Advanced Heat Transfer

1. Advanced Heat Transfer Lecturer: Dr. Du Chinese People’s Armed Police Force Academy

2. 教学计划 • 教学目的 • 掌握传热学的基本概念、基本理论和基本计算方法 • 了解传热学在火灾科学和消防工程中的应用 • 接触数值模拟研究方法 • 提高科技英语阅读和写作能力

3. 教学内容 • 热传导 • 热对流 • 热辐射 • 传热数值模拟初步

4. 教学方法 • 课堂讲授与讨论（30学时） • 数值模拟实践（10学时）

5. 传热学在火灾科学和消防工程中的应用 主要内容： 一、传热学概述 二、传热学在火灾科学和消防工程中的应用 三、火灾科学对传热学的期待

6. 火焰 可燃混气 D 预混火焰在管道内的传播 一、传热学概述 ● 研究热量传递规律的科学 热传导（导热） 热对流 热辐射 ● 热量传递的三种模式：

7. . = D - k E / RT Q H VK C e c C n A 二、传热学在火灾科学和消防工程中的应用 1. 谢苗诺夫热自燃理论 假设物体内部温度均匀分布，表面存在对流换热

8. 2. 弗兰克—卡门涅茨基热自燃理论 假设边界温度为环境温度，内部存在热传导 边界条件：T=T0 无线大平板： 无限长圆柱体： 球体： 立方体：

9. 3. 托马斯热自燃理论 假设内部热传导，边界对流换热 边界条件：

10. 4. 建筑钢构件保护层厚度的计算 在规定的耐火时间内，建筑钢构件的温度不得超过其设计的临界温度。根据火灾的温度上升曲线、临界温度和耐火时间，通过传热计算，可以得到保护层厚度。

11. 5. 建筑物之间的火焰传播 一座大楼着火，相邻的另一座大楼外表面接受的辐射热通量不得超过1.2×104W/m2。建筑物之间的允许间距就是以此为依据来计算的。

12. 6. 感温报警系统的性能化设计与分析 探测元件温度变化速率： 总传热速率： 目标： （1）性能化设计：对要求的火灾规模或探测时间，确定探测器的间距。 （2）性能化分析：对给定的探测器间距，确定探测时间和对应的火灾规模。 对流传热速率：

13. 三、火灾科学对传热学的期待 ——室内火灾发展过程中的传热学问题 问题4：烟的流动 问题1：材料热分解 问题5：烟气热辐射 问题2：多孔介质传热 问题3：烟的产生

14. 1. 多孔介质传热问题 受到多孔介质的结构，质量传递等因素的影响，而多孔介质的结构参数往往是未知的。

15. 2. 热烟气辐射传热问题 受到辐射气体和颗粒的浓度、温度和厚度等参数的影响。除开辐射理论本身的缺陷外，在实际火灾中，这些参数又受多种因素影响，很难准确掌握。

16. 传热学是火灾科学知识体系的重要组成部分。传热学的应用推动了火灾科学的发展，而火灾科学的发展又将不断向传热学的发展提出新的更高的要求。为了自身和学科的发展，让我们一起学习传热学的基本概念、基本理论和基本方法，以审视的眼光去发现火灾科学中的传热学问题，以期待的心情去注视传热学的最新进展，以探索的精神去尝试用传热学的知识推动火灾科学的发展。传热学是火灾科学知识体系的重要组成部分。传热学的应用推动了火灾科学的发展，而火灾科学的发展又将不断向传热学的发展提出新的更高的要求。为了自身和学科的发展，让我们一起学习传热学的基本概念、基本理论和基本方法，以审视的眼光去发现火灾科学中的传热学问题，以期待的心情去注视传热学的最新进展，以探索的精神去尝试用传热学的知识推动火灾科学的发展。

17. Chapter 1 Introduction Heat transfer is the science that seeks to predict the energy transfer that may take place between material bodies as a result of a temperature difference. Two aspects related to heat transfer: 1. Mechanism of thermal energy transfer 2. Thermal energy transfer rate

18. The relationship between heat transfer and thermodynamics Thermodynamics:deals with systems in equilibrium. • Two aspects: • To predict the amount of energy required to change a system from one equilibrium state to another. • First law: • 2. To predict the direction of change of a system. • Second law: energy is transferred from high temperature region to lower temperature region; if the form of thermal energy does not change, the amount of thermal energy remains constant

19. Iron Bar，M1 300oC Water，M2 20oC The Difference between Heat Transfer And Thermodynamics Thermodynamics: tm , Q Heat Transfer：

20. 1-1 Conduction Heat Transfer Definition: Thermal energy transfers between material bodies by the movement of molecules, atoms and free electrons Conduction heat transfer can take place in solids, liquids and gases

21. Characteristics of heat conduction 1. Temperature difference. 2. In one material or between contacting materials. 3. By the movement of molecules, atoms and free electrons. 4. Under earth gravity, the pure heat conduction takes place only in solids.

