循环流化床多组分颗粒气固两相流动模型和数值模拟

基于稠密气体分子运动论和颗粒动力学,考虑多组分颗粒中颗粒组分与颗粒组分、颗粒组分内颗粒之间的相互作用以及气体与颗粒之间的相互作用,提出多组分颗粒非等温颗粒气固两相流动模型.以颗粒压力、径向分布函数、黏度、颗粒碰撞耗散等耦合各颗粒组分间和颗粒间的相间作用.采用大涡模拟方法模拟气相湍流流动.提出了多组分颗粒的径向分布函数计算方法.对循环流化床上升管中双组分颗粒气固两相流动特性进行了数值模拟,模拟结果揭示了上升管中双组分颗粒气固两相流动的环-核流动结构,得到了平均颗粒粒径的轴向和径向分布规律,计算结果与文献中实验结果相吻合.     

IMPORTANCE OF DISPERSED SOLIDS IN BUBBLES FOR EXOTHERMIC REACTIONS IN FLUIDIZED BEDS

Ignition of activated carbon particles were measured in a vertical tube reactor of 4 cm ID, where single particles fell consecutively through a gas mixture containing oxygen.

A two dimensional fluidized bed reactor 24 cm wide, 51 cm high and 2.5 cm in thickness was used for visual observation through a wide front window 24 cm × 35 cm covered with a silica glass plate 1 cm thick. Activated carbon particles were fluidized incipientiy by air, and a gas mixture containing oxygen was injected upwards into the bed through a nozzle positioned 5 cm above the distributor, forming single bubbles intermittently.

It was observed that carbon particles dispersed in rising bubbles were ignited abruptly at emulsion phase temperatures above 550°C. Experimental findings from the fluidized bed were compared with those from the tube reactor, suggesting that the igniting conditions for particles dispersed in bubbles are nearly the same as for single particles falling in the tube reactor.     

IMPORTANCE OF DISPERSED SOLIDS IN BUBBLES FOR EXOTHERMIC REACTIONS IN FLUIDIZED BEDS

Ignition of activated carbon particles were measured in a vertical tube reactor of 4 cm ID, where single particles fell consecutively through a gas mixture containing oxygen.

A two dimensional fluidized bed reactor 24 cm wide, 51 cm high and 2.5 cm in thickness was used for visual observation through a wide front window 24 cm × 35 cm covered with a silica glass plate 1 cm thick. Activated carbon particles were fluidized incipientiy by air, and a gas mixture containing oxygen was injected upwards into the bed through a nozzle positioned 5 cm above the distributor, forming single bubbles intermittently.

It was observed that carbon particles dispersed in rising bubbles were ignited abruptly at emulsion phase temperatures above 550°C. Experimental findings from the fluidized bed were compared with those from the tube reactor, suggesting that the igniting conditions for particles dispersed in bubbles are nearly the same as for single particles falling in the tube reactor.     

气固流化床中颗粒分离研究进展

综述了气固流化床有关颗粒分离的研究概况及最新进展。既包括实验研究，又给出模型比较。讨论了不同研究结果之间的差异及存在的不足。指出了在这一领域中应该进一步进行深入研究的课题。     

PREDICTING APPARENT SHERWOOD NUMBERS FOR FLUIDIZED BEDS

ABSTRACT

Mass transfer data of bubbling fluidized beds have been reevaluated with a new model which is completely predictive. The model bases on a two-phase approach with active bypass, formally plug flow for the suspension gas and a consideration of backmixing in the main kinetic coefficient, i.e. in the apparent particle-to-fluid Sherwood number. A good agreement with experimental results of various authors with a broad range of Reynolds numbers and particle diameters is demonstrated.     

循环流化床中颗粒团聚物性质的PDPA测量

提出了一种利用相位多普勒粒子分析仪（PDPA）来测量气固循环流化床中颗粒团聚物性质的方法,并运用此方法初步考察了操作条件对循环流化床稀相区中颗粒团聚物性质的影响.在本实验操作条件下,颗粒团聚物的时间分率、频率、内部空隙率以及轴向速度等性质都存在轴径向的不均匀分布,具有较明显的环核特征;固体循环速率对颗粒团聚物性质径向分布影响不大,表观气速的变化可引起其轴向分布规律发生显著改变,但其径向的环核特性仍然存在.     

