传热传质学

Heat Conduction (导热)Chapter 2 Heat conduction equations2-1 Introduction,1 Basic concepts,Chapter 2 Heat Conduction Equations 2-1 1 Basic Concepts,1) Temperature field (温度场),T = f ( x, y, z, t ),change with time ( t ) ?,1 Basic concepts,Chapter 2 Heat Conduction Equations 2-1 1 Basic Concepts,1) Temperature field (温度场),Multi-dimensional (多维问题）,1 Basic concepts,Chapter 2 Heat Conduction Equations 2-1 1 Basic Concepts,1) Temperature field (温度场),Multi-dimensional (多维问题）,Rectangular coordinates (x, y, z ),Cylindrical coordinates (r, , z ),Coordinate System (坐标系),1 Basic concepts,Chapter 2 Heat Conduction Equations 2-1 1 Basic Concepts,1) Temperature field (温度场),1 Basic concepts,1) Temperature field (温度场),T+T,T,T-T,T-2T,T+2T,Chapter 2 Heat Conduction Equations 2-1 1 Basic Concepts,T+T,T,T-T,T-2T,T+2T,.,gradT,2) Temperature gradient (温度梯度),direction: of temperature ascent,Chapter 2 Heat Conduction Equations 2-1 1 Basic Concepts,Total amount of heat transfer (总热量),3) Heat flux (热流密度),Heat transfer rate (热流量),Chapter 2 Heat Conduction Equations 2-1 1 Basic Concepts,heat transfer direction : of temperature descent,Chapter 2 Heat Conduction Equations 2-1 1 Basic Concepts,the second law of thermodynamics (热力学第二定律),2 Fouriers law of heat conduction (傅立叶导热定律),Chapter 2 Heat Conduction Equations 2-1 2 Fouriers law of heat conduction,Jean Baptiste Fourier (1768-1830),French mathematician Originally wanted to become a priest but drawn to mathematics Contemporary of Laplace, Lagrange, Biot & Poisson Served as Precept under Napoleon Worked on: mathematics heat conduction,Yesterday was my 21st birthday, at that age Newton and Pascal had already acquired many claims to immortality.,e.g. Heat conduction through a large plate ( steady, 1-D ),Solution:,?,k,iron (铁) wood (木头),3 Thermal conductivity k (热导率),k,thermophysical parameters (热物性参数),e.g.,Chapter 2 Heat Conduction Equations 2-1 3 Thermal conductivity,A measurement of a materials ability to conduct heat.,Thermal conductivity,Gases : T , k Gases are the least conductive materials.e.g. air ( k = 0.0255 W/mC at 25C)Liquids: T , k ( Water : T , k )Solids: T , k ,Metals are good electrical and thermal conductors.e.g. Cu: 401 W/mC Al: 237 W/mC Crystalline solids are most conductive materials but poor electrical conductors. e.g. diamond ( k =2300 W/mC), semiconductors, siliconPure metals are good thermal conductors but alloys of those metals are usually poor conductors. e.g. Commercial bronze( 90% Cu and 10% Al): k = 52 W/mCSuperconductor: k at T near 0K. e.g. Cu: 20,000 W/mC at 20K (50 times ),Thermal conductivity,Thermal conductivity,Insulator (绝热材料) :,Opaque solid , k 0.12 W/(mk),Heat Conduction is Slowed by Insulation,Which home has more insulation in the attic?,Conductivity of Ice and Snow,Chapter 2 Heat Conduction Equations 2-1 3 Thermal conductivity,Vegetables can survive 0 C temperatures, but not -10 C temperatures.,Chapter 2 Heat Conduction Equations 2-1 3 Thermal conductivity,瑞雪兆丰年！,Before nightfall, the farmer sprayed his crops with water.,1. 孔隙中气相 导热,2. 固体骨架 导热,Chapter 2 Heat Conduction Equations 2-1 3 Thermal conductivity,占 50% 80%,porosity (孔隙率),pore size (孔隙尺寸),: 小,: 高,绝热性能好,Insulator (绝热材料),常 规,?,Nano porous super insulator（纳米孔超级绝热材料）,Chapter 2 Heat Conduction Equations 2-1 3 Thermal conductivity,Key words: porous medium ， insulator ， thermal conductivity ， aerogel （关键词： 多孔介质， 绝热材料， 热导率， 气凝胶 ）,e.g. 气凝胶材料,Experimental Setups for Measuring Conductivity,4 Thermal diffusivity (热扩散率),Chapter 2 Heat Conduction Equations 2-1 4 Thermal diffusivity,Thermal diffusivities of some materials: Table A-3 Table A-12,It represents how fast heat diffuses through a material.,5 Heat generation,Chapter 2 Heat Conduction Equations 2-1 5 Heat Generation,Heat generation is in fact conversion of one form of energy to thermal energy inside a medium.e.g. electrical/nuclear