Computational Fluid Dynamics Simulation Of Flow Research Articles

CFD Simulations of Multiphase Flows: Interaction of Miscible Liquids with Different Temperatures

The incorporation of new equations to extend the applicability of open-source computational fluid dynamics (CFD) software according to the user’s needs must be complemented with code verification and validation with a representative case. This paper presents the development and validation of an OpenFOAM®-based solver suitable for simulating multiphase fluid flow considering three fluid phases with different densities and temperatures, i.e., two miscible liquids and air. A benchmark “dam-break” experiment was performed to validate the solver. Ten thermistors measured temperature variations in different locations of the experimental model and the temperature time series were compared against those of numerical probes in analogous locations. The accuracy of the temperature field assessment considered three different turbulence models: (a) zero-equation, (b) k-omega (Reynolds averaged simulation; RAS), and (c) large eddy simulation (LES). The simulations exhibit a maximum time-average relative and absolute errors of 9.3% and 3.1 K, respectively; thus, the validation tests proved to achieve an adequate performance of the numerical model. The solver developed can be applied in the modeling of thermal discharges into water bodies.

CFD simulation of ozone gas flow for controlling Sitophilus zeamais in rice grains

Ozone gas (O3) is a prominent alternative for the integrated pest management of stored products. The present study fitted a computational fluid dynamics (CFD) model to elucidate the ozone gas flow dynamics in an aeration system, at a lethal concentration for Sitophilus zeamais Mots in rice grains. First, an experiment was carried out to understand the flow of ozone gas (2.2 mg L−1; 2 L min−1) in a cylindrical PVC column filled with 4 kg of rice grains. The ozone gas concentration was monitored every 12 h, at a point immediately above the grains. The gas flow was modeled using the CFD technique. The parameters used to feed the model and adjust it to the experimental data were the mass transfer coefficient and ozone-rice grain reaction constant. Then, a simulation of ozone gas injection in a silo with a static capacity of 69.6 tons of rice was performed. In the ozone gas injection experiment, 100% of insect mortality was observed on the sixth day, when the O3 concentration reached 0.1 mg L−1. In this case, the mass transfer coefficient was 0.55 m s−1, and the reaction constant was 0.78 s−1. At the concentration of 0.8 mg L−1, it is possible to treat 69.6 tons of rice grains within 9 days.

Enhanced wall shear stress prevents obstruction by astrocytes in ventricular catheters.

The treatment of hydrocephalus often involves the placement of a shunt catheter into the cerebrospinal ventricular space, though such ventricular catheters often fail by tissue obstruction. While diverse cell types contribute to the obstruction, astrocytes are believed to contribute to late catheter failure that can occur months after shunt insertion. Using in vitro microfluidic cultures of astrocytes, we show that applied fluid shear stress leads to a decrease of cell confluency and the loss of their typical stellate cell morphology. Furthermore, we show that astrocytes exposed to moderate shear stress for an extended period of time are detached more easily upon suddenly imposed high fluid shear stress. In light of these findings and examining the range of values of wall shear stress in a typical ventricular catheter through computational fluid dynamics (CFD) simulation, we find that the typical geometry of ventricular catheters has low wall shear stress zones that can favour the growth and adhesion of astrocytes, thus promoting obstruction. Using high-precision direct flow visualization and CFD simulations, we discover that the catheter flow can be formulated as a network of Poiseuille flows. Based on this observation, we leverage a Poiseuille network model to optimize ventricular catheter design such that the distribution of wall shear stress is above a critical threshold to minimize astrocyte adhesion and growth. Using this approach, we also suggest a novel design principle that not only optimizes the wall shear stress distribution but also eliminates a stagnation zone with low wall shear stress, which is common to current ventricular catheters.

