FLOW BOILING HEAT TRANSFER WITH FLUIDIZED SOLID PARTICLES

In order to solve the fouling problems in boiling processes,a boiling system was designedby adding solid particles to the boiling liquid In this paper.both theoretical analyses andexperimental studies on the boiling heat transfer in such a three-phase flow boiling were carried out.Based on the analysis of heat transfer characters of this three-phase flow boiling,a mathematical mod-el for the heat transfer coefficient of flow boiling was developed.The experiments show that,in thepresence of particles the boiling heat transfer is enhanced and is about 2 times that of the vaporliquid two phase one with better flow stability.The fluidized particles rub the heat transfer wall toprevent and to clean the fouling.

Pressure Drop of Liquid–Solid Two-Phase Flow in the Vertical Tube Bundle of a Cold-Model Circulating Fluidized Bed Evaporator

A cold-model vertical multi-tube circulating fluidized bed evaporator was designed and built to conduct a visualization study on the pressure drop of a liquid–solid two-phase flow and the corresponding particle distribution.Water and polyformaldehyde particle(POM)were used as the liquid and solid phases,respectively.The effects of operating parameters such as the amount of added particles,circulating flow rate,and particle size were systematically investigated.The results showed that the addition of the particles increased the pressure drop in the vertical tube bundle.The maximum pressure drop ratios were 18.65%,21.15%,18.00%,and 21.15%within the experimental range of the amount of added particles for POM1,POM2,POM3,and POM4,respectively.The pressure drop ratio basically decreased with the increase in the circulating flow rate but fluctuated with the increase in the amount of added particles and particle size.The difference in pressure drop ratio decreased with the increase in the circulating flow rate.As the amount of added particles increased,the difference in pressure drop ratio fluctuated at low circulating flow rate but basically decreased at high circulating flow rate.The pressure drop in the vertical tube bundle accounted for about 70%of the overall pressure drop in the up-flow heating chamber and was the main component of the overall pressure within the experimental range.Three-dimensional phase diagrams were established to display the variation ranges of the pressure drop and pressure drop ratio in the vertical tube bundle corresponding to the operating parameters.The research results can provide some reference for the application of the fluidized bed heat transfer technology in the industry.

Modeling of gas-solid flow in a CFB riser based on computational particle fluid dynamics

A three-dimensional model for gas-solid flow in a circulating fluidized bed（CFB） riser was developed based on computational particle fluid dynamics（CPFD）.The model was used to simulate the gas-solid flow behavior inside a circulating fluidized bed riser operating at various superficial gas velocities and solids mass fluxes in two fluidization regimes,a dilute phase transport（DPT） regime and a fast fluidization（FF） regime.The simulation results were evaluated based on comparison with experimental data of solids velocity and holdup,obtained from non-invasive automated radioactive particle tracking and gamma-ray tomography techniques,respectively.The agreement of the predicted solids velocity and holdup with experimental data validated the CPFD model for the CFB riser.The model predicted the main features of the gas-solid flows in the two regimes;the uniform dilute phase in the DPT regime,and the coexistence of the dilute phase in the upper region and the dense phase in the lower region in the FF regime.The clustering and solids back mixing in the FF regime were stronger than those in the DPT regime.

多流程循环流化床气固流动和燃烧的计算颗粒流体力学数值模拟

多流程循环流化床与传统流化床相比,能有效降低炉膛高度,实现小型化。以多流程循环流化床锅炉为研究对象,基于计算颗粒流体力学,对气固流动和燃烧过程进行了数值模拟,计算结果与中试装置结果吻合较好。结果表明:炉膛内颗粒相浓度由高到低依次为主燃烧室、副燃烧室、燃尽室,炉内颗粒分布呈现“环-核”结构;炉膛温度范围为1 000~1 200 K,主燃烧室的温度大于副燃烧室,副燃烧室的温度大于燃尽室与旋风分离器;增大一、二次风的配比使炉膛密相区的氧气含量增大,加剧了燃烧,使床温升高;由副燃烧室进行分级给料,有利于焦炭还原NO,使出口NO浓度明显下降,有利于减少氮氧化物的排放。

