气液固三相提升管中液相扩散特性

对气液固三相提升管内液相扩散行为进行了实验研究,考察了气速、液速以及颗粒循环量等操作因素对液相扩散系数的影响规律.实验研究结果表明,轴向、径向扩散系数随气速的增大均增大;轴向扩散系数随液速的变化基本保持不变,径向扩散系数随液速的增大而减小;轴向、径向扩散系数随颗粒循环量的增大均增大.与传统的气液固三相流化床相比,气液固三相提升管反应器更接近理想的平推流反应器.     

Hydrodynamics of binary fluidization in a riser: CFD simulation using two granular temperatures

A new computational fluid-dynamic (CFD) model with a separate granular temperature (2/3 random particle kinetic energy per unit of mass) equation for each phase or particle size was developed using constitutive equations derived earlier by Huilin, Gidaspow and Manger. In agreement with the experiment and model of Mathiesen, Solberg and Hjertager the new model computes the observed core-annular flow regime. It predicts the trends of the observed radial and axial particle diameter distributions. For elastic particles the computed particle velocity distributions are parabolic. They are close to the laminar type approximate analytical solution for flow in a pipe, where the mean velocity equals the inlet flux divided by the particle density and volume fraction. The computed turbulent intensity is lower for large particles than for small particles, as measured. This is in agreement with an approximate analytical solution for the granular temperature in the developed flow region of a riser for elastic particles. Computations show that for sufficiently inelastic particles the granular temperature in the center can be lower than near the wall resembling the measured particle fluctuating velocity distribution.     

Axial liquid dispersion in a liquid–solid circulating fluidized bed

Although axial liquid dispersion has been studied extensively in particulate fluidized beds, no data has been reported previously in a liquid–solid circulating fluidized bed (LSCFb). In this work, the axial liquid dispersions at various radial positions were measured in an LSCFB of 76 mm in diameter and 3.0 m in height using a dual conductivity probe. The results reveal that the axial liquid dispersion is affected not only by the operating conditions but by the radial positions as well. A local axial dispersion model is proposed to describe the axial liquid dispersion at various radial positions. The local axial liquid dispersion coefficients determined by the proposed model are greater at the axis than near the wall region of the riser. This nonuniformity of axial liquid dispersion is believed to be caused by the radial nonuniform distribution of liquid velocity, and bed voidage in the LSCFB can significantly affect the axial liquid dispersion.     

Phase distributions in a gas–liquid–solid circulating fluidized bed riser

The distributions of the three phases in gas–liquid–solid circulating fluidized beds (GLSCFB) were studied using a novel measurement technique that combines electrical resistance tomography (ERT) and optical fibre probe. The introduction of gas into a liquid–solid circulating fluidized bed (LSCFB), thus forming a GLSCFB, caused the increase of solids holdup due to the significantly decreased available buoyancy with the lower density of the gas, even with a somewhat increased liquid velocity due to the decreased liquid holdup giving space for the gas holdup. The gas passed through the riser in the form of bubbles, which tended to flow more through the central region of the riser, leading to more radial non‐uniformity in radial holdup of the phases. The gas velocity has the most significant effect on the gas phase holdup. While the gas velocity also has an obvious effect to the solids holdups, the liquid flow rate had a much more considerable effect on the phase holdups. The solids circulation rate also had a significant effect on the phase holdups, with increasing solids circulation rate causing much more increased solids holdup in the central region than close to the wall. A correlation was developed for the relative radial distributions of solids holdup in GLSCFB, as such radial profiles were found similar over a wide range of operating conditions, like those in a typical gas–solid circulating fluidized beds (GSCFB). Finally, the axial solids profiles in a GLSCFB was found to be much closer to those in an LSCFB which are very uniform, than those found in a GSCFB which are less uniform and sometime having a S shape. Water was used as the continuous and conductive phase, air was the gas phase and glass bead and lava rock particles were used as the solid and non‐conductive phase.     

多段分级转化流化床提升管速度场的研究

针对流化床煤气化过程中需要长气固接触时间和高固体浓度,开发了耦合灰熔聚流化床和提升管的多段分级转化流化床。为了研究多段分级转化流化床提升管中局部颗粒速度的径向、轴向分布,在不同的操作条件下,采用PV-6型颗粒速度测量仪在冷态实验装置中系统测定提升管内局部颗粒速度。实验结果表明:提升管中任何径向、轴向位置的颗粒速度随着操作气速的增大而增大,随循环量的增加而减小。操作条件对中心区颗粒速度变化的影响明显高于边壁区。颗粒的加速首先发生在提升管中心区域,然后向边壁区域扩展。颗粒速度径向分布的不均匀性沿轴向逐渐增大,并且受操作气速影响比较大。     

