Heat Transfer to an Isolated Bubble Rising in a High‐Temperature Incipiently Fluidized Bed

Very few experimental data are available on the heat transfer between bubble and emulsion phases; no data are available at high temperatures. The effects of bed particle type and size, bubble size, and bed temperature on the heat transfer coefficient were examined in this experimental and modelling study. The heat transfer coefficient was found to increase to a maximum, and then decrease as the bubble mass increased in the bubble mass range investigated. Increase in bed temperature led to a significant decrease in the heat transfer coefficient. The simple mathematical model developed earlier was extended to include operation at high temperature.     

The Effect of Bed Temperature on Mass Transfer between the Bubble and Emulsion Phases in a Fluidized Bed

The rate of interphase mass transfer between the bubble and emulsion phases of a bubbling fluidized bed is of primary importance in all models for fluidized bed reactors. Many experimental studies have been reported, however, all these investigations have been carried out in fluidized beds operated at room temperature. In this work, the effect of the bed temperature on the interphase mass transfer is reported. Single bubbles containing argon – used as a tracer – were injected into an incipiently fluidized bed maintained at the required temperature. The change in argon concentration in the bubble was measured using a suction probe connected to a mass spectrometer. The effects of bed particle type and size, bubble size, and bed temperature on the mass transfer coefficient were examined experimentally. The interphase mass transfer coefficient was found to decrease with the increase in bed temperature and bubble size, and increase slightly with increase in particle size. Experimental data obtained in this study were compared with some frequently used correlations for estimation of the mass transfer coefficient.     

Investigation of bubble behavior in fluidized beds with and without immersed horizontal tubes using a digital image analysis technique

Bubble characteristics such as shape, size, number and motion control the hydrodynamics and therefore heat transfer and chemical conversion in fluidized bed reactors. Thus understanding these characteristics is very important for the design and scale-up of fluidized beds. In this work a digital image analysis technique was developed to study the bubble behavior of two-dimensional bubbling beds with and without immersed horizontal tubes. Digital image analysis is a non-intrusive measurement technique which can simultaneously provide a great quantity of information without interfering with flow dynamics. The technique developed and implemented in this study allowed for the simultaneous measurement of various bubble properties, such as bubble diameter, rise velocity, aspect ratio and shape factor. A robust in-house code was developed to fully automate the image acquisition and data processing procedure. The experimental results obtained were validated and found to be in good agreement with available literature correlations.Moreover, based on the experimental results obtained new correlations for bubble growth and rise velocity as a function of bed height above the distributor were proposed. The models were in good agreement with the experimental data for a wide range of superficial velocities and particle sizes.     

脉冲气流对烯烃流化床气泡特性的影响

流化床具有良好的传热传质效率,因此广泛应用于煤粉燃烧、气力输送等场合。但是流化床在工作过程中会产生一定的恶性流动,脉冲的振动能量可以有效地提高流化床的传热系数,减少恶性流动现象发生。本文设计、搭建了烯烃流化床实验平台,通入混合脉冲气流。通过改变脉冲气流的相关参数,对静电信号进行功率谱密度函数分析。并基于静电信号估算气泡尺寸,从而获得流化床内气泡行为的变化。实验结果表明,脉冲气流的加入对气泡尺寸有一定的影响。随着脉冲频率的增加,气泡尺寸呈现先减小后增大的趋势,在脉冲频率为0.5Hz左右时气泡尺寸最小。脉冲气流加入后使得气泡尺寸减小,提高了颗粒流化效果,因此烯烃流化床内流化结束后的团聚颗粒质量明显降低。     

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。     

气固循环流化床提升管环核区颗粒质量传递系数的估计

Based on analysis of energy dissipation in the core region of gas-solid fluidized bed risers,a simplified model for determination of core-annulus solids mass transfer coefficient was developed according to turbulent diffu- sion mechanism of particles.The simulation results are consistent with published experimental data.Core-annulus solids mass transfer coefficient decreases with increasing particle size,particle density and solids circulation rate, but generally increases with increasing superficial gas velocity and riser diameter.In the upper dilute region of gas-solid fiuidized bed risers,core-annulus solids mass transfer coefficient was found to change little with the axial coordinate in the bed.     

Wall-to-bed heat transfer in three-phase fluidized beds of low density particles

Bed-to-wall heat transfer was measured in three-phase fluidized beds under conditions typical of biochemical process applications. The thermal resistance of the fluidized bed, which was significant in the absence of gas, became negligible when gas was introduced. Decreasing the particle density at constant gas and liquid velocity increased the bed-to-wall heat transfer coefficient. Previously published heat transfer correlations were used and gave poor predictions of our data. A new correlation was developed which predicted very well all the heat transfer coefficient results in this paper.     

