【作者】 李瑞霞；

【导师】 柳朝晖； 郑楚光；

【作者基本信息】 华中科技大学 ， 热能工程， 2006， 硕士

【摘要】 燃烧源可吸入颗粒物对于人体健康和自然环境有严重的危害,了解其在燃烧过程中生成和长大过程的机理是控制颗粒物生成的前提,其核心问题之一是颗粒-颗粒的碰撞和凝并过程。对于燃煤产生的细微颗粒,其碰撞过程主要受布朗运动和湍流作用影响,研究湍流作用下有布朗运动的细微颗粒的碰撞过程是目前两相流体力学和气溶胶动力学的研究热点之一。本文在系统总结有关颗粒平均碰撞核函数的研究进展和相关数学模型的基础上,基于高效的后溯法来检测碰撞,首先对Reλ约为51均匀各向同性湍流内St_k (St_k=τ_p/τ_k)为0到10的有限惯性颗粒(不考虑布朗力)的碰撞行为进行了直接数值模拟,研究了湍流对有限惯性颗粒碰撞的影响。结果表明,具有一定惯性颗粒的湍流碰撞率完全不同于零惯性的轻颗粒(St_k=0)和可忽略湍流作用的重颗粒(Stk=∞),其变化趋势极其复杂。使用同时考虑了湍流掺混效应和局部富集效应的圆球模式来估计湍流作用下的颗粒碰撞核函数,分别利用可能发生碰撞的颗粒对的径向相对速度<|w_r|>和径向分布函数g(R)来量化这两种效应。结果表明St_k≈1.0时局部富集效应最为强烈,使得颗粒的碰撞核函数在此时出现第一个峰值;湍流掺混效应则随着颗粒St_k的增大而渐进增大;局部富集和湍流掺混联合作用的结果,使得颗粒碰撞核函数在St_k≈3.0附近出现另一个峰值。通过对Stk在0到1之间的细微颗粒进行渐近性分析,结合数值模拟的结果,提出了适用于细微颗粒的简单的湍流碰撞核函数模型。在直接模拟湍流作用下的颗粒碰撞的基础上,本文结合布朗动力学模拟(BD)方法,初步模拟了均匀湍流中有布朗运动的细微颗粒的碰撞现象,模拟的颗粒直径范围在0.1-1.2微米之间,表明在0.6-1.2微米范围内,湍流对碰撞起主要作用,布朗运动作用并不明显;在0.1-0.4微米范围内,除了湍流作用外,布朗运动作用随着直径减小而增大,并在0.1微米时二者作用相当。更多还原

【Abstract】 Inhaled particulate matter from combustion does serious harm to human health and environment. To know how the particulate matter generates and grows in the combustion process is the prerequisite to control and prevent them. And one of the core problems is inter-particle collision and coagulation process. For fine particles, both turbulent effect and Brown motion effect play a signifinant role for the collision process. To simulate and predict the collision kernel under the two effects is the hotspot of two phase flow studies and the aerosol studies nowadays.After a brief summary of the research developments in particle average collision kernel model, correlated mathematic models and numerical simulation algorithms, direct numerical simulations (DNS) were conducted to study particle collisions in a stationary isotropic homogeneous turbulent flow(without Brownian motion), with the aim to investigate the influence of turbulence on particle collision kernels of various finite-inertia particles. It is found that the behavior of finite-inertial particle collision is very complicated, both the Saffman & Turner theory(St_k=τ_p/τ_k=0) and kinetic theory (St_k=∞) can’t predict it correctly. To further understand the mechanism of finite-inertia particle collision in isotropic turbulence, two major effects of turbulent flow on particle collision, namely turbulent mixing effect and preferential concentration effect, are investigated and are represented qualitatively using radial relative velocity <|wr|> and radial distribution function g(R) of colliding particle pairs respectively. Both effects tend to increase collision rates, leading to the observed complex behavior. The results showed that preferential concentration effect is the main contribution factor for the peak of particle collision rate near St_k~1, while both preferential concentration effect and turbulent mixing effect contributing to the peak near St_k~3, with much stronger turbulent mixing effect herein. To focus on the fine particle matters, particles with Stk between 0 and 1 were further investigated. After an asymptotic analysis, a simple turbulent collision kernel model for fine particles was derived from the DNS results.Combined with DNS method, Brownian dynamics simulation (BD) method was utilized to study the relative importance of turbulent effect and Brownian motion effect on fine particle collision process. For particles with diameter between 0.6-1.2μm, the results showed that turbulent effect is dominant, while for particles with diameter between 0.1-0.4μm, Brownian motion effect increases with diameter decreases. Turbulent effect and Brownian motion effect became comparable herein.更多还原