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     

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。     

一种改善循环流化床锅炉性能的方法

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有特性很强烈的影响,称为循环流化床锅炉出口端头效应。该文通过试验分析了循环流化床锅炉出口端头效应的机理。结果表明,采用合理的出口几何结构,可以使其具有初级内分离器的功能,进而增强颗粒的内循环,延长颗粒在炉内的停留时间,增强炉内传热,全面改善循环流化床锅炉的性能。     

气固流化床流体与颗粒的传热研究

本文介绍了双室流化床依靠固体颗粒循环实现双室间流体热量传递的流化床换热器模式及其热平衡试验;并在此基础上,分析了流化床内气体温度分布规律,理论计算值与实验值比较,吻合较好;讨论了流化床气固传热;提出新的准则数Nu_p·H_d/d及有效传热高度H_b的概念,研究表明,新的准则数Nu_p·H_d/d及特性尺度H_b更适合描述流化床气固传热规律。     

燃煤循环流化床模型与试验研究

利用循环流化床内气－固两相流动等基础方面的研究成果,根据本文床内气固浓－淡流动模型,建立适用不同结构参数的循环流化床燃烧模型,考虑了床内气体、固体颗粒的返混、循环过程,以及煤燃烧、ＮＯ的生成和分解、颗粒磨损等因素。在循环流化床燃烧试验台上进行实验研究,模型仿真结果和实验数据吻合良好,表明气固两相浓－淡流动模型所建立的循环流化床燃烧系统模型可以正确地模拟循环流化床的燃烧过程。     

半干法脱硫反应器内固体颗粒浓度分布的试验研究

在三维循环流化床脱硫反应器冷态模型上,研究了文丘里布风装置的循环流化床脱硫塔内气固两相流动中固体颗粒浓度的分布规律,以及不同操作参数:表观气速和颗粒循环流率对颗粒浓度在径向以及轴向分布的影响。试验中利用PV6D型光纤探针测量颗粒浓度,压力传感器测量反应器壁面压力。试验结果表明,在半干法循环流化床脱硫塔反应器中固体颗粒浓度的分布呈中间稀,边壁浓的趋势。截面平均颗粒浓度大体呈上稀下浓的分布。随表观气速的减小和颗粒循环速率的增大,颗粒浓度的径向分布不均性增大,截面平均颗粒浓度的轴向不均匀性增大。     

稠密气固两相流动过程模拟的改进模型与应用

提出了模拟稠密气固两相流动的改进模型。湍流流场采用改进的k-ε-εe模型，颗粒的聚合效应采用聚合力的当量直径折算模型计算。将颗粒团作为离散相，研究颗粒团的运动、碰撞、破碎与合并。应用上述模型数值模拟了循环流化床内的稠密气固两相流动。得到了床内气相速度、颗粒团分布、颗粒浓度分布及颗粒团大小分布等详细两相流场信息。计算结果合理，与前人实验结果相符。模拟结果详细揭示了循环流化床内稠密气固两相流动的基本特征。图8表2参8     

大型循环流化床锅炉固体颗粒流动及分布的数值模拟

对引进的４１０ｔ／ｈＰｙｒｏｆｌｏｗ商用循环流化床锅炉燃烧系统固体的流动过程及其分布特性进行了数字模拟,建立的流动模型采用小室模型模拟气固两相沿轴向的流动,采用“核心－环”流动结构模拟气固两相在径向的不均匀性,模拟时考虑了给料,固体颗粒磨损以及床料的宽筛分等特性,为引进大型商用循环流化床锅炉技术的消化吸收进行了初步的基础研究,并为进一步分析研究该循环流化床锅炉的整体特性奠定了基础。     

惯性分离器内气固两相流雷诺应力数值模拟

对循环流化床中气田惯性分离过程进行了详细的数值研究。湍流模型采用雷诺应力模型,研究对象为U型分离器内的气一固两相流动。为了真实描述固体颗粒与分离器壁面之间的碰撞过程,固体颗粒模拟采用颗粒轨道模型,引入壁面粗糙度的影响,同时考虑了固体颗粒在湍流中的扩散作用和颗粒之间的相互碰撞。模拟计算了不同入口速度、分离器挡板数目对颗粒分离效率和流体压降的影响,计算结果不仅给出了分离器内的气-固两相流动结构特性,而且给出了分离器效率、压降与入口主流速度和分离器结构参数的关系。     

