Quality of CFD models for jet flow analysis for the design of burners and boilers

CFD models are increasingly used for the design and optimisation of boiler combustion chambers. Numerous commercial codes are available, and the user is confronted with making a proper choice for a particular application. In this paper, the accuracy and effectiveness of the popular code FLUENT™ is investigated in terms of the different turbulence models and numerical schemes that are bundled in the software. The tests are performed for different simple experiments, involving classical hydrodynamic conditions with no combustion. The conclusion of these tests involves also the additional criterion of the computational time required for achieving a reasonable accuracy.     

Turbulence modelling of multiphase flow in high-pressure trickle-bed reactors

Computational fluid dynamics (CFD) has been used as a successful tool for single-phase reactors. However, fixed-bed reactors design depends overly in empirical correlations for the prediction of heat and mass transfer phenomena. Therefore, the aim of this work is to present the application of CFD to the simulation of three-dimensional interstitial flow in a multiphase reactor. A case study comprising a high-pressure trickle-bed reactor (30 bar) was modelled by means of an Euler-Euler CFD model. The numerical simulations were evaluated quantitatively by experimental data from the literature. During grid optimization and validation, the effects of mesh size, time step and convergence criteria were evaluated plotting the hydrodynamic predictions as a function of liquid flow rate. Among the discretization methods for the momentum equation, a monotonic upwind scheme for conservation laws was found to give better computed results for either liquid holdup or two-phase pressure drop since it reduces effectively the numerical dispersion in convective terms of transport equation.After the parametric optimization of numerical solution parameters, four RANS multiphase turbulence models were investigated in the whole range of simulated gas and liquid flow rates. During RANS turbulence modelling, standard k-ε dispersed turbulence model gave the better compromise between computer expense and numerical accuracy in comparison with both realizable, renormalization group and Reynolds stress based models. Finally, several computational runs were performed at different temperatures for the evaluation of either axial averaged velocity and turbulent kinetic energy profiles for gas and liquid phases. Flow disequilibrium and strong heterogeneities detected along the packed bed demonstrated liquid distribution issues with slighter impact at high temperatures.     

Development of a multi-scale simulation method for design of novel multiphase reactors

In this paper a multi-scale simulation method for modelling dispersions in a novel multiphase reactor is presented. This novel reactor is a continuous reactor which consists of repeated identical small mixing elements. The reactor is excellent for studying the effect of turbulence on drop size distributions since turbulence is continuously produced and dissipated along the reactor. Furthermore the energy dissipation within each element is very homogeneous. In addition it allows optical access at all positions along the reactor.Simulations were performed for a wide range of turbulence intensities for different dispersed phase hold-up. Each simulation was validated with measurements of the size distribution along the reactor. Good quantitative agreement was obtained at low hold-up in terms of prediction of the breakage rates and prediction of the size distributions. At higher hold-up the model gave reasonable predictions at low turbulence intensity however too large drops were predicted at high turbulence intensity. This can be a result of turbulence modulation and shows that reliable turbulence models for multiphase flows are necessary in this simulation method. The results show that physical models describing breakup and coalescence combined with CFD provide a good tool for efficient development and optimisation of novel multiphase reactors.     

CFD simulation of mixing in tall gas-liquid stirred vessel: Role of local flow patterns

In this work, we have used the computational fluid dynamics (CFD)-based models to investigate the gas-liquid flows generated by three down-pumping pitched blade turbines. A two-fluid model along with the standard k-ε turbulence model was used to simulate the dispersed gas-liquid flow in a stirred vessel. Appropriate drag corrections to account for bulk turbulence [Khopkar and Ranade, 2005. CFD simulation of gas-liquid flow in a stirred vessel: VC, S33 and L33 flow regimes. A.I.Ch.E. Journal, accepted for publication] were developed to correctly simulate different flow regimes. The computational snapshot approach was used to simulate impeller rotation and was implemented in the commercial CFD code, FLUENT4.5 (of Fluent. Inc., USA). The computational model has successfully captured the flow regimes as observed during experiments. The particle trajectory simulations were then carried out to examine the influence of the different flow regimes on the circulation time distribution. The model predictions were verified by comparing the predicted results with the experimental data of [Shewale and Pandit, 2006. Studies in multiple impeller agitated gas-liquid contactors. Chemical Engineering Science 61, 489-504]. The computational model and results discussed in this study would be useful for explaining the implications local flow patterns on the mixing process and extending the applications of CFD models for simulating large multiphase stirred reactors.     

