Large Vertical Pipe Research Articles

Development of a one-dimensional system code for the analysis of downward air-water two-phase flow in large vertical pipes

In nuclear thermal-hydraulic system codes, most correlations used for vertical pipes, under downward two-phase flow, have been developed considering small pipes or pool systems. This suggests that there could be uncertainties in applying the correlations to accident scenarios involving large vertical pipes owing to the difference in the characteristics of two-phase flows, or flow conditions, between large and small pipes. In this study, we modified the Multi-dimensional Analysis of Reactor Safety KINS Standard (MARS-KS) code using correlations, such as the drift-flux model and two-phase multiplier, developed in a plant-scale air-inflow experiment conducted for a pipe of diameter 600 mm under downward two-phase flow. The results were then analyzed and compared with those based on previous correlations developed for small pipes and pool conditions. The modified code indicated a good estimation performance in two plant-scale experiments with large pipes. For the siphon-breaking experiment, the maximum errors in water flow for modified and original codes were 2.2% and 30.3%, respectively. For the air-inflow accident experiment, the original code could not predict the trend of frictional pressure gradient in two-phase flow as ⟨jg⟩/⟨j⟩ increased, while the modified MARS-KS code showed a good estimation performance of the gradient with maximum error of 3.5%.

Characterizations of gas-liquid interface distribution and slug evolution in a vertical pipe

Large vertical pipes are key structures connecting subsea wells to offshore platforms. However, existing studies mainly focus on small vertical pipes. In a vertical acrylic pipe with 80 mm inner diameter and 11 m height, a high-speed camera was used to visually research the influences of pipe diameters, liquid properties and inlet effect on air-water co-flow characteristic. Different flow regime maps of vertical pipes (diameters are in the range of 50–189 mm) were compared and the critical gas velocity of the transition boundary from bubble to slug flow tended to increase with the increase of diameters at D ≥ 80 mm. Drift-flux models were established in different flow regimes and liquid properties have a significant effect on drift coefficients of bubble flow and slug flow (void fraction α ≤ 0.4). The influence of inlet turbulent effect on the gas-liquid interface distribution gradually weakened and disappeared from the pipe base to 85D, where the flow was fully developed. Slug frequency has a trend of increase first and then decrease with the gas Weber numbers increasing at low liquid superficial velocities (JL ≤ 0.31 m/s). And on the basis of this law, a new slug frequency correlation was proposed. It was found that there was an exponential relationship between the ratio of lengths of Taylor bubble to slug and the void fraction.

Plant-scale experiments of an air inflow accident under sub-atmospheric pressure by pipe break in an open-pool type research reactor

In an open-pool type research reactor with a downward forced flow in the core, pipes can be under sub-atmospheric pressure because of the large pressure drop at the reactor core in the atmospheric pool. Sub-atmospheric pressure can result in air inflow into the pipe from the pressure difference between the atmosphere and the inside of the pipe, which in a postulated pipe break scenario can lead to the breakdown of the cooling pump. In this study, a plant-scale experiment was conducted to study air inflow in large piping systems by considering the actual operational conditions of an advanced research reactor. The air inflow rate was measured, and the entrained air was visualized to investigate the behavior of air inflow and flow regime depending on the pipe break size. In addition, the developed drift-flux model for a large vertical pipe with a diameter of 600 mm was compared with other correlations. The flow regime transition in a large vertical pipe under downward flow was also studied using the newly developed drift-flux model. Consequently, the characteristics of two-phase flow in a large vertical pipe were found to differ from those in small vertical pipes where liquid recirculation was not dominant.

Computational Fluid Dynamics Modeling of the Pressure Drop of an Iso-Thermal and Turbulent Upward Bubbly Flow Through a Vertical Pipeline Using Population Balance Modeling Approach

Abstract A computational fluid dynamics (CFD) model of the two-phase flow is presented to simulate isothermal, turbulent, upward bubbly flow in a pipeline for the purpose of forecasting a mean pressure reduction along the pipe (a three-dimensional (3D) multiphase flow, by Eulerian–Eulerian strategy combined with population balance model (PBM)). A set of experimental data from the literature for water (liquid) and air (gas) in an isothermal pipe is used where the internal diameter of which is 200 mm; it tends to analyze radial void fraction and bubble diameter distributions as well as the axial pressure distribution of fluid flow. The CFD model is applied to grids of minimum control volumes. The interfacial forces, including nondrag and drag forces, where the former can be categorized into turbulent scattering, lift, and wall lubrication, have been noticed in simulations. The comparison between CFD forecasts with experimental data demonstrates that the coring phenomena plus observed wall peaking could be predicted with this CFD-PBM modeling approach. The primary aim of this study was to anticipate the axial pressure distribution of bubbly flow in the pipe by CFD modeling in a large vertical pipe. Acceptable agreement between models predictions and experimental data indicates that CFD can be a beneficial method for investigating pressure drop of an upward and multiphase flow.

