快速流化床颗粒团絮特征的MP-PIC数值模拟

为了研究快速流化床颗粒的团絮特征,建立了基于多相质点网格法的快速流化床气固多相流三维数理模型,气相场采用大涡湍流模型,通过求解牛顿运动方程得到颗粒相运动信息,气固间相互作用力采用Gidaspow曳力模型,固体间作用力通过计算颗粒应力梯度得到。基于该模型,对三维快速流化床上升管(H=3 m、d=0.1 m)气固流动开展了数值模拟,并与实验进行了校正,研究了在气速工况Ug=5.28 m?s-1下的颗粒(ρp=2650 kg?m-3、dp=250μm)团絮性质,实现了对上升管内颗粒团絮的基本类型(条形团絮、马鞍形团絮、U形团絮)的成功预测,并揭示了不同类型团絮在上升管内形成、发展、聚并直至破碎的演化规律。结果表明,上升管径向颗粒团絮的平均颗粒浓度分布呈现中间低两边高的环核结构,颗粒团絮速度的分布与其相反;随着轴向高度的增加,颗粒团絮的颗粒浓度逐渐降低而速度逐渐增加,但达到一定高度后变化减缓。

Co-firing of coal and biomass in oxy-fuel fluidized bed for CO2 capture: A review of recent advances

The co-firing of coal and biomass in oxy-fuel fluidized beds is one of the most promising technologies for capturing CO2.This technology has attracted wide attention from academia and industry in recent years as a negative emission method to capture CO2 produced by carbon contained in biomass.In the past decades,many studies have been carried out regarding experiments and numerical simulations under oxy-fuel combustion conditions.This paper firstly briefly discusses the techno-economic viability of the biomass and coal co-firing with oxycombustion and then presents a review of recent advancements involving experimental research and computational fluid dynamics(CFD)simulations in this field.Experimental studies on mechanism research,such as thermogravimetric analysis and tube furnace experiments,and fluidized bed experiments based on oxy-fuel fluidized beds with different sizes as well as the main findings,are summarized as a part of this review.It has been recognized that CFD is a useful approach for understanding the behaviors of the co-firing of coal and biomass in oxyfuel fluidized beds.We summarize a recent survey of published CFD research on oxy-fuel fluidized bed combustion,which categorized into Eulerian and Lagrangian methods.Finally,we discuss the challenges and interests for future research.

Seed-mediated synthesis of dendritic platinum nanostructures with high catalytic activity for aqueous-phase hydrogenation of acetophenone

This work reports a facile and efficient seed-mediated method for the synthesis of dendritic platinum （Pt） nanoparticles （NPs） at low temperatures of 55-60 ℃ in water, using L-ascorbic acid as a reducing agent and sodium citrate as a capping agent. It is found that the dendritic Pt NPs （10-150 nm） are composed of tiny Pt nanocrystals, which nucleate and grow through the introduced smaller Pt seeds with diameters of 3-5 nm. Further investigation shows that the dendritic Pt nanostructures display excellent catalytic performance in an aqueous-phase aromatic ketone hydrogenation reaction, including： （i） acetophenone conversion rate of 〉 90%, with smaller dendritic Pt NPs （10-46 nm） offering a higher conversion efficiency; （ii） high chemoselectivity toward carbonyl group （90.6%-91.5%）, e.g., the selectivity to l-phenylethanol is -90.1% with nearly 100% acetophenone conversion for 10 nm dendritic Pt NPs within 60 rain, under mild reaction conditions （20 ℃, 1.5 bar H2 pressure, and 1.5 tool% catalyst）. The high catalytic activity, selectivity and stability of the dendritic Pt nanostructures under the organic solvent-free conditions make them promising for many potential applications in green catalytic conversion of hydrophilic biomass derived compounds.

Tribological characteristics of dimpled surfaces filled with dopamine-modified MoS_(2)

To improve the tribological characteristics of dimples on the surface of 45 steel,the dimples were filled with MoS_(2)and MoS_(2)modified by dopamine(MoS_(2)@DA),and ball-disk friction and wear tests were conducted.Specifically,the dimple filling gap,abrasion depth,and surface cross-sectional area of 45 steel were measured.The wear morphology of the friction ball and exfoliation of MoS_(2)in the dimples and the bending characteristics of the specimens were studied.The surface friction coefficient of MoS_(2)@DA-filled specimen was 17.9%lower than MoS_(2)-filled specimen,and the dimple filling gap was 70.1%lower,the surface abrasion depth was 5.8%lower,and the abrasion cross-sectional area was 17.7%smaller.Moreover,the bending strength of the MoS_(2)@DA specimen was 3.27 times greater than that of the MoS_(2)specimen,and the exfoliation of MoS_(2)was slowed by filling with the MoS_(2)@DA.Finally,the tribological characteristics were also superior for the specimens prepared with MoS_(2)@DA.