Chaotic characteristics of local voidage fluctuation in a circulating fluidized bed

Local voidage fluctuations have been measured by using an optical transmittance probe at various axial and radial positions in a circulating fluidized bed riser with a 0.1 m i. d. and 10 m height. The chaotic time series analysis of the local voidage fluctuations has been adopted to characterize the nonlinear dynamics of the circulating fluidized bed riser. The variations of the correlation dimension and the Kolmogorov entropy of the voidage fluctuation were found to depend on the local time-average voidage. The axial and radial distributions of the correlation dimension and the Kolmogorov entropy were strongly affected by the solids flow structures (e.g. core-annulus flow) in various operating conditions. The correlation dimension of local voidage fluctuations increases along the riser, except the position near the distributor. Both, the correlation dimension and the Kolmogorov entropy of local voidage fluctuations near the wall, were found to be smaller than those at the center of the riser, independent of the solids circulation rate and the axial position.     

带中心管的两股催化剂颗粒混合预提升结构中的流动特性

在一套大型冷模实验装置上,采用光纤测量系统考察了带中心管的两股催化剂颗粒混合预提升结构内固含率和颗粒速度的分布,以径向不均匀指数对固含率和颗粒速度径向分布进行定量描述,并与不同的预提升结构进行对比. 结果表明,带中心管的预提升结构沿轴向可分为底部、中心管和提升管区,各区固含率和颗粒速度径向分布均匀程度为提升管区>底部区>中心管区;提升管区内1.3~2.3 m间存在一固含率和颗粒速度分布均匀截面;与无中心管的预提升结构和传统预提升结构相比,本实验结构固含率径向分布更均匀,有利于油剂接触;确定了截面平均固含率的影响参数为终端固含率es', Froude数Fr和轴向高度/提升管直径h/Dr;由实验数据回归出预提升结构内截面平均固含率轴向分布的经验模型,计算值与实验值吻合较好.     

Direct Measurements of Instantaneous Solid Flux in a CFB Riser using a Novel Multifunctional Optical Fiber Probe

With a novel optical fiber probe that can measure instantaneous local particle velocity and solid concentration simultaneously, extensive experiments were conducted to study transient flow structures in a 15.1‐m long circulating fluidized bed (CFB) riser of 100 mm in diameter. This study analyzed the radial and axial distributions of solid concentration, particle velocity, and their variations with nine operating conditions and at six axial levels. Instantaneous local solid concentration and particle velocity were found to be well correlated at most of the radial positions. The detailed time evolution, axial and radial distribution of instantaneous solid flux, and the variation of solid flux with operating conditions were also investigated. The radial solid flux profile showed a flat shape with a maximum at near wall area under most operating conditions. The instantaneous solid flux was found to have a strong fluctuation at a radial position of r/R = 0.8 ~ 0.9.     

Local hydrodynamic properties of gas phase in an internal-loop airlift reactor

The local hydrodynamic properties of the gas phase in an internal-loop airlift reactor were investigated in this study. The hydrodynamic properties including gas holdup, bubble velocity and bubble chord length were measured by dual electrical resistivity probes. The chord length distribution was then transformed to the bubble size distribution by modeling the bubbles as ellipsoids. It was found that the gas holdup increased with decreasing bubble velocity. In addition, most bubbles tended to rise along the riser central axis. Thus, the gas holdup in the axis was higher. The bubble size, bubble velocity and gas holdup were relatively constant in the axial direction of the riser except in the zones near the gas sparger and the gas–liquid separator. The bubble velocity became slower when the bubbles approached the gas–liquid separator. Moreover, the bubble size and bubble velocity for the three-phase system were relatively insensitive to the radial direction compared to those for the two-phase system. It was also found in this study that the bubble rise velocity and bubble size for the three-phase system were lower than that for the two-phase system. However, the gas holdup for the three-phase system were higher than that for the two-phase system due to bubble breakage caused by the solid particles.     

