热能与动力工程专业英语

Chapter 1 Introduction to Thermal

Science

第一章 热科学基础

Acoustic flow meter 声波流量计 Adiabatic [

]绝热的

Aerodynamics 空气动力学 Affiliation 联系 Airfoil 机翼，螺旋桨 Alternative 替代燃料 Anemometer 风速计

Angular speed 角速度 Area density 表面密度 Baffle 挡板 Bifurcation 分形 Blackbody 黑体 Blade 浆叶，叶片 Boiler 锅炉

Boundary layer 边界层 Carnot Cycle 卡诺循环 Cartesian coordinates 笛卡尔坐标系Celsius Degree 摄氏度 Compact heat exchanger 紧凑式换热器

Composition 成分，合成物 Compressed liquid 压缩液体 Compressibility 可压缩性，压缩率 Condensation 凝结 Condenser 冷凝器 Conduction 导热 Control volume 控制体

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Corrugated fin 波状散热片 Cross product 矢量积 Denominator 分母

Developed flow 充分发展流 Diffusion 扩散

Doppler effect 多普勒效应 Double-pipe heat exchanger 套管式换热器

Dry saturated vapor 干饱和蒸汽 Electrode 电极

Electrolyte 电解，电解液 Electrostatic 静电的 Emissivity 发射率 Equilibrium 平衡

Fluid mechanics 流体力学 Forced convection 强制对流 Free convection 自然对流 Friction loss 摩擦损失 Glass ceramic 微晶玻璃，玻璃陶瓷

Heat engine 热机

Heat pump 热泵 Hydrofoil 水翼

Hypersonic speed 高超音速 Infinitesimal 无穷小的 Inflating/deflating 充气/压缩 Internal combustion engine 内燃机 Isentropic 等熵的

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Convection 对流 Coriolis-accelaration flowmeter 科氏加速流量计 Isometric 等容的 Isothermal 等温的

Kinematic viscosity 运动黏度 Laminar 层流

Manuscript 手稿，原稿 Moisture 湿度，水分 Molecule (化学)分子 Molten polymer 熔融聚合物 Muti-disciplinary 多学科的 Newtonian Fluid 牛顿流体 Nominal temperature gradient 法向温度梯度

Numerator （数学）分子 Parallel flow 平行流动，并流 Pathline迹线 Phase change 相变 Plane flow 平面流，二元流 Plate and flame heat exchanger 板式换热器

Polymer solution 胶浆 Proof 校样

Propeller 螺旋桨，推进器 Pump泵 Qulity 干度

Qusi-equilibrium 准平衡、准静态 Radiation 辐射 Rankin Cycle 朗肯循环 Regenerative heat exchanger 蓄热/再生式换热器

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Isobaric 等压的

Isolated system 孤立体系的 Rough-wall tube 粗糙管 Saturation 饱和

Shear stress 剪切力、切应力 Shell-and-tube heat exchanger管壳式换热器

Specific volume 比容 Steady 稳态的，定常的 Stifling engine 斯特林机 Strain rate 变形速度，应变率 Streamline 流线 Strut 支撑，支柱

Subcooled liquid过冷液体

Superheated vapor 过热蒸汽 Surrounding 环境，外界 Thermal conductivity 热传导率 Thermal efficiency 热效率 Thermodynamics 热力学 Torsional 扭力的，扭转的 Trailing edge 机翼后缘、尾缘 Transmitter 传送装置、发送器 Turbine meter 涡轮流量计 Turbulent 湍流的 Ultrosonic 超声波的 Uniform flow 均匀刘 Vacuum 真空 View factor 角系数 Viscous 黏性的

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Reservoir 水库，蓄水池 Reversible 可逆的 Rotameter 转子流量计 Bi Biot number 比澳数 CFD 计算流体力学 CHF 临界热流量 COP 制冷系数 Eu 欧拉数 Fo 富立叶数 Fr 弗劳德数 Gr 格拉晓夫数 KE 动能

LMTD对数平均温差

Cortex shedding 漩涡脱落 Water faucet 水龙头，水嘴

NPSH 汽蚀余量 NTU 传热单元数 Nu 努谢尔特数 PE 势能 Pr 普朗特数 Ra 瑞利数 Re 雷诺数 Sc 施密特数

St 斯坦顿数 ， 斯特劳哈数 We 韦伯数

1.1 Fundamental of Engineering Thermodynamics

1.1 工程热力学基础

Thermodynamics is a science in which the storage, transformation and transfer of energy are studied. Energy is stored as internal energy (associated with temperature), kinetic energy (du to motion), potential energy (due to elevation) and chemical energy (due to chemical composition); it is transformed from one of these forms to another; and it is transferred across a boundary as either heat or work.

热力学是一门研究能量储存、转换及传递的科学。能量以内能（与温度有关）、动能（由物体运动引起）、势能（由高度引起）和化学能（与化学组成相关）的形式储存。不同形式的能量可以相互转化，而且能量在边界上可以以热和功的形式进行传递。

In thermodynamics, we will derive equations that relate the transformations and transfers of energy to properties such as temperature, pressure and density. Substances and their properties, thus, become very important in thermodynamics. Many of our equations will be based on experimental observations that have been organized into mathematical statements or laws, the first and second laws of

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thermodynamics are most widely used.

在热力学中，我们将推导有关能量转化和传递与物性参数，如温度、压强及密度等关系间的方程。因此，在热力学中，物质及其性质变得非常重要。许多热力学方程都是建立在实验观察的基础之上，而且这些实验观察的结果已被整理成数学表达式或定律的形式。其中，热力学第一定律和第二定律应用最为广泛。

1.1.1 Thermodynamic system and control volume 1.1.1 热力系统和控制体

A thermodynamic system is a fixed quantity of matter contained within some enclosure. The surface is usually an obvious one (like that surrounding the gas in the cylinder). However, it may be an imagined boundary (like the deforming boundary of a certain amount of mass as it flows through a pump).

热力系统是一包围在某一封闭边界内的具有固定质量的物质。系统边界通常是比较明显的（如气缸内气体的固定边界）。然而，系统边界也可以是假想的（如一定质量的流体流经泵时不断变形的边界）。

All matter and space external to a system is collectively called its surroundings. Thermodynamics is concerned with the interaction of a system and its surroundings, or one system interacting with another. A system interacts with its surroundings by transferring energy across its boundary. No material crosses the boundary of a system. If the system does not exchange energy with the surroundings, it is an isolated system.

系统之外的所有物质和空间统称外界或环境。热力学主要研究系统与外界或系统与系统之间的相互作用。系统通过在边界上进行能量传递，从而与外界进行相互作用，但在边界上没有质量交换。当系统与外界间没有能量交换时，这样的系统称为孤立系统。

In many cases, an analysis is simplified if attention is focused on a particular volume in space into which, or from which, a substance flows. Such a volume is a control volume. A pump, a turbine, and an inflating

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or deflating balloon are examples of control volume. The surface that completely surrounds the control volume is called a control surface.

在许多情况下，当我们只关心空间中有物质流进或流出的某个特定体积时，分析可以得到简化。这样的特定体积称为控制体。例如泵、透平、充气或放气的气球都是控制体的例子。包含控制体的表面称为控制表面。

Thus, we must choose, in a particular problem, whether a system is to be considered or whether a control volume is more useful. If there is mass flux across a boundary, then a control volume is required; otherwise, a system is identified.

因此，对于具体的问题，我们必须确定是选取系统作为研究对象有利还是选取控制体作为研究对象有利。如果边界上有质量交换，则选取控制体有利；反之，则应选取系统作为研究对象。

1.1.2 Equilibrium, process and cycle

平衡、过程和循环

When the temperature of a system is referred to, it is assumed that all points of the system have the same, or essentially the same temperature. When the properties are constant from point to point and when there is no tendency for change with time, a condition of thermodynamic equilibrium exists. If the temperature, say, is suddenly increased at some part of the system boundary, spontaneous redistribution is assumed to occur until all parts of the

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system are at the same temperature.

对于某一参考系统，假设系统内各点温度完全相同。当物质内部各点的特性参数均相同且不随时间变化时，则称系统处于热力学平衡状态。当系统边界某部分的温度突然上升时，则系统内的温度将自发地重新分布，直至处处相同。

When a system changes from one equilibrium state to another, the path of successive sates through which the system passes is called process. If, in the passing one state to the next, the deviation from equilibrium

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is infinitesimal, a quasi-equilibrium process occurs, and each state in the process may be idealized as an equilibrium state. Quasi-equilibrium processes can approximate many processes, such as the compression and expansion of gases in an internal combustion engine, with no significant loss of accuracy. If the system goes from one equilibrium state to another through a series of non-equilibrium states (as in combustion), a non-equilibrium process occurs.

