A Numerical Investigation of Heat Transfer Mechanisms in Gas-Solid Fluidized Beds Using the Two-Fluid Granular Temperature Model

In this work, the two-fluid granular temperature model is used to investigate the heat exchanged between a heated wall and a gas-solid fluidized bed. Numerical simulations were performed in 2-D and 3-D fluidized beds using a solid phase effective thermal conductivity correlation based on the granular temperature. The heat exchange in the bubbles' wake is investigated by tracking the train of bubbles that rises along the heated wall. Large heat transfer coefficients were obtained in the rear wake region of bubbles due to relatively larger granular temperature there and intense particle circulation.     

流化床气固传热特性的实验研究

通过对汉化床气固传热特性的简单分析,依据稳态工况活动段区气体温度分布求取了有效传热系统。实验结果和传统经验式数值吻合,从而论证了气固传热特性分析的合理性及计算流化床气固有效传热系数的可行性,为研究流化床气固传热特性的提供参考。     

循环流化床内气粒两相传热的萘升华模拟实验研究

在循环流化床试验台上对床中气体与颗粒两相间的传热特性进行了试验研究，试验中首次将萘升华热质类比技术应用于循环流化床内气粒两相间传热的研究中，考察了不同的固体颗粒循环量、一次风风速和床料平均粒径对气粒间换热的影响。试验表明：随着一次风风速的增加，循环床中气体和颗粒之间的表现热换系数变大，当固体颗粒循环量增加或颗粒平均粒径减小时，表征相间换热特性的Nu数增大。图5表1参4     

Combustion of isolated bubbles in an elevated-temperature fluidized bed

Combustion of isolated bubbles was investigated with a laboratory-scale fluidized-bed reactor. Two different combinations of oxygen and argon were employed as the fluidizing gas. Single bubbles of methane were injected into an incipiently fluidized bed maintained at elevated temperatures. Gas composition inside the bubbles was measured using a suction probe connected to an on-line mass spectrometer, and the temperature of the bubbles was monitored using a fast-response thermocouple. The effects of bed particle type, particle size, bubble size, bed temperature, and oxygen concentration in the emulsion phase were examined for bed temperatures between 923 and 1203 K. A theoretical model of homogeneous combustion within the bubble phase was developed for comparison to the experimental results. The model accounted for the heat and mass transfer between bubble and emulsion phases, but only considered combustion within the bubble. The results indicated that small bubble size and high oxygen concentrations in the emulsion phase enhanced bubble-phase combustion. The bed temperature also proved to be an important parameter, with higher temperatures promoting bubble combustion, but unlike some other investigations, no critical ignition temperatures were observed in either experiments or model results. The fluidized bed's particle size and particle composition influence the heat and mass-transfer coefficients, and therefore the bubble-phase combustion, but these have a smaller influence than bed temperature and bubble size. Model results for bubble-phase gas composition and temperature compared favorably with the experimental measurements.     

流化床煤燃烧与气化过程中的辐射换热

流化床反应器中颗粒与颗粒之间的传热在一定程度上决定了化学反应的速率及反应的中间历程。本文通过对气固流化床乳化相中颗粒群结构的进一步认识，建立了颗粒间的辐射换热模型，比较了不同颗粒直径、不同床层温度水平及不同流化工况下颗粒间辐射换热与通过气膜导热份额的大小，并预测了流化床反应器中反应颗粒与惰性床料之间的温差，对于流化床反应器选择合理的运行工况和进行操作参数优化具有参考价值     

Simulation study of the effect of the restitution coefficient on interphase heat transfer processes and flow characteristics in a fluidized bed

This article discusses a simulation study performed to investigate the effect of particle collision on inter-particle and gas–solid heat transfer processes, and other related bed flow characteristics. The effect of particle elasticity is presented using different values of the particle–particle coefficient of restitution. The simulation study was carried out using a two-dimensional model of a fluidized bed reactor incorporated to ANSYS Fluent 16.2 software. Two different materials, steel beads and sand particles, were used as the bed material fluidized by air. The simulation results are compared to those from previous studies on fluidized bed reactors containing a single bed material. The coefficient of restitution affected the bed hydrodynamics. Specifically, an increasing coefficient of restitution resulted in an increasing bed pressure drop and decreasing void fraction, granular temperature, particle velocity, and collision frequency. Conversely, increasing the particle coefficient of restitution resulted in decreasing the particle–particle heat exchange coefficient and the gas–particle heat transfer coefficient. The gas–particle heat transfer coefficient for sand particles was higher than that for steel beads. The effect of the coefficient of restitution on the flow characteristics from a binary mixture bed was quite similar to those of single material beds found in previous studies. This study demonstrated that the restitution coefficient clearly affected both the particle–particle and gas–particle heat transfer processes.     

