chap1-introduction传热传质

Fundamentals of Heat and Mass TransferWhy study heat transfer?How to studyChapter 1 Introduction Thermodynamics(热力学 ) deals with the end states of the process during which interaction (work or heat) occurs and provides no information concerning the nature of the interaction or the time rate at which it occurs. The objective of this text: to extend thermodynamic analysis through study of the modes of heat transfer and through development of relations to calculate heat transfer rates.Chapter 1 IntroductionThe objectives of this chapter: (1)to develop an appreciation for the fundamental concepts and principles that underlie heat transfer process;(2)to illustrate the manner in which a knowledge of heat transfer may be used with the first law of thermodynamics (conservation of energy) to solve problems relevant to technology and society.1.1 What and How? What is heat transfer? (Definition): Heat transfer (or heat) is thermal energy in transit due to a temperature difference.Whenever there exists a temperature difference in a medium or between media, heat transfer must occur.Difference between heat transfer and thermodynamics:Thermodynamics: equilibrium, reversible, end statesH.T: interaction, process, time rateHow is heat transferred? (H.T. Types, 3 modes)We refer to different types of heat transfer processes as modes.Conduction(导热 ) Convection(对流 ) Radiation (热辐射 )Conduction When a temperature gradient exists in a stationary medium, which may be a solid or a fluid, we use the term conduction to refer to the heat transfer that will occur across the medium. Convection In contrast, the term convection refers to heat transfer that will occur between a surface and a moving fluid when they are at different temperatures.Thermal radiation All surfaces of finite temperature emit energy in the form of electromagnetic waves. Hence, in the absence of an intervening medium, there is net heat transfer by radiation between two surfaces at different temperatures.1.2 Physical Origins and Rate Equationsn1.2.1 Conduction:Atomic in a conductor it is also due to the translational motion of free electrons. Rate equation (Fouriers law)For 1d plane wall shown in Fig1.3, rate equation is:heat flux, per unit area(热流密度 ,面积热流量 )heat rate, all areathermal conductivity, characteristic of the material temperature gradient For steady-condition n1.2.2 ConvectionThe convection mode is comprised of two mechanisms.Random molecular motion (diffusion)Bulk (macroscopic) motion of fluid Advection (对流 ):transport due to bulk fluid motionConvection: heat transfer is due to a superposition of energy transport by the random motion of the molecules and by the bulk motion of the fluid. Boundary layer (Fig.1.4)Hydrodynamic (velocity) boundary layerThermal boundary layerClass (according the nature of the flow)Forced convection: The flow is caused by external means, such as by a fan, or a pump. Fig 1.5aFree (nature) convection: The flow is induced by buoyancy forces, which arise from density differences caused by temperature variations in the fluid. Fig1.5 bMixed(combined) forced and nature convectionSensible (internal thermal) heat exchangeLatent heat exchange (phase change,boiling fig 1.5c, and condensation fig1.5d) Figure 1.5 Convection heat transfer processes.(a) Forced convection (b) Natural Convection(C) Boiling (d) CondensationRate equationNewtons law of cooling:convection heat flux the surface and fluid temperatures, respectivelyconvection heat transfer coefficientIt depends on conditions in the boundary layer, which are influenced by surface geometry, the nature of the fluid motion, and the assortment of fluid thermodynamic and transport properties.n1.2.3 Radiation (thermal radiation)Def: thermal radiation is energy emitted by matter that is at a finite temperature. The energy of the radiation is transported by electromagnetic waves(or photons), not need a material mediumConduction and convection need medium. Emissive power E,(W/m2)Stefan-Boltzmann lawthe absolute temperature (K) of the surfacethe Stefan-Boltzmann constantblackbody(ideal radiator)Real surface: 发射率 Emissivity, radiative property of the surfaceIrradiation G: radiation be incident on a unit area of the surfaceThe rate at which radiant energy is absorbed per unit surface area Gabs is: absorptivity(吸收率 ), radiative property , depend on the nature of irradiation and the surface.Other concepts: opaque, semitransparent; reflectivity , transmissivity .A special case that occurs frequently involves radiation exchange between a small surface at Ts and a much larger, isothermal surface that completely surrounds the smaller one. For the case of the fig 1.6(b), the net rate of radiation heat transfer from the surface (gray surface ) (per unit area) isRewrite:where The total rate of heat transfer from the surface (convection and radiation) is (1.10)1.3 the conservation of energy requirementThe first law (the law of conservation of energy)n1.3.1 Conservation of energy for a control volumecontrol volume (CV) 控制体 control surface (CS)Time basis: energy rates W (joules per second, at each and every instant of time t)the amount of all energy changes J (over any time interval t)The first law can be stated as :At an instant (t): The rate at which thermal and mechanical energy enters a control volume, plus the rate at which thermal energy is generated within the control volume, minus the rate at which thermal and mechanical energy leaves the control volume must equal