Comparison Of Computational Fluid Dynamics Research Articles

CFD Analysis of Flow and Heat Transfer in a Direct Transfer Preswirl System

The accuracy of computational fluid dynamics (CFD) for the prediction of flow and heat transfer in a direct transfer preswirl system is assessed through a comparison of CFD results with experimental measurements. Axisymmetric and three-dimensional (3D) sector CFD models are considered. In the 3D sector models, the preswirl nozzles or receiver holes are represented as axisymmetric slots so that steady state solutions can be assumed. A number of commonly used turbulence models are tested in three different CFD codes, which were able to capture all of the significant features of the experiments. A reasonable quantitative agreement with experimental data for static pressure, total pressure, and disk heat transfer is found for the different models, but all models gave results that differ from the experimental data in some respect. The more detailed 3D geometry did not significantly improve the comparison with experiment, which suggests deficiencies in the turbulence modeling, particularly in the complex mixing region near the preswirl nozzle jets. The predicted heat transfer near the receiver holes was also shown to be sensitive to near-wall turbulence modeling. Overall, the results are encouraging for the careful use of CFD in preswirl-system design.

Loss of vacuum accident (LOVA): Comparison of computational fluid dynamics (CFD) flow velocities against experimental data for the model validation

A recognized safety issue for future fusion reactors fueled with deuterium and tritium is the generation of sizeable quantities of dust. Several mechanisms resulting from material response to plasma bombardment in normal and off-normal conditions are responsible for generating dust of micron and sub-micron length scales inside the VV (Vacuum Vessel) of experimental fusion facilities. The loss of coolant accidents (LOCA), loss of coolant flow accidents (LOFA) and loss of vacuum accidents (LOVA) are types of accidents, expected in experimental fusion reactors like ITER, that may jeopardize components and plasma vessel integrity and cause dust mobilization risky for workers and public. The air velocity is the driven parameter for dust resuspension and its characterization, in the very first phase of the accidents, is critical for the dust release. To study the air velocity trend a small facility, Small Tank for Aerosol Removal and Dust (STARDUST), was set up at the University of Rome “Tor Vergata”, in collaboration with ENEA Frascati laboratories. It simulates a low pressurization rate (300 Pa/s) LOVA event in ITER due to a small air inlet from two different positions of the leak: at the equatorial port level and at the divertor port level. The velocity magnitude in STARDUST was investigated in order to map the velocity field by means of a punctual capacitive transducer placed inside STARDUST without obstacles. FLUENT was used to simulate the flow behavior for the same LOVA scenarios used during the experimental tests. The results of these simulations were compared against the experimental data for CFD code validation. For validation purposes, the CFD simulation data were extracted at the same locations as the experimental data were collected for the first four seconds, because at the beginning of the experiments the maximum velocity values (that could cause the almost complete dust mobilization) have been measured. In this paper the authors present and discuss the computer-simulation data and compare them with data collected in STARDUST.

Comparison of CFD and experimental performance results of a variable area ratio steam ejector

The advantages of numerical modelling compared with experimental studies (e.g. reduced cost, easy control of the variables, high yield etc.) are well known. Theoretical studies where experimental validation is also presented provide an important added value to numerical investigations. In the present paper, experimental and computational fluid dynamics (CFD) results for a 5-kW-rated capacity steam ejector, with a variable primary nozzle geometry, are presented and compared. The variable geometry was achieved by applying a movable spindle at the primary nozzle inlet. Relatively low operating temperatures and pressures were considered, so that the cooling system could be operated with thermal energy supplied by solar collectors (solar air-conditioning). The CFD model was based on the axi-symmetric representation of the experimental ejector, using water as a working fluid. The experimental entrainment ratio varied in the range of 0.1–0.5, depending on operating conditions and spindle tip position. It was found that the primary flow rate can be successfully adjusted by the spindle. CFD and experimental primary flow rates agreed well, with an average relative error of 8%. CFD predicted the secondary flow rate and entrainment ratio with good accuracy only in 70% of the cases.

An intercomparison of CFD models to predict lean and non-uniform hydrogen mixture explosions

The paper describes an exercise on comparison of Computational Fluid Dynamics (CFD) models to predict deflagrations of a lean uniform hydrogen–air mixture and a mixture with hydrogen concentration gradient. The exercise was conducted within the work-package “Standard Benchmark Exercise Problem” of the EC funded Network of Excellence “Safety of Hydrogen as an Energy Carrier”, which seeks to provide necessary quality in the area of applied hydrogen safety simulations. The experiments on hydrogen–air mixture deflagrations in a closed 1.5 m in diameter and 5.7 m high cylindrical vessel were chosen as a benchmark problem to validate CFD codes and combustion models used for prediction of hazards in safety engineering. Simulations of two particular experiments with approximately the same amount of hydrogen were conducted: deflagration of a uniform 12.8% vol. hydrogen mixture and deflagration of a non-uniform hydrogen mixture, corresponding to an average 12.6 % vol. hydrogen concentration (27% at the top of the vessel, 2.5% at the bottom of the vessel) with ignition at the top of the vessel in both cases. The comparison of the simulation results for pressure and flame dynamics against the experimental data is reported.

