2022年热能与动力工程专业英语-翻译

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1、1.3 The Characteristics of Fluids 流体的特征constituent ：组成的；tangential ：切向的；restrain ：限制、约束；equilibrium ：平衡，均衡；interface ：相互关系、分界面；molecule ：微小颗粒、分子；continuum ：连续体；vessel：容器；tar：焦油、柏油；pitch ：树脂；imperceptibly ： 发觉不到的， 细微的；restore ：恢复；subside ：下沉、沉淀、减退、衰减；hypothetically ：假设地、假想地；sphere ：球、球体；microvolume ：

2、微元体积；rarest：最稀罕的，虽珍贵的A fluid is a substance which may flow; that is, its constituent particles may continuously change their positions relative to one another. Moreover, it offers no lasting resistance to the displacement, however great, of one layer over another. This means that, if the fluid is at

3、 rest, no shear force (that is a force tangential to the surface on which it acts )can exist in it. A solid, on the other hand, can resist a shear force while at rest; the shear force may cause some displacement of one layer over another, but the material does not continue to move indefinitely. In a

4、 fluid, however, shear forces are possible only while relative movement between layers is actually taking place. A fluid is further distinguished from a solid in that a given amount of it owes its shape at any particular time to that of a vessel containing it, or to forces which in some way restrain

5、 its movement. 流体是可以流动的物质，也就是说，组成流体的质点可以连续的改变它们的相对位置。而且，不管层与层之间的相对位移有多大都不会产生持久的抵抗力。这意味着流体在静止状态下是不会存在剪切力的剪切力是与其作用外表相切的力。另一方面， 固体在静止时却可以抵抗剪切力，其中的剪切力也可以使层与层之间发生相对位移，但是固体材料却不一定会有连续的运动。然而在流体中，只有当层与层之间有相对运动产生时才会有剪切力存在。流体和固体的进一步区别还在于在特定的时刻，确定数量的流体其形状取决于承载它的容器，或者取决于一些限制其运动的力。The distinction between solids a

6、nd fluids is usually clear, but there are some substances not easily classified. Some fluids, for example, do not flow easily: thick tar or pitch may at times appear to behave like a solid. A block of such a substance may be placed on the ground, but, although its flow would take place very slowly,

7、yet over a period of time-perhaps several days-it would spread over the ground by the action of gravity, that is, its constituent particles would change their relative positions. On the other hand, certain solids may be made to flow when a sufficiently large force is applied; these are known as plas

8、tic solids. 固体和流体之间的区别通常是很明显的，但是也有些物质难于归类。比方说有些流体并不易流动，如重油和树脂有时候会表现得像固体一样，像这样的一块物质如果放在地面上，虽然它的流动发生的非常缓慢，要经过很长的一段时间也许要好几天，但是在重力的作用下它仍然会在地面上蔓延开来，也就是说，它的组成质点会改变它们之间的相对位置。另一方面，某些固体当足够大的力作用时也会“流动”，这就是我们所知的“塑性固体”。Even so, the essential difference between solids and fluids remains. Any fluid, no matter how

9、 thick or viscous it is, begins to flow, even if imperceptibly, under the action of the slightest net shear force. Moreover, a fluid continues to flow as long as such a force is applied. A solid, however, no matter how plastic it is, does not flow unless the net shear force on it exceeds a certain v

10、alue. For forces less than this value the layers of the solid move over one another only by a certain amount. The more the layers are displaced from their original relative positions, however, the greater are the forces resisting the displacement. Thus, if a steady force is applied, a state will be

11、reached in which the force resisting the movement of one layer over another balance the force applied and so no further movement of this kind can occur. If the applied force is then removed, the resisting forced will tend to restore the solid body to its original shape.即便如此，固体和流体之间依然有本质的差异。任何流体，无论多“

12、稠”或者粘性多大，在最微小的剪切力作用下都会流动，即便这种流动是极其细微的。而且，只要这种力持续作用，流体就会连续流动。然而对于固体，不管它的可塑性有多强，只有当作用其上的净剪切力超过一定数值后才会流动，而小于该值的力所引起的固体层之间的相对移动是有限的。层偏离原始位置的程度越大，抵抗这种变形的力也就越大。因此，当一恒定力作用时，就会到达这样一个状态：即会产生一个抵抗这种层间相对运动的力以平衡所施加的外力，所以不会产生进一步的运动。如果将所施加的外力移除，抵抗力将会使固体恢复到它的原始形状。In a fluid, however, the forces opposing the movemen

13、t of one layer over another exist only while the movement is taking place, and so static equilibrium between applied force and resistance to shear never occurs. Deformation of the fluid takes place continuously so long as a shear force is applied. But if this applied force is removed the shearing mo

14、vement subsides and, as there are then no forces tending to return the particles of fluid to their original relative positions, the fluid keeps its new shape. 然而，在流体中，只有当层间相对运动发生时才会存在这种阻止运动的力。所以不会存在这种外力和抵抗力之间的静态平衡。只要有剪切力作用，流精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 1 页，共 13 页体便会产生连续变形。但是如果将外力移除

15、，剪切运动便会减退，而且因为这时没有使流体质点回到它们初始位置的力，所以流体将保持它的“新”形状。Fluids may be sub-divided into liquids and gases. A fixed amount of a liquid has a definite volume which varies only slightly with temperature and pressure. If the capacity of the containing vessel is greater than this definite volume, the liquid occu

16、pies only part of the container, and it forms an interface separating if from its own vapour, the atmosphere or any other gas present. 流体可以划分为液体和气体。一定数量的液体其所占据的体积也是一定的，它随温度和压力的变化很小。如果容器的体积大于这个一定的体积，那么液体占据的只是容器的一部分，而且会形成一个分界面将液体与该液体的蒸汽、空气或其它存在的气体分开。On the other hand, a fixed amount of a gas, by itsel

17、f in a container, will always expand until its volume equals that of the container. Only then can it be in equilibrium. In the analysis of the behaviour of fluids the most important difference between liquids and gases is that, whereas under ordinary conditions liquids are so difficult to compress t

18、hat they may for most purposes be regarded as incompressible, gases may be compressed much more readily. Where conditions are such that an amount of gas undergoes a negligible change of volume, its behaviour is similar to that of a liquid and it may then be regarded as incompressible. If, however, t

19、he change in volume is not negligible, the compressibility of the gas must be taken into account in examining its behaviour.而另一方面，容器内一定数量的气体则通常都要膨胀到和容器相等的体积。这样它才会到达平衡状态。在分析流体特性时，液体和气体之间最重要的差异在于：鉴于在通常的条件下液体是难以压缩的，因此常常将液体认为是不可压缩流体，而气体则容易压缩的多。当一定量的气体其体积变化可以被忽略的情况下，气体的表现和液体类似，因此也可以认为是不可压缩的。然而，如果体积的变化不可忽

