Numerical investigation of the autoignition of turbulent gaseous jets in a high-pressure environment using the multiple-RIF model

The autoignition and subsequent flame propagation of initially nonpremixed turbulent system have been numerically investigated. The unsteady flamelet modeling based on the RIF (Representative Interactive Flamelet) concept has been applied to account for the influences of turbulence on these essentially transient combustion processes. In this RIF approach, the partially premixed burning, diffusive combustion and formation of pollutants (NO_x, soot) can be consistently modeled by utilizing the comprehensive chemical mechanism. To treat the spatially distributed inhomogeneity of scalar dissipation rate, the multiple RIFs are employed in the framework of Eulerian Particle Flamelet Model approach. Computations are made for the various initial conditions of pressure, temperature and fuel composition. The present turbulent combustion model reasonably well predicts the essential features of autoignition process in the transient gaseous fuel jets injected into high-pressure and high-temperature environments.     

Flow field and residence time distribution simulation of a cross-flow gas-liquid wastewater treatment reactor using CFD

A three-dimensional Eulerian-Eulerian two-phase approach has been used for the simulation of a cross-flow gas-liquid wastewater treatment reactor. Two different turbulence models have been tested: the k-ε and Reynolds Stress Model (RSM) models. Bubble induced turbulence source terms have been added to these models. Numerical results have been validated using Laser Doppler Velocimetry (LDV) measurements. Simulations with both turbulence models successfully predicted the hydrodynamics of the reactor. Then particle tracking with a stochastic approach has been used to calculate residence time distributions (RTD) with the flow previously simulated. It has been shown that dispersion in the reactor is primarily due to turbulence. Results have been compared with experimental RTD for various liquid and gas flowrates both on a bench scale and full scale plant. The RSM model accurately predicted the dispersion whereas the standard k-ε model slightly underestimated the dispersion.     

A Three-Dimensional Numerical Study of the Gas/Particle Interactions in an Industrial-Scale Spray Dryer for Milk Powder Production

Gas/particle interaction plays an important role in modern spray dryers and may have influences on wall deposition, agglomeration, powder degradation, etc. In the present study, the three-dimensional (3-D) transient multiphase flow in an industrial-scale spray dryer has been investigated using the CFD package FLUENT. The Eulerian–Lagrangian approach and two-way coupling method were used in the simulations. The reaction engineering approach (REA model) for milk particles has been implemented. Some new characteristics of the gas flow pattern and the particle behavior (e.g., temperature–time profiles) were identified from the numerical results; for example, the milk particles flow in such a way that makes the central jet oscillation more nonlinear. The discrete phase enhances the turbulence near the air/droplet inlet but damps it downstream. The transient turbulent flow causes significant uncertainties in the particle tracking, which presented some challenges in simulations. The study has highlighted the importance in performing 3-D transient simulations in order to understand the industrial-scale dryers.     

The impacts of standard wall function and drag model on the turbulent modelling of gas–particle flow in a circulating fluidised bed riser

A transient turbulence model was applied to simulate the gas–particle system in a circulating fluidised bed riser. The k–epsilon turbulent equations coupled with the fluctuating energy equation were used to simulate the gas–particle system in a riser. The simulation results were validated by the experimental data of a CFB system. A grid study was implemented to examine the impact of grid discretisation. A comparison between the conventional drag models and the EMMS model was also conducted. Other factors, like the restitution coefficient particle to particle, was also found to have a significant impact on the turbulence model. © 2013 Canadian Society for Chemical Engineering     

CFD SIMULATION ON INFLUENCE OF SUPERFICIAL GAS VELOCITY,COLUMN SIZE,SPARGER ARRANGEMENT,AND TAPER ANGLE ON HYDRODYNAMICS OF THE COLUMN FLOTATION CELL

The use of flotation columns in the mineral processing industry has experienced a remarkable growth over the years. The detailed hydrodynamics study of a column flotation cell demands the solution of mass, momentum, phase-transfer, and turbulence quantities. Simulations have been carried out to examine the influence of superficial air velocity, column size, column taper angle, and sparger arrangement on hydrodynamics of the column flotation cell. A commercial CFD software package (ANSYS CFX 10.0) has been used to predict the complex unsteady air-water flow. The k-ε turbulence model for shear-induced turbulence, Sato's eddy viscosity model for bubble-induced turbulence, and the effect of interfacial momentum transfer terms (lift force, wall force) were considered. Present findings suggest use of low height-to-diameter ratio, low airflow rate, small column taper angle, and uniformly distributed sparger to achieve good separation in a column flotation cell.     

