CFD Simulation Of Flow Research Articles

Flow Modeling in Symmetrically Narrowing Flood Plains

Flooding situation in river is a complex phenomenon and affects the livelihood and economic condition of the region. The modelling of such flow is primary importance for a river engineers and scientists working in this field. As a result of topography changes along the open channels, designing the converging compound channel is an essential. Fluvial flows are strongly influenced by geometry complexity and large overall uncertainty on every single measurable property, such as velocity distribution on different sectional parameters like width ratio, aspect ratio and hydraulic parameter such as relative depth. The geometry selected for this study is that of a non-prismatic compound channel having converging flood plain. For the research work the parameters, the water depth, incoming discharge of the main channel and floodplains were varied. Continuous variation of channel geometry along the flow path associated with secondary currents makes the depth averaged velocity computation difficult. In this paper the variations of depth average velocities for five different sections in between the convergence region of the converging compound channel are compared by taking two different flow depth conditions. And also shear stress distribution at the walls and on the bed of two stage converging compound channel is calculated by doing experimentation in laboratory as bed shear forces are useful for the study of bed load transfer whereas wall shear forces presents a general view of channel migration pattern. Then the numerical method is applied to analyse the flow conditions in non-prismatic compound channel configurations, the results of calculations show good agreement with the experimental data. As numerical hydraulic models can significantly reduce costs associated with the experimental models, an effort has been made through the present investigation to determine the different flow characteristics of a converging compound channel such as velocity distribution, depth averaged velocity distribution, boundary shear etc. In this paper a complete three dimensional and two-phase CFD model for flow distribution in a converging compound channel is investigated by using Large Eddy Simulation (LES) model for solving the turbulence equation. This study aims to validate CFD simulations of free surface flow or open channel flow by using Volume of Fluid (VOF) method by comparing the data observed in hydraulics laboratory of the National Institute of Technology, Rourkela.

A CFD Simulation of 3D Air Flow and Temperature Variation in Refrigeration Cabinet

Presented is a 3D simulation of the air flow and temperature variation in an ice drink refrigeration cabinet, during an automated dynamic cycle of switching on and switching off. To correctly model the effect of perforated plate separating evaporator and storage compartments, a benchmark problem is designed, and used as a reference to determine the parameters in the porous-jump model by parameter optimization. Subsequently, the dynamic cycle of initial switch on, automated switch off and automated switch on is simulated. Both the flow field distribution and temperature profile are analyzed and the time required for these three stages is obtained. Full 3D CFD simulation of refrigeration system provides not only detailed temperature distribution in the cabinet but also operational parameters of the system such as for switch on-off cycle. Those output from simulation are important for cabinet design to improve the storage quality of products while reducing the energy use.

Characterization and modeling of multiphase flow in structured microreactors: a post microreactor case study.

We study microreactors with internal fields of posts as typical examples of structured microreactors to elucidate flow fields and their implications for mass transfer. Laser-induced fluorescence (LIF) visualization combined with image analysis is used to systematically quantify key features such as interfacial area, phase holdup and the characteristics of the post-wetting layer. The subsequent mass transport analysis yields insight into how the posts contribute to the overall enhanced mass transfer performance compared to open channels, and provides predictions of mass transfer performance under varying operating conditions. Computational fluid dynamic (CFD) simulations of multiphase flow using the volume-of-fluid (VOF) method are in good agreement with experimentally observed multiphase flows.

Scalability of the parallel CFD simulations of flow past a fluttering airfoil in OpenFOAM

The paper is devoted to investigation of unsteady subsonic airflow past an elastically supported airfoil during onset of the flutter instability. Based on the geometry, boundary conditions and airfoil motion data identified from wind-tunnel measurements, a 3D CFD model has been set up in OpenFOAM. The model is based on incompressible Navier-Stokes equations. The turbulence is modelled by the Menter’s k-omega shear stress transport turbulence model. The computational mesh was generated in GridPro, a mesh generator capable of producing highly orthogonal structured C-type meshes. The mesh totals 3.1 million elements. Parallel scalability was measured on a small shared-memory SGI Altix UV 100 supercomputer.