粉煤灰强度效应因子的动态预测

提出了粉煤灰强度效应因子ｋ的动态预测模型,该模型建立了粉煤灰强度效应因子与粉煤灰火山灰活性的指数、粉煤灰掺量、水胶比、龄期等主要参数之间的直接联系,并考虑了粉煤灰品种、水泥品种等其它因素的影响,最后,对模型进行了实验验证,预测结果与实验结果吻合较好     

A THEORETICAL STUDY OF THE LIMITING HEAT TRANSFER COEFFICIENT IN FLUIDIZED BEDS

A new model is presented to describe the heat transfer process occurring in fluidized beds under the limiting condition of very short contact times between the heat transfer surface and the emulsion phase. Unlike other work in this area the proposed model assumes that the particles are in constant motion close to the surface throughout the heat transfer process. The effect of the non-continuum gas film assumption (the Smoluchowski effect) when particles are very close to the surface is found to be of secondary importance when compared with the movement of particles close to the surface. Expressions for both the instantaneous and time averaged heat transfer coefficients for individual and a spatial distribution of parlicles are presented.     

A NON-MARKOVIAN MODEL FOR PARTICLE MOTION IN FLUIDIZED BEDS

The notion of modeling particle motion in fluidized beds by stochastic methods is extended to include non-Markovian behavior. The inclusion of such “memory” terms should more adequately represent the key physical considerations. A general memory equation is first introduced. Two expressions for the so-called memory function are then suggested and subsequently used to generate autocorrelation functions which favorably represent the available experimental data.     

流化床中灰分对煤焦和生物质焦混合特性影响

在115 mm×2 110 mm有机玻璃圆柱型流化床中,选取具有实际意义的焦灰体系,引入描述灰分存在时煤焦和生物质焦两组分混合特性的的质量比混合指数,考察了灰分对煤焦和生物质焦混合特性的影响.结果表明,在操作气速不变的情况下,增加灰含量,松木屑焦的富集位置向上转移,煤灰的含量小于15.4%时,灰含量增加利于煤焦和松木屑焦的混合;减小灰分粒径,煤焦和松木屑焦的混合趋于均匀;提高操作气速有利于煤焦和松木屑焦的混合.此研究结果可以为煤与生物质共气化流化床气化炉的设计、运行和操作条件的确定提供一定参考.     

粘性颗粒流化床中聚团大小的计算模型

根据粘性颗粒流化床中聚团所受力的分析,提出了力平衡模型。并对模型中的参数进行了估算。据此模型计算了几种粘性颗粒的聚团大小,计算结果与实测的聚团大小较为接近。与其他研究者的模型比较,计算结果更接近实测值。     

循环流化床内颗粒的加速作用及气固两相间的动量交换

根据对气固两相流动特性的分析,表明在循环流化床提升管内,颗粒的加速运动及气固两相间的动量交换对床层各项参数具有明显的影响。由于颗粒聚集,减小了气固两相间的相互作用,使曳力系数小于单颗粒标准曳力系数(C_D/C_(DS)<1.0)。从而使颗粒具有较长的加速段,其加速压降在总压降中所占的分率不可忽略。考虑颗粒加速作用及由于颗粒聚集使气固两相相互作用力减小的事实后,用一维轴向稳态流动的模型可以获得截面平均空隙率、平均颗粒速度以及曳力系数等参数及其轴向分布规律,并与实验直接测定结果吻合很好。通过对大量数据的分析,表明C_D/C_(DS)可用下述经验关联式表示:C_D/C_(DS)=1.68(ε)~(0.253)(Re_(?)/Re_t)~(-1.213)(d_P/D)~(0.105)     

A MODEL FOR BED-TO-SURFACE HEAT TRANSFER IN CIRCULATING FLUIDIZED BEDS

Experiments on bed-to-surface heat transfer in the axial and radial directions were carried out in a cold flowmodel of the circulating fluidized bed of 244mm i.d.and with FCC catalysts(d_p=83.4μm)as the fluidized particles.A simplified gas-solid flow model comprising of the cluster-particulate phases upflowing cocurrentlywas proposed.By taking into account the alternative contacts of cluster and particulate phases with the immersedsurface,a composite heat transfer model was developed.The model gave good predictions as compared withthe experimental results of this work and those of others.     