20、略，那么在分析其行为特征时就必须考虑气体的压缩性。In considering the action of forces on fluids, one can either account for the behavior of each and every molecule of fluid in a given field of flow or simplify the problem by considering the average effects of the molecules in a given volume. In most problems in fluid dynami

21、cs the latter approach is possible, which means that the fluid can be regarded as a continuum-that is, a hypothetically continuous substance. 在考虑作用于流体上的力时，可以用给定流动区域内的流体上每个分子的行为来解释，也可以只考虑给定体积内分子的平均效应而使问题得以简化。在流体动力学的许多问题中后一种方法是可行的，这意味着将流体看作是连续介质即一种假想的连续介质。The justification for treating a fluid as a co

22、ntinuum depends on the physical dimensions of the body immersed in the fluid and on the number of molecules in a given volume. Let us say that we are studying the flow of air past a sphere with a diameter of 1 cm. A continuum is said to prevail if the number of molecules in a volume much smaller tha

23、n the spheres is sufficiently great so that the average effects ( pressure, density, and so on ) within the volume either are constant or change smoothly with time. 是否可以将流体看作是连续介质取决于浸入流体的物体的尺寸和给定体积内的分子数目。比方我们现在研究空气流过一个直径为1 厘米的球体的问题。如果在比球的体积小得多的体积内流体分子数目足够多以至于该体积的平均效应压力、密度等等为常数或者随时间缓慢变化，就认为该流体是连续介质。T

24、he number of molecules in a cubic meter of air at room temperature and sea-level pressure is about 1025. Thus the number of molecules in a volume of 10-19 m3 (about the size of a dust particle, which is very much smaller than the sphere) would be 106 . This number of molecules is so large that the a

25、verage effects within the microvolume are indeed virtually constant. On the other hand, if the 1 cm sphere were at an altitude of 305 km, there would be only one chance in 108 of finding a molecule in the microvolume, and the concept of an average condition would be meaningless. In this case, the co

26、ntinuum assumption would not be valid for fluid flow except in the rarest conditions, such as those encountered in outer space. 在室温、海平面压力下，每平方米空气中的分子数大约为1025 个，因此10-19 m3 体积中的分子数为106 个 10-19 m3 大约是一个灰尘的体积， 这要远远小于上面所说的球体。如此多的分子数目使得在该微元体积内的平均效应实际上为常数。另一方面， 如果 1 厘米的球体被置于海拔305 千米处，那么在该微元体积内发现一个分子的概率只有10

27、8 分之一， 所以平均状态的概念也就没有任何意义。在这种情况下，除了像在外层空间这种空气非常稀薄的情况之外，流体流动的连续性假设都是无效的。1.4 Scope Significance and Trend of Fluid Mechanics Fluid mechanics,as the name indicates,is that branch of applied mechanics which is concerned with the statics and dynamics of liquids and gases.Dynamics,the study of motion of ma

28、tter,may be divided into two part-dynamics of rigid bodies and dynamics of non-rigid bodies. The latter is usually further divided into two general classifications-elasticity(solid elastic body) and fluid mechanics. 流体力学，正如名字所指，是和液体，气体的静态和动态有关的应用力学的分支。动力学，物体运动的研究，可以精选学习资料 - - - - - - - - - 名师归纳总结 -

29、- - - - - -第 2 页，共 13 页分为两部分刚性物体动力学和无刚性物体动力学。后者通常进一步分为两大一般类别。弹性固定弹性物体和流动力学。The subject of fluid mechanics can be subdivided into two broad categories: hydrodynamics and gas dynamics. Hydrodynamics deals primarily with the flow of fluids for which there is virtually no density change,such as liquid f

30、low or the flow of gas at low speeds.Hydraulics,for example, the study of liquid flows in pipes or open channels, falls within this category. The study of fluid forces on bodies immersed in flowing liquids or in low-speed gas flows can also be classified as hydrodynamics. 流体力学学科可以再分为两大广泛的类别：水动力学和气体动

31、力学。水动力学主要处理的是实际没有密度变化的流体流动。例如液体和气体在低速情况下的流动。水动力学，例如在管道和开放式通道中液体流动的研究，属于这一类别。对浸入流动液体和低速流动其他的物体所受流体力的研究也可以分类为水动力学。Gas dynamics,on the other hand, deals with fluids that undergo significant density changes.High-speed gas flowing through a nozzle or over a body ,the flow of chemically reacting gases, or

32、 the movement of a body through the low density air of the upper atmosphere falls within the general category of gas dynamics. 气体动力学，另一方面，处理经历有效密度变化的流体。高速气体流动通过一个喷嘴或者掠过一个物体，发生化学反应气体的流动，物体通过更高大气中低密度空气中的运动属于气体动力学的一般类别。An area of fluid mechanics not classified as either hydrodynamics or gas dynamics is

33、 aerodynamics, which deals with the flow of air past aircraft or rockets,whether it be low-speed incompressible flow or high-speed compressible flow. 流体力学中即不归类水动力学也不属于气体动力学的那一领域称为航空动力学，它处理穿过飞机和火箭的空气流动，是否它是低速不可压缩流动或者是高速可压缩流动。There are, however,two major aspects of fluid mechanics which differ from so

34、lid-body mechanics.The first is the nature and properties of the fluid itself, which are very different from those of a solid. The second is that, instead of dealing with individual bodies or elements of known mass, we are frequently concerned with the behavior of a continuous stream of fluid, witho

35、ut beginning or end. 然而，流体力学不同于固体力学有两个主要方面。第一个是流体的本质和性能，他和那些固体有很大不同。第二个是，我们经常关注流体的一个连续单位的行为，没有首节和末端而不是处理单独物体或是已知固体的组分。Knowledge and understanding of the basic principles and concepts of fluid mechanics are essential in the analysis and design of any system in which a fluid is the working medium. Man

36、y applications of fluid mechanics make it one of the most vital and fundamental of all engineering and applied scientific studies. The flow of fluids in pipe and channels makes fluid mechanics of importance to civil engineers. The study of fluid machinery such as pumps,fans, blowers, compressors,tur

37、bines, heat exchangers,jet and rocket engines, and the like, makes fluid mechanics of importance to mechanical engineers.Lubrication is an area of considerable importance in fluid mechanics.The flow of air over objects,aerodynamics ,is of fundamental interest to aeronautical and space engineers in t

38、he design of aircraft,missiles and rockets.In meteorology,hydrology and oceanography the study of fluids is basic since the atmosphere and the ocean are fluids. And today in modern engineering many new disciplines combine fluid mechanics with classical disciplines. For example,fluid mechanics and el

39、ectromagnetic theory are studied together as magnetogas-dynamics. In new types of energy conversion devices and in the study of stellar and ionospheric phenomena, magnetogasdynamics is vital. 流体力学最基本的原理和概念的知识和理解在以流体作为工作介质的任何系统的分析和设计中是十分重要的。很多流体力学的应用是它成为所有工程和运用科学研究中最重要的基础的一环。流体在管子和隧道中的流动使流体力学对土木工程师很重