当系统从一个平衡状态转变为另一个平衡状态时，系统所经历的一系列由中间状态组成的变化历程称为过程。若从一个状态到达另一个状态的过程中，始终无限小地偏离平衡态，则称该过程为准静态过程，可以把其中任一个中间状态看作为平衡状态。准静态过程可近似视为许多过程的叠加结果，而不会显著减小其精确性，例如气体在内燃机内的压缩和膨胀过程。如果系统经历一系列不平衡状态（如燃烧），从一个平衡状态转变为另一个平衡状态，则其过程为非平衡过程。

When a system in a given initial state experiences a series of process and returns to the initial state, the system goes a cycle. At the end of the cycle, the properties of the system have the same values they had at the beginning.

当系统从一个给定的初始状态出发，经历一系列中间过程又回到其初始状态，则称系统经历了一个循环。循环结束时，系统中的各参数又与初始参数相同。

The prefix iso- is attached to the names of any property that remain unchanged in a process. An isothermal process is one in which the temperature is held constant; in an iso-baric process, the pressure remains constant; an isometric process is a constant-volume process.

在任一特性参数名称前加上前缀iso-，表示该参数在整个过程保持不变。等温（isothermal）过程中温度保持不变；等压（isobaric）过程中压强恒定；等容（isometric）过程中体积保持不变。

1.1.3 Vapor-liquid phase equilibrium in pure substance

纯物质的气-液相平衡

Consider as a system 1 kg of water contained in the piston or

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cylinder arrangement shown in Fig.1-1(a). Suppose the piston and weight maintain a pressure of 0.1MPa in the cylinder and that the initial temperature is 20℃. As heat is transferred to the water, the temperature increase appreciably, the specific volume increase slightly, and the pressure remains constant. When the temperature reaches 99.6℃, additional heat transfer results in a change of phase, as indicated in Fig.1-1 (b). That is, some of the liquid becomes vapor, and during this process both the temperature and pressure remain constant, but the specific volume increases considerably. When the last drop of liquid has vaporized, further transfer of heat results in an increase in both temperature and specific volume of the vapor, as shown in Fig.1-1(c).

如图1-1(a)所示，由活塞和气缸组成的装置中装有1kg水。假定活塞和其上的重物使气缸内压强维持在0.1Mpa，初始温度20℃。当有热量开始传递给水时，缸内水温迅速上升，而比容略有增加，气缸内压强保持恒定不变。当水温达到99.6℃时，如若再增加传热量，水将发生相变，如图1-1(b)所示。也就是说，一部分水开始气化变为蒸汽，在此相变过程中，温度和压强始终保持不变，但比容却有大幅度的增加。当最后一滴液体被气化时，进一步的加热将使蒸汽温度和比容均有所增加，如同1-1(c)所示。

图1-1 液体在常压下的蒸发过程

The term saturation temperature designates the temperature at which vaporization takes place at a given pressure. This pressure is called the saturation pressure for the given temperature. Thus, for water at 99.6℃, the saturation pressure is 0.1MPa, and for water at 0.1MPa the saturation temperature is 99.6℃.

在给定压强下发生气化的温度称为饱和温度，压强称为给定温度下的饱和压强。因此，99.6℃水的饱和压强是0.1MPa，0.1MPa水的饱和温度为99.6℃。

If a substance exists as liquid at the saturation temperature, it is called saturated liquid. If the temperature of the liquid is lower than the saturation temperature for the existing pressure, it is called either a subcooled liquid (implying that the temperature is lower

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than the saturation temperature for the given pressure) or a

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compressed liquid (implying that the pressure is greater than the saturation pressure for the given temperature).

如果某一工质为液态并处于其饱和温度和饱和压强下，则称该液体为饱和液体。如果液体温度低于当前压强下的饱和温度，则称该液体为过冷液体（表明液体的当前温度低于给定压强下的饱和温度）或压缩液体（表明液体的当前压强大于给定温度下的饱和压强）。

When a substance exists as part liquid and part vapor at the saturation temperature, its quality is defined as the ratio of the mass of vapor to the total mass. Thus, in Fig.1-1(b), if the mass of vapor is 0.2 kg and the mass of liquid is 0.8 kg, the quality is 0.2 or 20%. Quality has meaning only when the substance is in a saturated state.

若某一工质在饱和温度下以液、气共存的形式存在，则称蒸汽质量与总质量之比为干度。因此，如图1-1(b)所示，若蒸汽质量为0.2kg，液体质量为0.8kg，则其干度为0.2或20%。干度只有在饱和状态下才有意义。

If a substance exists as vapor at the saturation temperature, it is called saturation vapor (Some times the term dry saturation vapor is used to emphasize that the quality is 100%). When the vapor is at a temperature greater than the saturation temperature, it is said to exist as superheated vapor. The pressure and temperature of superheated vapor are independent properties, since the temperature may increase while the pressure remains constant.

若某一工质处于饱和温度下并以蒸汽形态存在，则称该蒸汽为饱和蒸汽（有时称为干饱和蒸汽，意在强调其干度为100%）。当蒸汽温度高于其饱和温度时，则称之为过热蒸汽。过热蒸汽的压强和温度是彼此的，因为温度上升时，压强可能保持不变。

Let us plot on the temperature-value diagram of Fig.1-2 the constant-pressure line that represents the states through which the water passes as it is heated from the initial state of 0.1 MPa and 20℃. Let state A represent the initial state, B the saturated-liquid state(99.6℃), and line AB the process in which the liquid is heated from the initial temperature to the saturation temperature. Point C is the

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saturated-vapor state, and line BC is the constant-temperature process in which the change of phase from liquid to vapor occurs. Line CD represents [process.

在图1-2所示的温度-比容图上作等压线，表示水由初压0.1MPa、初温20℃被加热的过程。点A代表初始状态，点B为饱和液态（99.6℃），线AB表示液体由初始温度被加热至饱和温度所经历的过程。点C表示饱和蒸汽状态，线BC表示等温过程，即液体气化转变为蒸汽的过程。线CD表示在等压条件下蒸汽被加热至过热的过程，在此过程中，温度和比容均增大。

] the process in which the steam is superheated

at constant pressure. Temperature and volume both increase during this

图1-2 温度-比容曲线 表1-1 一些物质的临界参数

In a similar name, a constant pressure of 10 MPa is represented by line IJKL, for which the saturation temperature is 311.1℃. At a pressure of 22.09MPa, represented by line MNO, we find, however, that there is no constant-temperature vaporization process. Instead, point N is a point of inflection with a zero slope. This point is called the critical point. At the critical point the saturated-liquid and saturated-vapor states are identical. The temperature, pressure and specific volume at critical point are called the critical temperature, critical pressure and critical volume. The critical-point data for some substances are given in Table 1-1.

类似地，线IJKL表示压强为10MPa下的等压线，相应的饱和温度为311.1℃。但是，在压强为22.09MPa条件下（线MNO），不存在等温蒸发过程。相反，点N是个转折点，在该点上，切线斜率为零，通常把N点称为临界点。在临界点处，饱和液体和饱和气体的状态都是相同的。临界点下的温度、压强和比容分别称为临界温度、临界压强和临界比容。一些工质的临界点数据如表1-1所示。

1.1.4 The first law of thermodynamics

The first law of the thermodynamics is commonly called the law of conservation of energy.

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In elementary physics courses, the study of conservation of energy emphasizes changes in kinetic and potential energy and their relationship to work. A more general form of conservation of energy includes the effects of heat transfer and internal energy changes. Other forms of energy could also be included, such as electrostatic, magnetic, strain and surface energy.

1.1.4 热力学第一定律

通常把热力学第一定律称为能量守恒定律。在基础物理课程中，能量守恒定律侧重动能、势能的变化以及和功之间的相互关系。更为常见的能量守恒形式还包括传热效应和内能的变化。当然，也包括其它形式的能，如静电能、磁场能、应变能和表面能。

Historically, [

] the first law of thermodynamics was

stated for a cycle: the net heat transfer is equal to the net work done for a system undergoing a cycle.

历史上，用热力学第一定律来描述循环过程：净传热量等于循环过程中对系统所做的净功。

1.1.5 The second law of thermodynamics

The second law of thermodynamics can be stated in a variety of ways. Here we present two: the Clausius statement and the Kelvin-Planck statement.

1.1.5 热力学第二定律

热力学第二定律有多种表述形式。在此列举两种：克劳修斯表述和凯尔文-普朗克表述。

Clausius statement

It is impossible to construct a device that operates in a cycle and whose sole effect is transfer of heat from a cooler body to a hotter body.