气固脉冲流化床流体力学特性的研究

在φ７０ｍｍ的流化床内，采用聚氯乙烯、玻璃珠和不规则天然刚玉等Ｂ类、Ｄ类颗粒，测定了０￣５．０Ｈｚ脉冲频率下气固流化床的基本流体力学特性，探讨了影响床层流化特性的一些主要因素，并根据实验数据对脉冲流化床的临界流化速度和临界流化压降的无因次准数式进行了关联。     

NUMERICAL INVESTIGATION OF A PHENOMENOLOGICAL MODEL FOR VIBRATED FLUIDIZED BEDS

This paper presents a comparative analysis of measured and calculated pressure fluctuations in a vibrated fluidized bed. The calculations are based on a phenomenological model of a vibrated system represented as a reciprocating porous piston in a cylinder. Bed porosity fluctuation is considered to be sinusoidal with pos-sible slip in phase with respect to the exciting vibration. Sat-is factory agreement is found between the measured and calculated results. Sensitivity of the model is analysed with regard to the pertinent model parameters. A nomogram is presented for practical; use to predict tie average bed pressure drip as a function of the vibrational intensity.     

确定性颗粒轨道模型在流化床模拟中的研究进展

随着计算机硬件的发展,基于颗粒尺度模拟稠密气固两相流的颗粒轨道模型发展很快.本文回顾了目前流化床模拟中确定性颗粒轨道模型的研究状况,概述了处理颗粒碰撞的硬球模型、软球模型以及DSMC方法在国内外流化床模拟中的研究成果,评述了颗粒间相互作用处理方法的优缺点以及适用的系统等,指出了确定性颗粒轨道模型发展亟待解决的问题,展望了确定性颗粒轨道模型未来的发展趋势.     

RADIAL DISPERSION AND BUBBLE CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of gas and liquid velocities, liquid viscosity and particle size on the radial dispersion coefficient of liquid phase (D_r) and the bubble properties in three-phase fluidized beds have been determined. A new flow regime map based on the drift flux theory in three-phase fluidized beds has been proposed.

In three-phase fluidized beds, D, increases with increasing gas velocity in the bubble coalescing and in the slug flow regimes, but it decreases in the bubble disintegrating regime. The coefficient exhibits a maximum value in the bed of small particles with increasing liquid velocity at lower gas velocities. However, it increases with increasing liquid velocity at higher gas velocities. In two and three-phase fluidized beds of larger particles (6,8 mm), D_r exhibits a maximum value with an increase in liquid viscosity at lower gas velocities, but it increases at higher gas velocities. The mean bubble chord length and its rising velocity increase with increasing gas velocity and liquid viscosity. However, the bubble chord length decreases with an increase in liquid velocity and it exhibits a maximum value with increasing particle size in the bed. The radial dispersion coefficients in the bubble coalescing and disintegrating regimes of three-phase fluidized beds in terms of the Peclet number in the present and previous studies have been well represented by the correlations based on the concept of isotropic turbulence theory.     

THE BEHAVIOR OF THE DISPERSED PHASE IN LIQUID-LIQUID EXTRACTION IN FLUIDIZED BEDS

An investigation was carried out on the effects of liquid flow rates, solid particle size, bed height, and voidage on the dispersed-phase holdup, the drop size and the behavior of the dispersed phase in liquid-liquid concurrent fluidized beds. The mass transfer coefficients were calculated from liquid-liquid extraction data and the calculated results could be satisfactorily explained by matching with the dynamic behavior of the liquids and the change of drop size. The increase of liquid-liquid extraction rates in liquid-liquid fluidized beds was mainly due to the increase of interfacial areas of the dispersed phase.