Non-drag Forces Research Articles

Air–Water Two-Phase Flow Dynamics Analysis in Complex U-Bend Systems through Numerical Modeling

This study aims to provide insights into the intricate interactions between gas and liquid phases within flow components, which are pivotal in various industrial sectors such as nuclear reactors, oil and gas pipelines, and thermal management systems. Employing the Eulerian–Eulerian approach, our computational model incorporates interphase relations, including drag and non-drag forces, to analyze phase distribution and velocities within a complex U-bend system. Comprising two horizontal-to-vertical bends and one vertical 180-degree elbow, the U-bend system’s behavior concerning bend geometry and airflow rates is scrutinized, highlighting their significant impact on multiphase flow dynamics. The study not only presents a detailed exposition of the numerical modeling techniques tailored for this complex geometry but also discusses the results obtained. Detailed analyses of local void fraction and phase velocities for each phase are provided. Furthermore, experimental validation enhances the reliability of our computational findings, with close agreement observed between computational and experimental results. Overall, the study underscores the efficacy of the Eulerian approach with interphase relations in capturing the complex behavior of the multiphase flow in U-bend systems, offering valuable insights for hydraulic system design and optimization in industrial applications.

CFD Modeling of Phase Change during the Flashing-Induced Instability in a Natural Circulation Circuit

Flashing-induced instability (FII) has a significant impact on the safe operation of a natural circulation circuit, a phenomenon frequently encountered in the cooling systems of advanced light water reactors. While one-dimensional system codes are commonly used for the engineering design and safety analysis of FII, there is a strong academic interest in understanding the underlying physical mechanisms. To address this, high-resolution computational fluid dynamics (CFD) simulations serve as a valuable tool. However, the current state of CFD modeling for two-phase flows with phase change, which are particularly highly transient fluctuating flashing flows, is still in its early stages of development. In this study, we establish a CFD model that focuses on interphase heat transfer to analyze the phase change during FII. By incorporating experimental data from the literature, we investigate the transient flow field and thermodynamic behavior in the riser of the GENEVA test facility. The study provides valuable insights into the non-equilibrium and interfacial transfer phenomena during the phase change as well as the effect of high-frequency fluctuation. Additionally, we discuss in detail the challenges associated with FII modeling and the limitations of the current model. We also provide suggestions for potential improvements in future numerical studies. The results show that the thermal phase change and heat transfer coefficient model adopted for the simulation reasonably captures the evaporation and condensation process. However, it tends to under-predict the evaporation rate, which results in a larger pressure drop through the riser. The observation that the void fraction close to the wall is higher than that in the riser center evidences that the reliable modeling of bubble size distribution as well as the inclusion of non-drag forces are important for predicting the transverse void distribution. Furthermore, it reveals that both the temperature and pressure change in an FII, and their effects on phase change should be taken into account simultaneously.

Verification and validation of CFD and its application in PWR fuel assembly

Fuel assembly is one of the most important components in Pressurized Water Reactor (PWR). Mixing Vane Spacer Grid (MVG) provides support for the fuel bundle. However, the most important role for MVG is to mix the coolant and decrease the temperature gradient, ultimately enhance the heat transfer and improve Critical Heat Flux (CHF). Hence the performance of MVG are directly related to the safety and economics of PWR. Due to the complexity of MVG and the broad range of thermal hydraulic conditions involved, full scale, time consuming and costly full length rod bundle experiments (usually 5 × 5 or 4 × 4 or 6 × 6) were performed to study the complicated thermal hydraulic phenomenon. Recently, with the development of computer technology, Computational Fluid Dynamics (CFD) has become one of the most popular tools to simulate the flow field of the fuel assembly. In this paper, the method based on CFD to study the MVG performance is reviewed. The main process for CFD in simulating flow field of MVG is divided into two parts. The first part is the verification and validation of CFD codes under single phase-and two-phase flow. The second is the application of CFD for exploring the MVG performance. For the verification of single phase flow CFD simulation, mesh sensitivity, turbulent model effects were examined, the validation including axial pressure drop, lateral temperature distribution, and lateral velocity distribution. For two phase flow verification and validation, the sensitivities of different drag and non-drag forces on void fraction distribution were tested. The experimental data with a tube and an annual flow fields under two phase boiling were chosen to validate the two phase flow simulation and to cover different aspects of internal and external wall boiling effects. The verified and validated CFD codes were then used to study the performance of MVG. Considering the complexity of MVG, a separated effect study approach is presented to evaluate the performance of MVG. Another systematic approach from simple geometry to 5 × 5 MVG model to study the effect of different components, including vane, dimple, and spring were proposed for the simulation of two phase flow. Some indexes were reviewed and used to evaluate the performance of MVG.