Aircraft Passenger Cabin ECS-Generated Ventilation Velocity and Mass Transport CFD Simulation: Mass Transport Validation Exercise

A collaborative research project established between the University of Tennessee (UT) computational fluid dynamics (CFD) lab and the Federal Aviation Administration (FAA) Civil Aerospace Medical Institute (CAMI) sought to validate CFD prediction of the environmental control system (ECS)-generated ventilation velocity vector field with induced mass transport. The results of the velocity field validation exercise were published previously. An FAA Cooperative Research and Development Agreement (CRADA) project generated an experimental database of time-dependent inserted mass distributions in the CAMI Aircraft Environmental Research Facility (AERF). The companion CFD mass transport simulations were conducted at the UT CFD Lab using both commercial and proprietary computer codes. This paper reports the qualified results of the mass transport validation exercise.

Aerodynamic and Heat Flux Measurements in a Single-Stage Fully Cooled Turbine—Part II: Experimental Results

This paper presents measurements and the companion computational fluid dynamics (CFD) predictions for a fully cooled, high-work single-stage HP turbine operating in a short-duration blowdown rig. Part I of this paper (Haldeman, C. W., Mathison, R. M., Dunn, M. G., Southworth, S. A., Harral, J. W., and Heltland, G., 2008, ASME J. Turbomach., 130(2), p. 021015) presented the experimental approach, and Part II focuses on the results of the measurements and demonstrates how these results compare to predictions made using the Numeca FINE/Turbo CFD package. The measurements are presented in both time-averaged and time-accurate formats. The results include the heat transfer at multiple spans on the vane, blade, and rotor shroud as well as flow path measurements of total temperature and total pressure. Surface pressure measurements are available on the vane at midspan, and on the blade at 50% and 90% spans as well as the rotor shroud. In addition, temperature and pressure measurements obtained inside the coolant cavities of both the vanes and blades are presented. Time-averaged values for the surface pressure on the vane and blade are compared to steady CFD predictions. Additional comparisons will be made between the heat transfer on cooled blades and uncooled blades with identical surface geometry. This, along with measurements of adiabatic wall temperature, will provide a basis for analyzing the effectiveness of the film cooling scheme at a number of locations.

Wall-to-particle heat transfer in steam reformer tubes: CFD comparison of catalyst particles

Computational fluid dynamics (CFD) was used to simulate non-reacting heat transfer in a steam reforming packed reactor tube of tube-to-particle diameter ratio ( N ) equal to 4, with cylindrical multi-hole catalyst particles. These simulations extend those of our previous study [Nijemeisland, M., Dixon, A.G., Stitt, E.H., 2004. Catalyst design by CFD for heat transfer and reaction in steam reforming. Chemical Engineering Science 59, 5185–5191] to provide accurate tube wall temperatures, runs at constant pressure drop in addition to those at constant mass flow rate and simulations of particles with different sizes of holes. At constant pressure drop, particles with higher void fractions allowed higher mass flow rates, resulting in tube wall temperatures and radial temperature profiles in order: solid cylinders > one-hole particles > multi-hole particles. Little difference was seen between three-hole and four-hole particles. The particles with multiple holes gave a substantial reduction in tube wall temperature, with only a small decrease in core tube heat transfer. The effect of hole size was small, for the cases investigated in this study.

Raking in gravity thickeners

Thickener rakes are essential in the transport of sediment bed material to the underflow in conventional thickeners, however very few studies of bed transport have been published. In this paper, results from pilot-scale thickener experiments with tailor-made yield stress slurries are presented and compared to companion Computational Fluid Dynamics (CFD) simulations. Rake torque is a key issue in thickener operation and it was found that the yield stress of the suspension is the major factor in determining rake torque. Over a range of rake speeds, the measured torque was an almost linear function of yield stress. CFD simulations of the experiments allowed torque to be estimated, and results are shown to be within 20% of the measured values in all cases except the lowest (zero) yield stress suspension. Residence time distributions of solids in the bed were also measured and unusual results were found in which the relationship between residence time and distance from the underflow is not linear (or even monotonic). CFD results clearly show that for uniform sized rake blades, the over-delivery of an outer blade (compared to the next inner blade) sets up recirculation in the bed, especially in the outer regions of the tank, and this can result in long material pathways and hence long residence times. This picture is further complicated by the relative contributions of rake delivery and underflow rate, and indicates that a simple picture of plug flow in the bed is far from reality. The study illustrates the value that can be obtained from validated CFD modelling of thickener rakes.