Implicitly coupled phase fraction equations for polydisperse flows

AbstractThis work presents the implementation, verification and the validation of an incompressible Eulerian multifluid model for polydisperse flows. The proposed model uses a novel monolithic, that is, implicitly coupled phase continuity equation for an arbitrary number of fluids, where the breakup source and sink terms are handled implicitly in the block‐system. The implemented model is tested for an upward bubbly flow inside a large vertical pipe. The selected flow conditions exhibit both breakup and coalescence. The grid refinement study is conducted on four structured grids with varying levels of refinement. In the validation section, the numerical results are compared to the TOPFLOW experimental measurements. The last presented test examines the performance of the novel implicitly coupled phase continuity equation to the corresponding segregated formulation and the standard segregated formulation. The performance is evaluated by comparing the conservation error over the nonlinear iterations. The presented model exhibits good agreement with the experimental measurements and gives stable results on various grids with different levels of refinement. Moreover, the implicit coupling reduces the conservation error during the calculation.

Poly-disperse simulation of condensing steam-water flow inside a large vertical pipe

The condensation of saturated steam bubbles in sub-cooled water inside a vertical pipe was studied by poly-disperse CFD simulations. Six test cases with varied pressure, liquid sub-cooling and diameter of the gas injection orifices were simulated. Baseline closures presented for non-drag forces in previous work were found to be reliable also in non-isothermal cases. The effect of bubble coalescence and breakup is over-weighting in the region close to steam injection in case of small orifice diameter. With the increase of orifice diameter, breakup becomes dominant in determining bubble size change. The effect of interphase heat transfer coefficient correlations was investigated. The widespread Ranz–Marshall correlation was found to under-estimate the condensation rate, especially at high pressure levels. In contrast, satisfying agreement with the experimental data was obtained by the Tomiyama correlation.

Classification of bubbles in vertical gas–liquid flow: Part 1 – An analysis of experimental data

In consideration of the practical importance regarding the application of gas–liquid flow in a vertical pipe and the quest towards the development of more robust physical models to accurately predict the essential interfacial parameters of the two-phase flow, comprehensive analysis of the characteristics and phase distribution patterns of such a flow have been performed on both experimental measurements and theoretical predictions. In this first part, analysis of experimental data in a large diameter pipe with an inner diameter of 195.3 mm via the wire-mesh senor measuring technique was presented. The experiments were performed at the TOPFLOW facility of Helmholtz-Zentrum Dresden-Rossendorf. In the present paper, measurements of local interfacial parameters which included the void fraction, volume equivalent bubble diameter, bubble size distribution and interfacial velocities were discussed. Test points covering flow regimes from bubbly to cap to slug to churn- turbulent flow were selected to investigate the flow of different bubble shapes and sizes and the significant bubble coalescence and break-up mechanism throughout the large vertical pipe. The radial and axial evolutions of the local flow structure were interpreted in terms of the classifications of different groups of bubbles (Group-1 and Group-2). In addition, the phase distribution patterns were analyzed through the concept of skewness, which essentially identified two fundamental patterns, namely, wall peak and core peak. In general, Group-1 bubbles being smaller spherical bubbles have shown to exhibit a wall peaking distribution while Group-2 bubbles being larger non-spherical bubbles corresponded to a core peaking distribution. The classification of bubbles that have been performed in this present study can be employed for the development of bubble coalescence and break-up mechanistic kernels and other interfacial force closure models for a two bubble group approach in the context of computational fluid dynamics.

