这门课学什么 ？.

Presentation on theme: "这门课学什么 ？."— Presentation transcript:

1 这门课学什么 ？

2 Reference Books 《传热学要点与解题》,王秋旺、曾敏编著,西安交通大学出版社
《传热学》（英文版，原书第10版），J.P.Holman著，机械工业出版社 《数值传热学》(第二版),陶文铨编著,西安交通大学出版社

3 以中文版的书为主，英文版辅助。 听课很重要，书上有的内容是不讲的，有的内容会补充。 公众号推送的每章重点难点要关注。 多练习。

4 传热学课程考核方法与学习要求 考核方法: 平时考勤占5%； 课堂提问占5%； 平时作业占15%； 实验成绩占5%； 期末考试成绩占70%。
学习要求：开放式的课堂； 按时、独立、认真完成作业； 缺课或不交作业次数超1/3，取消期末考资格。

5 辅助教学手段 微信学习交流群 微信公众平台 课程网站互动（这学期准备建设）

6 Contents Introduction Conduction Convection Radiation Heat Exchangers
steady-state conduction Conduction 稳态导热 unsteady-state conduction 导热 非稳态导热 without phase change Convection condensation 对流 with phase change 凝结 boiling Radiation 沸腾 辐射 Heat Exchangers 换热器

7 Chapter Introduction

8 重要概念… Heat Transfer (1) is the science that seeks to predict the energy transfer that may take place between material bodies as a result of a temperature difference. 传热学是研究由温差引起的热能传递规律的科学。它主要研究热量传递的规律以及控制和优化热量传递过程的方法。 温差！ 温差！ 温差！

9 Heat Transfer (1) is the science that seeks to predict the energy transfer that may take place between material bodies as a result of a temperature difference. (2) driving force: temperature difference. (3) three modes: conduction, convection & radiation.

10 热能动力 汽轮机 锅 炉 发电机 给水泵 凝汽器

11 热能动力

12 制冷、热泵与空调

13 冰箱 evaporator condenser Heat expansion device compressor

14 建筑节能 太阳直射辐射 大气长波辐射 太空散射辐射 对流换热 地面反射辐射 环境长波辐射 地面长波辐射 壁体得热

15 Heat Transfer & Thermodynamics
热力学 Heat Transfer + Thermodynamics Thermal Science = Predict the amount of energy required to change a system from one equilibrium state to another How heat energy may be transferred, heat-transfer rate 平衡态 Thermodynamics: hot steel water Heat transfer:

16 Research & Development
Heat transfer enhancement 传热强化

17 Micro-scale Heat Transfer
微尺度传热 Researches for new energy sources Fuel battery Solar energy Geothermy

18 Characteristics temperature difference exists
direct contact with materials occur in solids, liquids or gases

19 导热不仅仅发生在固体中。。。。。。

20 Characteristics temperature difference exists
direct contact with materials occur in solids, liquids or gases

21 导热不仅仅发生在固体中。。。。。。

22 Heat Conduction Mechanism
重要概念… 热传导 Mechanism 在温差作用下依赖介质微观运动传递能量的过程称为热传导。它的微观机理是电子的运动，晶格的振动等。 The achievement of heat transfer must have the aid of the motion of the microscopic particles.

23 Fourier’s Law of Heat Conduction
傅里叶导热定律 t  x  dx dt Φ = heat-transfer rate , W = thermal conductivity , W/m·℃ = area , m2 =temperature gradient = heat flux , W/m2 热流量 热流密度 导热系数(热导率) 温度梯度

24 Fourier’s Law of Heat Conduction
傅里叶导热定律 t  x  dx dt Φ = heat-transfer rate , W = thermal conductivity , W/m·℃ = area , m2 =temperature gradient = heat flux , W/m2 热流量 热流密度 导热系数(热导率) 温度梯度

25 常见错误： 导热系数的单位，有的书上写的是W/m·℃，有的书上写的是W/m·K，需要做单位的换算。 解释： 热力学温标与摄氏温标之间有换算关系： T=t+273（K），但是在导热系数里出现的是温差，即△T=△t，所以不需要再做单位的换算。即： 1W/m·K=1 W/m·℃。

26 例题1-1 一块厚度δ=50 mm 的平板， 两侧表面分别维持在 试求下列条件下的热流密度。 材料为铜，λ=375 w/(mK ); 材料为钢， λ=36.4 w/(mK ); 材料为铬砖， λ=2.32 w/(mK )； 材料为铬藻土砖， λ=0.242 w/(mK )。

27 相同条件下通过铜板的导热量比通过硅藻土砖的导热量大三个数量级。因而，铜是热的良导体，而硅藻土砖则起到一定隔热作用。
解：由一维稳态导热公式有 铜： 钢： 铬砖： 硅藻土砖： 讨论： 相同条件下通过铜板的导热量比通过硅藻土砖的导热量大三个数量级。因而，铜是热的良导体，而硅藻土砖则起到一定隔热作用。

