Heterogeneous Flow Conditions Research Articles

Research on the generalization issue of the heterogeneous QC-EMMS drag model for gas-solid fluidization

In the circulating fluidized bed (CFB), the non-uniform mesoscale structure formed by the particle clustering effect causes a substantial decrease in the gas-solid drag. Since the degree of particle clustering varies with operating conditions, an accurate drag model needs to be universal in different heterogeneous flow conditions. In this study, the relationships between the Ψ factor in clusters' solid holdup model that characterizes the flow non-uniformity and the operating parameters of CFB (including slip velocity Reynolds number, Re*, bed-averaged solid volume fraction, εs,bed, and solid mass circulation rate, Gs) are established. It improves the adaptability of the QC-EMMS drag model under different working conditions. In previous research, we established two types of models that related Ψ factor to Re* and εs,bed respectively. However, there exists a problem of high dispersion of points, indicating that the selected parameters cannot fully describe the flow non-uniformity. Therefore, Gs is reintroduced to modify the two types of models. The results show that the prediction accuracy of the modified models is improved and the relative error is <10%, indicating that the non-uniform factor Ψ has a strong correlation with Gs. In addition, the quantitative relation between Re*, εs,bed, and Gs is derived from modified models, and the trend of relation is highly consistent with the fluidization diagram proposed by Yerushalmi J., which verifies the accuracy of modified models. Finally, numerical simulation of typical CFB cases proves the adaptability of the modified models in wide operating conditions of fluidization.

Process Development of Heterogeneous Rh Catalyzed Carbene Transfer Reactions Under Continuous Flow Conditions.

A very simple Rh-based catalyst operates under heterogeneous flow conditions for the carbene transfer of methyl diazoacetate (MDA) with several substrates. Two different methods for heterogenizing the catalyst in a column reactor have been applied. Different X-H (X=O, S, Si, CH2 ) were successfully functionalized by the carbene and cyclopropenation was performed under very mild continuous flow conditions. Following these promising results, catalyst recycling experiments using both methodologies were conducted in which up to 5 catalytic cycles have been achieved for the carbene O-H insertion reaction and interestingly, a sequential transformation of different substrates with up to 10 consecutive runs per reactor were achieved with no loss in the catalytic activity, thus allowing the production of families of compounds.

The revised FLORIDyn model: implementation of heterogeneous flow and the Gaussian wake

Abstract. In this paper, a new version of the FLOw Redirection and Induction Dynamics (FLORIDyn) model is presented. The new model uses the three-dimensional parametric Gaussian FLORIS model and can provide dynamic wind farm simulations at a low computational cost under heterogeneous and changing wind conditions. Both FLORIS and FLORIDyn are parametric models which can be used to simulate wind farms, evaluate controller performance and can serve as a control-oriented model. One central element in which they differ is in their representation of flow dynamics: FLORIS neglects these and provides a computationally very cheap approximation of the mean wind farm flow. FLORIDyn defines a framework which utilizes this low computational cost of FLORIS to simulate basic wake dynamics. This is achieved by creating so-called observation points (OPs) at each time step at the rotor plane which inherit the turbine state. In this work, we develop the initial FLORIDyn framework further considering multiple aspects. The underlying FLORIS wake model is replaced by a Gaussian wake model. The distribution and characteristics of the OPs are adapted to account for the new parametric model but also to take complex flow conditions into account. To achieve this, a mathematical approach is developed to combine the parametric model and the changing, heterogeneous world conditions and link them with each OP. We also present a computationally lightweight wind field model to allow for a simulation environment in which heterogeneous flow conditions are possible. FLORIDyn is compared to Simulator for Offshore Wind Farm Applications (SOWFA) simulations in three- and nine-turbine cases under static and changing environmental conditions. The results show a good agreement with the timing of the impact of upstream state changes on downstream turbines. They also show a good agreement in terms of how wakes are displaced by wind direction changes and when the resulting velocity deficit is experienced by downstream turbines. A good fit of the mean generated power is ensured by the underlying FLORIS model. In the three-turbine case, FLORIDyn simulates 4 s simulation time in 24.49 ms computational time. The resulting new FLORIDyn model proves to be a computationally attractive and capable tool for model-based dynamic wind farm control.