40、要。流体机械的研究，例如泵，风扇，鼓风机，压缩机，涡轮机，热交换器，喷气飞机和火箭引擎等等，使流体力学对机械工程师很重要。润滑在流体力学中是相当重要的一个领域。掠过物体的空气流动，航空动力学是空气动力学和空间工程师在飞机，导弹，火箭的设计中的基本爱好。在气象学，水文学，海洋学中，流体的研究是基本因为大气和海洋是流体。在当今现代工程中，许多新学科是结合了流体力学和传统学科。例如，流体力学和电磁理论放在一起研究称为磁性气体动力学。在能量转换装置的新类型和在星球的研究以及电离现象中，磁性气体动力学很重要。精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 3

41、 页，共 13 页On the contrary ,the collapse of the Tacoma Narrows Bridge in U,S.A. is evidence of the possible consequences of neglecting the basic principles of fluid mechanics .On a memorable day in November 1940.,Nature decided to teach us all a lesson.The wind could not even be considered strong on t

42、hat day,but it happened to disturb the great Tacoma Narrows suspension bridge cyclically with a frequency close to the bridges natural frequency of vibration.The entire bridge started to dance. Traffic was stopped;and an astonished public watched the bridge itself to pieces. 相反地， 美国塔科马海峡吊桥的垮塌是无视流体动力

43、学基本原理的可能结果的证明。在 1940 年 9 月，难忘的一天，大自然决定教我们一堂课，在那天风据说不是很强，但是它刚好以一个接近桥自然振动频率的频率周期性的打搅宏伟的塔科马吊桥。整座桥开始摇晃，交通停止，一个震惊的公民看着桥身分裂成碎片。We see that a good familiarity with fluid mechanics is essential to the modern engineer and scientist,and it is probably obvious that fluid mechanics and its applications is a bro

44、ad subject with far-flung fields of specialization.What we should do is to master the basic concepts and principles of fluid mechanics.Once these fundamentals are mastered more advanced books and research literature may be studied to increase ones understanding of more specialized aspects of fluid m

45、echanics. 我们看到，很好的熟悉流体力学对现代工程师和科学家至关重要。并且很显然，流体力学和他的应用是具有专业户的长远领域的广泛学科。我们应该做的是掌握最基本的流体力学的概念和原理。一旦这些基础的东西被掌握，更多先进的书籍和研究文献可以被研究用来增加流体力学更多专业化方面的理解The significance of fluid mechanics becomes apparent when we consider the vital role it plays in our everyday lives.When we turn on our kitchen faucets,we ac

46、tivate flow in a complex hydraulic network of pipes,valves, and pumps.When we flick on a light switch ,we are drawing energy either from a hydroelectric source that operates by the flow of water through turbines or from a thermal power source derived from the flow of steam past turbine blades. 流体力学的

47、意义变得很明显，当我们思考它在我们日常生活中起到的重要作用。当我们打开我们厨房的插口，我们推动在一个复杂水力管网，阀和泵中的流动。当我们弹开一个灯的开关时，我们正在引导自由通过涡轮机的水流动操作的水力源或者来自起源于流过涡轮叶片蒸汽流动的热力源。When we drive our cars, pneumatic tires provide suspension, hydraulic shock absorbers reduce road shocks,gasoline is pumped through tubes and later atomized,and air resistance c

48、reates a drag on the auto as a whole;and when we stop, we are confident in the operation of the hydraulic brakes,Very complex fluid processes are also involved in the manufacture of the paper on which this book is printed, And our very lives depend on a very important fluid mechanic process-the flow

49、 of blood through our veins and arteries. 当我们驾驶我们的车时，气动轮胎提供悬浮，水力振动吸收装置减小路面振动，汽油由泵的作用通过管束，并且分裂，空气阻力从总体来说自动创造了一个拖力。并且当我们停车时，我们对水力刹车的运行有信心。很复杂的流体过程还牵涉到了这本书打印所用纸的制造，并且，我们完整的生命依赖一个很重要的流体力学过程通过我们静脉和动态的血液流动。Some of the most significant environmental problems facing society today involve fluid mechanics,For

50、 example, coastal cities often discharge their wastewater(usually treated) into the sea, near the sea bed, far enough from shore so that the wastes become sufficiently diluted with the ambient sea water to render the resulting mixture harmless. The process involves mixing the wastewater with the amb

51、ient liquid, a complex turbulence phenomenon,The degree of mixing is a function of the characteristics of the wastewater and the ambient liquid (such as density ) as well as the discharge velocity of the wastewater,Also involved in this process are the velocity and pattern of coastal currents.In add

52、ition to the fluid mechanics of such a problem, the contaminants in the mixture may change both chemically and biologically in the process.Thus sophisticated models linking the basic flow model with other aspects of the problem are required to design a satisfactory waste disposal system,Such models

53、are generally developed and used by multidisciplinary teams that may include engineers, mathematicians, chemists,and bioscientists.There is an increasing need for engineers who have the ability and mathematical skills to assist in the generation of, and to use,sophisticated computational models of t

54、his type. Other problems, similar in nature, that involve fluid mechanics include air pollution and underground hazardous waste problems. 今天，一些有价值的直面社会的环境问题涉及流体力学，例如沿海城市经常排放他们的废水通常被处理到大海中，海床附近，离海滨足够远以便于这些废物在周围海水作用下充分稀释来缓解最终混合物的危害。这个过程涉及精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 4 页，共 13 页了废水和周围液

55、体的混合，一个很复杂的紊流现象。混合的程度是周围液体和废水性质的一个指标例如密度还有废水的排放速率，还有涉及到这个过程的是沿海气流的速率和形式。另外，这样的一个问题的流体力学，在这个过程中混合物的成分可能发生化学性和生物性的改变。因此，连着问题其他方面的基本模型的复杂模型被要求设计一个符合要求的废物处理系统，这些模型发展普遍并且被训练过的团队使用，可能包括工程师，数学家，化学家，和生物学家。对那些有能力并且具有帮组构建和使用这种复杂计算模型的数学技巧的工程师的需要也越来越大。其他问题，本质上类似，涉及流体力学包括空气污染和地下危险废物问题。Modern developments in flui

56、d mechanics, as in all fields, involve the use of high-speed computers in the solution of problems. Remarkable progress is being made in this area, and the use of the computer in fluid dynamic design is increasing.In the design of aircraft, computers are used to predict the flow over engine nacelles

57、 and appendages in order to select configurations that minimize aerodynamic drag.The NASA publication on wind tunnels(1) explains the role of computers in aircraft design,Computational solutions for wind forces on buildings and structures are used to complement measurements on wind tunnel models to

