CFD Studies of Pressure Drop and Increasing Capacity in MellapakPlus 752.Y Structured Packing

Packed columns equipped with structured packings are widely used in separation processes. In this study, the hydrodynamics of MellapakPlus 752.Y was investigated using a computational fluid dynamics (CFD) approach. This packing includes short smooth bends at both ends of each corrugated sheet. Two adjacent sheets of a whole packing module were considered as computational domain. The CFD results indicated that the gas phase should be simulated using a turbulent model for F factors higher than 0.8. Thus, various two‐equation turbulence models were evaluated for the gas phase in the CFD model. It was shown that the baseline k‐ω (BSL) model leads to a slightly improved prediction of the pressure drops compared with the experimental data. The effects of the bends on the structured packing were studied by the model. It was found that using bends in the packings is useful for increasing the capacity and decreasing the pressure drop of the systems.     

Experimental and Numerical Studies of the Flow Field in a Stirred Tank Equipped with Multiple Side‐Entering Agitators

The flow field and the macro‐mixing process in a stirred tank equipped with four side‐entering agitators were investigated experimentally and numerically. Experiments were conducted using two‐dimensional particle image velocimetry (PIV) measurements to characterize the flow field at different positions in the vessel. The computational fluid dynamics (CFD) simulation was performed by the software Fluent 6.3, using the standard k‐ϵ turbulent model and the multiple reference frame together with the sliding mesh technique. The macro‐mixing process was also discussed using CFD and decolorization experiments. The effects of the tracer detection positions and some mounting parameters in the mixing system were discussed. The results show that the mixing process was dominated by the flow field pattern in the stirred tank. According to the mixing times under different conditions using CFD simulation, the mounting parameters including inclination angle, plunging length and mounting height of the shaft were optimized.     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

CFD simulations of hydrodynamic/thermal coupling phenomena in a bubble column with internals

CFD simulations have been carried out in a full three‐dimensional, unsteady, Eulerian framework to simulate hydrodynamic/thermal coupling in a bubble column with internals. A first part of the study, dedicated to the hydrodynamic/thermal coupling in liquid single‐phase flows, showed that assuming constant wall temperature on the internals constitutes a reasonable approximation in lieu of comprehensive simulations encompassing shell flow and coolant flow together. A second part dealing with the hydrodynamics of gas–liquid flows in a bubble column with internals showed that a RNG k–ε turbulence model formulation accounting for gas‐induced turbulence was a relevant choice. The last part used these conclusions to build a hydrodynamic/thermal coupling model of a gas–liquid flow in a bubble column with internals. With a per‐phase RNG k–ε turbulence model and assuming constant wall temperature, it was possible to simulate heat transfer phenomena consistent with experimentally measured heat transfer coefficients. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Heat transfer and flow pattern in co‐current downward steam condensation in vertical pipes‐II: Comparison with published work

The condensation of pure steam flowing downward inside a vertical tube has been extensively studied. Considerable amount of experimental and analytical efforts can be found due to the significance of this subject in practice. In this work, a critical review of the most important experimental, analytical and computational fluid dynamic (CFD) investigations have been presented. CFD simulations for the geometries of Goodykoontz and Dorsch [Goodykoontz and Dorsch, NASA TN D‐3326, 1966; Goodykoontz and Dorsch, NASA TN D‐3953, 1967], Kim and No [Kim and No, Int. J. Heat Mass Transf. 2000;43:4031–4042] and the present work have been performed and compared with the experimental data reported in these investigations. CFD predictions of the pressure drop and the heat transfer coefficient (HTC) were in close agreement with the experimental values. A preliminary regime map has been constructed for downward flow steam condensation inside pipes. Finally, all the published semi‐empirical correlations for the HTC have been critically analysed and compared with the CFD predictions. An attempt has been made to make specific recommendations. © 2012 Canadian Society for Chemical Engineering     

应用SST k-ω湍流模型计算Rushton搅拌釜流场

应用SSTkω-湍流模型对Rushton搅拌釜流场进行了计算,并与文献报道实验数据以及标准k-ε模型、RNGkω-模型预测结果进行了比较。结果表明SSTkω-模型所计算的排除流量准数与实验值相差较小,桨叶区速度场与实验数据吻合较好,略优于标准k-ε模型,同时也给出了近壁区的计算结果。在湍动动能的预报上,SSTk-ω模型值与标准k-ε模型比较接近,优于RNGkε-模型,但与实验值都还不同程度地存在着一些差异。     

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.     