A Novel Model for Predicting the Dense Phase Behavior of 3D Gas‐Solid Fluidized Beds

A novel phenomenological discrete bubble model was developed and tested for prediction of the hydrodynamic behavior of the dense phase of a 3D gas‐solid cylindrical fluidized bed. The mirror image technique was applied to take into account the effects of the bed wall. The simulation results were validated against experimental data reported in the literature that were obtained by positron emission particle tracking. The time‐averaged velocity profiles of particles predicted by the developed model were found to agree well with experimental data. The initial bubble diameter had no significant influence on the time‐averaged circulating pattern of solids in the bed. The model predictions clearly indicate that the developed model can fairly predict the hydrodynamic behavior of the dense phase of 3D gas‐solid cylindrical fluidized beds.     

Heat Transfer between Immersed Horizontal Tubes and Aerated Vibrated Fluidized Beds

Heat transfer coeffients between an immersed horizontal tube and an aerated vibrated fluldlzed bed are measured. There is a maximum value in the h-Г experlmental curve. The heat trander coefllcient increases with decreases in particle diameter in the fully fluidized region. The particle density has less effect on the heat transfer coetftclents. High smplltude and low frequency, or low amplitude and high frequency are favorable to heat transit. Exceedingly high gas veloclty is unfavorable to the surface-bed heat transfer. A model based on the ‘pocket‘ theory was proposed for predicting the surface-to-bed heat trausfer coefllclents in fully fluldlzed region. The predlctlons from the model were compared with observed data The reasonable fit suggests the adequacy of the model.     

Influence du passage d'une bulle sur le coefficient instantané de transfert de chaleur à une paroi immergée dans un lit fluidisé

This paper deals with the study of the vigorous particle mixing around an artificial bubble injected in a fluidized bed using a hot film anemometric technique.The fluctuations of the heat transfer coefficient have been studied as a function of the bubble characteristics and position with respect to the probe.In the bubble injection axis the heat transer coefficient depends very much on the bubbling frequency but only slightly on the bubble diameter. On the other hand, the distance x between the transfer surface and the bubble track was found very important. In this study we also determined the critical values of the bubble diameter and frequency which characterize the beginning of interaction between two successive bubbles.We propose a velocity potential model so as to take into account the form of the agitation zone of particles around the bubbles.     

Kinetic studies of the pyrolysis of sewage sludge by tga and comparison with fluidized beds

The pyrolysis of sewage sludge was examined by thermogravimetric analysis using different heating rates, particle sizes and final temperatures. A semi-empirical model was developed with four global consecutive/competitive reactions to volatile and solid products including also a heat balance. This model was extrapolated to isothermal reactor conditions and compared to experimental data on fluidized bed pyrolysis. Depending on the particle size and reactor temperature, about 90% of the maximum conversion in a fluidized bed takes place under external heat transfer control, the rest under kinetic control. Incomplete conversion occurs at short particle residence times, which were calculated approximately by the model.     

Particle scale study of heat transfer in packed and bubbling fluidized beds

The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle‐fluid flow, is extended to study particle‐particle and particle‐fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid‐particle convection, particle‐particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

模拟三相流化床中颗粒尺寸对单气泡传质系数的影响

本文在模拟三相流化床中,测量了单气泡的传质系数,实验结果表明颗粒直径对气-液传质系数影响显著。颗粒直径较大时,传质系数有明显提高。     

On the heat transfer mechanism in three phase fluidized beds

A two resistance model is proposed for the heat transfer between a coaxially mounted heater and a three phase fluidized bed. Effects of gas and liquid velocity and particle size on individual heat transfer resistances in the heater and in the fluidized bulk zones have been determined. The optimum bed porosity at which the maximum heat transfer coefficient occurred coincided with the bed porosity at which the boundary layer thickness around the heater attained a minimum value. The fluidized bed resistance attained its minimum value when the maximum heat transfer coefficient is achieved in two and three phase fluidized beds. The heat transfer in the zone adjacent to the healer is found to be the rate controlling step since the contribution of fluidized bed resistance was found to be less than 10% of the heater zone resistance in two and three phase fluidized beds. The heat transfer resistances in liquid and three-phase fluidized beds have been represented by a modified Stanton and Peclet numbers based on the heat transfer resistances in the heater zone and in the fluidized bulk zone in series.     