A hybrid CFD—reaction engineering framework for multiphase reactor modelling: basic concept and application to bubble column reactors

The coupling of turbulent mixing and chemical phenomena lies at the heart of multiphase reaction engineering, but direct CFD approaches are usually confronted with excessive computational demands. In this hybrid approach, the quantification of mixing is accomplished through averaging the flow and concentration profiles resulting from a CFD flow field calculation and a computational (“virtual”) tracer experiment. Based on these results, we construct a mapping of the CFD grid into a generalised compartmental model where the chemistry calculations can be efficiently carried out. In contrast to the empirical models used in the residence time distribution (RTD) approach, the compartmental model in this methodology, owning to its CFD origins, retains the essential features of the equipment geometry and flow field. A procedure for extracting the mixing information from k-ε based CFD codes is outlined, but the main concept of the approach is not restricted to any particular type of turbulence modelling, and will therefore benefit from future developments. A phenomenological model of mass transfer and chemical reaction, based on the penetration theory, is employed to simulate the interfacial phenomena in gas-liquid reactors, and a study of CO₂ absorption into alkali solution is presented to demonstrate the method.     

The influence of an anisotropic Langevin dispersion model on the prediction of micro- and nanoparticle deposition in wall-bounded turbulent flows

This paper deals with the issues of stochastic dispersion models and associated best practice responses for the investigation of micro- and nanoparticle deposition in turbulent flows. For such applications, Reynolds averaged turbulence models are widely used in combination with particle Lagrangian tracking, due to their relative simplicity and computational efficiency. Such approaches imply to generate the instantaneous velocity of the fluid at particle location to reproduce the effect of turbulence on particle transport. The default dispersion model used in most CFD codes is an eddy lifetime model, which frequently overestimates the deposition rates. In this work, a simple method is proposed to implement a three-dimensional stochastic dispersion model based on the Langevin equation in the Fluent^® commercial code. Comparisons are provided between this model, complemented by the simulation of Brownian effects, and available numerical data obtained using either an eddy lifetime model or a simple Langevin model. Computations are carried out in horizontal and vertical channel flows and in circular pipe flows as well. The use of the proposed anisotropic Langevin model is shown to improve the accuracy of deposition prediction in the whole range of particle inertia.     

Experimental and numerical study of stratified viscous oil–water flow

Stratified two-phase flows of oil and water are important to the energy industry, and models capable of predicting this type of flow are primordial. Many studies focus on fluids with low viscosity, but a high viscosity oil in the mixture significantly changes its behavior. We gathered experimental data of pressure drop, volumetric fractions, and flow-pattern data of a stratified liquid–liquid flow with high viscosity ratio. In addition, a wire-mesh sensor provided tomographic views of the flow. The data were compared with computational fluid dynamics (CFD) models using OpenFOAM and a one-dimensional model. CFD simulations used an interface capturing method, and turbulence damping was introduced to avoid high eddy viscosity at the interface region. Reynolds Average Navier–Stokes and large eddy simulations were used to account for turbulence, and they showed significant differences. The comparisons showed good overall results for pressure drop, volumetric fractions, and phase distributions between CFD and experiments.     

Computational fluid dynamic simulations of coal-fired utility boilers: An engineering tool

The objective of this study was to develop an engineering tool by which the combustion behavior of coals in coal-fired utility boilers can be predicted. We presented in this paper that computational fluid dynamic (CFD) codes can successfully predict performance of- and emission from- full-scale pulverized-coal utility boilers of various types, provided that the model parameters required for the simulation are properly chosen and validated. For that purpose we developed a methodology combining measurements in a 50 kW pilot-scale test facility with CFD simulations using the same CFD code configured for both test and full-scale furnaces. In this method model parameters of the coal processes are extracted and validated. This paper presents the importance of the validation of the model parameters which are used in CFD codes. Our results show very good fit of CFD simulations with various parameters measured in a test furnace and several types of utility boilers. The results of this study demonstrate the viability of the present methodology as an effective tool for optimization coal burning in full-scale utility boilers.     

Development of a CFD Model of Bubble Column Bioreactors: Part Two – Comparison of Experimental Data and CFD Predictions

The application of computational fluid dynamics (CFD) as a tool to simulate bubble column bioreactors is investigated. A three‐dimensional model utilizing the Euler‐Euler approach is evaluated. The role of various terms, i.e., lift, drag, bubble‐induced turbulence, and volume fraction correction terms for drag, is determined. Good agreement between experimental data and simulation results was obtained by means of a single‐bubble size model provided that bubble‐induced turbulence and the reduction in drag due to the presence of other bubbles were taken into account.     

