【作者】 任聪静；

【导师】 阳永荣； 王靖岱；

【作者基本信息】 浙江大学 ， 化学工程， 2010， 博士

【摘要】 在复杂多变的多相流体系中,准确检测各种操作参数的关键是如何用最佳的检测手段提取体系中的有效信息进行分析。利用反映多相流动特性的波动信号,通过从中提取出的特征信息,从而实现多相流参数的测量一直是多相流检测领域的一个研究热点。在多相流体系中,广泛存在各种临界现象,其转变方式和规律是多相流反应器的重要调控依据。因此,将各种临界现象作为一种特殊的问题加以研究,分析不同操作条件下临界现象的产生和转变规律,建立波动信号与各种临界参数的直接关联,不仅极富挑战性,同时对于掌握多相流体系的流动状态以解决实际生产中涉及多相流体系的各种具体问题具有重要意义。针对多相流体系中存在的各种临界现象以及关键参数,本研究选择了具有实时在线、安全环保、简便准确、不侵入流场以及适合工业装置等恶劣外部环境特点的声发射测量技术作为手段,结合频谱分析、小波分析以及多尺度分析方法等多种分析手段,通过单通道和多通道测量方法,分析了时域和空间两个层面存在的各种临界现象,获得了多相流体系中气固两相体系和液固两相体系的关键参数与声信号存在的直接关联。首先,通过单通道随时域变化的信号特征分析,建立了包括临界流化速度、临界湍动速度、临界悬浮转速等临界参数的判断准则；其次,通过多通道随空间变化的信号特征分析,建立了包括颗粒流动结构、流化质量恶化、淤浆悬浮高度等临界现象的判断准则；最后,以多相流体系中的气固流化床为例,基于临界现象理论,针对不同颗粒流动结构的本质及其转变规律,建立了颗粒流动结构的调控依据。分别获得了包括分布板结构、布气方式、外加搅拌设备以及床结构等因素对气固体系颗粒流动结构的影响规律,提出了不同因素影响下相应的流化质量改善策略,为工业生产提供了重要依据。本论文的主要研究成果如下：1.基于单通道声发射技术,研究了多相流体系中随时域变化的各种临界现象。通过分析声信号的时域特征变化,建立了各种时域临界参数的判断准则。其中包括代表气固流化床流型临界转变的临界流化速度、临界湍动速度和代表液固搅拌体系流型临界转变的临界悬浮转速。1)在气固流化床体系中,采用频谱分析,得到了气固流化床中固体颗粒碰撞壁面产生声信号的机理和特征。实验结果表明,声信号的能量和方差随气速呈现规律性的变化,能量值和方差值在临界流化速度和临界湍动速度附近均有阶跃性的增长。基于此,提出了能量比和方差比的概念,研究发现在流型临界转变处存在能量比和方差比的极值。由此,得到了基于声发射技术的气固流化床中流型临界现象转变的判断依据,即当声能量比或方差比达到最大值时对应的表观气速为临界流化速度,能量比或方差比达到次最大值时对应的表观气速为临界湍动速度。同时,实验考察了不同Geldart分类颗粒对流型临界转变的影响,对比分析了不同分类颗粒的流化特性。2)在液固搅拌釜体系中,结合声信号的频谱分析、小波分解和R/S分析,得到了搅拌釜反应器中固体颗粒碰撞壁面产生声信号的机理和特征。根据各小波尺度下声信号的Hurst指数变化规律,获得了液固搅拌釜中代表固体颗粒相运动特征的声信号特征频段(S1尺度)。实验结果表明,声信号特征频段能量随搅拌转速呈现规律性的变化。基于此,选取特征频段的能量分率值作为特征参数,根据s1尺度能量分率值随搅拌转速的变化规律,提出了搅拌釜反应器中流型临界转变的判断依据,即当声信号特征频段能量分率快速减少并开始趋于稳定时所对应的搅拌转速为临界悬浮转速。同时,实验考察了粒径、表观浆液浓度和桨叶离底高度等操作条件对临界悬浮转速的影响规律。结果表明,临界悬浮转速随着粒径、浆液浓度和桨叶离底高度的增加而增加。2.基于多通道声发射测量技术,研究了多相流体系中随空间变化的各种临界现象。通过分析声信号的空间特征变化,建立了各种空间临界现象的判断准则。其中包括代表气固流化床中流化质量临界转变的颗粒流动结构和代表液固搅拌体系两相混合均匀程度临界转变的淤浆悬浮高度。1)在气固流化床反应器中,沿壁面不同高度设置多通道声发射探头,结合声信号能量分析,得到了固体颗粒不同分布和运动方式呈现的循环流动结构。利用声信号能量的空间分布,获得了表征流化质量临界转变的流动结构的判断依据,揭示了聚乙烯流化床内存在大、小两个循环的整床流动模式,两个循环的分界即为颗粒流化滞留区域。根据声信号能量沿床高的变化规律可以确定滞留区的位置,在滞留区位置声能量存在极小值,对应为颗粒运动相对不活跃的区域,聚合物团聚和结块现象容易在此区域产生。同时,通过考察不同操作条件如表观气速、颗粒粒径、静床高以及颗粒种类对颗粒流动结构临界转变的影响,得到了相应的流动结构转变规律。实验发现,除静床高外,表观气速、粒径大小和颗粒种类均对流动结构存在较大影响,当上述操作条件变化时,颗粒流动结构往往随之发生改变,流动结构在单循环和双循环之间转变。研究表明,利用不同操作条件下声信号能量在空间的分布规律,获得整床颗粒的流动结构和滞留区位置,可以为工业生产过程提供稳定操作的依据,以改善反应器的流化质量。由于滞留区高度对应为颗粒运动不活跃的位置,在实验室冷模装置中考察了结块存在时,颗粒流动结构的临界转变规律。通过人为模拟结块产生(在流化床中添加不同重量的结块),考察了结块尺寸对循环模式的影响。结果表明,当结块尺寸达到一定程度时,聚乙烯颗粒流动结构出现了临界转变,由单循环流动结构转变为双循环流动结构,而结块所处高度对应于大、小循环界面的滞留区位置。实验结果从另一角度阐明了滞留区存在与结块产生的因果联系。由此说明,当流化床内存在的结块到达一定量以后,结块影响的区域变大,甚至改变了整床的颗粒分布和运动方式,转变了循环模式。同时,基于气固流化床颗粒流化结构的理论,提出了利用“颗粒温度”概念来定量表征流化床不同位置颗粒运动活跃程度的理念。根据颗粒作用于器壁产生声信号的机理以及“颗粒温度”的本质特征,首次建立了基于“颗粒温度”和声发射技术的定量表示颗粒运动活跃程度的理论模型。结合实验数据,推导得出了颗粒温度的计算公式,获得了流化床内不同区域(主流区和滞流区)的颗粒温度空间分布。基于此,提出了“临界颗粒温度”和“临界操作气速”的概念,进而得到了气固流化床良好流化的判据,即当流化床内所有区域的颗粒温度均大于临界颗粒温度(r*≥T*cr)时,床内颗粒具有一定的活跃程度,整床流化质量良好,没有出现团聚结块的流化床故障。基于判据,得到了不同颗粒粒径下的临界操作气速。利用临界颗粒温度和临界操作气速,可对工业上生产监控和操作优化起到指导作用,由此实现了声发射技术定量监测反应器流化质量及其临界转变的实时在线测量。2)在液固搅拌釜体系中,沿反应器壁面不同高度设置多通道声发射探头,研究了声信号在空间的分布规律。根据颗粒运动碰撞搅拌釜壁面产生声信号的机理,结合频谱分析、小波分解和R/S分析,发现代表颗粒相运动特性的声信号特征频段能量值与液固两相的混合程度紧密相关。随着搅拌釜轴向高度位置的增加,声能量值呈现规律性的变化。基于此,提出了声发射技术测量搅拌釜淤浆悬浮高度的判据,即当声信号特征频段能量或能量比出现阶跃性变化时对应的高度为淤浆悬浮高度,利用此判据,实现了基于声发射技术的液固体系混合程度临界转变的实时在线测量。同时,通过改变颗粒尺寸、搅拌桨的形式(盘式涡轮和桨式叶轮)等操作条件,验证了声发射技术测量淤浆悬浮高度的判据,实验结果与目测法很好地吻合,两种桨型实验的平均相对误差分别为6.5%和8.9%,具有较高精度。3.以多相流体系中的气固流化床为例,基于临界现象理论,建立了颗粒流动结构的临界调控依据,分别获得了包括外加搅拌设备、不同布气方式、不同分布板结构以及不同床结构下颗粒流动模式的临界转变规律,并提出了相应的流化质量改善策略。