On the mechanisms of secondary flows in a gas vortex unit

The hydrodynamics of secondary flow phenomena in a disc‐shaped gas vortex unit (GVU) is investigated using experimentally validated numerical simulations. The simulation using ANSYS FLUENT^® v.14a reveals the development of a backflow region along the core of the central gas exhaust, and of a counterflow multivortex region in the bulk of the disc part of the unit. Under the tested conditions, the GVU flow is found to be highly spiraling in nature. Secondary flow phenomena develop as swirl becomes stronger. The backflow region develops first via the swirl‐decay mechanism in the exhaust line. Near‐wall jet formation in the boundary layers near the GVU end‐walls eventually results in flow reversal in the bulk of the unit. When the jets grow stronger the counterflow becomes multivortex. The simulation results are validated with experimental data obtained from Stereoscopic Particle Image Velocimetry and surface oil visualization measurements. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 1859–1873, 2018     

Mixing and micromixing times in the forced vortex region of unbaffled mixing devices

Unbaffied mixing devices are characterized by a very particular hydrodynamics. A well defined depression in the liquid surface is formed, resulting from the combined vortex established in the liquid. The combined vortex is composed of one central Forced Vortex Region (FVR) and one annular free vortex region. Because the FVR behaves like a confined mixing zone, this region inside an unbaffled mixing device appears to be quite interesting to carry out certain chemical reactions. In this paper the influence of operating conditions on the mixing time of two reagent feed streams arriving continuously in the FVR of unbaffled mixing devices is investigated. Process parameters investigated are?: stirrer speed, reagent flow rates and feed pipe position. Mixing time correlations are established, which are useful for the scale‐up of this reactor.     

Continuous flow characterization of solid biomass in a reciprocating/rotating scraper tube: An experimental study

The performance of reciprocating/rotating scrapers has been assessed in a visualization study of the continuous flow hydrodynamics of air‐fluidized solid biomass under varying conditions of air flow rate and scraping velocities. A combination of low air flow rates and high scraping velocities results in more uniform flow of both types of biomass investigated. Power consumed by the reciprocating action of the scrapers increases with the scraping velocity but typically represents no more than 20% of the overall power consumption at the highest air flow rate applied. We also demonstrate that rotation of the scrapers superimposed on their reciprocating action gives higher flow rate of biomass and better mixing within the bulk solid compared to reciprocating action alone. The application of the reciprocating/rotating scraper technology described in this study represents a viable step forward in developing a continuous, large‐scale process for the microwave‐assisted decomposition of solid biomass to produce bio‐oils. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3732–3738, 2014     

Mixing study in an inductive plasma reactor: Comparison between model calculations and experimental results

This study compares the predictions of a multicomponent reactive model of inductive plasma reactor mixing and experimental data obtained with the enthalpy probe technique. Central injection of two different gases, helium and nitrogen, into the core of an inductive (h.f.) plasma discharge permits us to analyze the influence of the nature of the central gas on the reactor's mixing patterns. This work highlights diffusion in a multicomponent situation. The model considers the dissociation of H₂, present in the sheath gas.     

An experimental and numerical study on fuel atomization characteristics of high-pressure diesel injection sprays

An experimental and numerical study was performed to investigate the macroscopic spray structure and the spray characteristics of common rail type high-pressure injector for the diesel engine. The global spray structure and microscopic characteristics of high-pressure diesel injector were investigated when fuel was injected at various injection pressures. Spray developing process and spray tip penetration were obtained by particle motion analysis system, and the quantitative spray characteristics such as Sauter mean diameter (SMD) and axial mean velocity were measured by using the phase Doppler particle analyzer. To consider the difference between primary and secondary breakup, KH–RT hybrid model, in which it is assumed that Kelvin–Helmholtz and Rayleigh–Taylor instability cause primary and secondary breakup, respectively, is used. After optimizing the breakup constants based on the axial distribution of SMD, the comparisons between predicted and experimental results were conducted.     

An experimental and computational study of the vortex shape in a partially baffled agitated vessel

The vortex shape in a non-standard partially baffled agitated vessel in the form of a glass-lined, under-baffled stirred vessel has been investigated using both experimental and numerical approaches for an air/water system for different rotation speeds of the agitator. A simple and flexible experimental strategy was developed for determination of the time-averaged location of the unstable free surface using a process involving superimposition of images. CFD simulations were made to predict the vortex shape by using an Eulerian-Eulerian multiphase model coupled with a homogenous turbulence model. The simplifying assumptions of a constant bubble size, a constant drag coefficient and use of the k-ε turbulence model were made. An assessment of the capability of the numerical method to predict the vortex shape was carried out through comparison between experimental data and numerical results. Considering for comparison purposes a water isosurface volume fraction equal to 0.9, to account for the existence of air/water mixture present at the interface in the experiments, instead of the classical value of 0.5, gave very good agreement with the experimental data.     