Gas–liquid flows in medium and large vertical pipes

Gas–liquid bubbly flows with wide range of bubble sizes are commonly encountered in many industrial gas–liquid flow systems. To assess the performances of two population balance approaches – Average Bubble Number Density (ABND) and Inhomogeneous MUlti-SIze-Group (MUSIG) models – in tracking the changes of gas volume fraction and bubble size distribution under complex flow conditions, numerical studies have been performed to validate predictions from both models against experimental data of Lucas et al. (2005) and Prasser et al. (2007) measured in the Forschungszentrum Dresden-Rossendorf FZD facility. These experiments have been strategically chosen because of flow conditions yielding opposite trend of bubble size evolution, which provided the means of carrying out a thorough examination of existing bubble coalescence and break-up kernels. In general, predictions of both models were in good agreement with experimental data. The encouraging results demonstrated the capability of both models in capturing the dynamical changes of bubbles size due to bubble interactions and the transition from “wall peak” to “core peak” gas volume fraction profiles caused by the presence of small and large bubbles. Predictions of the inhomogeneous MUSIG model appeared marginally superior to those of ABND model. Nevertheless, through the comparison of axial gas volume fraction and Sauter mean bubble diameter profiles, ABND model may be considered an alternative approach for industrial applications of gas–liquid flow systems.

Development of a generalized coalescence and breakup closure for the inhomogeneous MUSIG model

In the present study we propose new coalescence and breakup closures for the inhomogeneous MUltiple bubble Size Group (MUSIG) model. The major purpose is to consider bubble coalescence and breakup due to different mechanisms in turbulent gas–liquid mixtures and to develop a generally applicable constitutive model for CFD applications. For bubble coalescence the new model includes coalescence due to turbulence, laminar shear, wake-entrainment and eddy-capture. Bubble breakup mechanisms encompass turbulent fluctuation, laminar shear and interfacial slip velocity. The new model was implemented in the commercial CFD software (CFX-12.0) and applied to the case of turbulent air–water mixtures in a large vertical pipe (DN 200). Simulation results for the evolution of bubble size distributions were compared to the standard closure model of Luo and Svendsen (1996) and Prince and Blanch (1990) as well as the experimental data of TOPFLOW test facility. Better prediction is accomplished by the proposed new model.

A new database on the evolution of air–water flows along a large vertical pipe

A comprehensive database was obtained for stationary upward air–water flows in a vertical pipe with an inner diameter of 195.3 mm using the wire-mesh sensor technology. During the experiments the sensor was always mounted on the top of the test section while the distance between gas injection and measuring plane was varied to up to 18 different L/ D by using gas injection chambers at different vertical positions. The gas was injected via holes in the pipe wall. The pressure was kept at 0.25 MPa (absolute) at the location of the active gas injection while the temperature was constant at 30 °C ± 1 K. This procedure exactly represents the evolution of the flow along the pipe, as it would be observed for an injection at a constant height position and a shifting of the measurement plane. The experiments were done for 48 combinations of air and water superficial velocities varying from 0.04 m/s to 1.6 m/s for water and 0.0025 m/s to 3.2 m/s for air. From the raw data time-averaged data as: radial gas volume fraction profiles, bubble size distributions, radial volume fraction profiles decomposed according to the bubble size and the radial profiles of the gas velocity were calculated. Due to the combination of the new experimental procedure with the high spatial and temporal resolution of the wire-mesh sensor technology the data have new quality especially regarding their consistency in the evolution with increasing L/ D. This closes a gap for data suitable for CFD code development and validation for two-phase flows, especially for models on bubble coalescence and break-up.

The characteristics of gas/liquid flow in large risers at high pressures

Although most of the work reported on two-phase flows are limited to small pipe diameters, two-phase flow in large risers are increasingly being encountered in the petroleum and nuclear industries. In the present work, a wire mesh sensor was employed to obtain void fraction and bubble size distribution data and visualizations of steam/water flow in a large vertical pipe (194 mm in diameter) at 46 bar. For comparison purposes, measurements were made at similar phase velocities and physical properties to a dataset for nitrogen/naphtha flow in a similar-sized riser. There exist significant differences between both sets of data. Churn-turbulent flow is observed in the present work instead of slug flow, and this differs from the intermittent and semi-annular flow patterns reported for nitrogen/naphtha data. The mean void fraction of the nitrogen/naphtha data is higher than that of the present steam/water data due to the differences in purity in the liquid phases. Furthermore, core peak distributions are observed for the present work in contrast to the flatter profiles deduced for the nitrogen/naphtha using a power law relationship.

