大学精品课件：Heat Transfer.ppt

1、Bilingual Program Heat Transfer,Instructor: Dr. Tan Sichao,College of Nuclear Science and Technology, Harbin Engineering University,Heat Transfer,Instructor CV,College of Nuclear Science and Technology, Harbin Engineering University,Si-chao Tan, Prof. of CNST, HEU B.S. CPNE, HEU (2001) M.S. CPNE, HE

2、U (2003) Ph.D. CNST, HEU (2006) Post-doctoral at CNST, XJTU (2006-2009) Visiting scholar at Dept. of NE, TAMU, USA (2009-2010) Associate Professor at CNST, HEU (2007-2009) Professor at CNST, HEU (2009-Present) Mobile: 13091441949 Office： 3A205, 1#527 E-mail: tansichao,Heat Transfer,Objectives,Why yo

3、u choose this course What do you expect from this course How we organize the course,College of Nuclear Science and Technology, Harbin Engineering University,Heat Transfer,学习要求,质疑发现问题态度的转变,推理分析问题思维的转变,实证解决问题行为的转变,自学“学”会还是“教”会 学什么-兴趣 怎么学-探究 为何学-乐趣 自治“主”动还是“被”动 治什么-惰性 怎么治-习惯 为何治责任 自学+自治=自由,“培养学生对学术领域问题

4、和现实生活问题的批判思考能力不仅是教育的重要目标，这对在当前复杂多变的世界，培养会思考的公民和有能力的劳动者，进而维护民主社会都意义深远。” 美国国家教育目标报告 “Inquiry, investigation, and discovery are the heart of the enterprise Everyone at a university should be a discoverer, a learner.” Reinventing Undergraduate Education,课程要求,预习课程内容发现问题,掌握重点、理解难点应用数学知识从具体工程问题中抽象科学问题并进行数学

5、描述,课后作业解决问题,课堂提问有效表达,完成实验实验和理论分析的相互反馈,Grading,Homework 5% In-class quiz 5% Group works 10% Exam 80%,College of Nuclear Science and Technology, Harbin Engineering University,Heat Transfer,Heat transfer and Nuclear Engineering,Heat transfer and heat transport Heat transfer and fluid mechanics Heat tra

6、nsfer and reactor thermal hydraulics Heat transfer and reactor physics Heat transfer and nuclear safety,课程学习主要以理论分析为主,Question:,What is heat transfer?, The science of heat transfer seeks not merely to explain how heat energy may be transferred, but also to predict the rate at which the exchange will

7、 take place under certain conditions,The difference between Heat transfer and Thermodynamics!,Thermodynamics deals with systems in equilibrium and measures the energy required to change one equilibrium state to another;,Heat Transfer deals with how fast a change will take place since the system is n

8、ot in equilibrium during the process,Chapter 1 Introduction,1-1 Conduction heat transfer 1-2 Thermal conductivity 1-3 Convection heat transfer 1-4 Radiation heat transfer 1-5 Dimensions and Units 1-6 Summary,structure,Figure 1 sketch showing direction of heat flow,A temperature gradient exists!,Fig.

9、1-1 sketch showing direction of heat flow,An energy transfer from the high-temperature region to the low temperature region.,We say that an energy is transferred by conduction!,1-1 conduction heat transfer,The heat-transfer rate per unit area is proportional to the normal temperature gradient:,Inser

10、t the proportionality constant:,Fouriers law of heat conduction,q: the heat-transfer rate; unit: watts (W) k: thermal conductivity; unit: W/m,:the temperature gradient .,Using Fouriers law, we determine the 1-dimensional governing equation of heat transfer,Energy conducted in the left face+ Heat gen

11、erated within element= Change in internal energy+ Energy conducted out right face,These energy quantities are given as follows:,Energy in the left face=,Energy generated within element=,Change in internal energy=,Energy out right face=,Combining the relations above gives:,The equation above is the 1

12、-dimensional heat-conduction equation, as for the 3-dimensions, the energy balance yields:,the energy quantities are given by:,So, the 3-dimensional heat-conduction equation is:,When the k is const, we get,The quantity,is called thermal diffusivity,1.The larger value of it, the faster heat will diff