1)通过在气固流化床中引入外加搅拌装置,指出了搅拌桨叶作用对颗粒流动结构具有较大影响。由于搅拌桨的作用,床内粒子不断被带入气泡内部,床内气泡不断破碎,使得大气泡、沟流和节涌减少甚至消失,保持了流化床良好的流化状态。实验发现,对比没有搅拌作用时的颗粒流动结构,搅拌桨叶的引入强化了流化过程,在加强气泡相和颗粒相相互作用的同时,消除了滞留区域,减少了流化的不均匀和不稳定,同时使得料位更加稳定。2)通过改变孔板型气体分布板的布气方式,设计了内圆-外环相同面积的两路不同进气方式,系统分析了不同表观气速下的声能量空间分布与流动结构之间的变化关系。实验发现,颗粒流动结构受到表观气速大小和布气方式的共同影响。同时利用声信号能量的空间分布规律,获得了不同布气方式下整床的颗粒运动方向和气泡分布情况。结果表明,内环和外环两种不同的布气方式造成了截然不同的颗粒运动结构。3)针对四块不同结构的气体分布板,利用声发射技术研究了装载不同分布板的流化床颗粒死区分布情况以及流动结构的临界转变。通过不同区域的声信号能量和方差分析,得到了颗粒死区分布和流化均匀性的判断依据。同时,实验对比并分析了不同分布板结构对流动结构的影响及其内在原因,筛选得出了满足压降稳定性及流化质量良好的抗沉积分布板,提供了分布板优化设计的方法。4)针对二维气固流化床,实验考察了床结构对颗粒循环模式的影响。结果表明,二维流化床由于受到边壁效应的影响,其流动结构与三维流化床存在较大差别,气泡只能在二维空间发展,床内更容易出现大尺寸扁平气泡。因此,在二维床中容易形成节涌等不稳定流化状况,并且不能形成类似三维床的整体循环。同时,分析了不同表观气速、静床高等操作条件下声信号能量的变化规律。研究表明,表观气速和静床高的增加均使声能量相应增加。结合不同高度声能量和标准偏差分析,证明了声发射检测方法能真实反映反应器内的颗粒运动规律。更多还原

【Abstract】 The key factor of accurate determination of operation parameters in complicated multi-phase flow systems is how to use appropriate technique and how to extract useful signals. Extracting characteristic information based on fluctuating signals representing multi-phase flow hydrodynamics in order to determine the crucial parameters is one of the significant ways in the multi-phase research. There are various critical phenomenal parameters in multi-phase flow systems, which are the important control principle of reactors operation. Therefore, taking the critical phenomenon as a special issue to study and analyzing the appearance and transition of critical phenomenon based on the direct relationship between the fluctuating signals and critical parameters play a crucial role to understand the flow condition and supply useful advice to solve the specific problems in industrial process, which is also a big challenge.Our study employed acoustic emission (AE) measurement that is a reliable, safe, on-line, accurate and well-established technique for machinery condition and has been proved sensitive, environment friendly and non-invasive to study all the critical phenomenon and key parameters in multi-phase flows. We studied the temporal and spatial critical phenomenon and obtained the direct relationship between AE signals and critical parameters in multi-phase flows including gas-solid flow and liquid-solid flow by using single-channel and multi-channel measurement based on FFT analysis, wavelet analysis and multi-scale analysis. Firstly, the criterion to determine critical fluidized velocity, critical turbulent velocity and critical suspension speed was presented by analyzing single-channel AE signals varied temporally. Secondly, the criterion to detect particle fluidization pattern, the agglomeration and slurry suspension height was presented by analyzing multi-channel AE signals varied spatially. Finally, we established the control rule of particle fluidization pattern based on relationship between fluidization pattern and flow quality in gas-solid fluidized bed. By studying, specific cases such as the influences of improved distributor structure, various gas distribution styles, added agitated equipment on fluidization pattern and different bed structure, we got important operation guidance to the industrial process. The main achievements of this paper are