A numerical study of pellet model consistency with respect to molar and mass average velocities,pressure gradients and porosity models for methanol synthesis process: Effects of flux models on reactor performance

The objective of this work is to compare mass- and mole based diffusion flux models, convection, fluid velocity and pore structure models for methanol synthesis process. Steady-state models have been derived and solved using least-squares spectral method (LSM) to describe the evolution of species composition, pressure, velocity, total concentration and diffusion fluxes in porous pellets for methanol synthesis. Mass diffusion fluxes are described according to the rigorous Maxwell Stefan model, dusty gas model and the more simple Wilke model. These fluxes are defined with respect to molar- and mass averaged velocities. The different effects of choosing the random- and parallel pore models have been investigated. The effects of Knudsen diffusion have been investigated. The result varies significantly in the dusty gas model. The effectiveness factors have been calculated for the methanol synthesis process for both mass- and mole based pellet models. The values of effectiveness factors for both mass- and mole based pellet models do not vary so much. The effect of Wilke-, Maxwell–Stefan- and dusty gas mass diffusion fluxes on the reactor performance have been studied. Steady-state heterogeneous fixed bed reactor model is derived where the intra-particle mass diffusion fluxes in the voids of the pellet are described by Wilke-, Maxwell–Stefan- and dusty gas models. Furthermore, the total computational efficiency of the heterogeneous fixed bed reactor model is calculated with several closures for the intra-particle mass diffusion fluxes. The model evaluations revealed that:

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The mass- and mole based pellet models are not completely consistent. However, the small deviation (less than 2%) between mass- and mole based pellet models is due to the model equations are not fully consistent. If one pellet model is to be chosen for the methanol synthesis process, the optimal diffusion flux model is the Maxwell–Stefan model.     

Experimental and numerical investigation of the reaction of 2,4-thiazolidinedione and p-methoxybenzaldehyde in microreactors for the production of drugs for diabetes mellitus type 2 treatment

The use of microreactors (MRs) in chemical and pharmaceutical industries allows for a series of advantages due to their reduced sizes regarding conventional batch reactors. In the present paper, the transposition of the reaction between 2,4-thiazolidinedione (TZD) with p-methoxybenzaldehyde from batch to a continuous capillary MR was carried out. The microdevice performance was evaluated experimentally and numerically by computational fluid dynamics (CFD). The batch process yielded 92% in 480 min using piperidine for equimolar reactant feed, while the pyrrolidine promoted a 100% yield in a 50 min, both using solvent ethanol. Kinetic and thermodynamic parameters of the synthesis using piperidine and pyrrolidine were also obtained from experimental data. In the transposition to flow chemistry, ethanol was also used as solvent and a product yield of 100% (140°C, pyrrolidine) was obtained for a residence time of 20 min, representing a reduction of 24 times in the reaction time. In the numerical simulations by CFD, two mathematical models were elaborated: a transient batch and a steady-state continuous flow. Both models exhibited good agreement with experimental data. The average relative deviations of TZD conversion and the reaction yields in MRs were, respectively, 0.23% and −7.1% (78°C) and 1.7% and 1.2% (140°C).     

Solubility and mass transfer of H2, CH4, and their mixtures in vacuum gas oil: An experimental and modeling study

In this work, the solubility data and liquid-phase mass transfer coefficients of hydrogen (H₂), methane (CH₄) and their mixtures in vacuum gas oil (VGO) at temperatures (353.15-453.15 K) and pressures (1-7 MPa) were measured, which are necessary for catalytic cracking process simulation and design. The solubility of H₂ and CH₄ in VGO increases with the increase of pressure, but decreases with the increase of temperature. Henry's constants of H₂ and CH₄ follow the relation of ln H=-413.05/T + 5.27 and ln H=-990.67/T + 5.87, respectively. The molar fractions of H₂ and system pressures at different equilibrium time were measured to estimate the liquid-phase mass transfer coefficients. The results showed that with the increase of pressure, the liquid-phase mass transfer coefficients increase. Furthermore, the solubility of H₂ and CH₄ in VGO was predicted by the predictive COSMO-RS model, and the predicted values agree well with experimental data. In addition, the gas-liquid equilibrium (GLE) for H₂ + CH₄ + VGO system at different feeding gas ratios in volume fraction (i.e., H₂ 85% + CH₄ 15% and H₂ 90% + CH₄ 10%) was measured. The selectivity of H₂ to CH₄ predicted by the COSMO-RS model agrees well with experimental data. This work provides the basic thermodynamic and dynamic data for fuel oil catalytic cracking processes.     

