Fluidized bed freeboard measurements of heat transfer

Local surface-to-suspension heat transfer coefficients in the freeboard of a fluidized bed reactor were measured by a newly designed miniature probe. Four types of plate orientations were used to test directional effects of surface on heat transfer rate. Variations of heat transfer coefficients with radial position are more significant at lower freeboard. For plate parallel both to the axial and radial directions of the column, the heat transfer coefficient (h) is higher at the center than at wall. For plate with exposed surface parallel to the axial and perpendicular to the radial directions, the plate gives nearly the same value of h at the center and the wall. At low heights of freeboard, center-facing surface exhibits higher heat transfer rate than wall-facing one. The phenomena of higher solids concentration and center to wall movement of particles account for this observation. Variations of heat transfer coefficients with elevation in the freeboard are as much as an order of magnitude, decreasing to gas convection with increasing elevation. These variations are relatively insensitive to the types of plate orientation investigated.     

Analysis of local heat transfer and hydrodynamics in a bubble column using fast response probes

A fast response probe is used to measure local heat transfer in a bubble column. It captured the variations in local heat transfer coefficients due to changes in local hydrodynamic conditions in radial and axial directions. These measurements have been used to identify flow regime transitions, variations in flow patterns and local hydrodynamic structure as obtained with different gas distributors and varying gas velocity. Standard deviations of pressure measurements obtained with a fast response probe have been compared with heat transfer coefficient fluctuations for the first time and the similarities and differences have been pointed out. Variations in average heat transfer coefficients and standard deviations in radial and axial directions point to different hydrodynamic conditions and are compared with literature studies. Relationships between local heat transfer measurements and hydrodynamic conditions are shown.     

快速流化床内探头设置方向对传热系数测量结果的影响

本文在一高8m,直径186mm 的快速流化床中采用小型电加热探头测定了床层与换热面之间的局部传热系数,实验中通过将探头向上和向下设置考察了探头设置方向对局部传热系数的影响,实验表明探头在床中的设置方向不同将对传热系数的测定值产生一定的影响:另外在同一床层浓度下,操作气速和固体循环速率的变化将对传热系数产生直接的影响     

快速流化床内床层与浸润表面间的传热研究——传热系数的径向分布规律

本文对低温快速流化床内传热系数的径向分布规律进行了实验研究.推荐以下关联式来计算床内各轴、径向位置处的传热系数:式中a、n_1、n_2均为径向位置的函数,(?)为轴向位置的函数.     

BOILING TWO-PHASE FLOW HEAT TRANSFER IN A HORIZONTAL BEND

A paucity of heat transfer rate data for boiling two-phase flow through bends was noted after an extensive literature survey, The present work was undertaken to redress this shortcoming.

A boiling water loop, capable of being operated at pressures up to 1300 kpa, was used. The lest section was in the form of a U-tube with two straight horizontal sections connected by a 180^° return bend. Using this loop, pressure drop and heat transfer data were gathered over a wide range of mass and heat fluxes. steam qualities and system pressures.

The data obtained were used to investigate the variation in heal transfer coefficients around the radial positions of the bend. Correlations for heat transfer coefficients for four different radial positions (top. bollom, inside and outside of the bend) have been presented for the first time. A possible explanation for the observed variations in heat transfer coefficients has also been suggested.     

BOILING TWO-PHASE FLOW HEAT TRANSFER IN A HORIZONTAL BEND

Abstract

A paucity of heat transfer rate data for boiling two-phase flow through bends was noted after an extensive literature survey, The present work was undertaken to redress this shortcoming.

A boiling water loop, capable of being operated at pressures up to 1300 kpa, was used. The lest section was in the form of a U-tube with two straight horizontal sections connected by a 180^° return bend. Using this loop, pressure drop and heat transfer data were gathered over a wide range of mass and heat fluxes. steam qualities and system pressures.

The data obtained were used to investigate the variation in heal transfer coefficients around the radial positions of the bend. Correlations for heat transfer coefficients for four different radial positions (top. bollom, inside and outside of the bend) have been presented for the first time. A possible explanation for the observed variations in heat transfer coefficients has also been suggested.     

高粘变物性流体垂直管内流动传热

对高粘变物性流体管内传热进行了计算机仿真计算.结果表明,高粘流体传热时径向温差很大,采用较小的加热管径可以有效地减小径向温差.     

快速流化床床层与内浸表面间的传热特性实验研究

在内径186mm、高8m的快速流化床内,采用特殊设计的组合式传热探头,考察了局部传热系数沿换热表面长度方向的分布规律.结果表明,只要传热表面积足够大,设置于床层径向的任何位置,都可能诱导颗粒聚集,并在传热表面形成从上向下运动的絮状物层,从而使局部传热系数沿换热表面向下逐渐降低.实验结果预示,对具有较大尺寸的换热表面,影响传热系数的主要因素是局部颗粒密度及颗粒在表面的更新颇率.气体对传热的贡献主要是通过改变颗粒在传热表面的更新频率,影响颗粒非稳态导热过程而实现的.相比之下,气体对流传热的贡献可以忽略.     