欧拉-欧拉双流体模型中颗粒黏性对液固散式流态化的影响

采用无黏性双流体模型、基于颗粒动理学的双流体模型以及Brandani和Zhang简化双流体模型探究了颗粒黏性对液固散式流化特性的影响规律。经与文献中实验数据和Gibilaro公式计算值对比后发现:3种双流体模型均能较好地预测整体固含率,与实验值相对偏差在3%以内,其中无黏性双流体模型和简化双流体模型对固含率预测更符合散式流态化特点;3种双流体模型对时均颗粒轴向速度预测均呈现整体环核结构的固有属性,其中简化双流体模型在较低和较高两组液速工况下预测的相对偏差平均值分别为0.277和1.028;当入口液速突然变化后,收缩过程中简化双流体模型对响应时间预测准确度略高,而膨胀过程中因低浓度区界面不稳定以及膨胀过程中床内颗粒形成的由浓到稀过渡段延长了稳定时间,3种模型预测均与理想过程存在一定偏差,其中液速较低时三者差异不明显,但液速较高时简化双流体模型准确性略高。基于颗粒动理学的双流体模型对动态过程模拟的计算耗时最长。

Local solids flow structure in a countercurrent liquid-upward and solids-downward fluidized bed(CCLSFB)system

The local solids holdup and local particle velocity in a Countercurrent Liquid-upward and Solids-downward Fluidized Bed(CCLSFB)were investigated in details using optical fiber probes with two different models in a Plexiglas column of 1.5 m in height and 7.0 cm in inner diameter.A new flow regime map including fluidized bed,transition,and flooding regimes was established.The axial solids holdup distribution is almost uniform at low liquid velocity and/or solids flowrate and becomes less uniform with higher solids holdup at the top of the column after the operating liquid velocity is reaching the flooding velocity.The radial solids holdup profile is also nearly flat with a slightly lower solids holdup in the near-wall region at low liquid velocity and solids flowrate but becomes nonuniform as the operating liquid velocity approaches the flooding velocity.Two equations were also proposed to correlate radial local solids holdups.The descending particle velocity in CCLSFB increases with the decrease of the liquid velocity and the increase of the solids flowrate.A generally uniform particle velocity distribution was found in the axial direction,as well as in the radial direction except for a small decrease near the wall.These results on the local solids flow structure would provide basic information and theoretical supports for the design and industrial application of CCLSFB.

Experiments on particle cluster behaviors in a fast fluidized bed

A three-dimensional(3D) fast fluidized bed with the riser of 3.0 m in height and 0.1 m in inner diameter was established to experimentally study the cluster behaviors of Geldart B particles. Five kinds of quartz sand particles(dp= 0.100, 0.139, 0.177, 0.250 and 0.375 mm and ρp= 2480 kg·m^(-3)) were respectively investigated, with the total mass of the bed material kept as 10 kg. The superficial gas velocity in the riser ranges from 2.486 to 5.594 m·s^(-1) and the solid mass flux alters from 30 to 70 kg·((m^(-2)·s))^(-1). Cluster characteristics and evolutionary processes in the different positions of the riser were captured by the cluster visualization systems and analyzed by the self-developed binary image processing. The results found four typical cluster structures in the riser,i.e., the macro stripe-shaped cluster, saddle-shaped cluster, U-shaped cluster and the micro cluster. The increasing superficial gas velocity and particle sizes result in the increasing average cluster size and the decreasing cluster time fraction, while the solid mass flux in the riser have the reverse influences on the cluster size and time fraction. Additionally, clusters in the upper region of the riser often have the larger size and time fraction than that in the lower region. All these effects of operating conditions on clusters become less obvious when particle size is less than 0.100 mm.