联合两段氧化制合成气/F-T合成的GTL工艺和催化剂

A novel process for catalytic oxidation of methane to synthesis gas (syngas),which consists of two consecutive fixed-bed reactors with air introduced into the reactors,integrated Fischer-Tropsch synthesis,was investigated.At the Same time,a catalytic combustion technology has been investigated for utilizing the F-T offgas to generate heat or powr energy.The results show that the two-stage fixed reactor process keep away from explosion of CH4/O2.The integrated process is fitted to produce diesel oil and lubricating oil in remote gas field.     

Pore‐size evaluation and gas transport behaviors of microporous membranes: An experimental and theoretical study

A modified gas‐translation (GT) model based on a GT mechanism was successfully applied to the pore‐size evaluation and gas transport behavior analysis of microporous membranes with different pore‐size distributions. Based on the gas permeation results of three microporous membranes derived from different alkoxides, the effects of activation energy and the selection of a standard gas on the pore‐size evaluation were discussed in a comparative study. The presence of nano‐sized defects had an important influence on the gas permeation performance of microporous membranes, depending largely on the original pore size of the membrane in question. Moreover, the gas‐separation effect of the pore‐size distribution in a silica membrane was theoretically studied and revealed a significant increase in gas permeance for relatively large gas species but not for small ones. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2268–2279, 2015     

An experimental, numerical and analytical investigation of gas flow characteristics in concrete

A series of tests are reported which used existing pressure cell apparatus to measure nitrogen gas flow in concrete. A finite difference numerical model is described which is used to investigate the time to reach steady state and the long-term transient response of the experimental system. An analytical solution is derived for the cell-pressure-time function from which a formula is derived for the intrinsic permeability coefficient. The permeabilities calculated from the experimental data have variations consistent with those from other investigations. The analytical solution matches the experimental pressure-time decay curves closely in the range of interest but there is a tendency for the curves to diverge for lower cell pressures. This is attributed to gas slippage and modifications to both the numerical and analytical solutions are described to account for this factor. Revised formulae are derived which give both the intrinsic permeability and Klinkenberg factor. These are then applied to the present experimental data and results compared with those from the literature.     

Monitoring concept for structural integration of PZT-fiber arrays in metal sheets: a numerical and experimental study

ABSTRACT

The technique joining by forming allows the structural integration of piezoceramic fibers into locally microstructured metal sheets without any elastic interlayers. A high-volume production of the joining partners causes in statistical deviations from the nominal dimensions. A numerical simulation on geometric process sensitivity shows that the deviations have a high significant influence on the resulting fiber stresses after the joining by forming operation and demonstrate the necessity of a monitoring concept. On this basis, the electromechanical behavior of piezoceramic array transducers is investigated experimentally before, during and after the joining process. The piezoceramic array transducer consists of an arrangement of five electrical interconnected piezoceramic fibers. The findings show that the impedance spectrum depends on the fiber stresses and can be used for in-process monitoring during the joining process. Based on the impedance values the preload state of the interconnected piezoceramic fibers can be specifically controlled and a fiber overload.     

液相中气泡上升行为与界面传质:实验研究与数值计算

在工业生产过程中，气泡在液相中的上升行为及气液界面的传质行为极为常见。本文针对不同条件下气泡上升过程的实验研究方法以及数值计算方法进行了总结。从实验与数值计算的角度，综述了单气泡上升过程的影响因素、多气泡上升过程聚并与破裂的现象和机理以及工业装置中气液两相流型和气泡特性，并对传质模型进行了归纳，主要关注了气侧-界面传质模型的研究现状。综述结果表明：当前对于单气泡上升行为的研究较为充分，而对于多气泡的行为机理的研究尚需深入。此外，受到研究手段的限制，进行气侧-界面传质模型研究具有一定挑战性。针对当前的相关研究进展和存在的问题，对今后气泡上升行为和传质行为的研究提出以下建议，即开展气泡聚并与破裂可控性研究，强化对气侧-界面传质过程的研究，包括泡内流体行为可视化研究和相关传质模型的建立。     