Flow Optimization of Gas Quenching Processes Using Perforated Plates

Abstract Industrial gas quenching is a cost-effective, environmental-friendly alternative to conventional fluid-based quenching processes used for the heat treatment of workpieces mainly in the automotive industry. The influence of the heat exchanger geometry in a novel gas quenching chamber on the gas flow was investigated through measurement in parallel to CFD-simulations. With reduced quenching chamber dimensions, a flow guiding system utilizing a perforated plate placed between the heat exchanger and the batch has been found to play a significant role on the velocity and turbulence distribution of the gas flow. The use of various perforated plates in this gas quenching process focuses on the exploration of two main parameters: plate porosity and perforation hole diameters. By combining in situ flow and CFD-simulations of heat and fluid flow, optimized parameters have been determined indicating improved uniformity and intensity of the velocity distribution in the quenching chamber, thus obtaining the potential of an overall improvement of the gas quenching process. Geometric parameters of perforated plates introduced into the gas quenching chamber facility are investigated as, e.g., the distance between the plate and the batch. More generally, the use of a perforated plate system to optimize gas flows may be extended to other applications where tube bundle heat exchangers in gas flow processes are involved.

CFD simulation of flow and mixing in‐inline rotor‐stator mixers with complex fluids

The objective is to apply CFD methodology for the validation of the scale‐up of rotor‐stator units formed by flat blades and used as in‐line mixers under laminar flow conditions. The comparison between simulated and experimental data as a function of rotor geometry, rotation speed and flow rate has shown a good agreement in terms of power input and RTD curves both for Newtonian and power‐law fluids with a flow index between 0.2 and 1. The applicability of the virtual Couette analogy has been validated quantitatively and explained by the analysis of the local flow in the mixer. As a result, a shear coefficient independent of fluid rheology has been deduced from CFD data. This has been shown to depend only on the dimensionless gap when the length‐to‐diameter ratio is higher than 2.5. In this case, fast 2D simulations can provide a good approximation of the shear coefficient obtained from 3D calculations from which a master power curve can be deduced, but these are not able to predict flow transition, contrary to 3D computations. Finally, original correlations able to estimate the power and shear coefficients as a function of the mixer geometry have been established.

Investigation of the corrosion behaviour of 13CrMo4–5 for biomass fired boilers with coupled online corrosion and deposit probe measurements

High-temperature corrosion in biomass fired boilers is still an insufficiently explored phenomenon which causes unscheduled plant shutdowns and hence, economical problems. To investigate the high-temperature corrosion and deposit formation behaviour of superheater tube bundles, online corrosion probe as well as deposit probe measurements have been carried out in a specially designed fixed bed/drop tube reactor in order to simulate a superheater boiler tube under well-controlled conditions. The investigated boiler steel 13CrMo4–5 is commonly used as steel for superheater tube bundles in biomass fired boilers. Forest wood chips and quality sorted waste wood (A1–A2 according to German standards) as relevant fuels have been selected to investigate the influence on the deposit formation and corrosion behaviour. The following influencing parameter variations have been performed during the test campaigns: flue gas temperature between 650 and 880°C, steel temperature between 450 and 550°C and flue gas velocity between 2 and 8 m/s. One focus of the work presented is the detailed investigation of the structure and the chemical composition of the deposits formed as well as of the corrosion products. A further goal of the work presented was the development of an empirical model which can be used within CFD simulations of flow and heat transfer to calculate and evaluate the local corrosion potential of biomass fired plants already at the planning stage. The corrosion probe measurements show a clear dependency on the parameters investigated and the empirical function developed reproduces the measured corrosion behaviour sufficiently accurate. Since the additional calculation time within the CFD simulation is negligible the model represents a helpful tool for plant designers to estimate whether high-temperature corrosion is of relevance for a certain plant or not, when using fuels with similar compositions and the steel 13CrMo4–5.

Assessment of a methodology to include differential diffusion in numerical simulations of a turbulent flame

A newly developed methodology to incorporate differential diffusion effects in CFD simulations of turbulent reactive flows is applied to the ‘H3’ benchmark flame of the Turbulent Non-premixed Flames (TNF) workshop series. The fuel consists of species with vastly different mass diffusivities. The accuracy of the proposed methodology is assessed and the influence of inclusion of differential diffusion is examined. Good agreement is obtained between numerical simulation results and experimental data if differential diffusion effects are considered. Differential diffusion effects are shown to be significant in the region near the inlet and have a great influence on the stabilization mechanism of the flame, as well as on the predicted profiles of temperature and species concentration. If differential diffusion is modeled, temperatures above the adiabatic flame temperature are recovered. Ignoring differential diffusion, this is not the case and large discrepancies between numerical simulations and experiments are observed.