as follows.1. Based on single-channel AE measurement, we studied the critical phenomenon varied temporally in multi-phase flows. By temporal transition of AE signals, we presented the criterion including the phenomenal parameters such as critical fluidized velocity and critical turbulent velocity that represent the flow regime transitions in gas-solid fluidized bed and the critical suspension speed that represent the flow regime transition in liquid-solid stirred tank respectively.1) In gas-solid fluidized bed, we obtained the mechanism and characteristic of AE signals from impact between particles and wall based on FFT analysis. It appears that the average energy E increased with superficial velocity Ug and sudden increase took place when Ug=Umf or Ug=Uc distinguishingly. Moreover, by the definition of energy ratio RE, a criterion to determine the flow regime transition based on AE method was obtained. That is, the velocity was the critical fluidized velocity when the ratio of the AE energy reached the maximum first, while it secondly reached the maximum the corresponding velocity was the critical turbulent velocity. The influence of different particle Geldart classification was also considered, in order to analyze the different fluidization characteristic.2) In a liquid-solid stirred tank, a characteristic frequency band (S1 scale) of AE signals which represented the motion of solid particles was obtained by using FFT, wavelet transform wavelet and R/S analysis. Then it was observed that the energy fraction of AE signals on different frequency changed regularly with the increasing impeller speed in a stirred tank. Taking the energy ratio of characteristic frequency band (S1 scale) as variable, a new method for determining the flow regime critical transition (critical suspension speed) was presented based on this regular behavior of energy ratio in S1 scale as speed increases. When the AE energy fraction in the characteristic frequency band decreased rapidly and began to level off, the corresponding impeller speed was the critical suspension speed. We also investigated the influences of particle size, slurry concentration and impeller height on the critical suspension speed. It showed that the critical suspension speed increases with the particle size, slurry concentration and impeller height.2. Based on multi-channel AE measurement, we studied the critical phenomenon varied spatially in multi-phase flows. By spatial transition of AE signals, we presented the criterion to determine spatial critical phenomenon including particle fluidization pattern and flow quality that represent the flow quality and slurry suspension height that represent the mixing degree m liquid-solid stirred system.1) In gas-solid fluidized bed. sensors were located along the bed in order to detect the particle fluidization pattern under different conditions based on AE energy analysis. It was found that the particle fluidization structure performs multi-circulation pattern based on the AE energy axial profile, including main circulation in upper bed, smaller circulation above the distributor and stagnant-zone between them. The location of stagnant-zone can be detected by AE energy