28 思考题： One face of a copper plate 3 cm thick is maintained at 400℃, and the other face is maintained at 100℃. How much heat is transferred through the plate? 一块3 cm厚的铜板，一个表面维持在400℃，另一个表面维持在100℃，通过这块铜板传递了多少热量？

29 Solution Fourier’s law From Appendix 2: Integrating gives q copper
3cm x Integrating gives

30 传热机理是什么呢？

31 Heat Convection Characteristics Convection Heat Transfer
热对流 Characteristics direct contact, temperature difference, macroscopic motion 宏观运动 only occur in fluid Convection Heat Transfer 对流传热 重要概念…

32 the temperature gradient at the wall depends on the flow field
convection heat transfer process conduction why at the wall 速度 the velocity of the fluid layer at the wall is zero the temperature gradient at the wall depends on the flow field high velocity large temperature gradient

33 Newton’s Law of Cooling
牛顿冷却公式 = convection heat-transfer coefficient , W/m2·℃ = Area , m2 表面传热系数(对流换热系数) = temperature of the wall , ℃ = temperature of the fluid , ℃

34 电影《泰坦尼克号》里为什么Jack冻死了，而Rose没有？

35 Convection heat-transfer coefficient
(1) It is relation to velocity, thermal properties of the fluid, geometric shape and size of the wall, etc. (2) Must be determined experimentally.

36 Example： A electric current is passed through a wire 1mm in diameter and 10cm long. The wire is submerged in liquid water at atmospheric pressure, and the current is increased until the water boils. For this situation h=5000W/m2·℃,and the water temperature will be 100℃. 电流通过一根直径为1mm长度为10cm的电线，把电线浸入水中，一个大气压下加热至水沸腾，在这种情况下h=5000W/m2·℃，水温为100℃。 为了使电线表面温度维持在114℃，需要提供多大的电功率？ How much electric power must be supplied to the wire to maintain the wire surface at 114℃?

37 The heat transfer rate is therefore
Solution q d=1mm L=10cm Newton’s law The heat transfer rate is therefore and this is equal to the electric power which must be applied.

38 重要概念… Thermal Radiation 热辐射

39 Radiation heat transfer
Definition Thermal radiation is that electromagnetic radiation emitted by a body as a result of its temperature. 物体因为热的原因而发出辐射能的现象称为热辐射。 电磁辐射 Characteristics Occurs when T＞0K; Can be transferred through vacuum region; 真空 Energy conversion. 转换 Radiation heat transfer 辐射换热

40 研究辐射换热的非常重要的概念… Black body 黑体 Reflected Absorbed Transmitted

41 Radiation emitted by a blackbody ——Stefan-Boltzmann Law
斯忒藩-玻耳兹曼定律 Stefan-Boltzmann constant Radiation emitted by non-blackbodies ε- emissivity 发射率(黑度)

42 Radiant exchange

43 同样是25℃的房子，为什么夏天可以穿衬衫，而冬天却要穿毛衣？

44 Two infinite black plates at 800℃ and 300℃ exchange heat by radiation.
Example： Two infinite black plates at 800℃ and 300℃ exchange heat by radiation. Calculate the heat transfer per unit area. 两块无限大平板的温度分别为800℃和300℃，它们之间通过辐射交换热量。 计算单位面积的换热量为多少？

45 Solution Stefan-Boltzmann law Heat exchange between the two black plates

46

47

48 Overall heat transfer process
传热过程 重要概念… For steady-state heat transfer 稳态传热 Φ

49 Overall heat-transfer coefficient
传热系数 重要概念… Compared with The overall heat-transfer coefficient is

50 temperature difference
重要概念… Thermal resistance 热阻 compared with Ohm’s law in electric-circuit Rth heat-transfer rate Flow = driving force resistance for heat transfer Heat flow = temperature difference thermal resistance

51 How to calculate the temperature t2 and t3?
x Φ Φ How to calculate the temperature t2 and t3?

52 例题1-4 一台氟里昂冷凝器传热过程:管内水对流传热表面换热系数h1=8700W/(m２·K),管外氟里昂凝结换热表面换热系数h2=1800W/(m2·K),换热管壁厚δ=1.5mm.管子材料导热系数λ=383Ｗ/(ｍ·Ｋ)的铜． 试计算三个环节的热阻及冷凝器的总传热系数。 欲增强传热应从哪个环节入手？

53 一房屋的外墙近似由4英寸厚的砖( ),和1.5英寸厚的石膏灰泥( ) 组成。若想让墙体的失热量(或得热量)减少80%，需要加入多厚的岩棉保温层( )？
思考题： An exterior wall of a house may be approximated by a 4-in layer of common brick ( ) followed by a 1.5-in layer of gypsum plaster ( ). What thickness of loosely packed rock-wool insulation ( ) should be added to reduce the heat loss (or gain) through the wall by 80 percent? 砖 石膏灰泥 岩棉 1in=25.4millimeters

54 gypsum plaster gypsum plaster x rock-wool brick brick x Φ Φ

55 思考题 习题： 1-4，1-27

56 作业 习题： 1-11，1-12，1-21