22. Basic law of heat conduction --Fourier’s Law Degree Celsius ：Heat flux through area A,[W] q：heat transfer rate per unit area, A：the area perpendicular to the heat flow. Thickness of the plane [m]； Thermal conductivity temperature difference between two walls

23. Governing equations One-dimensional heat conduction equation: Energy conducted in left face+heat generated within element=change in internal energy+energy conducted out right face Energy in left face Energy generated within element Change in internal energy Energy conducted out right face Combining the above relations gives: or

24. is called thermal diffusivity. The larger the value of , the faster heat will diffuse through the material. For constant thermal conductivity, the above equation is written

25. 3-dimensional heat conduction equations Cartesian coordinates: （正交坐标系） Cylindrical coordinates: （柱坐标系） Spherical coordinates:（球坐标系）

26. Reduced forms of heat conduction equations： Steady-state one-dimensional heat flow without heat sources Steady-state one-dimensional heat flow in cylindrical coordinates without heat sources Steady-state one-dimensional heat flow with heat sources Two-dimensional steady-state heat conduction without heat sources

27. Thermal Conductivity Thermal conductivity equals to the heat conduction rate through unit area per unit time when the local temperature gradient is unity. It is one of material properties, indicates how fast heat will flow in a given material, and can be determined experimentally or theoretically.

28. Physical mechanism of heat-conduction Conduction may be viewed as the transfer of energy from the more energetic to the less energetic particles of a substance due to interactions between the particles The mechanism in gases: The temperature at any point can be associated with the energy of gas molecules in proximity of the point. The energy is related to the random transitional motion, as well as the rotational and vibrational motions, of the molecules.

29. The dependence of thermal conductivities of gases on temperature The faster the molecules move, the faster they will transport energy. Therefore the thermal conductivity of a gas should be dependent on temperature. The thermal conductivity of a gas varies with the square root of the absolute temperature. For most gases at moderate pressures the thermal conductivity is a function of temperature alone.

30. The mechanism of heat conduction in liquids is similar to that in gases, but the molecules are more closely spaced and the interactions between molecules are more stronger and more frequently.

31. Two modes of heat conduction in solids: Lattice vibration (晶格振动): energy transfer may be attributed to atomic activities in the form of lattice vibrations. Free electrons transport（自由电子传输）: large number of free electrons are moving about in the lattice structure, and carry thermal energy from a higher temperature region to a lower temperature region.

32. In a nonconductor, the energy transfer is exclusively via lattice vibration; In a conductor, thermal energy can be transferred by both modes, but the transport by free electrons is much larger than that by lattice vibration transport. Conclusion: good electrical conductors are almost always good heat conductors

33. The relative orders of magnitude of thermal conductivity for solids, liquids and gases.

34. 1-2 Convection Heat Transfer Convection: when bulk movement of fluid occurs in liquid, heat will be transferred from one part of the fluid to another. Convection heat transfer: heat transfer between fluid and solid surface.

35. 1-2 Convection Heat Transfer Two mechanisms of convection heat transfer: (1) energy transfer due to random molecular motion (2) energy transfer by bulk, or macroscopic motion of the fluid. Convection is accompanied by conduction in the fluid.

36. Note that Convection heat transfer is not a basic mode of heat transfer. it consists of convection and conduction Characteristics of convection heat transfer • It is a complex process of heat transfer (2) The fluid must contact with the surface, and there exists relative motion and temperature difference between the fluid and the surface (3) Due to the viscous action, the velocity will be reduced to zero at plate surface. At this point, the heat is transferred from the surface to the fluid by conduction.

37. Newton’s law — heat flux [W]  h —Convection heat transfer coefficient q — heat flow density, A — contacting area between fluid and surface, — temperatures of solid surface and the fluid,

38. Convection heat transfer coefficient Affecting factors: flow velocity, fluid properties, geometry of the surface. Heat transfer rate per unit surface when the temperature difference is 1 ºC

39. Classification of convection heat transfer Forced convection heat transfer: the flow of fluid is caused by external force. Natural convection heat transfer: the movement of the fluid is caused as a result of density gradients near the plate.

40. 1-3 Thermal Radiation Thermal radiation is the energy emitted by matter which is at a finite temperature. The higher the temperature of the matter, the more energy can be emitted. The ability of thermal radiation is dependent on the type of the material and the surface condition. Radiation heat transfer is heat transfer between two bodies due to thermal radiation.

41. CHARACTERISTICS OF THERMAL RADIATION The energy form is changed in the process of radiation heat transfer. thermal energy electro-magnetic radiation thermal energy Two materials need not contact with each other for radiation heat transfer. The energy can be transferred in vacuum. No matter what temperature is, the material is constantly emitting electro-magnetic radiation. Black body: it is an ideal material, which can absorb all radiation falling on its surface

42. Stefan-Boltzmann law — emissive power of black body, — absolute temperature of surface， — Stefan-Boltzmann constant， Real body emissive power is less than that of black body at same surface temperature. — emissivity of grey body，0~1； Emissivity is dependent on the type of material, surface condition and the surface temperature.

43. is an emissivity function is a geometric “view factor”function(几何视角因子函数) Radiant energy flux emitted by a blackbody surface is Net radiant exchange between two surface is

44. 讨论: • 举例说明传热学在消防中的应用 • 传热学要解决的主要问题是什么？ • 传热学与热力学的主要区别是什么？ • 导热、导热机理、导热基本定律 • 导热控制方程的推导 • 导热系数与导温系数（热扩散系数）的物理意义 • 气体的导热机理、导热系数与温度的关系 • 固体的饿导热机理、导体与非导体的导热机理的区别 • 热对流与对流换热、对流换热机理 • 粘性对对流换热的饿影响 • 对流换热的影响因素 • 热辐射与辐射传热