Three-dimensional Computational Fluid Dynamics Simulations Research Articles

Numerical study of desalination by vacuum membrane distillation – Transient three-dimensional analysis

The performance of vacuum membrane distillation (VMD) modules can be optimized through careful selection of design parameters. The present study examines how the addition of cylindrical filaments in the feed channel increases momentum mixing and the overall performance of VMD modules under different operating inlet conditions. Three-dimensional transient Computational Fluid Dynamics (CFD) simulations are conducted using Wall-Adapting Local Eddy-Viscosity (WALE) subgrid-scale Large Eddy Simulation (LES) turbulence model. Local concentration, temperature, and flux are coupled at the membrane surface to predict the rate of water vapor diffused through the membrane by Knudsen and viscous diffusion mechanisms. The predicted and measured vapor flux agrees reasonably well; validating the employed model. The small-scale eddies induced by the presence of spacer filaments promote mixing in the module, thus the temperature and concentration polarization is alleviated and the water vapor flux is immensely improved. The insertions of filaments in the feed channel increase the water permeate rate by more than 50% at higher feed flow rates and inlet temperatures. The pressure drop by the spacer reduces the allowable module length by one order of magnitude, but the module length increases two folds at feed temperature 80°C. Even though the power consumption of the module containing the filaments is increased, the addition of filaments is strongly recommended since the required power for the process could be supplied from readily available low-grade heat source.

Numerical modeling of thermophoretic deposition on cylinder liner of a diesel engine using a sectional soot model

Soot deposition on combustion chamber walls of a diesel engine is one of the key concerns among diesel engine manufacturers as it affects the durability and wear of the power cylinder components. The deposition of soot on the liner also gets scraped to the crank case and leads to soot in the lubrication oil. In this work, a modeling framework is presented to estimate the deposition of soot on the liner walls. Three-dimensional computational fluid dynamic combustion simulations are performed at two operating loads for a direct injection heavy duty diesel engine. A sectional soot model coupled with gas phase chemical kinetics is used to model soot in the combustion chamber. The combustion and soot models are validated by comparing in-cylinder pressure, heat release rate and soot mass from simulations with the experimental data. A thermophoretic soot deposition model is presented which transports soot from combustion to the liner walls. Detailed in-cylinder soot mass fraction contours are used to explain the deposition phenomenon. The deposition of soot on the liner is qualitatively compared to measured soot in the lubrication oil and a good agreement is observed. The proposed modeling framework shall be helpful for optimizing combustion systems to reduce degradation of lubrication oil.

Numerical investigations on flow characteristics and energy separation in a Ranque Hilsch vortex tube with hydrogen as working medium

In the last decades, the theory of energy separation in vortex tubes is debated broadly based on the heat transfer and work transfer between core and peripheral flow layers. Many parameters were considered in the literature. However, the present study involves the inlet energy considered collectively towards energy separation. In this paper, three-dimensional computational fluid dynamic simulations are discussed in vortex tube to analyze the energy separation phenomena in different cases by varying the working medium such as hydrogen and air having specific heat variation. The energy at the inlet is maintained same in both cases by adjusting the inlet mass flow rate. The results from this study are validated with recently published literature using hydrogen as a working medium. Vortex tube with hydrogen as working medium yields a temperature separation of 8 K lower than air as working medium. Further studies on vortex tube with hydrogen as a working fluid is explored at different inlet temperatures relative to the room temperature. Vortex tube with hydrogen at an inlet temperature of 400 K gives better temperature separation as compared to other inlet temperatures considered in this study.