STUDY OF FLUID DYNAMICS AND SULFUR MASS TRANSFER BETWEEN STEEL AND SLAG IN A LADLE FURNACE CONSIDERING DRAG AND NON-DRAG FORCES

In this work fluid dynamics and a basic study of the sulfur transfer at the steel/slag interface in the ladle during argon gas agitation was developed. Mass transfer and chemical reaction models coupled with Computational Fluid Dynamics (CFD) were employed. The multiphasic simulation was solved using the Eulerian model considering drag and non-drag forces, and the flow pattern was validated through Particle Image Velocimetry (PIV) technique. The sulfur transfer rate was tracked by two approximations: (1) unidirectional constant rate Mass Transfer Model (MTM), and (2) unidirectional constant rate Mass Transfer Model coupled with Chemical Reaction Model (MTM+CRM) using Arrhenius equation. It was found that including the non-drag forces affects the fluid dynamics structure. Otherwise, the desulfurization rates increase as the argon gas flow rate increases, finding that the MTM model predicts ~15% less sulfur in the steel than the MTM+CRM, whose results were compared with plant measurements reports.

Investigation of subcooled boiling wall closures at high pressure using a two-phase CFD code

This study validates the applicability of the CUPID code for simulating subcooled wall boiling under high-pressure conditions against number of DEBORA tests. In addition, a new numerical technique in which the interfacial momentum non-drag forces are calculated at the cell faces rather than the center is presented. This method reduced the numerical instability often triggered by calculating these terms at the cell center. Simulation results showed good agreement against the experimental data except for the bubble sizes in the bulk. Thus, a new model to calculate the Sauter mean diameter is proposed. Next, the effect of the relationship between the bubble departure diameter (Ddep) and the nucleation site density (N) on the performance of the Wall Heat Flux Partitioning (WHFP) model is investigated. Three correlations for Ddep and two for N are grouped into six combinations. Results by the different combinations show that despite the significant difference in the calculated Ddep, most combinations reasonably predict vapor distribution and liquid temperature. Analysis of the axial propagations of wall boiling parameters shows that the N term stabilizes the inconsistences in Ddep values by following a behavior reflective of Ddep to keep the total energy balance. Moreover, ratio of the heat flux components vary widely along the flow depending on the combinations. These results suggest that separate validation of Ddep correlations may be insufficient since its performance relies on the accompanying N correlations.

CFD-PBM Simulation of Bubble Columns: Sensitivity Analysis of the Nondrag Forces

In this work, the effect of non-drag forces (lift force, turbulent dispersion force, wall lubrication force and virtual mass force) and bubble induced turbulence (BIT) on the CFD-PBM simulation of ...

Computational fluid dynamics of rectangular external loop airlift reactor

AbstractThe novel design of a rectangular external loop airlift reactor is at present the most used large-scale reactor for microalgae culture. It has a unique future for a large surface to volume ratio for exposure of light radiation for photosynthesis reaction. The 3D simulations have been performed in rectangular EL-ALR. The Eulerian–Eulerian approach has been used with a dispersed gas phase for different turbulent models. The performance and applicability of different turbulent model’s i.e., K-epsilon standard, K-epsilon realizable, K-omega, and Reynolds stress model are used and compared with experimental results. All drag forces and non-drag forces (turbulent dispersion, virtual mass, and lift coefficient) are included in the model. The experimental values of overall gas hold-up and average liquid circulation velocity have been compared with simulation and literature results. It is seemed to give good agreements. For the different elevations in the downcomer section, liquid axial velocity, turbulent kinetic energy, and turbulent eddy dissipation experimental have been compared with different turbulent models. The K-epsilon Realizable model gives better prediction with experimental results.

Numerical investigation of flow boiling of refrigerant-based nanofluids and proposing correlations for heat transfer

The numerical simulation of subcooled flow boiling of R-113 working fluid has been done for two different nanofluids (R-113/Al2O3, R-113/TiO2) under different volume concentrations (0.5%, 1%, and 3%). The numerical results were compared with experimental results obtained by previous researchers, and the comparison shows that the numerical results are in good accordance. Nucleation site density, bubble departure frequency, and bubble departure diameter, which are three key parameters, are investigated in this study. The results express that these three parameters have the highest variation at low Reynolds numbers. The influence of different nanoparticles concentrations on the variation of the heat transfer coefficient is studied. The results indicate there is an insignificant difference between the effect of 1% and 3% concentrations on the heat transfer coefficient that means an increase of nanoparticles more than 1% concentration cannot improve heat transfer. The effect of different non-drag forces such as lubrication force, turbulent dispersion force, and lift force is also studied. Two correlations are proposed for predicting the convective heat transfer coefficient.

