气固鼓泡床的全尺度直接数值模拟

颗粒全尺度直接数值模拟由于不引入任何相间作用力封闭模型、同时又考虑了颗粒相与流体相之间的四向耦合,因而具有极高的计算精度和准度。本文采用内嵌边界多重直接力算法结合软球碰撞模型对实验室尺度的气固鼓泡流化床装置进行了全尺度直接数值模拟。计算结果真实还原了鼓泡流态化的流动情形,且能有效地捕捉装置内部流场的详细运动情况和涡结构。对流化床内颗粒在传统的离散元模型框架下进行曳力统计发现,平均曳力比全尺度模拟结果小约20%~30%。其具体数值因选取比较的曳力模型和统计网格而异。改进传统曳力模型需考虑颗粒群的非均匀性以及颗粒拟温度。     

气固两相流FR-DNS模拟的颗粒统计分析

由于颗粒全尺度直接数值模拟(FR-DNS)不引入任何相间作用力封闭模型,同时又考虑了颗粒相与流体相的四向耦合,因而其数据具有极高的计算精度和预测准确度,在研究颗粒流动的机理方面有重大价值。本文采用FR-DNS方法模拟实验室尺度的鼓泡流化床条件下单颗粒、双颗粒及9240颗粒系统,对时间序列下系统统计特性进行对比,得出其数据的分布规律差异。结果表明多颗粒系统的颗粒平均水平和垂直运动数据统计结果均呈现多峰性,结合鼓泡床条件下多颗粒运动的特征得到了合理的解释,旨在通过统计学观点理解密相流化床系统的颗粒运动规律。     

鼓泡流化床埋管磨损量及其分布的数值研究

本文采用离散颗粒单元法对流化床内颗粒运动及其与固定埋管受热面的相互作用进行颗粒直接数值模拟,其中颗粒之间的碰撞采用Tsuji等提出的软球碰撞模型处理,而流场的计算采用大涡模拟,其亚网格应力为Smagorinsky涡黏性模型,流动工况为两维鼓泡流化床.磨损量的估计是基于祝京旭等人的埋管磨损试验研究的结论,并结合本文数值模拟的结果,揭示了流化床埋管磨损量及其分布的若干规律.     

稠密颗粒物质系统鼓泡现象的微观结构研究

通过重新整理气相控制方程,实现了基于非结构化网格的离散颗粒与连续流体的非线性耦合求解方法.应用该求解方法模拟计算了二维气固脉冲流化床(pulsed fluidized bed, PFB)单个鼓泡过程,数值计算结果与实验结果相符合.通过剖析两种典型床宽PFB的气固两相微观结构,观察到颗粒起动瞬间力链断裂"解锁"现象和系统压降脉冲现象,发现床内气体的流动是一个由双主涡到多涡共存再到双次涡的发展过程,颗粒的运动呈现三种形态,即抬升,沿鼓泡边界下滑和角落内"滞缓"运动. 关键词： 离散颗粒 脉冲流化床 鼓泡 力链     

燃煤旋涡流化床燃烧特性研究

燃煤旋涡流化床燃烧特性研究金保，刘坤磊，赵长遂，徐益谦（东南大学热能工程研究所）一、前言旋涡流化床燃烧技术是在鼓泡床和循环床的基础上发展起来的第三代流化床燃烧技术，通过在鼓泡床的悬浮空间加入四角切圆二次风，组成强旋涡流场，分离从床内飞溅上来的细颗粒，...     

速度进口边界条件对鼓泡床流动影响研究

进口边界条件是影响鼓泡流化床流体动力行为的重要因素之一.本文基于欧拉一欧拉双流体气固两相流模型,对具有中心射流进口的鼓泡流化床进行了数值模拟研究.分别采用脉动和定常进口气体速度边界条件,对鼓泡流化床内气泡的形成、长大、分离,气泡的大小进行了分析和研究.模拟结果与Kuipers等的实验结果进行了比较,研究结果表明考虑进口速度的脉动能够更加合理地预测和分析床内流体动力行为.     