58、insure the safety and structural integrity of the full-scale structures. 流体力学的现代发展，和所有领域一样，在问题解决方案中涉及高速电脑的使用。在这个领域取得了巨大进步，并且电脑在流体动力学设计中的使用不断增多。在飞机设计中，电脑被用来预测掠过发动机机舱和附属物的流动为了选择结构来最小化航空动力拉力。NASA 出版物关于风道说明了电脑在飞机设计的作用，对于作用在建筑物和结构的风力计算解决方案被采用来补充完善风道模型的测量来确保安全性和全规模结构的完整性。The ever-increasing speed and memo

59、ry capacity of modern computers are leading to even more exciting applications of computers in fluid mechanics.Computer solutions for the motion of terrestrial winds and weather fronts are leading to more accurate forecasting of local weather conditions.The coupling of fluid mechanics with heat tran

60、sfer and chemical kinetics in computational solutions will lead to improved designs for industrial power and propulsion systems.As space stations and space travel become more feasible, computers will play a vital role in the design of flow systems in microgravity environments that are difficult to e

61、xamine through terrestrial experimentation.The application of computers to the analysis of flows in biological systems is only beginning but it will continue to grow as the mechanics of flows in these systems becomes better understood. 现代电脑的高速和记忆功能正引领流体力学中电脑更加振奋的应用。关于大陆风移动和天气朝向的电脑解决方法正促进更多精确的当地气候条件的

62、预测。电脑化的带有热量转换和化学运动的流体机械的联轴器将促进提升工业动力和推进系统的设计。当宇宙空间站和宇宙旅游具有更多可能性，电脑将会在通过陆地实验很难检测的微重力环境中的流动系统的设计起到重要作用。生物系统中的流动分析的电脑应用只是开始，但他会不断发展，当这些系统中的流体力学规律被更加了解。The science of fluid mechanics is developing at a rapid rate. Armed with more detailed measurements and numerical models, fluid mechanicians have develo

63、ped higher levels of understanding that have led to sophisticated designs and applications of fluid systems. Still,there are many areas in which only rudimentary information and physical models are available. Turbulence is a prime example. Even though we presently have high_speed computers at our di

64、sposal, the solutions are only as valid as the equations we use to describe the basic flow phenomena. And there is currently no general analytic model that completely describes the nature of turbulence.We have good data on turbulence in straight pipes, so reliable empirical formulas have been develo

65、ped to describe the turbulence in such a simple case. But turbulence in high-shear flows,buoyant flows,and compressible flows is still the subject of extensive study.Analyses of the flow of multiphase mixtures such as solids in a liquid (slurries )and bubbles in a liquid still rely heavily on empiri

66、cism.In oil recovery operable liquids,such as oil in water, which is not well understood. These are areas which represent exciting challenges to current and future practitioners of fluid mechanics. 流体力学技术正快速发展。由于储备了更多详细的测量方法和数学模型，流体力学专家们发展了更高的理解水平，这种水平促进了产生流体系统的复杂设计和应用。而且有许多只有基本信息和物理模型可使用的领域。紊流就是最好的

67、一个例子， 即使我们目前有高速电脑可操作，但这种解决方式只与我们用来描述基本流动现象的等式一样有效。并且当前没有一般的分析模型，可以完整描述紊流的本质。我们有关于在直管中紊流的完整数据，因此，可靠的经验公式已经发展来描述在这样一个简单情况下的紊流。但是，在高切变流动，浮升流动和可压缩流动中的紊流仍是扩展研究的学科。多相混合的流动的分析例如固体在液体中和气泡在液体中仍要着重依赖于经验理论。在油恢复操作中，工程师遇到了不容混液体流动的问题例如油在水中，这些还不是很好理解。这些是将带给现在和未来流体力精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 5 页

68、，共 13 页学从业者激动挑战的领域。1.5 The Principles of Fluid Machines 流体机械原理vise versa：反过来也这样；reciprocate ：互换位置、 往复移动；compressor ：压缩机；blower ：鼓风机、风扇；piston ：活塞、柱塞；cylinder ：汽缸、圆筒；rotor ：转子、旋转部；hydro-static ：流体静力学的；decelerate ：减速；clearance ：余隙；churn ：搅拌；functioning ：机能；ensure ：确保、保证；diaphragm ：横膈膜，振动膜；impeller ：叶轮

69、；momentum ：动力、要素、动量；reduction ： 减少；discharge ：卸下、放出、流出；casing：包装、保护性的外套；bulky ：大的，容量大的；bring into play：发挥、发动；positive-displacement：容积式的；rotodynamic ： 旋转式、转动式；diaphragm pump：膜片泵；gear pump ：齿轮泵；close fitting ：紧贴的；exemplified ：例证、例示，以. 作为例子；A fluid machine is a device either for converting the energy h

70、eld by a fluid into mechanical energy or vice versa. The mechanical energy is usually transmitted by a rotating shaft : a machine in which energy from the fluid is converted directly to the mechanical energy of a rotating member is known as a turbine ( from the Latin turbo, a circular motion ); if,

71、however, the initial mechanical movement is a reciprocating one the term engine or motor is used. A machine in which the converse processthe transfer of energy from moving parts to the fluidtakes place is given the general title of pump. When the fluid concerned is a gas other terms may be used. If

72、the primary object is to increase the pressure of the gas, the machine is termed a compressor. On the other hand, a machine primarily used for causing the movement of a gas is known as a fan or blower. In this case the change in static pressure is quite smallusually sufficient only to overcome the r

73、esistance to the motionand so the variation of density is negligible and the fluid may be regarded as incompressible. No attempt will be made here to describe constructional details or the practical operation of any of these machines. Our concern is simply with the basic principles of mechanics of f

74、luid machines which are brought into play. 流体机械是这样的一种装置：它将流体所具有的能量转化为机械能，或者反过来将机械能转化为流体的能量。机械能通常通过旋转的轴来传输：将流体的能量直接转化为旋转部件的机械能的机器称为涡轮该词起源于拉丁文字turbo ，旋转运动的意思 ；然而，如果初始的机械运动是一种往复的运动，则被称为马达或发动机。相反的过程即能量从运动部件传输给流体所发生的机器则通常称为“泵”。当所采用的流体为气体时，则要用到其它的名词。如果主要的目的是为了提升气体的压力，那么这种机器称为压缩机。而另一方面，主要用来使气体运动的机器则称为“鼓风机”

75、或者“送风机”。在这种情况下，静压的变化非常小通常只够克服运动阻力因此密度的变化也可以忽略，流体可以认为是不可压缩的。在这里我们不会对以上任意一种机器进行构造细节和实际操作上的描述。我们所关心的仅仅是这些流体机械运作的机械原理。Although a great variety of fluid machines is to be found, any machine may be placed in one of two categories: the positive-displacement group or the rotodynamic group. The functioning o

76、f a positive-displacement machine derives essentially from changes of the volume occupied by the fluid within the machine. This type is most commonly exemplified by those machines, such as reciprocating pumps and engines, in which a piston moves to and from in a cylinder (a suitable arrangement of v