循环床提升管中粗重颗粒浓度的轴向分布

在10m高提升管中对空气-沙子体系的压力梯度进行系统测试,研究了粗重颗粒平均颗粒浓度云的轴向分布及操作条件对它的影响。结果表明,粗重颗粒的^εs在相同操作条件下显著低于FCC颗粒;随操作条件的不同,沙子颗粒表现出与FCC显著不同的轴向分布形态。高气速下粗重颗粒^εs的轴向分布与FCC相似表现为单调下降或直线形关系;但在表观气速Ug降低至某一临界值后,粗重颗粒^εs的轴向分布呈现出波动形式,表明沙子颗粒在提升管中的流动是一个加速-减速-再加速直至充分发展的过程。随Ug减小或Gs增大,提升管各截面上云升高;当^εs的轴向分布为波动形式时,提升管底部截面和中部颗粒聚集截面上^εs的变化较其它截面更为显著。     

耦合流化床提升管内固含率径向分布及沿轴向的发展

针对催化汽油辅助反应器改质降烯烃工艺,结合提升管与流化床的特点,建立了一套提升管与流化床耦合反应器大型冷态实验装置. 在不同操作条件下,采用PV-4A型光纤密度仪测定了提升管内固含率沿径向的分布规律. 结果表明,固含率径向分布整体上呈现中心小、边壁大的环-核结构分布特征;沿轴向向上,各径向位置上的固含率在颗粒加速区逐渐降低,在充分发展区趋于稳定,在颗粒约束返混区又有所升高;各径向位置上的固含率随表观气速增大或颗粒循环强度减小而减小,且均匀性变好;提升管上部流化床内颗粒静床高度只对颗粒约束返混区内固含率径向分布有影响,而对颗粒加速区和充分发展区的固含率径向分布影响较小;当表观气速较低或颗粒循环强度较大时,颗粒约束返混区上部局部固含率最大值出现在无因次半径f=r/R=0.7附近,此时局部无因次固含率es*=es/ 沿轴向在H>5.33 m时不再具有相似性;通过比较径向不均匀指数,得到轴向各区固含率径向分布趋于均匀的程度依次为：充分发展区>颗粒约束返混区>颗粒加速区. 利用实验数据回归出了局部固含率径向分布关联式,其平均相对误差在6%以内.     

不同粒度分布高硫焦颗粒在强混燃烧器内的流动特性

利用一套新型强混燃烧器,对3种不同粒度分布的高硫焦颗粒的流动特性进行了较为详尽的考察. 反应器主要由预混段和提升段组成,两部分结构尺寸分别为f240 mm′3000 mm和f70 mm′9000 mm. 实验操作条件为：提升管表观气速8~18 m/s,颗粒循环速率60~200 kg/(m2×s). 通过对3种不同粒度分布的颗粒在强混燃烧器内颗粒固含率、颗粒速度轴径向分布的测定,发现随掺入细颗粒比例的增加,截面平均颗粒速度降低;在较粗的高硫焦颗粒中适当掺入细粉,可使床层内颗粒固含率增加,返混加剧,停留时间延长,有利于高硫焦的充分燃烧.     

流化床-提升管耦合反应器内沥青颗粒的流动特性

针对重油残渣(沥青颗粒)气化制氢工艺,在流化床-提升管耦合反应器大型冷模实验装置上,考察了不同操作条件下沥青颗粒体系在耦合反应器内截面平均密度的轴向分布. 结果表明,对单组分沥青颗粒体系,耦合反应器适宜的操作条件为：提升管表观气速ug,r=0.70~1.76 m/s;与A类颗粒相比,沥青颗粒在耦合反应器内的流动特性呈现不同的特点,整个反应器沿轴向可分为底部流化床密相区、提升管底部低密度区、提升管颗粒密度重整区、提升管加速区、充分发展区和出口约束区6个区域;反应器内截面平均密度随颗粒质量流率增大而增大,随表观气速增大而减小;确定了耦合反应器内提升管区域截面平均固含率的影响参数为ep', Fr及H/Dr,并利用实验数据回归了平均固含率的轴向分布经验模型,其计算值与实验值吻合较好.     

提升管内气固两相双组分颗粒流动的离散颗粒硬球模型的模拟

基于离散颗粒(DPM)硬球模型,数值模拟提升管内双组分颗粒气固两相湍流流动行为。应用Vreman的亚格子尺度（SGS）模型模拟气体湍流,建立考虑不同颗粒加速度效应的两颗粒碰撞最小时间计算模型。数值模拟预测了大颗粒和小颗粒的速度和浓度分布。研究结果表明小颗粒具有高的轴向速度和脉动速度,而大颗粒具有低的轴向速度和脉动速度。在床中心区域,小颗粒轴向速度分布出现3个峰值,对于大颗粒轴向速度仅出现两个峰值。在壁面区域大颗粒和小颗粒速度均出现两个峰值。沿床径向方向呈现床中心颗粒浓度低、壁面区域颗粒浓度高的环核流动结果。随着表观气速的增大,颗粒浓度沿径向和床高分布趋于均匀。在床中心区域模拟计算轴向颗粒速度、颗粒浓度和RMS速度与文献实验结果相吻合。在提升管内气体湍流对小颗粒流动具有一定的影响,颗粒间碰撞作用对颗粒相流动的影响大于气相湍流的影响。     

