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1、主要内容8.1 自由剪切层流动自由剪切层流动8.2 热射流热射流8.3 热尾流热尾流远离固体壁面的自由紊流的动量传递和换热特性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：研究内容：自由紊流自由紊流自由剪切层流动自由剪切层流动热射流热射流热尾流热尾流自然对流热尾流自然对流热尾流强制对流热尾流强制对流热尾流8.1 自由剪切层流动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y0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
2、流。二维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轴对称圆形射流虚拟原点时间平均界面环状剪切流环状剪切流圆形射流区圆形射流区剪切流区域剪切流区域射流区域射流区域数学模型数学模型8.3 热尾流热尾流自然对流热尾流自然对流热尾流大尺寸涡大尺寸涡环境环境热源热源虚拟原点时间平均界面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浮升力数学模型数学模型8.1 Physical ConsiderationsFree convection: Buoyance forces Forced convection: Flu
3、id motion Forced forces fluid density gradient Body forceBuoyance forces Temperature gradientGravitational forceUnstable fluid circulationStable (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 s
4、urface 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
5、the existence of free convection currents.8.2 The Governing EquationsQuiescent fluid gFigure 8.1 boundary layer development on a heated vertical plateAssumptions:1. Steady-state. 2. Two-dimensional. 3. Constant property. 4. Incompressible.Equations:In the quiescent region outside the boundary layer.
6、 u=v=0Buoyancy forceThr volumetric thermal expansion coefficientStrongly coupledFor a perfect gas, 9.3 Similarity ConsiderationNondimensionalizing Grashof number GrLThe Grashof number indicates the ratio of the buoyancy force to the viscous force acting on the fluid.ConclusionIt is important to note
7、 that such correlations are pertinent only when Forced- and free-convection effects are comparable.Effects of free and forced convection must be consideredEffects of free convection may be neglectedEffects of forced convection may be neglected8.4 Laminar Free Convection on a Vertical SurfaceGovernin
8、g equationsBoundary conditionsMathematical model8.5 The Effects of TurbulenceThermal instabilityTurbulence flow Quiescent fluid gFigure 8.2 Free convection boundary layer transition on a vertical plate.Laminar Turbulent Transition Rayleigh numberTransition in a free convection boundary layer depends
9、 on the relative magnitude of the buoyance and viscouse forces in the fluid. 8.6 Empirical Correlations: External Free ConvectionThe correlations are suitable for most engineering calculations and generallt of the form8.7 Combined Free And Forced ConvectionIn dealing with forced convection, we ignor
10、ed the effects of free convection. Similarly, in the preceding sections of this chapter, we assumed that forced convection effects were negligible. The ratio of buoyance and inertial force may be used to determine the combined convection.The effect of buoyancy on heat transfer in a forced flow is strongly influenced by the direction of the buoyance force relative to that of the flow. Same directions (assisting flow), opposite directions (opposing flow), and perpendicular directions (transverse flow)