Three-dimensional numerical modelling of a fast fluidized bed biomass gasifier to generate energy from wastes

The modelling of a biomass fluidized bed gasification system, one of the most effective ways to produce energy from biomass resources and wastes, has been performed in this study. The effect of the turbulence phenomena, including calculations relating to flow turbulence, chemical fuel reactions, and energy and momentum exchange between multiple solid and gas phases, has been taken into account in the current research as a novel approach. A computational fluid dynamics case study model that combines equations with comprehensive geometry has been considered. Results have been compared with published operational records of an existing power plant to validate the model. The solid particle distribution, the velocity of the mixture and gas phase, the turbulent flow viscosity ratio, and the temperature distribution in the model indicated the accuracy of the simulation performance compared with the experimental studies. The production of the molar fraction of the constituent elements of the synthesis gas has been evaluated in transient conditions. Additionally, 35 s after the process began, the system's performance was estimated, and the results indicated the average molecular weights of hydrogen, carbon monoxide, carbon dioxide, and methane are 26%, 23%, 12.5%, and 3.3%, respectively, which presented high precision with the experimental results.     

小尺度受限空间内瓦斯湍流爆燃大涡模拟

构建了150 mm × 150 mm × 500 mm小尺度受限空间三维模型,基于火焰表面密度模型和Charlette湍流燃烧模型,对两侧连续障碍物条件下瓦斯爆燃火焰与湍流耦合过程进行了大涡模拟(LES)。模拟结果均与实验结果进行了比较。结果表明:大涡模拟可以很好预测瓦斯爆燃过程中的火焰结构、火焰锋面位置、火焰传播速度及超压,验证了大涡模拟及湍流燃烧模型对于瓦斯爆燃的适用性。此外,通过Karlovitz数定量描述了瓦斯爆燃火焰与湍流之间的相互作用及其变化规律,并对不同时刻的火焰模态进行了判别,在两侧连续障碍物条件下瓦斯湍流爆燃火焰先后经历波纹小火焰和薄反应区两种模态。     

FLOW GENERATED BY PITCHED BLADE TURBINES II: SIMULATION USING κ-ε MODEL

Experimental data on average velocity and turbulence intensity generated by pitched blade downflow turbines (PTD) were presented in Part I of this paper. Part II presents the results of the simulation of flow generated by PTD

The standard κ-ε model along with the boundary conditions developed in the Part 1 have been employed to predict the flow generated by PTD in cylindrical baffled vessel. This part describes the new software FIAT (Flow In Agitated Tanks) for the prediction of three dimensional flow in stirred tanks. The basis of this software has been described adequately. The influence of grid size, impeller boundary conditions and values of model parameters on the predicted flow have been analysed. The model predictions successfully reproduce the three dimensionality and the other essential characteristics of the flow. The model can be used to improve the overall understanding about the relative distribution of turbulence by PTD in the agitated tank     

Hydrodynamic and heat transfer characteristics of a centrally heated cylindrical enclosure: CFD simulations and experimental measurements

Natural convection in enclosures is of importance in many engineering applications. The stratification arising out of natural convection may be desirable/undesirable depending on applications. In order to control the degree of stratification, understanding of flow pattern and temperature profiles is required. In the present work, transient natural convection in a cylindrical enclosure has been investigated for water with CFD simulations and flow visualization [using particle image velocimetry (PIV) and hot film anemometry (HFA)] over a wide range of parameters namely Rayleigh number (1.08 × 10¹¹ ≤ Ra ≤ 3.76 × 10¹³) and aspect ratio (1 ≤ H/R ≤ 2). The effect of various parameters like pressure, tube diameter and aspect ratio on the extent of stratification has been studied. PIV measurements have been performed to understand the transient flow behavior. Multiple thermocouples were used to measure the temperature profiles. CFD simulations have been performed using SST k–ω model and the results have been compared with the PIV measurements. The CFD simulations have been carried out for 2D axi-symmetric cases and the effect of boundary conditions (free-slip and no-slip) has been investigated. An excellent agreement was found between the CFD predictions and the experimental measurements of flow and temperature patterns. The extent of stratification has been quantified using dimensionless parameters like stratification number and stratification time. The kinetic energy profiles and kinetic energy dissipation profiles show that almost 75% of the enclosure is stratified (after different times depending on Ra number and the aspect ratio). The turbulence parameters were found to weaken with time in the stratified region and these predictions are corroborated with HFA measurements.     