profile, which was a minimum in the energy value. In stagnant-zone, particles have less motion energy, where agglomeration often forms. Furthermore, the effects of operating variables such as particle size, superficial gas velocity, static bed height and particle classification were considered. The results show that particle size, particle classification and superficial gas velocity have greater effect on the fluidization pattern while static hold-up has little influence contrastively. The particle fluidization pattern varied between single-pattern and multi-pattern. It was concluded that, the particle fluidization pattern and location of stagnant-zone obtained from the AE energy spatial distribution under different operation conditions can supply advice to the stable operation of industrial process, in order to improve the flow quality.Moreover, experiments were taken out by adding different mass particle agglomeration in the fluidized bed in order to investigate the influence of agglomeration size on particle fluidization pattern. It showed that the single circulation flow pattern of polyethylene particles in fluidized bed would change to the multi-circulation flow pattern when the ratio of agglomeration reached to certain amount. The location of agglomeration was corresponding to the location of stagnant zone. That is to say, the influence area became larger and the particle distribution and movement change if the agglomeration size reached to certain amount.We also addressed issues related to granular temperature with respect to particle velocity as well based on particle fluidization pattern in gas-solid fluidized bed. Through the mechanism of particle impact on wall and characteristic of granular temperature, we established a model to quantitatively detect the particle motion energy based on granular temperature and AE technique. By analyzing of spatial energy distribution, we got the formula to calculate granular temperature in different zones (main zone and stagnant-zone). Consequently, by the definition of critical granular temperature and critical operation velocity, we presented a criterion to monitor the fluidization quality in gas-solid fluidized bed, namely, particles are properly active and there is no agglomeration, indicating good fluidization quality when the granular temperature in all regions satisfies T*>T*cr. In addition, the definition of critical operation velocity was introduced which provide significant direction to production in pilot-scale.2) In a liquid-solid stirred tank, we discussed the AE signals spatial distribution by setting multi-channel sensor along the bed height. We presented the characteristics frequency band of AE signals on behave of the particles movement by spectrum analysis, wavelet transform and R/S analysis based on the mechanism of interaction between particles and the wall of tank. A criterion of slurry suspension height in stirred tank was established based on the relationship between acoustic emission energy on the characteristics frequency band from the collision between the moving particles and inside wall and the height of the investigation location in stirred tank. The criterion was that the location with