【作者】 闫洁；

【导师】 樊建人； 岑可法；

【作者基本信息】 浙江大学 ， 工程热物理， 2009， 博士

【摘要】 关于气固两相湍流中颗粒与流体相互作用以及其中颗粒之间的碰撞问题,多年来一直是国内外关于气固两相流动的研究热点和难点问题。气固两相平面射流作为一种典型的自由剪切流动,在自然界与实际的工程应用中广泛存在。但是迄今为止,对三维气固两相平板间湍流射流中相间耦合的研究还很少,其中采用直接数值模拟(DNS)进行的研究更是几乎没有。在这样的背景下,本学位论文采用DNS方法集中对不可压缩的三维气固两相平板间湍流射流中相间作用规律进行了研究。其中对流场不引入任何湍流模型,采用DNS方法直接求解完整的三维非定常的Navier-Stokes方程组,对颗粒的跟踪分别采用基于单向和双向耦合的拉格朗日方法,颗粒之间的碰撞采用确定性的硬球模型,颗粒与流体之间的动量耦合采用点力描述。首先在单向耦合情况下,单独考察了不同Stokes的颗粒间碰撞及其对颗粒扩散特性的影响。模拟发现不同Stokes数的颗粒碰撞与其扩散特点及流场拟序涡结构的演化有着密切的关系,尽管整个流场的颗粒平均浓度较低,但是在颗粒局部聚集的较高浓度区域都可以观察到颗粒碰撞的发生。同时发现,颗粒的局部聚集效应是较小惯性颗粒发生碰撞的主导因素,而较大惯性颗粒的碰撞现象则主要受湍流输运作用的影响。此外,颗粒碰撞使得颗粒在流场中的分布更加均匀,扩散程度也有所增加。对于流场中颗粒尺寸按照Rosin-Rammler函数分布时的研究发现,不同粒径颗粒混合的情形下,流场中颗粒发生碰撞的次数明显变多了碰撞对颗粒扩散统计结果的影响也明显了。接着采用双向耦合的方法,对流场中颗粒与流体之间的相互作用进行了详细的研究。计算时,为了得到更为真实准确的结果,没有采用计算颗粒的概念,而是对不同载荷下流场中的实际颗粒数目进行了跟踪。模拟结果表明不同Stokes数的颗粒都改变了流场拟序涡结构的发展演化,对于同一种Stokes数的颗粒而言,颗粒固相载荷越大,对流场涡结构的影响越明显。颗粒使得流场流向平均速度剖面变高变窄了,降低了流场的横向平均速度和脉动速度。相同载荷的情况下,Stokes数为0.5和1的两种颗粒对流场平均速度的改变程度要更明显。同时发现,颗粒使射流中心线上的流向平均速度衰减变慢了,且在相同载荷的情况下,Stokes数为0.5的较小颗粒影响更明显。所有颗粒都增加了流场横向两侧的湍流强度,减小了流场湍动能和雷诺应力。此外,还发现双向耦合效应对颗粒运动及扩散的影响也不容忽视,双向耦合效应使不同Stokes数颗粒的扩散特点都发生了变化,并且改变了颗粒在流场中的扩散程度、运动速度、浓度以及能量分布情况。最后,考虑四向耦合效应对较低载荷情况下的三维气固两相射流中的相间耦合作用作了进一步的探讨。通过对颗粒间的碰撞进行了定量统计,发现碰撞主要集中分布在流场的中心区域、颗粒碰撞次数与颗粒在流场中的浓度、运动速度和颗粒粒径都有关系,其中stokes数为100的颗粒在流场中碰撞发生的次数最多。同时发现颗粒碰撞对流场统计特性产生了不同程度的影响,各种Stokes数的颗粒碰撞作用都加快了气相中心速度的衰减、增强了流场的总湍流强度。但是在较低固相载荷的情形下,四向耦合不会从根本上改变流场的性质。此外,对四向耦合情况下颗粒在流场中的运动速度及浓度分布进行了统计,发现不同Stokes数的颗粒在流场中的速度及浓度分布与其在流场中的运动扩散特点有密切的关系。同时也初步探讨了颗粒碰撞对其在射流中心线上运动特性的影响,发现碰撞改变了颗粒沿横向的运动速度和方向。更多还原