Numerical simulation of turbulent flow in a baffled stirred tank with an explicit algebraic stress model

An explicit algebraic stress model (EASM) was used to simulate anisotropic turbulent flows in baffled stirred tanks equipped with a standard Rushton turbine. The quantitative predictions of velocity components, turbulence kinetic energy, Reynolds stresses and turbulence energy dissipation rate in the context of anisotropic turbulence were conducted to assess the comprehensive performance of the EASM. A lot of efforts have been made to ensure numerical stability during the calculations such as using a good initial flow field, manipulating source terms and adjusting under-relaxation factors. The predicted results were also compared with experimental data and other simulation results obtained using the standard k–ε model, algebraic stress model (ASM), Reynolds stress model (RSM) and large eddy simulation (LES). All the simulations were run with in-house codes. The simulation results show that agreement between the EASM predictions and experimental values is satisfactory. The EASM is consistently superior to the standard k–ε model when predicting both peak values and trend of variation in velocities and turbulence quantities. In comparison to the RSM, the EASM has almost the same predictive accuracy. The EASM is inferior to the LES on the prediction of turbulence kinetic energy. Nevertheless, the computational cost of the EASM is significantly lower than that of the LES, which is an obvious advantage in practical applications.     

The modified dynamic model of a riser type fluid catalytic cracking unit

The dynamic model developed by Ali and Rohani (1997) to describe the transient behavior of a fluid catalytic cracking (FCC) unit is modified (i) to incorporate the effect of volumetric expansion of the feed and product gases flowing in the riser reactor (ii) to consider the enhancement of mass and heat transfer coefficients due to high turbulence in the regenerator and (iii) to model the reactor and stripper as a continuous stirred tank. The modified model is validated using steady-state plant data from an industrial unit (Consumer's Co-operative Refineries Ltd., Regina, SK) and the results are found to be in good agreement.     

Numerical Investigation of Three‐Dimensional Bubble Column Flows: A Detached Eddy Simulation Approach

Numerical simulations were performed employing detached eddy simulation (DES) in a three‐dimensional transient Euler‐Euler framework for bubble columns, and all the computational fluid dynamics results were compared with a k‐? model and available experimental data. The numerical results are in good agreement with the experiments in predicting the time‐averaged axial velocity and turbulent kinetic energy profiles. The flow‐resolving capabilities of the DES model are highlighted, and it is shown that the DES turbulence model can be efficiently used for simulating flow field and turbulent quantities in the case of bubble columns.     

FLOW GENERATED BY PITCHED BLADE TURBINES I: MEASUREMENTS USING LASER DOPPLER ANEMOMETER

Mean flow and turbulence intensities have been measured using laser Doppler anemometer for pitched blade downflow turbines (PTD). Fully baffled, flat bottomed cylindrical vessels of 300 and 500 mm internal diameter were employed. The effect of impeller clearance on flow characteristics have been investigated. The influence of geometry of PTD, that is blade angle (30-60°), blade width (0.2D-0.4D) and impeller diameter (0.25T-0.5T) on the flow have been studied in detail. The energy balances have been established around all the impellers and the hydraulic efficiency values have been reported. This part provides a right set of boundary conditions to the model as well as provides necessary and sufficient set of data to evaluate the model performance     

A BASIC CODE FOR THE PREDICTION OF TRANSIENT THREE-DIMENSIONAL TURBULENT FLOWFIELDS

A primitive pressure-velocity finite difference code has been developed to predict transient three-dimensional turbulent flow. The code is a simplified yet effective prediction procedure for use by persons with little experience in computational fluid dynamics, and into which user-oriented complexities can easily be added. The method is based on the transient 2-D Los Alamos SOLA prediction technique for laminar flows. Turbulence is simulated by. means of the two-equation κ - ε turbulence model; species diffusion and buoyancy are also included. Two applications of the code are presented to local destratification near the release structure of a reservoir and to the deflection of a jet entering normally into a uniform cross-flow. Predicted results exhibit good agreement with experimental data, showing that a useful characterization of fully three-dimensional flows is now available     

Effect of the shaft eccentricity on the hydrodynamics of unbaffled stirred tanks

The aim of this work is to investigate the effect of the shaft eccentricity on the hydrodynamics of unbaffled stirred vessels. The difference between coaxial and eccentric agitation is studied using a combination of experiments carried out by particle image velocimetry, that provide an accurate representation of the time-averaged velocity, and computational fluid dynamics simulations, that offer a complete, transient volumetric representation of the three-dimensional flow field, once a proper modelling strategy is devised. The comparison of the experimental and simulated mean flow fields has demonstrated that calculations based on Reynolds-averaged Navier-Stokes equations are suitable for obtaining accurate results. Depending on the position of the shaft, steady-state or transient calculations have to be chosen for predicting the correct flow patterns. Care must be exerted in the choice of turbulence models, as for the unbaffled configurations the results obtained with the Reynolds stress model are superior to that of the k-ε model.     