烟气中灰粒对锅炉过热器,省煤器传热影响的研究

采用模拟含尘烟气实验研究了烟气中含尘浓度，颗粒粒度和烟气速度等关键因素对锅炉过热器、省煤器传热的影响，得出了具有工程应用价值的经验关联式，并用于计算分析煤粉炉、两级分离的特 流化床锅炉和中温分离的循环流化床锅炉的过热器、省煤器等对流管束在考虑固体粒子影响作用后烟后侧的传热系数。通过传热机理分析，提出了可用于锅炉对流受热面设计的多相流传热模型，为循环流化床锅炉过热器和省煤器的设计计算提供了依据和方法     

Local Heat and Mass Transfer for Gas—Solid Two Phase Flow in CFB

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Numerical modeling of axial bed-to-wall heat transfer in a circulating fluidized bed combustor

The water-wall surfaces located above the secondary air inlet within the circulating fluidized bed (CFB) combustor are exposed to the axial bed-to-wall heat transfer process. In the current work, the axial bed-to-wall heat transfer coefficients are estimated for three different axial voidage profiles (covering three widely occurring average particle concentrations) in order to investigate the effect of voidage, time, initial and fixed temperature of the bed and annulus, and gas gap between wall and solid particles; on the axial heat transfer process. A 2D thermal energy balance model is developed to estimate the axial heat transfer values for the gas–solid suspension along the height of the riser column with horizontally changing mass distribution. The gas–solid mass distribution is fixed with time thus providing a spectrum of changes in axial bed-to-wall heat transfer profile with time. The current work provides an opportunity to understand the axial heat transfer relationship with particle concentration and instantaneous behaviour. The results from the work show that: (i) first few seconds of the suspension temperature near the wall has maximum energy thus providing a small time frame to transfer more heat to the surface (CFB wall); (ii) both axial and horizontal particle concentrations (influenced by the operating conditions) affect the axial heat transfer locally; (iii) initial temperature of the bed between average and maximum values provide end limits for the axial heat transfer; (iv) annulus region has higher thermal energy than the core due to increased particle presence; and (v) a particle-free zone near the wall (gas gap) having a maximum thickness of 1 mm, tends to reduce up to 25% of axial heat transfer value. The model trends have close agreement with experimental trends from published literature; but the model values differ when correlating with real values due to inconsistencies in riser diameter and nature of variation in parameters.     

Investigation of catalyst particle hydrodynamic and heat transfer in three phase flow circulating fluidized bed

Vaporization of gas oil droplets has significant effects on the gas-solid flow hydrodynamic and heat transfer characteristic. A three dimensional CFD model of the riser section of a CFB have been developed considering three phase flow hydrodynamic, heat transfer and evaporation of the feed droplets. Several experiments were performed in order to obtain the data needed to evaluate the model using a pilot scale CFB unit. The Eulerian approach was used to model both gas and catalyst particle phases comprising of continuity, momentum, heat transfer and species equations as well as an equation for solid phase granular temperature. The flow field and evaporating liquid droplet characteristics were modeled using the Lagrangian approach. The catalyst particle velocity and volume fraction were measured using a fiber optic probe. The comparison between model predictions of catalyst particle velocity and volume fraction with the experimental data indicated that they were in good agreements and the Syamlal-O'Brien was the most accurate drag equation. The CFD model was capable of predicting the main characteristic of the complex gas-solids flow hydrodynamic and heat transfer, including the cluster formation of the catalyst particles near the reactor wall. In addition, the simulation results showed droplet vaporization caused reduction of catalyst particle residence time. Moreover, the higher ratios of the feed to catalyst flow rates led to the lower values of the catalyst temperature profile minimum.     

3D modelling of radiative heat transfer in circulating fluidized bed combustors: influence of the particulate composition

A three-dimensional model is developed to predict the bed-to-wall radiative heat transfer coefficient in the upper dilute zone of circulating fluidized bed (CFB) combustors. The radiative transfer equation is solved by the discrete ordinates method and Mie scattering theory is applied to calculate the absorption and scattering efficiency factors of particles existing in CFB combustors. Empirical correlations calculate both spacial variation of solid volume fraction and temperature distribution at the wall. The model considers the influences of the particle properties (including particle size distribution, particle optical constants and solid composition) on the radiative heat transfer coefficient. Simulation results show that the particle properties have significant influences on the bed-to-wall radiative heat transfer coefficient in CFB combustors. A very good agreement of predicted results is shown with experimental data.     