Compact waterjets for high-speed ships

The article describes the development and validation of a compact waterjet propulsion system for high-speed vessels. The development was conducted in four discrete phases over a period of 4 years: Phase 1 involved studies of pump-type options for compact units; Phase 2 used advanced Computational Fluid Dynamics (CFD) to design the preferred pump type for a high-speed ship design; Phase 3 involved the manufacture and performance/cavitation testing of a model of the pump designed in Phase 2; and Phase 4 involved the construction and testing, in a towing tank, of a self-propelled model to determine the critical interaction effects between the hull and the waterjet inlet. Following an introduction that serves to define the challenge, the article discusses what is good and not so good about marine waterjet propulsion. It then describes the design approach, the design tools used, the testing procedures, and the validation comparison of CFD predicted and model test results. In addition, it describes the whole-ship impact of using these advanced pumps in terms of the significant improvements possible in ship speed, range or payload. The overall program was considered to be an outstanding success with the potential of having major benefits to future high-speed ships.

Pneumatic Transport of Granular Materials in an Inclined Conveying Pipe: Comparison of Computational Fluid Dynamics−Discrete Element Method (CFD−DEM), Electrical Capacitance Tomography (ECT), and Particle Image Velocimetry (PIV) Results

This paper presents three flow patternsdispersed flow, reverse flow, and half-ring flowin the post-bend region of a 45° inclined pneumatic conveying pipe. Solid concentration and velocity distribution were measured using electrical capacitance tomography (ECT), particle image velocimetry (PIV), and a high-speed camera. The axial velocity of particles and solid transverse motion in the pipe cross section were obtained using a modified cross-correlation method from the ECT data for the three flow patterns. The axial velocity profile over the pipe cross section was compared with data obtained from the PIV system. In particular, emphasis has been placed on the characterization of the reverse-flow pattern, where negative values of solid velocities were observed. By analyzing the images captured with the high-speed camera, three distinct regions in the solid phase of the reverse-flow regime may be discerned. A dense region with high solid concentration formed next to the bottom wall of the inclined conveying pipe, while a dilute region with a relatively low solid concentration formed near the center of the pipe. Reverse flow was observed to occur predominantly in the dense region and a transition region between the two. Finally, the dynamic forces acting on single particles were analyzed for the three flow patterns. This analysis showed that the phenomena of reverse-flow and half-ring flow structure formation may be attributed to the effects of electrostatics. This finding has also been validated with results of numerical simulations performed in the present study.

Experimental Validation of Numerical Simulations: A Comparison of Computational Fluid Dynamics and the Oil Film Method

Today Computational Fluid Dynamics (CFD) is used for simulating flow in many applications. The quality of the results, however, depends on various factors, like grid quality, boundary conditions and the computational model of the fluid. For this reason, it is important to validate the performed computation with experimental results. In this work, a comparison of numerical simulation with the oil film method was performed for two cardiovascular applications. The investigations were conducted at various geometries, such as a bended cannula tubing, an impeller of a magnetically levitated rotary blood pump and tips of inflow cannulas. The oil film for the experimental validation was composed of black oil color and varnish. In the numerical simulation, color abrasion was displayed with a special post-processing tool by means of wall-attached pathlines. With the proper choice of numerical parameters, the computer simulations and the oil film method demonstrated good correlation. Improper generation of the simulation grid did lead to divergent results between the numerical simulation and the experiment. For the pump impeller as well as for the inflow cannulas, the calculation and the experiment showed similar flow patterns with backflow and stall zones. The oil film method represents a fast and simple approach to help validate numerical simulations of fluid flow. The experimentally generated near wall flow patterns can be easily compared with the solution of the CFD analysis.