富氧燃烧中气体辐射模型对燃烧与换热数值模拟的影响

富氧燃烧过程中,由于使用再循环烟气代替空气中N₂作为稀释剂,烟气中存在大量CO₂和H₂O。CO₂和H₂O作为非极性三原子分子,具有N₂没有的辐射能力,导致富氧燃烧中气体辐射特性发生变化。在数值模拟过程中,气体辐射模型是一个重要的子模型。前人提出多种修改后适用于富氧燃烧的气体辐射模型,但不同气体辐射模型在不同富氧燃烧工况数值模拟中的影响尚未有统一研究。为了研究不同炉型下,气体燃烧和煤粉燃烧中气体辐射模型对燃烧换热模拟结果的影响,通过编程,将一种考虑CO影响的气体辐射模型以及文献中的6种典型气体辐射模型耦合入数值模拟计算。结果表明,在气体富氧燃烧中,气体辐射模型影响了火焰结构。同时,燃烧温度分布有所变化,不同模型结果之间差别最高可到500 K。气体与壁面之间的辐射换热受到影响。气体辐射模型对炉膛中心火焰区域影响较大,而对非火焰区域影响较小。在煤粉富氧燃烧过程中,当有效辐射层厚度在0.3 m左右时,如在100 kW下行炉中,气体辐射模型对煤粉燃烧数值模拟结果几乎没有影响。这可能是由于颗粒辐射在辐射换热计算中占主导地位。而当有效辐射层厚度在16 m左右时,如1000 MW塔式炉中,气体辐射模型对炉内切圆燃烧火焰温度以及组分浓度影响较大,温度差别可到100 K左右。而气体辐射模型对炉膛中心模拟结果没有影响。     

Visual study on the characteristics of liquid and droplet in a novel rotor-stator reactor

Rotor–stator reactor (RSR), an efficient mass transfer enhancer, has been applied in many fields. However, the hydrodynamic characteristics of liquid flow in RSR are still a mystery despite they are fundamental for the mass transfer performance and processing capacity. In view of the above, this paper studies the liquid–liquid flow and liquid holdup in RSR under various conditions with a high-speed camera. The paper firstly demonstrates two flow patterns and liquid holdup patterns that we obtained from our experiment and then presents in succession a flow pattern and a liquid holdup criterion for the transition of film flow to filament flow and complete filling to incomplete filling. It is found that experimental parameters, including rotor–stator distance, rotational speed and volume flow rate exert great influence on the average droplet diameter and size distribution. Besides, by comparison and contrast, we also find that the experimental values match well with our previous predicted calculations of the average diameter, and the relation between the average diameter and the mean energy dissipation rate.     

An activity, stability and selectivity comparison of propioin synthesis by thiamine diphosphate-dependent enzymes in a solid/gas bioreactor

Enzymatic carboligation in a solid/gas bioreactor represents a new challenge in biotechnology. In this paper, the continuous gas-phase production of propioin from two propanal molecules by using thiamine diphosphate-dependent enzymes was studied. Two enzymes were used, namely benzaldehyde lyase (BAL) from Pseudomonas fluorescens and benzoylformate decarboxylase (BFD) from Pseudomonas putida. The enzymes are homologous and catalyze carboligase and carbolyase reactions in which no external cofactor regeneration is needed. The influence of water and substrate activity on the initial reaction rate and biocatalyst stability was investigated. An increase in water activity raised the initial reaction rates to the maximal values of 250 and 80 U g(-1) for BAL and BFD, respectively. The half-life showed the same trend with maximal values of 50 and 78 min for BAL and BFD, respectively. The increase in the half-life by increasing water activity was unexpected. It was also observed that BFD is more stable than BAL in the presence of the substrate propanal. Both enzymes showed substrate inhibition in the kinetic studies, and BAL was also deactivated during the reaction. Unexpectedly, the stereoselectivity of both enzymes (ee of 19 % for BAL and racemic mixture for BFD) was significantly impaired in the gas phase compared to the liquid phase.     

Numerical and experimental study on the effect of solid particle sphericity on cyclone pressure drop

The continuous flow inside cyclone separator is usually simulated by solving the Reynolds averaged Navier–Stokes equations in Eulerian reference frame whereas the dispersed phase is modeled using Lagrangian approach. Although these methods have had a remarkable success, more advanced ideas are needed to model particulate phase in cyclones, especially the non-spherical shaped particles. Numerical simulation is verified with experimental results for the gas-solid flow in a cyclone separator. Reynolds Averaged Navier–Stokes equations (RANS) employing the RNG-based k–ε turbulence model are used to simulate the gas phase. 3-D particle tracking procedure is used for the solid phase. Three different equations for the drag coefficient are utilized in the numerical modeling to acquire more understanding of the behavior of non-spherical particles in cyclones. Computations resulted in the difference of pressure between the inlet and exit of the cyclone, and results are compared with experimental data. Experiments included measuring the separation efficiency of different shapes and sizes of particles. The results indicate that the CFD simulation can effectively reveal the pressure drop behavior as well as separation efficiency of gas-non-spherical particle flow in cyclone.