过热蒸汽/热载体/生物质三相混合数值模拟

建立了工业尺度数值试验的过热蒸汽/热载体/生物质三相欧拉数理模型。水蒸气设为主连续相,采用k-ε方程描述;热载体和生物质设为两种不同的拟流体相,其黏度和压力采用颗粒动理论方程进行封闭;气固相互作用力采用Symbol-O'Brein曳力模型描述,固固之间作用力采用morsi-alexander模型描述;气固之间传热采用Gunn模型描述。基于该模型的数值试验,实现了喷动流化床混合器的优化设计使热载体与生物质的混合程度达70%以上;并研究了物料性质对生物质终温的影响,生物质终温受生物质粒径的影响最为显著。

基于XPTV的流化床异形颗粒分布特性研究

构建了X光颗粒示踪(X-ray particle tracking velocimetry,XPTV)测量系统,建立了示踪颗粒的制备方法以及示踪颗粒位置与姿态取向的测量和误差评估方法。基于以上方法获得了不同气速下圆柱颗粒位置和姿态取向的分布规律,并阐述了以上分布规律与流型之间的内在联系。结果表明,流化床处于鼓泡流化时圆柱颗粒会形成内循环,颗粒中心线与竖直方向多呈倾斜状态;而处于湍动流化时圆柱颗粒则会在全床内趋于均匀分布,颗粒中心线趋于竖直方向。

湿颗粒喷动床颗粒聚团特征的实验研究

建立了内径140 mm、锥角60的可视化锥柱喷动整床和半床实验系统,研究了液含量、颗粒粒径、形状等对床内湿颗粒聚团的影响规律。实验表明:湿颗粒喷动床内有块喷动、死床和非均匀结块三种聚团类型,聚团的出现类型与粒径dp有很大的关系;当dp>1 mm,液体饱和度S>0.2时,容易出现块喷动;当dp<1 mm,S>0.05时,容易出现死床、非均匀结块。在此基础上,从微观颗粒受力分析出发,提出了一种预测湿喷动床内聚团倾向的方法。

湿颗粒流化床流动特性的实验研究

基于可视化湿颗粒流化床实验系统,研究了多种Geldart-D类颗粒在不同含液量时的流动特性变化规律,包含流型、床层压降、最小流化速度。实验结果表明:1)在湿流化床内,颗粒出现聚团结块行为,气泡呈现不规则形状;湿颗粒流化过程的稳定性较干颗粒床明显下降,床内近壁面区域易出现局部沟流。2)固定床阶段,由于空隙率的增加,相同气速条件下,湿颗粒床层压降低于干颗粒;基于厄贡公式,得出了湿颗粒固定床床层压降的计算方法。3)湿颗粒的最小流化速度U_(mf)高于干颗粒,且随粒径d_p的增加而增加;导出了预测D类湿颗粒最小流化速度的半经验公式,Re_(mf)=0.279.Ar^(0.5),该式计算结果与实验测量结果偏差小于15%。

Synthesis of Li_4Ti_5O_(12) nanostructural anode materials with high charge–discharge capability

This study demonstrates a facile and efficient hydrothermal method to prepare spindle titanate(Li4Ti5O12 denoted as LTO)and/or carbon-LTO nanocomposites(CLTO),in which the LTO or C-LTO microspheres have diameters of a few micrometers,composed of numerous nanosheets with thickness of*30 nm and edge length of hundreds of nanometers.The morphology and size control of these nanoparticles could be achieved by varying experimental parameters including concentration of titanium butoxide,lithium hydroxide,and cetyltrimethylammonium bromide,as well as reaction temperature and time.These micro-nanostructures were characterized by several advanced techniques,such as transmission electron microscopy,scanning electron microscopy,X-ray diffraction,energy dispersive spectroscopic analysis,surface area,and electrochemical measurements.The LTO and C-LTO microstructures were examined in the charge–discharge capacity at a rate of 50 C,as well as the stability after 100 cycles at a rate of 10 C.The excellent capability may be attributed to good conductivity,large surface area,and stable assembly structure of such micro-nanostructures,which could be explored as a promising anode material for lithium-ion batteries.

CFD-DEM modelling of mixing and segregation of binary mixtures of ellipsoidal particles in liquid fluidizations

Solid-liquid fluidized beds of binary mixtures are widely used in many industries.Particle segregation may occur as particles can differ in size,density,or shape.Extensive studies have been conducted in the past to understand the effects of particlesize and density on the mixing and segregation,but the effect of particle shape has not been well addressed.Therefore,in the present work,CFD-DEM approach is employed to perform a numerical analysis of the effect of particle shape on the particle mixing and segregation phenomenon in liquid fluidization system.Different particle shapes from oblate to prolate are produced by varying aspect ratio of ellipsoids from 0.25 to 3,and eight binary mixtures of spheres and ellipsoids are examined.The results show that when oblate or prolate particles are added to spheres,the segregation takes place.The segregation degree increases with particle aspect ratio diverging from 1.0 and also liquid superficial velocity.The relationship of mixing index with aspect ratio under different liquid velocities is established,and a detailed explanation is given.It is revealed that increasing the projected area and hence the drag force results in the separation of ellipsoidal particles from spheres.