Optimization of Mixing and Mass Transfer in In-line Multi-Jet Ozone Contactors Using Computational Fluid Dynamics

Typical ozone mixing and mass transfer calculations are lumped approaches based on ideal operating conditions and can misrepresent behavior in real-life installations. This article models the effect of local hydrodynamics and mixing on the overall mass transfer of ozone into water with the aid of multiphase computational fluid dynamics (CFD). CFD models were validated with measured data from a pipeline ozone contactor installation which was optimized for more rapid, uniform mixing and mass transfer. Results emphasize the sensitivity of mixing quality to nozzle placement, size, orientation and spacing relative to main pipeline diameter and flows.     

Studies on thermal responses of sprinkler heads in atrium buildings with fire field models

Sprinklers are commonly sited in the high headspace atrium of a building where such systems are required. It was questioned whether sprinkler heads would be activated in the case of a fire. A fire modelling technique was applied to estimate the actuation time of sprinkler heads located in the atrium ceiling. The response time of sprinkler heads to a fire in a shop at a high level adjacent to the atrium roof was calculated. A self‐developed FORTRAN code together with the CFD model CFX4.2 were used to simulate the atrium fire environment. The standard k‐ε model was used with two key parameters, diffusion coefficient and effective Prandtl number tuned. A comparison with some experiments reported in the literature was made. Satisfactory agreement between the predicted and measured results was found. Copyright © 2001 John Wiley & Sons, Ltd.     

Simulation of Radiant Syngas Coolers and Comparison with Various Arrangements of the Entrained‐Flow Gasifier

A 3D numerical model was developed for studying the multiphase flow and heat transfer process in a radiant syngas cooler (RSC). Realizable k‐? turbulent and discrete random walk models were adopted to simulate gas phase and particle phase flow fields, respectively. The surface temperature of the membrane wall was calculated by heat flux balance equations. The calculated temperature distribution was validated by comparing calculated values with measured data of an industrial RSC. Four different membrane wall arrangements of RSC, namely, ordinary membrane wall (OMW), partial division wall (PDW), annular division wall (ADW), and fin division wall (FDW), were designed for a specific condition.     

Development of an efficient CFD-simulation method to optimize the structure parameters of an airlift sonobioreactor

A two-dimensional CFD model was developed for optimizing the structure design of an airlift sonobioreactor for hairy root culture, and a height to diameter ratio of 4 was found suitable for hairy root culture because both oxygen interphase mass transfer rate and liquid flow rate were high in the whole culture process. The installation position of ultrasound transducers was optimized with the help of CFD simulation, the ultrasound transducers set at three layers around the bioreactor wall gave the highest TKE value, turbulent dissipation rate and liquid flow rate among all installation positions tested, which is positive for the dissolved oxygen mass transfer into the center of hairy root clumps. The optimized structure of the airlift sonobioreactor provided great mass transfer performance and sharply reduced the gradient of dissolved oxygen in root domain. The detection results of dissolved oxygen concentration in hairy root clumps before and after ultrasound stimulation proved that ultrasound could enhance the mass transfer of dissolved oxygen into root clumps, which is an advantage for hairy root growth and secondary metabolite accumulation.     