克劳修斯表述：制造一台唯一功能是把热量从低温物体传给高温物体的循环设备是不可能的。

图1-3 第二定律的违背

This statement relates to a refrigerator (or a heat pump). It states

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that it is impossible to construct a refrigerator that transfers energy from a cooler body to a hotter body without the input of work; this violation is shown in Fig.1-3(a).

以冰箱（或热泵）为例，不可能制造一台不用输入功就能把热量从低温物体传给高温物体的冰箱，如图1-3(a)所示。

It is impossible to construct a device that operates in a cycle and produces no other effect than the production of work and the transfer of heat from a single body.

凯尔文-普朗克表述：制造一台从单一热源吸热和做功的循环设备是不可能的。

In other words, it is impossible to construct a heat engine that extracts energy from reservoir, does work, and does not transfer heat to a low-temperature reservoir. This rules out any heat engine that is 100 percent efficient, like the one shown in Fig.1-3(b).

换句话说，制造这样一台从某一热源吸热并对外做功，而没有与低温热源进行换热的热机是不可能的。因此，该表述说明了不存在工作效率为100%的热机，如图1-3(b)所示。

1.1.6 The Carnot Cycle

The heat engine that operates most efficiently between a high-temperature reservoir and a low-temperature reservoir is the Carnot engine. This is an ideal engine that uses reversible process to form its cycle of operation; such a cycle is Carnot cycle. The Carnot engine is very useful, since its efficiency establishes the maximum possible efficiency of any real engine. If the efficiency of a real is significantly lower than the efficiency of Carnot engine between the same temperature limits, then additional improvements may be possible. 1.1.6 卡诺循环

卡诺机是低温热源和高温热源间运行效率最高的热机。卡诺机是一个理想热机，利用多个可逆过程组成一循环过程，该循环称为卡诺循环。卡诺机非常有用，因为它的运行效率为任何实际热机最大可能的效率。因此，如果一台实际热机的效率要远低于同样条件下的卡诺机效率，则有可能对该热机进行一些改进以提高其效率。

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图1-4 卡诺循环

The ideal Carnot cycle in Fig.1-4 is composed of four reversible processes: 12: Isothermal expansion: 23；Adiabatic reversible expansion:34；Isothermal compression：41；Adiabatic reversible compression. The efficiency of a Carnot cycle is

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1TL THNote that the efficiency is increased by raising the temperature TH at which heat is added or by decreasing the temperature TL at which heat is rejected.

理想的卡诺循环包括四个可逆过程，如图1-4所示：1→2等温膨胀；2→3绝热可逆膨胀；3→4等温压缩；4→1可逆绝热压缩。卡诺循环的效率为

1TL (1-1) TH注意，提高TH（提高吸热温度）或降低TL（降低放热温度）均可使循环效率提高。

1.1.7 The Rankine cycle

The first class of power cycles that we consider are those utilized by the electric power generating industry, namely, power cycles that operates in such a way that the working fluid changes phases from a liquid to vapor. The simplest vapor-power cycle is called the Rankine cycle, shown schematically in Fig.1-5(a). A major feature of such cycle is that the pump requires very little work to deliver high-pressure water to the boiler. A possible disadvantage is that the expansion process in the turbine usually enters the quality region, resulting in the formation of liquid droplets that may damage the turbine blades. 1.1.7 朗肯循环

我们所关心的第一类动力循环为电力生产工业所采用的，也就是说，动力循环按这样的方式运行：工质发生相变，由液态变为气态。最简单的蒸汽-动力循环是朗肯循环，如图1-5(a)所示。朗肯循环的一个主要特征是泵耗费很少的功就能把高压水送入锅炉。其可能的

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缺点为工质在汽机内膨胀做功后，通常进入湿蒸汽区，形成可能损害汽轮机叶片的液滴。

图1-5 朗肯循环

The Rankine cycle is an idealized cycle in which friction losses in each of the four components are neglected. The losses usually are quite small and will be neglected completely in initial analysis. The Rankine cycle is composed of the four ideal processes shown on the T-s diagram in Fig.1-5(b):12: Isentropic compression in a pump；23: Constant-pressure heat addition in a boiler; 34: Isentropic expansion in a turbine; 41: Constant pressure heat rejection in a condenser.

朗肯循环是一个理想循环，其忽略了四个过程中的摩擦损失。这些损失通常很小，在初始分析时可完全忽略。朗肯循环由四个理想过程组成，其T-s图如图1-5(b)所示：1→2为泵内等熵压缩过程；2→3为炉内定压吸热过程；3→4为汽轮机内等熵膨胀做功过程；4→1为凝汽器内定压放热过程。

The pump is used to increase the pressure of the saturated liquid. Actually, states 1 and 2 are essentially the same, since the high-pressure lines are extremely close to the saturation curve; they are shown separated for illustration only. The boiler (also called a steam generator) and the condenser are heat exchangers that neither require nor produce any work.

泵用于提高饱和液体的压强。事实上，状态1和状态2几乎完全一样，因为由2点开始的较高压强下的吸热过程线非常接近饱和曲线，图中仅为了解释说明的需要分别标出。锅炉（也称蒸汽发生器）和凝汽器均为换热器，它们既不需要功也不产生功。

If we neglect kinetic energy and potential energy changes, the net work output is the area under the T-s diagram, represented by area 1-2-3-4-1 of Fig.1-5(b); this is true since the first law requires that

Wout=Qnet. The heat transfer to the working substance is represented by area a-2-3-b-a. Thus, the thermal efficiency η of the Rankine cycle is

area 12341

area a23baThat is, the desired output divided by the energy input (the purchased

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energy). Obviously,

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the thermal efficiency can be improved by increasing the numerator or by decreasing the nominator. This can be done by increasing the pump outlet pressure p2, increasing the boiler outlet temperature T3, or decreasing the turbine outlet pressure p4.

如果忽略动能和势能的变化，输出的净功等于T-s图曲线下面的面积，即图1-5(b)中1-2-3-4-1所包围的面积，由用热力学第一定律可证明WnetQnet。循环过程中工质的吸热量对应面积a-2-3-b-a。因此，朗肯循环的热效率可表示为

面积12341 (1-2)

面积a23ba即，热效率等于输出能量除以输入能量（所购能量）。显然，通过增大分子或减小分母均可以提高热效率。这可以通过增大泵出口压强p2，提高锅炉出口温度T3，或降低汽机出口压强p4来实现。

1.1.8 The Reheat cycle

It is apparent that when operating in a Rankine cycle with a high boiler pressure or a low condenser pressure it is difficult to prevent liquid droplets from forming in the low-pressure portion of the turbine. Since most metal cannot withstand temperatures above about 600℃, the reheat cycle is often used to prevent liquid-droplet formation: the steam passing through the turbine is reheat at some intermediate pressure, thereby raising the temperature to state 5 in the T-s diagram of Fig.1-6. The steam then passes through the low-pressure section of the turbine and enters the condenser at state 6. This controls or completely eliminates the moisture problem in the turbine. The reheat cycle dose not significantly influences the thermal efficiency of the cycle, but it does result in a significant additional work output, represented in the figure by area 4-5-6-4’-4 of Fig.1-6. The reheat cycle demands a significant investment in additional equipment, and the use of such equipment must be economically justified by the increased work output. If reheat is not used to avoid droplet formation, the condenser pressure must be quite high, resulting relatively low cycle efficiency. In that sense, reheat

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significantly increase cycle efficiency when compared to cycle with no reheat but with the higher condenser pressure.

1.1.8 再热循环

对于一个处于高锅炉压强和低凝汽器压强条件下的朗肯循环，显然，很难阻止液滴在汽轮机低压部分的形成。由于大多数金属不能承受600℃以上的高温，因此，通常采用再热循环来防止液滴的形成。再热过程如下：经过汽轮机的部分蒸汽在某中间压强下被再热，从而提高蒸汽温度，直至达到状态5，如图1-6所示。然后这部分蒸汽进入汽轮机低压缸，而后进入凝汽器（状态6）。再热循环方式可以控制或者完全消除汽轮机中的湿蒸汽问题，因此，通常汽轮机分成高压缸和低压缸两部分。虽然再热循环不会显著影响循环热效率，但带来了显著的额外的输出功，如图1-6中的面积4-5-6-4-4所示。当然，再热循环需要一笔可观的投资来购置额外的设备，这些设备的使用效果必须通过与多增加的输出功进行经济性分析来判定。如果不采用再热循环来避免液滴的形成，则凝汽器出口压强必须相当地高，因而导致循环热效率较低。在这种意义上，与无再热循环且高凝汽器出口压强的循环相比，再热可以显著提高循环效率。

图1-6 再热循环

1.2 Fundamental of Fluid Mechanics

Fluid motions manifest themselves in many different ways. Some can be described very easily, while others require a thorough understanding of physical laws. In engineering applications, it is important to describe the fluid motions as simply as can be justified. This usually depends on the required accuracy. Often, accuracies of ±10% are acceptable, although in some applications higher accuracies have to be achieved. The general equations of motion are very difficult to solve; consequently, it is the engineer’s responsibility to know which simplifying assumptions can be made. This, of course, requires experience and, more important,

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a understanding of the physics involved.