Core thermal hydraulic CFD support for liquid metal reactors

Knowledge of the heat transport in the core is important for design and safety assessment of all nuclear reactors including liquid metal cooled reactors. In the past, design and safety calculations with respect heat transport in the core for such liquid metal cooled reactors were largely one-dimensional and based on experimental data. Nowadays, with modern state-of-the-art computer power and tools, three-dimensional Computational Fluid Dynamics (CFD) simulations allow designers and safety specialists to obtain much more detailed information on the heat transport in liquid metal cooled fuel assemblies, obviously supported by necessary experimental campaigns. This may lead to new insights possibly decreasing the safety margins.To this respect, an overview will be provided on the necessary activities in the frame of design and safety support using CFD for liquid metal reactors accompanied and illustrated by examples from NRG in the Netherlands. These examples include validation efforts for fuel assemblies as they are designed on the drawing board for ‘cold’ conditions. However, in reality, even under normal operational condition, a fuel assembly may deform. Therefore, an assessment of the effect of deformations resulting from operational conditions is necessary and will be shown. Another aspect possibly occurring during operational conditions is vibration. State-of-the-art coupled CFD and finite element method fluid structure interaction techniques have been developed and applied to a wire wrapped fuel assembly, providing insights in the vibration behavior of such assemblies. However, design and safety analysts will not only have to cope with operational conditions, but also have to show the heat transport behavior under accident conditions. For this, an assessment of the effect of internal and inlet blockages will be presented.

Three-Dimensional CFD Simulation and Experimental Assessment of the Performance of a H-Shape Vertical-Axis Wind Turbine at Design and Off-Design Conditions

The paper presents the results of a computational study on the aerodynamics and the performance of a small-scale Vertical-Axis Wind Turbine (VAWT) for distributed micro-generation. The complexity of VAWT aerodynamics, which are inherently unsteady and three-dimensional, makes high-fidelity flow models extremely demanding in terms of computational cost, limiting the analysis to mainly 2D or 2.5D Computational Fluid-Dynamics (CFD) approaches. This paper discusses how a proper setting of the computational model opens the way for carrying out fully 3D unsteady CFD simulations of a VAWT. Key aspects of the flow model and of the numerical solution are discussed, in view of limiting the computational cost while maintaining the reliability of the predictions. A set of operating conditions is considered, in terms of tip-speed-ratio (TSR), covering both peak efficiency condition as well as off-design operation. The fidelity of the numerical predictions is assessed via a systematic comparison with the experimental benchmark data available for this turbine, consisting of both performance and wake measurements carried out in the large-scale wind tunnel of the Politecnico di Milano. The analysis of the flow field on the equatorial plane allows highlighting its time-dependent evolution, with the aim of identifying both the periodic flow structures and the onset of dynamic stall. The full three-dimensional character of the computations allows investigating the aerodynamics of the struts and the evolution of the trailing vorticity at the tip of the blades, eventually resulting in periodic large-scale vortices.

Temperature distribution in a photovoltaic module at various mounting and wind conditions: A complete CFD modeling

In this study, a complete three-dimensional computational fluid dynamics simulation of fluid flow and heat transfer from a Photovoltaic (PV) module is developed. Since the electrical efficiency of photovoltaic modules is a function of its temperature, the present research tries to calculate the temperature distribution in the module by numerically solving the governing momentum and energy equations with the finite element method. The three dimensional wind flow field around the system is simulated in different geometrical and ambient conditions to account for realistic and nonuniform convection heat transfer from the module surfaces. The electrical output power is calculated as a function of PV module temperature and is modeled as a negative source term in the energy equation. Results show that the present method is very successful in predicting the temperature distribution and output power of photovoltaic modules at various geometrical and environmental conditions. Specifically, the effect of wind direction and tilt angle of the PV module is investigated. By increasing the tilt angle at wind velocities higher than 1 m/s, the average PV temperature increases by about 4 K; as a result, its efficiency decreases. The most favorable wind direction is 90° as a 3 K cooling in the average PV module temperature can be achieved. The proper choice of mounting orientation is also important, and it depends on the wind direction. In most cases, the horizontal mounting (i.e., the longer side of the panel is parallel to the ground) results in better cooling.

