第一章基本概念 Chapter 1. Basic Concepts

第一章基本概念Chapter 1. Basic Concepts • 热力系统（Thermodynamic System） • 系统的状态参数（Properties of A System） • 状态及平衡状态（State and Equilibrium） • 过程与循环（Process and Cycles）

§1.1 热力系统Thermodynamic System • 热力系统的定义（Definition ）系统就是指被选做研究对象的物体或空间。（A quantity of matter or a region in space chosen for study.） For example: 汽轮机Steam turbine 气缸-活塞装置 Cylinder-piston equipment

2. 系统，外界和边界 System, Surroundings and boundary • 外界（Surroundings） The mass or region outside the system （2）边界（Boundary） The real or imaginary surface that separates the system from its surroundings. (3)系统与外界之间的质量及能量交换都必须穿越边界 The mass or energy exchange between system and its surroundings must cross the boundaries.

(4) Characteristics of Boundary • 边界是人为选定的 Boundaries are selected subjectively. • 边界可以是固定的，也可以是可移动的Boundaries can befixed or movable. • 边界可以是真实的，也可以是假想的Boundaries can bereal or imaginary.

热力系统选取的人为性 过热器(reheater) 汽轮机(turbine) 只交换功锅炉boiler 发电机(generator) 既交换功也交换热凝汽器(condenser) 只交换热给水泵(pump)

固定、活动 fixed 、 movable real 、 imaginary 真实、虚构

3.热力系统分类 Types of systems 物质流进和流出热力系统物质交换能量交换热力系外界传热和作功两种形式 Energy can cross the boundary, in the form of heat and work.

(1) 以系统与外界关系划分： 有无是否传质开口系闭口系是否传热非绝热系绝热系是否传功非绝功系绝功系是否传热、功、质非孤立系孤立系

A. 闭口系统和开口系统 Closed system and Open system 闭口系与外界无物质交换 A Closed system (a control mass 控制质量) consists of a fixed amount of mass, and no mass can cross its boundary. That is, no mass enters or leave a closed system. such as, Piston-cylinder device (汽缸-活塞装置）

开口系与外界有物质交换 An Open system (ora control volume 控制体积）is a properly selected region in space. Both mass and energy can cross the boundary of a control volume. such as, A Water heater, a turbine and a compressor, etc

B. 绝热系统和孤立系统 Adiabatic system and Isolated system 绝热系统与外界之间无热量交换 Adiabatic systemis that no heat cross the boundary or heat is negligible compared with work cross the boundary 孤立系统与外界之间无任何物质和能量交换 Isolated systemis a special case that no mass and energy cross the boundary.

m 2 1 Q W 4 3 1 开口系 1+2 闭口系 1+2+3 绝热闭口系 1+2+3+4 孤立系非孤立系＋相关外界＝孤立系

（2）其它分类方式 物理化学性质均匀系（Even System) 非均匀系(Uneven System) 单元系 (Single substance system) 工质种类其它分类方式多元系(multi substance system) 单相 (Single Phase) 相态多相(Multiple Phase)

简单可压缩系统 Simple compressible system 最重要的系统只交换热量和一种准静态的容积变化功 Moving Boundary Work 容积变化功 Compression Work 压缩功膨胀功 Expansion Work

§1.2状态及状态参数State and State Properties • 状态：某一瞬间热力系所呈现的宏观状况 State: it reveals the physical condition of a system. Consider a system not undergoing any change. At this point, all the properties can be measured or calculated throughout the entire system. It can be described by a set of properties

2. Property （状态参数） （1）状态参数：描述系统宏观物理状况的物理量,简称参数 Properties are used to depict any characteristic of a system. such as Pressure P （压力），temperature T（温度）， volume V（体积），mass m（质量）， internal energy U（内能），enthalpy（焓）， entropy（熵）， viscosity（粘度）， thermal conductivity（导热系数）。

（2）状态参数的特征Characteristics of State Properties • A. 状态确定，则状态参数也确定，反之亦然 • Properties of a state are determined by the state. • If the state is specified, its properties are fixed, or vise versa. • B. 状态参数的积分特征：状态参数的变化量与路径无关，只与初终态有关 • The magnitude of the change in property is independent of the path (route), but just depend on the initial and final states.