Bubble behavior in the slab continuous casting mold: Physical and mathematical model

A two phase population balance is used to predict the polydispersed bubble flow and size distribution in a slab continuous casting mold and Submerged Entry Nozzle (SEN) system. Multiple Size Group (MUSIG) with a suitable breakage and coalescence model (Ansys CFX) was adopted to account for the polydispersed gas flow. Initial bubble size distribution as determined for two industrial refractories have been taken in consideration. A two way coupling model including the effect of the drag force and non-drag forces such as virtual mass force and turbulent dispersion force was considered. The results are compared with gas distribution in a 1:1 scale water — air mold model running under conditions of fluidynamic similarity to validate the model. The simulations have then been extended to describe the actual steel — argon flow, considering a thermal expansion factor for argon bubbles. The effect of gas distribution on the flow field of liquid inside the mold and other metallurgical aspects are discussed.

The profiles of the local void fraction close to the heated wall in the subcooled flow boiling

Flow boiling in the subcooled liquid condition is characterized by the complex interplay of phenomena such as bubble detachment from the wall, condensation in the flow, and the interaction between bubbles on which non-drag forces are exerted. However, the fundamental characteristics of the flow boiling in the subcooled liquid condition remain uncharted. In particular, a distribution of the bubbles close to the heated wall is not well understood. Here, we measure the void fraction in the steam-water mixture which is representing the distribution of bubbles by using a single sensor optical fiber probe with fine radial spacing in the close vicinity of the heated wall. We show that the profile of the void fraction along the radial direction of the flow channel is altered by the flow condition. In particular, the peak of the void fraction can be shifted towards the heated wall when there are fast bubbles close to the heated wall.

Hydrodynamics in unbaffled liquid-solid stirred tanks with free surface studied by DEM-VOF method

DEM-VOF simulations of particle suspension dynamics in unbaffled liquid-solid stirred tanks with free surface were performed in conjunction with RSM turbulence model. The two-way coupling of particle and fluid was achieved by solving Gidaspow’s drag force, non-drag force and source term in the momentum equation. The calculation of particle volume in the fluid was realized by the porous model. Particle-particle and particle-wall interactions were also considered. A four-blade 45° pitched blade turbine was used, and the sliding-grid has been adopted to simulate impeller rotating. DEM-VOF coupling method was validated by theories and experiments. The effects of tank geometries, impeller rotating speed, particle density, diameter and volume fraction on the free surface vortex, flow patterns and particle suspension dynamics were investigated by DEM-VOF method for the first time. Results indicate that the particle-fluid interaction has important effects on the flow field and particle distribution. The modifications of tank geometries substantially influence flow patterns and particle suspension behaviors. The profiled bottom could eliminate sharp flow diversions completely. It can give rise to off-bottom suspension of particles and an elimination of the surface vortex due to particle-fluid interaction for large solid holdup and large particle diameter. Furthermore, the base of the profiled bottom tank was optimized. The reported research data has important guiding significance for the design of stirred tanks.

Comparison of Eulerian QBMM and classical Eulerian–Eulerian method for the simulation of polydisperse bubbly flows

AbstractThe spatial gas distribution of poly‐disperse bubbly flows depends greatly on the bubble size. To reflect the resulting polycelerity, more than two momentum balance equations (typically for the gas and liquid phases) have to be considered, as done in the multifluid approach. The inhomogeneous multiple‐size group model follows this approach, also combined with a population balance model. As an alternative, in a previous work, an Eulerian quadrature‐based moments method (E‐QBMM) was implemented in OpenFOAM; however, only the drag force was included. In this work, different nondrag forces (lift, wall lubrication, and turbulent dispersion) are added to enable more complex test cases to be simulated. Simulation results obtained using E‐QBMM are compared with the classical E–E method and validated against experimental data for different test cases. The results show that there is good agreement between E‐QBMM and E–E methods for mono‐disperse cases, but E‐QBMM can better simulate the separation and segregation of small and large bubbles.