生物质洁净能源研究中的流化床动力学模型

分别对最小流化床、鼓泡流化床和腾涌流化床及相应的全混模型、鼓泡模型、气泡汇集模型等加以综述 ,分析其优缺点 ,并在此基础上提出动力学模拟研究的新思路 .根据流化床内在的本质———流化态的不同 ,将流化床分为最小流化床、鼓泡流化床和腾涌流化床三种 .总结了前人针对各种流化床提出的全混模型、鼓泡模型、气泡汇集模型等思想 ,建议今后可以在以下几个方面进行深入研究 :⑴使得模型更有普适性 .⑵由于气泡有效直径尚不能在理论上求得 ,可以在理想气泡直径变化公式的基础上 ,加入非线性化学的计算 .⑶确定不同情况下的参数 ,使得工作更有延续性 ,也使得模型更加具有生命力 .⑷从高压的角度去进行模型的计算 ,并得到相应的试验数据支持 .     

流化床焚烧炉中颗粒扩散特性理论研究

本文建立了流化床焚烧炉中粒子扩散方程,通过求解方程,得到了鼓泡床和内旋流床中颗粒扩散特性,并与试验结果进行了对比,两者符合较好。结果表明鼓泡床横向扩散能力较差,而内旋流床有很好的横向扩散特性,非常有利于废弃物在床内的快速扩散。     

喷动流化床的数值模拟研究

本文借鉴 Ｃｕｎｄａｌｌ＆ Ｓｔｒａｃｋ提出的碰撞模型,将欧拉方法和拉格朗日法结合起来,建立了喷动流化床气固两相流动的数学模型,在计算机上对试验台进行了模拟,探讨了床内颗粒的运动特征。按１：１的比例建立了喷动流化床的模型试验台并进行了试验,对模拟结果和试验结果进行了比较与分析。颗粒在喷动床内的运动特征,模拟结果都能够再现出来,模拟结果与试验结果取得了很好的一致性。     

气粒两相流传热问题的光滑离散颗粒流体动力学方法数值模拟

在气粒两相流动问题中,颗粒间以及气体与颗粒间的传热问题不可忽略.光滑离散颗粒流体动力学(SDPH)模型作为一种新的求解气粒两相流动问题的方法,已经成功应用于模拟风沙运动等问题.在此基础上,提出了SDPH方法的热传导模型,模拟了气粒两相流动问题中的热传导过程以及颗粒蒸发过程.首先引入各相的能量方程,利用有限差分与光滑粒子流体动力学一阶导数相结合的方法,处理各相内部热传导项中的二阶导数问题,基于气粒两相间温度差及对流换热系数计算颗粒与气体间的热传导量,推导得到了含热传导模型的气粒两相流SDPH计算方程组,模拟计算了圆盘形颗粒团算例及鼓泡流化床内部热传导算例,并与双流体模型计算结果进行对比,结果基本符合;其次利用离散液滴模型中的颗粒蒸发传质传热定律计算颗粒的蒸发过程,数值模拟了颗粒射流蒸发过程,并与离散颗粒模型结果进行对比,两者符合得较好,验证了该方法的准确性及实用性.     

Heat recovery by perforated-plate multistage fluidized bed exchangers

An experimental and theoretical study of a perforated plate multistage fluidized bed heat exchanger for particle heating or cooling is presented. Based on experimental data (with three-stage and four-stage columns), two design correlations are proposed for predicting loading and flooding for Froude numbers ranging from 100 to 400. The heat transfer is described using a residence time distribution model which takes into account the solid and gas bypasses and the solid entrainment. Predicted values are compared to experimental data in the range 373–773 K for estimating the three parameters.     

Simulation of a full-scale pressurized fluidized bed combustor by using semi-empirical pilot plant correlations

A model, based on semi-empirical correlations, was developed to simulate the behavior of a pressurized fluidized bed combustor (PFBC) under various operating conditions. This paper presents the results of a comprehensive survey of one-dimensional empirical correlations applicable to a PFBC and which were used to assemble a 1-D predictive model of the complete performance of a pressurized fluidized bed combustor. The model was adjusted in order to reproduce process and in-bed data taken at Escatrón PFBC power station (Spain) under a wide range of operating conditions. The novelty of this approach lies in the use of full-scale information to validate empirical correlations, the objective being to draw specific conclusions on the reliability of empirical PFBC modeling. The model was validated using experimental power plant data. Good agreement was found between the computed results and actual plant data of bed temperature profiles, bed density, char and sorbent elutriation, fuel feed rate, in-bed and entrainment particle size distributions, oxygen and carbon dioxide concentrations, heat transfer coefficients along the combustor height and heat transfer to the tube bundles under different operational regimes. This paper describes the model, as well as its results, validation and prospects for future work.     