After a series of change, the state returns to the original one, then the changes in its properties are ‘0’ C. 状态参数的微分特征：全微分 Properties are functions of state, there exist differential for properties

状态参数的微分特征 dz是全微分设 z =z (x , y) Total differentials 充要条件：可判断是否是状态参数

3. 基本状态参数 Basic State Properties 压力p、温度 T、比容 v（容易测量） (1)密度和比容 Density and Specific Volume 密度指单位体积所含物质的多少。 Density is defined as mass per unit volume.

比体积 单位质量的物质所占有的体积，用v表示。 Specific volume is the reciprocal of density and is defined as volume per unit mass. 比体积是表示物质内部分子疏密程度的状态参数。

① 定义 Definition:垂直作用于单位面积上的力 It is defined as the force exerted by a fluid vertically on a surface of unit area. (for solid is stress: person stand on foot) ②压力的单位 Unit of pressure It has the unit of Newtons per square meter （N/m2) 1 Pa=1 N/m2 1 kPa= 103 Pa 1 MPa= 106 Pa (2) 压力 Pressure 绝对压力 SI单位制

其他单位 Other units 1 bar =105 Pa=100 kPa =0.1 MPa 标准大气压 Standard atmosphere 1 atm= 101325 Pa=101.325 kPa 工程大气压 Engineer atmosphere 1 at=1 kgf/cm2 =9.807 N/cm2 =9.807*104Pa 液柱高度Height of liquid column 1 atm=760 mm Hg 1 at =10 m H2O 1mmHg=1ρgh=133.3 Pa

③ 绝对压力和相对压力 Absolute pressure and relative pressure 绝对压力指系统中工质的真实压力。 Absolute pressure isthe actual pressure at a given position P . 相对压力反映绝对压力与当地大气压力的差值。 Relative pressure indicates the difference between the absolute pressure and the local atmospheric pressure .

④ 表压与真空度 Gage Pressure and Vacuum Pressure • 表压力：当高于大气压时，压力计显示的绝对压力超出大气压力的部分。 Gage pressure（表压力）is denoted as Pg 真空度表示绝对压力低于大气压力的量值 Vacuum pressure （真空度）：Pressures below atmosphere pressure. It is the pressure difference between atmospheric and system pressure when system pressure is lower than atmospheric And it is denoted as PVAC or H. 表压力=绝对压力-大气压力真空度=大气压力-绝对压力

绝对压力与相对压力 absolute pressure relative pressure 表压力 pg 当p>pb Gage pressure 当p < pb 真空度 pv Vacuum pressure pe pv p pb p

Attentions 注意 • 测量压力的仪表通常处于大气环境中，不能直接测量出绝对压力，显示的是绝对压力和当时当地大气压的差值。 • 如果大气压力发生变化，即使工质的绝对压力不变，测压计的对数也会变，所以只有绝对压力才是状态参数。 • 工程计算中，必须选取绝对压力

Variation of pressure with Depths • Pressure is a scalar quantity. （压力是一个标量） At any point in a fluid, Pressure is the same in all directions. • Pressure in a fluid increases linearly with depth. （液体中的压力随液体的深度线性增加） • Pressure is the same at all points on a horizontal plane in a given continuous fluid at rest. （帕斯卡定律）.

注意：只有绝对压力 p 才是状态参数 示意图 ④ 压力p的测量 Pressure Measuring 绝对压力与环境压力的相对值 ——相对压力 U-tube manometer Bourdon Tube

大气压力 Atmospheric pressure barometric 注意：环境压力一般为大气压，但不一定。 h barometer 环境压力与大气压力环境压力 Environmental pressure 指压力表所处环境

大气压力Atmospheric pressure 大气压随时间、地点变化物理大气压1atm= 760mmHg 1mmHg=ρgh= 133.322Pa

A.The Manometer （压力计） • Manometers measure a pressure difference by balancing the weight of a fluid column between the two pressures of interest.