【Abstract】 With regard to the fluid-particle interaction and particle-particle collision in gas-solid turbulence, it has been the focus and difficulty of investigation of gas-solid two-phase flow at home and abroad for several decades. Gas-solid two-phase plane jet, which is one of the typical free-shear flows, exists widely in the nature and engineering applications. But, up to now, few researches are devoted to the inter-phase coupling interaction in three-dimensional gas-solid two-phase plane turbulent jet, especially for numerical researches using direct numerical simulation.Under the above background, the present thesis uses direct numerical simulation to investigate incompressible three-dimensional gas-solid two-phase plane turbulent jet, focusing on the inter-phase interactions. The fluid motion is without any turbulence model, whereas the full three-dimensional unsteady Navier-Stokes equations are directly solved. The particle motion is traced in Lagrangian frame based on the one way and two-way coupling method respectively, and particle-particle collision is simulated by using the hard-sphere model in the deterministic way. Additionally, the momentum coupling between fluid and particle is depicted by point-force approximation.For the one-way coupling, the effect of particle-particle collision as well as particle dispersion at different Stokes numbers is investigated respectively. The simulation results indicate the close correlation between the characteristics of particle-particle collision and dispersion for particles with different Stokes numbers and the evolution of fluid coherent structure. Although the mean particle concentration is kept low, particle-particle collision is observed in the accumulation regions with relatively high particle concentration. Meanwhile, the local accumulation effect is the dominant factor of particle-particle collision for particles with small inertia, whereas for particles with large inertia, collision phenomenon is mainly influenced by the turbulent transport effect. Moreover, particle-particle collision causes the particle distribution in the fluid field become more uniformly and the degree of dispersion enhanced slightly. With regard to the particle size distribution based on the Rosin-Rammler function, it is observed that the particle-particle collision is enhanced by mixing between different size particles, and the effect of particle-particle collision on enhancement of particle dispersion becomes more evident.For the two-way coupling, detailed investigations on fluid-particle collision interaction are carried out. In order to obtain more accurate results, the concept of computational particles is not taken into account. Alternatively, actual numbers of particles are traced based on the mass loading. Simulation results indicate that the evolution of fluid coherent structure is changed by particles with different Stokes numbers. For the same Stokes number, larger particle mass-loading causes greater changes in fluid vortex structure. The mean streamwise velocity profile becomes higher and narrower by the additive of particles, and the mean lateral velocity and fluctuation are decreased. With the same mass-loading, the changes in mean streamwise velocity are more evident for particles with St=0.5 and St=1 than that for other Stokes numbers. Meanwhile, the decay of mean streamwise velocity in the jet axis is attenuated, and it is more evident for St=0.5. The turbulence intensity at both sides of the lateral are increased, whereas the turbulence energy and Reynolds stress are decreased by additive of particles. Moreover, the effect of two-way coupling on particle motion and dispersion cannot be neglected too. The characteristics of particle dispersion in the fluid field, such as the degree of dispersion, motion velocity, concentration and the energy distribution, are all changed by the two-way coupling effect.Finally, for the four-way coupling, the inter-phase coupling effects in the three-dimensional gas-solid two-phase jet with low mass-loadings are investigated. By carrying out quantitative statistics of particle collision, it is indicated that particle-particle collision occurs mainly in the central region, and the collision times is related to the concentration, the motion velocity and the size of particles. The particle collision changes the statistical results of fluid field in different degrees. Collision enhances the decay of fluid in the jet axis and the total turbulence intensity for all particles. However, for the low mass loading, the four-way coupling interaction doesn’t change the property of fluid field radically. Moreover, statistics of particle motion velocity and concentration distribution in the fluid field are also carried out for four-way coupling case. It is observed that the motion velocity and concentration of particles are closely related to the dispersion characteristics of particles. Additionally, the effect of particle collision on the particle motion in the jet axis is preliminarily interpreted, such as the particle velocities in the lateral direction are changed, etc.更多还原