Modeling and Simulations of Deteriorated Turbulent Heat Transfer in Wall-Heated Cylindrical Tube

Abstract This paper discusses the modeling and simulations of deteriorated turbulent heat transfer (DTHT) for a wall-heated fluid flows, which can be observed in gas-cooled nuclear power reactors during pressurized conduction cooldown (PCC) event due to loss of force circulation flow. The DTHT regime is defined as the deterioration of normal turbulent heat transport due to increase of acceleration and buoyancy forces. The computational fluid dynamics (CFD) tools such as Nek5000 and STAR-CCM+ can help to analyze the DTHT phenomena in reactors for efficient thermal-fluid designs. Three-dimensional (3D) CFD nonisothermal modeling and simulations were performed in a wall-heated circular tube. The simulation results were validated with two different CFD tools, Nek5000 and STAR-CCM+, and validated with an experimental data. The predicted bulk temperatures were identical in both CFD tools, as expected. Good agreement between simulated results and measured data were obtained for wall temperatures along the tube axis using Nek5000. In STAR-CCM+, the under-predicted wall temperatures were mainly due to higher turbulence in the wall region. In STAR-CCM+, the predicted DTHT was over 48% at outlet when compared to inlet heat transfer values.

Numerical investigation of wind turbines and turbine arrays on highways

High-speed moving vehicles can produce strong wakes which contain considerable amount of localized wind energy. Previous studies have proved the feasibility of using vertical axis wind turbines (VAWTs) to recover energy from the wake of vehicles on highways. However, two key problems remains unsolved considering the highway wind turbines. What is the best type of VAWTs for highway wind energy recovery? And what is the best gap between VAWTs if they are planted in an array? This study aims at solving the above two problems using three-dimensional computational fluid dynamics (CFD) simulations. First, the performance of three different rotors are evaluated and compared, it is found that the Banki rotor operates best in the wake of a moving vehicle. Then, simulations are performed on an array that composed of three Banki rotors. The performances of the array is evaluated at different rotor gaps and the optimal rotor gaps are provided for different situations.

A review on computational fluid dynamics modeling and simulation of horizontal axis hydrokinetic turbines

Hydrokinetic energy conversion devices provide the facility to capture energy from water flow without the need of large dams, impoundments, channels or deviation of the water as in conventional hydroelectric centrals. Hydrokinetic systems are intended to be used in streams, either natural (rivers, estuaries, marine currents) or artificially built channels. This article reviews the advances made over the last 10–15 years regarding the three-dimensional computational fluid dynamics modeling and simulation of this type of turbines. Technical aspects of model design, employed boundary conditions, solution of the governing equations of the water flow through the hydrokinetic turbine and assumptions made during the simulations are thoroughly described. We hope that this review will encourage new computational investigations about hydrokinetic turbines that contribute to their continuous improvement, development and implementation aimed to sustainable use of water resources and addressed to solve the problem of lack of electricity supply in small, isolated populations.

Numerical simulation and experimental investigation on tribological performance of micro-dimples textured surface under hydrodynamic lubrication

Aiming at establishing an effective and efficient optimization method of texture parameters to further enhance the tribological performance of mechanical equipment, numerical simulation and experimental studies were undertaken to assess the tribological performance of textured surface. Pressure distributions and velocity distributions of the lubricant flow were analyzed by three-dimensional computational fluid dynamic simulation to comprehensively understand the effects of geometric parameters and operating conditions. It is found that the tribological performance of the textured surface is significantly affected by the micro-dimple geometric parameters, and the optimum texturing geometric parameters are highly dependent on the operating conditions. The comparative tribological experiments of untextured and textured specimens were conducted, and the specimen with area density of 28.26% achieves a maximum friction reduction rate of 26.61% under the tested conditions. The numerical simulation and experimental results were cross-checked and consistent with each other to verify that the textured surface can achieve a remarkable friction reduction effect. The friction reduction mechanism is mainly attributed to the improvement of carrying capacity, lubricant reservation, contact area reduction and secondary lubrication. Results can provide theoretical guidance for the optimization of texture geometric parameters under different application conditions.

