Counter-current Two-phase Flow Research Articles

Flooding phenomena in vertical counter-current annular two-phase (gas-liquid) flow

Flooding phenomena in vertical counter-current annular two-phase (gas-liquid) flow

Two-phase countercurrent flow in a model of a pressurized water reactor hot leg

The onset of flooding or countercurrent flow limitation (CCFL) determines the maximum rate at which one phase can flow countercurrently to another phase. In the present study, the experimental data of the CCFL for gas and liquid in a horizontal pipe with a bend are investigated. The different mechanisms that lead to flooding and that are dependent on the liquid flow rate are observed. For low and intermediate liquid flow rates, the onset of flooding appears simultaneously with the slugging of unstable waves that are formed at the crest of the hydraulic jump. At low liquid flow rates, slugging appears close to the bend; at higher liquid flow rates, it appears far away from the bend, in the horizontal section. For high liquid flow rates, no hydraulic jump is observed, and flooding occurs as a result of slug formation at the end of the horizontal pipe. The effects of the inclination angle of the bends, the liquid inlet conditions and the length of the horizontal pipes are of significance for the onset of flooding. A mathematical model of Ardron and Banerjee is modified to predict the onset of flooding. Flooding curves calculated by this model are compared with present experimental data and those of other researchers. The predictions of the onset of flooding as a function of the length-to-diameter ratio are in reasonable agreement with the experimental data.

Mechanism and Effects of Predominant Parameters Regarding Limitation of Falling Water in Vertical Countercurrent Two-Phase Flow

In this study, an investigation was carried out to clarify the mechanism of countercurrent flow limitation (CCFL) or flooding, that is, limitations in the falling water mass flux in countercurrent two-phase flow in vertical channels, and to identify the effects of predominant parameters regarding CCFL, adopting the criterion that the CCFL condition be given by an envelope of momentum equation applied for the entire length of the channel with respect to any void fraction. As a result, it was found that the analytical model proposed could adequately predict all existing experimental results investigated in this study. In the channel configuration, circular, rectangular, and annular or planar channels, channel dimensions of diameter, gap size, width or circumference, and length, interfacial and wall friction, water injection mode, and inlet water subcooling were dominant parameters. Therefore, both the mechanism and the quantitative effects of CCFL have been identified.

Visualization and correlation analysis of counter-current two-phase flow in a thermosyphon by neutron radiography

We visualized the working fluid behavior in a bent thermosyphon by a neutron radiography and analyzed it quantitatively with an image processing technique. Working fluid velocity was evaluated by the two-point correlation function and also the spatio-temporal correlation was calculated, which led to a clarification of the pass route of working fluid in the thermosyphon. The probability density function of the void fraction was calculated.

The influence of fluid properties and inlet geometry on flooding in vertical and inclined tubes

Flooding during adiabatic two-phase countercurrent flow in a vertical and 60° inclined (to the horizontal) 30 mm i.d. tube is investigated. Experiments are conducted with air, argon, helium, hydrogen, water, methanol, isopropanol and aqueous methanol solutions. An improved flooding correlation is proposed which accurately correlates the data. In the past, a number of proposed dimensionless groups often only succeeded in correlating their own data. The present correlation excellently correlates the data of three previous investigations. The findings clearly show that some flooding data had been misinterpreted in the past. The fluid densities have the strongest effect on the flooding gas velocity. The effect of the liquid viscosity is relatively stronger than that of the surface tension. Flooding is independent of the gas viscosity.

The Effect of Gas Injection Configuration on Two-Phase Countercurrent Flow Limitation in Short Vertical Channels

Countercurrent flow limitation in channels with evaporation taking place inside them is examined. Countercurrent flow limitation is short, small-diameter channels subject to purely axial, purely radial, and combined axial and radial gas injection is studied. Experiments were performed using air and water, with channel diameters 0.475 to 1.91 cm and channel lengths 1.27 to 5.72 cm. Purely axial gas injection data are shown to agree with Wallis`s correlation but with coefficients that strongly depend on channel dimensions. Purely radial gas injection data and data obtained with combined axial and radial gas injection result in flooding curves significantly different from those representing the purely axial gas injection data and indicate that near complete flooding (zero liquid penetration) can occur in small-diameter and short channels due to relatively small radial gas injection rates. Flooding curves for long or large-diameter channels are insensitive to the gas injection configuration, however.