300MW循环流化床锅炉屏式受热面传热系数计算及其变化规律

根据300MW循环流化床（CFB）锅炉现场测试数据并结合以往CFB锅炉传热系数的研究成果,建立了屏式受热面烟气侧的传热模型,包括辐射传热模型和对流传热半经验公式．利用该模型对某300MWCFB锅炉在94％锅炉最大连续蒸发量（BMCR）工况下炉膛内屏式受热面的传热系数进行了计算,分析了屏式受热面管间节距、炉膛温度、工质温度、壁面黑度及烟气速度等因素对传热系数的影响．结果表明：烟气速度、炉膛温度和壁面黑度对传热系数的影响较大,所建立的传热模型能够合理地反映主要因素对CFB锅炉屏式受热面传热的影响．     

Prediction of heat transfer coefficient in the cyclone separator of a CFB

This paper deals with the study of heat transfer behaviour in the cyclone separator of a circulating fluidized bed (CFB) and how it is affected by relevant operating parameters. The experiments are conducted in a 140 mm diameter cyclone of a cold bed set‐up of 102 mm×102 mm bed cross‐section, 5.25 m height CFB. The cyclone separator is designed according to the high‐efficiency Lapple design and made to accommodate two identical heat transfer probes. From the experimental results, the heat transfer coefficient is found to increase with increase in solid circulation rate, as well as gas superficial velocity. The effect of bed inventory and heat flux on heat transfer coefficient has also been investigated. An empirical equation has been developed to predict the heat transfer coefficient in the cyclone separator based on dimensional analysis. The experimental results are compared with the predicted results and a good agreement has been observed. There is an optimal distance from the entry of the cyclone where the local heat transfer coefficient is maximum. The collection efficiency of the cyclone separator has been measured for all the operating conditions. Copyright © 2000 John Wiley & Sons, Ltd.     

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.     

循环流化床锅炉物料浓度分布的影响因素分析

循环流化床(CFB)锅炉炉膛物料浓度分布直接影响炉膛内的传热特性和锅炉负荷。基于CFB锅炉炉膛内气固两相流动特性,建立了密相区以上区域的物料浓度数学计算模型。以1台1 060 t/h CFB锅炉为研究对象,数值计算分析不同煤种、给煤粒度分布、流化风速、分离器效率及床压降对炉膛物料浓度分布的影响,计算分析结果表明,CFB锅炉运行中,针对不同煤质,可以通过优化给煤粒度分布和床压降来调节炉膛上部的物料浓度分布,低挥发分煤种的给煤粒度分布中细颗粒的比例要大一些。分离器性能越好,床质量越好,在相同的流化风速和床压降下,炉膛出口处物料浓度随d50、d99的减小而增大。     

Numerical modeling of bed-to-wall heat transfer in a circulating fluidized bed combustor based on cluster energy balance

The bed-to-wall heat transfer in a circulating fluidized bed (CFB) combustor depends on the heat transfer contributions from particle clusters, dispersed/gas phase and radiation from both of them. From the available CFB literature, most of the theoretical investigations on cluster and bed-to-wall heat transfer are based on mechanistic models except a few based on mathematical and numerical approaches. In the current work a numerical model proposed to predict the bed-to-wall heat transfer based on thermal energy balance between the cluster/dispersed phase and the riser wall. The effect of cluster properties and the thermal boundary conditions on the cluster heat transfer coefficient are analyzed and discussed. The fully implicit finite volume method is used to solve the governing equations by generating a 2D temperature plot for the cluster and the dispersed phase control volumes. From this 2D temperature profile, space and time averaged heat transfer coefficients (for cluster, dispersed phase and radiation components) are estimated for different operating conditions. The results from the proposed numerical simulation are in general agreement with published experimental data for similar operating conditions. The results and the analysis from the current work give more information on the thermal behavior of the cluster and dispersed phases, which improves the understanding of particle and gas phase heat transfers under different operating conditions in CFB units.     

Modeling of heat transfer coefficient in the furnace of CFB boilers by artificial neural network approach

The present work introduces a way of predicting the local heat transfer coefficient in the combustion chamber of the circulating fluidized bed boiler (CFB) by the artificial neural network (ANN) approach.Neural networks have been successfully applied to calculate the local overall heat transfer coefficient for membrane walls, Superheater I (SH I, Omega Superheater) and Superheater II (SH II, Wing-Walls) in the combustion chamber of the 260 MWe CFB boiler. The previously verified numerical model has been used to obtain the overall heat transfer coefficients, necessary for training and testing the ANN. It has been shown, that the neural networks give quick and accurate results as an answer to the input pattern. The local heat transfer coefficients evaluated using the developed ANN model have been in a good agreement with numerical and experimental results.