气固流化床的离散颗粒运动-碰撞解耦模型与模拟

基于分子动力学和气固两相流体动力学，建立流化床稠密气-固两相离散颗粒运动-碰撞解耦模型，采用硬球模拟方法处理颗粒与颗粒之间的碰撞，及大涡模拟方法处理气相湍流流动．单颗粒运动满足牛顿第二定律，颗粒相和气相相间相互作用的双向耦合由牛顿第三定律确定，数值模拟二维鼓泡流化床内稠密气-固两相流动，得到了气泡的形成、发展及颗粒的流化过程，计算结果表明颗粒弹性恢复系数影响气-固两相流动特性。     

Numerical prediction of heat transfer in fluidized beds by a kinetic theory of granular flows

In dense gas–solid two-phase flows of bubbling fluidized beds the particle-to-particle interactions cannot be neglected and an Eulerian approach has been used to predict the fluid dynamics as well as the heat transfer. The physical properties of the solid phase can be modeled with the kinetic theory of granular medias and the governing equations are solved numerically. The present work compares different physical models for the thermal transport coefficients of the solid phase for a lab-scaled two-dimensional fluidized bed filled with mono-disperse glass beads. The numerical results show a strong correlation between fluid dynamics and the instantaneous heat transfer similar to the so-called packet theory by Mickley and Fairbanks [1].     

Combustion in crater beds

A preliminary study of a new type of heat-recirculating particulate bed combustor has been carried out with a view to overcoming some limitations arising in the scaling up of spouted bed combustors. A fountain of particles, somewhat similar to that produced in spouted beds, is raised by a jet of gaseous reactants flowing downward into a crater formed below the bed surface. Reactants are preheated prior to combustion both by heat transfer from the products across the walls of the inlet tube and by the interaction of reactants with the inert particles heated by the flame. Combustion characteristics are studied for single and multiple inlet tubes in small laboratory burners and in a much larger 50 KW system. Crater beds are compared with other two-phase combustors, especially spouted beds. They readily lend themselves to scaling up by the use of multiple jets, from one or from several inlets, and recirculate heat between products and reactants both within the craters and by heat transfer to the inlet tubes. Tests show that they can be used either with gaseous reactants entering through the jet or with only oxidant introduced in that way, fuel being included within the bed. Stability limits established for the laboratory burners show a leaner burning capability than is achievable in spouted or fluidized beds and the device offers several advantages over other circulating two-phase systems.     

Opportunities and challenges in using particle circulation loops for concentrated solar power applications

Concentrated Solar Power (CSP) is an electricity generation technology that concentrates solar irradiance through heliostats onto a small area, the receiver, where a heat transfer medium, currently a fluid (HTF), is used as heat carrier towards the heat storage and power block. It has been under the spotlight for a decade as one of the potential or promising renewable and sustainable energy technologies.Using gas/solid suspensions as heat transfer medium in CSP has been advocated for the first time in the 1980′s and this novel concept relies on its possible application throughout the full CSP plant, i.e., in heat harvesting, conveying, storage and re-use, where it offers major advantages in comparison with the common heat transfer fluids such as water/steam, thermal fluids or molten salt. Although the particle suspension has a lower heat capacity than molten salts, the particle-driven system can operate without temperature limitation (except for the maximum allowable wall temperature of the receiver tubes), and it can also operate with higher hot-cold temperature gradients. Suspension temperatures of over 800 °C can be tolerated and achieved, with additional high efficiency thermodynamic systems being applicable. The application of high temperature particulate heat carriers moreover expands the possible thermodynamic cycles from Rankine steam cycles to Brayton gas cycles and even to combined electricity generating cycles.This review paper deals with the development of the particle-driven CSP and assesses both its background fundamentals and its energy efficiency. Among the cited systems, batch and continuous operations with particle conveying loops are discussed. A short summary of relevant particle-related properties, and their use as heat transfer medium is included. Recent pilot plant experiments have demonstrated that a novel bubbling fluidized bed concept, the upflow bubbling fluidized bed (UBFB), recently adapted to use bubble rupture promoters and called dense upflow fluidized bed (DUFB), offers a considerable potential for use in a solar power tower plant for its excellent heat transfer at moderate to high receiver capacities.For all CSP applications with particle circulation, a major challenge remains the transfer of hot and colder particles among the different constituents of the CSP system (receiver to storage, power block and return loop to the top of the solar tower). Potential conveying modes are discussed and compared. Whereas in solar heat capture, bubbling fluidized beds, particle falling films, vortex and rotary furnaces, among others, seem appropriate, both moving beds and bubbling fluidized beds are recommended in the heat storage and re-use, and examined in the review.Common to all CSP applications are the thermodynamic cycles in the power block, where different secondary working fluids can be used to feed the turbines. These thermodynamic cycles are discussed in detail and the current or future most likely selections are presented.Since the use of a back up fuel is recommended for all CSP systems, the hybrid operation with the use of alternative fuel back-up is also included in the review.The review research is concluded by scale-up data and challenges, and provides a preliminary view into the prospects and the overall economy of the system. Market prospects for both novel concentrated solar power are expected to be excellent. Although the research provided lab- and pilot-scale based design methods and equations for the key unit operations of the novel solar power tower CSP concept, there is ample scope for future development of several topics, as finally recommended.     