Comparison of Large Eddy Simulation Predictions with Particle Image Velocimetry Data for the Airflow in a Generic Cabin Model

A comparison of computational fluid dynamics (CFD) predictions and experimental data for the airflow in a generic cabin model is presented in this paper. The CFD predictions were generated using the large eddy simulation (LES) model, while the particle image velocimetry (PIV) technique was used to obtain the experimental data. A brief summary of the test setup and the experimental data are described herein. The main focus of this study is to analyze the temporal variation of the experimental data. The time series of the PIV measured velocity components with sampling frequencies of 0.1 and 5 Hz were compared with the CFD predictions, sampling at 20 Hz. Energy spectral analysis, through fast Fourier transformation (FFT) on the PIV data and their CFD counterparts, was performed and is presented in this study. By using direct comparisons of the velocity data, good agreement on the range of the velocity components was obtained at all monitoring locations, hence validating the LES predictions. A similar conclusion can be reached from the results of the energy spectral analysis. The energy-spectrum function calculated from the LES predicted velocity magnitudes has excellent correlation with the Kolmogorov spectrum law in the universal equilibrium range. For the smaller wave numbers, the PIV data taken at 0.1 Hz clearly reveal the characteristic motion of the largest eddy in the flow domain, and it correlates very well with the CFD predictions.

CFD Reliability Issues in the Prediction of Airflows in a Naturally Ventilated Building

The potential for prediction error when using computational fluid dynamics (CFD) for investigating internal building airflows is assessed in the current paper. The ability of a proprietary CFD code, CFX, to simulate buoyant and forced airflow regimes, typical of a naturally ventilated building, are investigated using two experimental case studies from the literature. Comparisons are then made between simulated and measured airflows for a naturally ventilated building. Results from the two case studies indicate that structured meshes are less dependent on mesh density and yield consistent convergence and accuracy when coupled with the k-ϵ or k-ω turbulence models. Comparison of CFD predicted airflows with the full-scale building airflows was challenging due to scatter in the measured data. A structured mesh in conjunction with either the k-ϵ or k-ω turbulence models, showed reasonable correlation with the measured airflows, with both models performing equally.

Analysis of isothermal annular jets: Comparison of computational fluid dynamics and experimental data

AbstractA computational fluid dynamics technique was developed for the simulation of airflow through an annular jet. The technique used a commercial simulation package with a Reynolds stress model for the simulation of turbulent flows. The model parameters were calibrated using available experimental data for circular and annular jets. It was found that, after this calibration, the computational results agreed well with experimental data (specifically, with the velocity magnitude, velocity decay rate, and the velocity spreading rate). The jet geometry studied was based on industrial melt‐blowing nozzles. The velocities studied varied from the low subsonic incompressible range to nearly sonic conditions. Based on both the computational and experimental results, a correlation was proposed that predicts the centerline velocity profiles in both the near‐ and far‐field regions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 909–922, 2004

Comparison of Computational Fluid Dynamics and Particle Image Velocimetry Data for the Airflow in an Aeroengine Bearing Chamber

Investigations into the single-phase velocity field of a model aeroengine bearing chamber are presented. Adequately resolving the airflow field is important to subsequent computational modeling of two-phase fluid transport and heat transfer characteristics. A specially designed test rig, representing the features of a Rolls Royce Trent series aeroengine bearing chamber, was constructed. Experimental data for the airflow field was obtained using particle image velocimetry (PIV). The results show a strong influence of shaft rotation and chamber geometry on the flow features within the bearing chamber. A computational fluid dynamics (CFD) simulation was carried out using the commercial CFD code FLUENT 6. Flow features were adequately modeled, showing the features of secondary velocities. Turbulence modeling using the differential Reynolds stress (RSM) model shows good agreement with the experimental data.

CFD Comparison to 3D Laser Anemometer and Rotordynamic Force Measurements for Grooved Liquid Annular Seals

A pressure-based computational fluid dynamics (CFD) code is employed to calculate the flow field and rotordynamic forces in a whirling, grooved liquid annular seal. To validate the capabilities of the CFD code for this class of problems, comparisons of basic fluid dynamic parameters are made to three-dimensional laser Doppler anemometer (LDA) measurements for a spinning, centered grooved seal. Predictions are made using both a standard and low Reynolds number κ-ε turbulence model. Comparisons show good overall agreement of the axial and radial velocities in the through flow jet, shear layer, and recirculation zone. The tangential swirl velocity is slightly under-predicted as the flow passes through the seal. By generating an eccentric three-dimensional, body fitted mesh of the geometry, a quasi-steady solution may be obtained in the whirling reference frame allowing the net reaction force to be calculated for different whirl frequency ratios, yielding the rotordynamic force coefficients. Comparisons are made to the rotordynamic force measurements for a grooved liquid annular seal. The CFD predictions show improved stiffness prediction over traditional multi-control volume, bulk flow methods over a wide range of operating conditions. In cases where the flow conditions at the seal inlet are unknown, a two-dimensional, axisymmetric CFD analysis may be employed to efficiently calculate these boundary conditions by including the upstream region loading to the seal. This approach is also demonstrated in this study.