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Lesson Five
Heat Transfer
Topics
Words learning1
Introduciton of heat transfer2
Working site pictures33
Text lerning44
heat transfer equipment 传热设备 heat exchanger 热交换器boiler 锅炉 condenser 冷凝器 radiator 散热器 refrigerator 冰箱solar collector 太阳能集热器 heat transfer rate 传热( 速 ) 率temperature difference 温差 factor of safety 安全系数experimental error 实验误差 operating condition 运行条件heat transfer coefficient 传热系数under these adverse conditions 在最不利工况下
three forms of heat transfer: conduction 传导 convection 对流 radiation 辐射
heat transfer equipment 传热设备 heat exchanger 热交换器boiler 锅炉 condenser 冷凝器 radiator 散热器 refrigerator 冰箱solar collector 太阳能集热器 heat transfer rate 传热( 速 ) 率temperature difference 温差 factor of safety 安全系数experimental error 实验误差 operating condition 运行条件heat transfer coefficient 传热系数under these adverse conditions 在最不利工况下
three forms of heat transfer: conduction 传导 convection 对流 radiation 辐射
Topic one:Frequently used words and phrases
Heat Sensible Heat Latent Heat
due to molecular thermal motion Solidificationfusion
Liquefactionevaporation
Topic two:Introduciton of heat transfer
Conduction
Convection
Radiation
Three forms of
heat transfer
For steady-state heat conduction in one dimension, the Fourier law is
dx
dtAq
temperature gradientcross-sectional area normal to flow
thermal conductivity
heat flow rate
Equation (1) states that the heat flow rate q in the x direction is directly proportional to the temperature gradient dt/dx and the cross-sectional area A normal to the heat flow. The proportionality factor is the thermal conductivity λ. The minus sign indicates that the heat flow is positive in the direction of decreasing temperature. Conductivity values are sometimes given in other units, but consistent units must be used in Equation (1).
Equation (1) states that the heat flow rate q in the x direction is directly proportional to the temperature gradient dt/dx and the cross-sectional area A normal to the heat flow. The proportionality factor is the thermal conductivity λ. The minus sign indicates that the heat flow is positive in the direction of decreasing temperature. Conductivity values are sometimes given in other units, but consistent units must be used in Equation (1).
(1)
The fundamental equation for unsteady-state conduction in solids or fluids in which there is no substantial motion is
where thermal diffusivity α is the ratio λ/ρcp; λ is thermal conductivity;ρ, density; and cp, specific heat. If α is large (high conductivity, low density and specific heat, or both), heat will diffuse faster.
Is there any
mistakes?
Heat transfer involving motion in a fluid due to the difference in density and the action of gravity is called natural convection or free convection. Heat transfer coefficients associated with gases for natural convection are generally much lower than those for forced convection.
Forced air coolers and heaters, forced air- or water-cooled condensers and evaporators, are examples of equipment that transfer heat primarily by forced convection.
When fluid flows over a flat plate, a boundary layer forms adjacent to the plate. The velocity of the fluid at the plate surface is zero and increases to its maximum free stream value just past the edge of the boundary layer.
Heat transfer involving motion in a fluid due to the difference in density and the action of gravity is called natural convection or free convection. Heat transfer coefficients associated with gases for natural convection are generally much lower than those for forced convection.
Forced air coolers and heaters, forced air- or water-cooled condensers and evaporators, are examples of equipment that transfer heat primarily by forced convection.
When fluid flows over a flat plate, a boundary layer forms adjacent to the plate. The velocity of the fluid at the plate surface is zero and increases to its maximum free stream value just past the edge of the boundary layer.
Radiation, one of the basic mechanisms for energy transfer between different temperature regions, is distinguished from conduction and convection in that it does not depend on an intermediate material as a carrier of energy but rather is impeded by the presence of material between the regions.
The rate of thermal radiant energy emitted by a surface depends on its absolute temperature. A surface is called black if it can absorb all incident radiation. The total energy emitted per unit time per unit area of black surface Wb to the hemispherical region above it is given by the Stefan-Boltzmann law.
where Wb is the total rate of energy emission per unit area, and σ is the Stefan-Boltzmann constant [5.670 × 10−8 W/(m2 ·K4)].