CFD simulation of stirred tanks: Comparison of turbulence models. Part I: Radial flow impellers

A critical review of the published literature regarding the computational fluid dynamics (CFD) modelling of single‐phase turbulent flow in stirred tank reactors is presented. In this part of review, CFD simulations of radial flow impellers (mainly disc turbine (DT)) in a fully baffled vessel operating in a turbulent regime have been presented. Simulated results obtained with different impeller modelling approaches (impeller boundary condition, multiple reference frame, computational snap shot and the sliding mesh approaches) and different turbulence models (standard k ? ε model, RNG k ? ε model, the Reynolds stress model (RSM) and large eddy simulation) have been compared with the in‐house laser Doppler anemometry (LDA) experimental data. In addition, recently proposed modifications to the standard k ? ε models were also evaluated. The model predictions (of all the mean velocities, turbulent kinetic energy and its dissipation rate) have been compared with the experimental measurements at various locations in the tank. A discussion is presented to highlight strengths and weaknesses of currently used CFD models. A preliminary analysis of sensitivity of modelling assumptions in the k ? ε models and RSM has been carried out using LES database. The quantitative comparison of exact and modelled turbulence production, transport and dissipation terms has highlighted the reasons behind the partial success of various modifications of standard k ? ε model as well as RSM. The volume integral of predicted energy dissipation rate is compared with the energy input rate. Based on these results, suggestions have been made for the future work in this area.     

Particle deposition in turbulent duct flows—comparisons of different model predictions

Numerical studies of transport and deposition of nano- and micro-particles in turbulence flow field have been studied in the past few decades. In most current industrial applications, Reynolds averaged turbulence models were used due to its relative simplicity and computational efficiency. In this work, a series of numerical simulations were conducted to study the transport and deposition of nano- and micro-particles in a turbulent duct flow using different turbulence models. Commercial software (FLUENT^TM 6.1.22) was used for turbulence mean flow simulation. Simulations of the instantaneous turbulence fluctuation with and without turbulence near wall correction, and particle trajectory analysis were performed with the in-house PARTICLE (object-oriented C++) code, as well as with FLUENT^TM code with and the use of user's defined subroutines. The simulation results for different cases were compared with the available experimental data, and the accuracy of various approaches was evaluated. In addition, the importance of turbulence model, boundary conditions, and turbulence fluctuation particularly near wall on particle transport and deposition were carefully evaluated. It was shown that when sufficient care was given to the modeling effort, the particle deposition rates could be predicted with reasonable accuracy. The presented results could provide guidelines for selecting appropriate procedure for simulating nano- and micro-particle transport and deposition in various applications.     

Comparison of several turbulence models for predicting flow patterns within confined jets

Numerical solutions of the differential equations governing isothermal and nonisothermal confined jets have been obtained for three different models of the turbulence correlation terms. The models used were 1) mixing length model 2) k-l model 3) k-e model A comparison of the predictions with experimental data taken in a confined jet system indicates that in the absence of a measured distribution of the kinetic energy of turbulence at the entrance section the mixing length model provided the best results for the isothermal case. For the nonisothermal jets, however, it was found that the k-e model gives better prediction than the other two models in spite of the lack of measured distributions for the kinetic energy of turbulence at the entrance.     

Assessment of large eddy and RANS stirred tank simulations by means of LDA

Large eddy simulations (LES) and Reynolds-averaged Navier-Stokes (RANS) calculations were performed on the flow in a baffled stirred tank, driven by a Rushton turbine at Re=7300. The LES methodology provides detailed flow information as velocity fluctuations are resolved down to the scale of the numerical grid. The Smagorinsky and Voke subgrid-scale models used in the LES were embedded in a numerical lattice-Boltzmann scheme for discretizing the Navier-Stokes equations, and an adaptive force-field technique was used for modeling the geometry. The uniform, cubic computational grid had a size of 240³ grid nodes. The RANS calculations were performed using the computational fluid dynamics code CFX 5.5.1. A transient sliding mesh procedure was applied in combination with the shear-stress-transport (SST) turbulence closure model. The mesh used for the RANS calculation consisted of 241464 nodes and 228096 elements (hexahedrons). Phase-averaged and phase-resolved flow field data, as well as turbulence characteristics, based on the LES and RANS results, are compared both mutually and with a single set of experimental data.