Kinematics and shear-induced alignment in confined granular flows of elongated particles

The kinematics and the shear-induced alignment of elongated particles in confined, heterogeneous flow conditions are investigated experimentally. Experiments are conducted in an annular shear cell with a rotating bottom wall and a top wall permitting confinement of the flow. Flow kinematics and particle orientation statistics are computed by particle tracking using optical imaging. Translational velocity profiles show an exponential decay, and surprisingly, only the slip velocity at the bottom is influenced by the particle shape. Rotations are highly frustrated by particle shape, more elongated particles showing, on average, a lower angular velocity. In addition, a clear shear-rate dependency of the proneness of a particle to rotate is observed, with a stronger inhibition in low shear zones. The average orientation of the particles does not correspond to the main flow direction, they are slightly tilted downwards. The corresponding angle decreases with the particles’ elongation. Orientational order was observed to increase with particles’ elongation, and surprisingly was not affected by the applied confinement. A weak but systematic decrease of the orientational order was observed in regions of higher shear rate. At the particle-scale, angular velocity fluctuations show a strong correlation with local particle orientation, particles being strongly misaligned with the preferential particles’ orientation rotating faster. This correlation becomes stronger for more elongated particles, while is almost unaffected by the applied confinement.

The Dominant Control of Relief on Soil Water Content Distribution During Wet‐Dry Transitions in Headwaters

AbstractThe redistribution of hillslope soil water during and after rainstorms is affected by soil properties and topography. Therefore, understanding how soil‐terrain attributes affect the soil volumetric water content (VWC) distribution under various catchment storages is a prerequisite for accurate hydrological modeling. Herein, the relationships between soil‐terrain attributes and soil VWC were examined in a steep (average slope = 60%), forested, zero‐order catchment. Detailed topography, soil properties, and runoff, and high frequency (6‐min) soil moisture data observed from July 2016 to November 2017 were employed along with resampled VWC data in three time‐steps (daily, hourly, and 6‐min intervals) for correlation analysis between soil‐terrain attributes and measured VWC. The results showed that daily aggregated data overlooked the highly heterogeneous flow conditions under dry‐wet transitions, leading to a constantly high correlation between VWC and the topographic wetness index (TWI). In comparison, the 6‐min data captured short‐lived flow processes, highlighting that, in reality, VWC‐TWI correlations fluctuated frequently as a function of catchment storages and precipitation characteristics, with four wetting patterns identified. Under wet‐dry transitions, the VWC‐TWI correlations increased significantly (from 0.3 to 0.6) as the hillslope gradually drained of lateral subsurface water. Soil water behavior consists of diurnal oscillations superimposed on a declining trend, which demonstrates the existence of unsaturated flow and which contributed to the increase in the strength of the correlations. Finally, through correlation analysis of all drying (n = 4) and wetting (n = 45) periods, we found that soil‐terrain attributes and VWC correlations can be used to identify the dominant soil water processes in various catchment storages.

Mass Flux Analysis of Abiotic Tetrachloroethene Dense Nonaqueous Phase Liquid Pool Dissolution in a Heterogeneous Flow Environment