77、alves ensures that the fluid always moves in the direction appropriate to either a pump or an engine). Also in this category are diaphragm pumps, in which the change of volume is brought about by the deformation of flexible boundary surfaces (an animal heart is an example of this form of pump ), and

78、 gear pumps in which two rotors similar to gear wheels mesh together within a close-fitting housing. Although hydrodynamic effects may be associated with a positive-displacement machine, the operation of the machine itself depends only on mechanical and hydrostatic principles. This is not to say tha

79、t such a machine is easy to design, but since few principles of the mechanics of fluids are involved our consideration of positive-displacement machines in there will be very brief. 虽然流体机械种类繁多，但它们都可以归为以下两类：容积式的或者转动式的。容积式机器的机能本质上源自机器内部流体体积的变化。这种类型的机器最常见的例子有：往复式的泵和发动机。在这些机器中有一活塞在汽缸内往复运动安装有合适的阀门从而确保流体流

80、动的方向总是适合于该泵或发动机。 属于这一类型的机器还有膜片泵，这种机器中体积的变化是由边界面的弹性变形产生的动物的心脏即是这种类型的泵。另外还有齿轮泵，在这种机精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 6 页，共 13 页器中，类似于齿轮的两个转轴在一个紧贴的机架内啮合在一起。虽然在容积式机器中也存在水力效应，但机器自身的运动还是仅仅取决于机械原理和流体静力学原理。这并不是说这种类型的机器容易设计，而是说既然这种机器涉及到的流体力学的原理较少，我们对容积式机器的考虑就较为粗略。Rotodynamic machines, on the oth

81、er hand, do present hydrodynamic problems. All these machines have a rotor, that is, a rotating element through which the fluid passes. In a turbine this rotor is called the runner; for a pump the term impeller is more often used. The fluid has a component of velocity and therefore of momentum in a

82、direction tangential to the rotor, and the rate at which this tangential momentum is changed corresponds to a tangential force on the rotor. In a turbine there is a reduction of the tangential momentum of the fluid in the direction of movement of the rotor; thus energy is transferred from the fluid

83、to the rotor and hence to the output shaft. In a pump, energy from the rotor is used to increase the tangential momentum of the fluid; subsequent deceleration of the fluid produces a rise in pressure. 而另一方面，转动式机器则表达了现在的水力学问题。这种类型的机器都有一个转子，也就是一个旋转部件，流体流经该部件。 在涡轮中这种轴被称为“转子”， 在泵中则通常称为“叶轮”。流体在与轴相切的方向上有速

84、度分量，因此也就具有动量，该切向动量的变化速度与作用于转子上的切向力一致。在汽轮机中，沿着转子运动方向流体的切向动量是减小的，因此能量便由流体传递给了转子，然后再传输给输出轴。在泵中，来自转子的能量被用来增加流体的切向动量，随之流体速度减小从而使流体压力增大。Rotodynamic machines have a number of advantages over the positive-displacement type. The flow from most positive-displacement machines is unsteady whereas, for normal co

85、nditions of operation, that from a rotodynamic machine is essentially steady. Most positive-displacement machines require small clearances between moving and stationary parts, and so are unsuited to handling fluids which may contain solid particles; in general, rotodynamic machines are not restricte

86、d in this way. If discharge from a positive-displacement pump is preventedfor example, by the closing of a valve the pressure within the pump rises and so either the pump stops or some part of the casing bursts; if the discharge valve of a rotodynamic pump is closed, however, the rotating impeller m

87、erely churns the fluid round, and the energy consumed is converted to heat. Moreover, for dealing with a given overall rate of flow a rotodynamic machine is usually less bulky than one of positive-displacement type. 转动式机器要优于容积式机器。大多数容积式机器内的流动都是不稳定的，然而在通常的运行情况下，转动式机器内的流动基本上都是稳定的。大所属容积式机器要求运动和静止部件之间的余

88、隙很小，所以不适用于携带有固体颗粒的流体。通常，转动式机器则没有这方面的限制。如果容器式泵的卸流受限比方关闭阀门那么泵内的压力将会升高，这样或者泵将停止工作，或者壳体的某些零件将会爆裂。然而，如果将转动式泵的卸出阀关闭，旋转的叶轮仅仅只是带动流体旋转，消耗的能量将转变为热能。而且，在流体流动速度相同的情况下，转动式机器所占的体积通常要小于容积式的。2.2 Thermodynamic Systems 热力学系统isolate 使隔离，使独立；additional 额外的， 附加的， 补充的；imaginary 虚构的，想象德；envelope 壳层、外壳、包裹物；cylinder 圆筒、圆柱体、

89、汽缸；boundary 界线、分界、边界；location 地点、位置、场地；solely 单独的，单独的；steam condenser 冷凝器，凝汽器feed-water pump 给水泵；feed-water heater 给水加热器；steam generator 蒸汽发生器 锅炉individual 个别的，单独的，一个人的；In the engineering world, objects normally are not isolated from one another. In most engineering problems many objects enter into

90、a given problem. Some of these objects, all of these objects, or even additional ones may enter into a second problem. The nature of a problem and its solution are dependent on which object are under consideration. Thus, it is necessary to specify which objects are under consideration in a particula

91、r situation. In thermodynamics this is done either by placing an imaginary envelope around the objects under consideration or by using an actual envelope if such exists. The term system refers to everything lying inside the envelope. The envelope, real or imaginary, is referred to as the boundaries

92、of the system. It is essential that the boundaries of the system be specified very carefully. For example, when one is dealing with a gas in a cylinder where the boundaries are located on the outside 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 7 页，共 13 页of the cylinder, the system includes both t

93、he cylinder and its contained gas. On the other hand, when the boundaries are placed at the inner face of the cylinder, the system consists solely of the gas itself. 在工程领域中，对象与对象之间通常并不是相互孤立的。在许多工程问题中某一给定的问题会包含多个对象，而这些对象中的一部分、全部甚至它们的附加的对象往往也会出现在其它的问题中。问题及其解决问题的关键就在于要确定所要研究的对象。因此，有必要指明在特定的场合下所要研究的是哪些对

94、象。在热力学中可以通过给所要研究的对象设置一个假想的将其包围的外壳或者如果实际存在则应用实际存在的外壳来到达这一目的。“系统”这个术语指的是此外壳内的一切物质。而该外壳，无论是实际存在的还是假想的，都可以认为是系统的边界。必须非常谨慎确实定系统的边界，这一点是非常重要的。比方，当我们研究气缸内的气体时，如果将边界取在气缸的外外表上，那么该系统将同时包含气缸以及它内部的气体。然而如果将边界取在气缸的内外表上，那么该系统包含的则仅仅是气体本身。When the boundaries of a system are such that it cannot exchange matter with t