Gas-solid flow in a high-density circulating fluidized bed riser with Geldart group B particles

We carried out experiments to explore and characterize the gas-solid flow dynamics of Geldart group B particles in a dense circulating fluidized bed riser. By reducing the pressure drop across the solid control valve and increasing the solid inventory in the storage tank, a high solid circulation rate and a solid holdup above 0.075 throughout the riser were simultaneously achieved. At a solid-to-gas mass flux ratio of approximately 105, flow transitioned from fast fluidization to a dense suspension upflow. In the axial direction of the riser, solid holdup had an exponential profile, increasing with increasing solid circulation rate and Jot decreasing superficial gas velocity. From the riser＇s center to its wall, the solid holdup increased markedly, exhibiting a steep parabolic profile. Increasing the solid circulation rate increased the radial non-uniformity of the solid concentration, while increasing the superficial gas velocity had the opposite effect, In our dense circulating fluidized bed riser, Geldart group B particles had similar slip characteristics to Geldart group A particles,

液-固流化床最小流态化速度的数值模拟与验证

为使用计算颗粒流体力学(CPFD)的数值模拟法代替实验法,建立单管的液-固两相流化床实验系统;实验测量5种颗粒的最小流化速度;应用CPFD方法对单管液-固流化床模型进行数值模拟,比较5种颗粒的最小流态化速度的实验值与模拟值,并进行误差分析,验证数值模拟方法的正确性。结果表明:5种实验颗粒的最小流态化速度实验值分别为0.0216、0.0367、0.0293、0.0555、0.0845 m/s;2种公式验算值与实验值的最大相对误差小于10%,平均相对误差小于5%,证明实验结果是可靠的;5种模拟颗粒的最小流态化速度模拟修正值分别为0.024、0.044、0.041、0.069、0.062 m/s;颗粒S_(1)、S_(2)、S_(4)、S_(5)的模拟修正值与颗粒E_(1)、E_(2)、E_(4)、E_(5)的实验值之间的最小流态化速度的误差分别为11.1%、19.9%、24.3%、26.6%,均为正向偏差且在工程允许范围内,证明CPFD数值模拟方法是可靠的;最小流态化速度随固体颗粒的密度和粒径的增大而增大。

流化床反应器内碳纳米管流态化技术的研究进展

碳纳米管作为一种战略新型碳材料以其独有的高强度、高导热、高导电等诸多优异性能,可应用于航空、电子器件、新能源汽车等国防军事及民生领域,而限制其广泛应用主要受制于高质高产制备。流化床反应器作为制备碳纳米管的核心部件,研究其内碳纳米管制备过程的流态化技术已愈发重要。综述了从碳纳米管的制备技术、生长机理及操作条件对其制备过程的影响到不同流化床内碳纳米管的流态化研究等这一系列围绕宏量制备碳纳米管的技术,提出了反应器内碳纳米管流态化特性研究中存在及未解决的问题,为将来更详细深入地研究碳纳米管的流态化技术提供了指导与方向。

基于人工神经网络的非均匀固相应力模型

最小多尺度理论EMMS已经被引入多相质点网格法MP-PIC中,建立了非均匀EMMS固相应力模型.但现有的非均匀固相应力模型计算中,中间步骤繁琐且花费时间长.采用人工拟合的方式能获得非均匀固相应力表达式,但需要人为确定拟合变量和拟合函数,且针对于非均匀固相应力这种高度非线性函数所得到的拟合精度不高、用于MP-PIC模拟的结果相比原EMMS固相应力模型结果存在偏差.针对上述问题,本文提出通过机器学习的方法,规避对固相体积分数的局部分布情况的表征,并提出和建立能考虑颗粒浓度详细分布的人工神经网络ANN固相应力模型.首先,基于局部颗粒浓度和颗粒非均匀分布指数建立了双变量的ANN固相应力模型;进一步将当前网格及其周边网格颗粒浓度组成的序列来详细表征颗粒浓度分布,并建立相应的ANN固相应力模型.然后,将两种模型与EMMS固相应力模型进行了对比并测试了网格分辨率和粗化率对模型的影响.研究表明:基于ANN固相应力模型的模拟结果对EMMS固相应力模型结果有较高的还原度,同时具有一定的网格分辨率无关性和粗化率无关性.