1.2 流体力学基础

流体运动表现出多种不同的运动形式。有些可以简单描述，而其它的则需要完全理解其内在的物理规律。在工程应用中，尽量简单地描述流体运动是非常重要的。简化程度通常取决于对精确度的要求，通常可以接受±10%左右的误差，而有些工程应用则要求较高的精度。

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描述运动的一般性方程通常很难求解，因此，工程师有责任了解可以进行哪些简化的假设。当然，这需要丰富的经验，更重要的是要深刻理解流动所涉及的物理内涵。

Some common assumptions used to simplify a flow situation are related to fluid properties. For example, under certain conditions, the viscosity can affect the flow significantly; in others, viscous effects can be neglected greatly simplifying the equations without significantly altering the predictions. It is well known that the compressibility effects do not have to be taken into account to predict wind forces on buildings or to predict any other physical quantity that is a direct effect of wind. After our study of fluid motions, the appropriate assumptions used should become more obvious. Here we introduce some important general approaches used to analyze fluid mechanics and give a brief overview of different types of flow.

一些常见的用来简化流动状态的假设是与流体性质有关系的。例如，黏性在某些条件下对流体有显著的影响；而在其它条件下，忽略黏性效应的影响可以大大地简化方程，但并不会显著改变计算结果。众所周知，气体速度很高时必须考虑其压缩性，但在预测风力对建筑物的影响程度，或者预测受风力直接影响的其它物理量时，可以不计空气的压缩性。学完流体运动学之后，可以更明显地看出采用了哪些恰当的假设。这里，将介绍一些重要的用来分析流体力学问题的一般性方法，并简要介绍不同类型的流动。

1.2.1 Lagrangian and Eulerian Descriptions of Motion

In the description of a flow field, it is convenient to think of individual particles of which is considered to be a small mass of fluid, consisting of a large number of molecules that occupies a small volume that moves with the flow. If the fluid is incompressible, the volume does not change in magnitude but may deform. If the fluid is compressible, as the volume deforms, it also changes its magnitude. In both cases the particles are considered to move through a flow field as an entity. 1.2.1 拉格朗日运动描述和欧拉运动描述

描述流场时，将着眼点放在流体质点上是非常方便的。每个质点都包含了微小质量的流体，它由大量分子组成。质点占据很小的体积，并随流体流动而移动。对不可压缩流体，其体积大小不变，但可能发生形变。对可压缩流体，不但体积发生形变，而且大小也将改变。

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在上述两种情况下，均将所有质点看作一个整体在流场中运动。

In the study of particle mechanics, where attention is focused on individual particles, motion is observed as a function of time. The position, velocity and acceleration of each particle are listed as s(x0,y0,z0,t), V(x0,y0,z0,t) and a(x0,y0,z0,t) and quantities of interest can be calculated. The point (x0,y0,z0) locates the starting point the name-of each particle. This is the Lagrangian description, named after Joseph L. Lagrange, of motion that is used in a course on dynamics. In the Lagrangian description many particles can be followed and their influence on one another noted. This becomes, however, a difficult task as the number of particles becomes extremely large, as in a fluid flow.

质点力学主要研究单个质点，质点运动是时间的函数。任一质点的位移、速度和加速度可表示为s(x0, y0, z0, t)，V(x0, y0, z0, t)，a(x0, y0, z0, t)，其它相关参量也可计算。坐标(x0, y0, z0)表示质点的起始位置，也是每个质点的名字。这就是拉格朗日运动描述，以约瑟夫Ｌ拉格朗日的名字命名，该描述方法通常用于质点动力学分析。拉格朗日法跟踪多个质点的运动过程并考虑质点间的相互作用。然而，由于实际流体包含质点数目巨大，因而采用拉格朗日法研究流体流动则非常困难。

An alternative to following each fluid particle separately is to identify points in space and then observe the velocity of particles passing each point; we can observe the rate of change of velocity as the particles pass each point, that is, V/x，V/y，V/z and we can observe if the velocity is changing with time at each particular point, that is,

V/t .In this Eulerian description, named after Leonhard Euler, of motion,

the flow properties, such as velocity, are functions of both space and time. In rectangular, Cartesian coordinates the velocities expressed as

V=V(x, y, z, t). The region of flow that is considered is called a flow field.

与分别跟踪每个流体质点不同的另一种方法是将着眼点放在空间点上，然后观察质点经过每个空间点时的质点速度，由此可以得到质点流经各空间点时的速度变化率，即V/x，V/y，V/z；还可以判断某一点上的速度是否随时间变化，即计算V/t。这种描述方法称为欧拉运动描述，以莱昂哈德欧拉的名字命名。在欧拉法中，速度等流动参数是空间和

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时间的函数。在直角笛卡儿坐标系中，速度表示为V=V(x, y, z, t)。我们所研究的流动区域称为流场。

1.2.2 Pathlines and streamlines

Two different lines help us in describing a flow field. A pathline is the focus of points traversed by a given particle as it travels in a field of flow; the pathline provides us with a “history” of the particle’s locations. A photograph of a pathline would required a time exposure of an illuminated particle.

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1.2.2 迹线和流线

可采用两种不同的流动线来帮助我们描述流场。迹线是某一给定质点在流场中运动时所经过的不同空间点形成的轨迹，它记录了质点的“历史”位置。一定曝光时间下可以拍得发亮粒子的运动迹线。

A streamline is a line in the flow possessing the following property: the velocity vector of each particle occupying a point on the streamline is tangent to the streamline, that is, V×dr=0. Since V and dr are in the same direction; recall that the cross product of two vectors in the same direction is zero. A photograph of a streamlines cannot be made directly. For a general unsteady flow the streamlines can be inferred from photographs of short pathlines of a large number of particles.

流线是流场中具有这样特性的线：任一质点在流线上某点处的速度矢量与该流线相切，即Vdr=0。这是因为V和dr具有相同的方向，而具有相同方向的两个矢量的叉乘积等于零。同迹线相比，流线不能直接由相机拍摄获得。对于一般的非定常流动，根据大量质点的短迹线相片可以推断出流线的形状。

1.2.3 One-, two-, and three-dimensional flows

In the Eulerian description of motion the velocity vector, in general, depends on three space variables and time, that is, V=V(z,y,z,t). Such a flow is a three- dimensional flow, because the velocity vector depends on three space coordinates. The solutions to problems in such a flow are

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very difficult and are beyond the space of an introductory course. Even if the flow could be assured to be steady [i.e, V=V(x,y,z)], it would remain a three-dimensional flow. 1.2.3 一维、二维和三维流动

一般来说，欧拉运动描述中的速度矢量取决于三个空间变量和时间变量，即V=V(x, y,

z, t)。这样的流动称为三维流动，因为速度矢量依赖于三个空间坐标。三维流动的求解非

常困难，并且也超出了序言的范围。即使假设流动为定常的（如，V=V(x, y, z)），该流动仍为三维流动。

Often a three dimensional flow can be approximated as a two-dimensional flow. For example, the flow over a wide dam is three-dimensional because of the end conditions, but the flow in the central portion away from the ends can be treated as two-dimensional. In general, a two-dimensional flow is a flow in which the velocity vector depends on only two space variables. An example is a plane flow, in which the velocity vector depends on two spatial coordinates, x and y, but not z [i.e. V=V(x,y)].

三维流动常常可以近似成二维流动。例如，对于一个很宽的大坝，受坝两端条件的影响，水流经大坝时的流动为三维流动；但远离坝端的中间部分的流动可看作是二维的。一般来说，二维流动是指其速度矢量只取决于两个空间坐标的流动。平面流动即是如此，速度矢量只依赖于x，y两个空间坐标，而与z坐标无关（如，V=V(x, y)）。

A one-dimensional flow is a flow in which the velocity vector depends on only one space variable. Such flows occur in long, straight pipes or between parallel plates. The velocity in the pipe varies only with r i.e., u=r(r). The velocity between parallel plates varies only with the coordinate y i.e. , u=u(y). Even if the flow is unsteady so that u=u(y,t), as would be the situation during the startup, the flow is one-dimensional.