95、he surroundings, the system is said to be a closed system (see Fig. 2-1 a). The system, however, may exchange energy in the form of heat or work with the surroundings. The boundaries of a closed system may be rigid or may expand or contract, but the mass of a closed system cannot change. Hence, the

96、term control mass sometimes is used for this type of system. When the energy crossing the boundaries of a closed system is zero or substantially so, the system may be treated as an isolated system (Fig. 2-1b). 如果系统的边界与环境没有质量的交换，则该系统被称为闭口系统如图2-1a 所示。然而，系统与环境却可以以热和功的形式交换能量。闭口系统的边界可以是刚性的，也可以是可膨胀的或可收缩的。

97、但闭口系统内的质量却不会改变。因此，有时用“控制质量”这个词来描述这种类型的系统。当穿过闭口系统边界的能量为零或足够小时，该系统可以当作孤立系统来对待(如图 2-1b 所示 )。In most engineering problems, matter, generally a fluid, crosses the boundaries of a system in one or more places. Such a system is known as an open system (see Fig. 2-1c). The boundaries of an open system are s

98、o placed that their location does not change with time. Thus, the boundaries enclose a fixed volume, commonly known as the control volume. Sometimes a system may be a closed system at one moment and an open one the next. For example, consider the cylinder of an internal combustion engine with the bo

99、undaries at the inner walls. With the valves closed, the system is a closed one. However, with either or both the valves open, the system becomes an open system. 在许多工程问题中，物质通常为流体会从一个或多个方位穿过系统，这样的系统即为开口系统如图2-1c 所示 。开口系统的边界的选择应该是其位置不会随时间而变。因此， 边界封闭了一固定的体积，所以通常也称之为控制体积。有时候某个系统会在某个时刻为闭口系统而在另一时刻为开口系统。比方对

100、边界取在气缸内壁的内燃机汽缸来说，当阀门关闭时，该系统为闭口系统，然而随着一个或两个阀门的开启，该系统则变为开口系统。Frequently the total system to be considered may be large and complicated. The system may be broken down into component parts and an analysis of the component part made. Then the performance of the entire system can be determined by the summ

101、ation of the performance of the individual component system. For example, consider the liquid-vapor part of a steam power plant as an entire system. This system, which is closed, contains the steam generator, the steam turbine, the steam condenser, the feed-water pumps, and the feed-water heaters. A

102、ny or all of these units may be considered separately by throwing a boundary around them. Since a fluid enters and leaves each of these smaller systems, each one is an open system and must be analyzed as such. 通常我们所研究的完整系统是庞大而复杂的。这样的系统则常被分解为各个组成部分，然后对各个组分进行分别地分析。整个系统的性能则可以通过对各分系统的性能进行加总求和来确定。比方，要研究热

103、电厂的汽- 水系统这一完整系统。该闭口系统包括蒸汽发生器锅炉、汽轮机、凝汽器和给水加热器。该系统内的任何一个或所有单元都可以通过设置相应的边界来进行单独的分析。因为流体在这些小的系统内都有流进和流出，所以这些小的系统都是作为开口系统来分析的。2.3 General Characteristics of Heat Transfer 传热的基本特征momentum 动量、运动量；friction 摩擦力、阻力；lattice 格子，晶格；laminar 成薄层的，薄层状的；diffusion 扩散、散布；nonhomogeneous 不均匀的；精选学习资料 - - - - - - - - - 名师

104、归纳总结 - - - - - - -第 8 页，共 13 页analogy 类比，类推；heating 加热器、air-conditioning 空气调节装置；oscillation 振动、摆动；state 陈述、说明；opaque 不透明的，不传热的；dominate 支配、占优势；eddy 旋转、漩涡；buffer 缓冲器；dominant 占优势的，支配的；electromagnetic 电磁的；fundamental 基本原理；collision 猛烈相撞，抵触意见 ，冲突correlation 相互关联、交互作用、关联式refrigeration equipment制冷装置Heat

105、or thermal energy is transferred from one region to another by three modes; conduction, convection and radiation. Each is important in the design or application of heating, air-conditioning or refrigeration equipment. 热能通过三种模式从一个区域传递到另一个区域，即导热、对流和辐射。在加热器、空调和冰箱的设计和应用中这三者都是非常重要的。Heat transfer is among

106、 the transport phenomena that include mass transfer, momentum transfer or fluid friction and electrical conduction. Transport phenomena have similar rate equations and flux is proportional to a potential difference. In heat transfer by conduction and convection, the potential difference is the tempe

107、rature difference. Heat, mass and momentum transfer, because of their similarities and interrelationship in many common physical processes, receive unified treatment in some textbooks. 传热是众多传输现象中的一例，这些传输现象包括传质、动量传输或流体摩擦传输以及导电等。传输现象都有相同的速度方程，并且流量都正比于某个势差。在对流和导热传热中，该势差为温度差。热量、质量和动量的传输，因为它们在许多共同的物理过程中的

108、相似性和相互联系，在一些教科书中都被统一处理。Thermal conduction is the mechanism of heat transfer whereby energy is transported between parts of a continuum from the transfer of kinetic energy between particles or groups of particles at the atomic level. In gases, conduction is a result of elastic collision of molecules;

109、 in liquids and electrically nonconducting solids, it is believed to be caused by longitudinal oscillations of the lattice structure. Thermal conduction in metals occurs like electrical conduction, through motions of free electrons. The second Law of Thermodynamics states that thermal transfer occur

110、s in the direction of decreasing temperature. In solid opaque bodies, the significant heat transfer mechanism is thermal conduction, since there is no net material flow in the process. With flowing fluids, thermal conduction dominates in the region very close to a solid boundary where the flow is la

111、minar and parallel to the surface, and there is no eddy motion. 导热是由于在原子级别上质点或质点组之间进行动能传输而引起连续介质之间能量传输的一种传热机理。对于气体而言，导热是分子弹性碰撞的结果。对于液体和非导电固体而言，则被证实是由于晶格的纵向振动引起的。金属的导热和导电类似，是通过电子的自由运动产生的。热力学第二定律说明导热沿温度降低的方向进行。对于不透明的固体物质，导热是主要的传热形式，因为在此过程中没有发生物质的净流动。随着流体的流动，在接近固体边界的区域导热占主导地位，因为此处流体呈层流状态且与固体外表相平行，并且没有漩

112、涡运动。Thermal convection may involve energy transfer by eddy mixing and diffusion in addition to conduction. Consider heat transfer to a fluid flowing inside a pipe. If the Reynolds number is sufficiently great, three different flow regions will exist. Immediately adjacent to the wall is a laminar sub

113、layer where heat transfer occurs by thermal conduction; outside the laminar sublayer is a transition region called the buffer layer, where both eddy missing and conduction effects are significant; beyond the buffer layer and extending to the center of the pipe is the turbulent region, where the domi