CFD Analysis for Two Phase Performance in 5×5 Rod Bundle with 7 Spacer Girds

A 5×5 rod bundles with 7 spacer girds which structure mixing gird (MG) and mid span mixing gird (MSMG) are arranged alternant are studied by two phase computational fluid mechanic(CFD), and bubbles coalescence and breakup, and heat transfer are considered in the calculation, but not the interface mass change. In order to choose the reasonable two phase model parameters, 5×5 rod bundles with 2 spacer girds (MG and MSMG) based on AFA3G fuel assembly isstudied by two phase CFD, and the sensitivity and uncertainty analysis is conducted on max bubble size, bubbles coalescence and breakup model parameters, non-drag force model, drag force model parameters, inlet bubble diameter and inlet void fraction distribution. Use this model setting, the study of the two phase performance is conducted on AFA3G fuel assembly with 7 spacer grids. The calculation shows that the bubble through the spacer grid is without periodicity, but the void fraction fields of every MSMG upstream are similar and the MG upstream shows the same situation. This research is useful to choose the model setting and geometry size for the rod bundle with spacer girds in the two phase calculation, for geometry size it is possible to analyze the two phase performance on rod bundle with 2 or 3 spacer grids. Finally, the void fraction distribution of AFA3G fuel assembly and advanced fuel assembly are compared, and evaluated in terms of the improvement of the CHF. It agrees well with the experiment, which verifies the evaluation method, the research is the base for the establishment of the evaluation guideline for the thermal hydraulic performance of the fuel assembly by two phase CFD calculation.

Validation of a closure model framework for turbulent bubbly two-phase flow in different flow situations

Abstract In the present paper a set of closure relations for interphase momentum exchange and for bubble-induced turbulence within the Euler-Euler framework is presented and validated against a set of tests performed at the HZDR-facility MT-Loop. The facility was equipped with wire-mesh sensors that allow cross sectional distributions of gas fraction, gas velocity, and bubble sizes to be measured at different distances from the gas injection. The here applied models were derived for the simulation of fully developed turbulent vertical upward bubbly flow in a pipe. The radial gas fraction profile for this kind of flow is the result of the ratio of the radial force components of the so-called non-drag forces and can be used for model validation. However, only the ratio, not the absolute value of the bubble forces can be tested in this way. In the present paper more detailed information from the experiment is exploited by consideration of the evolution of gas fraction distribution particularly after the gas injection region. The change of cross sectional gas volume fraction distribution is the result of the action of non-drag forces. In addition to vertical pipe flow tests further insight is obtained from the investigation of the effect of a slight tube inclination which shifts the gas distribution. Here the disturbance of the cylindrical symmetry of a vertical pipe gives hints on the absolute value of the non-drag force components. The results show that the presented model framework at least is able to describe the phenomena qualitatively. Possible reasons for quantitative deviations are discussed and require further investigations.

Assessment of the CUPID Code for Bubbly Flows in Horizontal Pipes

Two-phase flow in a horizontal pipe has a pronounced feature; that is, two-phase-flow parameters are highly nonsymmetric because gravity is perpendicular to the mean flow direction. Thus, three-dimensional analysis is necessary for the accurate prediction of two-phase flow in a horizontal pipe, such as the hot leg and cold leg of a pressurized water reactor and the pressure tubes in a CANDU reactor. In this study, we simulated bubbly flows in horizontal pipes using the CUPID code, which adopts a two-fluid, three-field model for two-phase flow. In the preliminary calculations, it was found that the particle-averaged two-fluid momentum equation, rather than the standard two-fluid momentum equation, predicts a physically reasonable slip ratio and nondrag forces, except turbulent dispersion forces have negligible effects on the radial void distribution when the particle-averaged two-fluid momentum equation is used. Based on the results, we selected the physical models and computational mesh for subsequent code assessment using various bubbly flow experiments in horizontal pipes. The turbulent dispersion force model was improved to take into account the large void fraction change at the top. The results of the code assessment show good predictions for the axial pressure drop, liquid velocity, and turbulent kinetic energy profile and predict reasonably well the effects of jl and jg on two-phase-flow parameters. However, additional studies are needed for more accurate prediction of the nonsymmetric distribution of gas velocity and turbulent kinetic energy.

A new measuring concept to determine the lift force for distorted bubbles in low Morton number system: Results for air/water

The lift force, which strongly influences the spatial bubble distribution, is one of the most important non-drag forces. However, measurements in systems with a low Morton number are limited. In the present work, a time-averaging measurement method with which this gap can be closed is discussed. The experimental setup is kept as simple as possible, avoiding any moving parts. The single bubble movement through a linear shear field was observed in three-dimensions over 75 min. In total, 85 measurement points cover 13 bubble sizes at 7 different shear rates. The results reveal that former empirical correlations obtained from experiments and simulations in predominantly high Morton number systems are applicable. In this context, the characteristic length scale that is used to describe the lift force needs to be carefully defined. From the present results, the major axis seems to be the most reasonable choice for wobbling bubbles. However, the major axis might be dependent on the flow properties, which leads to a flow dependent lift force formulation.