Experimental and numerical study of single-phase pressure drop in downhole shut-in valve

In this work an experimental and numerical analysis of single-phase pressure drop in a downhole shut-in valve is performed. The main goal was to develop a 1D numerical model suitable for both compressible and incompressible flow. For the experimental study a mock-up of the shut-in valve was built and instrumented with pressure sensors and flowmeters. The pressure drop along the different sections of the valve were recorded for various flow rates using water, oil and air as working fluids. For the numerical analysis a two-step approach was used. First a commercial CFD package was used for 3D simulations of the flow, and different turbulence models were compared. Then a 1D model was developed based on a spectral element method, with minor loss factors derived from the CFD simulations. Both the 3D CFD simulations and the 1D model simulations provided a good comparison with the experimental data. The small difference in the simulation results can be attributed to the difference in the frictional coefficient which showed a discrepancy of about 20% compared with the measurements. Minor loss factors derived from CFD simulations of incompressible flow are found to be valid also for 1D simulation of compressible flow of air. The 1D model is developed for future simulation of compressible multiphase flow.

CFD Simulation of Air Flow through the Annular Distributor of a Swirling Fluidized Bed

Fluidized beds are widely used in a variety of industrial processes. The air distributor being an integral part of the fluidized bed, the air flow characteristic through it is of great importance as far as the design and working of it is concerned. Inappropriate design will lead to an incomplete fluidization and improper mixing of the air and bed particles. The current study was carried out to investigate the flow characteristics and predict non-uniformity in flow through the annular distributor of a Swirling Fluidized Bed and its variation with various aspects of the distributor like blade overlap angle and blade inclination. In this study, the commercial CFD package FLUENT 6.3 was used for analysis. The velocity and pressure profiles for various blade designs were investigated at the distributor outlet based on several operating variables including air inlet velocities, blade overlap angles (9o, 12o, 15o, and 18o), blade inclinations (10oand 15o), along with variations in the opening between the distributor blades. The most significant finding of this work is that the fluid tends to flow through a path with least resistance. The flow path with the largest cross section area and shortest path length has been identified and explains the velocity profile at blade exit.

Multiphase CFD-simulation of bubbly pipe flow: A code comparison

CFD simulations of dispersed bubbly flow on the scale of technical equipment become feasible within the Eulerian two-fluid framework of interpenetrating continua. For practical applications suitable closure relations are required which describe the interfacial exchange processes. Implementations of such closures have been provided in major commercial codes for years, but more recently there is a growing interest also in open source packages among which in particular OpenFOAM has become widely known.In the present work a set of closure relations suitable for adiabatic bubbly flow has been implemented in OpenFOAM. Selection of closure models has been based on previous experience with ANSYS-CFX. Great effort has been made to match all details of the models so that only residual differences due to different numerical procedures would be expected in the results. Unfortunately this was not the case and the value of one empirical model parameter had to be changed in order to obtain similar results. Under this provision the new open source implementation is validated and shown to be comparable to commercial codes.

Prediction of hydrodynamic entrance length for single and two-phase flow in helical coils

The hydrodynamic entrance length in helical coils for single and two-phase bubbly flow was studied experimentally and numerically. Development region length and the detailed characteristics of fluid flow have been investigated by varying helical coil parameters such as tube diameter, coil diameter and void fraction. For the CFD simulation of the two-phase fluid flow, the Eulerian–Eulerian model was employed. To calculate the turbulent fluctuations, the SSTk−ω turbulence model has been used. The experimental and numerical simulation of the local parameters demonstrates that the hydrodynamic developing length (L/D) increases when Reynolds number increases in the single and two-phase flows. The obtained results show that the development entrance length increases with the increase of pipe diameter and decreases with the increase of coil diameter. Also, entrance length decreases while curvature ratio of helical coil and void fraction increase. A correlational equation has been suggested to predict the hydrodynamic entrance length as functions of various parameters of the helical coil.

CFD simulation of turbulent flow in a rod bundle with spacer grids (MATIS-H) using STAR-CCM+

This paper presents the CFD simulation of the turbulent flow generated by a model PWR spacer grid within a rod bundle. The investigation was part of the MATIS-H benchmark exercise, organized by the OECD-NEA, with measurements performed at the KAERI facilities in Daejeon, Korea. The study employed the CD-Adapco code Star-CCM+. An initial sensitivity study was conducted to attempt to assess the importance to the overall flow of components such as the outlet plenum and the end support grid; these were shown to be able to be safely neglected, but the tapered end portion of the rods was found to be significant, and this was incorporated in the model analyzed. A RANS model using any of K-epsilon, K-omega and Reynolds-stress turbulence models was found to be adequate for the prediction of mean velocity profiles, but they all three underestimate the time-averaged turbulent velocity components. Vorticity seems to be better predicted, although the measured values of vorticity are only presented via colored contour plots, making quantitative comparison rather difficult. Circulation, calculated via an integral for each channel, seems to be well predicted by all three models.