经孔板形成旋转流的管壳式换热器壳程传热及流阻的研究

对螺旋折流孔板管壳式换热器壳程的传热与流体阻力做了研究 ,给出换热器壳程传热与流阻的计算关联式 ,并采用实验模型对换热器壳程流体旋转流的阻力系数与传热管的局部传热系数做了测试 ,且对光滑和菱形翅片两种管型作了对比     

Thermal dispersion and wall heat transfer in packed beds

A new analysis showing the effect of axial and radial thermal dispersion and wall thermal resistance upon heat transfer to fixed beds of solids is presented. By application of this theory and non-linear regression, coefficients of axial and radial dispersion and wall heat transfer coefficients are calculated from experimental measurements of radial temperature profiles in fixed beds heated at the wall.The experiments have been performed for beds packed with glass and with metallic particles within the particle Reynolds number range from 1 to 400.The calculated coefficients are compared with experimental values reported by other workers. Some differences are attributed to the neglect of axial dispersion in the work of others, but other differences are significant in that, for example, thermal characteristics of fixed beds of metallic particles differ from those of non-metallic particles.     

Local hydrodynamics and heat transfer in fluidized beds of different diameter

Heat transfer coefficients were measured in fluidization columns of 0.29 m and 1.56 m ID at fixed distance above the distributor, using an identical vertical heater, the same alumina particles and geometrically scaled bubble-cap distributors. The magnitude of the maximum heat transfer coefficients was found to be unaffected by the column diameter, but occurred at higher superficial velocities in the larger column. When the local flow structure near the heater corresponded to the turbulent flow regime of fluidization, the local heat transfer coefficients were independent of radial position in both columns.     

鼓泡塔列管传热系数的测量与估算

在φ500 mm×5000 mm的冷模实验装置中,使用自制传热探头,对无内构件的空塔和安装31根竖直换热列管的鼓泡塔内列管传热系数进行了测量.实验表明,列管传热系数随表观气速的增加而增大,传热系数沿径向呈抛物型分布,垂直列管内构件的加入使得传热系数的径向分布变得更为陡峭.基于表面更新理论,结合鼓泡塔内气含率和液速分布的测量及计算结果,提出了计算传热系数的数学模型.该模型既可以用于空塔的局部传热系数与平均传热系数计算,也可以用于安装列管束的局部传热系数与平均传热系数计算.模型计算值与实验数据符合良好,最大相对误差为5.62％.     

Heat transfer study in a pilot-plant scale bubble column

The effects of superficial gas velocity on heat transfer coefficient and its time-averaged radial profiles along the bed height have been investigated in a pilot-plant scale bubble column of 0.44 m diameter using air-water system. Notable differences were observed in heat transfer coefficients along the bed axial locations particularly between the sparger (Z/D = 0.28) and the fully developed flow (Z/D = 4.8) regions. In the fully developed flow region larger heat transfer coefficient values were obtained compared to those in the sparger region. About 14-22% increase in heat transfer coefficients measured in the fully developed flow region has been observed compared to those measured in the distributor region when the superficial gas velocity increases from 0.05 to 0.45 m/s. The heat transfer coefficients in the column center for all the conditions studied are about 9-13% larger than those near the wall region. It has been noted that in the fully developed flow region, the axial variation of the heat transfer coefficients was not significant.     

Wall to fluid heat transfer in liquid fluidised beds: Part 2

Wall to fluid heat transfer coefficients and radial temperature profiles have been obtained for beds of hydrodynamically similar spheres fluidized with water in a 2.058 inch pipe at a constant heat flux. From packed bed to open pipe conditions, heat transport occurs mainly by turbulent mixing, although conduction through the particles and possibly particle convection have some effect at low porosities. This result contradicts a previously published prediction based on model calculations using erroneous temperature profiles⁽²⁴⁾. The model predicts a minor role for particle convection when appropriate temperature profiles are used. A series model based on the observed shift of thermal resistance from the wall region to the bulk of the bed with decreasing porosity is used to correlate heat transfer coefficients. The shift in resistance largely accounts for the maximum in heat transfer coefficient plots.     

