传热学-第五章

1、2020/7/11,1,第五章 对流传热的理论基础,Chapter 5 Convective heat transfer fundamental Non-dimensional parameter Nu , Re , Pr,2020/7/11,2,5.1 概述,一、影响因素,2020/7/11,3,Convection heat transfer strongly depends on the fluid properties:,Also depends on the fluid velocity,It also depends on the geometry and roughness of

2、 the solid surface, in addition to the type of fluid flow (such as being streamlined or turbulent).,When a fluid is forced to flow over a solid surface, it is observed that the fluid layer in contact with the solid surface sticks to the surface. This is a very thin layer of fluid and is assumed to b

3、e zero velocity at the wall.,2020/7/11,4,dimensionaless convection heat transfer coefficient,heat transfer coefficient,Characteristic length,Nusselt number,In fluid flow, this phenomenon is known as the no-slip condition. An implication of the no-slip condition is that heat transfer from the solid s

4、urface to the fluid layer adjacent to the surface is by pure conduction, since the fluid layer is motionless, it can be expressed as,thermal conductivity of the fluid,2020/7/11,5,What is the physical significance of the Nusselt number?,The Nusselt number represent the enhancement of heat transfer th

5、rough a fluid layer as a result of convection relative to conduction across the same fluid layer.,The larger the Nusselt number, the more effective is the convection.,2020/7/11,6,二、分析方法,b. 近似积分法,d. 比拟法,三、研究内容,1.强制对流,（3）管外流体流动,（2）管内流体流动,（1）流体外掠平板,2.自然对流,3.相变对流,2020/7/11,7,Velocity boundary and bounda

6、ry layer,2020/7/11,8,Velocity boundary and boundary layer,Reynolds number,The value of critical Reynolds number is different for different geometries.,The Reynolds number at which the flow becomes turbulent is called the critical Reynolds number.,Characteristic length,2020/7/11,9, 分类,层流边界层：,紊流边界层：,惯

7、性力粘性力,惯性力 粘性力,Velocity boundary and boundary layer,The critical Reynolds number,critical Reynolds number,2020/7/11,10,Laminer sublayer, 边界层中的质量流量,二、热边界层,Thermal boundary and boundary layer,2020/7/11,11,Thermal boundary and boundary layer,2020/7/11,12,Thermal boundary and boundary layer,Prandtl numbe

8、r,Is a property of temperature,甘油,2020/7/11,13,三、 与 关系,The Prandtl number is a measure of the relative magnitudes of the diffusivity of momentum (and thus the development of the velocity boundary layer) and the diffusivity of heat (and thus the development of the thermal boundary layer). The Pr is a

9、 fluid property, and thus its value is independent of the type of flow geometry, The Pr changes with temperature, but not pressure.,2020/7/11,15,5.3 流体外掠平板对流换热分析解,一、微分方程组的一般形式,假设：,（1）二维,（2）不可压缩性,（3）稳定,（4）常物性,（5）粘性耗散热忽略不计,（一）质量守恒定律,（连续性方程）,2020/7/11,16,（二）动量守恒定律,2020/7/11,17,（三）能量守恒定律,（四）换热微分方程,2020/

10、7/11,18,二、简化,1. 稳定流动：,2. 强制对流：,4. 层流边界层：,3. 外掠平板：,2020/7/11,19,5. 很小：,6. 无内热源,2020/7/11,20,未知变量,2020/7/11,21,三、定解条件,1. 速度边界层,Velocity Boundary Layer,2020/7/11,22,三、定解条件,2. 热边界层,Thermal Boundary Layer,2020/7/11,23,四、求解结果,1. 局部表面传热系数,2. 平均表面传热系数,层流边界层,2020/7/11,24, 讨论,1. 称为努塞尔数,2. 特征长度 ： 板长,3. 定性温度：,4

11、. 适用范围：层流边界层,意义：大小反映平均对流换热的强弱,25,1) 速度场,2) 流动边界层厚度,2020/7/11,26,27,3) 摩擦系数,由速度分布求出局部粘性切应力,局部摩擦系数,平均摩擦系数,4)流动边界层与热边界层之比,2020/7/11,28,复习,边界层,15/10/2012,Thermofluids-II- L3 Y.Y. Yan,29,A simple question,1.,2.,2020/7/11,30,5.4 流体外掠平板的近似积分求解及比拟理论,一、求解,边界条件,（一）流体外掠平板传热层流分析解,2020/7/11,31,二、求,2020/7/11,32,三

12、、求,边界条件,2020/7/11,33,四、求,2020/7/11,34,（二）比拟法（紊流）,动量,热量,The relation between fluid friction and heat transfer. (for turbulent boundary layer ),比较无量纲动量微分方程式和能量微分方程式,35,当Pr=1时，两方程的形式完全相同。 对于同一对流换热现象，二者具有相同的几何条件和物理条件，如果边界条件也相同，如：Y=0:U=0、=0；Y:U1、1，则无量纲速度分布和无量纲温度分布完全相同。 这种流动边界层与热边界层的类似是由于动量传递与热量传递具有完全相同的机理所至，因此描写这两种现象的物理量之间一定存在必然联系。,36,前面温度场的求解结果,引进新的无量纲特征数斯坦顿(Stanton)数：,比较,以上两式称为契尔顿科尔本比拟式 ，建立了摩擦系数与对流换热表面传热系数之间的关系。,The Stanton number,The Chilton-Colburn Analogy,传热因子,j factor,37,如果Pr =1，则,以