【作者】 龚希武；

【导师】 胡国新；

【作者基本信息】 上海交通大学 ， 工程热物理， 2008， 博士

【摘要】 作为流态化技术的一个分支,喷动床技术的应用领域已从最初主要用于粗大颗粒的干燥过程,逐渐扩展到用于粘性强或粗块状颗粒的表面涂层、涂料、悬浮液以及干燥、粉碎、造粒、煤燃烧和气化、热解、石油热裂化等工艺。正是由于该技术应用范围的不断推广及其研究的逐渐深入,许多的研究者对传统的柱锥型喷动床进行了改造,提出了各种各样的改进型喷动床,适应了各种应用场合的需要。本文通过对喷动床技术的机理研究,将喷动床技术与煤的综合利用技术结合起来,设计建造了一种新型的喷动床-多喷口环形喷动床。并采用实验研究与数值模拟相结合的方法,系统地研究了该新型喷动床体内的气体和颗粒流动特性。采用可视化观测、床内压力波动信号分析等实验研究手段开展了对床体喷动行为的研究,为合理实施工业应用提供依据。基于对颗粒相的“拟流体化”,采用双流体模型,模拟了床内的气-固速度场、压力场等,通过特征信息的量化分析了床体喷动机理与过程。本课题研究内容主要包括:新型多喷口环形喷动床体的进料装置的设计与研究;床体内喷动过程的流体动力学特性;喷动床体内喷动过程的流型与转变;不同运行条件下,床层内压力波动变化特征;由于多喷口的存在所导致的喷口间的相关性研究;建立该新型床体内的气相和颗粒相的流动平衡方程,进行数值模拟研究。在新型多喷口环形喷动床体结构中,为保证实验物料在整个喷动床体环向的喷口位置处较为均匀地播撒,设计中引入旋转锥体作为喷动床体的给料装置。作为实验物料气化的预处理阶段,实验物料在旋转锥体内可以进行气化的前处理,提高气化效率。针对颗粒在旋转锥体内停留时间短、存在静止滞留区的问题,在锥体设计上作了一些改进。锥体增设竖立环壁以增加颗粒的停留时间,而搅拌器的设立会使锥体内的所有颗粒都处于动态滑移混合区,从而使得颗粒得到较好的混合与输运。实验研究结果显示:在设立竖立环壁及搅拌器的条件下,锥体内动态存料量和停留时间与锥体旋转速率以及溢流口面积成反比,但动态存料量与输送给料速率成正比,颗粒在锥体内的停留时间与输送给料速率成反比。在三维喷动床中,研究了干物料状态下该新型喷动床的流体动力学特性。实验结果表明:床内物料的喷动与流化存在分区现象,随着喷嘴尺寸或颗粒粒径的增加,最大可喷动床层高度相应地减小。通过实验手段,分析了静止床层高度、喷嘴尺寸、颗粒粒径等因素对最小喷动速度和最大床层喷动压降的影响,推导了最小喷动速度和最大床层喷动压降的实验关联式,实验关联式的计算值与实验值具有较好的吻合性。采用CCD可视化实验手段详细分析了床内喷动过程的发展情况,区分了床体不同的流动形态及相应的流型转变特征。三种明显、稳定的流动形态可以得到确认,它们是内部射流,射流喷动,完全喷动。此外,还观察到两种过渡的流型,即单口内部射流和单口射流喷动。当床内静止床高大于最大可喷动床高时,一种不稳定的流型-鼓泡与腾涌也能被观察到。最后,绘制出了床内流型的变迁图和床内沿床高方向的压力变化图。通过统计和频率分析手段对床体内不同运行条件(喷动气速、静止床高和测点位置高度)下的压力波动信号进行了研究。随着喷动气速的不断增加,整个床层的压降表现出与传统喷动床类似的特征。起初,整个床层压降随着喷动气速的增加而不断增加,当喷动气穿透床层形成喷动后,床内整个床层的压降突然下降到一个低值,之后,即使喷动气速再进一步增大,但整个床层压降的平均值基本保持一个常数,床层压降围绕着这个平均值上下波动。在各个测试点位置,压力波动的标准差总是随着Us/Ums的增加而增大,随着测点位置高度的增加而减小。根据对压力波动信号直接FFT变换所做出的功率谱图的分析,确定了床内的三个压力信号波动源,即喷射气流在床内运动的影响、由于不同喷口间的扰动引起的喷射气流的高频脉动和床层颗粒互相的碰撞、扰动。随着喷动气速、测点位置高度、静止床层高度的增加,压力波动信号的主频趋于减少。引入偏相关系数和聚类分析,考察了各喷口之间的压力波动相关性和压力信号相似性。各喷口间的相关性都表现为弱相关。这说明,多喷口环形喷动床能较好地保证各送风喷口的均衡与稳定,各个喷口喷动区域的相互干扰较小,能够满足工程操作的要求。利用神经网络,建立了各个喷口相互间的相关系数模型,通过与实验结果比对,模型具有较好的吻合性。此外,聚类分析结果也对喷口进行了相似度划分,根据要求分组数的不同可以将所有喷口根据相似性并成不同的组别。通过聚类分析,可以对喷口的差异性进行定性区分,并可以做出必要的设计改进,保证各喷口实验数据的相近性,减少床体内不均衡现象的发生。结合颗粒动力学理论,文章最后采用FLUENT软件对喷动床内气固两相流动进行了冷态数值模拟。模拟结果全面完整地揭示了床内喷动气流的形成、发展、直至喷动气流穿透床层产生喷动的一系列发展过程。并将模拟结果和实验图片进行了对照,无论对于最小喷动速度,或是对于最大喷动床层压降,实验值与模拟值的相对误差都不超过10%。这表明模拟所得的结果与实验数据有较好的吻合性。随着喷动气速的增大,床内中心喷动区内颗粒的体积浓度不断减小,在中心喷动区,颗粒运动的速度值明显增大。但在两侧密相区,气体的速度反而随着喷动气速的增加有所下降。静止床层高度大的床体,床料对喷动气流的反作用力也越大,导致喷动气流向密相区扩散的量越多。此时,对比低静止床层高度的床体,高静止床层高度的床体在中心喷射区域的气体射流速率更小,中心喷射区两边的密相区域内的气体流动速率更大。更多还原

【Abstract】 As a branch of fluidization, spouted beds technology has been applied initially to dryness of coarse particles, and gradually expand to a wide variety of chemical processes related to dryness, comminution, prilling, gasification, combustion, pyrolysis, petroleum thermal cracking etc. With the extension of spouted beds technology, many modified spouted bed designs have been proposed by researchers. These modified spouted beds have been applied to various situations. Base on the mechanism research of spouted bed technology, a novel annular spouted bed with multiple air nozzles has been proposed in this thesis, which combines spouting and coal usage. The flow characteristics of the novel spouted beds have been studied systematically by experiment method and numerical simulation. Visual experiments and pressure fluctuation