13、use through the material; 2.units:,By using standard calculus techniques, the heat-conduction equation could be translated into either cylindrical or spherical coordinates:,Cylindrical coordinates,Spherical coordinates,For some special cases, we get,Steadystate one-dimensional heat flow (no heat gen

14、eration) :,Steadystate 1-dimensional in cylindrical coordinates heat flow (no heat generation):,Steadystate one-dimensional heat flow with heat sources:,2-dimensional steadystate heat conduction without heat sources:,1-2 thermal conductivity,The defining equation for thermal conductivity is the Four

15、iers heat conduction equation :,For different materials, experimental measurements are adopted to determine the thermal conductivity; however, in some cases, some theories are available for predicting the thermal conductivities.,For the gases,mechanisms: the motion of the gas molecules, which collid

16、ing with one another and energy and momentum,Clearly, the thermal conductivity of a gas highly depend upon temperature!,For the liquids,Mechanisms: Qualitatively the same as in gases, but more complex for the closely-fixed liquid molecules,Therefore, the thermal conductivity of a liquid also depend

17、upon temperature!,For the solids,Mechanisms:,Two modes,Lattice vibration,Transport by free electrons,In general, the energy transferred by the free electrons transportation is faster than the lattice vibration, so : good electrical conductors are almost always good heat conductors!,1-3 convection he

18、at transfer,What is the convection heat transfer?!?,This kind of process are called: Convection heat transfer!,We employ an example to explain the mechanisms of the convection heat transfer,The velocity of the fluid layer at the wall is zero, so the heat must be transferred only by conduction at tha

19、t point.,If so, what is the different between the conduction and convection heat transfer,The ANSWER is:,The temperature gradient is dependent upon the rate at which the fluid carries the heat away!,To consider the overall effect of convection, we use Newtons law of cooling,The quantity h is called

20、the convection heat-transfer; (2) h is sometimes called the film conductance; (3) h could be determined analytically for some systems, complex situations must be determined experimentally; (4) The units of h is: W/m2s (5) h also depends upon the viscosity of the fluid,Convection energy balance on a

21、flow channel,The equation of Newtons law of cooling is used to evaluate the convection loss of pipe flow, FOR EXAMPLE:,Figure 1-8 convection in a channel,Using the energy conservations, we get,1-4 radiation heat transfer,As for the conduction and convective heat transfer, a material medium is involv

22、ed. If a medium is not exist, could the heat be transferred?,The ANSWER is yes!,This mode of heat-transfer is radiation,Mechanisms: electromagnetic radiation, which is also called thermal radiation,找副辐射的图片here!,Blackbody radiation,The blackbody emit energy at a rate to the fourth power of the absolu

23、te temperature of the body and directly proportional to its surface area. this kind of radiation is called Blackbody radiation,Here,is the Stefan-Boltzmann const, with a value of,The equation above governs only radiation emitted by a blackbody, when come to the radiation between two surfaces, it bec

24、omes,The equation above is about the black surface, HOWEVER, not all the types of surfaces in the real world could be seen as black surfaces, and they can not radiate as much energy as the black surfaces;,ON the other hand, not all the radiation leaving from one surface will reach the other surface

25、,since electromagnetic radiation travels in straight lines, and some will lose to the surroundings.,Taking into the consideration of the two aspects, we get,Here,:is an emissivity function,:is a geometric ”view factor”,WHEN a heat-transfer surface at temperature T1 completely enclosed by a much larg

26、er surface maintained at T2 .the equation becomes,1-5 dimensions and units,Definitions,Dimension:,A dimension is a physical variable used to specify the behavior or nature of a particular system.,Units:,When we need to determine the dimensions of something, we give the units with which we choose to

27、measure the dimension.,The SI quantities in heat transfer are showed in table 1-4 on the right side,The SI system also specifies standard multiples to be used to conserve space when numerical values are expressed. they are summarized in Table 1-3,1-6 summary,Heat transfer includes 3 different kind modes: conduction, convection and radiation,The mechanisms are showed as follows:,example,As shown in figure 1-9. in this case the heat conducted through the plate is removed from the plate surface by a combination of convection and radiation.,An energy balance would give,Thats all Thank you!,