【作者】 桂南；

【导师】 樊建人； 岑可法； 罗坤；

【作者基本信息】 浙江大学 ， 工程热物理， 2010， 博士

【摘要】 本文的研究内容分成三部分,分别对颗粒流、流化床及旋转射流中颗粒碰撞及其相关现象采用了离散元-大涡模拟／直接模拟耦合方法(DEM-LES/DNS)进行了数值研究。其中,颗粒流的模拟方法主要离散元模拟,同时采用了实验手段作为参照和验证；流化床的模拟方法主要为离散元-大涡模拟耦合方法；旋转射流的模拟方法主要为离散元-直接模拟耦合方法。在颗粒流的研究方面,本文首先采用高速摄影和PTV算法对惯性颗粒的碰撞率的统计特性进行了研究,获得了颗粒碰撞率与颗粒浓度、直径、平均相对速度的统计关系式,并对重力场的效应进行了分析讨论。其次,本文采用直接模拟及离散元硬球碰撞模型耦合的数值手段,对均匀各向同性湍流中不同St数的颗粒碰撞率进行了进一步研究,以研究湍流影响下颗粒碰撞率的统计关系式,并进一步分析了小颗粒的碰撞凝并效应。接着,本文采用离散元硬球碰撞模型对波形转鼓内颗粒运动特性采用能量分析和角动量分析的手段进行了研究,分析了波形鼓边界的波数、振幅和转速对颗粒运动的影响。最后,本文采用离散元软球碰撞模型对圆形转鼓内颗粒混合特性,包括混合界面的分形维、混合过程的信息熵特性等进行了分析研究。在流化床的研究方面,本文首先采用离散元-大涡模拟耦合方法对三维小尺度带埋管流化床进行了模拟研究,分析了流化床内气固两相的相间作用的变化特性和颗粒-埋管碰撞及埋管磨损的分布特性。接着,本文采用同样的方法对两维脉动流化床进行了模拟研究,研究了脉动流态化对气固两相作用力、床层压降、颗粒脉动特性等的调制作用,并分析了脉动流态化对颗粒-埋管碰撞特性的影响。最后,本文采用同样的方法对局部气流扰动对颗粒-埋管碰撞特性的影响进行了细致研究,分别对两维少数埋管、两维管排、三维侧风等情形进行了研究,论证并揭示了局部气流扰动对颗粒-埋管碰撞规律变动的机理。在旋转射流的研究方面,本文首先采用直接模拟方法对气相旋流的涡破裂现象进行了直接模拟研究,揭示了涡破裂区域流动标量的拟序振荡特性。接着采用同样的方法对气相旋流的涡核三维结构进行研究,揭示了不同旋流数下涡核的拓扑结构；并对旋流场的基本统计特性,包括平均速度分布、脉动速度分布曲线等进行了研究。然后,本文对气固两相旋流中颗粒受力特性、扩散及分布特性、扩散的影响因素等进行了系统研究,定量分析了不同特性颗粒的受力情况、揭示了不同特性颗粒扩散的分布形态及影响颗粒的扩散的主导因素等。最后,本文采用硬球模型-直接模拟耦合方法对气固两相旋流中的不同St数颗粒碰撞的分布及于颗粒速度的关系、碰撞与湍流性质的依赖性和关联性、碰撞对湍流的调制效应、湍流雷诺应力对颗粒碰撞的相关性等等进行了细致研究。更多还原

【Abstract】 The research is divided into three main parts, using the coupling methods of discrete element method-large eddy simulation/direct numerical simulation (DEM-LES/DNS) to investigate the particle-particle collision and related phenomena in particulate/granular flows, fluidized beds and swirling jets, respectively. To say specifically, the DEM is used for simulation of particulate/granular flows; the DEM-LES method is used for simulation of fluidized beds; and the DEM-DNS method is used for simulation of swirling jets.Firstly, for the study of particulate/granular flows, an experimental study of particle-particle collision rates is carried out by using high speed camera and and the PTV algorithm. The statistical correlation between the particle-particle collision rate and particle concentration, diameter and mean relative velocity is obtained. Meanwhile, the effect of gravity on the particle-particle collision rate is analyzed and discussed. Secondly, a numerical study on particle-particle collision rates in homogeneous and isotropic turbulence for particles with different Stokes numbers are performed to investigate the effect of turbulence on the statistical correlations of collision rates by using the direct numerical simulation and hard sphere model. Meanwhile, the effects of particle-particle collision on particle clusing/coagulation phenomenon for small Stokes numbers are illustrated. Then, the hard sphere model is used to study the characteristics on particle motion in a rotating tumbler with wavy boundaries based on the energy analysis and angular momentum analysis. The effects of wave number, amplitude and velocity of rotation of the wavy boundary on the motion pattern and collision pattern of particle clusters are analyzed. Finally, the soft-sphere model is used to study the mixing characteristics of particles within a tumbler, including the fractal feature of mixing interface and the information entropy characteristics of mixing, etc.For the study of fluidized beds, at first, the DEM-LES coupling method is developed to simulate the small scale three-dimensional fluidized beds with immersed tubes, analyzing the variation characteristics of gas-particle forces and the distribution characteristics of particle-tube collisions and tube erosions. Then, the same method is used to simulate a two-dimensional pulsed fluidized bed to investigate the effects of pulsed fluidization on modulation of the characteristics of gas-particle interactions, pressure drops and particle fluctuations, etc. Meanwhile, the effect of pulsed fluidization on change of distribution characteristics of particle-tube collision is analyzed. Finally, the effect of locally blowing gas flow on the particle-tube collision characteristics is studied in detail by the DEM-LES coupling method, including the cases with several immersed tubes and a tube bank in a two-dimensional bed respectively, and the case with immersed tubes in the three-dimensiona bed. The mechanisms of modification of particle-tube collision characteristics by the disturbance of locally blowing gas are illustrated.For the swirling jets, a direct numerical simulation study of vortex breakdown is carried out at first, and the coherent oscillation characteristics of flow variables within the breakdown region are observed. Secondly, the same method is used to study the spatial structure of vortex cores of swirling jets, and the instrinsic topology structures of vortex cores at different swirl numbers are illustrated. Moreover, the statistical characteristics of the flow fields are studied, including the mean velocity profiles, fluctuation profiles, etc. Then, the forces on particles, particle distribution and dispersion, the influencing facters on particle dipersion are studied systemically. A quantification analysis of the relative importance of particle forces is carried out. The distribution patterns of particles and the influencing facters on particle dispersion are revealed and discussed. Finally, the distribution charactersics of particle-particle collision in gas-solid swirling jets are investigated in detail by the DEM-DNS coupling method, including the collision distribution in the configuration space and velocity space, the relationship, e.g. correlation and modulation, between collision and turbulence, and the effects of Reynolds stress on particle-particle collision etc.更多还原