AbstractPorous aquifer materials are often characterized by layered heterogeneities that influence groundwater flow and present complexities in contaminant transport modeling. Such flow variations also have the potential to impact the dissolution flux from dense nonaqueous phase liquid (DNAPL) pools. This study examined how these heterogeneous flow conditions affected the dissolution of a tetrachloroethene (PCE) pool in a two‐dimensional intermediate‐scale flow cell containing coarse sand. A steady‐state mass‐balance approach was used to calculate the PCE dissolution rate at three different flow rates. As expected, aqueous PCE concentrations increased along the length of the PCE pool and higher flow rates decreased the aqueous PCE concentration in the effluent. Nonreactive tracer studies at two flow rates confirmed the presence of a vertical flow gradient, with the most rapid velocity located at the bottom of the tank. These results suggest that flow focusing occurred near the DNAPL pool. Effluent PCE concentrations and pool dissolution flux rates were compared to model predictions assuming local equilibrium (LE) conditions at the DNAPL pool/aqueous phase interface and a uniform distribution of flow. The LE model did not describe the data well, even over a wide range of PCE solubility and macroscopic transverse dispersivity values. Model predictions assuming nonequilibrium mass‐transfer‐limited conditions and accounting for vertical flow gradients, however, resulted in a better fit to the data. These results have important implications for evaluating DNAPL pool dissolution in the field where subsurface heterogeneities are likely to be present.

Design and analysis of a wake model for spatially heterogeneous flow

Abstract. Methods of turbine wake modeling are being developed to more accurately account for spatially variant atmospheric conditions within wind farms. Most current wake modeling utilities are designed to apply a uniform flow field to the entire domain of a wind farm. When this method is used, the accuracy of power prediction and wind farm controls can be compromised depending on the flow-field characteristics of a particular area. In an effort to improve strategies of wind farm wake modeling and power prediction, FLOw Redirection and Induction in Steady State (FLORIS) was developed to implement sophisticated methods of atmospheric characterization and power output calculation. In this paper, we describe an adapted FLORIS model that features spatial heterogeneity in flow-field characterization. This model approximates an observed flow field by interpolating from a set of atmospheric measurements that represent local weather conditions. The objective of this method is to capture heterogeneous atmospheric effects caused by site-specific terrain features, without explicitly modeling the geometry of the wind farm terrain. The implemented adaptations were validated by comparing the simulated power predictions generated from FLORIS to the actual recorded wind farm output from the supervisory control and data acquisition (SCADA) recordings and large eddy simulations (LESs). When comparing the performance of the proposed heterogeneous model to homogeneous FLORIS simulations, the results show a 14.6 % decrease for mean absolute error (MAE) in wind farm power output predictions for cases using wind farm SCADA data and a 18.9 % decrease in LES case studies. The results of these studies also indicate that the efficacy of the proposed modeling techniques may vary with differing site-specific operational conditions. This work quantifies the accuracy of wind plant power predictions under heterogeneous flow conditions and establishes best practices for atmospheric surveying for wake modeling.

Simulating Scale Dependencies on Dispersive Mass Transfer in Porous Media Under Various Boundary Conditions

Studies on mass transfer and transport of dissolved contaminants through heterogeneous hydrogeology rely on realistic expressions of their functional parameters. Hydrodynamic dispersion is one such parameter inheriting significant ambiguities in its estimation when subjected to heterogeneous flow conditions. In this study, a numerical model has been developed to explicitly evaluate the variability of dispersion coefficient on time-based and space-based scales for simulating contaminant transport mechanism in saturated porous media under nonlinear sorption mass transfer. Four types of inlet (solute source) conditions, viz., constant continuous, pulsating, exponentially decaying and sinusoidal varying were considered, and the resulting concentration profiles were compared. The governing partial differential equations were solved using an implicit finite difference technique. The results from sensitivity analysis suggest that the magnitude of the fluid velocity was influenced by the type of dispersion coefficient for different inlet boundary conditions. However, sorption partition coefficients and porosities were generally insensitive to the nature of dispersion under these boundary conditions. When time-dependent dispersion coefficient was adopted, an asymptotic increase in retained solute mass was observed for increased fluid velocity irrespective of the boundary conditions. The behavior of the concentration profile obtained using distance-dependent dispersion coefficient was unique when sinusoidal decaying solute boundary condition was used. The study rightly emphasizes the need for simultaneously updating mass transfer and transport parameters while accounting for the variation in solute inlet conditions.

