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1、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：,8.1 自由剪切层流动,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,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,数学模型,8.3 热尾流,自然对流热尾流,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,(a),(b),(a)The density dec
2、reases 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
3、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,Assumptions:1.Steady-state.2.Two-dimensional.3.Constant property.4.Incompressible.,Equations:,In the qui
4、escent region outside the boundary layer.u=v=0,Thr volumetric thermal expansion coefficient,For a perfect gas,9.3 Similarity Consideration,Nondimensionalizing,Grashof number GrL,The Grashof number indicates the ratio of the buoyancy force to the viscous force acting on the fluid.,Conclusion,8.4 Lami
5、nar Free Convection on a Vertical Surface,8.5 The Effects of Turbulence,Rayleigh numberTransition in a free convection boundary layer depends on the relative magnitude of the buoyance and viscouse forces in the fluid.,8.6 Empirical Correlations:External Free Convection,The correlations are suitable
6、for most engineering calculations and generallt of the form,8.7 Combined Free And Forced Convection,In dealing with forced convection,we ignored the effects of free convection.Similarly,in the preceding sections of this chapter,we assumed that forced convection effects were negligible.The ratio of b
7、uoyance 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),