CFD simulation of turbulent flow of drill cuttings and parametric studies in a horizontal annulus

Understanding the mechanisms of cuttings transport and investigating the effects of different parameters is necessary to design an efficient cuttings transport process. Improper hole cleaning leads to the accumulation of drill cuttings in the well annulus and causes many problems such as rotational speed reduction, stuck pipe, circulation loss, and drill bit abrasion especially in horizontal drilling. In this study, computational fluid dynamics (CFD) were used for simulation of turbulent two phase flow in a horizontal annulus including a Herschel–Bulkley drilling fluid. The Lagrangian–Eulerian/discrete element method approach was adopted for description of the particle flow, interparticle and particle–wall interactions. Simulation results were compared and validated against published experimental data in the literature and j simulation results indicate that at constant drilling rate, the major factors that affects hole cleaning in horizontal wellbores are drill pipe rotational speed, drilling fluid rheology and velocity. Results of this study show the reliability of CFD analysis to simulate cuttings transport process which can minimize the difficult experimental measurements.

CFD Simulation of Two-Phase Flows in Helical Coils

The behavior of two-phase flow and corresponding flow regimes in helical tubes significantly differ when compared to two-phase flows in straight tubes due to centrifugal and torsion effects. In order to gain physical insight and gather data for validating computational models, a large number of experiments were performed on a helical coil experimental setup operated with a mixture of water and air. The experimental data were used to assess the predictive capabilities of current two-phase Computational Fluid Dynamics (CFD) models based on the Volume of Fluid (VOF) approach. In the present paper, a comparison of the CFD simulation results with the high-resolution experimental data is discussed, with special emphasis on two-phase pressure drops and void fraction distributions. It is shown that the CFD VOF model is able to correctly capture the occurrence of five flow regimes observed in the experiments, namely bubbly flow, plug flow, slug flow, slug-annular flow, and annular flow. However, a good quantitative agreement for pressure drops and void fraction distributions is found in slug flow and slug-annular flow regimes only.

Computation of Global and Local Mass Transfer in Hollow Fiber Membrane Modules

Computational fluid dynamics (CFD) provides a flexible tool for investigation of separation processes within membrane hollow fiber modules. By enabling a three-dimensional and time dependent description of the corresponding transport phenomena, very detailed information about mass transfer or geometrical influences can be provided. The high level of detail comes with high computational costs, especially since species transport simulations must discretize and resolve steep gradients in the concentration polarization layer at the membrane. In contrast, flow simulations are not required to resolve these gradients. Hence, there is a large gap in the scale and complexity of computationally feasible geometries when comparing flow and species transport simulations. A method, which tries to cover the mentioned gap, is presented in the present article. It allows upscaling of the findings of species transport simulations, conducted for reduced geometries, on the geometrical scales of flow simulations. Consequently, total transmembrane transport of complete modules can be numerically predicted. The upscaling method does not require any empirical correlation to incorporate geometrical characteristics but solely depends on results acquired by CFD flow simulations. In the scope of this research, the proposed method is explained, conducted, and validated. This is done by the example of CO2 removal in a prototype hollow fiber membrane oxygenator.

Application Progress of Computational Fluid Dynamic Techniques for Complex Viscous Flows in Ship and Ocean Engineering

Complex flow around floating structures is a highly nonlinear problem, and it is a typical feature in ship and ocean engineering. Traditional experimental methods and potential flow theory have limitations in predicting complex viscous flows. With the improvement of high-performance computing and the development of numerical techniques, computational fluid dynamics (CFD) has become increasingly powerful in predicting the complex viscous flow around floating structures. This paper reviews the recent progress in CFD techniques for numerical solutions of typical complex viscous flows in ship and ocean engineering. Applications to free-surface flows, breaking bow waves of high-speed ship, ship hull–propeller–rudder interaction, vortex-induced vibration of risers, vortex-induced motions of deep-draft platforms, and floating offshore wind turbines are discussed. Typical techniques, including volume of fluid for sharp interface, dynamic overset grid, detached eddy simulation, and fluid–structure coupling, are reviewed along with their applications. Some novel techniques, such as high-efficiency Cartesian grid method and GPU acceleration technique, are discussed in the last part as the future perspective for further enhancement of accuracy and efficiency for CFD simulations of complex flow in ship and ocean engineering.