Pressure Drop in High-Density Porous Metals via Tomography Datasets

This study combines three-dimensional advanced imaging techniques and computational fluid dynamic modelling and simulation (CFD) to characterise the pressure drop of flowing fluid across high-density porous metals utilising high-resolution X-ray computed tomography data. The modelling approach quantifies the combined effects of pore volume fraction, pore connectivity, pore size and morphology on the flow behaviour of porous metals and to study in more detail the pressure drop behaviour characterised by the sudden change in pore volume by stacking of differential porous samples at the pore-level. The resulting predicted values of the pressure drop as a function of superficial fluid velocity ranging from Darcy to turbulent fluid flow regimes were used to account for the permeability (k0) and Form drag coefficient (C) of these materials. Supportable agreement between CFD modelled data against empirical measurements available in the literature was substantiated. Therefore it is considered that this approach could lead practically to minimizing the number of design iterations required for the processing of novel-attributing porous metallic materials for applications involving fluid flow.

Effect of colloidal properties on the particle characteristics in the flame-assisted spray-drying process

Spherical silica particles have been synthesised from sodium silicate (Na2SiO3) via flame-assisted spray-drying. The parameters of colloidal properties, such as pH and concentration of a solution (ranging 8–11 and 0.1–0.3 mol/L, respectively), were studied to understand their effects on droplet size and the resultant morphological changes in the silica particles. A three-dimensional multiphase computational fluid dynamic (CFD) simulation for the flame-assisted spray-drying process has been developed to predict the temperature distribution in flame reactors and the generated average particle size. The pH and concentration of the solutions were found to affect the precursor physical properties, such as density, viscosity, and surface tension. This observation was validated by the presence of non-spherical doughnut-like particles using the colloidal precursor at pH 8 and solution concentration 0.1 mol/L. An approach that permits the direct estimation of the average droplet size produced from an ultrasonic nebulizer is presented in this paper. Good agreement, with an approximately 5% negative discrepancy, was observed between the experimental results and the theoretical values. A higher discrepancy of approximately 33% occurred in the particles using the colloidal precursor at pH 8 owing to morphological changes in the silica particles.

Numerical investigation on the effects of valve timing on in-cylinder flow, combustion and emission performance of a diesel ignition natural gas engine through computational fluid dynamics

In the present study, a diesel engine was modified to a diesel pilot ignited natural gas engine and the influences of intake valve closing timing on in-cylinder flow, combustion and emission performance of engine were investigated by three-dimensional computational fluid dynamics simulation. Based on the geometric model and basic parameters of this engine, the simulation model was built under three operating conditions and then validated by experimental data. On this basis, the validated model was applied to investigate the effects of the intake valve closing timing strategy. The simulation results indicated that, the volumetric efficiency decreases with the retarding of intake valve closing timing in three cases while it increases as the intake valve closing timing is advanced by 10°CA at 1200 rpm. The in-cylinder turbulence kinetic energy decreases with the retarding of intake valve closing timing. The peak in-cylinder pressure decreases when the intake valve closing timing is either advanced or retarded at 50% load. Nevertheless, the maximum peak in-cylinder pressure occurs at advancing 10°CA intake valve closing timing at 1200 rpm and 100% load, which rises by 4.5 bar compared with that at the original intake valve closing timing. Additionally, the maximum heat release rate appears at advancing 10°CA intake valve closing timing at 100% load, which is 26 J/deg higher than that at the original intake valve closing timing. Simultaneously, the shortest combustion duration occurs at advancing 10°CA intake valve closing timing at 1200 rpm and 100% load. For the emissions, the NOx emissions decrease with the retarding of intake valve closing timing but the variation becomes unobvious with the advancing of the intake valve closing timing. Besides, the intake valve closing timing strategy has little effect on HC and CO emissions.