CFD Study of Effects of Module Geometry on Forced Convection in a Channel with Non‐Conducting Fins and Flow Pulsation

CFD simulations were carried out to investigate the effects of the module geometry on forced convection in a rectangular channel containing series of regularly spaced non‐conducting baffles with flow oscillation. The simulations were performed at constant wall temperature. Steady‐flow Reynolds numbers Re in the range of 200 and 600 were studied. The results of the CFD simulations show that, for the effect fin spacing to be significant on heat transfer enhancement in finned system with oscillating flow, the oscillating flow velocity must be higher than the mean flow velocity. Superposition of oscillation yields increasing heat transfer performance with increasing fin height. Fin geometry with pyramidal shape yields highest performance in terms of the heat transfer effectiveness.     

CFD Prediction of Flow and Homogenization in a Stirred Vessel: Part I Vessel with One and Two Impellers

A simulation of flow field and tracer homogenization was performed using the commercial CFD software FLUENT 6.1. The aim is to investigate the potential of CFD software to predict concentration distribution of added tracer in cylindrical vessels. The calculated results – dimensionless velocity profiles, power and pumping numbers, dimensionless concentration curves, and mixing times – were compared with experiments in stirred vessels. In Part I, the study was performed for vessels agitated by one or two impellers on a centric shaft. Two different impellers were used – a 6‐bladed 45° pitched blade turbine and a standard Rushton turbine. The standard k‐? turbulence model and multiple reference frames method were used for the simulations. The influence of the grid type was also investigated; three types of grid – a structured, unstructured and a special user‐defined grid – were studied.     

Efficiencies of Sieve Tray Distillation Columns by CFD Simulation

A 3‐D two‐fluid CFD model in the Eulerian‐Eulerian framework was developed to predict the hydrodynamics and heat and mass transfer of sieve trays. Interaction between the two phases occurs via interphase momentum and heat and mass transfer. The tray geometries are based on the large rectangular tray of Dribika and Biddulph and FRI commercial‐scale sieve tray of Yanagi and Sakata. In this work a CFD simulation is developed to give predictions of the fluid flow patterns, hydraulics, and mass transfer efficiency of distillation sieve trays including a downcomer. The main objective has been to find the extent to which CFD can be used as a design and prediction tool for real behavior, concentration and temperature distributions, and efficiencies of industrial trays. Despite the use of simple correlations for closure models, the efficiencies obtained are very close to experimental data. The results show that values of point efficiency vary with position on the tray because of variation of affecting parameters, such as velocities, temperature and concentration gradients, and interfacial area. The simulation results show that CFD can be used as a powerful tool in tray design and analysis, and can be considered as a new approach for efficiency calculations and as a new tool for testing mixing models in both phases. CFD can be used as a “virtual experiment” to simulate tray behavior under operating conditions.     

Modelling and simulation of trickle‐bed reactors using computational fluid dynamics: A state‐of‐the‐art review

Trickle‐bed reactors (TBRs), which accommodate the flow of gas and liquid phases through packed beds of catalysts, host a variety of gas–liquid–solid catalytic reactions, particularly in the petroleum/petrochemical industry. The multiphase flow hydrodynamics in TBRs are complex and directly affect the overall reactor performance in terms of reactant conversion and product yield and selectivity. Non‐ideal flow behaviours, such as flow maldistribution, channelling or partial catalyst wetting may significantly reduce the effectiveness of the reactor. However, conventional TBR modelling approaches cannot properly account for these non‐ideal behaviours owing to the complex coupling between fluid dynamics and chemical kinetics. Recent advances in the application of computational fluid dynamics (CFD) to three‐phase TBR systems have shown promise of achieving a deeper understanding of the interactions between multiphase fluid dynamics and chemical reactions. This study is intended to give a state‐of‐the‐art overview of the progress achieved in the field of CFD simulation of TBRs over the past two decades. The fundamental modelling framework of multiphase flow in TBRs, advances in important constitutive models, and the application of CFD models are discussed in detail. Directions for future research are suggested.     