固定床内传热参数的估计与分析

本文在系统地考察了各种因素对床层径向有效导热系数和壁传热系数影响的实验研究与计算机模拟的基础上,对文献中发表的固定床内传热参数估值离散的原因进行了分析。结果表明,Bi取值在5.0左右时,对床层出口温度比较敏感,是造成h_(?)估值离散的重要原因之一。床层填充的随机性影响床层温度分布,低床层、低Re_p条件下估计参数误差要大。d_(?)取值范围不同,壁冷却和壁加热条件下估计出的参数均会不同。     

Heat transfer in aggregative and particulate liquid-fluidized beds

Wall-to-bed heat transfer in liquid fluidized beds, particulately and aggregatively fluidized, was studied. Glass particles fluidized with water gave particulate fluidization and lead particles with water gave aggregative fluidization. Local heat transfer coefficients and bed temperature profiles were measured. Heat transfer coefficients were found to be strongly dependent on particle size and porosity and increased with increasing particle size, but were independent of the height of the heater surface from the grid. Any variations in local bed properties, such as porosity do not affect wall-to-bed heat transfer. The heat transfer coefficients show a characteristic, maximum at porosities near 0.7 for both systems. Bed temperature profiles deviate considerably from open-pipe values.A two-resistance model for the heat transfer resistance agrees well with the data. Bed resistance is modeled by a radial eddy diffusivity, which indicates the mixing effectiveness in the core of the bed. Glass beds (particulate) show a maximum mixing effectiveness at porosities near 0.7 and the mixing effectiveness increases with particle diameter. Lead beds (aggregative) show two maxima in mixing effectiveness, the first between porosities of 0.5 and 0.6, and the second between porosities of 0.7 and 0.8. Mixing is greatest at an intermediate particle size in the case of lead beds. In both systems the fraction of the total resistance in the bed core increases as porosity decreases towards packed bed conditions.     

THERMAL DISPERSION AND HEAT TRANSFER IN NONISOTHERMAL BUBBLE COLUMNS†

Local axial and radial temperatures were measured at steady-state conditions in a 0.078-m-I.D. bubble column heat exchanger. Nitrogen and water superficial velocity ranges were 0-0.6 m/s and 0-0.02 m/s, respectively. Average column pressures were 3.0, 5.1, and 7.1 atm. The axial temperature profile varied significantly with all conditions encountered. Radial temperature profiles were found to be nearly constant, indicating very good radial mixing.

An axial thermal dispersion heat transfer model, capable of representing nonisothermal systems, was employed to characterize the measured bubble column temperature profiles. Thermal dispersion was apparent from large temperature changes in the entrance of the bubble column. Heat transfer coefficients depended on the gas and liquid flow rates. However, the thermal dispersion coefficients depended on linear gas velocity and were a weak function of liquid flow rates. The thermal dispersion coefficients obtained in this study were found to be consistent with other investigations. In addition, they were compared to the mass dispersion coefficients obtained by other studies and found to be in good agreement     

竖直管气固鼓泡流化床传热机理的CPFD模拟

针对细颗粒气固鼓泡流化床中床料与竖直传热管壁面间的传热行为,在前期实验的基础上,采用计算颗粒流体力学(CPFD)方法从颗粒在传热壁面更新的角度,深入分析了传热特性与壁面气固流动行为之间的关联性。结果表明,模拟得到的传热管壁面颗粒更新通量和基于颗粒团更新模型的颗粒团平均停留时间均能很好解释实验测得的传热系数变化规律,这证实颗粒团更新是影响传热过程的控制性因素。模拟还发现随加热管从床层中心向边壁的移动,加热管周向方向上颗粒更新通量和传热系数的不均匀性都呈增大趋势。随着表观气速的增大,气泡行为导致床层颗粒内循环流率增大,这是导致颗粒团在加热管壁面上的更新频率增大以及床层与壁面间传热系数增大的根源。     

Investigation of the heat transfer intensification mechanism for a new fluidized catalyst cooler

A small cold model was employed to investigate the heat‐transfer mechanism for a new fluidized catalyst cooler. Local heat‐transfer coefficients (h) and tube surface hydrodynamics were systematically measured by a specially designed heat tube and an optical fiber probe. The higher total h further validated the feasibility of the heat transfer intensification method used in the new catalyst cooler, which indicated that the induced higher packet renewal frequency due to the nonuniform gas distribution played a dominant role in its increased hs. Strongest heat transfer intensification effect was located at r/R_w>0.8 below the heat transfer intensification height. The changes of the mean packet residence time in the radial and axial directions and with superficial gas velocity were all agreeable with the measured hs according to the packet renewal theory. This further demonstrated the feasibility of the experimental method for tube surface hydrodynamics. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2415–2427, 2015     

固定床吸附过程传热传质耦合影响数值模拟

引言 多孔介质中物质传递与热量传递相互影响,这种现象最早由Soret和Dufour提出[1].由温度梯度作用产生的传质效应称为Soret效应(热附加扩散效应),它代表由温度场的不均匀性而导致的传质现象;由浓度梯度作用产生的传热效应称为Dufour效应(扩散附加热效应),它代表由浓度场的不均匀性而导致的传热现象.