一维流动的速度矢量只依赖于一个空间坐标。这类流动常发生在长直管内和平行平板间。管内流动的速度只随到管轴的距离变化，即u=u(r)。平行平板间的速度也只与y坐标有关，即u=u(y)。即使流动为非定常流动，如启动时的情形，u=u(y, t)，但该流动仍是一维的。

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As for developed flows, the velocity profiles do not vary with respect to the space coordinate in the direction of flow. This demands that the region of interest be a substantial distance from an entrance or a sudden change in geometry. There are many engineering problems in fluid mechanics in which a flow field is simplified to a uniform flow: the velocity and other fluid properties, are constant over the area. This simplification is made when the velocity is essentially constant over the area, a rather common occurrence. Examples of such flows are relatively high speed flow in a pipe section, and flow in a stream. The average velocity may change from one section to another; the flow conditions depends only on the space variable in the flow direction.

对于完全发展的流动，其速度轮廓线并不随流动方向上的空间坐标而改变。这要求研究区域要远离入口处或几何形状突然改变的区域。有许多流体力学方面的工程问题，其流场可以简化为均匀流动：速度和其它流体特性参数在整个区域内均为常数。这种简化只对速度在整个区域内均保持不变时才成立，而且这种情况非常普遍。例如：管内的高速流动和溪水的流动。平均速度可能从一个断面到另一个断面有所不同，而流动条件仅取决于流动方向上的空间变量。

1.2.4 Newtonian fluid and non-Newtonian fluid

A Newtonian fluid is a fluid whose stress versus rate of strain curve is linear and passes through the origin. The constant of proportionality is known as the viscosity. A simple relation to describe Newtonian fluid behavior is τ=μdu/dy. τ is the shear stress exerted by the fluid, μ is the fluid viscosity, du/dy is velocity gradient perpendicular to the direction of shear.

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1.2.4 牛顿流体和非牛顿流体

牛顿流体是指应力与变形率关系曲线为过坐标圆点的直线的流体。直线的斜率称为黏度。用τ=μdu/dy这个简单的关系式来描述牛顿流体的特性。τ为流体施加的切向应力，

μ为流体的动力黏度，du/dy为垂直于切应力方向上的速度梯度。

If a fluid does not obey this relation, it is termed as non-Newtonian

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fluid, of which there are several types, including polymer solutions, molten polymers, many solid suspensions and most highly viscous fluids. In a non-Newtonian fluid, the relation between the shear stress and the strain rate is nonlinear, and can even be time-dependent. Therefore a constant coefficient of viscosity can not be defined. A ratio between shear stress and rate of strain (or shear-dependent viscosity) can be defined, this concept being more useful for fluid without time-dependent behavior.

如果流体不满足上述关系式，则被称为非牛顿流体，它包括以下几种类型：聚合物溶液、聚合物熔体、固体悬浮物和高黏度流体。在非牛顿流体中，切向应力和变形率成非线性关系，甚至可能是非定常的，因此不能定义恒定的黏度系数。但可以定义切向应力和变形率的比值(或随切向应力变化的黏度)，这个概念对不具有时间相关性行为的流体非常有用。

1.2.5 Viscous and inviscid flow

A fluid flow may be broadly classified as either a viscous flow or an invisid flow. An inviscid flow is one in which viscous effects do not significantly influence the flow and are thus neglected. In a viscous flow the effects of viscosity are important and can not be ignored. 1.2.5 黏性和非黏性流动

流体的流动可大致分为黏性流动和非黏性流动。非黏性流动是指黏性作用对流动的影响很小、可被忽略的流动。而在黏性流动中，黏度的影响极为重要，不容忽视。

To model an inviscid flow analytically, we can simply let the viscosity be zero: this will obviously make all viscous effects zero. It is more difficult to create an in viscid flow experimentally, because all fluids of interest (such as water and air) have viscosity. The question then becomes: Are there flows of interest in which the viscous effects are negligibly small? The answer is “Yes, if the shear stresses in the flow are small and act over such small areas that they do not significantly affect the flow field. ” This statement is very general, of course, and it will take considerable analysis to justify the inviscid flow assumption.

为了模拟分析非黏性流动，简单地让黏度为零即可，这显然忽略了一切黏性作用。在实

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验室中，制造非黏性流动则非常困难，因为所有的流体（例如水和空气）都有黏性。然后问题变为：是否存在我们感兴趣的、且黏性影响微乎其微的流动？答案是：“存在，只要流动中的切向应力很小，而且其作用范围小到不会显著影响流场就可以”。当然，这种描述非常笼统，需要大量的分析以证明无黏性流动假设是正确的。

Based on experience, it has been found that the primary class of flows, which can be modeled as inviscid flows, is external flows, that is, flows which exist exterior to a body. Inviscid flows are of primary importance in flows around streamlined bodies, such as flow around an airfoil or a hydrofoil. Any viscous effects that may exist are confined to a thin layer, called a boundary layer, that is attached to the boundary, such as that shown in Fig.1-7; the velocity in a boundary layer is always zero at a fixed wall, a result of viscosity. For many flow situations, boundary layers are so thin that can simply be ignored when studying the gross features of a flow around a streamlined body. For example, the inviscid flow solution provides an excellent prediction to the flow around the airfoil, except inside the boundary layer and possibly near the trailing edge. Inviscid flow is also encountered in contractions inside piping systems and in short region of internal flows where viscous effects are negligible.

图1-7 围绕空气翼的流动

根据经验，发现可以用于模拟非黏性流动的基本流动为外部流动，即存在于物体外部的流动。非黏性流动对于绕流线型物体的研究非常重要，如绕流机翼或水翼。任何可能存在的黏性影响只限于薄薄的一层之内，称之为边界层，它紧贴物体的表面，如图1-7所示。受黏性的影响，边界层内固定壁面处的速度始终为零。对于许多流动情形，边界层非常薄，当研究绕流线型流动的总体特征时，可以忽略边界层的影响。例如，对绕翼型的流动，除了边界层内和可能接近尾缘的区域之外，非黏性流动解与实际情况非常吻合。管道系统中收缩段的流动，以及内部流动中黏性影响均可忽略不计的小段区域都可简化成非黏性流动。

Viscous flows include the broad class of internal flows, such as flows in pipes and conduit [

] and in open channels. In such flows viscous

effects cause substantial “losses” and account for the huge amounts of energy that must be used to transport oil and gas in pipelines. The no-slip

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condition resulting in zero velocity at the wall, and the resulting shear stresses, lead directly to these losses.

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内流中的很大一部分情形都属于黏性流动，如管道流、暗渠流以及明渠流。在这些流动中，黏性作用造成相当大的“损失”，以此解释了管道输运石油和天然气必定耗费大量的能源。无滑移条件使得壁面处的速度为零，由此产生的切应力，直接导致这些损失的产生。

1.2.6 Laminar and turbulent flows

A viscous flow can be classified as either a laminar flow or a turbulent flow. In a laminar flow the fluid flows with no significantly mixing of neighboring fluid particles. If dye were injected into the flow, it would mot mix with the neighboring fluid except by molecular activity; it would retain its identity for a relatively long period of time. Viscous shear stress always influences a laminar flow. The flow may be highly time dependent or be steady. 1.2.6 层流和紊流

黏性流动可分为层流和紊流。在层流中，流体与周围流体质点无明显的混合。如果在流动中注入染料，除了分子运动的影响外，流体质点不与周围流体混合，并将在相当长的一段时间内保持其状态。黏性切应力始终影响层流流动。层流可以是高度非定常的，也可以是定常的。

In a turbulent flow fluid motions very irregularly so that quantities such as velocity and pressure show a random variation with time and space coordinates. The physical quantities are often described by statistical averages. In this sense we can define a “steady” turbulent flow: a flow in which the time-average physical quantities do not change in time. A dye injected into a turbulent flow would mix immediately by the action of the randomly moving fluid particles; it would quickly lose its identity in this diffusion process. A laminar flow and a turbulent flow can be observed by performing a simple experiment with a water faucet. Turn the faucet on so the water flows out very slowly as silent stream. This is laminar flow. Open the faucet slowly and observe the flow becoming turbulent. Note that a turbulent flow develops with a relatively small

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flow rate.

