第八章自由紊流流动和换.ppt

主要内容 8.1 自由剪切层流动 * 8.2 热射流 8.3 热尾流 远离固体壁面的自由紊流的动量传递和换热特性 研究内容： 自由紊流 自由剪切层流动 热射流 热尾流 自然对流热尾流 强制对流热尾流 8.1 自由剪切层流动 层流剪切层 过渡 紊流剪切层 周围环境 虚拟原点 两个自由剪切流 解决问题： 剪切层厚度变化 速度分布 温度分布及通过y＝0平面的热流 边界层性质 假设 数学模型 模型求解 8.2 热射流 剪切层区域 热射流区域 虚拟原点 时间平均界面 两个自由剪切流层会合的流动为热射流。 二维热射流 热射流区域的流动具有边界层的性质 轴对称圆形射流 虚拟原点 时间平均界面 环状剪切流 圆形射流区 剪切流区域 射流区域 数学模型 8.3 热尾流 自然对流热尾流 大尺寸涡 环境 热源 虚拟原点 时间平均界面 集中热源上方的自然对流热尾流及时间平均的漏斗形流场 浮升力 数学模型 8.1 Physical Considerations Free convection: Buoyance forces Forced convection: Fluid motion Forced forces fluid density gradient Body force Buoyance forces Temperature gradient Gravitational force Unstable fluid circulation Stable (a) (b) (a) The density decreases in the direction of the gravitational forces. This condition is unstable. Heat transfer occurs from the bottom to the top surface by free convection. (b) The density no longer decreases in the direction of the gravitational forces. This condition is now stable. Heat transfer occurs from the bottom to the top surface by conduction. Note that the presence of a fluid density gradient in a gravitational field does not ensure the existence of free convection currents. 8.2 The Governing Equations Quiescent fluid g Figure 8.1 boundary layer development on a heated vertical plate Assumptions: 1. Steady-state. 2. Two-dimensional. 3. Constant property. 4. Incompressible. Equations: In the quiescent region outside the boundary layer. u=v=0 Buoyancy force Thr volumetric thermal expansion coefficient Strongly coupled For a perfect gas, *