signal analysis are carried out to investigate the spouting behaviors in the bed. It can provide some evidences of industry application. In numerical simulation, particle phase is considered to“quasi-fluid”. The two-fluid model is adopted to simulate the gas-particle velocity field and pressure field. The spouting mechanism and spouting process are analysis by the quantization of characteristic information.Present research mainly consists of following sections:(i) design and investigation of feeding system for the novel annular spouted bed with multiple air nozzles; (ii) spouting characteristics in the spouting bed; (iii) flow patterns and transitions of the novel annular spouted bed; (iv) pressure fluctuation signal analysis in the bed and correlation among nozzles; (v) numerical simulation study based on flow equation of gas and particle phase in the bed.In the novel spouted bed, as feeding equipment, rotating cone is adopted to ensure equably distribution for experimental materials. To obviate some disadvantages for the rotating cone, such as short residence time and‘passive’layer for particles, adding the upright ring wall and the stirrer will ensures suitable residence time and all particles remain in‘active’layer. It will helpful to improve on particle mixing and transportation. Experimental results show that the dynamic hold-ups and the residence time of particles are inversely proportional to the cone rotational frequency and the areas of the overflow gate. The dynamic hold-ups increase with the increasing of feed rate, while the residence time of particles in the rotating cone tends to decrease with the increasing of feed rate.Dry materials were used to study flow characteristics in the three-dimension spouted bed. The results show that there exist three different zones for the particle flow in the annular spouted bed. With the increase of nozzle diameter or particle size, the maximum spoutable bed height tends to decrease. The effects of the static bed height, nozzle diameter and particle size on the minimum spouting velocity and the maximum spouting bed pressure drop are studied by experimental method. The minimum spouting velocity and the maximum spouting bed pressure drop were determined experimentally and correlated empirically. The calculated data is similar to the experimental data.By the digital CCD camera, experimental study on the flow patterns and transitions in the novel annular spouted bed with multiple air nozzles was carried out. Three distinct stable flow patterns, i.e. internal jet, jet-spouting and fully developed spouting were identified. In addition, two transitional flow patterns and flow instabilities, single internal jet, single jet-spouting, and bubbling or slugging were found. Schematic diagrams and typical flow pattern images obtained by a digital CCD camera were presented for classifying these flow patterns. Typical flow pattern map at various static bed heights and spouting gas velocity were plotted for describing the transitions between flow patterns.Under different operating conditions (spouting velocity, static bed height, measuring point height), the