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     

循环流化床的传热机理——颗粒絮团更新模型

本文论述了循环流化床的传热机理——颗粒絮团更新模型。对不同来源模型中的壁面被颗粒絮团覆盖的面积分率、颗粒絮团在壁面的滞留时间以及颗粒絮团与壁面间的气膜厚度等关键参数的计算和设定进行了分析。对比了各模型的预测精度。     

循环流化床的传热机理

本文论述了循环流化床的传热机理－－颗粒絮团更新模型，对不同来源模型中的壁面被颗粒絮团覆盖的面积分率、颗粒絮团在壁面的滞留时间以及颗粒絮团与壁面间的气膜厚度等关键参数的计算和设定进行了分析。对比了各模型的预测精度。     

填充床中受热面的换热模型

详细分析了高温填充床与冷却壁面的换热过程,建立了高温填充床与低温壁面传热模型,模型主要考虑了近壁面颗粒和壁面的接触导热热阻以及热边界层中的导热热阻两个环节,并提出了模型中所涉及参数的具体确定方法。模型预测数据与实验数据的比较表明:建立的模型是可信的,可以用于填充床物料冷却过程的冷却管传热系数计算。     

300MW循环流化床锅炉稀相区的传热研究

本文针对燃烧煤矸石的循环流化床锅炉的传热情况展开研究,以山西平朔电厂1台300 MW的循环流化床锅炉为实例,采用环核模型和颗粒团更新模型,对稀相区的传热系数分布进行建模计算,本文所建模型考虑炉内床温实际分布特征,根据现场温度实测数据对模型进行修正,研究了对流和辐射换热系数在不同负荷下沿炉膛高度的变化情况。锅炉在较高负荷下运行时,负荷的波动对颗粒团壁面覆盖时均份额影响较小,继而对炉内对流换热影响较小。炉内环形区温度沿床高的偏差随负荷升高略有减小,且对辐射换热影响比对流换热大。随着负荷升高,对流换热系数沿炉高下降增大,而辐射换热系数沿炉高下降减小,高负荷时炉内总换热系数沿炉高下降25%左右,低负荷时沿炉高下降28%左右,高负荷下炉内沿高度温差更小,传热更稳定。     

循环流化床中的气固两相传热模型

循环流化床中存在着分散固体颗粒的连续上升气相和相对紧密的颗粒团两部分,颗粒团聚物对气固两相传热有着重要的影响。采用拟Boltzmann动力学方法描述循环流化床中颗粒团的动力学行为,建立了循环流化床中气固两相间传热过程的理论模型,对气体表观流速、固体颗粒循环率等对气固传热系数沿床高分布规律的影响进行了分析和讨论。模型计算结果与参考文献中的实验数据进行了比较,两者符合较好。     

循环流化床床层与壁面间时均局部传热系数的计算

以循环流化床传热的颗粒絮团更新理论为基础,建立了一个计算床层与壁面同时均局部传热系数的通过将循环流化床床层沿高度分段的方法,构想了颗粒絮团在壁面的形成与运动过程,考虑了床层的轴向分布对时均局部传热系数的影响。模型计算结果与有关试验数据的对比表明该模型基本有效。     

颗粒组合角度对近壁颗粒传热特性的影响

为了探究颗粒堆积结构变化对近壁面颗粒传热过程的影响,构建了近壁面两颗粒的非稳态传热模型,研究了不同初始温度条件下颗粒组合角度变化对近壁面颗粒传热特性的影响规律。结果表明：初始温度越高,所需换热时间越长,颗粒组合角度增加,使得换热时间明显减少。初始温度为1073K时,换热面平均热流密度值呈先迅速下降后缓慢下降的趋势。同一时刻,组合角度越大,颗粒平均温度越低。不同组合角度颗粒的固相传热率均达到0.8以上。颗粒组合角度越大,固相传热占比越小,辐射传热占比越大。在换热前期,辐射传热率最高可达0.57,对传热过程的影响不可忽略。     

球床中传热与流动数值模拟研究

针对球床中气固两相传热与流动过程,建立了考虑Forchheimer阻力以及孔隙率分布的多孔介质计算模型。计算表明,Da固定时,随着Ra的增加Nuavg随之显著上升。有效热导率是球床的重要参数,改进后的ZBS模型与实验结果一致,在高温堆正常运行工况下(≈900℃)辐射传热起主导作用。随着球床加热功率的升高以及气体压力的降低,气体流动传热强度逐渐降低。     

富氧条件下循环流化床锅炉炉内传热特性

在富氧气氛下对循环流化床锅炉炉内的传热特性研究,考虑锅炉产生的烟气成分变化,在传热系数的计算中考虑了气体的辐射特性,在密相区采用以前的鼓泡床传热传质模型,稀相区膜式水冷壁颗采用粒团更新模型,对模型求解,研究并分析了运行参数(如床料的直径、空隙率、烟气成分以及烟气温度等)在富氧燃烧条件下对传热的影响。     

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.     

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.     

Analysis of particle motion and heat transfer in circulating fluidized beds

In this study, the forces affecting the motion of particle clusters near the wall of a CFB were theoretically analysed. The motion trajectory and the contact time of clusters were determined from the proposed model for two cases, steel ball having density of 6980 kg m^?3 and sand having density of 2500 kg m^?3. Computational results showed that the construction and operational parameters such as the bed equivalent diameter, the gas velocity and the bed temperature have great influence on the contact time of clusters. Based on analysis of the contact time of clusters, a theoretical model was developed for predicting the particle–gas convection heat transfer coefficient. The results were compared with experiments and were a quite agreement with the measured data in the open literature which suggests that the theoretical analysis conducted in this work can very well describe the convection heat transfer in a CFB. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of dilute and dense phase operating conditions on bed-to-wall heat transfer mechanism in a circulating fluidized bed combustor

In the present paper investigations are conducted on bed-to-wall heat transfer to water-wall surfaces in the upper region of the riser column of a circulating fluidized bed (CFB) combustor under dilute and dense phase conditions. The bed-to-wall heat transfer depends on the contributions of particle convection, gas convection and radiation heat transfer components. The percentage contribution of each of these components depends on the operating conditions i.e., dilute and dense phase bed conditions and bed temperature. The variation in contribution with operating conditions is estimated using the cluster renewal mechanistic model. The present results contribute some fundamental information on the contributions of particle convection, gas convection and radiation contributions in bed-to-wall heat transfer under dilute and dense phase conditions with bed temperature. This leads to better understanding of heat transfer mechanism to water-wall surfaces in the upper region of the riser column under varying load conditions i.e., when the combustor is operated under dilute and dense phase situations. The results will further contribute to understanding of heat transfer mechanism and will aid in the efficient design of heat transfer surfaces in the CFB unit.     

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.     

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.     

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．