传热学第四版 杨世铭 陶文铨 第一章

1、2022-6-271传传 热热 学学青岛科技大学 热能与动力工程专业2022-6-2722022-6-273教材教材参考书参考书2022-6-274第一章第一章 绪论绪论2022-6-275 2022-6-276q分类：就物体温度与时间的依存关系而言,可以分为稳态过程和非稳态过程2022-6-277物体中各点的温度不随时间而改变的热传递过程例如：各种热力设备在持续稳定运行时的热传递稳态过程2022-6-278非稳态过程物体中各点的温度随时间而改变的热传递过程例如：启动、制动、停机过程中所经历的热传递过程2022-6-279二、二、 传热学和工程热力学的关系传热学和工程热力学的关系2022-6-

2、27102022-6-27112022-6-27121-2 1-2 热量传递的基本方式热量传递的基本方式 2022-6-2713 2022-6-2714 2022-6-2715一一 热传导（导热）热传导（导热）Heat conductionHeat conduction 2022-6-2716u 导热机理导热机理2022-6-2717u 导热基本定律导热基本定律tf1, h1 tw1tw2tf2, h2 :C)(mWW tA2 mWtAq2mW:21wwtttC2m2022-6-2718u 热导率热导率 (导热系数导热系数) (Thermal conductivity) tqtAC)(mW 2

3、022-6-2719导热系数导热系数的定义式 由傅里叶定律的数学表达式给出:测量方法有稳态和非稳态两种导热系数的数值取决于物质的种类和温度等因素习惯上把导热系数小的材料称为保温材料,效能高的保温材料都是蜂窝状多孔性结构的材料.实用计算（工程计算）可以用线性关系来表达2022-6-2720 WttAA u 热流量和导热热阻热流量和导热热阻2022-6-2721二二 热对流热对流 Heat convectionHeat convection WttGcp )(122022-6-27222022-6-2723u 对流换热的基本计算式对流换热的基本计算式() WwfhA tt2() W m wfqht

4、tA2mW2W (mC) 2mwt Cft C2022-6-2724 ()wfhA ttC)(mW2 1 ()htthARu 对流换热系数对流换热系数 1httqhr2022-6-2725hhrthtqRthAt 1 )(1 )(1WChARh 12WCmhrh2022-6-2726三、热辐射三、热辐射（Thermal radiationThermal radiation）2022-6-2727u 斯蒂芬斯蒂芬-玻尔兹曼定律玻尔兹曼定律4 W A T T K-824=5.67 10 W (m K )A2m 4 W A T 2022-6-2728u 辐射换热辐射换热 Radiation heat

5、 transfer2022-6-2729T1T2QA )(4241TTAQ2022-6-2730t2022-6-2731)(111wfttAQ111()fwttQA12()/wwQAtt12()wwttQA 222()wfQAtt222()wfttQAf1f212t - t111tQAAAAk12111k2022-6-2732u 热阻热阻123kttQRRRR RUI/Rt/ 12111kRAkAhAAh2022-6-273312111krk tf2tf1tw1tw21/(A1h1) /(A )1/(A2h2)2211111hAAhAAkRktAkRtk2022-6-27342022-6-27

6、35温差是热量传递的推动力温差是热量传递的推动力There is no energy flow There is no energy flow between two objects at between two objects at the same temperature.the same temperature.2022-6-2736分子、原子和自由电子的运动分子、原子和自由电子的运动2022-6-2737Conduction in gasConduction in gas2022-6-2738Conduction in MetalsConduction in Metals2022-6-

7、2739Conduction in Non-MetalsConduction in Non-Metals 2022-6-2740Most houses have radiators to heat their rooms. Most houses have radiators to heat their rooms. This is a bad name for them - as they give off heat This is a bad name for them - as they give off heat mainly bymainly by convectionconvect

8、ion! ! The ceiling forces this air to circulateThe ceiling forces this air to circulate as shownas shown, , warming the air around it.warming the air around it. Finally, when the air has cooled, itFinally, when the air has cooled, it falls downwardsfalls downwards, , completing the pleting the cycle. The convector heater The convector heater warms the air in contact warms the air in contact with it. This becomes less with it. This becomes less dense, anddense, and rises.rises.2022-6-2741why