CFD simulation of bubble column flows: An explicit algebraic Reynolds stress model approach

CFD study of flow hydrodynamics has been conducted in transient Euler–Euler environment by using commercial simulation software Ansys CFX 14.0. First, three different wall lubrication force models are compared. Afterwards, the influence of turbulent dispersion force models has been analyzed. Finally, the performance of Explicit Algebraic Reynolds Stress Model (EARSM) combined with k-ε and Baseline (BSL) models has been tested. Three different combinations of EARSM were included with two cases incorporating the Bubble Induced Turbulence (BIT) as well (i.e. EARSM k-ε BIT, EARSM BSL, EARSM BSL BIT). All simulations were compared for average velocity and turbulent kinetic energy profiles with experiments. The performance of these models was also compared with Re-Normalization Group (RNG) and k-ε models. EARSM k-ε model was unable to capture the axial liquid as well as gas phase velocities close to experimental values. On contrary, EARSM BSL and EARSM BSL BIT were more successful in predicting the velocity profiles and showed good agreement with experiments, with EARSM BSL slightly less accurate in predicting axial velocity profiles. All models predicted the turbulent kinetic energy profiles reasonably good both quantitatively as well as qualitatively, while near the bottom of the column; EARSM k-ε slightly under predicted the turbulent quantities. Thus, EARSM combined with BSL and BIT can be effectively exploited for simulating flow fields and turbulent quantities.

Development and validation of a radial variable geometry turbine model for transient pulsating flow applications

This paper presents the development and validation of a one-dimensional radial turbine model able to be used in automotive turbocharger simulations. The model has been validated using results from a numerical 3D CFD simulation of stationary and pulsating flow in a variable geometry radial turbine. As the CFD analysis showed, the main non-quasi-steady behavior of the turbine is due to the volute geometry, so special care was taken in order to properly model it while maintaining low computational costs. The flow in the volute has been decomposed in its radial and azimuthal direction. The azimuthal flow corresponds to the flow moving along the volute, while the radial flow is computed by coupling its flow with a stator model. Although the stator caused fewer accumulation effects than the volute, a small accumulation model has been used for it, which also allows to compute the evolution of the flow inside the turbine with lower costs. The flow in the moving rotor can be considered quasi-steady, so a zero-dimensional model for the rotor has been developed. Several losses models where implemented for both the stator and the rotor. The results show good agreement with the CFD computations.

Artificial Neural Networks for Modeling of Chemical Source Terms in CFD Simulations of Turbulent Reactive Flows

The main goal of the work presented here was to develop, implement and test a highly efficient numerical algorithm for the evaluation of the chemical reaction source terms that appear in the Navier - Stokes equations when a turbulent, premixed, reactive flow is simulated using a finite rate chemistry combustion model. The approach was based on employing Artificial Neural Networks (ANN) that were designed, trained and incorporated into an existing LEM – LES numerical algorithm. Two numerical simulations of reacting flows have been carried out using several techniques for the estimation of the LES filtered reaction rate for the chemical species in laminar and turbulent, premixed, reactive flows, and the results were compared in terms of numerical accuracy and computational speed. It was concluded that the ANN approach provides a significant speedup of the numerical simulation while preserving acceptable accuracy.

Accuracy quantification metrics for CFD simulation of in-vessel flows

Within more general frameworks dealing with uncertainty evaluation and code assessment, well assessed methodologies exist for the accuracy evaluation of system thermal hydraulic analysis results. As long as multi-dimensional transient data is involved, such as space and time distributions obtained from CFD calculations and experiments, the task of quantifying the accuracy and defining acceptance criteria becomes harder, and there seems to be a lack of systematic approaches available to that purpose.An example of such multi-dimensional data is represented by the time and space distribution of coolant properties (e.g. temperature, boron concentration) at the reactor core inlet, which is relevant to reactor safety as local perturbations potentially induce power excursions. An attempt is made here to tackle the accuracy quantification issue for that class of data, in a somewhat empirical way, i.e. by proposing a set of parameters that can be used, after proper qualitative analysis of measured data and code simulation results, to characterize the target variable time and space distributions, and to quantify the deviation of the simulation from the experimental data. The advantage of using such kind of metrics based on many parameters relies in the ability to cover different relevant features of the target variables and thus provide a more complete assessment and allow statements about the overall agreement.