114、nant mechanism of transfer is eddy mixing. 除了导热之外，对流换热还包括通过漩涡的混合和扩散进行的能量的传输。我们现在来研究流体在管内流动时的换热。如果雷诺数足够大，将会存在三个不同的流动区域。紧贴管壁的是一层流底层，此处热量的传输主要通过导热进行。在层流底层之外是一过渡区域称为缓冲层，在此区域内漩涡的混合和导热效应都非常显著。在缓冲层之外向管中心延伸即为紊流区域，此处占主导地位的传热机理是漩涡的混合。In most equipment, the main body of fluid is in turbulent flow, and the lami

115、nar layer exists at the solid walls only. In cases of low velocity flow in small tubes, or with viscous liquids such as oil (i.e. , at low Reynolds numbers), the entire flow may be laminar with no transition or eddy region. 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 9 页，共 13 页在大多数设备中，流体的主体都处于紊流区

116、域，层流底层仅仅存在于固体边壁处。对于小型管道内的低速流动，或者对于像油一样的粘性流体即雷诺数较小的情形，全部的流动则可能都表现为层流，而没有过渡区和涡流区。When fluid currents are produced by sources external to the heat transfer region, for example, a blower or pump, the solid-to-fluid heat transfer is termed forced convection. If the fluid flow is generated internally by n

117、onhomogeneous densities caused by temperature variation, the heat transfer is termed free of natural convection. 如果流体的流动是由传热区域之外的源比方鼓风机或水泵引起的，固体液体之间的换热则被称为强迫对流换热。如果流体流动是由内部温度变化造成密度不均而引起的，这种换热则被称为自然对流换热。In conduction and convection, heat transfer takes place through matter. For radiant heat transfer,

118、 there is a change in energy form; from internal energy at the source to electromagnetic energy for transmission, then back to internal energy at the receiver. Whereas conduction and convection are affected primarily by temperature difference and somewhat by temperature level, the heat transferred b

119、y radiation increase rapidly as the temperature increases. 在导热和对流换热中，热量的传输要通过物质才能发生。对于辐射换热则在能量形式上有所改变。在热源处为内能，传输过程中为电磁能，在接受处又重新变为内能。尽管导热和对流换热主要受温差的影响，只稍微受温度水平的影响，然而，辐射换热则随温度的升高而迅速增加。Although some generalized heat transfer equations have been mathematically derived from fundamentals, usually they are

120、 obtained from correlations of experimental data. Normally, the correlations employ certain dimensionless numbers from analyses such as dimensional analysis or analogy. 尽管已通过应用基本原理用数学的方法得到了一些通用的传热方程，但是这些方程主要还是从试验数据的关联式中得到的。通常，这些关联式使用的是通过量纲分析和类推得到的一些无量纲参数。2.6 Radiation Every free surface emits energy

121、 in the form of electromagnetic waves; the amount of energy is a function of the surface temperature. This emitted energy is known as radiant thermal energy. The nature of this radiant energy is not completely understood, but laws have been formulated that describe its behavior. It is recognized tha

122、t, as with other forms of radiant energy , radiant heat energy is transmitted in the form of electromagnetic waves. The complete formulation of the laws governing radiant heat energy must consider that this energy is quantized, that is, the energy is transferred in quanta. In contrast with other mod

123、es of heat transfer, no medium is required to transmit radiant energy. In fact some gases, for instance, carbon dioxide and water vapor, absorb some of the radiant energy passing through them. 每一个自由外表都会以电磁波的形式发射能量，能量的量是外表温度的函数。这种发射的能量被称为辐射热能。这种辐射能的性质是不完全理解的，但已经制定了法律，描述其行为。这是公认的，与其他形式的辐射能，辐射热能以电磁波形式传

124、播。控制辐射能的法律的制定必须考虑到这种能量是量子化的，即，能量转移量子。与其他模式相比，传热，无需介质传输辐射能。事实上，例如，二氧化碳和水蒸气，吸收一些辐射能通过它们。For a fixed set of conditions, any free surface emits radiant energy of varying wavelengths. The frequency of vibration (v) of radiant waves is dependent solely on the source of radiation and is independent of the

125、medium through which they pass. The velocity of radiant waves is a function solely of the medium through which they pass. Thus, the wavelength is a function of both the source and the medium. 对于一个固定的条件，任何自由外表发射的辐射能的不同波长。辐射波的振动频率是完全依赖于辐射源，是独立的介质，通过它们传递。辐射波的速度是一个函数的介质，通过它们传递。因此，波长是源和介质的函数。All free sur

126、faces receive radiant energy from all other surfaces that they can “see, ” that is , surfaces in direct line of sight. Most problems in radiant deal with the net radiant energy exchanged between a given surface and those that surround it. In common parlance, the term ”heat exchanged by radiation” is

127、 used. It must be emphasized, however, that radiation is not heat. Heat is conducted to a surface. By virtue of the temperature of a surface, electromagnetic 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 10 页，共 13 页waves transmit energy from the surface. When these strike another surface, part of t

128、he energy will be absorbed, tending to increase the temperature of the surface struck by them, and part will be reflected. When the object is transparent, or partially so, to radiant waves, some of all of the radiant energy received by the surface will pass into the object . The transparency of an o

129、bject to radiant energy is a function of the wavelength of the radiant waves. 所有的自由外表都能接收到他们能“看到”的所有其他外表的辐射能，也就是说，中间没有阻碍的外表。辐射中的大多数问题是在给定的外表和周围的空间之间交换净辐射能。在共同的说法，“辐射热交换”的使用。但是，必须强调的是，辐射是不热的。热传导到外表。借助于外表的温度，电磁波从外表发射能量。当这些撞击到另一个外表时，部分能量会被吸收，倾向于增加外表的温度，并且部分将被反射。当对象是透明的，或部分如此，辐射波，部分的辐射能接收到的外表，将传递到对象。一个物

130、体的透明度是辐射波的波长的函数。these statements relating to the radiant energy received by a surface may be put in equation from as follows:这些与外表接收到的辐射能有关的陈述可以从如下的方程中被放置：Where a=absorptivity, or the portion of the radiant energy that is absorbed; P=reflectivity , or the portion of the radiant energy that is reflec

131、ted; T=transmissivity, or the portion of the radiant energy that is transmitted. =吸收，或是吸收的辐射能量的一部分；=反射率，或反射的辐射能的一部分；= 透射率，或是发射的辐射能量的一部分。A black surface has an absorptivity close to unity. For this reason the term blackbody has been used to designate an imaginary object whose surface has an absorptiv

132、ity of unity. Since no known surface completely absorbs radiant energy ,the term blackbody refers to an ideal surface. kirchhoff conceived a method of completely absorbing radiant energy. Assume that a hollow sphere contains a very small opening , an is indicated in fig2.4 Radiation entering this op

133、ening will be received by the back wall of the sphere. Here it will be partially absorbed and partially reflected to other parts of the walls of the sphere. The reflected waves are , in turn ,partially reflected , so that each reflected portion is a progressively smaller portion of the energy enteri