在紊流中，流体运动作不规则地变化，速度和压强等参数的大小在时间和空间坐标上呈现随机变化，这些物理量往往通过统计平均值来描述。在这个意义上，可定义“定常”紊流：即时均值不随时间变化的紊流。注入紊流中的染料在流体质点随机运动的作用下，迅速与周围流体进行掺混，染料在此扩散过程中很快就会消散而变得无法识别。层流和紊流可用一个水龙头进行简单实验来观察其流动状态。打开水龙头，这时的水流正如静静的小溪一样，流动得非常缓慢，此时的流动状态就是层流；慢慢开大水龙头，观察到流动逐渐变得紊乱。注意，紊流从相对较小的流量下开始发展而成。

The flow regime depends on three physical parameters describing the flow conditions. The first parameter is a length scale of the flow field, such as the thickness of a boundary layer or the diameter of a pipe. If this length scale is sufficiently large, a flow disturbance may increase and the flow may be turbulent. The second parameter is a velocity scale such as a spatial average of the velocity; for a large enough velocity the flow may be turbulent. The third parameter is the kinetic viscosity; for a small enough viscosity the flow may be turbulent.

流动状态依赖于三个描述流动条件的物理参数。第一个参数是流场的特征长度，如边界层厚度或管道直径。如果这个特征长度尺度足够大，流动中的扰动可能会逐渐增大，从而使得流动转变为紊流。第二个参数是特征速度，如空间平均流速，足够大的流速将导致紊流的产生。第三个参数是运动黏度，流体的黏性越小，紊流的可能性越大。

The three parameters can be combined into a single parameter that can serve as a tool to predict the flow regime. This quantity is the Reynolds number, named after Osborne Reynolds, a dimensionless parameter, defined as Re=VL/ν, where L and V are a characteristic length and velocity, respectively, andνis the kinematic viscosity. If the Reynolds number is relatively small，the flow is laminar; if it is large, the flow is turbulent. This is more precisely stated by defining a critical Reynolds number, Recrit, so that the flow is laminar if Re< the minimum critical Reynolds number and is used for most engineering applications. If the pipe wall is extremely smooth and free of vibration, the critical Reynolds number can be increased as the fluctuation level in the flow is decreased;

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values in excess of 40,000 have been measured. The critical Reynolds number is different for every geometry, e.g. , it is 1500 for flow between parallel plates using the average velocity and the distance between the plates.

上述三个参数可以整理成一个参数，用于预测流动状态。这个参数就是雷诺数，以奥斯本雷诺的名字命名，该参数为无量纲参数，定义为Re=VL/，式中，L和V分别为特征长度和特征速度，为运动黏度。例如，在管道流中，L为管径，V为平均速度。如果雷诺数相对较小，流动为层流；如果雷诺数较大，则为紊流。通过定义临界雷诺数Recrit，可更加精确地进行表述，当ReIn a boundary layer that exist on a flat plate, due to a constant-velocity fluid stream, the length scale changes with distance from the upstream edge. A Reynolds number is calculated using the length x as the characteristic length. For a certain xT, Re becomes Recrit and

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the flow undergoes transition from laminar to turbulent. For a smooth rigid plate in a uniform flow with a low free-stream fluctuation level, values as high as Recrit=106 have been observed. In most engineering applications we assume a rough wall, or high free stream fluctuation level, with an associated Reynolds number of approximately 3×105.

对于平板上的边界层，由于来流为均匀来流，其特征长度随到前缘点的距离x而变化。计算雷诺数时采用长度x作为特征长度。在某一特定的xT下，Re变为Recrit，流动从层流过渡到紊流。处于均匀流中的光滑刚性平板，且自由来流的脉动水平较低时，已观测到的临界雷诺数高达10。在大多数工程应用中，通常假设壁面为粗糙壁面，或者自由来流的脉动水平较高时，相应的临界雷诺数约为3×10。

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1.2.7 Incompressible and compressible flows

An incompressible flow exists if the density of each fluid particle remains relatively constant as it moves through the flow field, that is ,

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Dρ/Dt=0. This does not demand that the density is everywhere constant. If the density is constant, then obviously, the flow is incompressible, but that would be a more restrictive condition. Atmospheric flow, in which ρ=ρ(z), where z is vertical height, and flows that involve adjacent layers of fresh and salt water, as happens when rivers enter the ocean, are examples of incompressible flows in which the density varies. 1.2.7 不可压缩和可压缩流动

如果任一流体质点在通过流场时密度保持相对恒定，即D/Dt=0，则该流动为不可压缩流动。这并不要求各处的密度值均相等。如果流场中各处的密度值均相等，则很明显，流动是不可压缩的，但那是一种更加严格的情况。密度发生变化的不可压缩流动的例子有大气流动，=(z)，z为垂直方向的坐标，以及江河流入海洋时淡水与盐水相邻的分层流动。

In addition to liquid flows, low-speed gas flows, such as the atmospheric flow referred to above, are also considered to be incompressible flows. The Mach number, named after Ernst Mach, is defined as M=V/c, where V is the gas speed and the wave speed c=(κRT)0.5. If M<0.3, density variation are at most 3% and the flow is assured to be incompressible; for standard air this corresponds to a velocity below about 100m/s. If M>0.3, the density variations influence the flow and compressibility effects should be accounted for; such flows are compressible flows.

除液体流动之外，低速气体流动也被视为不可压缩流动，例如上文提到的大气流动。马赫数，以厄恩斯特马赫的名字命名，定义为M=V/c，V是气体流速，波的传播速度为cRT。如果M＜0.3，密度的最大变化为3%，此时流动可认为不可压缩的；对于标准状态下的大气，这种情况对应的气体流速低于100 m/s。如果M＞0.3，密度的变化将影响流动，则必须考虑流体压缩性带来的影响，这样的流动就是可压缩流动。

Incompressible gas flows include atmospheric flows, the aerodynamics of landing and takeoff of commercial aircraft, heating and air-conditioning airflows, flow around automobiles and through radiations, and the flow of air around buildings, to name a few. Compressible flows include the aerodynamics of high-speed aircraft, airflow through jet engines, steam flow through the turbine in a power

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plant, airflow in a compressor, and the flow of the air gas mixture in an internal combustion engine.

不可压缩的气体流动包括大气流动、商用飞机着陆和起飞时的气体流动、供暖和空调系统中的气流、绕流汽车周围的流动、通过散热器的气流以及绕流建筑物的气体流动等等，不胜枚举。可压缩流动包括高速飞行器周围的气体流动，通过喷气式发动机的气体流动，电站中通过汽轮机的蒸汽流动，压缩机中的气体流动以及内燃机中空气和燃气混合物的流动。

1.3 Fundamentals of heat transfer

Heat transfer is the science that seeks to predict the energy transfer that may take place between material bodies as a result of temperature difference. Thermodynamics teaches that this energy transfer is defined as heat. The science of heat transfer seeks not merely to explain how heat energy may be transferred, but also to product the rate at which the exchange will take place under certain specified conditions. The fact that a heat-transfer rate is the desired objective of an analysis points out the difference between heat transfer and thermodynamics. Thermodynamics deals with systems in equilibrium; it may be used to predict the amount of energy required to change a system from one equilibrium state to another; it may not be used to predict how fast a change will take place since the system is not in equilibrium during the process. Heat transfer supplements the first and second principles of thermodynamics by providing additional experimental rules which may be used to establish energy-

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transfer rate. Here we introduce three modes of heat and different types of heat exchanger.

1.3 传热学基础

传热学是一门研究在存在温差的物体间发生能量传递的科学。热力学中将这种方式传递的能量定义为热量。传热学不仅可以解释热量传递是如何传递的，而且可以计算在特定条件下的传热速率。事实上，传热速率正是一个分析所期望的目标，它指明了传热学和热力学间的差别。热力学处理的是平衡状态下的系统，它可计算当系统从一个平衡状态过渡到另一个平衡状态时所需要的能量，但不能解决系统处于过渡过程的非平衡状态时能量变化的快慢程度。传热学提供了可用于计算传热速率的实验关联式，从而对热力学第一定律和第二定律进

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行补充。这里，我们介绍热量传递的三种方式和不同型式的换热器。

1.3.1 Conduction heat transfer

When a temperature gradient exists in a body, experience has shown that there is an energy transfer from the high-temperature region to the low-temperature region. We say that the energy is transferred by conduction and that the heat transfer rate per unit area is proportional to the normal temperature gradient: q/A~T/x. When the proportionality constant is inserted

qAT (1-3) xWhere q is the heat transfer rate and T/x is the temperature gradient in the direction of heat flow. The positive constant λ is called the thermal conductivity of the material, and the minus sign is inserted so that the second principle of thermodynamics will be satisfied; i.e., heat must flow downhill on the temperature scale. Equation (1-3) is called Fourier’s law of heat conduction after the French mathematical physicist Joseph Fourier, who made very significant contributions to the analytical treatment of conduction heat transfer. It is important to note that Equation (1-3) is the defining equation for the thermal conductivity and that λhas the units of watts per meter per Celsius degree in a typical system of units in which the heat flow is expressed in watts. 1.3.1 热传导