开孔阳极铝电解槽熔体中气液两相流数值模拟

引言目前的电解铝生产工艺以氧化铝为原料,在直流电的作用下,熔融电解质中的氧化铝在高温环境下与碳阳极发生复杂的电化学反应,主要在阳极底掌析出二氧化碳[1]。二氧化碳气体从阳极底掌产生到逸出电解质表面的过程中,一方面会不断搅动电解质熔体,阳极气泡与电解质相互发生复杂的传热传质作用,有利于氧化铝颗粒在高温熔体中均匀快速地溶解和扩散运动;另一方面,由于     

Particle capture by a rotating disk in a kitchen exhaust hood

Abstract

Chinese cooking produces large numbers of particles that can cause both indoor and outdoor air quality problems. To reduce the extraction of particles to the outdoor air, this investigation studied capture efficiency of a rotating disk in an exhaust hood. The studies were performed experimentally in a wind tunnel and numerically by computational fluid dynamics (CFD) models with the Lagrangian method for tracking particle trajectories. The experimental data were used to identify the best turbulence model among the three tested in the CFD simulations. The results show that the capture efficiency increased with disk rotation speed and particle size but decreased with exhaust airflow rate. The CFD simulations provided detailed information about the mechanisms by which particles of different sizes were captured by the rotating disk. CFD was then used to explore two methods for improving the capture efficiency: adding more wires to the middle and outer zones of the disk, and using two layers of disks. Both methods can increase the capture efficiency of the rotating disk at an acceptable pressure loss.

Copyright © 2020 American Association for Aerosol Research     

CFD modeling of gas entrainment in stirred tank systems

In the present work, CFD modeling was used to study the phenomenon of gas entrainment in stirred tank systems. Two types of impellers (DT, PBTD) were simulated. VOF method was used as surface tracking technique along with LES model to study interfacial behavior at the onset of gas entrainment. Simulations were performed to study cause of entrainment and underlying interfacial mechanism at the location of entrainment. CFD simulations clearly showed differences in onset and non onset conditions in terms of the magnitudes of interfacial turbulence. As per the predictions, phenomenon of surface aeration in stirred tank systems was characterized by exchange of momentum across the interface from water side to air side. Magnitudes of instantaneous axial velocities on air side, strain rates on air side and vorticities on air side exhibited a threshold at the onset of entrainment and reduced substantially after the onset.     

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.     

CFD modeling of pilot-scale pump-mixer: Single-phase head and power characteristics

The present work involves single-phase computational fluid dynamics (CFD) simulations of continuous flow pump-mixer employing top-shrouded Rushton turbines with trapezoidal blades. Baffle—impeller interaction has been modeled using sliding mesh and multiple reference frame approaches. Standard k-ε model has been used for turbulence modeling. Several CFD runs representing different combinations of geometric and process parameters have been carried out. Results of CFD simulations have been used to find out two macroscopic performance parameters of pump-mixer—power consumption and head generated by the impeller. The simulation results have been compared with the experimental data obtained on a pilot-scale setup. Good agreement between CFD predictions and experimental results is observed. In most cases, sliding mesh approach is found to perform better than multiple reference frame approach. Details from CFD simulations have been used to have an insight into the pumping action of the impeller.     

Assessing floc strength using CFD to improve organics removal

Floc characteristics play a major role in the removal of contaminants from water in physico-chemical treatment processes. The efficiency of the main removal processes is a function of floc size, strength and density. Changes in these parameters affect floc removal and hence the removal of adsorbed organic matter. Coagulation and flocculation efficiency and floc strength are often assessed using a jar tester. Here, CFD was used to model the flow field within a standard jar test apparatus and, using a Lagrangian particle trajectory model, to study the effects of turbulence on individual flocs. Combining numerical and experimental data, velocity gradient values at which floc breakage occurs are postulated for three different floc suspensions. Although the threshold values are determined using jar test and CFD data in combination, they are based on the flocs’ resistance to induced velocity gradients. This is a significant result, as previous breakage thresholds have been expressed in terms of mixing speed and cannot be applied at full scale. The results shown here can be adopted for use in other situations and can be used to assess the performance of existing flocculators or to design new installations.     