Infiltration
zero energy building
low energy building
Typical Features
• Increased ceiling and wall insulation
• Radiant barrier
• Low air infiltration rate
• High-performance windows
• Engineered HVAC
• Tight duct installation
• All-fluorescent lighting
• Photovoltaic (PV) solar-electric system
Topic three: Woking site pictures
Topic three: Woking site pictures
Topic three: Woking site pictures
1. Introduction
heat transfer problems encountered in practice
(1) rating (2) sizing
two kinds of approach for the study of heat transfer
(1) experimentally (2) analytically
2. Modeling in Heat Transfer
modeling in heat transfer and the problems encountered
a very accurate but complex model
a simple but not-so-accurate model
3. Engineering Software Packages
Neither will engineering software packages replace the traditional engineering courses. And why?
Topic Four: Text learning
1. The heat transfer problems encountered in practice can be considered in two groups: (1) rating and (2) sizing problems.
2. However, this approach is expensive, time-consuming, and often impractical.
3. the size of a heating system of a building must usually be determined before the building is actually built on the basis of the dimensions and specifications given.
encountered in practice: 现实中遇到的rating problem: 速率问题 sizing problem: 容量选择的问题
time-consuming: 耗时的
dimension: 规模尺寸、维 3-D ：？？？
specification: （工程）说明书
4. the results obtained are subject to the accuracy of the assumptions and idealizations made in the analysis.
5. In heat transfer studies, often a good compromise is reached by reducing the choices to just a few by analysis, and then verifying the findings experimentally and doing some fine-tuning.
subject to: 受…支配，取决于…
make a assumption (idealization) in: 在…中作了假设（理想化）
reach a compromise: 达到一种折衷或平衡
fine: 细微地 , 精巧地 tune: 调整
fine-tuning ：？？？
6. In the limiting case of infinitesimal or differential changes in variables, we obtain differential equations that provide precise mathematical formulations for the physical principles and laws by representing the rates of changes as derivatives.
7. The equation itself is very instructive as it shows the degree of dependence of some variables on others, and the relative importance of various terms.
infinitesimal: 无穷小、无穷小的 infinite: 无穷大、无穷大的variable: 变量 constant: ？？differential equation: 微分方程 integral ：积分、积分的calculus: 微积分 derivative: 派生的事物，导数
instructive: 指导性的 various terms: 不同变量
8. Many processes that seem to occur in nature randomly and without any order are, in fact, being governed by some visible or not-so-visible physical laws.
9. This is where the power of analysis lies.
10. An analyst working on an engineering problem often finds himself or herself in a position to make a choice between a very accurate but complex model, and a simple but not-so-accurate model.
occur randomly: 随机发生 without any order: 无序 not-so-visible: 不（那么）可见的 not-so-+adj
lie: 存在 The solution lies in research. ????
make a choice between: 在…之间作出选择（抉择）
11. For example, the process of baking potatoes or roasting a round chunk of beef in an oven can be studied analytically in a simple way by modeling the potato or the roast as a spherical solid ball that has the properties of water.
12. For example, when purchasing a heat exchanger that will handle hard water, we must consider that some calcium deposits will form on the heat transfer surfaces over time, causing fouling and thus a gradual decline in performance.
model…as…: 将…模型化为…，将…视为… spherical: 球形的hemi-: “ 半” hemispherical:??? hemisphere:???
calcium deposits: 钙的沉积物 calcium carbonate: 碳酸钙fouling: 结垢 decline in performance: 性能的降低
13. At the minimum, the model should reflect the essential features of the physical problem it represents.
At the minimum: 至少 essential features: 基本特征
14. There are many significant real-world problems that can be analyzed with a simple model.
real-world problem: 实际问题
Shell and tube heat exchangerPlate heat exchanger
Finned Tube Heat Exchanger
15. But it should always be kept in mind that the results obtained from an analysis are as accurate as the assumptions made in simplifying the problem.