丝状生物质颗粒在矩形提升管内的分布特性

采用欧拉-拉格朗日法对生物质颗粒群在提升管内流动与分布特性方面展开研究。根据生物质颗粒结构特征建立链状可弯曲三维颗粒模型,考察表观气速(5.71、6.89、7.58、9.10 m/s)、颗粒质量流量(0.018、0.0216、0.0259、0.0311 kg/s)对丝状颗粒在提升管内轴向和径向分布特性的影响。研究发现:丝状颗粒在提升管轴向呈下密上疏的分布规律,在径向上随高度的增加颗粒分布由不均匀的倒C形分布向ω形转变。随着表观气速的增加,颗粒体积分数沿提升管径向逐渐降低。当表观气速u_(g)=9.10 m/s时,颗粒处于快速流化状态,气固间相互作用增强、混合良好,沿提升管轴向分布更均匀。当颗粒质量流量增加,丝状颗粒聚集区域向上延伸,颗粒体积分数沿径向升高,且各水平截面的分布特征沿轴向差别不大。

Experimental Investigation on the Hydrodynamic Characteristics of Fluidized Bed Particle Solar Receiver with Gas-Solid Countercurrent Flow Pattern

To design a particle solar receiver(PSR),a vital energy conversion system,is still a bottleneck for researchers.This study presents a novel PSR based on countercurrent fluidized bed(CCFB)technology,named CCFB receiver.In this design,downward-moving particles are subjected to the action of an up-flow gas to reduce the falling speed and enhance the radial disturbance,and hence increase the residence time of particles and improve the heat transfer.A cold-mold visual experimental setup is established.The influence factors are investigated experimentally,including the superficial gas velocity,solid flux,aeration gas,particle size and transport tube diameter.The results indicate that the maximum solid holdup can exceed 9%or so with fine particles of diameter d_(p)=113.5 μm and a tube diameter of 40 mm.It is proved that the CCFB can operate stably and adjust the solid flux rapidly.The results of this study provide a new structure for PSRs in the concentrated solar power field and could fill the research insufficiency in the gas-solid counterflow field.

EXPERIMENTAL RESEARCH OF FLOW STRUCTURE IN A GAS-SOLID CIRCULATING FLUIDIZED BED RISER BY PIV

Particle Imaging Velocimetry （PIV） techniques were applied to investigate the particle motion and cluster properties in a gas-solid two-phase flow in a circulating fluidized bed riser. Visual images and micro-structure of various clusters were captured. After the boundary of clusters was determined by the gray level threshold method, clusters were classified by the distance between particles and the shape and position of clusters. In addition, the process of clusters forming and breaking up was described, and the sizes of clusters were also obtained. With the Minimum Quadric Difference （MQD） cross-correlation algorithm suitable for high-density particles, the axial velocities of the particles were obtained in the dilute phase section. The features of particle motion were revealed by investigating statistically the magnitude and distribution of particle axial velocity in the radial direction. At most radial cross-sections, there exists a parabola-shaped distribution of upward axial velocity of particles, namely, the magnitude of axial velocity in the core region is higher than that near the wall region of the riser.

Numerical prediction of flow hydrodynamics of wet molecular sieve particles in a liquid-fluidized bed

The Eulerian-Eulerian framework was used in the numerical simulation of liquid hydrodynamics and particle motion in liquid-fluidized beds. The kinetic theory of granular flow, which accounts for the viscous drag influence on the interstitial liquid phase, was used in combination with two-fluid models to simulate unsteady liquid-solid two-phase flows. We focus on local unsteady features predicted by the numerical models. The solid fraction power spectrum was analyzed. A typical flow pattern, such as core annular flow and particle back-mixing near the wall region of liquid-solid fluidized beds is obtained from this calculation. Effects of the restitution coefficient of particle-particle collisions on the distribution of granular pressure and temperature are discussed. Good agreement was achieved between the simulated results and experimental findings.