乙烯裂解炉耦合模拟中湍流模型的影响分析

配备底部烧嘴和侧壁烧嘴的乙烯裂解炉应用越来越广泛，不同燃烧模式影响着炉膛内湍流流动状态，考虑到裂解炉中湍流流动与燃气喷料、燃烧和传热有较强的非线性耦合作用，为此探究不同湍流模型在裂解炉/反应器耦合模拟中的影响对于裂解炉的精确设计和优化至关重要。针对不同湍流模型对某十万吨工业乙烯裂解炉进行了耦合模拟，利用CFD数值模拟对采用标准k-ε模型、RNG k-ε和Realizable k-ε模型所建立的湍流流动模型进行评估。将三种湍流模型的模拟结果与工业数据进行比较，重点分析了裂解炉内的速度、温度、湍流能力等参数的分布情况，表明Realizable k-ε模型在火焰稳定性、反应效率等方面优于其他两种模型，且基于Realizable k-ε湍流方程的反应管模型在热通量、炉管外壁温度分布计算结果更接近实际工况。     

Performance of stress-transport models in the prediction of particle-to-fluid heat transfer in packed beds

Computational fluid dynamics (CFD) as a simulation tool allows obtaining a more complete view of the fluid flow and heat transfer mechanisms in packed bed reactors, through the resolution of 3D Reynolds averaged transport equations, together with a turbulence model when needed. This tool allows obtaining mean velocity and temperature values as well as their fluctuations at any point of the bed. An important problem when a CFD modeling is performed for turbulent flow in a packed bed reactor is to decide which turbulence model is the most accurate for this situation. Turbulence models based on the assumption of a scalar eddy viscosity for computing the turbulence stresses, so-called eddy viscosity models (EVM), seem insufficient in this case due to the big flow complexity. The use of models based on transport equations for the turbulence stresses, so-called second order closure modeling or Reynolds stress modeling (RSM), could be a better option in this case, because these models capture more of the involved physics in this kind of flow.To gain insight into this subject, a comparison between the performance in flow and heat transfer estimation of RSM and EVM turbulence models was conducted in a packed bed by solving the 3D Reynolds averaged momentum and energy equations. Several setups were defined and then computed. Thus, the numerical pressure drop, velocity, and thermal fields within the bed were obtained. In order to judge the capabilities of these turbulence models, the Nusselt number (Nu) was computed from numerical data as well as the pressure drop. Then, they were compared with commonly used correlations for parameter estimations in packed bed reactors. The numerical results obtained show that RSM give similar results as EVM for the cases checked, but with a considerably larger computational effort. This fact suggests that for this application, even though the RSM goes further into the flow physics, this does not lead to a relevant improvement in parameter estimation when compared to the performance of EVM models used.     

Modeling hydrodynamic cavitation in venturi: influence of venturi configuration on inception and extent of cavitation

Hydrodynamic cavitation (HC) is useful for intensifying a wide variety of industrial applications including biofuel production, emulsion preparation, and wastewater treatment. Venturi is one of the most widely used devices for HC. Despite the wide spread use, the role and interactions among various design and operating parameters on generated cavitation is not yet adequately understood. This article presents results of computational investigation into the cavitation characteristics of different venturi designs over a range of operating conditions. Influence of the key geometric parameters such as the length of venturi throat and diffuser angle on the inception and extent of cavitation is discussed quantitatively. Formulation and solution of multiphase computational fluid dynamics (CFD) models are presented. Appropriate turbulence and cavitation models are selected and solved using a commercial CFD code. Care was taken to eliminate the influence of numerical parameters like mesh density, discretization scheme, and convergence criteria. The computational model was validated by comparing simulated results with three published data sets. The simulated results in terms of velocity and pressure gradients, vapor volume fractions and turbulence quantities, and so on, are critically analyzed and discussed. Diffuser angle was found to have a significant influence on cavitation inception and evolution. The length of the venturi throat has relatively less impact on cavitation inception and evolution compared to the diffuser angle. The models and simulated flow field were used to simulate detailed time–pressure histories for individual vapor cavities, including turbulent fluctuations. This in turn can be used to simulate cavity collapse and overall performance of HC device as a reactor. The presented results offer useful guidance to the designer of HC devices, identifying key operating and design parameters that can be manipulated to achieve the desired level of cavitational activity. The presented approach and results also offer a useful means to compare and to evaluate different designs of cavitation devices and operating parameters. © 2018 American Institute of Chemical Engineers AIChE J, 65: 421–433, 2019