Example 1: U-tube manometer using multiple fluid column (采用多种液柱的U型管压力表） Pressure in a continuous static fluid is the same at any horizontal level so, For the left hand arm For the right hand arm

B. Bourdon tube （弹簧管式压力表） • It is a mechanical pressure measurement device and consists of a hollow metal tube bent like a hook whose end is closed and connected to a dial indicator needle. C-Type

Exercise.1 (练习1） As shown in the following figure，it is known that pb=101325Pa, the height difference is H=300mm for murcury liquid. The gauge pressure of B is 0.2543MPa, Then what is the pressure for side A, and what is the value of Pg,A? 已知大气压pb=101325Pa，U型管内汞柱高度差H=300mm，气体表B读数为0.2543MPa，求：A室压力pA及气压表A的读数pgA

解：

(3) Temperature （温度） 传统：冷热程度的度量。感觉，导热，热容量微观：衡量分子平均动能的量度 T 0.5m w 2 热力学第零定律（R.W. Fowler in 1931）如果两个系统分别与第三个系统处于热平衡，则两个系统彼此必然处于热平衡。温度测量的理论基础 B 温度计

Equality of temperature is a necessary and sufficient condition for thermal equilibrium, i.e. no transfer of heat. （温度相等是热平衡的充要条件）

Temperature measurement 物质（水银，铂电阻）特性（体积膨胀，阻值）温度计基准点 Reference state 刻度Scale 温标Temperature scale

温标 Temperature scale • 热力学温标（绝对温标）Kelvin scale • 摄氏温标Celsius scale Temperature 273.16 K 0.01 ℃ The temperature interval on both scales is the same. • 华氏温标Fahrenheit scale (German, • G. Fahrenheit, 1686-1736) • 朗肯温标Rankine scale(W. Rankine, • 1820-1872)

常用温标之间的关系 摄氏℃ 绝对K 华氏F 朗肯R 水沸点 100 212 373.15 671.67 559.67 37.8 100 发烧水三相点 273.16 0.01 32 491.67 273.15 冰熔点 0 盐水熔点 -17.8 459.67 0 -273.15 -459.67 0 0

温标的换算

Property Symbol S.I. Unit Temperature T Kelvin (K) Pressure P Pascals (Pa) 5.强度参数和广延参数 Intensive and Extensive Properties （1）强度参数:与质量的多少无关的参数，不具有可加性 Intensive properties are those independent of the size of a system, such as temperature, pressure and density.

(2) 广延参数 广延参数:与物质的量有关的参数，具有可加性） Extensive properties are those whose values depend on the size or extent of the system 如质量m、容积V、内能U、焓H、 such as mass, volume, internal energy and enthalpy

Property Definition Symbol S.I.Unit Volume Volume of a substance V m3 Internal Energy The translational, rotational and vibrational kinetic energy of a substance U Joules (J) Enthalpy U + PV H Joules (J) Entropy The entropy is a measure of the lack of structure or the amount of disorder in a system S Joules/Kelvin (J / K)

比参数: • 比参数是单位质量的广延参数Specific properties is extensive properties per unit mass. • 比容 • 比内能 • 比焓 • 比熵

强度参数与广延参数 动能速度（强）（广） Velocity Kinetic Energy 位能高度（强）（广） Height Potential Energy 温度（强）内能（广） Temperature Internal Energy

§1.3 平衡状态和状态公理 Equilibrium State and State Postulate (1) 定义 Definition 所谓平衡状态就是指在没有外界影响的情况下，系统的状态不随时间而发生变化。 A system in equilibriumexperiences no changes with timewhen it isisolated from its surroundings. 1.平衡状态（Equilibrium State）

Monatomic gases Permanent diatomic gases Polyatomic gases

(2)如何实现热力学平衡 How to fulfill thermodynamic equilibrium? • 平衡状态下，系统内外不存在不平衡势差（推动力In an equilibrium state there are no unbalance potentials (or driving forces) B. A system is not in thermodynamic equilibrium unless the condition of all the relevant types of equilibrium are satisfied.

第一章基本概念 Chapter 1. Basic Concepts