Hydrodynamics, mixing and mass transfer in a pilot-scale bubble column with dense internals

Bubble column reactors with exothermic reactions are often equipped with dense tube bundle heat exchangers. While there is some knowledge about the impact of such internals on hydrodynamics and mass transfer for narrow columns, its role in pilot-scale columns is less clear. In this paper we report on a study of hydrodynamics and mass transfer in a BCR of 4.2 m height and 0.392 m diameter. We investigated different tube arrangements with triangular and square pitch and tube diameters of 32 × 10−3 m and 45 × 10−3 m at the same cross-sectional coverage (∼25%). The column was operated at homogeneous and heterogeneous flow conditions. A customized three-layer wire-mesh sensor was utilized to visualize gas phase dynamics and liquid mixing characteristics in the column’s cross-section. We found that sub-channel size is the most crucial geometric design parameter. Tracer mixing experiments revealed that internals reduce the mixing time due to the induction of large-scale liquid circulation. Mass transfer was studied with the oxygen stripping method. Here we found, that the effect of the internals on the gas-liquid mass transfer is almost negligible. Eventually, correlations for gas holdup, axial liquid dispersion and the volumetric gas-liquid mass transfer coefficient are given, which take the internals’ geometry into account.

Two-Bubble Class Approach Based on Measured Bubble Size Distribution for Bubble Columns with and without Internals

The complex flow patterns in bubble columns can be phenomenologically described by the two-bubble class approach. For the first time, this approach is applied to bubble columns with dense internals. Internals of square and triangular pitch tube patterns of two tube sizes (8 × 10–3 and 13 × 10–3 m) with flat and U-tube bottom design and cross-sectional occupation of ∼25% were inserted in a bubble column of 0.1 m diameter and 2 m height. Contrary to the well-known gas disengagement technique, dual-plane ultrafast X-ray computed tomography data have been used for the bubble class allocation. Experiments were performed at superficial gas velocities ranging from 0.02 to 0.20 m s–1 to cover homogeneous and heterogeneous flow conditions. The contributions of small and large bubble classes on total gas holdup, flow structure and bubble rise velocities were determined. Furthermore, the regime transition onset was determined based on the two-bubble class approach. Eventually, new correlations for regime transition,...

Landscapes of facilitation: how self-organized patchiness of aquatic macrophytes promotes diversity in streams.

Spatial heterogeneity plays a crucial role in the coexistence of species. Despite recognition of the importance of self-organization in creating environmental heterogeneity in otherwise uniform landscapes, the effects of such self-organized pattern formation in promoting coexistence through facilitation are still unknown. In this study, we investigated the effects of pattern formation on species interactions and community spatial structure in ecosystems with limited underlying environmental heterogeneity, using self-organized patchiness of the aquatic macrophyte Callitriche platycarpa in streams as a model system. Our theoretical model predicted that pattern formation in aquatic vegetation - due to feedback interactions between plant growth, water flow and sedimentation processes - could promote species coexistence, by creating heterogeneous flow conditions inside and around the plant patches. The spatial plant patterns predicted by our model agreed with field observations at the reach scale in naturally vegetated rivers, where we found a significant spatial aggregation of two macrophyte species around C.platycarpa. Field transplantation experiments showed that C.platycarpa had a positive effect on the growth of both beneficiary species, and the intensity of this facilitative effect was correlated with the heterogeneous hydrodynamic conditions created within and around C.platycarpa patches. Our results emphasize the importance of self-organized patchiness in promoting species coexistence by creating a landscape of facilitation, where new niches and facilitative effects arise in different locations. Understanding the interplay between competition and facilitation is therefore essential for successful management of biodiversity in many ecosystems.