循环流化床内颗粒运动的PIV测试

作为一种瞬时全场测速技术,PIV测试技术被尝试用于测量循环流化床(CFB)内颗粒运动,以进一步了解循环流化床内复杂的气固两相流动特性。初步实验已在一个截面为200mm×200mm高为4m的冷态循环流化床实验台上完成,并运用二值化互相关图像处理算法,获得了床内截面上的颗粒流动矢量图。初步测试结果较好地反映了循环流化床内颗粒流动的一些特性,表明PIV技术在循环流化床气固两相流体特性研究中具有较好的应用前景。

气固流化床中声发射和流动模式关系

The flow pattern of a fluidized bed is a key factor for heat transfer and new reactor design.The flow pattern of the gas-solid fluidized bed was experimentally measured by a novel acoustic emission(AE)technique.The acoustic energy generated by the collision of solids particles on the reactor wall or in the local space is a reflection of the speed and frequency of collision,and consequently the flow pattern of particles.The flow pattern of polyethylene particles with the average size 460 μm in a  150 mm fluidized bed in the range of superficial gas velocity from 0.3 m·s-1 to 0.7 m·s-1 was multi-circulation pattern with main-circulation zone,sub-circulation zone and stagnant zone.The multi-circulation flow pattern would transit to the single circulation flow pattern if superficial gas velocity exceeded 0.8 m·s-1 or average particle size decreased to below 365 μm.The height of stagnant zone remained unchanged with the increase of static bed height.It was also found that the type of the distributor had an evident effect on flow pattern.The single circulation flow pattern in the fluidized bed with a perforated stainless steel plate distributor would change to the multi-circulation flow pattern for the same particles and the same fluidized bed with a cone-shaped distributor.An empirical equation calculating the height of stagnant zone was presented and the experimental data could be correlated with the empirical equation with fairly good accuracy.

提升管气固两相流中颗粒速度的实验研究

用新型 5 -光纤速度探头对 16m高循环床提升管 8个截面上的颗粒速度进行了直接测量。结果表明 :沿提升管轴向 ,颗粒速度径向分布不均匀性逐渐增加 ,并最终趋于较为稳定的抛物线分布 ;通常认为近壁区颗粒速度Vp <0的情形 ,只能在离开提升管底部一定距离且在Gs相对较高或Ug 相对较低的操作条件下才会出现。表观气速Ug 对颗粒速度的影响明显大于颗粒循环量Gs的影响 ,Ug提高 ,提升管内颗粒速度都将显著增加 ;提高Gs将使颗粒速度降低 ,但对提升管充分发展段中心区影响较小。

汽-液-固多管循环流化床蒸发器中固体颗粒的分布

为研究三相循环流化床蒸发器加热管束内固体颗粒的分布规律,设计并构建了汽-液-固透明多管循环流化床蒸发系统.利用CCD图像采集和数据处理系统,研究了液体循环流量、热通量、颗粒加入量及颗种类等参数对于加热管束中固体颗粒分布的影响.结果表明:随着液体循环流量、热通量和颗粒加入量的增加,颗粒分布的不均匀度降低;而随着颗粒沉降速度的增加,颗粒分布的不均匀度增大.研究结果为进一步解决固体颗粒在加热管束中良好分布的问题奠定了基础.

惰性固体颗粒清除和防止硅渣结垢的研究

用流化床传热技术 ,研究惰性固体颗粒清除和防止铝土矿熟料溶出液脱硅过程中的硅渣结垢。硅渣结垢机理为结晶结垢和微颗粒沉积结垢混合机理。研究表明 ,有机聚合物固体颗粒能有效抑制硅渣垢层的生长 ,但不能完全清除已有的硅渣结垢 ;无机和复合材质的固体颗粒不仅能完全控制垢层的生长 ,而且能完全清除已有的硅渣垢层 ;无机非金属惰性固体颗粒的防垢除垢效果最好 ;