传热学-绪论.pptVIP

传热学Heat Transfer 北京工业大学环境与能源工程学院 夏国栋 Chapter 1 绪论(Introduction) What is heat transfer? Heat transfer (or heat) is energy in transit due to a temperature difference. Whenever there exists a temperature in a medium or between media, heat transfer must occur. 1.1 Heat transfer modes(传热方式) There are three heat transfer modes. Conduction（热传导） Advection（热对流） Thermal radiation（热辐射） Convection=Advection+Conduction 对流换热 = 热对流 + 热传导 The convection heat transfer mode is comprised of two mechanisms. Heat transfer modes Mechanisms of heat transfer modes Heat conduction 热流量：Heat transfer rate（?，W） 热流密度：Heat flux（q，W/m2） 导热系数：Thermal conductivity（?，W/m·K） 热阻：Thermal resistance(R，K/W) 温度差：Temperature difference（?t， K） 热流量方程：Heat transfer rate equation Boundary layer 受迫对流：Forced convection 自然对流：Free or natural convection 沸腾：Boiling 凝结：Condensation 对流换热系数：Convection heat transfer coefficient(h, W/m2·K) Heat convection 辐射力：Emissive power(E, W/m2) 投射辐射：Irradiation (G, W/m2) 吸收辐射：Absorbed radiation (Gabs, W/m2) 黑体：Blackbody(an ideal radiator) 灰体：Greybody(an ideal radiator) 发射率：Emissivity(?) 吸收率：Absorptivity (?) 辐射换热量：Radiation exchange rate 斯蒂芬-波尔茨曼：Stefan-Boltzman Law 斯蒂芬-波尔茨曼常数：Stefan-Boltzman Constant(?b) Heat radiation 1.2 Heat transfer process(传热过程) 图1为三种太阳能热水器的元件：图a为充满水的金属管；图b为在图a的管外加一玻璃罩，玻璃罩和金属管间有空气；图c为在图a的管外加一玻璃罩，但罩与金属管间抽真空。试分别用框图表示三元件的传热过程，并论述其效率由图a向图c逐步提高的原因。 答案: * Energy transfer by electromagnetic waves Radiation Diffusion of energy due to random molecular motion plus energy transfer due to bulk motion (advection) Convection Diffusion of energy due to random molecular motion Conduction *