the bubble-slug transition occurred at a lower air flow rate in tubes with wire coil than in the smooth tube. The average void fraction was clarified using the drift flux model. Furthermore, the experimental results of the friction pressure drop were compared with the Lockhart-Martinelli correlation. As a result, a correlation for the constant C in Chisholm's equation was obtained as a function of wire coil pitch-to-diameter ratio.

Experimental study on transition of flow pattern and phase distribution in upward air–water two-phase flow along a large vertical pipe

In order to investigate the dependency of gas–liquid two-phase flow on pipe scale, the transition characteristics of flow pattern and phase distribution were studied experimentally in upward air–water two-phase flow along a large vertical pipe (inner diameter D: 0.2 m, the ratio of pipe length to diameter L/ D: 61.5). The experiments were conducted under the flow rate: 0.03 m/s ≤ superficial air velocity (at top of test section) ≤ 4.7 m/s, 0.06 m/s ≤ superficial water velocity J L ≤ 1.06 m/s. Flow pattern was observed and measurements were performed on axial differential pressure, phase distribution, bubble size and bubble and water velocities. The scale effect was discussed with small-scale data ( D: 0.025–0.038 m). The flow conditions at which coalescence starts are almost the same as those found in small-scale pipes, but no large bubbles are observed in the region L/ D<20 which corresponds to the developing region of the axial differential pressure curves. The large coalescent bubbles were generated in L/ D>20. The churn flow is dominant in the large vertical pipe under the conditions where small-scale pipes have slug flow. The transition of phase distribution corresponds to the change of flow pattern. Large coalescent bubbles affect the phase distribution as similar to small-scale pipes but the following remarks are concluded as the scale effect: (1) under a low J L where small-scale pipes have a wall-peak phase distribution, a core-peak phase distribution is established, where some large eddies including bubble clusters fill up the pipe, (2) the large coalescent bubbles are developed along the test section via the churn bubbly flow where the phase distribution is a core peak one, whereas Taylor bubbles in small-scale pipes are generated at the vicinity of gas–liquid mixing region or are developed from the bubbly flow with a wall-peak phase distribution, (3) the wall-peak in the large vertical pipe is lower even under the same bubble size. The lower peak is considered to be related to the lower radial velocity gradient of water and the larger turbulent dispersion force.

Flow Characteristics of Air-Water Two-Phase Flow in the Vertical Twisted-Tape-Inserted Tubes. Friction Pressure Drop and Average Void Fraction.

Experiments were performed to investigate the friction pressure drop of upward air-water two-phase flow in vertical twisted-tape-inserted tubes. Total pressure drop and average void fraction for hydrostatic pressure drop were measured at the same time, from which the friction pressure drop was calculated. The friction pressure drops of twisted-type-inserted tubes were 1.2∼5 times as large as that of empty one. However, the two-phase friction multiplier of the former was smaller than that of the latter and decreased with decrease of twist ratio (more tightly twisted). Such trends were predicted well by the well-known Chisholm's equation with modified value of the constant C based on the present experimental data. A correlation for the constant C was obtained as a function of twist ratio, with which all the present experimental data of friction pressure drop in the twisted-tape-inserted tubes were predicted well within ±30 percent error range.

Experiment and numerical simulation of bubbly two-phase flow across horizontal and inclined rod bundles

Experimental and numerical analyses were carried out on vertically upward air-water bubbly two-phase flow behavior in both horizontal and inclined rod bundles with either in-line or staggered array. The inclination angle of the rod bundle varied from 0 to 60° with respect to the horizontal. The measured phase distributions indicated non-uniform characteristics, particularly in the direction of the rod axis when the rods were inclined. The mechanisms for this non-uniform phase distribution is supposed to be due to: (1) Bubble segregation phenomenon which depends on the bubble size and shape; (2) bubble entrainment by the large scale secondary flow induced by the pressure gradient in the horizontal direction which crosses the rod bundle; (3) effects of bubble entrapment by vortices generated in the wake behind the rods which travel upward along the rod axis; and (4) effect of bubble entrainment by local flows sliding up along the front surface of the rods. The liquid velocity and turbulence distributions were also measured and discussed. In these speculations, the mechanisms for bubble bouncing at the curved rod surface and turbulence production induced by a bubble were discussed, based on visual observations. Finally, the bubble behaviors in vertically upward bubbly two-phase flow across horizontal rod bundle were analyzed based on a particle tracking method (one-way coupling). The predicted bubble trajectories clearly indicated the bubble entrapment by vortices in the wake region.

Flow Patterns of Air-Water Two-Phase Flow in Vertical Twisted-Tape-Inserted Tubes.

Experiments were performed to investigate the flow patterns which occur in upward air-water two-phase flow in vertical twisted-tape-inserted and circular empty tubes. Three basic flow patterns, i. e., bubbly, intermittent and annular flow, were defined based on the observations in the present experiments. At high water flow rate and small twist ratio, the bubbly-intermittent transition occurred at a lower air flow rate in the twisted-tape-inserted tubes than in the empty tube due to the centrifugal force of swirl flow which concentrated bubbles at the center of tube. The effect of centrifugal force was successfully modeled to obtain the criterion equation of the bubbly-intermittent transition. The intermittent-annular transition could be predicted well by the existing criterion equation developed for empty tube.

Characteristics and behavior of the interfacial wave on the liquid film in a vertically upward air-water two-phase annular flow

The behavior of individual interfacial waves on liquid film in vertically upward air-water annular flows has been visualized, observed and analyzed by a pigment luminance method(PLM) which was calibrated with a fiber-optic liquid film sensor. By means of this technique, we distinguished three different types of interfacial waves, i.e. the ripple wave, the ring wave and the disturbance wave. Furthermore we measured the characteristics of these three different kinds of waves, and in particular those of the disturbance wave: i.e. its propagation velocity, its frequency in passing and the distance between two adjacent waves, and then obtained the dependency of these characteristics on the air and water volumetric fluxes j g and j l . These results agreed well with the results obtained by other investigators, using an electric needle contact method. A probable mechanism of the occurrence of the ring and the disturbance waves was posited.