CFD simulations of wind flow and mean surface pressure for buildings with balconies: Comparison of RANS and LES

Façade geometrical details can substantially influence the near-façade airflow patterns and pressures. This is especially the case for building balconies as their presence can lead to multiple separation and recirculation areas near the façades and hence large changes in surface pressure distribution. Computational fluid dynamics (CFD) has been widely used to investigate the impact of building balconies, mainly based on the steady Reynolds-averaged Navier-Stokes (RANS) approach. The objective of the present study is to evaluate the performance of steady RANS and large-eddy simulations (LES) in predicting the near-façade airflow patterns and mean surface pressure coefficients (Cp) for a building with balconies for three wind directions θ = 0°, 90°, 180°, where 0° is perpendicular to the façade under study. The evaluation is based on validation with wind-tunnel measurements of Cp. The results show that both RANS and LES can accurately predict Cp on the windward façade for θ = 0° with average absolute deviations of 0.113 and 0.091 from the measured data, respectively. For the other two wind directions, LES is clearly superior. For θ = 90°, the average absolute deviations for RANS and LES are 0.302 and 0.096, while these are 0.161 and 0.038 for θ = 180°. Large differences are found in the computed flow fields on the balcony spaces. Because RANS systematically underestimates the absolute values of both Cp and mean wind speed on the balconies, it is suggested that building design based on RANS might result in excessive ventilation and in too high wind nuisance level.

CFD Simulation of Two-Phase Flow in a Hybrid Pulsed Sieve-Plate Solvent Extraction Column: Prediction of Holdup and Axial-dispersion Coefficients

ABSTRACTTwo-phase computational fluid dynamics (CFD) models for a hybrid pulsed sieve-plate solvent extraction column, as well as a standard pulsed sieve-plate column, have been developed with commercial software ANSYS FLUENT. Hydrodynamic performance including two-phase distribution and velocity fields are generated with the models and comparisons are made between two columns. Important parameters including holdup and axial-dispersion coefficients are studied systematically, and CFD successfully predicts the higher holdup and lower axial-dispersion coefficients for the hybrid pulsed sieve-plate column as measured in the experiments. CFD also gives reasonable predictions for the effect of pulsation intensity, dispersed-phase velocity, and continuous-phase velocity on holdup, except for the effect of pulsation intensity in low pulsation region, and the cause has been discussed from the perspective of droplet breakage and coalescence. Comparison with literature data shows that CFD underestimates the holdup of hybrid pulsed sieve-plate column and standard pulsed sieve-plate column by 23.3% and 31.4%, respectively, and the cause has been discussed from the perspective of drag law. CFD gives good prediction of axial-dispersion coefficients for the hybrid pulsed sieve-plate column and the standard pulsed sieve-plate column with ARD of 12.0% and 14.3%, respectively. This study shows CFD to be a useful tool to predict performance for the novel hybrid pulsed sieve-plate column as well as the standard pulsed sieve-plate column.Abbreviations: CFD: computational fluid dynamics; RDC: rotating-disc contactor; ARD: average relative deviation

Determination of Discharge Coefficients for Valves of High Voltage Circuit Breakers

Nowadays, high voltage circuit breaker (CB) simulations are mostly based on Computational Fluid Dynamics (CFD) models. Such simulations require significant computer resources. An alternative approach is to use enthalpy flow models, which do not use space discretization of the interrupter unit chambers and valves. Gas flow is calculated based on state of gas in adjacent chambers and valve settings. However, the valve shape has significant influence on the effective flow cross section between chambers. Therefore, in order to ensure correct simulation results, it is necessary to determine the correct values of discharge coefficients for all valves in the interrupter unit of a circuit breaker. In this paper, the discharge coefficients were determined by combining a series of CFD and enthalpy flow simulations for each valve in the interrupter unit. After that, discharge coefficients are used as input for further simulations based on the enthalpy flow model. This way, benefits from both models are combined: more precise gas flow calculation and faster simulations. The proposed novel approach is validated in a high-power laboratory by pressure measurements on a 420 kV 63 kA self-blast circuit breaker.