16. Therefore, the solution obtained should not be applied to situations for which the original assumptions do not hold.
hold: 使有效、可行或真实
The theory holds.
This is an observation that still holds true.
be kept in mind: 牢记
17. A solution that is not quite consistent with the observed nature of the problem indicates that the mathematical model used is too crude.
18. Thus any solution to a problem should be interpreted within the context of how that problem is formulated.
19.After all, almost all the heat transfer problems we are likely to encounter in practice can be solved using one of several sophisticated software packages readily available in the market today.
be consistent with: 与…一致 crude: 粗略的，天然的、未加工的crude oil: ?? crude salt:???
software package: 软件包 readily: 容易地
interpret: 解释、说明 context: 上下文、背景formulate: 用公式表示
AIRPAK FLUENT
20.These software packages not only give the desired numerical results, but also supply the outputs in colorful graphical form for impressive presentations.
21.It is unthinkable to practice engineering today without using some of these packages.
impressive: 给人印象深刻的 presentation: 表达、表现
practice: 从事 engineering: 工程（学） the College of Civil Engineering: ??Built Environment and Service Engineering: ????the Chinese Academy of engineering: ???the Chinese Academy of Sciences: ???the Chinese Academy of Social Sciences: ???
22.This tremendous computing power available to us at the touch of a button is both a blessing and a curse.
at the touch of a button: 按一下按键
23. In the hands of poorly educated people, these software packages are as dangerous as sophisticated powerful weapons in the hands of poorly trained soldiers.
poorly educated ：未受过良好教育的 highly educated: 受过良好教育的sophisticated: 尖端的、先进的 poorly trained: 未受过良好培训的
24. If it were true that the engineering students do not need all these fundamental courses they are taking because practically everything can be done by computers quickly and easily, then it would also be true that the employers would no longer need high-salaried engineers since any person who knows how to use a word-processing program can also learn how to use those software packages.
engineering students: 工科学生 science students: 理科学生arts students: 文科学生 high-salaried: 高薪的word-processing program: 文字处理程序
25. But the statistics show that the need for engineers is on the rise, not on the decline, despite the availability of these powerful packages.
26. We should always remember that all the computing power and the engineering software packages available today are just tools, and tools have meaning only in the hands of masters.
statistics: 统计数字 统计学 mathematical statistics: ??? theory of probability:???show: 显示、表明 results show that: 结果表明
how to translate???
27. Having the best word-processing program does no make a person a good writer, but it certainly makes the job of a good writer much easier and makes the writer more productive.
make 注意用法
how to translate???
28. That is, more time will be spent in the classrooms discussing the physical aspects of the problems in greater detail, and less time on the mechanics of solution procedures.
mechanics: 技巧， 机械学、力学
29. Neither will engineering software packages replace the traditional engineering courses.
30. They will simply cause a shift in emphasis in the courses from mathematics to physics.
traditional: 传统的 conventional:???
traditional method traditional model traditional approach
shift: 转变、轮（换）班 , 轮班职工 ; 轮班工作时间
an eight-hour shift: 八小时的工作班
work in three shifts of eight hours: 每班八小时的三班制
the day shift: 日班 the night shift: 夜班
worked the night shift: 值夜班
31. All these marvelous and powerful tool available today put an extra burden on today’s engineers.
32. They must still have a thorough understanding of the fundamentals, develop a “feel” of the physical phenomena, be able to put the data into proper perspective, and make sound engineering judgments, just like their predecessors.
put an burden on: 增加负担
physical phenomenon: 物理现象
perspective: 观点、看法
33. The engineers in the past had to rely on hand calculations, slide rules, and later hand calculators and computers.hand calculations: 手工计算 slide rules: 计算尺 hand calculators: 计算器
34. The easy access to such power and the possibility of a simple misunderstanding or misinterpretation causing great damage make it more important today than ever to have a solid training in the fundamentals of engineering.
35. In this text we make an extra effort to put the emphasis on developing an intuitive and physical understanding of heat transfer phenomena instead of on the mathematical details of solution procedures.
access to: 有权使用 solid: 坚实的
make a effort to do: 努力去做某事put the emphasis on: 将重点放在…，强调…