压力循环流化床中的传热研究

本文从理论与试验上研究压力循环流化床中床－壁面传热系数的变化规律。对常压循环流化床中的传热模型进行修正，发展了压力循环流化床中的传热模型。试验测试了床内传热系数随床内固体颗粒浓度、颗粒径和运行风速的变化规律。提出计算床－壁面传热系数的无量纲经验公式。     

The effect of pelletization on the attrition of wood under fluidized bed combustion and gasification conditions

This work reports on a study, carried out in a lab-scale fluidized bed apparatus, on fragmentation and attrition of two biomass fuels, namely wood chips and wood pellets, under both combustion and gasification conditions. The aim was to highlight the effect of their different mechanical strength on the fuel particle size distribution and overall carbon conversion. Primary fragmentation tests showed that for wood pellets limited fragmentation occurred during devolatilization, with a fragmentation probability around 30% and particle multiplication factor of 1.4. On the contrary, wood chips were subject to extensive fragmentation as witnessed by large values of the particle multiplication factor and of the fragmentation probability.Results of char attrition experiments carried out under inert, combustion and gasification conditions showed that the carbon loss by elutriation is critical only during gasification, especially for the wood chips char. A gasification-assisted attrition mechanism was proposed to explain the experimental results, similar to the well known combustion-assisted attrition patterns already documented for coal under oxidizing conditions. The higher mechanical strength of the wood pellets appears to be beneficial for reducing carbon elutriation and for obtaining a higher carbon conversion.     

Attrition of lignite char under fluidized bed gasification conditions: The effect of carbon conversion

The effect of carbon conversion on the attrition of lignite char particles during fluidized bed gasification by CO₂ was studied in a lab-scale apparatus. The influence of bed temperature and inlet CO₂ concentration on carbon conversion and, consequently, on attrition was studied. The mechanical resistance of the char particles was also characterized at different stages of char conversion by specific attrition experiments. A predictive kinetic model for CO₂ gasification of the lignite char was developed from the experimental results, that was able to correctly predict the evolution of carbon conversion versus time. On this basis a semi-empirical model was developed in order to simulate the evolution of carbon elutriation rate with carbon conversion degree, i.e. the gasification-assisted attrition enhancement effect.     

Study of gas-fluidization dynamics with laser-polarized 129Xe

We report initial NMR studies of gas dynamics in a particle bed fluidized by laser-polarized xenon (129Xe) gas. We have made preliminary measurements of two important characteristics: gas exchange between the bubble and emulsion phases and the gas velocity distribution in the bed. We used T2* contrast to differentiate the bubble and emulsion phases by choosing solid particles with large magnetic susceptibility. Experimental tests demonstrated that this method was successful in eliminating 129Xe magnetization in the emulsion phase, which enabled us to observe the time dependence of the bubble magnetization. By employing the pulsed field gradient method, we also measured the gas velocity distribution within the bed. These results clearly show the onset of bubbling and can be used to deduce information about gas and particle motion in the fluidized bed.     

热烟气启动流化床的数学模型

1引言在流化床锅炉能够正常运行之前，必须进行点火操作，把惰性床料流化加热到足够高的温度，以使燃料能够开始燃烧。用高温热烟气经由布风板加热床层，传热效率较高，易于实现自动控制，能够明显缩短启动时间，减少辅助燃料消耗，目前国外普遍采用这种方法山。在我国第一台中试PFBC锅炉的启动中，就将采用热烟气启动的方式。为了对热烟气启动增压流化床进行模拟研究，提出了一种热烟气启动流化床的简化模型二本文提出的简化模型，考虑了扬析、磨损等因素，力求物理概念正确、简单实用、功能强；特别是借鉴对于间隙扬析过程的处理方法，…     

喷动床内颗粒浓度的实验测量和理论模拟

颗粒浓度对于稠相流动体系中颗粒应力本构方程建立具有决定性的作用。本文采用双流体模型,并结合颗粒流动力学理论来描述固相应力的基础上实现了对喷动床的数值模拟,探讨了从喷射区到环隙区颗粒浓度的理沦变化趋势。同时采用自主开发的电容探针技术,不仅颗粒浓度测量值和理论值吻合较好,而且也准确地验证了颗粒非均匀夹带这一现象。电容探针还可灵敏地分辨出环隙区内不同床高处以及接近壁面处的颗粒浓度的细微变化。