Gas distributor and bed material effects in a cold flow model of a novel multi-stage biomass gasifier

A new three-stage biomass gasifier is proposed to generate product gas with low tar content. The first stage is dedicated for biomass pyrolysis. While the main roles of the second and third stages, which are superposed and separated by a gas distributor plate, are product pyrolysis gases combustion and gasification, respectively; noting that gasification is realized in a fluidized bed. In order to study and improve the process fundamentals, a cold flow model is developed to understand system dynamics and parameters. A computational fluid dynamics (CFD) study has been conducted using the Eulerian-multiphase model approach applied within the combustion and gasification zones. A complete CFD simulation of flow through the gas distributor is realized. Three values of the distributor perforated area: 1%, 3% and 5% and three bed materials were investigated to study the effect of the solid density and particle size on the fluidized bed. The aim of this work is to study the effect different parameters such as the distributor design, gas flow rate, fluidization velocity, solid density and solid particle size. In this work, gas and solid velocities above the distributor, pressure drop across the distributor, distributor to bed pressure drop ratio, solid volume fraction, bed pressure drop and the bed expansion ratio have been examined. Results show that the perforated area of a distributor has a great influence on the uniformity of the fluidization. The hydrodynamics of solid-gas fluidized beds are shown strongly affected by the gas flow rate and the properties of the bed material.

CFD simulation of methane flow through an orifice plate

In this paper the effects of errors in the flow rate and discharge coefficient when Methane (CH 4 ) flow through orifice plates with flange tapping has been investigated by the Computational Fluid Dynamics (CFD). The location of vena- contracta was also estimated from CFD simulations. Outcomes of the CFD simulations are in terms of profiles of pressure and velocity. In this study a numerical investigation of the flow characteristics behavior caused by a Methane (CH 4 ) flow through an orifice configuration is reported. This study investigates the following parameters; orifice pressure drop, flow rate and shift in discharge coefficient. In the present work, k-e turbulence model with density based solver has been used to predict velocity fields, pressure loss, shift in flow rate and discharge coefficient around this device. It is observed that the flow is jet-like flow in the core field and the presence of recirculation zone.

CFD Simulation and High-Speed Photography of Liquid Flow in the Outer Cavity Zone of a Rotating Packed Bed Reactor

The outer cavity zone, having numerous moving liquids, is an important mass transfer zone in a rotating packed bed (RPB) reactor. However, investigation of the liquid flow in the outer cavity zone is scarce. In this work, liquid flow in the outer cavity zone of the RPB reactor with nickel foam packing was studied by computational fluid dynamics (CFD) simulation and then verified by high-speed photography technology. Large eddy simulation and volume of fluid model were adopted to predict the liquid flow pattern, average diameter, and velocity of liquid droplets in the outer cavity zone. A high-speed photography method, called “Sudoku”, with eight shooting windows was employed to capture the liquid flow of different zones in imaging experiments. Simulation results of liquid droplet velocity agreed well with the experimental results within a deviation of ±10%. This CFD simulation assisted with Sudoku photography was helpful to comprehensively understand the liquid flow in the outer cavity zone of the RPB rea...