当物体内部存在温度梯度时，经验表明，就有能量从高温区向低温区传递。我们说，此时的能量通过传导进行传递，单位面积上的传热速率与法向温度梯度成正比，即q/A~T/x。引入比例系数，则有

qAT (1-3) x其中q是热流量，T/x是热流方向上的温度梯度，正常数称为材料的导热系数。方程中插入的负号表示热传导过程应满足热力学第二定律，即热量必须沿温度降低的方向传递。式(1-3)称为傅立叶导热定律，以法国数理学家约瑟夫傅立叶的名字命名，傅立叶在导热的分析处理方面做出了极其重大的贡献。值得注意的是，式(1-3)也是导热系数的定义式，在典型的单位体系中，当热流量q的单位为W时，的单位为W/(m℃)。

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1.3.2 Convection heat transfer

It is well known that a hot plate of metal will cool faster when placed in front of a fan then when exposed to still air. We say that heat is convected away; and we call the process convection heat transfer. The term convection provides the reader with an intuitive notion concerning the heat-transfer process; however, this intuitive notion must be expanded to enable one to arrive at anything like an adequate analytical treatment of the problem. For example, we know that the velocity at which the air blows over the hot plate obviously influences the heat transfer rate. But does it influence the cooling in a linear way; i.e., if the velocity is doubled, will the heat transfer double? We should suspect that the heat transfer rate must be different if we cooled the plate with water instead of air, but, again, how much difference would there be? These questions may be answered with the aid of some rather basic analyses. For now, we sketch the physical mechanism of convection heat transfer and show its relation to the conduction process.

图1-8 对流换热

1.3.2 对流换热

众所周知，与热金属板放置在静止的空气中相比，放置在转动的风扇前的热金属板会更快地冷却。我们说热量通过对流进行传递，称此类换热过程为对流换热。对流这个术语给读者提供了有关传热过程的直观概念，然而，必须扩展这种直观概念，使我们可以达到对某一问题进行充分的分析和处理。例如，我们知道流过热平板的空气速度会明显影响其传热量，但它是以线性方式影响冷却的吗？即如果速度增加一倍，传热量也会增加一倍吗？我们猜想，如果用水代替空气冷却热平板，传热量可能有所不同，但是，二者的差异会有多少呢？这些问题在了解一些非常基本的分析后，可得以回答。现在，我们来简要描述对流换热的物理机理，并且说明它和传导过程的联系。

图1-8

Consider the heat transfer plate shown in Fig.1-8. The temperature of the plate is Tw and the temperature of the fluid is T∞. The velocity of the flow will appear as shown, being reduced to zero at the plate as a result of viscous action. Since the velocity of the fluid layer at the wall will be zero, the heat must be transferred only by conduction at that point. Thus we might compute the

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heat transfer, using Equation (1-3), with the thermal conductivity of the fluid and the fluid temperature gradient at the wall. Why, then, if the heat flows by conduction in this layer, do we speak of convection heat transfer and need to consider the velocity of the fluid? The answer is that the temperature gradient is dependent on the rate at which the fluid carries the heat away; a high velocity produces a large temperature gradient, and so on. Thus the temperature gradient at the wall depends on the flow field, and we must develop in our later analysis an expression relating the two quantities. Nevertheless, it must be remembered that the physical mechanism of heat transfer at the wall is a conduction process. 被加热的平板如图1-8所示，平板的温度为Tw，流体的温度为T∞。速度分布如图所示，

受黏性作用，平板上的速度减小为零。因为壁面处流动薄层的速度为零，因此，在该点上热量只能以导热方式传递。因此，可以利用式(1-3)，以及壁面上的流体导热系数和温度梯度来计算传热量。如果热量在该层经导热传递，那么，为什么我们要谈及对流换热以及需要考虑流体速度的影响呢？答案是，温度梯度依赖于流体带走热量的速度，较高的流速将产生较大的温度梯度。因此，壁面上的温度梯度依赖于流场的变化，在以后的分析中，我们将建立这二者间的关系。然而，必须记住，壁面上传热的物理机理是一导热过程。

To express the overall effect of convection. We use Newton’s law of cooling:

qhA(TwT) (1-4)

Here the heat-transfer rate is related to the overall temperature difference between the wall and fluid and the surface area A. The quantity h is called the convection of heat-transfer coefficient, and Equation (1-4) is the defining equation. An analytical calculation of h may be made for some systems. For complex situations it must be determined experimentally. From Equation (1-4) we note that the units of h are in watts per square meter per Celsius degree when the heat flow is in watts.

为描述对流换热的整体效应，应用牛顿冷却定律 qhA(TwT) (1-4)

这里，热流量与壁面和流体间的整体温度差以及表面积A有关。参数h称为对流换热系数，式(1-4)是其定义式。对某些传热过程，可获得h的分析表达式，而复杂情形下的传热系数

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必须通过实验研究来确定。式(1-4)表明，当热流量的单位为W时，h的单位为W/(m℃)。

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If a heat plate were exposed to ambient room air without an external source of motion, a movement of the air would be experienced as a result of the density gradients near the plate. We call this natural, or free, convection as opposed to forced convection, which is experienced in the case of the fan blowing air over a plate. Boiling and condensation phenomena are also grouped under the general subject of convection heat transfer.

如果将热平板置于没有外部风源的房间空气中，平板附近的密度梯度将造成空气运动。我们称此换热过程为自然对流，以区别于风扇吹扫平板表面时形成的强制对流。沸腾和凝结现象也属于对流换热的范畴。

1.3.3 Radiation heat transfer

In contrast to the mechanisms of conduction and convection, where energy transfer through a material medium is involved, heat may also be transferred through regions where a perfect vacuum exists. The mechanism in this case is electromagnetic radiation. We shall limit our discussion to electromagnetic radiation which is propagated as a result of a temperature difference; this is called thermal radiation. 1.3.3 辐射换热

对于导热和对流换热，其热量传递需要介质才得以进行，与此不同的是，热量也可以在完全真空中传递，其传热机理是电磁辐射。我们将讨论限定在由温差导致的电磁辐射，即所谓的热辐射。

Thermodynamic considerations show that an ideal thermal radiator, or blackbody, will emit energy at a rate proportional to the fourth power of the absolute temperature of the body and directly proportional to its surface area. Thus

4 qemittedAT (1-5

Where is σ the proportionality constant and is called the Stefan-Boltzmann constant with the value of 5.669×10-8W/(m2·K4). Equation (1-5) is called the Stefan- Boltzmann law of thermal radiation, and it

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applied only to blackbodies. It is important to note that this equation is valid only for thermal radiation; other types of electromagnetic radiation may not be treated so simply.

热力学研究表明，对于理想的热辐射体或黑体，其辐射力正比于物体绝对温度的四次方及其表面积，因此有

4 qemittedAT (1-5)

式中，为比例系数，称为斯忒藩－玻耳兹曼常数，其值为5.669×10 W/(m·K)。式(1-5)称为热辐射的斯忒藩－玻耳兹曼定律，该式仅适用于黑体。值得注意的是，该表达式仅适用于热辐射，其它类型的电磁辐射要比该式复杂得多。

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Equation (1-5) governs only relation emitted by a blackbody. The net radiant exchange between two surfaces will be proportional to the difference in absolute temperatures to the fourth power; i.e.

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式(1-5)只能用于确定单个黑体的辐射能。两个表面间的净辐射换热量与其绝对温度四次方的差成正比，即

qnet　exchangeA(T14T24) (1-6)

We have mentioned that a blackbody is a radiate energy according to the T4 law. We call such a body black because black surface, such as a piece of metal converted with carbon black, approximate this type of behavior. Other types of surfaces, such as a glossy painted surface or a polished metal plate, do not radiate as much energy as the blackbody; however, the total radiation emitted by these bodies will generally follow the T41 proportionality. To take account of the “gray” nature of such surfaces we introduce another factor into Equation (1-5), called the emissivity ε, which relates the radiation of the “gray” surface to that of an ideal black surface. In addition, we must take into account the fact that not all the radiation leaving one surface will reach the other surface since electromagnetic radiation travels in straight lines and some will

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be lost to the surroundings. We therefore introduce two new factors in Equation (1-5) to take into account both situations, so that

qFFGA(T14T24) (1-7)

Where Fε is an emissivity function and FG is a geometric “view factor” function. It is important to alter the reader at this time, however, to the fact that these functions usually are not independent of one another as indicated in equation (1-7).