according to the sharp decrease of the AE energy on the characteristics frequency band or the ratio of the energy on the characteristics frequency band at one location and the average energy of all of the samples was the slurry suspension height of stirred tank. By using this criterion, a on-line measurement method based on AE technique to determine the liquid-solid mixing degree was established. The criterion was also suitable with different particle size and impeller structure. The relative error of both impeller structures were 6.5% and 8.9% respectively, which were well agree with the result of observation.3. Concerning the gas-solid fluidized bed, the transition and control rule of particle fluidization pattern under various conditions such as added agitated set up, various gas flow distribution, various distributor structure and different bed structure were observed in order to establish some useful experience to practical production and reactor design.1) Agitator was added in a fluidized bed to monitor the particle fluidization pattern, which showed great influence. The particles were brought to the bubble inside and the bubbles were broken continuously with the effect of impeller, leading to the decrease of huge bubble, channeling and slugging and good fluidization condition. By comparing with the condition without impeller, the addition of impeller intensified the fluidization and mixing between bubbles and particles, which decreased the stagnant-zone and unstable fluidization.2) Particle fluidization pattern was studied by changing gas flow distribution style in the distributor with equal area of inside and outside parts. The results showed that, superficial velocity and gas distribution (inside/outside gas ratio) would affect the fluidization pattern. Both the conditions of various outside gas fluxes with constant inside gas flux and various inside gas flux with constant outside gas flux generated complex pattern. Moreover, the particle fluidization direction and bubble distribution under various gas distributions were obtained based on the AE energy spatial distribution. The inside and outside gas flow form different fluidization structure respectively.3) The influence of distributor structure on fluidization pattern was observed by AE technique. By analyzing the AE signals in the distributor, the particle distribution and fluidization quality were also presented, which gave the result of anti-deposit distributor with good fluidization quality.4) We studied the AE signals in a 2D gas-solid fluidized bed in order to investigate the influence of bed structure. The results showed that, the fluidization pattern in 2D was obviously different with 3D since the influence of wall effect in 2D with bigger bubbles developing in 2D space. Often,2D fluidized bed easily produces slugging and unsteady fluidization and has no multi-circle structure as in 3D fluidized bed. Experiments were taken out to observe AE energy profile under different velocity and static bed height. It showed that AE energy increased with velocity and static bed height, and the symmetry results proved that AE measurement can represent the multi-phase flow information inside the reactor.更多还原