Liquid circulation hydrodynamics in an external loop airlift reactor

The radial profiles of axial liquid velocity and gas hold‐ups are investigated in the riser of a pilot plant scale external loop airlift reactor (ELAR) using a modified Pavlov tube and differential pressure technique. The experimental investigation reveals that there exist two different kinds of liquid circulation structures in an ELAR, which has rarely been reported in the literature, namely internal liquid circulation, which exists only in the riser and external liquid circulation, which circulates through the downcomer. A power–law relationship is used to correlate the gas hold‐up and superficial gas velocity, which gives good agreement with experimental data. Experiments for axial liquid velocity profiles are analysed in analogy to a model described for a conventional bubble column. The results predicted by the model are in excellent agreement with the experimental data obtained under various operating conditions. © 2011 Canadian Society for Chemical Engineering     

Behavior of gas bubbles in a concentric cylindrical airlift column

Using a light transmission optical probe, the effect of superficial gas velocity on bubble properties(bubble size, bubble rising velocity, bubble frequency and local gas holdup) at axial and radial positions was determined in the riser and the downcomer of a concentric cylindrical airlift reactor. The vertical bubble length, the bubble rising velocity and the bubble frequency at axis in the riser increased with increasing superficial gas velocity and the bed height. The radial distribution of the local gas holdup, vertical bubble length and bubble frequency in the riser and the downcomer were found to be non-uniform. The profiles of the local gas holdup, vertical bubble length and bubble rising velocity in the riser were shown as parabolic shapes. The local gas holdup, the vertical bubble length and the bubble frequency in the downcomer changed with superficial gas velocity and the distance from the top of the draft tube.     

Measurement of three-dimensional velocity field in the translational region of a co-rotating twin-screw extruder

The velocity field in the screw channels of a co-rotating twin-screw extruder was measured using laser Doppler anemometry. Velocity distributions were measured for two screw elements having pitches of 14 and 28 mm, respectively. The magnitude of radial velocity component for both elements was no more than 10% of the magnitude of total velocity. The radial and the axial velocity components were similar for both screw elements. Wider range of tangential velocity values and steeper gradients near the flights were observed for smaller pitch screw element.     

气固循环床提升管内的局部颗粒浓度及流动发展

采用反射式光纤浓度探头对f100mm×15.1m循环床提升管8个轴向截面上11个径向位置的局部颗粒浓度进行了测量, 分析研究了颗粒浓度径向分布的不均匀性及其沿轴向的发展变化。结果表明:提升管内气固两相流的发展并不同步,而是一个由核心区向边壁区逐渐扩展,并最终达到总体充分发展的过程,该过程主要受边壁区发展过程所控制;相对于核心区,边壁区的发展不仅显著缓慢,而且受操作条件的影响也较显著。实验还发现:在颗粒加速段,无因次颗粒浓度的径向分布不具有相似性,不仅与径向位置有关,而且还与床层截面高度有关。     

循环流化床提升管中颗粒速度的径向分布及其沿轴向的发展

采用5光纤速度探头对f100mm?5.1m循环床提升管8个高度截面上11个径向位置的局部颗粒速度进行了实验测试,并采用径向不均匀指数(RNI)对颗粒速度径向分布的不均匀性及其沿轴向的变化进行了定量描述。研究结果表明:在高气速、高颗粒循环量操作时,操作条件对颗粒上升速度和下降速度的径向分布的影响在加速段和充分发展段呈现出不同的规律;颗粒上升速度和下降速度沿轴向的变化在核心区和边壁区也表现出不同的趋势。当颗粒循环速率大于200 kgm-2s-1时,颗粒的加速段长度大大延长,以至于大于提升管的高度(15.1m)。颗粒速度径向分布的不均匀性沿轴向是逐渐增大的,并且与截面平均颗粒速度存在很强的相关性。     

60米高循环流化床内物料浓度分布的冷态试验

利用电厂循环流化床锅炉现有的结构和设备, 搭建提升管高度60m、内径400mm的超高循环流化床冷态实验台, 重点研究了流化风速和颗粒密度对提升管内轴向和径向空隙率分布的影响。实验结果表明:空隙率分布形式与流化风速和物料密度密切相关, 对于一定的床料高度, 在底部密相区一直有床料堆积的情况下, 随着流化风速的增加, 提升管底部密相区空隙率增大, 上部稀相区的空隙率减小并且其在径向的分布变得更加不均匀;在一定的流化风速下, 密度较小的物料将更多的被带入上部稀相区, 上部稀相区的空隙率减小, 其在径向分布将变得更加不均匀。