Heat transfer characteristics in a pulsating fluidized bed in relation to bubble characteristics

A pulsating fluidized bed is operated with two sequential durations designated as an on‐period with injecting fluidization gas and an off‐period without it. The heat transfer coefficient between a vertically immersed heater and bed in a pulsating fluidized bed is measured under various pulse cycles and fluidized particles. The obtained results are compared with those in a normal fluidized bed with continuous fluidization air injection. The relationship between heat transfer coefficients and bubble characteristics, evaluated using a digital video camera, has also been investigated. For certain fluidized particles and operating pulse cycles, the fluidization of particles and the increment of heat transfer coefficients can be obtained under a mean air velocity based on a pulse cycle duration smaller than the minimum fluidization air velocity in a normal fluidized bed. Under the pulse cycles where a static bed through the whole bed is formed in the off‐period duration, the improved heat transfer rate over that in a normal fluidized bed can be measured. This may be attributed to large bubble formation. As heat transfer in the pulsating fluidized bed is obstructed with increasing time to keep a static bed due to the excessive off‐period duration, it is indicated that there is an optimum off‐period duration based on the heat transfer rate. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 307–319, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10038     

Theoretical performance analysis of the multi‐stage gas–solid fluidized bed air preheater

The multi‐stage fluidized bed can be used to preheat the combustion air by recovering the waste heat from the exhaust gas from industrial furnaces. The dilute‐phase fluidized bed may be formed to exclude the excessive pressure drop across the multi‐stage fluidized bed. But, in this case, the solid particles do not reach to the thermal equilibrium due to relatively short residence time in each layer of fluidized bed. In this study, a theoretical analysis on the dilute phase multistage fluidized bed heat exchanger was performed. A parameter related to the degree of thermal equilibrium between gas and solid particles at the dilute‐phase fluidized beds was derived. Using this parameter, a relatively simple expression was obtained for the thermal efficiencies of the multi‐stage fluidized bed heat exchanger and air preheater. Copyright © 2001 John Wiley & Sons, Ltd.     

Bubble induced heat transfer in two phase gas-liquid flow

The influence of gas bubbles on heat transfer in two phase gas-liquid systems has been investigated. Platinum wires have been used as heat-transfer probes and the two phase flow has been simulated by generating a single continuous stream of discrete gas bubbles into a stationary liquid. The contribution of various modes of heat transfer has been determined. It has been found that transient conduction into the liquid is the predominant mode of the bubble induced heat transfer and is responsible for about 75 per cent of heat transfer. Convection contributes the remainder. A theoretical model of the bubble induced heat transfer based on the surface renewal and penetration theory has been developed.     

Mitigation of ice crystallization fouling in stationary and circulating liquid–solid fluidized bed heat exchangers

Liquid–solid fluidized bed heat exchangers are attractive ice crystallizers since they are able to mitigate ice crystallization fouling and exhibit high heat transfer coefficients. Experiments show that the fouling removal ability of stationary fluidized beds increases with decreasing bed voidage (95–80%) and increasing particle size (2–4 mm). The removal of ice crystallization fouling appears to be more effective in circulating fluidized beds, especially at high circulation rates. Fouling removal is realized by both particle–wall collisions and pressure fronts induced by particle–particle collisions. A comparison between ice crystallization experiments and impact characteristics shows that the removal rate is proportional to the impulse exerted on the wall. A model based on these phenomena is discussed and predicts the transition temperature difference for ice crystallization fouling in both stationary and circulating fluidized beds with an average absolute error of 9.2%.     