pressure fluctuation signals in a novel annular spouted bed were studied by statistical and frequency analysis. Experimental results show that the total bed pressure drop appears similar characteristics observed in a conventional spouted bed, with the increasing of the spouting velocity. Initially, the whole bed pressure drop increase with the increasing of the spouting velocity. After the spouting formation, the mean value of total bed pressure drop decline to a low value, and almost keeps constant but with fluctuations around this mean value. For all measuring point, The pressure fluctuation standard deviation increases with the increasing of the spouting gas velocity or the static bed height, while it decreases with increasing measuring point height. According to the power spectral density determined by Fast Fourier Transformation (FFT) of pressure fluctuation signals, there exist three pressure fluctuation sources, namely, the spouting jet, interaction among all nozzles and particle interaction. The pressure fluctuation major frequency decreases when increasing the spouting gas velocity, the static bed height or the measuring point height.Two statistical methods, partial correlation coefficient and cluster analysis are firstly introduced to assess the correlation and similarity of pressure fluctuation signals. There are less correlation among nozzles, which shows the novel spouted bed can adopted to industry process due to the equalization and stabilization of supplied gas for all nozzles. The correlation model for nozzles is found by the artificial neural network (ANN).Compared to the experimental results, the model have better application. In addition, by the cluster analysis, all nozzles are distributed to different group according to different similarity. Based on the results of cluster analysis, it can improve on the design of the novel spouted bed, and decrease the unbalance among all nozzles.Combined to the dynamics theory of particle, finally, the dissertation held a cold-state numerical simulation for the gas-solid two phases flow in the novel spouted bed with FLUENT CFD software. Three spouting velocity and three static bed height is used as the simulating condition. The numerical simulation displayed completely the formation, development, broken up of the bubble as well as the formation, development, disappearment of the ejection until the gas-solid steady flow in the spouted-fluidization bed. The numerical simulation results are basically in accordance with the experiment. With the spouting gas velocity increases, bulk concentration of particles in the center of spouting zone of bed, while the velocity of particle increases. In the dense-phase zone of bed, the velocity of gas decreases with the increasing of the spouting gas velocity. The higher static bed height, the bigger resistance of bed materials to flow, result in more spouting gas diffuse to the dense-phase. The jet velocity in the center of spouting zone is less for a spouted bed with high static bed height than for low static bed height, while the gas flow velocity in the dense-phase zone is bigger.更多还原