STAR-CCM+를 이용한 썬루프 버페팅 유동 소음 해석

썬루프 버페팅 소음의 고유특성을 의미하는 속도에 따른 소음강도의 증-감쇠 현상을 살펴보고자, HSM(Hyundai simplified model) 형상에 대해서 유동소음 해석 프로그램인 STAR-CCM+을 통하여 전체 차속 범위에 걸쳐 시험과 비교 검토하였다. 차량 내부의 재질에 따른 압축성 효과 및 감쇠효과는 인공 압축성과 감쇠 보정 기법인 FRET(frequency response test)를 이용하였다. 시뮬레이션 결과는 특정 속도에서 나타나는 소음 강도의 증-감쇠 시험결과를 매우 잘 예측하였으며 최대 SPL 수치도 정확히 예측하였다. 이는 썬루프 개방에 의해 발생하는 전단면에서의 유동 박리 주파수를 유동 소음 해석인 STAR-CCM+가 전 차속에 걸쳐서 매우 잘 예측하고 있음을 나타낸다. CFD flow simulation of vehicles with open sunroof and passenger window help the automotive OEM(original equipment manufacturer) to identify the low frequency noise levels in the cabin. The lock-in and lock-off phenomena observed in the experimental studies of sunroof buffeting is well predicted by CFD speed sweep calculations over the operating speed range of the vehicle. The trend of the shear layer oscillation frequency with vehicle speed is also well predicted. The peak SPL from the CFD calculation has a good compromise with the experimental value after incorporating the real world effects into the CFD model by means of artificial compressibility and damping correction. The entire process right from modeling to flow analysis as well as acoustic analysis has been performed within the single environment i.e., STAR-CCM+.

Hydrodynamic numerical simulation of diffuser for horizontal axis marine current turbine based on CFD

In order to comprehensively study the hydrodynamic characteristics of diffuser for marine current turbine with a postpositive bulb, a geometric model of the turbine was established. Three-dimensional CFD simulation of turbulent flow was performed based on the incompressible continuity equation, the Navier-Stokes equations and the Spalart-Allmaras turbulence model. The influence of diffuser was calculated and analyzed by numerical results, which were also compared with model test results. Results showed that the numerical results agree fairly well with model test and the maximum error of impeller efficiency and power is 1.5% and 1.8% in the rated water velocity condition, which are less than 5.8% and 5.5% under other cases respectively. The new type marine current turbine with a bulb for erecting motor which is different from regular, and the diffuser can aggregate water flow, raise inlet water velocity more than 3% and efficiency of impeller effectively increased. After diffuser was added, the power coefficient curve rose over the full range, so the high power area became widely, and then remarkably prolonged power time as well as increased generated energy, it is also significant for efficient utilization of marine current energy and environmental pollution remission.

Comprehensive experimental and numerical analysis of instability phenomena in pump turbines

The changes in the electricity market have led to changed requirements for the operation of pump turbines. Utilities need to change fast and frequently between pumping and generating modes and increasingly want to operate at off-design conditions for extended periods. Operation of the units in instable areas of the machine characteristic is not acceptable and may lead to self-excited vibration of the hydraulic system. In turbine operation of pump turbines unstable behaviour can occur at low load off-design operation close to runaway conditions (S-shape of the turbine characteristic). This type of instability may impede the synchronization of the machine in turbine mode and thus increase start-up and switch over times. A pronounced S-shaped instability can also lead to significant drop of discharge in the event of load rejection. Low pressure on the suction side and in the tail-race tunnel could cause dangerous separation of the water column. Understanding the flow features that lead to the instable behaviour of pump turbines is a prerequisite to the design of machines that can fulfil the growing requirements relating to operational flexibility. Flow simulation in these instability zones is demanding due to the complex and highly unsteady flow patterns. Only unsteady simulation methods are able to reproduce the governing physical effects in these operating regions. ANDRITZ HYDRO has been investigating the stability behaviour of pump turbines in turbine operation in cooperation with several universities using simulation and measurements. In order to validate the results of flow simulation of unstable operating points, the Graz University of Technology (Austria) performed detailed experimental investigations. Within the scope of a long term research project, the operating characteristics of several pump turbine runners have been measured and flow patterns in the pump turbine at speed no load and runaway have been examined by 2D Laser particle image velocimetry (PIV). For several wicket gate positions, the flow fields in the vane-less space at runner inlet observed in the experiment are compared with the results of unsteady CFD flow simulations. Physical phenomena are visualized and insight to flow phenomena is given. Analyses using both results of simulation and measurement allow deriving a consistent explanation of the fluid mechanical mechanisms leading to the S-shaped instability of pump turbines.