我们已经提到，黑体是按四次方定律辐射能量的物体。因其黑色的表面我们称之为黑体，如覆盖炭黑的金属片，就近似具有这种辐射特性。其它类型的表面，如有光泽的漆面或抛光的金属板，并不具有黑体那样大的辐射力，然而，这些物体的辐射力仍大致与T14成正比。为了考虑这些表面的“灰”特性，在式(1-5)引入另一个参数，称为发射率ε，发射率将这些“灰”表面的辐射与理想黑体的表面辐射联系起来。此外，我们必须考虑这样一个事实，并非一个表面发出的所有辐射都可以到达到另一个表面，因为电磁辐射是沿直线传播的，将有部分能量散失到周围环境中。因此，考虑到这两种情况，式(1-5)引入另外两个新的参数，则有

qFFGA(T14T24) (1-7)

式中，Fε是发射率函数，FG是几何角系数。此时，值得提醒读者的是，式(1-7)中的这两个函数通常并不是相互的。

1.3.4 Types of heat exchangers

The simplest type of heat exchanger consists of two concentric pipes of different diameters, called the double-pipe heat exchanger. One fluid in a double pipe heat exchanger flows through the smaller pipe while the other fluid flows through the annular [环形] space between the two pipes. Two types of flow arrangement are possible in a double pipe heat exchanger; in parallel flow, both the hot and cold fluid enters the heat exchanger at the same end and move in the same direction. In counter flow, on the other hand, the hot and cold fluids enter the heat exchanger at opposite ends and flow in opposite directions. 1.3.4 换热器的类型

最简单的换热器是由两个不同直径的同心圆管组成，称为套管式换热器。套管换热器中的一种流体流经细管，另一种流体流经两管间的环形区域。套管换热器中包括两种不同类型的流动方式：一种为顺流，即冷、热流体从同一端进入换热器，并沿同一方向流动；另一种

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为逆流，即冷、热流体从相反的两端进入换热器，且沿相反方向流动。

Another type of heat exchanger, which is specifically designed to realize a large heat transfer surface area per unit volume, is the compact heat exchanger. The ratio of the heat transfer surface area of a heat exchanger to its volume is called the area density β. A heat exchanger with β>700m2/m3 is classified as being compact. Example of compact heat exchangers are car radiators (β≈1000m2/m3), glass ceramic gas turbine heat exchangers (β≈6000m2/m3), the regenerator of a Stifling engine (β≈15,000m2/m3), and human lung (β≈20,000m2/m3). Compact heat exchangers enable us to achieve high heat transfer rates between two fluids in a small volume, and they are commonly used in applications with strict limitations on the weight and volume of heat exchangers.

另一类换热器，被专门设计成单位体积内有很大的换热面积，称为紧凑式换热器。换热器的换热面积与其体积之比称为面积密度β。β＞700 m/m的换热器归为紧凑式换热器。例如汽车散热器（β≈1000 m/m）、燃气轮机中的玻璃陶瓷换热器（β≈6000 m/m）、斯特林机的回热器（β≈15,000 m/m）以及人的肺部（β≈20,000 m/m）。紧凑式换热器能实现小容积内两种流体的高换热率，通常用于换热器重量和容积受到严格的场合。

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The large surface area in compact heat exchangers is obtained by attaching closely spaced thin plate or corrugated fins to the walls separating the two fluids. Compact heat exchangers are commonly used in gas-to-gas and gas-to-liquid (or liquid-to-gas) heat exchangers to counteract the low heat transfer coefficient associated with gas flow with increased surface area. In a car radiator, which is a water-to-air compact heat exchanger, for example, it is no surprise

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that fins are attached to the air side of the tube surface.

图1-9

紧凑式换热器通过在分离两种流体的壁面上附加间隔紧密的薄板或波纹翅片来扩展其表面。紧凑式换热器通常用于气－气和气－液（或液－气）换热器，通过增加传热面积来抵消气侧低传热系数所带来的影响。例如，汽车散热器是水－气紧凑式换热器的典型例子，通

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常管子气侧表面装有翅片。

Perhaps the most common type of heat exchanger in industrial applications is the shell-and-tube heat exchanger, shown in Fig.1-9. Shell-and-tube heat exchangers contain a large number of tubes (sometimes several hundred) packed in a shell with their axes parallel to that of the shell. Heat transfer takes place as one fluid flows inside the tube while the other fluid flows outside the tubes through the shell. Baffles are commonly placed in the shell to force the shell-side fluid to flow across the shell to enhance heat transfer and to maintain uniform spacing between the tubes. Despite their widespread use, shell-and-tube heat exchangers are not suitable for use in automotive and aircraft applications because of their relatively large size and weight. Note that the tubes in a shell-and-tube heat exchanger open to some large flow areas called headers at both ends of the shell, where the tube-side fluid accumulates before entering the tubes and after leaving them.

图1-9 管-壳式换热器简图

工业应用中最常见的换热器也许是管壳式换热器，如图1-9所示。管壳式换热器外壳里封装有大量的管束（有时为数百根），其轴线与外壳轴线平行。当一种流体在管内流动，另一种流体在管外流动并穿过壳体时，就进行了热交换。壳内通常布置有挡板，用于使壳侧流体沿壳流动以强化传热，并保持均匀的管间距。虽然管壳式换热器应用广泛，但因其相对较大的尺寸和重量，因而并不适用于汽车和航空器领域。注意，管壳式换热器的管束两侧开口处的较大流动区域称为封头，它位于壳体两端，管侧流体流入、流出管子前后都在此汇集。

Shell-and-tube heat exchangers are further classified according to the number of shell and tube passes involved. Heat exchangers in which all the tubes make one U-turn in the shell, for example, are called one-shell-pass and two-tube-passes heat exchangers. Likewise, a heat exchanger that involves two passes in the shell and four passes in the tubes is called a two-shell-passes and four-tube-passes heat exchanger.

管壳式换热器依据所含管程和壳程的数目可进一步分类。例如，换热器壳内的所有管束采用一个U型布置的称为单壳程双管程换热器（1-2型换热器）。同样地，含有双壳程和四管程的换热器叫做双壳程－四管程型换热器（2-4型换热器）。

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An innovative type of heat exchanger that has found widespread use is the plate and frame (or just plate) heat exchangers, which consist of a series of plates with corrugated flat flow passages. The hot and cold fluids flow in alternate passages and thus each cold fluid stream is surrounded by two hot fluid streams, resulting in very effective heat transfer. Also, plate heat exchangers can grow with increasing demand for heat transfer by simply mounting more plates. They are well suited for liquid-to-liquid heat exchange application provided that the hot and cold fluid streams are at about the same pressure.

一种广泛使用的新型换热器是板翅式（或板式）换热器，它由一系列平板组成，并形成波纹状的流动通道。冷、热流体在间隔的每个通道中流动，每一股冷流体被两股热流体所包围，因此换热效果非常好。此外，板式换热器可通过简单添加更多的平板来满足增强换热的需求。该类型换热器非常适用于液－液式换热场合，但需要冷、热液流的压强大致相等。

Another type of heat exchanger that involves the alternate passage of the hot and cold fluid stream through the same flow area is the regenerative heat exchanger. The static-type regenerative heat exchanger is basically a porous mass that has a large heat storage capacity, such as a ceramic wire mesh. Hot and cold fluid to the matrix of the regenerator during the flow of the hot fluid, and from the matrix to the cold fluid during the flow of the cold fluid. Thus, the matrix serves as a temporary heat storage medium. The dynamic-type regenerator involves a rotating drum and continuous flow of the hot and cold fluid through different portions of the drum so that any portion of the drum passes periodically through the hot

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stream, storing heat, and then through the cold stream, rejecting this stored heat. Again the drum serves as the medium to transport the heat from the hot to the cold fluid stream.

另一类冷、热流体交替通过同一流动面积的换热器为蓄热式换热器。静态型蓄热式换热器基本上由多孔介质组成，其热容量大，如陶瓷铁丝网。冷、热流体交替地流经这些多孔介

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质，热量先由流过的高温流体传递到换热器的换热基体，再由基体传递给接着流过的低温流体。因此，基体充当了临时储热介质的作用。动态型蓄热式换热器内有转筒，冷、热流体连续流动通过转筒的不同部分，使得转筒的任一部分周期性地通过热流体，存储热量，再通过冷流体，释放存储的热量。转筒作为热量从热流体传递到冷流体的媒介。

Heat exchangers are often given specific names to reflect the specific application for which they are used. For example, a condenser is a heat exchanger in which one of the fluids is cooled and condenses as it flows through the heat exchanger. A boiler is another heat exchanger that transfers heat from the hot fluid (flue gas) to the surrounding space by radiation.

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换热器往往被赋予特定的名称来反映它们的特定用途。例如，冷凝器是流体流经它时会发生冷却凝结的一种换热器。锅炉是另一类换热器，流体在其内吸热并汽化。空间辐射器是以辐射方式将热流体的热量传递到周围空间的换热器。

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