CFD Study on the Local Mass Transfer Efficiency in the Gas Phase of Structured Packing

Visualization of local mass transfer coefficients over the dry surface of corrugated‐sheet structured packing is essential for optimizing the existing geometry of structured packing and for improving mass transfer efficiency to develop new structured packing. The local flow patterns between packing sheets and the gas‐phase mass transfer coefficient at each point over the surface are illustrated by employing a wall‐surface reaction model. Different turbulence models are utilized, i.e., a standard κ‐? model and three different low‐Re‐κ‐? models. The numerical calculation results with the Lam‐Bremhorst low‐Re‐κ‐? turbulence model is found to agree well with experimental data. There are three similar regions with enhanced mass transfer efficiency in each mass transfer unit cell of structured packing.     

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.     

下行床气固两相流动计算流体力学模拟

基于颗粒相动力学理论 ,对层流机制表达的气固两相流体力学模型采用Reynolds平均的方法获得气固两相流的湍流模型描述 .其中 ,气相湍流行为以k -ε模型描述 ;颗粒相的碰撞行为以颗粒流的动力学模型表达 ;而湍流行为以kp 模型描述 .因此建立的k -ε -Θ -kp 模型综合考虑了气相和颗粒相的湍流运动以及颗粒的碰撞行为 .依据此模型建立了三维流体力学求解程序并对下行床气固两相流动行为进行了模型预测 .讨论了恢复系数的选取及壁效应假设 ,从机理上分析并考察了 3种模型的预测能力 .针对内径 1 40mm、高 7m的下行床冷态设备 ,在较宽的操作范围内 ,对比了详细的颗粒浓度和速度径向分布以及轴向参数分布 ,并对下行床的放大行为进行了预测 .     

An integral criterion for turbulence suppression in swirling flows

A numerical study of flow in rotating pipes was conducted to elucidate the relative importance of convection and turbulence. CFD (Computational Fluid Dynamics) simulations of flow inside a rotating pipe (D = 2 cm and L/D = 20) were carried out, using the Reynolds Stress Model, for four different Reynolds numbers and a range of rotation numbers. The objective was to gain a deeper understanding of the interaction between fluid forces in swirling flows. This widely‐studied model problem was used to ascertain the conditions under which computationally cheaper turbulence models such as the k‐? model should be accurate. We identified a dimensionless rotation parameter that delineates the condition at which decreasing turbulence force equals increasing convective force as rotational speed increases. This dimensionless number establishes a criterion for knowing which forces are dominant, and thereby a rational basis for choosing turbulence models that are both cost‐effective and accurate. We found a universal, critical threshold that determines when convective forces dominate over turbulence forces. This threshold determination is based on an ‘integral measure criterion’ of local forces in the radial direction. The threshold itself is defined by a dimensionless rotation number, N, based on the ratio of the circumferential and axial flow velocities. The critical value was found to be N_cr = 0.45. Above this, convection dominates; below it, turbulence dominates. This finding will facilitate selection of CFD models to optimize cost and accuracy for modelling swirling flows. For example, k‐? models suffice when N_cr < 0.45, but more complex models are required for higher values.

     

Flow and heat transfer in serpentine channels

Serpentine channels are often used in microchannel reactors and heat exchangers. These channels offer better mixing, higher heat and mass‐transfer coefficients than straight channels. In the present work, flow and heat transfer experiments were carried out with a serpentine channel plate comprising of 10 units (single unit dimensions: 1 × 1.5 mm² in cross section, length 46.28 mm, D_h 1.2 mm) in series. Pressure drop and heat‐transfer coefficients were experimentally measured. Flow and heat transfer in the experimental set‐up were simulated using computational fluid dynamics (CFD) models to understand the mechanisms responsible for performance enhancement. The CFD methodology, thus, developed was applied to understand the effect of various geometrical parameters on heat transfer enhancement. A criterion was defined for evaluation of heat transfer performance (heat transfer per unit pumping power), thus, ensuring due considerations to required pumping power. The effect of geometrical parameters and the corresponding mechanisms contributing for enhancement are discussed briefly. Based on the results, a design map comprising different serpentine channels showing heat transfer enhancement with pumping power was developed for Reynolds number of 200 which will be useful for further work on flow and heat transfer in serpentine channels. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1814–1827, 2013