Initiation conditions of liquid ascent of the countercurrent two-phase flow in vertical pipes (in the presence of two-phase mixture in the lower portion)

Experiments on the countercurrent two-phase flow of air and water were conducted using vertical pipes of 10–26 mm in diameter to investigate the initiation conditions of liquid ascent. When a liquid film flowed down to a bubbling two-phase mixture in the lower portion of the pipe, liquid ascent began at much lower gas velocities than usual flooding velocities without the bubbling two-phase mixture. In most cases, liquid ascent occurred in a slug flow state. The initiation conditions of the liquid ascent were analyzed physically by considering the level swell of the two-phase mixture fluctuating around the mean height.

Study of injected water subcooling effect on falling water limitation in countercurrent two-phase flow in vertical channels

This study analytically investigated the subcooling effect of injected water on the falling water limitation in countercurrent two-phase flow (CCFL) in vertical channels, by applying a new model of momentum balances for both liquid and gas phases over the entire length of the channel. The subcooling effect of injected water on CCFL, which is one of the dominant parameters, had been clarified neither analytically nor experimentally because the CCFL phenomena is very complicated thermodynamically. As a result of the present study, it has been clarified that the analytical model proposed here could give good predictions of the existing data on the subcooling effect of experiments simulating the performance of emergency core-cooling water injection during a loss-of-coolant accident in pressurized light-water reactors.

Study of Subcooling Effect of Injected Water on Falling Water Limitation in Countercurrent Two-Phase Flow in Vertical Channels.

This study analytically investigated the subcooling effect of injected water on falling water limitation in countercurrent two-phase flow (CCFL) in vertical channels, by applying a new model of momentum balances for both liquid and gas phases to the entire length of the channel. The subcooling effect of injected water on CCFL, which is one of the dominant parameters, had been clarified neither analytically nor experimentally because the CCFL phenomena is so complicated thermodynamically. As a result of the present study, it was clarified that the analytical model proposed here could give good predictions of the existing data on the subcooling effect of experiments simulating the performance of emergency core cooling water injection during a loss-of-coolant accident in pressurized light-water reactors.

Flooding of Counter-Current Two-Phase Flow of Gas and Liquid. 1st Report. On Flooding in Vertical Circular and Annular Channel.

Experinlental investigations arc performed on flooding which occurs in vertical circular and annular channels. Water film flows down along the channel surface through porous cylinders, and air is raised from the channel bottom by a suction pump. The ooding velocity of the circular tube is proportional to the diameter ill agreement with earlier investigations For the annular channel, a ratio of liquid flow rates of both channel surfaces has a strong effect on the flooding velocity. The conventional method cannot predict the flooding velocity. In the present study, a characteristic diameter of the annular channel is determined by the ratio of liquid flow rates for Reynolds number of liquid and gas. The flooding velocity is approximately indicated as a uniform curvature with the Reynolds number of liquid.

Flooding in Gas-Liquid Countercurrent Two-Phase Flow in Parallel Vertical Pipes.

Flow regime transition and pressure drop in gas-liquid countercurrent two-phase flow was studied in three vertical parallel channels. The conditions for flooding initiation in parallel-channel systems could be predicted well using Imura's correlation for a single channel. After flooding, the flow patterns were different in each channel. Aflow pattern classification diagram for the three-channel system was developed based on the flow patterns observed under various conditions of gas and liquid flow rates. It was seen from the results of pressure measurement in the three-channel system that the maximum pressure developed in each channel had a serious effect on the average pressure in the lower plenum, and that the frictional loss coefficient, estimated from the time-averaged pressure in the lower plenum, could be correlated to the relative gas Reynolds number.