Heat transfer within dynamically structured bubbling fluidized beds subject to vibration: A two-fluid modeling study

Bubbling fluidized beds are often used to achieve a uniform particle temperature distribution in industrial processes involving gas and particles. However, the chaotic bubble dynamics pose significant challenges in scale-up. Recent work (Guo et al., 2021, PNAS 118, e2108647118) has shown that using vibration can structure the bubbling pattern to a highly predictable manner with the characteristic bubble properties independent of system width, opening opportunities to address key issues associated with conventional bubbling fluidized beds. Herein, using two-fluid modeling simulations, we studied heat transfer characteristics within the dynamically structured bubbling fluidized bed and compared to unstructured bubbling fluidized beds and packed beds. Simulations show that the structured bubbling fluidized bed can achieve the most uniform particle temperature distribution because it can achieve the best particle mixing while maintaining a global heat transfer coefficient similar to that of a freely bubbling fluidized bed.     

Heat transfer in three-phase (G/L/S) circulating fluidized beds with low surface tension media

Characteristics of heat transfer were investigated in a three-phase circulating fluidized bed whose diameter and height were 0.102 m (ID) and 2.5 m, respectively. Effects of gas and liquid velocities, particle size (0.5–3.0 mm), solid circulation rate (2.0–6.5 kg/m² s), and surface tension (47.53–72.75×10⁻³ N/m) of liquid phase on the heat transfer coefficient were examined. It was found that the heat transfer coefficient (h) between the immersed vertical heater and the riser proper of the three-phase circulating fluidized bed increased with increase in gas and liquid velocities, but did not change considerably with a further increase in liquid velocity, even in the higher range. The value of heat transfer coefficient increased gradually with increase in the size of fluidized solid particles without exhibiting the local minimum, which represented that there was no bed contraction in three-phase circulating fluidized beds due to the higher liquid velocity. The heat transfer system could attain a stabilized condition more easily with increase in particle size. The value of heat transfer coefficient increased with increase in solid circulation rate in all the cases studied due to the increase of solid holdup in the riser. The value of heat transfer coefficient decreased with increase in surface tension of liquid phase, due to the decrease of bubbling phenomena and bubble holdup. The decrease in liquid surface tension could lead to an increase in elapsed time from which the temperature difference between the heater surface and the riser became an almost constant value. The experimentally obtained values of heat transfer coefficient were well correlated in terms of dimensionless groups as well as operating variables.     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

汽液固三相流动沸腾传热计算与实验研究

对汽液固三相循环流化床中流动沸腾传热进行了理论分析和实验研究 ,在此基础上结合渐进模型及表面更新机理建立了汽液固三相流动沸腾传热模型 ,模型计算值和实验数据吻合较好 ,最大偏差在 18%以内     

Investigation of Temperature Distribution and Heat Transfer in Fluidized Bed Using a Combined CFD-DEM Model

Fluidized beds are widely used in many industries because they are effective for both mixing and drying. The distinct element method (DEM) has recently received more attention for investigating the phenomena of multiphase flow because the technique is effective in gathering detailed information on complex phenomena without physically disturbing the flows. However, most studies have focused on the aerodynamics of the particles. In this study, a combined computational fluid dynamics (CFD)-DEM model, which allows prediction of gas and particle temperature profiles and heat transfer coefficients in a two-dimensional fluidized bed, was developed. The predicted results were compared with the experimental results at the superficial gas velocities of 2.04, 2.22, and 2.41 m/s and at the controlled inlet temperature of 343 K. Based on the comparison between the predicted and experimental results, it was found that the developed model performed adequately in predicting the gas temperature profiles, and the predicted particle temperature profiles were higher than the experimental data. The predicted heat transfer coefficient was slightly higher than the experimental data. However, the predicted and experimental results had a similar trend in which the heat transfer coefficient increased as a function of an increase in superficial gas velocity. In addition, the minimum fluidization velocity predicted by the developed model agreed well with the experimental data. Such predictions can provide essential information on temperature and heat transfer coefficients inside the fluidized bed for design and scale-up.     

循环流化床的沸腾传热

A model is proposed to predict boiling heat transfer coefficient in a three-phase circulating fluidized bed (CFB), which is a new type of evaporation boiling means for enhancing heat transfer and preventing fouling. To verify the model, experiments are conducted in a stainless steel column with 39 mm ID and 2.0 m height, in which the heat transfer coefficient is measured for different superficial velocities, steam pressures, particle concentrations and materials of particle. As the steam pressure and particle concentrations increase, the heat transfer coefficient in the bed increases. The heat transfer coefficient increases with the liquid velocity but it exhibits a local minimum.The heat transfer coefficient is correlated with cluster renewed model and two-mechanism method. The prediction of the model is in good agreement with experimental data.