The analysis of distribution of the reaction mixture in ammonia oxidation reactor

Abstract This article describes the influence of various design modifications of the ammonia oxidation reactor operating in nitric acid plant TKIV in Kędzierzyn-Koźle on flow distribution of an air-ammonia mixture. The CFD (Computational Fluid Dynamics) simulations of turbulent flow were carried out with SST k-ω turbulence model to close the system of RANS (Reynolds Averaged Navier-Stokes) equations. The simulation results show that the properly selected perforated plate screen and the conical diffuser ensure uniform flow of gas on the ammonia oxidation catalysts and on the catalysts for nitrous oxide decomposition. It was proved experimentally achieving uniform temperature of nitrous gases in different locations under the catalytic gauzes and high efficiency of ammonia oxidation and nitrous oxide decomposition

BEPU Method Applied to CFD Simulation of Mixing Flows

Prediction of highly turbulent flows using the computational fluid dynamics (CFD) tools is not an easy task. Besides the uncertainty in the choice of turbulence model parameters, the physical properties of the fluid and experimental boundary conditions also can be largely affected by uncertainties. The objective of the study is uncertainty quantification of CFD simulation to obtain figures of merits, downstream velocity, and turbulence kinetic energy. The water-mixing experiment in the GEneric MIxing Experiment (GEMIX) facility performed at Paul Scherrer Institute is used as a benchmark case. The NEPTUNE_CFD code that solves Reynolds-averaged Navier Stokes equations with k-eps turbulence model has been used to perform a series of simulations. For uncertainty quantification with the Monte Carlo method the Optimal Statistical Estimator (OSE) was used for response surface (RS) generation from the set of CFD calculations. The results of the uncertainty analysis show that OSE is a very suitable method for RS generation, which is then used in uncertainty analysis using the Monte Carlo method to determine the 5% lower limit and 95% upper limit with 95% confidence level. In this way, the impact of some sources of uncertainty is evaluated. Also, OSE can reproduce the CFD simulation with high accuracy at the CFD calculation points, even in the case when only 5 out of 40 calculation points are used for RS generation. The results further suggest that it is very important to perform accurate reference calculation and select appropriate ranges of variation for uncertain input parameters.

Computer-assisted simulation of haemodynamic parameters of carotid artery bifurcation after carotid endarterectomy

Computer-aided simulation appropriately using the method of computational fluid dynamics (CFD) makes it possible to determine the elevated-risk zones of most probable formation of restenosis. The main idea of the method described in our article consists in a possibility of creating by the model of the geometrical shape of the vessel and characteristics of the flow at inlets and outlets the parameters of flow in each point of a vessel. The curves of velocity are used to create the curves of pressure at inlets and outlets, which are used in a CFD model. The resulting from CFD simulation of blood flow are nonstationary three-dimensional fields of pressure and velocity in the area under study. Visual examination of the dynamics of these fields makes it possible to judge about possible problem zones inside the area of flow and on the internal wall of the vessel. This article also presents a clinical case report illustrating the use of this technique.

Computational Fluid Dynamics Flow Simulations in Discrete Element Method-Resolved Packed Beds

This paper presents computational simulations of flows in packed beds and compares the computational pressure-drop results with those given by the Ergun correlation. The computational methodology used in this work follows the combined discrete element method (DEM) and computational fluid dynamics (CFD) technique. DEM is used to predict the locations and packing structure of the particles in the bed, while CFD is used to predict the flow field in the void space surrounding the packed particles. The computational results obtained for irregular packed beds show that the local packing-structure parameters have significant effects not only on the local velocity and pressure fields but also on macroscopic quantities, such as the average pressure gradient along the length of the packed column. The computational results also show that classical correlations based on averaged values, such as the Ergun correlation, have poor predictive accuracy for macroscopic variations along a packed column, and this is mainly because such correlations do not account for local packing-structure parameters. The computational results confirm the existence of sections with linear variation of macroscopic parameters along the length of the packed column, and this leads to the conclusion that accurate results from DEM-CFD methods on shortened columns can be extrapolated to full-length columns. Moreover, it was found that unlike regularly packed beds, the predicted pressure for randomly packed beds experiences an apparent strong recovery near the downstream end of the packed bed, and then experiences a strong dip down to the plateau leading to the exit pressure.

