Flow boiling heat transfer in circulating fluidized bed

In order to enhance heat transfer and mitigate contamination in the boiling processes, a new type of vapor-liquid-solid (3-phase) circulating fluidized bed boiling system has been designed, combining a circulating fluidized bed with boiling heat transfer. Experimental results show an enhancement of the boiling curve. Flow visualization studies concerning flow hydrodynamics within the riser column are also conducted whose results are presented and discussed.     

Study on boiling heat transfer in liquid saturated particle bed and fluidized bed

An investigation on the effects of solid particles on boiling heat transfer enhancement is performed. The range of particle diameter is from millimeter to nanometer. The experimental results show that boiling heat transfer can be enhanced greatly by adding the solid particle into the liquid whether in fixed particle bed or in fluidized particle bed. The boiling enhancement is closely related to the particle size, the initial bed depth and the heat flux applied. The experiments show that boiling characteristics are greatly changed when a particle layer is put on the heated surface. The major effects of fixed particle bed on nucleate pool boiling heat transfer are the nucleation, bubble moving and thermal conductivity effect. A boiling heat transfer correlation is obtained to predict the boiling heat transfer coefficients in a liquid saturated porous bed. A volumetric convection mechanism of boiling heat transfer enhancement by fluidized particles is proposed. The calculated results from the model suggested in this paper agree reasonably with the experimental values.     

利用出口几何结构改善循环流化床锅炉性能

循环流化床锅炉出口几何结构对炉内气固两相流动特性和传热特性有强烈的影响,称为循环流化床锅炉出口端头效应。试验结果表明,采用合理的出口几何结构,可以增强颗粒的内循环和炉内传热。     

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。     

大型循环流化床锅炉中的传热

分析了典型的大型循环流化床锅炉炉膛传热数据及其相关参数关系.以12 MW、50MW和135 MW循环流化床锅炉为例,讨论大型循环流化床锅炉炉膛传热问题.此外,还讨论了炉膛悬吊受热面、汽/水冷旋风分离器、外置式换热器和尾部受热面的传热规律,并给出大型循环流化床锅炉传热系数计算的经验公式.图10表2参12     

循环流化床锅炉设计中若干问题探讨

循环流化床燃烧技术是一种新型、高效、清洁的燃烧技术,有许多其它热工设备无法比拟的优势。 但在循环流化床锅炉设计方面还有一些方面亟待解决,文中就循环流化床锅炉设计方面的一些问题,如 气固两相流体动力特性对其设计的影响;一、二次风比例,燃烧份额对其设计的影响;炉内传热系数对其 设计的影响等方面作了一些阐述。     

循环流化床锅炉热力计算方法探讨

循环流化床锅炉与鼓泡流化床相比 ,有许多新的特点 ,床内的燃烧和受热面的传热过程很复杂。由循环倍率决定的颗粒浓度是循环流化床锅炉的一个重要参数 ,它对传热强度、燃烧与燃尽、过热器的汽温、负荷的调节范围以及分离器的设计等都产生重要影响。以 65t/h油页岩流化床锅炉的热力计算为例 ,从循环倍率入手 ,着重讨论了循环倍率以及由它决定的飞灰浓度对床内燃烧传热及过热器传热的影响     

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     

Effect of operating parameters on heat transfer in the lean phase region of a CFB riser column

The radial variation of the heat transfer coefficient across the bed width, including the effect of fins and fin shapes (surface roughness), on the heat transfer characteristics in the lean phase region of a circulating fluidized bed riser column are investigated. Three test sections (bare horizontal tube, V-fin tube and square fin tube) are employed for the investigation. The experimental unit consists of a riser column of 102×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions, and both made up of plexiglas columns. For the same operating conditions, the provision of fins results in a drop in heat transfer coefficient, but increases total heat transfer owing to increased surface area. The present experimental data are compared with published literature and good agreement has been observed. The experimental data also corresponds to the two-zone hydrodynamics (i.e. the core–annulus structure) of a circulating fluidized bed. © 1997 John Wiley & Sons, Ltd.     

Augmentation of heat transfer by deflectors on circulating fluidized bed membrane walls

It is shown experimentally that the addition of angled deflectors to the fin region of membrane water-wall heat exchanger surfaces in circulating fluidized beds can lead to a significant increase in local and overall suspension-to-wall heat transfer. The experiments were carried out in the 12 MWth circulating fluidized bed (CFB) boiler at Chalmers University. The results are consistent with calculations based on renewal of packets traveling along the fin.     

循环流化床锅炉水冷壁的热流密度分布

超临界循环流化床锅炉的一个主要技术关键是炉膛受热面的横向热流分布.在管内工质温度不同、容量不同的3台循环流化床锅炉上,测量了不同高度上膜式水冷壁的金属壁温.将有限元算法用于水冷壁的换热分析,得到了循环流化床锅炉炉膛内烟气向水冷壁的换热系数分布.该结果为超临界CFB锅炉的设计提供了依据.     