Filter performance with non-uniformly distributed concentration of dust-evidence from experiments and models

Cleanable fabric filters such as bag filters are widely used in industrial processes to remove large quantities of solid particles from gas streams. Intrinsically, such a filtration process is as non-uniform as it is semi-continuous; that is, filter cake build-up processes are successively followed by a short-duration cleaning step. Moreover, heterogeneous flow conditions and, thus, concentrations of particles entering the filter housing and flowing toward the filter contribute to rather non-uniform filter conditions. Since cleaning is not necessarily performed in a homogeneous manner, and patches of cake might remain on the filter fabric, the subsequent filtration is also non-uniform. Finally, the filter medium itself is largely heterogeneous certainly on micro-, but often also on a macro-scale.The influence of a heterogeneous dust concentration as a common filter non-uniformity is investigated on filter performance, i.e., pressure drop increase as a measure for filter cycle cleaning frequency being a precursor for filter life-time, dust emissions and operating costs.Laboratory tests were performed whereby the pressure increase curves are recorded at basically constant gas flow. As a baseline the entire filter area was exposed to a homogeneous dust concentration. Comparatively the filtration area was split into two zones where cake formation occurred under a high and a low particle concentration mimicking a distinctive heterogeneous dust concentration situation. The transient pressure increase profiles of both, the homogeneous and heterogeneous cake formation situation, were compared with each other.The apparent filter cake resistance was slightly higher under homogeneous dust concentration conditions, i.e., the pressure drop increased faster compared with the situation of a heterogeneous dust concentration. These results suggest that, in practical terms, a filter exposed to non-uniform dust concentrations exhibits a better overall performance. Consequently aiming for a most uniform dust concentration is to no avail, though often intended in industrial operation.

Corner and sloped culvert baffles improve the upstream passage of adult European eels (Anguilla anguilla)

Installation of baffles intended to improve fish passage through culverts can reduce discharge capacity and trap debris, increasing flood risk. A sloping upstream face may reduce this risk, but new designs must be tested for fish passage efficiency. The European eel (Anguilla anguilla) is a critically endangered species, yet the suitability of even common baffle types to aid upstream movement has not been tested. This study compared the water depth, velocity, turbulent kinetic energy (TKE), and upstream passage performance of adult yellow-phase eels, between three 6m long culvert models: smooth and unmodified (control); containing corner baffles (treatment 1); and with prototype sloped baffles installed (treatment 2). Passage of individual fish was assessed during 25 one-hour trials per model. Performance was quantified as entrance efficiency, number of entries per fish, passage efficiency, and overall efficiency. Total and passage delay, and successful passage time were also evaluated. Despite some individuals being able to swim against unexpectedly high water velocities (>1.5ms−1 for 4m), passage performance in the control was poor, with an overall efficiency of 28%. Compared to the control, both treatments increased the mean centreline water depth by approximately 0.11m, created heterogeneous flow conditions with low velocity resting areas, and reduced maximum velocities. As a result, entrance rate and all efficiency parameters were higher for the treatments than for the control (overall efficiency=84%), despite longer passage delay. The TKE was slightly higher in treatment 2 than 1, but there was no difference in water depth or overall efficiency. The findings show that both corner and sloped baffles can mitigate for impeded upstream adult eel movement. The extent to which the sloping upstream face will improve debris transport should be explored further.

Uncertainty in applying the temperature time-series method to the field under heterogeneous flow conditions