Gravity-driven countercurrent two-phase flow during filling of an unvented vessel

Countercurrent two-phase flow associated with filling of sealed vessels via gravity-driven liquid injection through inclined channels was experimentally studied and analytically modeled. Experiments were performed using transparent tubular test sections connected at one end to the bottom of a large, open water tank, and at the other end to an unvented tank. The test section parameters (including the channel diameter (1.27–2.54cm), length (30.5–122 cm), angle of inclination with respect to horizontal plane (0–30°), and the empty volume in the sealed vessel) were systematically varied. Flow regimes in the test section were recorded and transient flow rates were measured during the experiments. Oscillatory, and intermittent stratified slug, were dominant flow regimes in most tests. The quasi-steady liquid superficial velocity in the test section was sensitive to the test section dimensions, and varied in the range 0.04–0.95 m s −1. These flow regimes were mechanisally modeled. The models are shown to satisfactory predict the measured hydrodynamic parameters.

Countercurrent Two-Phase Flow Regimes and Void Fraction in Vertical and Inclined Channels

Hydrodynamic characteristics of countercurrent two-phase flow in vertical and inclined channels are investigated. Experiments are performed using air and water at room temperature (25 to 27° C) and...

Multidimensional two-phase flow regime distribution in a PWR downcomer during an LBLOCA refill phase

The multidimensional countercurrent two-phase flow regimes that occur in a pressurized-water reactor (PWR) vessel downcomer during the refill phase of a large-break loss-of-coolant accident are studied using a transparent 1/10 scale model of a PWR vessel. The various flow regimes and their distribution in the downcomer have been identified and mapped for a range of air-water flooding experiments. The two-phase flow patterns that are identified in the downcomer included various types of film flows, droplet flows, countercurrent churn flows and cocurrent flows depending on the flooding condition. Through observation of the two-phase flow dynamics it was deduced that the physical mechanisms associated with the flooding processes could be separated into a liquid entrainment process and a film flow reversal process. In addition to the above exercise, the effect of non-uniform injection of water into the downcomer via different combinations of cold leg was studied similarly by determining flooding curves and flow pattern maps. It was found that differences in the flooding characteristic were noticeable for various water inlet configurations when compared with the uniform injection case. The differences could be explained qualitatively in terms of the flooding mechanisms identified previously by examining the flow patterns in the downcomer for the non-uniform injection tests.

Modeling of Gravity-Driven Oscillatory Countercurrent Two-Phase Channel Flows

Gravity-driven countercurrent two-phase flow, in channels connected to a sealed tank at one end and open to the atmosphere at the other end, was analytically studied. This type of gravity-driven co...

Experimental investigation of two-phase countercurrent flow limitation in a bend between horizontal and inclined pipes

The countercurrent flow limitation (CCFL) or onset of flooding determines the maximum rate at which one phase can flow countercurrently to another phase. In the present study, the experimental data of the countercurrent flow limitation for air and water in a bend between a horizontal pipe and a pipe inclined to the horizontal are investigated. Water is introduced into the upper leg and flows downward while the air injected into the horizontal leg flows countercurrently. The flow patterns are visualized. The different mechanisms that lead to flooding and that are dependent on the water flow rate are observed. For low and intermediate water flow rates, the onset of flooding appears simultaneously with the slugging of unstable waves that are formed at the crest of the hydraulic jump. At low water flow rates, slugging appears close to the bend; at higher water flow rates, it appears far away from the bend in the horizontal section. For high water flow rates, no hydraulic jump is observed, and flooding occurs as a result of slug formation at the water flow outlet close to the end of the horizontal pipe. The influence of the inclination angle of the bends, the water inlet conditions, and the length of the horizontal pipes is of significance for the onset of flooding.

Limitation of Falling Water in Countercurrent Two-Phase Flow in Vertical Circular Tubes.