CUPID 코드 기반 수소거동 해석 모델 방법론 연구

In the wake of Fukushima accident, controlling the hydrogen risk has become a priority for the safety of nuclear power plants. While lumped-parameter system codes are computationally efficient, they do not provide precise information on the local hydrogen distribution. On the other hand, computational fluid dynamics (CFD) codes provide detailed information on local phenomena. In this work, a three-dimensional CFD simulation of the PANDA facility is developed using the CFD code, CUPID. Specifically, the simulation of the thermal-hydraulic experiments OECD/SETH-2 ST1 test series is presented in the current paper. The code predictions of the concentration during the mixing and stratification phases are compared to the experimental data. This effort is part of an ongoing effort to understand the local behavior of steam-hydrogen releases in nuclear reactor containment in support of accident management and simultaneously serves as a validation of the CUPID code to assess it applicability for containment analyses.

Improved double-multiple-streamtube approach for H-Darrieus vertical axis wind turbine computations

Accurate predictions on vertical axis wind turbine performance are important for the design process but also challenging because the flow is highly unsteady involving a strong variation of the angle of incidence. In the present work, a double-multiple-streamtube (DMS) approach is improved by analytical corrections. First, two-dimensional and three-dimensional computational fluid dynamics (CFD) simulations are performed for wind turbines at various tip speed ratios. The CFD results are used as a database to investigate the velocity field of the turbines and to derive reasonable physical basis for the modelling approach. Second, new analytical models are introduced to correct the angle of incidence seen by the airfoil section together with correction models for the streamtube expansion and dynamic stall effects. DMS simulations are then carried out and compared with experimental data for turbines with different rotor solidity and various operating ranges. The prediction accuracy is significantly improved by the new model not only for the power coefficient but also for the azimuthal variations of the blade loads.

3D CFD Simulation of Water Hammer in High Pressure Common Rail System of Diesel Engine

The water hammer pressure triggered by injection in the high pressure common rail system play a major role in precise controlling of diesel injection. In order to obtain a deep insight into the existing transient three-dimensional effects, a three-dimensional computational fluid dynamics (3D CFD) simulation of water hammer was performed. An adequate mesh and suitable physical model was generated using a commercial code. The transition SST model was applied to simulate the three-dimensional hydrodynamic calculation of the oil supply lines of high-pressure common rail systems. The accuracy of simulation was verified by comparing the results with experimental data. The fluctuation characteristics of pressure and velocity in high pressure oil pipeline and the three-dimensional flow field effect of pipe radial direction were analyzed in the course of water hammer pressure fluctuation propagation in high pressure common rail system. 3D CFD simulation can show more details than traditional methods.

Numerical investigation of the convection heat transfer driven by airflows in underground tunnels

This study presents a fundamental investigation of the convection heat transfer phenomenon driven by airflows in underground tunnels, with reference to the development of the so-called thermal and velocity boundary layers. The purpose of the work is to expand on the knowledge of non-isothermal problems of internal flow common to applications involving pipes, pipelines and conventional tunnels, as well as to innovative applications involving so-called energy tunnels that can provide structural support and energy supply to built environments. The study is based on three-dimensional thermo-hydraulic Computational Fluid Dynamics simulations accounting for the actual fluid flow that occurs in the aforementioned environments, and addresses the investigated phenomena by considering tunnels with different (i) shapes of the cross-section, (ii) inlet airflow velocities and (iii) surface wall(s) roughness. The results of this investigation highlight a crucial influence of the previous variables on the convection heat transfer phenomenon, which is inherently related to the development of the thermal and velocity boundary layers. The work also presents novel relationships between the convection heat transfer coefficient and the average airflow velocity that may be used in the analysis and design of the addressed non-isothermal problems of internal flow.