Effect of packing method on packing formation and the correlation between packing density and interparticle force

The packing of cohesive particles is of paramount importance in many industries because the packing structure is closely related to process performance.A general relation between packing density and interparticle force was previously proposed based on packing structures formed without dynamic fluid flows.Its universality is examined here in two different packings,formed in settling and defluidization of static and dynamic fluids,respectively.First,it is shown that the packings of the same particles formed by two different methods have different structures because of different impact-induced pressures.Nevertheless,a one-to-one relationship between packing density and structural properties still holds regardless of the different packing methods,and the force distribution in those packings obeys similar rules.Finally,the packing densities obtained by the different methods are demonstrated to be universally correlated with the ratio of the interparticle force to the effective gravity.These findings indicate that different phenomena of particulate systems at a macro-or meso-scale may share similar microscopic origins,with the interparticle force playing a crucial role.

DEM simulation of particle percolation in a packed bed

The phenomenon of spontaneous particle percolation under gravity is investigated by means of the discrete element method. Percolation behaviors such as percolation velocity, residence time distribution and radial dispersion are examined under various conditions. It is shown that the vertical velocity of a percolating particle moving down through a packing of larger particles decreases with increasing the restitution coefficient between particles and diameter ratio of the percolating to packing particles. With the increase of the restitution coefficient, the residence time and radial dispersion of the percolating particles increase. The packing height affects the residence time and radial dispersion. But, the effect can be eliminated in the analysis of the residence time and radial dispersion when they are normalized by the average residence time and the product of the packing height and packing particle diameter, respectively. In addition, the percolation velocity is shown to be related to the vertical acceleration of the percolating particle when an extra constant vertical force is applied. Increasing the feeding rate of percolating particles decreases the dispersion coefficient.

Rapid synthesis and growth of silver nanowires induced by vanadium trioxide particles

This study demonstrates a novel approach for rapid synthesis of silver （Ag） nanowires induced by vana- dium trioxide （V2O3） particles in aqueous solution at room temperature. Silver nanowires have an average diameter of 20 nm and length up to a few micrometers by parametric optimization. The micro- structure of the silver nanowires was characterized by TEM, HRTEM, SEM, and XRD techniques. The optical property of the as-prepared product was measured by ultraviolet-visible （UV-vis） spectroscopy. The possible growth mechanism of Ag nanowires via oriented attachment of Ag nanocrystals was dis- cussed. The present approach shows several unique features such as rapid （a few minutes）, reproducible and high-yield reaction with no need of any modifiers. V2O3 rods were reported for the first time to be used for synthesis of silver nanowires, playing multiple roles as reducing agent, template, and cata- lyst. The silver nanowires produced are promising for optical applications （e.g., SERS） due to their rough surface.

交互循环双流化床的气/固运动三维数值模拟

双流化床作为一种具有燃料高效利用和分级转化优势的新型能源技术,其内部的物理结构和流动特性复杂。本文以交互循环双流化床为研究对象建立了三维数值模型,采用了基于欧拉-拉格朗日方法的气固两相流数值模拟方法,气相采用大涡湍流模型(LES),颗粒相采用多相流网格质点群(MP-PIC)方法.实现了三维双流化床全场气固循环、交互和连续稳定运行。在与实验验证的基础上,通过模拟,研究了不同的操作条件对流动结构、颗粒分布等流体力学特性的影响,获得了一系列实验难以得到的规律。

Special issue on conveying and handling of particulate solids–Challenges of discrete element simulation, application and calibration

A series of international Conveying and Handling of Particulate Solids （CHOPS） conferences was initiated in Herzliya, Israel in 1995 and has since proceeded with six successive conferences held in Jerusalem （1997）, the Dead Sea （2000）, Budapest （2003）, Sorrento （2006）, Brisbane （2009） and Friedrichshafen （2012）. The conference series covers processes involvin particulate solids, for example,

Numerical investigation of separation efficiency of the cyclone with supercritical fluid-solid flow

The utilization of hydrogen is gaining increasing attention due to its high heating value and environmentally friendly combustion product.The supercritical water circulating fluidized bed reactor is a promising and potentially clean technology that can generate hydrogen from coal gasification.Cyclone is a vital part of the reactor which can separate incomplete decomposition of pulverized coal particles from mixed working fluid.This paper aims to gain in-depth understanding of the cyclone separation mechanisms under supercritical fluid by computational fluid dynamics(CFD).Although the amount of supercritical carbon dioxide in mixed working fluid is minor,it obviously influences the flow fields and separation efficiency of a cyclone.The simulation results suggest that both the decreasing content of supercritical carbon dioxide and adding the extra dipleg cause the promoting performance of cyclones.Research findings could refine the design of supercritical fluid-solid cyclones.