بهینهسازی هندسی اجکتور فراصوتی برای بهبود عملکرد آن در سیستمهای تبرید

Due to an increasing need for refrigeration systems and their growing electrical demand and greenhouse gases production, using ejector refrigeration systems would be a suitable substitution for conventional cooling systems. Main drawback of ejector refrigeration systems is their low coefficient of performance. The key component to improve the cycle performance is the ejector. Prerequisite of improving ejector performance is an accurate computational fluid dynamics simulation for predicting its entrainment ratio. In this study, a two dimensional, axisymmetric, steady state, compressible flow computational fluid dynamics simulation of a supersonic ejector is performed. In the second part of this study, geometrical optimization of the simulated ejector for two different objective functions is performed. The first objective function considered was the ejector entrainment ratio. The optimization with this objective function led to 53% relative improvement in the entrainment ratio with a negligible decrease in critical pressure. The second, objective function considered was the exergy efficiency in which the optimization showed 39.6% relative improvement. The exergy efficiency is used for the first time in the literature as the objective function for optimization of ejector geometry.

CFD simulation of gas and particle flow and a carbon capture process using a circulating fluidized bed (CFB) reacting loop

Circulating fluidized bed (CFB) loop-based reactors are widely used in the chemical and energy industries. CFB reactors are excellent candidates for chemical looping of solid sorbents for CO2 capture and sorbent regeneration processes because they ensure a continuous carbon dioxide removal process in a relatively compact unit. Computational fluid dynamics (CFD) provides an excellent approach to model the carbonator and regenerator reactors and the entire CFB loop design in a systematic and computationally feasible way. The simulation of individual parts of the system was recently performed and provided a qualitative estimation of the system behavior. In order to use CFD to perform simulations of the CO2 capture regenerative process, a model based on the multiphase flow equations taking into account the sorption/regeneration is needed. Therefore, in this work, a CFD approach was developed and applied to simulate gas and solid flows and CO2 sorption and regeneration in the entire CFB loop. The CFB's geometry used in this study is the same as National Energy Technology Laboratory‘s experimental setup.Our calculations showed that the CFB hydrodynamic behavior reached to a statistical steady state after about 10 s in a three-dimensional domain. The time to reach statistical steady state strongly depends on the initial condition of the simulations, and the geometry and the size of the system. Three-dimensional CFD simulations demand significantly high computational time; thus, we used only the three-dimensional domain for understanding the hydrodynamics of the system and to verify the dynamics of the same system in a two-dimensional CFD domain (no reaction was included in the three-dimensional domain). The two-dimensional domain requires considerably less computational time and allowed us to ultimately add two heterogeneous reactions to the circulating fluidized bed's CFD simulation for carbonation and regeneration of MgO-based solid sorbents. The results of our simulation showed the excellent capability of our CFD approach in describing the CO2 conversion profiles and flow behavior in a reacting CFB system that includes the entire range of solid volume fraction from very dilute (i.e., riser) to very dense (i.e., standpipe) ranges. The effect of CO2 sorption and regeneration at elevated temperature and pressure was also studied.

CFD simulation of flow behind overflooded obstacle

Abstract This paper deals with studying of two topics – measuring of velocity profile deformation behind a over-flooded construction and modelling of this velocity profile deformation by computational fluid dynamics (CFD). Numerical simulations with an unsteady RANS models - Standard k-ε, Realizable k-ε, Standard k-ω and Reynolds stress models (ANSYS Fluent v.18) and experimental measurements in a laboratory flume (using ADV) were performed. Results of both approaches showed and affirmed presence of velocity profile deformation behind the obstacle, but some discrepancies between the measured and simulated values were also observed. With increasing distance from the obstacle, the differences between the simulation and the measured data increase and the results of the numerical models are no longer usable.