summary Due to the irregular distributions of aquifer hydraulic properties, the detail on the characterization of flow field cannot be anticipated. There can be a great degree of uncertainty in the prediction of heat transport processes anticipated in applying the traditional deterministic transport equation to field situations. This article is therefore devoted to quantification of uncertainty involving predictions over larger scales in terms of the temperature variance. A stochastic frame of reference is adopted to account for the spatial variability in hydraulic conductivity and specific discharge. Within this framework, the use of the firstorder perturbation approximation and spectral representation leads to stochastic differential equations governing the mean behavior and perturbation of the temperature field in heterogeneous aquifers. It turns out that the mean equation developed in this sense is equivalent to the traditional deterministic transport equation and the temperature variance gives a measure of the prediction uncertainty from the traditional transport equation. The closed-form expression for the temperature variance developed here indicates that the controlling parameters such as the correlation scale of specific discharge, which measures the spatial persistence of the flow field, and the periodicity of the source term tend to increase the variability in temperature field in heterogeneous aquifers. The uncertainty of the traditional heat transport model increases as the penetration depth of thermal front through the aquifer increases. This suggests that prediction of temperature distribution using the traditional heat transport model in heterogeneous aquifers is expected to be subject to large uncertainty at a large depth (in the downstream region). For the management purpose, the variance of temperature could serve as a calibration target when applying the traditional model to field situations. It may be more reasonable to make conclusions from, say, the mean temperature with one or two standard deviations rather than only the mean temperature drawn from the traditional heat transport equation.

Fish responses to flow velocity and turbulence in relation to size, sex and parasite load

Riverine fish are subjected to heterogeneous flow velocities and turbulence and may use this to their advantage by selecting regions that balance energy expenditure for station holding while maximizing energy gain through feeding opportunities. This study investigated microhabitat selection by guppies Poecilia reticulata in terms of flow characteristics generated by hemisphere boulders in an open channel flume. Velocity and turbulence influenced the variation in swimming behaviour with respect to size, sex and parasite intensity. With increasing body length, fish swam further and more frequently between boulder regions. Larger guppies spent more time in the areas of high-velocity and low-turbulence regions beside the boulders, whereas smaller guppies frequented the low-velocity and high-turbulence regions directly behind the boulders. Male guppies selected the regions of low velocity, indicating possible reduced swimming ability owing to hydrodynamic drag imposed by their fins. With increasing Gyrodactylus turnbulli burden, fish spent more time in regions with moderate velocity and lowest turbulent kinetic energy which were the most spatially and temporally homogeneous in terms of velocity and turbulence. These findings highlight the importance of heterogeneous flow conditions in river channel design owing to the behavioural variability within a species in response to velocity and turbulence.

Variability of unsaturated Bromide fluxes as measured through a layered volcanic vadose zone in New Zealand

The measured drainage fluxes through a layered volcanic vadose zone exhibited high spatial variability as a consequence of heterogeneous flow conditions. The drainage flux variability was quantified using automated equilibrium tension lysimeters, installed in close-proximity and resulted in high variability in the Br masses recovered from a conservative tracer experiment. The primary cause of the heterogeneous flow was attributed to textural changes occurring at the interface between volcanic layers, resulting in development of funnel-flow patterns, and further enhanced by the existence of hydrophobic conditions. The Br recoveries in individual automated equilibrium tension lysimeters were used to determine the corresponding variable sizes of the surface areas contributing drainage to the lysimeters. The tracer experiment confirmed the existence of unsaturated lateral transport occurring at the sloping interface of the coarse Taupo Ignimbrite material with the silty Palaeosol layer at approximately 4.2 m depth. This study demonstrates that measurements of both flux and solute concentrations at multiple locations are essential when heterogeneous flow is suspected to be present, to be able to determine reliable estimates of contaminant leaching through the vadose zone at the plot scale. Copyright © 2013 John Wiley & Sons, Ltd.

CFD Modeling of Flow Pattern and Phase Holdup of Three Phase Fluidized Bed Contactor

The present work has been carried out to characterize the dynamics of three-phase flow in cylindrical fluidized bed, run under homogeneous bubble flow and heterogeneous flow conditions using CFD (Computational Fluid Dynamics) simulation. The investigation has been done to study the flow pattern of three-phase fluidized bed along with parametric studies. The simulations were performed for air-water-glass beads in a fluidized bed of H = 0.6 m, Di = 0.1 m and ds = 0.05 m to study the flow pattern. Eulerian-Eulerian, three-phase simulations with k-? turbulence for liquid phase were carried out using the flow simulation software CFD-6.2.16, with a focus on characterizing the dynamics properties of gas-liquid-solid flows. Detail study of the flow pattern in three-phase fluidized bed has been carried out and the flow pattern has been presented in the form of contour plots. The results presented are useful for understanding the dynamics of gas-liquid-solid flows in fluidized bed and provide a basis for further development of CFD model for three phase systems.