Evolution of the structure of a gas–liquid two-phase flow in a large vertical pipe

The evolution of the structure of a gas–liquid flow in a large vertical pipe of 195 mm inner diameter was investigated at the TOPFLOW test facility in Rossendorf. Wire-mesh sensors were used to measure sequences of two-dimensional distributions of local instantaneous gas fraction within the complete pipe cross-section. The sensors own a resolution of 3 mm at a frequency of 2500 Hz. Superficial velocities were varied in a range covering flow regimes from bubbly to churn-turbulent flow. The distance between the gas injection and the sensor position was changed using a so-called variable gas injection system. It consists of six gas injection units, each equipped with three rings of injection orifices in the pipe wall (orifice diameter: 1 and 4 mm), which are fed from ring chambers. The gas flow towards these distributor chambers is individually controlled by valves. Measured bubble-size resolved radial gas fraction profiles reveal differences in the lateral migration of bubbles of different size starting from the injection at the wall. The evolution of bubble-size distributions allows to study bubble coalescence and break-up. The influence of the physical properties of the fluid was studied by comparing cold air–water experiments with steam–water tests at 65 bar.

Modeling the Evolution of Bubbly Flow along a Large Vertical Pipe

A detailed experimental database, obtained for a 195-mm inner diameter, 9-m-long pipe was used for the validation of models applied in computational fluid dynamics codes for the simulation of bubbly flow. Since the bubbles were injected via holes at the pipe wall, very useful information on the bubble migration from the pipe wall toward the pipe’s center was obtained by measurements at different distances between gas injection and measuring plane. The bubble migration is determined by the forces acting on the bubbles. The multibubble-size group test solver, introduced earlier but with some new extensions, was used to analyze the data. A comparison of results from a simulation and the experimental findings indicate that the turbulent dispersion force according to the Favre averaged drag model is too strong compared with the drag in the radial direction. No appropriate models for bubble coalescence and breakup, which can be applied for a wide range of gas and liquid volume flow rates, are available as yet. Nevertheless, for selected combinations of volume flow rates, the calculated bubble size distributions and radial gas volume fraction profiles show an acceptable agreement with the experimental data.

The influence of physico-chemical material properties on erosion processes in the badlands of Basilicata, Southern Italy

Piping (tunnelling) appears to be one of the more significant methods of erosion in the badlands in the Plio-Pleistocene marine-sourced clays of the Basilicata area in Southern Italy. A detailed field investigation of the pedological, textural, mineralogical and geochemical properties of the clay-rich terrains widely outcropping in two selected badland field sites (the Fossa Bradanica area and the Bacino di Sant'Arcangelo) was undertaken to further elucidate process variability across the complex site. Paralleling findings from other piped badlands, certain physico-chemical properties of the clays were found to influence the different erosional processes in fundamental ways. The very dispersive nature of the materials enhances pipe enlargement so that subsurface flow rapidly becomes the dominant process, causing pipe enlargement by a process of positive feedback. The paper speculates on the progressive role of surface and subsurface processes in the genesis of all evolutional badland forms (calanchi, calanchi mammellonari and biancane). In an early stage, the calanchi mammellonari are produced, as slope mounds become increasingly separated by a dense network of small inclined pipes in the intermediate part of the slopes, a site which is presumed to favour elevated subsurface flow. When the overburden collapses, many cones are isolated in these portions of the slope. A second stage follows in which overland flow dominates, with the water canalizing into small gullies generated by the collapsed crust. At the base of many slopes, this same mechanism produces biancane as residual cones. Because of the structural weakness of piped materials, mass movements cannot be ruled out. In a few locations, landsliding may be involved, especially in the formation of calanchi mammellonari. In this case the intersection of the vertical pipes with the impermeable substratum focuses subsurface flow, and collapse of the surface along this failure plane. Subsequent remodelling occurs by surface processes, since the ‘catchment volume’ for large pipes no longer exists. This interpretation differs from that of others who have argued that in other locations the biancane formation is linked to the development of large vertical pipes along tectonic joints.