Fundamental heat transfer mechanism between bed‐to‐membrane water‐walls in circulating fluidized bed combustors

In the present work, the fundamental mechanism between bed‐to‐membrane water‐walls in the riser column of a circulating fluidized bed (CFB) combustor is presented. The bed‐to‐membrane water‐wall heat transfer depends on the contributions of particle heat transfer, dispersed phase heat transfer and radiation heat transfer. The fundamental mechanism of particle heat transfer and the effect of fraction of wall exposed to clusters and gas gap thickness between cluster and wall on particle heat transfer coefficient and bed‐to‐wall heat transfer coefficient are investigated. The influence of operating parameters like cross‐sectional average volumetric solids concentration and bed temperature on particle and bed‐to‐wall heat transfer are also reported. The present work contributes some fundamental information on particle heat transfer mechanism, which is responsible for increasing the bed‐to‐wall heat transfer coefficient (apart from dispersed phase convection and radiation heat transfer). The details on particle heat transfer mechanism will enable to understand the basic heat transfer phenomena between bed‐to‐membrane water‐walls in circulating fluidized bed combustors in a detailed way, which in turn will aid for better design of CFB combustor units. The particle heat transfer mechanism is significantly influenced by the fraction of wall exposed to clusters and gas gap thickness between clusters and wall. Copyright © 2003 John Wiley & Sons, Ltd.     

循环流化床锅炉水冷壁的传热系数

对循环流化床锅炉内水冷壁的传热机理进行了分析,验证了循环流化床炉内受热面的传热计算经验公式,可供设计、调试和运行时参考。     

循环流化床流体动力学研究进展

论述了国内外在循环流化床流体动力学研究领域的进展,对循环流态化颗粒流动问题、颗粒聚集及传热以及循环流态经数学模型等热点问题进行了重点回顾,并指出了当前循环流化床流体动力学研究的一些新动向。     

循环流化床传热研究的综述

本文综述了关于循环流化床床内介质与受热面之间传热的主要研究成果,分析了循环床操作参数对传热的影响规律和各种传热计算关联式及其使用条件,并对研究中存在的问题和今后的研究方向提出了看法。     

循环流化床气固两相间传热特性的实验研究

在小型实验台上用双热电偶测温法测定床层温度。实验表明：气固间传热主要在循环床下半部进行;同时气体表观流速提高、颗粒循环率增大及颗粒粒径减小有利于气固两相间的传热,获得了相应的无因次方程。     

Effect of pressure on thermal aspects in the riser column of a pressurized circulating fluidized bed

In the present paper the effect of pressure on bed‐to‐wall heat transfer in the riser column of a pressurized circulating fluidized bed (PCFB) unit is estimated through a modified mechanistic model. Gas–solid flow structure and average cross‐sectional solids concentration play a dominant role in better understanding of bed‐to‐wall heat transfer mechanism in the riser column of a PCFB. The effect of pressure on average solids concentration fraction ‘c’ in the riser column is analysed from the experimental investigations. The basic cluster renewal model of an atmospheric circulating fluidized bed has been modified to consider the effect of pressure on different model parameters such as cluster properties, gas layer thickness, cluster, particle, gas phase, radiation and bed‐to‐wall heat transfer coefficients, respectively. The cluster thermal conductivity increases with system pressure as well as with bed temperature due to higher cluster thermal properties. The increased operating pressure enhances the particle and dispersed phase heat transfer components. The bed‐to‐wall heat transfer coefficient increases with operating pressure, because of increased particle concentration. The predicted results from the model are compared with the experimentally measured values as well as with the published literature, and a good agreement has been observed. The bed‐to‐wall heat transfer coefficient variation along the riser height is also reported for different operating pressures. Copyright © 2005 John Wiley & Sons, Ltd.     

Augmentation of boiling heat transfer from horizontal cylinder to liquid by movable particles

This paper presents a series of experimental results on a passive augmentation technique of boiling heat transfer by supplying solid particles in liquid. A cylindrical heater 0.88 mm in diameter is placed in saturated water, in which a lot of mobile particles exist, and the nucleate and film boiling heat transfer characteristics are measured. Particle materials used were alumina, glass, and porous alumina, and the diameter ranged from 0.3 mm to 2.5 mm. Particles are fluidized by the occurrence of boiling without any additive power, and the heat transfer is augmented. The maximum augmentation ratio obtained in this experiment reaches about ten times the heat transfer coefficient obtained in liquid alone. The augmentation ratio is mainly affected by the particle material, diameter, and the height of the particle bed set at no boiling condition. The augmentation mechanism is discussed on the basis of the experimental results. © 2001 Scripta Technica, Heat Trans Asian Res, 31(1): 28–41, 2002     

The impact of bed temperature on heat transfer characteristic between fluidized bed and vertical rifled tubes

In the present work, the heat transfer study focuses on assessment of the impact of bed temperature on the local heat transfer characteristic between a fluidized bed and vertical rifled tubes (38mm-O.D.) in a commercial circulating fluidized bed (CFB) boiler. Heat transfer behavior in a 1296t/h supercritical CFB furnace has been analyzed for Geldart B particle with Sauter mean diameter of 0.219 and 0.246mm. The heat transfer experiments were conducted for the active heat transfer surface in the form of membrane tube with a longitudinal fin at the tube crest under the normal operating conditions of CFB boiler. A heat transfer analysis of CFB boiler with detailed consideration of the bed-to-wall heat transfer coefficient and the contribution of heat transfer mechanisms inside furnace chamber were investigated using mechanistic heat transfer model based on cluster renewal approach. The predicted values of heat transfer coefficient are compared with empirical correlation for CFB units in large-scale.     

循环流化床传热系数的计算模型

本文在循环流化床流动模型的基础上建立了传热模型，流动模型根据实际运行情况考虑了颗粒的宽筛分，并把床层在轴向上分为密相床和稀相床两部分。在密相床内，传热按照鼓泡床传热微型进行计算；在稀相床内，传热模型建立在颗粒团更新的假设基础上，根据假设，床层由颗粒浓度很低的上升稀相和相对颗粒浓度较大的颗粒团两部分组成，两部分交替地与床壁面接触，床层和受热面间局部换热系数和颗粒浓度及两部分接触壁面的份额有关。模化结