South China karst aquifer storm-scale hydrochemistry.

The peak cluster and peak forest karst regions of Southeast Asia form one of the earth's most extensive karst regions. Although there exists a rich, descriptive tradition of geomorphic work performed there, little quantitative study has been made of carbonate hydrochemistry and related aquifer/landscape behavior and evolution. In this paper, high-resolution measurements of ground water carbonate chemistry and flow were made and analyzed at two adjacent locations within the subtropical peak cluster karst of the Guilin Karst Experimental Site in Guangxi Province, China. While waters from a large, perennial spring represent the exit for the 2 km2 catchment's conduit flow, a nearby well (within 5 m) measures water in the conduit-adjacent, fractured media. Results indicate that within peak cluster karst aquifer flow systems, spatially heterogeneous flow conditions can exist with respect to timing, magnitude, and, in some cases, direction of responses, as different controls can operate in the different flow system components. Storm-scale chemical responses are controlled by dilution from rapid infiltration of rain water, CO2 gas sources and sinks, and water-carbonate rock interactions. At this particular location, there is also an influence from high pH recharge, apparently buffered by atmospheric limestone dust. An example of the varying controls on storm-scale responses within the flow system is that within the fractured medium, variations in the ground water calcite saturation index, a key parameter influencing rates of aquifer/landscape evolution, are small and controlled by CO2 gas, while in the conduit they are more significant and dominated instead by dilution with rain water.

Gas Holdup in a Fiber Suspension

AbstractThis paper investigates the effects of fiber mass fraction and length on gas holdup and flow regime transition. The experiments are performed using three different Rayon fiber lengths (3, 6, and 12 mm) over a range of superficial gas velocities (Ug ⩽ 18 cm/s) and a range of fiber mass fractions (0 ⩽ C ⩽ 1.8%) in a 15.24 cm diameter bubble column, in which air is continuously bubbled through a fixed volume of a fiber suspension. Experimental results show that increasing the fiber mass fraction tends to decrease the overall gas holdup when the fiber mass fraction is less than a certain value, which depends on fiber length. Fiber length influences the gas holdup when C ⩽ 1.4%, and increasing fiber length reduces the overall gas holdup. Homogeneous, transitional, and heterogeneous flow conditions are observed at low fiber mass fractions, but when C ⩽ 0.6%, purely heterogeneous flow conditions are observed. The flow regime transition is not significantly affected by fiber length.

Improvement and Assessment of the CATHARE 2 Three-Dimensional Module Compared with the UPTF Downcomer Test 7

The CATHARE 2 three-dimensional module is assessed in comparison with the Upper Plenum Test Facility downcomer test 7, which was performed to obtain full-scale data on downcomer and lower plenum refill behavior during the refill phase of a loss-of-coolant accident. New discretizations for the equation of motion, named Mods. D and R, are suggested and implemented in the three-dimensional module. Mod. A is also investigated, which defines a new junction void fraction used to calculate interfacial friction. Using the standard and the modified three-dimensional modules, the four experiments, test 7 runs 200 through 203, are simulated with the downcomer nodalized as an 8 × 1 × 8 mesh. Sensitivity calculations associated with interfacial friction, condensation, and nodalization are also performed. The calculation results show that the discretization of the momentum convection is very important in strongly heterogeneous flow conditions. Mod. D + A gives the best results so far, and Mod. R + A yields the smallest scatter in the predicted water deliveries to the lower plenum. The results of the sensitivity calculations show that the interfacial friction coefficient of CATHARE 2 is somewhat overestimated and the 8 × 1 × 8 mesh downcomer is fine enough for test 7.