Experimental research of radiative heat transfer in fluidized beds

A new method to measure the radiative heat transfer in fluidized beds was presented. Experiments were carried out on a 0.8 th⁻¹ fluidized bed combustion boiler. The residual slag of fired coal was operated in a fluidized bed at room temperature. As the radiative heat transfer at room temperature is insignificant, its contribution at high temperatures might be obtained by the comparison of experimental results at high and low temperatures. On experimental study, a radiative contribution was given as a function of bed temperature and particle size. The results were compared with those in other references.     

A modelling study to the heat transfer between immersed surfaces and large-particle fluidized beds

A new theoretical model is proposed for the heat transfer between immersed surfaces and large-particle fluidized beds. The heat transfer of the emulsion phase to the immersed surface is treated as the sum of the convective part of the interstitial fluid flow and the conductive part of the solid particles. The heat transfer of bubbles to the surfaces is also considered. The theoretical calculating formulae obtained are in fairly good agreement with experimental data in a large region.     

流化床密相区流动特性的数值模拟

流化床内气固两相流动一直是实验研究和数值模拟的热点。基于Eulerian双流体模型,本文建立了流化床内的气固两相流动模型,采用FLUENT软件对流化床密相区两相流动特性、床内气泡的产生运动和爆裂等特性进行了数值模拟。模型中,将颗粒相看作是连续介质,建立与气相相同形式的数学模型;采用了离散介质动力理论,引入颗粒温度来描述固相粘性应力,并用气固曳力进行气固两相耦合。模拟得到了气泡产生、运动和爆裂的变化过程,与实验结果相一致。采用不同的曳力模型对流化床稠密两相流动进行了模拟,与Kuipers实验对比,结果表明采用Gidaspow曳力模型描述流化床稠密两相流动特性更准确。     

利用Monte Carlo方法对循环流化床锅炉炉膛传热的数值计算

作者对循环流化床锅炉炉膛传热进行数值研究,所建模型考虑轴向和径向颗粒浓度分布的影响。模型计算揭示烟气温度和热流密度在炉膛内的分布变化,计算结果表明在循环流化床锅炉炉膛传热计算中,颗粒相对流换热不能忽略。     

Discontinuity of particle contact with the surface and heat transfer in fluidized beds

The finite-element method has been applied to solve the unsteady-state heat transfer equation for solid particles heated at the surface. The results are given as a series of temperature profiles for these systems and are used to estimate the depth of the heat penetration region in the particles and the heat transfer coefficients between a fluidized bed and an immersed heating surface. From calculations it appears that for the typical conditions of bubbling fluidization, i.e. residence times at the heat transfer surface τ = 0.1 s, this region exceeds many times the one found experimentally. This provides theoretical support for the earlier hypothesis that particles in fluidized beds are separated from the heating surface by an air gap. The close agreement between this result and the experimental values requires the assumption of a gap thickness δ = 01d_p. Owing to the long contact times of the particles with the heating surface in the electrostatically charged fluidized bed the particle penetration theory of the heat transfer from the surface has been also verified. The heat transfer coefficients derived from this theory agree with the authors' previous experimental work. On the basis of the results obtained some assumptions concerning a generalization of the heat transfer theories have been formulated.     

Mechanisms of heat transfer between a surface and a gas-fluidized bed for combustor application

There is currently considerable interest in the development of fluidized-bed boilers for efficient and environmentally clean combustion of coal. Fluidized beds have the advantages of high heat transfer rates and intimate mixing of additives such as limestone or dolomite for sulfur dioxide absorption produced as a result of combustion of coal containing sulfur. However, design for optimum heat transfer remains uncertain and essentially empirical. The mechanisms of heat transfer are complicated because of the many variables in a commercial combustion operation such as particle size distribution, particle shape, particle and gas thermal properties, reactor geometry and boiler tube design.An understanding of the mechanisms of bed to tube heat transfer is essential to sound design and interpretation of empirically derived correlations. Here we will review and criticize the major mechanisms of heat transfer that have been proposed. These mechanisms are proposed and developed from two schools of thought: (a) The principal resistance to heat transfer is a fluid film, and the moving fluidized particles scour the film to reduce the resistance to heat transfer; (b) Heat is absorbed by the fluidized particles and the rate of heat transfer depends on the rate of heat absorption.Radiant heat transfer is also discussed in this review in detail. Heat transfer by radiation is an important consideration in combustors but has received limited attention. The results of theoretical calculations are given which have been recently reported on the basis of the alternate-slab model of Gabor.The review will predominantly deal with the mechanistic models of heat transfer and various correlations developed over years will not be covered as this topic is dealt with in another review article by Saxena, Grewal, Gabor, Zabrodsky and Galershtein.