The mechanism of countercurrent flow limitation (CCFL) was investigated analytically and quantitatively for the existing experimental results for vertical circular tubes, adopting a criterion that the CCFL condition is given by maximizing falling water mass velocity with respect to void fraction for the whole flow channel. Through the analysis, it was found that significant factors in the CCFL condition were the interfacial friction factor between liquid and gas based on the relative velocity, wall friction factors for laminar, transition and turbulent liquid flows and the effects of tube diameter and length. Analytical results gave very good predictions of the experimental results for vertical circular tubes of 19 to 140mm in diameter and 12.7 to 1520mm in length, under atmospheric pressure.

Analytical Study on Characteristics of Falling Water Limitation in Countercurrent Two-Phase Flow in Vertical Annular Channels.

This study shows quantitatively that existing experimental data on countercurrent flow limitation (CCFL) or flooding in vertical annular channels can be predicted well by applying a criterion that the CCFL condition be given by maximizing falling water mass velocity with respect to falling water film thickness for the entire flow channel, which is the same criterion as that which the author successfully applied to the CCFL conditions for both vertical circular tubes and vertical rectangular channels. Quantitative analyses were carried out for the previous experiments of both air-water and steam-water countercurrent flow systems under atmospheric pressure in the vertical annular channels of 0.46 to 1.22m in length, 0.91 to 1.85m in circumference and 12.7 to 50.8mm in gap size in both cases where water is injected into the upper plenum and through nozzles, simulating a vertical channel in PWR pressure vessel.

An Experimental Study of Gravity-Driven Countercurrent Two-Phase Flow in Horizontal and Inclined Channels

Countercurrent two-phase flow in horizontal and inclined channels, connecting a sealed liquid-filled reservoir to the atmosphere, is experimentally studied. This type of gravity-driven countercurrent two-phase flow can occur during the operation of passive safety coolant injection systems of advanced reactors. It can also occur in the pressurizer surge line of pressurized water reactors during severe accidents when the hot leg becomes voided.Four distinct flow regimes are identified: (a) stratified countercurrent, which mainly occurs when the channel is horizontal; (b) intermittent stratified-slug; (c) oscillating, which occurs when the angle of inclination is ≥30 deg; and (d) annular countercurrent. The characteristics of each regime and their sensitivity to important geometric parameters are examined. The superficial velocities in the stratified countercurrent and oscillating regimes are empirically correlated.

Buoyantly-driven two-phase countercurrent flow in liquid discharge from a vessel with an unvented gas space

This paper details experiments and analyses regarding the phenomenon of liquid discharge into a gaseous atmosphere from the bottom of a vessel with an unvented, upper gas space. The primary goal is the development of a simple model that predicts the rate of liquid discharge under the prevailing unvented condition. A literature survey of previous work on this phenomenon yielded only simple experiments and analyses that were limited in scope. Experiments were subsequently undertaken with an air-water system, using a larger volume and a wide range of drain line diameters. In addition to flowrate data, visual information was acquired regarding the physical mechanism possibly governing the prevalent flow regimes. The governing physical mechanism is identified as the stability of a gas-liquid interface, perturbed by buoyancy, at the drain line entrance. G.I. Taylor's fundamental analysis of interfacial stability lead to the determination of criteria for flow regime transition among the three prevalent flow regimes, corresponding to so-called small, medium, and large diameters. Also, analysis of the growth of interfacial instabilities lead to the application of flooding models for drainage rates within each regime. The models for moderate and large diameters were then compared against data, which confirmed their success in predicting discharge rates under the unvented condition. The motivation for this effort, besides the basic scientific significance of studying such a fundamental phenomenon, was its numerous applications, one of which is commercial nuclear reactor systems. Specifically, the phenomenon prevails in liquid coolant discharge from a PWR pressurizer, with an unvented steam volume, into a steam atmosphere existing in the adjoining hot coolant leg. Such a phenomenon could occur as part of a transient, or severe accident, scenario, entailing saturated conditions and steam production in the normally subcooled primary heat transport loop. The developed model was implemented in the Modular Accident Analysis Program (MAAP), a computer code designed to predict reactor system behavior in response to postulated off-normal conditions, including severe accident scenarios.