Influence of the Pipe Diameter on the Structure of the Gas-Liquid Interface in a Vertical Two-Phase Pipe Flow

Air-water two-phase flow tests in a large vertical pipe of 194.1-mm inner diameter (i.d.) are reported. Close to the outlet of a 9-m-tall test section, two wire-mesh sensors are installed that deliver instantaneous void fraction distributions over the entire cross section with a resolution of 3 mm and 2500 Hz used for fast-flow visualization. Void fraction profiles, gas velocity profiles, and bubble-size distributions were obtained. A comparison to a small pipe of 52.3-mm i.d. (DN50) revealed significant scaling effects. Here, the increase of the airflow rate leads to a transition from bubbly via slug to churn-turbulent flow. This is accompanied by an appearance of a second peak in the bubble-size distribution. A similar behavior was found in the large pipe; though the large bubbles have a significantly larger diameter at identical superficial velocities, the peak is less high but wider. These bubbles move more freely in the large pipe and show more deformations. The shapes of such large bubbles were characterized in three dimensions. They can be rather complicated and far from ideal Taylor bubbles. Also, the small bubble fraction tends to bigger sizes in the large pipe.

Prediction of Void Fraction in a Large Vertical Pipe under pressure 3-9 MPa by TRAC-BF1 code

Prediction of Void Fraction in a Large Vertical Pipe under pressure 3-9 MPa by TRAC-BF1 code

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.

An experimental study on developing air-water two-phase flow along a large vertical pipe: effect of air injection method

The flow structure in a developing air-water two-phase flow was investigated experimentally along a large vertical pipe (inner diameter, D h: 0.48 m, ratio of length of flow path L to D h: about 4.2). Two air injection methods (porous sinter injection and nozzle injection) were adopted to realize an extremely different flow structure in the developing region. The flow rate condition in the test section was as follows: superficial air velocity: 0.02–0.87 m s (at atmospheric pressure) and superficial water velocity: 0.01–0.2 0.01–0.2 m/s, which covers the range of bubbly to slug flow in a small-scale pipe ( D h ⩽ about 0.05 m). No air slugs occupying the flow path were recognized in this experiment regardless of the air injection methods even under the condition where slug flow is realized in the small-scale pipe. In the lower half of the test section, the axial distribution of sectional differential pressure and the radial distribution of local void fraction showed peculiar distributions depending on the air injection methods. However, in the upper half of the test section, the effects of the air injection methods are small in respect of the shapes of the differential pressure distribution and the phase distribution. The comparison of sectional void fraction near the top of the test section with Kataoka's correlation indicated that the distribution parameter of the drift-flux model should be modeled including the effect of D h and the bubble size distribution is affected by the air injection methods. The bubble size distribution is considered to be affected also by L D h based on comparison of results with Hills' correlation.

The course and magnitude of water stress in Lolium perenne and Dactylis glomerata

SUMMARYSimulated swards of Dactylis glomerata (var. S.37) and Lolium perenne (var. S.23) were grown in large lysimeters or vertical pipes of 15 cm diameter, both sufficiently deep to allow largely unrestricted root development.Rainfall was excluded, and the effect of a drying cycle on the plant water balance was compared with irrigated controls in a sequence of sampling harvests at increasing soil water deficits.Leaf water potential (ΨL) fell during the day, both in treatments and controls, to levels which might be expected to reduce extension growth and, frequently, stomatal diffusion. Rapid recovery occurred in the evening to levels which might allow normal functioning of growth processes not dependent on sunlight. Defoliation reduced plant stress and stomatal restriction.The amelioration of plant water stress appeared to require a reduction in atmospheric evaporative demand, and irrigation had relatively little effect. The possibility is discussed that the major benefits of irrigation are other than through the relief of water stress within the plant. The significance of this is considered in relation to conventional irrigation techniques.The leaf water status was more sensitive to drought, transpiration was reduced more, and the root system extended more slowly in Dactylis than in Lolium. Consequently, the onset of permanent wilting due to exhaustion of water from the profile was delayed compared with Lolium. It is deduced that this characteristic might enhance the survival of Dactylis in prolonged drought, but prove disadvantageous in terms of growth during short droughts, when reduced stomatal opening might limit CO2 uptake. This would not be an impediment, however, if investigations suggesting that partial closure has a minor effect on CO2 uptake compared with that on transpiration were to be confirmed.

Hydraulic Design of the Sandow Pumping Plant

Vertical turbine pump installation provides make-up water for cooling pond serving circulating water system of 300 mw steam power plant of Aluminum Co of America near Rockdale, Tex; pumping facility is unusual in its combination of large vertical pumps, long and irregular pipe line profile and surge and flow control problems; hydraulic features of plant described; solutions to some unusual design problems.