134、ng the sphere until ultimately all of it is absorbed. Strictly speaking , some of the reflected radiant energy will pass out through the hole. However ,the surface area of the sphere is .Hence , when the diameter of the sphere is chosen to be 50 times that of the opening, the inside surface area is

135、10000 times that of the opening, and it may be assumed that the hollow sphere absorbs all of the radiant energy. 黑色外表的吸收率接近于1。为此，该词已被用来指定黑体的一个虚构的物体外表有一个统一的吸收率。因为没有已知的外表完全吸收辐射能，术语黑体是一种理想的外表。基尔霍夫设想了一种完全吸收辐射能的方法。假定一个空心球体包含一个非常小的开放，一个是fig2.4 辐射进入这个开放将由球体的后墙送达。在这里，它将部分地被吸收，部分地反映到球体的其他部分。反射波，反过来，部分反射，因此，每

136、个反射部分是一个逐渐变小的部分的能量进入球体， 直到最终所有的它被吸收。严格地说， 一些反射的辐射能通过这个孔。然而， 球体的外表积是，因此，当球体的直径被选择为50 倍，开口，内部的外表积是开放的10000 倍，并且它可以假定，中空球体吸收所有的辐射能。The amount of radiant energy emitted by a surface is a function of the nature of the surface and its temperature. The term blackbody is also used to denote a surface that e

137、mits the maximum conceivable amount of radiant energy at any given temperature. There is no actual surface that is a perfect emitter, but the hollow-sphere concept may be used to establish a standard. The process of emission from the inner surface of the sphere is the reverse of that of absorption.

138、由外表发射的辐射能的量是外表的性质和温度的函数。该术语也用于表示黑体外表放射出最大数量的辐射能可能在任何给定的温度。没有实际的外表，这是一个完美的发射器，但空心球概念可以用来建立一个标准。从球体内外表发射的过程是吸收的反向。The total radiant energy emitted in a unit time by a unit area is known as the total emissive power and is designated by E. since radiant energy is emitted over a range of wavelengths, Whe

139、re E is the monochromatic emissive power. It is assumed that E in Eq2.8 is a continuous function of . 总辐射能量在单位时间内通过单位面积称为总发射功率是由E.由于辐射能量发射的波长范围内，其中 E是单色辐射功率。它是假定在eq2.8 是一个连续函数。3.1Heat of combustion 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 11 页，共 13 页In a boiler furnace (where no mechanical wor

140、k is done) the heat energy evolved from the union of combustible elements with oxygen depends on the ultimate products of combustion and not on any intermediate combinations that may occur in reaching the final result. A simple demonstration of this law is the union of 1 lb of carbon with oxygen to

141、produce a specific amount of heat. The union may be in one step to form the gaseous product of combustions, CO2, or under certain conditions the union may be in two steps, first to form CO, producing a much smaller amount of heat and, second the union of the CO so obtained to form CO2,releasing 9735

142、 Btu. However, the sum of the heats released in the two steps equals the 14100 Btu evolved when carbon is burned in one step to form CO2 as the final product. That carbon may enter into these two combinations with oxygen (and it) is of utmost importance in the design of combustion equipment. Firing

143、methods must assure complete mixture of fuel and oxygen to be certain that all of the carbon burns to CO2 and not to CO. Failure to meet this requirement will result in appreciable losses in combustion efficiency and in the amount of heat released by the fuel, since only about 28% of the available h

144、eat in the carbon is released if CO is formed instead of CO2. Measurement of heat of combustion In boiler practice the heat of combustion of a fuel is the amount of heat, expressed in Btu, generated by the complete combustion, or oxidation, of a unit weight (1 lb in the United States) of fuel. Calor

145、ific value or “fuel Btu value” are terms also used. The amount of heat generated by complete combustion is a constant for any given combination of combustible elements and the compounds and is not affected by the manner in which the combustion takes place, provided it is complete. The heat of combus

146、tion of a fuel is usually determined by direct measurement in a calorimeter of the heat evolved during combustion. Combustion products within a calorimeter are cooled to the initial temperature and the heat absorbed by the cooling medium is measured to determine the higher or gross heat of the combu

147、stion. For solid fuels and most liquid fuels, calorimeter of the “bomb ” type, in which combustible substance are burned in a constant volume of oxygen, give the most satisfactory results. With bomb calorimeters properly operated, combustion is complete, all of the heat generated is absorbed and mea

148、sured, and heat from external sources either can be excluded or proper corrections can be applied. For gaseous fuels, calorimeters of the continuous or constant-Flow type are usually accepted as standard. The principle of operation is the same as for the bomb calorimeter except that the heat content

149、 is determined at constant pressures rather than at constant volume. For most fuels, the difference in the heating values from the constant-pressure and the constant-volume determination is small and is usually neglected. For accurate heat values of solid and liquid fuels calorimeter determinations

150、are required. However, approximate heat values may b e determined for most coals if the ultimate chemical analysis is known. Dulongs formula gives reasonably accurate results (within 2 to 3%) for most coals and is often used as routine check of values determined by calorimeter: Btu/lb=14544C+62028(H

151、2-O2/8)+4050S (3.1) In this formula, the symbols represents the proportionate parts by weight of the constituents of the fuelcarbon, hydrogen, oxygen, and sulfur-as determined by an ultimate analysis; the coefficients represent the approximate heating values of the constituents in Btu per lb. The te

152、rm O2/8 is a correction applied to the hydrogen in the fuel to account for the hydrogen already combined with the oxygen in the form of the moisture. This formula is not generally 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 12 页，共 13 页suitable for calculating the Btu values of gaseous fuels. High

153、 and low heat values Water vapor is one of the products of combustion for all fuels which contain hydrogen. The heat content of a fuel depends on whether this water vapor is allowed to remain in the vapor state or is condensed to liquid. In the bomb calorimeter the products of combustion are cooled

154、to the initial temperature and all of the water vapor formed during combustion is condensed to liquid. This gives the high, or gross, heat content of the fuel with the heat of vaporization included in the reported value. For the low, or net heat of combustion, it is assumed that all products of comb

155、ustion remain in the gaseous states. While the high, or gross, heat of combustion can be accurately determined by established (ASTM) procedures, direct determination of the low heat of combustion is difficult. Therefore, it is usually calculated using the following formula: QL=QH-1040W Where: QL=low

156、 heat of combustion of fuel, Btu/lb QH=high heat of combustion of fuel, Btu/lb W=lb water formed per lb of fuel 1040=factor to reduce high heat of combustion at constant volume to low heat of combustion at constant pressure.In the United States the practice is to use the high heat of combustion in boiler combustion calculations. In Europe the low heat value is used. 精选学习资料 - - - - - - - - - 名师归纳总结 - - - - - - -第 13 页，共 13 页