The Computational fluid dynamics Performance Analysis of Horizontal Axis Wind Turbine

<span lang="EN-US">Computational fluid dynamics (CFD) simulations were performed in the present study using ANSYS Fluent 18.0, a commercially available CFD package, to characterize the behaviour of the new HAWT. Static three-dimensional CFD simulations were conducted. The static torque characteristics of the turbine and the simplicity of design highlight its suitability for the GE 1.5xle turbine. The major factor for generating the power through the HAWT is the velocity of air and the position of the blade angle in the HAWT blade assembly. The study presents the effect of The blade is 43.2 meters long and starts with a cylindrical shape at the root and then transitions to the airfoils S818, S825 and S826 for the root, body and tip, respectively. This blade also has pitch to vary as a function of radius, giving it a twist and the pitch angle at the blade tip is 4 degrees. This blade was created to be similar in size to a GE 1.5xle turbine by Cornell University [1]. In addition, note that to represent the blade being connected to a hub, the blade root is offset from the axis of rotation by 1 meter. The hub is not included in our model. The experimental analysis of GE 1.5xle turbine, so that possible the result of CFD analysis can be compared with theoretical calculations. CFD workbench of ANSYS is used to carry out the virtue simulation and testing. The software generated test results are validated through the experimental readings. Through this obtainable result will be in the means of maximum constant power generation from HAWT.</span>

Use of High-Resolution Numerical Models and Statistical Approaches to Understand New Zealand Historical Wind Speed and Gust Climatologies

AbstractThis paper describes how a combination of high-resolution numerical modeling, a robust homogenization algorithm, and local pressure observations have been used to understand and reconcile time series of daily, seasonal, and annual peak wind gusts recorded at observing sites in the Cook Strait region of New Zealand. The homogenization algorithm consists of corrections for the relocation of masts, changes in instrumentations, data acquisition and signal processing, surrounding surface roughness, and measurement heights. In addition, a statistical method, penalized maximal F test (PMFT), was used to assess the homogeneity of the wind speed time series and detect and eliminate all remaining, undocumented, artificial (i.e., nonclimatic) breakpoints. A three-dimensional time-dependent computational fluid dynamics (CFD) simulation was carried out using the Gerris model to characterize the turbulence environment at the mast sites and to estimate topographic speedup effects. Trends in magnitudes and frequencies of the homogenized seasonal and annual peak wind gusts are evaluated and presented for the Cook Strait region. The pressure gradients between pairs of stations were used to study the correlation between the gust wind speeds and the pressure field. A high-resolution convection-resolving numerical weather prediction model [the New Zealand Convective-Scale Model (NZCSM)] was employed to aid the interpretation of results and analyze wind trends. The trend in gust speeds is also shown to be consistent with larger-scale NCEP–NCAR reanalysis pressure trends. The homogenization algorithm showed promising results in eliminating the artificial breakpoints and trends. Overall, strong correlations were found between the homogenized gust speeds, the pressure field across the region, and NZCSM predictions.

3D Eulerian-Eulerian modeling of a screw reactor for biomass thermochemical conversion. Part 1: Solids flow dynamics and back-mixing

Three-dimensional (3D) computational fluid dynamics (CFD) simulations were performed to study solids flow dynamics and solids back-mixing behavior in a screw reactor (designed for thermal conversion of dry biomass particles) based on the Eulerian-Eulerian method. Simulation results were compared against experimental data with respect to filling degree and mean residence time of particles. The mean deviations for filling degree and for mean residence time between simulation and experiment were about 0.01 and 11.4 s, respectively, which shows that the model is reasonably accurate in predicting solids flow behavior in the screw reactor. The solids flow dynamics inside the reactor were discussed. The solids residence time distribution (RTD) was calculated and the degree of solids back-mixing in the forward transportation direction of the reactor was analyzed. It was found that solids being flung over the shaft and solids back-leakage, resulting from the low solids forward transportation velocity at the clearance between the flight and the bottom shell of the screw reactor, were responsible for solids back-mixing. The degree of solids back-mixing can be reduced at higher screw rotating speeds when keeping inlet mass flow rate of solids constant.