Study of the Gas Distribution in a Multiphase Rotodynamic Pump Based on Interphase Force Analysis

The performance of multiphase pumps has a remarkable influence on the related industrial application. In order to understand the flow field and gas-liquid phase interaction characteristics of a multiphase rotodynamic pump, detailed numerical analysis of the pump with a medium of air-water combination was carried out for the whole flow passage by means of a structured mesh using ICEM_CFD and TurboGrid. The results for 21% inlet gas void fraction (IGVF = 21%) condition showed that the magnitude ratio of non-drag forces to drag in impeller and guide vane passages was generally less than 1, whereas it was always less than 0.2 for the magnitude ratio of turbulent dispersion force to drag. When the IGVF was increased, the variation range of interphase forces in the impeller was greater than that in the guide vane. In addition, the gas in the impeller mainly accumulated near the suction surface in the outlet region. Further, with increased IGVF, the degree of aggregation increased as well as the gas inhomogeneity, and consequently the interphase forces in the impeller increased. Due to the divergent structure of the guide vane, obvious vortexes emerged at the hub and gradually moved toward the blade pressure surface along the streamwise direction.

하향 가열면 경사채널의 유동 비등 연구

The primary object of this study is to evaluate the sub-models of wall boiling model and inter-phase momentum transfer models to flow boiling with downward heated wall in an inclined channel. Based on eulerian two-fluid method, kurul and podowski’s heat partitioning method is applied to numerical simulation with star-ccm+ v.11.06.010-R8 commercial code. The inlet mass flux is 200 ㎏/㎡·s and heat flux is 100 kw/㎡. Based on the experimental wall superheat result, nucleate site number density, bubble departure diameter, interaction length scale(bubble size), area coefficient, drag force and non-drag force are compared with each other. After then, summarized all models are applied to code and the numerical results show a good agreement with experimental results.

Computational fluid dynamics (CFD) analysis of airlift bioreactor: effect of draft tube configurations on hydrodynamics, cell suspension, and shear rate.

The biomass productivity of microalgae cells mainly depends on the hydrodynamics of airlift bioreactor (ABR). Thus, the hydrodynamics of concentric tube ABR was initially studied using two-phase three-dimensional CFD simulations with the Eulerian-Lagrangian approach. The performance of ABR (17 L) was examined for different configurations of the draft tube using various drag models such as Grace, Ishii-Zuber, and Schiller-Naumann. The gas holdups in the riser and the downcomer were well predicted using E-L approach. This work was further extended to study the dispersion of microalgae cells in the ABR using three-phase CFD simulations. In this model (combined E-E and E-L), the solid phase (microalgae cells) was dispersed into the continuous liquid phase (water), while the gas phase (air bubbles) was modeled as a particle transport fluid. The effect of non-drag forces such as virtual mass and lift forces was also considered. Flow regimes were explained on the basis of the relative gas holdup distribution in the riser and the downcomer. The microalgae cells were found in suspension for the superficial gas velocities of 0.02-0.04ms-1 experiencing an average shear of 23.52-44.56s-1 which is far below the critical limit of cell damage.

Experimentally Validated CFD Model for Gas-Liquid Flow in a Round-Bottom Stirred Tank Equipped with Rushton Turbine

AbstractHydrodynamics of gas-liquid flow in a round-bottom stirred tank is modelled at two gas flow rates, constant bubble size and agitator speed of 300 rpm. A round-bottom tank equipped with four baffles and a Rushton turbine was chosen to represent a typical reactor used in hydrometallurgical processes operating under pressure. The applicability of different momentum interchange models and the Realizable k-ε, SST k-ω, and RSM turbulence models was studied using CFD software. The results were compared and validated against experimental data from Particle Image Velocimetry measurements by means of liquid and gas velocity distributions. In addition, energy balance between power input and dissipation energy was compared for the different turbulence models. The CFD model was found to be in good agreement with the measurements. Of the turbulence models studied, the Realizable k-ε model showed best agreement with the measured velocity profiles. Popular drag force models proposed in the literature were assessed, as was the influence of inclusion of non-drag forces. Gas flow was found to affect the liquid phase flow in the tank by generating an additional secondary circulation loop in the upper part of the reactor.